Water and Waste Management Systems for
Stanford University’s Green Dorm
Final Report
Client: Dr. Sandy Robertson, Stanford Green Dorm
LCC Design, Inc.
Team Coordinator: Charlotte Helvestine (helvesti@stanford.edu)
Christine George (hikachic@stanford.edu)
Leah Yelverton (leah.yelverton@gmail.com)
Tuesday, June 12 2007
EXECUTIVE SUMMARY
LCC Design, Inc. is pleased to present our final design recommendation for the Stanford
Green Dorm water and wastewater system. The design meets all of our established goals for the
project and represents the cutting edge of water and wastewater management innovation. First,
the design provides plans for a water and waste management system that meets the public
demand in the Green Dorm. Second, the design minimizes the environmental impacts of the
building and the net usage of water by outlining methods for water treatment and reuse. Finally,
the design maximizes the opportunities for research education and user health, safety, and
satisfaction within the Green Dorm. Figure 1 shows our final design schematic. Following the
figure are detailed descriptions of each of the three facets of the design: the Greywater System,
the Blackwater System, and the Green Roof and Rainwater Harvesting System.
| Inflow | Water Uses |
3000
gal/mo
(summer)
Treatment
Rainwater Storage
600 gpd
Showers
variable
Settling/
Sedimentation
Tank
|
TERTIARY
Biological
Treatment
600 gpd
Coarse
Filtration
Bathroom Faucet
200 gpd
Clothes Washer
180 gpd
Membrane
Filtration
Disinfection
180 gpd
Outflow
Green Roof Irrigation
variable
200 gpd
| Products |
80 gpd
Urine-diverting Toilet
40 gpd
Composting Toilet
20 gpd
Grey Storage
to inflow
Irrigation
Storage
to inflow
Urine Storage
to local farms
Compost
to Green Roof
TBD
80 gpd
460 gpd
Standard Toilet
470 gpd
50 gpd
50 gpd
Dish Washer
50 gpd
310 gpd
Kitchen Faucet
310 gpd
TBD
Subsurface Irrigation
130 gpd
Leaks
SEWER
with
optional
diversion
to
MBR lab
Figure 1. LCC Final Design Schematic.
2
Greywater System
Greywater flows will be taken from the bathroom sink, shower, and automatic clothes
washers. These flows will be treated using a sand filter and a gravity tank to collect
sedimentation, and water from this stage can then be used for subsurface irrigation. The next step
will be biological treatment, membrane filtration, and disinfection. This will allow the water to
reach the required treatment standards for toilet flushing and laundry reuse.
The red and orange dashed arrows on the Final Design Schematic represent flows of
treated greywater. The expected available supply of treated greywater from the showers,
bathroom faucets, and clothes washers (1000 gpd) is larger than the laundry and toilet demand
for recycled greywater (220-640 gpd, depending on the types of toilets used). However, in the
actual Green Dorm, the supply of greywater will probably be smaller than this because the
treatment efficiency will not be 100% (i.e., there will be some water loss in the treatment
process). There will be between 360-780 gpd of water available for irrigation storage depending
on the chosen design scenario for the Green Dorm.
Blackwater System
LCC recommends a combination of standard toilets, dual-flush urine-diverting toilets,
and micro-flush composting toilets for the Green Dorm. Tertiary treated greywater from the
greywater system will be used for flushing toilets. The dual-flush urine-diverting toilets will
require approximately 0.03 gallons per flush for urine and 0.8 gallons per flush for feces. Urine
from these toilets will be diverted to storage tanks for future use as fertilizer. Feces from the
urine-diverting toilets will be discharged to the sewer. The micro-flush composting toilets will
use one pint of water per flush. Waste from the composting toilets will travel to several central,
active composters in the Green Dorm. These composters will produce solid compost that could
potentially be used as fertilizer, and will produce liquid leachate, the extra fluid that runs through
the composters. The leachate is untreated and will be discharged to the sewer. Kitchen and
faucet wastes will also be discharged to the sewer. In the long term, LCC recommends that all
sewer discharges be diverted instead to an anaerobic digester or membrane bioreactor, either on
the Green Dorm site or in the Civil and Environmental Engineering labs on the Stanford campus.
The flow quantities shown in Figure 1 give the daily flows that would result if all toilets
in the Green Dorm were that particular type. For example, if all Green Dorm toilets were urine-
3
diverting toilets, 80 gallons of flushwater would be required per day, 80 gallons of wastewater
would be released per day, and 20 gallons of urine and urine flushwater would be collected for
storage.
These toilet technologies combined with anaerobic digestion of sewer wastes will provide
significant savings in water use by the Green Dorm, will allow capture of nutrients in the waste
stream for reuse as fertilizer, and will provide an energy source for the Green Dorm.
Combinations of the technologies could result in a 0 – 92% reduction in required flushwater
inflow and a 0 – 89% reduction in toilet wastewater volumes; in addition, 9 – 800 liters of
methane could be produced daily.
Green Roof and Rainwater Harvesting System
Stormwater collected from the green roofs and standard roofs can augment the treated
greywater supply to be reused for toilet flushing, laundry washing machines, and irrigation.
Assuming a tentative total roof catchment area of approximately 10,500 sf and an example green
roof layout presented in the report, LCC estimates an annual runoff volume of 65,000 gallons
from all roof surfaces. The runoff from the different roofs can be stored in a rainwater storage
tank of 15,000 gallons – the volume estimated to meet the dry season irrigation demands of the
green roof – with the remaining runoff being reused with the treated greywater. Small amounts
of runoff can be diverted to the laboratory for further research and testing. Primary treatment of
settling and coarse filtration is recommended before reusing the rainwater, and further filtration
and disinfection is possible through the greywater treatment system. There is ample opportunity
to test different treatment methods with the long-term goal of reusing rainwater for potable uses.
The volume of rainwater available for reuse will vary seasonally, and calculations from
local weather data suggest that total runoff volume will range from 0 gallons per month in the
summer to 18,000 gallons per month in the winter. These estimates along with further
calculations suggest that the intensive and extensive green roof sections will retain an average of
at least 55% and 30% of annual rainfall, respectively, which are mostly consistent with the
annual retention figures found in existing extensive green roofs that range from 50-70%.
Between May and October, when evapotranspiration rates are higher and precipitation is lower,
it is expected that 100% of the rainwater landing in the green roofs will be retained, so the
rainwater available for reuse during those months will come only from the standard roofs.
4
Because of the variability in rainwater availability, rainwater should be used to augment the
water reuse system but should not be considered reliable.
LCC is confident that our design recommendations will allow significant savings in water
use, minimize environmental impacts through nutrient capture and water use reduction, and
provide a fertile setting for future innovations in research. We are excited to be a part of the
Green Dorm project and look forward to its implementation.
5
TABLE OF CONTENTS
EXECUTIVE SUMMARY………………………………………………………….…………..2
TABLE OF CONTENTS………………………………………………………………………..6
1. INTRODUCTION……………………………………………………………...……………..8
2. WATER BALANCE…………………………………………………………………..……..10
2.1. Initial Water Balance………………………………………………………………..10
2.2. LCC Adjustments…………………………………………………………………...10
2.2.1. Water Usage Survey………………………………………………………11
2.2.2. Toilet Systems………………………………………………….………….11
2.2.3. Adjusted LCC Water Balance……………………………………………..12
2.3. Use of Recycled Water for On-Site Irrigation..…………………………….……….13
3. REGULATIONS……………………………………………………………………………..14
3.1. Greywater and Blackwater Regulations Summary…………..………………...……14
3.1.1. State Requirements………………………………………………………..14
3.1.2. Greywater Regulations………………………………………….…………14
3.1.3. Blackwater Regulations………………………………………….………..15
3.2. Rainwater Harvesting Regulations Summary……………..……………….…..……15
4. GREYWATER SYSTEM………………………………………………………………...…16
4.1. Greywater Treatment Technologies…………………………………………………17
4.1.1. Treatment for Subsurface Irrigation Reuse...…………………………...…17
4.1.2. Proposed Packaged Technologies for Tertiary Treatment………...………18
4.2. Living Laboratory Component………………………………………………...……23
5. BLACKWATER SYSTEM……………………………………………………………….…24
5.1. Goals ………………………………………………………………………..………24
5.2. Methods……………………………………………………………………...………24
5.3. Technologies………………………………………………………………...………25
5.3.1. Urine-diverting Toilets………………………………………….…………25
5.3.2. Composting Toilets…………………………………………………..……26
5.3.3. Anaerobic Digesters and Membrane Bioreactors…………………………29
5.4. Scenario Analysis……………………………………………………………………29
5.4.1. Scenario 1: “Business as Usual”……………………………………..……30
5.4.2. Scenario 2: Urine-diverting Toilets……………………………………….31
5.4.3. Scenario 3: Composting Toilets……………………………………...……32
5.4.4. Comparison of Scenarios……………………………………………….…33
5.4.5. The Optimal Scenario?................................................................................34
5.5. Conclusion: Blackwater Design Recommendations…………………………...……36
6. GREEN ROOF AND RAINWATER HARVESTING SYSTEM……………………...…37
6.1. Green Roof Background………………………………………………………….…37
6.2. Stanford Green Roof Layout………………………………………………...………39
6.2.1. Intensive Roof Design………………………………………………..……40
6.2.2. Extensive Roof Design……………………………………………………40
6.3. Green Roof Stormwater Retention…………………………………………..………40
6.3.1. Green Roof Case Studies: Observed Stormwater Retention………………41
6.3.2. Estimated Runoff from Stanford Green Roof……………………..………42
6
6.3.3. Living Laboratory Component……………………………………………44
6.4. Rainwater Harvesting for the Green Dorm………………………………….………45
6.4.1. Estimated Available Runoff…………………………………….…………46
6.4.2. Quality Concerns………………………………………………….………47
6.4.3. System Design and Treatment Suggestions………………….……………49
6.4.4. Living Laboratory Component……………………………………………52
6.5. Temperature and Energy Effects of the Green Roof…………………..……………52
6.5.1. Green Roof Case Studies: Energy…………………………………………52
6.5.2. Observed Temperature Effects……………………………………………53
6.5.3. Observed Energy Effects………………………………….………………53
6.5.4. Estimated Effects of Stanford Green Roof…………………..……………55
6.5.5. Living Laboratory Component ……………………………………...……56
7. LIVING LABORATORY: FUTURE RESEARCH OPPORTUNITIES……...…………57
8. CONCLUSIONS……………………………………………………………………..………62
APPENDIX A: WATER USAGE SURVEY……………………………….…………………63
APPENDIX B: REGULATIONS…………………………………………………...…………64
APPENDIX C: CONTEXT OF WASTE MANAGEMENT SYSTEMS…………….…...…66
APPENDIX D: URINE REUSEABILITY BASED ON STORAGE
TEMPERATURE AND TIME…………….…………………….……….……68
APPENDIX E: (HANDWRITTEN BLACKWATER CALCULATIONS)
APPENDIX F: GREEN ROOF RUNOFF ASSUMPTIONS AND CALCULATIONS……70
APPENDIX G: RECOMMENDED MINIMUM RAINWATER TREATMENT……….....73
REFERENCES…………………………………………………………………….……………74
7
1. INTRODUCTION
The Green Dorm at Stanford University is a bold step forward in the field of
sustainability. The project promises the incorporation of current research ideas to minimize and
hopefully eliminate the building’s negative effects on the natural environment. The research
team for the Green Dorm project states four overarching goals for the completed building: a
living laboratory for research, measurable environmental performance, the most desirable
housing on campus, and economic sustainability1. In addition, there is an unstated goal of
publicizing green buildings and sustainable development with a successful end product that
embodies the sustainability goals.
The water and wastewater systems of the Green Dorm will be vital to the overall
sustainability of the building. Existing dorms on campus use potable water for most indoor uses
and discharge the used water to the wastewater stream, which places burden on our limited water
resources and the capacity of the wastewater treatment plants. With the Green Dorm, there is
vast room for improvement in water recycling and usage efficiency.
With this report, LCC Design, Inc. addresses the goals of the research team with
suggestions for an effective water and waste management system. The specific goals of LCC are
to:
-
Provide plans for a water and waste management system that meets the public demand
-
Minimize (1) the environmental impacts of the building, and (2) the net usage of water by
outlining methods for water treatment and reuse
-
Maximize (1) the opportunities for research and education, and (2) user health, safety, and
satisfaction
To achieve these goals, LCC Design, Inc. focuses on three system areas: greywater, blackwater,
and green roof and rainwater harvesting.
Greywater System
The greywater system will provide a means of substantially reducing the water sent to the
public sewage system each day while simultaneously lessening the strain on potable water
sources, thus reducing the ecological footprint of the Green Dorm. The treatment and reuse of
greywater will bring the Green Dorm closer to achieving its goal of closing the water loop. The
greywater collected daily can be reused in place of valuable potable water for subsurface
irrigation of outdoor landscaping, toilet flushing, and laundry machines. Most importantly, the
8
greywater treatment system will provide an opportunity for research investigating the water
quality, health, and environmental impacts of various treatment technologies. This research will
hopefully contribute to the widespread implementation of greywater recycling systems in other
campuses and buildings worldwide.
Blackwater System
Through the introduction of several innovative technologies, the blackwater system will
allow ample savings in water usage, allow capture of nutrients in the wastewater stream, allow
methane production, and will provide exciting research opportunities for Green Dorm residents
and collaborators. The LCC design recommendation for the blackwater system focuses on toilet
waste streams; Stanford-specific data for kitchen water use is needed before detailed design
recommendations can be made for kitchen wastewater streams.
Green Roof and Rainwater Harvesting System
The green roof for the Stanford Green Dorm (from here on referred to as the “Stanford
green roof”) will not only provide aesthetic benefits for the residents of the dorm, but will also
improve the stormwater management system and provide tremendous opportunity for monitoring
and research. LCC expects that the green roof will lower the overall volume of stormwater
runoff, demonstrating that large-scale application of green roofs in urban areas can improve local
water quality and prevent sewer overflows. Through rainwater harvesting, stormwater runoff can
be treated and reused for indoor non-potable uses and outdoor irrigation – uses that typically rely
on potable water. This reuse will bring the Stanford Green Dorm closer to its goal of closing the
water loop. Within the green roof and rainwater harvesting system there are opportunities for
research since these systems are not currently widespread. LCC anticipates that within the Green
Dorm these systems will provide vital insight into water quality, runoff characteristics, and
thermal benefits from green roofs.
These systems do not represent a total analysis of the potential water and wastewater
system strategies, but can be used to inform work on other systems. In the following sections,
LCC presents a comprehensive water balance for the indoor water uses, pertinent regulatory
information, and details of the greywater, blackwater, and green roof and rainwater harvesting
systems. At the end of the report LCC concludes with suggestions for future research that are of
particular interest to the client. It is our aim that at the end of the report the client is left with a
clear understanding of the importance of these systems to the goals of the Stanford Green Dorm.
9
2. WATER BALANCE
Understanding the flows of water through the Green Dorm is particularly important to the
water conservation goals of the project. With a comprehensive water balance, the users can
understand where the water is going and identify areas of potential improvement. With this in
mind, LCC has estimated a comprehensive water balance for the indoor uses of water with the
specific purpose of quantifying the flows between our three specific systems. In this section LCC
presents the initial water balance taken from a study by the American Water Works Association,
our modifications to that data, and a final comprehensive water balance. LCC then briefly
assesses the surplus of treated greywater supply as it relates to outdoor irrigation.
2.1. Initial Water Balance
LCC used water balance data from the American Water Works Association (AWWA)2,
as modified by Jiang, Bela, and Murray Associates (JBM)3, as a basis for our modified water
balance. This AWWA Residential End Use Survey characterizes the flows of water in a typical
residential house. To reflect the differences between a residential setting and a college
dormitory, JBM made a few changes to the AWWA data that LCC has critically analyzed. First,
JBM modified the AWWA toilet flow value to reflect the lower volume per flush of Stanford
toilets (1.5 gpf). Second, they added bath flows to the “Showers” category, noting that Stanford
residences only contain showers. Finally, they divided the “Faucet” category into kitchen and
bathroom; to do this, they assumed that kitchen water use (including dishwashers) is 11% of total
household water use based on several outside references. This is particularly important to the
Green Dorm’s research goals because the kitchen faucet produces blackwater while the bathroom
faucet produces greywater – flows that should be accounted for separately. LCC agrees with
these decisions, however we believe that further modifications are necessary to reflect the
differences between water use in a residence and water use in the Stanford Green Dorm.
2.2. LCC Adjustments
LCC proposes several additional modifications to the AWWA/JBM water balance data.
The AWWA study is based on households, not college dormitories; occupants spend different
amounts of time per day in private residences versus college dormitories. We do not expect this
difference to significantly affect leaks, shower, kitchen, and bathroom faucet flows. However,
we do expect the number of toilet flushes per day to be higher in college dormitories because
students spend more time per day in the dorm. Furthermore, we expect that college students
10
wash fewer loads of laundry per week. To assess these predictions, LCC partnered with two
other design firms to collect water use survey data from Stanford students.
2.2.1. Water Usage Survey
A description of the Stanford Water Usage Survey and its statistical findings are
presented in Appendix A. The survey asks for information regarding shower time, faucet usage,
drinking fountain use, toilet flushing, personal dishwashing, and laundry usage; however LCC
uses only the data for toilet flushing and laundry usage because the other information was more
subjective and difficult to measure. It is simpler to count the number of in-dorm toilet flushes per
day and laundry loads per week, and these figures also tend to be more constant from day to day.
The survey was conducted over a two week period in various residences across campus.
Members of LCC and other design groups also participated in the survey. The final figures for
in-dorm toilet flushes per person per day and laundry loads per person per week were taken from
20 individuals who completed the survey. LCC used the mean values for these figures to adjust
the AWWA/JBM water balance. LCC also adjusted the laundry appliance usage to reflect the
water efficient, side-loading machines used in Stanford residences4. One final modification is
that the JBM water balance assumed 55 inhabitants in the Green Dorm; however, the number of
students has since been estimated to be 471.
2.2.2. Toilet Systems
LCC recommends experimenting with different types of toilets in addition to the lowflow variety used in the JBM water balance, so the LCC water usage from the toilets is
substantially different. Urine-diverting toilets are expected to use 0.03 gal/flush for a urine flush
and 0.8 gal/flush for a feces flush; composting toilets should use 0.13 gal/flush. Details of these
toilet types are described in detail in the Blackwater System discussion later in this report. For
this water balance, LCC presents a range of toilet water usage values without making decisions
regarding the number of each toilet type.
Table 1 summarizes LCC Design’s modifications of the AWWA/JBM water balance.
Table 1: LCC Design modifications
JBM Water Balance
Altered Categories
Appliance Usage
Personal Usage
LCC Water Balance
Appliance Usage
Personal Usage
Toilets
1.5 gal/flush
5.05 flushes/day
0.03 - 1.5 gal/flush*
6.5 flushes/day
Laundry
40 gal/load
2.6 loads/week
25 gal/load
1.1 loads/week
Inhabitants
55
47
*toilet water usage depends on the type of toilet, discussed in detail in the Blackwater System section
11
2.2.3. Adjusted LCC Water Balance
With the adjustments described above, LCC has created a modified final water balance,
shown in Tables 2 and 3. The JBM/AWWA data is given in Table 2 for comparison. The
volume/day values for Shower, Bathroom Faucet, Kitchen Faucet, Dishwater, and Leaks are
calculated by multiplying the per capita usage by 47 people. The remaining categories are
calculated similarly but with the modifications specified in Table 1. In Table 2, the toilet flows
per day are specified as if each type of toilet satisfied the entire toilet demand (i.e., if all toilets in
the Green dorm were composting toilets, the daily expected toilet flow is 40 gal/day for the 47
people).
Table 2. Final LCC Water Balance
Source
Category
potable
potable
potable, grey
potable
potable
potable
grey
grey
grey
grey
Item
JBM
Flow
(gpcd)
LCC Water Balance
Flow (gpcd)
Flow (gal/day)
Dishwashers
Other Domestic
Leaks
Faucet - Kitchen
Faucet - Bathroom
Shower
Clothes Washers
Toilets - Low-Flow 1.5 gpf
Toilets - Urine-Diverting
Toilets - Composting
Total
1.0
1.6
2.7
6.7
4.2
12.8
15
7.6
------------52
1.0
1.6
2.7
6.7
4.2
12.8
3.9
9.8
1.7
0.8
34 - 43
50
80
130
310
200
600
180
460
80
40
1600 - 2000
End
Category
black
n/a
n/a
black
grey
grey
grey
black
black
black
Table 3. Summary of greywater and blackwater flows.
Total greywater available for reuse (shower, bathroom faucet, clothes washer) (gal/day)
Total greywater demand (toilet flushing and clothes washers) (gal/day)
Leftover water for irrigation (gal/day)
Sewage discharge (toilet, kitchen faucet, dishwasher) (gal/day)
Total rainwater available (gal/day)
*estimated average volume available during the wet season (Oct-April)
1000
220 - 640
360 - 780
400 - 800
200 - 250*
Estimates for the total rainwater available are described in detail in the Rainwater Harvesting
section of this report. It is important to note that the rainwater is an unreliable source of reusable
water since daily precipitation is unpredictable; the estimates are based on historical monthly
average precipitation values. Rainwater should be used to supplement greywater reuse but should
not be depended on as a reliable source of water.
12
2.3. Use of Recycled Water for On-Site Irrigation
Supply
According to the water balance presented above, the Green Dorm greywater system could
supply 360-780 gallons per day of excess treated greywater and variable amounts of harvested
rainwater for outdoor irrigation. Although LCC chose not to focus on outdoor landscaping and
irrigation, we have conducted a brief analysis to determine if this recycled greywater and
rainwater could feasibly cover irrigation demand.
Demand
Based on the Green Dorm feasibility study by EHDD Architects and Stanford University,
LCC estimates the total potential outdoor irrigation area at 16,000 square feet1. According to
Ryan Navratilov of the San Francisquito Watershed Council (the San Francisquito watershed
includes parts of Stanford University), one square foot of lawn grass demands 30 gallons of
irrigation water per square foot per year5. Lawn grass approximates the top of the range of
possible landscape irrigation demands; native species, for example, require much less water.
Comparison
LCC converted these demand numbers into units of gallons per day, shown in Table 4,
assuming that all 16,000 square feet are landscaped. These results suggest that greywater supply
would not be able to meet the irrigation demand of lawn grass. However, harvested rainwater
could increase irrigation water supply, and native plant species could decrease irrigation water
demand. LCC recommends these two strategies to enable all irrigation demand to be met using
recycled water.
Table 4. Irrigation Supply vs. Demand
Quantity
(gal/day)
Greywater Supply
360 – 780
Irrigation Demand
0 – 1000
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3. REGULATIONS
The extent that the Stanford Green Dorm can implement water reuse technologies
depends on the governing regulations. In this section LCC presents pertinent state regulations for
the reuse of treated greywater, blackwater, and rainwater. Please see Appendix B for more
detailed regulation information.
3.1. Greywater and Blackwater Regulations Summary
3.1.1. State Requirements
The onsite treatment and reuse of water is governed on a state-by-state basis and falls into
the realm of public health. In California, it is governed by Title 22 (“Social Security”), Division
4 (“Environmental Health”) of the California Code of Regulations. The Code describes specific
filtration methods to reach the Disinfected Tertiary level in order to control turbidity. However,
the Code also states that other treatment methods can be used to meet the water quality
requirements, as long as those methods are approved by the State Department of Health. This
section summarizes the regulations for greywater and blackwater treatment and reuse.
3.1.2. Greywater Regulations
Greywater will be diverted to three uses: subsurface irrigation, toilet flushing, and
laundry. Subsurface irrigation requires no treatment and both toilet flushing and laundry reuse
require tertiary treatment. Table 5 summarizes the Title 22 California Code of Regulations for
recycled water reuse.
Table 56. Greywater Reuse Treatment Level Summary
Treament Levels
Greywater Use
Subsurface Irrigation
Toliet Flushing
Laundry Reuse
Disinfected
Tertiary
Recycled Water
Allowed
Allowed
Allowed
No Treatment
Required
Allowed
Not Allowed
Not Allowed
This section summarizes the requirements for tertiary recycled water taken from the Title 22
California Code of Regulations, California Recycled Water Criteria Section 60301.2306. The
water needs to be coagulated and sent through bed filter media, the flow rate through the system
should not exceed 2 GPM/ ft2, and the turbidity should not exceed 2 NTU. For the membrane
filtration system the NTU should not exceed 0.2 NTU more than 5% of the time. The chlorine
disinfection step should have a CT (the product of total chlorine residual and modal contact time
14
measured at the same point) of 450 mg-min/l and modal contact time of not less than 90 minutes.
The median concentration of total coliform bacteria measured in the effluent should not exceed
an MPN of 2.2 per 100 milliliters6.
Subsurface Irrigation Regulations
Below is a summary of the California Title 24, Part 5 (California Administrative Code G
11: Irrigation Field Construction) for subsurface irrigation that must be followed for the Green
Dorm. There is no treatment required if the following criteria are followed. The minimum
filtration size is 115 micrometers, and user controls such as valves, switches, and times are
required. The irrigation system should be constructed out of polyethylene tubing or PVC piping.
The California Code of Regulations, Title 27 (2.1.7.k) has further requirements for
subsurface irrigation systems. There needs to be a minimum of two feet of continuous soil under
the dispersal system. The system must be a closed loop distribution system, and the pipes should
be periodically flushed. All the distribution lines should be the color purple to identify them as
non-potable water. A permit is required and the systems must be placed 6 inches below the soil.
3.1.3. Blackwater Regulations
The onsite treatment and reuse of blackwater is not governed under any federal
legislation. The California Title 22 regulations suggest that treated blackwater can be reused
provided it meets the water quality standards described in the Title. The reuse of compost from
composting toilets is limited under Part 503 (“Biosolids”) of Section 405 of the Clean Water Act,
but the reuse of urine from urine-diverting toilets is not regulated7,27.
3.2. Rainwater Harvesting Regulations Summary
At this time there are no specific regulations pertaining to the harvesting and reuse of
rainwater. There are, however, some quality concerns, such as high concentrations of nitrogen
and phosphorus from compost on the green roof, heavy metals and chemicals like zinc or lead
from the regular roofing materials, and atmospherically deposited particulates and microbes.
These quality concerns are addressed in detail later in this report. Because of the risk of
contamination, it is suggested that rainwater be treated to the primary level before indoor nonpotable reuse so that large particles can settle out8. A higher level of treatment will be required if
the Green Dorm pursues potable reuse of rainwater.
15
4. GREYWATER SYSTEM
This section characterizes the greywater flows that will be collected for greywater use
which include the following: clothes washers, showers, and bathroom sinks. This is important
because it affects the type of treatment that will be necessary. For example the COD (Chemical
Oxygen Demand):BOD (Biological Oxygen Demand) ratio may be as high as 4:1 for greywater
flows from the clothes washer which is much higher than municipal sewage9. The low values of
biodegradable organic matter may affect the nutrient balance and limit the effectiveness of the
biological treatment9. Table 6 characterizes the greywater flows that could be found in the Green
Dorm.
Table 610. Characteristics of Greywater Flows
Greywater Source
Automatic Clothes
Washer
Characteristics
Bleach, foam, high pH, hot water, nitrate, oil and grease, high BOD, phosphate, salinity,
soaps, sodium, suspended solids, turbidity
Shower
Bacteria, hair, hot water, odor, oils, high BOD, soaps, suspended solids, and turbidity
Bathroom Sink
Bacteria, hair, hot water, odor, oils, organic matter, high, BOD, soaps, and turbidity
Storage of greywater before treatment can promote the growth of micro-organisms which
causes a risk of odor and possible health concerns11. A study done by A. Dixon et al. to model
untreated greywater found rapid growth of coliform in all samples within a few days of storage11.
Another study conducted by Ottoson et al. found that fecal coliform increased within 48 hr in
stored greywater from different sources12. The study also showed that greywater provides a
potential route for viruses which only require low doses to cause infections12. A study by E.
Friedler et al. reported that domestic greywater does indeed have to potential to pose a health
risk13. The levels of TSS (Total Suspended Solids) and BOD found in domestic greywater tested
range between 55-70% of that found in municipal sewage9. The highest pollutant levels were
found in washing machine flows, and the highest fecal coliform were found in bath and shower
water13.
Table 7 presents the water quality of untreated greywater flows from various sources. The
table shows that there is more COD present than BOD in the greywater flows, which as stated
earlier may affect the nutrient balance and has the potential to interfere with the effectiveness of
the biological treatment9. The highest turbidity was found in the laundry greywater flow. The
median total coliform concentrations were all much higher than the 2.2 (MPN/100ml) which is
required by the state of California Regional Water Quality Control Board for tertiary treatment 14.
16
Table 7. Water Quality in Greywater Flows
Shower15
Handbasin15
Laundry16
146
420
65.3
84.8
155
587
99
164
48-290
50-210
0.3
6800(cfu/100ml)
0.4
9420(cfu/100ml)
.062-42
MPN/100ml (2.3 x 103-3.3x105)
BOD5 (ppm)
COD (ppm)
TOC (ppm)
Turbidity (NTU)
PO4- (ppm)
Median Total Coliform
4.1. Greywater Treatment Technologies
Greywater flows will be taken from the bathroom sink, shower, and automatic clothes
washer to be used for water reuse in the green dorm. The first step in the treatment process
(shown in Figure 2) will be the removal of suspended solids from the greywater flow. This will
involve filtration through a backwashing sand filter and then a gravity tank to collect heavy
particles. This water will be used for subsurface irrigation. The next treatment step will be
biological treatment, membrane filtration, and disinfection using bromine, chlorine or ultraviolet
disinfection. This step will allow us to meet the California Regional Water Quality Control
Board tertiary treatment standards for water reuse. This section will explore the various treatment
options to determine which ones best suit the needs of the green dorm.
Rainwater Storage
TERTIARY
Showers
Bathroom Faucet
Gravity Tank
Coarse
Filtration
Backwash
sand filter
•
Clothes Washer
Toilet Flushing
Biological
Treatment
Membrane
Filtration
Clothes Washer
Disinfection
Grey Storage
Subsurface Irrigation
Figure 2. Diagram of Greywater Flows in the Green Dorm.
4.1.1 Treatment for Subsurface Irrigation Reuse
The first step of treatment will involve a gravity tank and a backwashing sand filter to
take out floating and settable material. We will use this method for subsurface drip irrigation.
Below are the advantages of this method.
Advantages

Because the greywater is being treated before entering the storage tank this should cut
down on the amount of pathogens that are present in the greywater used for irrigation9,11.
17

The water remains high in organic load and turbidity; and the nutrient load could
contribute to plant growth and vitality.

The treatment process will remove particles that could clog the subsurface drip irrigation
system.
Figure 3 shows the proposed subsurface irrigation design. This system uses a gravity tank to
collect heavier particles and a back-washing sand filter. This design layout was taken from the
website of the Santa Monica Green Buildings Program17.
Backwash
Sand Filter
Untreated
Greywater
Gravity
Pump
Irrigation
System
Tank
Figure 3. Proposed irrigation water treatment design.
4.1.2 Proposed Packaged Technologies for Tertiary Treatment
The technologies in this section are being proposed for toilet flushing and laundry reuse
which the California Regional Water Quality Control Board states must have a tertiary level of
treatment. The packaged systems each included biological treatment, membrane filtration, and
disinfection.
Equaris Total Household Water Recycling and Wastewater Treatment System
This system is a full packaged treatment system including a series of membrane filters
and UV disinfection18. The greywater first flows into an aerated storage tank, the water is then
pumped through ultraviolet (UV) light, sent through carbon filtration and nanofiltration,
subjected to UV light again, and than an ultrafilter and a reserve osmosis membrane18. The brine
from the filtration steps is sent through the system again, reducing the amount of brine
produced18. This system will allow the Stanford Green Dorm to meet tertiary water quality
standards which are necessary for water that will be used for toilet flushing and laundry reuse.
The system also monitors the pressure gauge, water level, and has alarm reporting19. Figure 4
shows the schematic of the system.
18
Figure 419. Equaris Total Household Water Recycling and Wastewater Treatment System
This Equaris system has been successfully used to treat greywater in the Natural
Resources Defense Council building in Santa Monica, CA18. The capacity of the greywater
system is 500 gallons/day, therefore two systems would be needed for the Green Dorm to reach a
capacity of 1000 gallons/day (assuming that roughly 220-640 gallons of water per day will be
needed for toilet flushing and laundry reuse, as estimated in the Water Balance)18. Each system
takes up 40 cubic feet, so a total of 80 cubic feet of space will be needed for the Green Dorm19.
Each system costs $25,000 and two systems will be needed so there will be a total cost of
$50,000 plus the cost of installation. The comparison of packaged treatment technologies in
Table 8 shows the water quality achieved by the system.
19
Z-MOD S: Below Ground Packaged Plant
The Z-MOD S packaged system includes the ZeeWeed MBR and UV disinfection. The
water from this system can be used for toilet flushing and laundry reuse20. The system has a
capacity of 2900 gpd, well within the range necessary for the Green Dorm. The wastewater is
first pumped from the collection tank through trash grinder pumps in to the bioreactor. The
bioreactor uses a suspended biological growth reactor with an ultrafiltration membrane21. The
Zeeweed MBR combines aeration, secondary clarification and filtration in one process21. A
vacuum is used to bring the treated water through a hollow fiber ultrafiltration membrane21. The
pore size for the MBR is 0.1 micrometers and the filter is able to remove most bacteria and
viruses21. Because the membranes are immersed within the bioreactor in direct contact with a
mixed liquor space is saved21. For the final steps the treated water then travels through an
activated carbon filter and subjected to UV disinfection21. Figure 5 shows a diagram of the
Zeeweed MBR. Figure 6 shows the below ground package plant. The system takes up 845 square
feet. The treatment results for the Z-MOD S are listed in the Comparison of Packaged Treatment
Technologies in Table 8. The Z-MOD S was successfully used in the Earth Rangers Centre in
Woodbridge, Ontario; the building received the Global Ecomagination Leadership Award and is
LEED-Gold certified22. The center recycles up to 2,600 gallons of water daily and uses the water
for irrigation, floor washing, and toilet flushing22.
Figure 523. Diagram of Zeeweed MBR.
20
Figure 623. Z-MOD S: Below Ground Packaged Plant
Aqua Reviva
The Aqua Reviva system is another possible packaged treatment option that could be
used for toilet flushing and laundry reuse in the Green Dorm. The system has a steel control box
mounted next to the treatment cell which has an automatic overflow to the sewer24. The
greywater is pumped to the treatment cell using a 12V pump. Biological treatment occurs in the
treatment cell, which is divided into 3 canisters, and the cell uses a fixed growth reactor with
filtration. This is then followed by disinfection using bromine24. The average cost per system
ranges between $10,000 and $15,000 which includes the cost of installation and storage. The
capacity for the system is 185 gallons/day. Therefore to meet the needs of the Green Dorm 4
systems will be necessary providing a total capacity of 740 gallons/day25. Thus the expected
system cost is $40,000-$60,000. The system has been successfully used in Australia for
greywater reuse, however the Green Dorm contractors will need to apply for approval from the
state of California Regional Water Quality Control Board to install the system. Figure 7 shows a
photo of the treatment system and control box for the Aqua Reviva system. The treatment system
and control box together have a footprint of 16 square feet each, therefore for four systems this
would be an area of 64 cubic feet plus the volume of the greywater storage container26.
Figure 726. Aqua Reviva treatment system and control box.
21
Table 8. Comparison of Packaged Treatment Technologies
22
Water Quality
All three technologies can potentially be used for greywater reuse for toilet flushing and
laundry reuse if they are able to meet the tertiary treatment standards of the California Regional
Water Quality Control Board. All of the packaged technologies except for Aqua Reviva have
been successfully used in the United States. Although Aqua Reviva has been used successfully in
Australia it is unclear from the website if the system will be able to meet stringent California
regulations. The Equaris system is the only system that has been successfully installed in
California. The current tertiary treatment standard for the California Regional Water Quality
Control Board for total coliform is 2.2 MPN (Most Probable Number)/100ml. For the Equaris
Total Household Water Recycling and Wastewater treatment systems the water quality achieves
the tertiary treatment standard. For the Z-MODS and the Aqua Reviva the companies will need
to be contacted to determine the level of total coliform present.
System Footprints
All of the selected systems are able to meet the capacity of the Green Dorm, 220-640
gal/day. However in the case of the Equaris and Aqua Reviva multiple systems will need to be
installed in parallel. The Aqua Reviva system takes up the smallest volume at 64 cubic feet. The
Equaris and the Aqua Reviva system are fairly similar in price. A strong advantage of the
Equaris system however is that it has been implemented in California so we know it will meet
regulatory standards. The Z-MOD S takes up the most space at 845 square feet, more than 10
times any of the other technologies, a potential drawback to the system.
4.2. Living Laboratory Component
Because the green dorm is a living laboratory multiple systems could be installed over
time. The Equaris has a strong advantage in comparison to the other technologies in terms of
successful past use in California so this technology should be implemented first. However the ZMOD S is able to achieve a higher water quality and can accommodate a greater load without
requiring multiple systems, and the Aqua Reviva system has the smallest footprint. So we
propose budget permitting that these technologies be implemented as well, and three systems
could be researched in parallel.
23
5. BLACKWATER SYSTEM
5.1. Goals
The Green Dorm comes at a turning point in the history of waste management. In both
the developing world and the developed world, researchers have identified problems with the
current waste management system and are looking for new solutions. In both settings, on-site
treatment technologies are emerging as viable waste management alternatives. LCC’s goal for
the Green Dorm Blackwater System is to test and analyze small-scale, on-site waste management
systems in an interactive fashion. Based on the current climate of wastewater management and
Stanford’s goals for the Green Dorm, LCC recommends the following research priorities for the
Green Dorm wastewater management system.
1. Reduce volume and nutrient loading of wastewater stream.
2. Compare and contrast onsite, small-scale treatment methods to municipal treatment
methods through comprehensive testing and monitoring.
3. Address the research needs of the developing world.
4. Provide a setting to test scale models of new treatment technologies.
5. Engage students in thinking about the management of their own wastes.
(Please see Appendix C to learn more about the origins and state of the current waste
management system.)
5.2. Methods
LCC defines "blackwater" as flows from kitchen faucets, dishwashers, and toilets. Our
design for the Blackwater System focuses on toilet waste streams; Stanford-specific data for
kitchen water use is needed before detailed design recommendations can be made for kitchen
wastewater streams. LCC considered three technologies in order to fulfill our goals for the
Green Dorm Blackwater System: urine-diverting toilets, composting toilets, and anaerobic
digesters and membrane bioreactors. Section 5.3 introduces these technologies and summarizes
LCC’s design considerations for what specific types would best fit the Green Dorm. Using these
specific types as examples, LCC then quantitatively analyzed three different design scenarios for
the Green Dorm: a standard toilet scenario, a urine-diverting toilet scenario, and a composting
toilet scenario. Results of the analyses show the savings of each system on a per capita basis.
24
Section 5.4 describes our scenario analyses, and Section 5.5 gives our conclusions for the Green
Dorm Blackwater System.
5.3. Technologies
5.3.1. Urine-diverting Toilets
Urine-diverting toilets attempt to capture the nutrient value of human waste and reduce
the loading of municipal treatment plants. Urine-diverting toilets use a separate catchment for
urine at the front of the toilet bowl to divert urine before it comes into contact with feces. In this
way, the nutrients in urine can be captured and recycled with minimal treatment27. Urine
contains the majority of the nitrogen and phosphorus in human waste. It consists of mostly urea
and ammonium, two unstable nitrogen compounds. These nitrogen compounds cannot be used
directly by plants, but can be broken down by aerobic microorganisms in the soil into stable
forms that plants can use. Urine can therefore be used as a fertilizer and is used in many
countries around the world27.
Safety and Reuse of Captured Urine
Urine is sterile in the bladder, and only small risks can result from handling pure urine.
The main risk in handling urine and using it for fertilizer is if it has been contaminated with fecal
matter28,29. Although urine-diverting toilets contain separate sections for urine and feces, fecal
contamination may accidentally occur. Pathogens from feces may include bacteria, protozoa,
and viruses. The survival of pathogens in urine depends on temperature, pH, ammonia, dilution,
and time29. The high pH of urine kills bacteria and protozoa over time. Viruses are able to
survive the longest, “with no inactivation in urine at 5ºC and T90 values of 35-71 days at 20ºC.”30
T90 values indicate the amount of time it takes for a 90% reduction in the pathogen level.
The risk of potential pathogens in urine can be reduced to insignificant levels through
long-term storage at 20ºC for 6 months31. Appendix D contains more detailed information about
urine storage temperatures and times.
Design Considerations
Several specific issues must be considered when designing a urine-diverting toilet system
for the Green Dorm. First, the high pH of urine causes a sludge of precipitates to form; urinediversion pipes must have at least a 1% slope and be sufficiently wide in order for the sludge to
flow to storage. Secondly, tanks and pipes should not be ventilated; otherwise, volatilized
25
nitrogen can escape, reducing the nutrient value of the urine. Third, urine is highly corrosive,
and pipes and storage containers should therefore be made of non-metals such as plastic or
fiberglass28.
An additional concern is acceptance by Green Dorm residents. With social acceptance in
mind, LCC recommends dual-flush urine-diverting toilets for the Green Dorm instead of
waterless toilets. Dual-flush toilets more closely imitate standard toilets and have minimal
odors27. Few if any commercial dual-flush urine-diverting toilets are available in the United
States; however, technologies are available in Europe and may soon be available in the US. One
well-known model currently sold in Sweden is the Wost Man Ecology porcelain model, shown
in Figure 8. LCC uses this model as the basis of our flush volumes for urine-diverting toilets in
the scenario analysis in Section 5.4.2 of this report.
Figure 832. WM-DS Urine diverting porcelain toilet from Wost Man Ecology.
5.3.2. Composting Toilets
Composting toilets use microorganisms to convert human waste into compost that can be
applied as fertilizer to plants. The microorganisms, mainly aerobic bacteria and fungi, break
down the carbon compounds present in human waste to derive energy, and break down the
nitrogen compounds to create proteins for cell growth. The carbon and nitrogen compounds that
remain after the breakdown process are stable, meaning they will not be broken down further if
released into the environment27.
A typical composting toilet consists of a toilet stool and an area for composting wastes
(the “composter”), as shown in Figure 9. Composting toilets require a ventilation fan to control
odors and to remove carbon dioxide that accumulates as residual of the microbial reactions.
They also require an outlet for leachate, the liquid urine and moisture that accumulates at the
26
bottom of the composter. This liquid is untreated and must undergo further treatment or be
discharged to the sewer27.
Figure 933. Diagram of composting toilet.
The microorganisms in composting toilets require particular levels of temperature,
moisture, and aeration in order to create compost effectively. Based on these concerns, the
following list of requirements is necessary for an effective composting toilet system27:
1. Adequate air circulation to supply oxygen to aerobic microorganisms. Bulky, carbonaceous
material such as wood chips should be added periodically to provide larger pore spaces in
the compost.
2. Adequate moisture content to keep microorganisms alive. The optimum moisture level is 4570% moisture. At less than 45%, decomposition stops and excrement accumulates, and
at greater than 70%, leachate accumulates at the bottom of the composter and can cause
anaerobic digestion.
3. Adequate temperature to stimulate high rates of microbial activity. The standard temperature
for manufactured composting toilets is between 93 and 113ºF.
4. Adequate carbon-to-nitrogen ratio to ensure complete waste digestion.
Microorganisms
require a carbon-to-nitrogen ratio of 25:1 in order to break down waste effectively.
5. Adequate operation and maintenance procedures to ensure that the preceding four conditions
are met.
27
Design Considerations
One of Stanford’s goals for the Green Dorm is for it to be the most desirable housing on
campus; another goal for the Green Dorm is for it to be an interactive research laboratory1. LCC
therefore suggests choosing a composting toilet technology that is both likely to succeed without
odor problems and that involves a bit of operation and maintenance that students can carry out.
LCC recommends active, centralized composting toilets with micro-flush toilets for the
Green Dorm. Active toilets ventilate, aerate, and heat the composter manually through energy
application; passive toilets rely on natural conditions27. Active composting toilets are less likely
to fail than passive composting toilets, so we recommend them for the Green Dorm. Centralized
systems direct excrement from several composting toilets to one central composter. Individual
systems contain one composter per toilet, often within the same small area so as to fit in a
bathroom. Centralized composting toilet systems have less odor problems than individual
composting toilets, because there is a greater distance between the toilet stools and the
composting process27. Furthermore, centralized systems allow the compost from many toilets to
be maintained at once. Composting toilets can either be dry, requiring no flush water, or can use
a small quantity of flush water. Micro-flush toilets are the closest to the standard toilet
experience; they look very much like standard toilets and flush like them, but only require one
pint of flushwater. LCC suggests choosing the SeaLand micro-flush toilet; it is the most widelyused model in the United States and is inexpensive compared to other toilet stools27. Figure 10
shows a picture of a Sealand toilet.
Figure 1034. SeaLand micro-flush toilet.
LCC further recommends toilets that require some maintenance but not the physical
raking of compost, and suggests choosing a manufactured system instead of building one on-site.
28
5.3.3. Anaerobic Digesters and Membrane Bioreactors
The waste composting processes described in the composting toilet section are aerobic
processes in which microorganisms convert carbon and nitrogen compounds to stable forms
using oxygen, creating carbon dioxide and water as bi-products. However, other
microorganisms can carry out the same conversions without using oxygen, creating methane as
an additional bi-product. Methane is natural gas, and can be used as an energy supply.
LCC considered two waste treatment technologies that use anaerobic microorganisms to
break down wastes: anaerobic digesters and membrane bioreactors. Membrane bioreactors
(MBRs) contain anaerobic microorganisms on a film through which the waste stream passes; the
organisms digest the waste as it moves through, depositing stable sludge onto the film and
producing almost clear water at the end. Anaerobic digesters are simpler versions of MBRs.
They are digestion chambers filled with microorganisms that also convert waste nutrients into
stable compounds; however, the effluent from anaerobic digesters is turbid35.
Design Considerations
Stanford’s Civil and Environmental Engineering Department contains an experimental
MBR as part of the Criddle Group labs36. In the long term, LCC recommends either installing an
anaerobic digester or MBR on-site or recommends diverting the Green Dorm’s wastewater
stream to the Criddle Group MBR for testing and processing. LCC assumes that all Green Dorm
wastes discharged to the sewer could be rerouted to an anaerobic digester or MBR. Our
estimates of possible methane production under the scenario analyses in Section 5.4 of this report
assume that sewer wastes are processed by an anaerobic digester or MBR.
5.4. Scenario Analysis
LCC analyzed three toilet technology scenarios in order to compare the advantages and
disadvantages of different technology choices. Each scenario considers the water usage, nutrient
capture potential, and end products resulting from the use of a particular toilet technology by one
person in a given day. Scenario 1 analyses standard toilets, Scenario 2 analyses urine-diverting
toilets, and Scenario 3 analyses composting toilets.
For these three scenarios, we make the following assumptions. First, we assume that all
toilet waste released per person per day is released into dorm toilets. In reality, some of the
29
waste will be released into toilets outside the dorm. Our nutrient estimates therefore represent
the maximum values possible and are likely lower by approximately 20-30%.
Second, we assume that each person flushes 6.5 times per day in the dorm and that two of
these flushes include fecal matter. We base these assumptions on a water use survey of Stanford
students, as described in Section 2.2.1 and Appendix A of this report, and on physiological
estimates of the number of fecal events per person per day37. We assume that all fecal events
take place in the dorm.
Third, we assume that all wastes sent to the sewer are diverted to an anaerobic digester or
MBR and that the full chemical oxygen demand of those wastes are processed anaerobically to
create methane. We assume that 0.35 liters of methane can be produced for every gram of COD
processed by the anaerobic digester or membrane bioreactor35,38.
Appendix E (attached handwritten notes) shows details of scenario calculations and gives
references for values and assumptions used.
5.4.1. Scenario 1: “Business as Usual”
This scenario assumes 1.5 gallon per flush toilets, the current standard at Stanford, and
assumes that all wastewater is discharged to the sewer. Figure 11 shows a diagram of the flows
in this scenario.
Standard
Toilet
Source
Sewer
Figure 11. Scenario 1 model.
Nutrient Capture
Table 9 shows the fate of nutrients according to this scenario. All of the nutrient value of
human waste is lost to the sewer in this case.
Table 9. Fate of waste nutrients under Scenario 1.
End location (g/ppd)
Nutrient
Sewer
Urine Storage
Compost
N
12
P
1.5
K
2.9
BOD5
18.5
COD
48
TS
109
30
Water Usage and Sewer Discharge
Table 10 shows the volume of flush water necessary and the volume of wastewater
released to the sewer per person per day according to this scenario. The volume of wastewater is
higher than the flushwater volume, because it contains urine and feces.
Table 10. Water and Wastewater Volumes under Scenario 1.
Volume
(L/ppd)
Flushwater
37
Wastewater
38
Energy Potential
According to the conditions of this scenario, 17 liters of methane could be produced per
person per day if all sewer wastes were processed by an anaerobic digester or MBR.
5.4.2. Scenario 2: Urine-diverting toilets
This scenario assumes dual-flush urine diverting toilets with flush volumes of 0.1 liters
for the short flush and 3 liters for the long flush (based on Wost Man WS-DS toilet32). Urine and
urine flushwater (short flush) are diverted to urine storage tanks, and feces and feces flushwater
(long flush) are discharged to the sewer. Figure 12 shows a diagram of the flows in this scenario.
Urine to Storage
Source
Urine-diverting
Toilet
Feces to Sewer
Figure 12. Scenario 2 model.
Nutrient Capture
Table 11 shows the fate of nutrients according to this scenario. The numbers in
parentheses indicate the percent of the original amount of the nutrient that is captured in urine
storage. For example, 92% of the original 12 grams of nitrogen excreted per person per day as
waste is captured through urine diversion and retained in urine storage. Table 11 shows high
percentage captures for nitrogen, phosphorus, and potassium, and also for total solids.
Table 11. Fate of waste nutrients under Scenario 2.
End location (g/ppd)
Nutrient
Sewer
Urine Storage
Compost
N
1.6
11 (92%)
P
0.5
1 (67%)
K
0.8
2.2 (76%)
BOD5
11
7.5 (40%)
COD
33
15 (31%)
TS
44
65 (60%)
31
Water Usage and Sewer Discharge
Table 12 shows the volume of flush water necessary and the volume of wastewater
released to the sewer per person per day according to this scenario. Additionally, 2 liters of
urine/flushwater are diverted to storage per person per day.
Table 12. Water and Wastewater Volumes under Scenario 2.
Volume (L/ppd)
Flushwater
6 (84% reduction)
Wastewater
6 (84% reduction)
Energy Potential
According to the conditions of this scenario, 12 liters of methane could be produced per
person per day if all sewer wastes were processed by an anaerobic digester or MBR.
5.4.3. Scenario 3: Composting Toilets
This scenario assumes composting toilets with micro-flush toilet stools that use 0.47 liters
per flush (based on SeaLand micro-flush toilet stool mentioned in Section 5.3.2). Leachate is
discharged to the sewer. Figure 13 shows a diagram of flows in this scenario.
Source
Micro-flush
Toilet
Compost to Garden
Composter
Leachate to Sewer
Figure 13. Scenario 3 model.
Nutrient Capture
Table 13 shows the fate of nutrients according to this scenario; most of the nutrients are
retained in the compost with minimal nutrients discharged to the sewer. The numbers in
parentheses indicate the percent of the original amount of the nutrient that is captured in
compost. LCC would like to stress that the sewer numbers in Table 13 are based on only one
estimate of the nutrient content of leachate, and assume 1-2 liters of leachate per person per
day27,39. The nutrient content of leachate likely varies considerably depending on the type of
composting toilet technology and composting environment (for example, higher temperatures
will cause more leachate to evaporate, higher C:N ratio may cause more nutrients to be processed
before leaching out of the composting toilet, etc.)27. We therefore are not able to draw specific
conclusions from this data, but instead use it as a general illustration of a composting toilet
scenario.
Table 13 shows very high nutrient capture for nitrogen, phosphorus, potassium, chemical
oxygen demand, and total solids. These values assume that all nutrients that aren’t leached out
32
are retained within the compost. In reality, some of the nitrogen will volatilize and leave through
the system’s ventilation mechanism27; further research is necessary to determine if phosphorus
could leave the system, as well.
Regardless of their form, however, the nutrients, chemical oxygen demand, and total
solids that do not leave the system through leachate are successfully removed from the waste
stream.
Table 13. Fate of waste nutrients under Scenario 3*.
End location (g/ppd)
Nutrient
Sewer
Urine Storage
Compost
N
0.09
12 (~100%)
P
0.06
1.4 (93%)
K
BOD5
COD
0.5
47 (98%)
TS
0.1
109 (~100%)
*Based on one study and requires further research39.
Water Usage and Sewer Discharge
Table 14 shows the volume of flush water necessary and the volume of wastewater
released to the sewer per person per day according to this scenario.
Table 14. Water and Wastewater Volumes under Scenario 3.
Volume (L/ppd)
Flushwater
3 (92% reduction)
Wastewater
4 (89% reduction)
Energy Potential
Because so few wastes are discharged to the sewer under this scenario, only 0.2 liters of
methane could be produced per person per day if all sewer wastes were processed by an
anaerobic digester or MBR.
5.4.4. Comparison of Scenarios
Table 15 shows a summary of the nutrient capture, water and wastewater volume, and
energy potential information for the three scenarios. Table 16 gives a qualitative summary of the
information in Table 15. Scenario 1 has the highest potential for methane production, yet also is
the highest water user and provides no nutrient capture or COD removal. Scenario 2 provides
high nitrogen capture and relatively high phosphorus capture and COD removal; it also has a
relatively high energy potential and uses a low amount of water. Scenario 3 also uses a low
amount of water and provides high nutrient capture and COD removal; however, this scenario
has a very low energy potential.
33
N capture (g/ppd)
P capture (g/ppd)
COD removal
(g/ppd)
Inflow vol
(L/ppd)
Discharge vol
(L/ppd)
CH4 potential
(L/ppd)
Table 15. Scenario Summary - Quantitative.
Scenario 1
Scenario 2
11
0
(92% capture)
1
0
(67% capture)
15
0
(31% removal)
37
6
(0% reduction)
(84% reduction)
38
6
(0% reduction)
(84% reduction)
Scenario 3
12*
(~100% capture)
1.4*
(93% capture)
47*
(98% removal)
3
(92% reduction)
4*
(89% reduction)
17
12
*Based on one study and requires further research39.
0.2*
Table 16. Scenario Summary – Qualitative.
Scenario 1
Scenario 2
Scenario 3
N capture
None
High
High*
P capture
None
Medium-High
High*
COD removal
None
Medium
High*
Water use
High
Low
Low
Sewer use
High
Low
Low*
Energy Potential
High
Medium-High
Low*
*Based on one study and requires further research39.
These preliminary results suggest Scenario 2 to be the best choice, because it provides
both high nutrient capture and relatively high energy potential. Furthermore, the nutrients
captured in urine may have more potential for re-use than those captured in compost; as
mentioned in Section 3, the reuse of compost as fertilizer is limited by Part 503 (“Biosolids”) of
Section 405 of the Clean Water Act27, whereas the use of urine as fertilizer is not regulated in the
United States. However, composting toilets are very useful for research purposes, and they
require the least water usage and remove the most COD of any of the scenarios. Additionally,
the standard toilet scenario may be useful for its methane production potential. LCC therefore
recommends a combination of standard, urine-diverting, and composting toilets for the Green
Dorm; further analysis is required to determine the optimal allocation of each toilet type. In
Section 5.4.5, LCC describes the information necessary to complete this analysis.
5.4.5. The Optimal Scenario?
LCC has presented the preceding scenario results in per capita form so that future Green
Dorm researchers can optimize the number of standard, urine-diverting, and composting toilets
they would like for the dorm. In order to solve this optimization problem, students and
34
researchers will need to decide how much they value each of the functions provided by the
scenarios: nitrogen capture, phosphorus capture, COD removal, inflow reduction, wastewater
reduction, and methane production.
In order to account for the Green Dorm’s focus on environmental sustainability and
social innovation, researchers should consider environmental and social value in addition to
economic value. Table 17 shows factors researchers may wish to consider for each function and
value category. The factors shown in this table are not all-inclusive and are presented here to
serve as a springboard for other ideas that Green Dorm researchers may have.
N capture
Table 17. Valuation factors to consider.
Environmental
Social
Research applicability to
Reduced emissions to
agriculture in developing
water bodies, reduced
countries, research on
manufacturing of
comparing/contrasting municipal
chemical fertilizers
treatment to on-site tech.
P capture
Reduced emissions to
water bodies, reduced
manufacturing of
chemical fertilizers
COD removal
Reduced emissions to
water bodies
Inflow reduction
Wastewater
reduction
Methane
production
Economic
Fertilizer
replacement,
municipal
treatment savings
Research applicability to
agriculture in developing
countries, research on
comparing/contrasting municipal
treatment to on-site tech.
Fertilizer
replacement,
municipal
treatment savings
Reduced environmental
impact of water supply
Reduced emissions to
water bodies
Research on comparing municipal
treatment to on-site tech.
Research on long-term sustainable
water practices, helps Stanford
meet water budget goals
Increase student awareness of
volume of waste loads
Municipal
treatment savings
Savings in cost of
water transport and
pumping
Municipal
treatment savings
Lower fossil fuels use
Research applicability to energy in
the developing world
Energy savings
Once individual function values are determined, Equation 5.4.5 shows a method of
optimizing the total value of the Green Dorm toilet system. In this equation, x is the number of
students using Scenario 1, y is the number of students using Scenario 2, and z is the number of
students using Scenario 3. The quantities in brackets are the value of each function that each
scenario provides. For example, “N cap.” in the first set of brackets represents the value that
Scenario 1 provides by capturing nitrogen. “N cap.” in the last set of brackets is the total
nitrogen capture value provided by the entire toilet system. Green Dorm researchers may use
this equation to optimize the total value of individual functions (for example, they may optimize
the value of methane production without considering nutrient capture or flow reduction), or they
35
may optimize the total value of all functions. The total value of all functions is optimized when
the sum of the numbers in the “Total” brackets is greatest.
x
N cap.
P cap.
COD rem.
Inflow red.
Wastewater red.
Methane prod. 1
+y
N cap.
P cap.
COD rem.
Inflow red.
Wastewater red.
Methane prod.
+z
2
N cap.
P cap.
COD rem.
Inflow red.
Wastewater red.
Methane prod.
=
3
N cap.
P cap.
COD rem.
Inflow red.
Wastewater red.
Methane prod.
Total
Equation 5.4.5. Determining numbers of students using Scenarios 1, 2, and 3. Here x + y + z = 47, the
number of people living in the Green Dorm, and the quantities in brackets represent the value of each
function provided by each scenario per person.
5.5. Conclusion: Blackwater Design Recommendations
Based on an analysis of three different toilet technology scenarios, LCC recommends a
combination of standard, urine-diverting, and composting toilets for the Green Dorm, as well as
optional sewage diversion to an anaerobic digester or membrane bioreactor. LCC recommends
dual-flush urine-diverting toilets and centralized, active composting toilets with micro-flush
toilet stools.
LCC has provided scenario analysis results on a per person basis so that future Green
Dorm researchers may easily compare different allocations of these toilet technologies. LCC has
also provided a method that researchers may use to choose an optimal allocation based on
valuation of the functions the toilet systems provide: nitrogen capture, phosphorus capture,
chemical oxygen demand reduction, inflow reduction, wastewater reduction, and methane
production. Table 18 summarizes the range of daily savings possible for the Green Dorm under
various combinations of toilet technologies.
Table 18. Range of possible daily savings for Green Dorm.
Percent
Quantity per
Total Quantity
Improvement
person per day
per day
Nitrogen capture
0 - 100% capture
0 - 12 g
0 - 560 g
Phosphorus capture
0 - 93% capture
0 - 1.4 g
0 - 66 g
COD removal
0 - 98% removal
0 - 47 g
0 - 2200 g
Inflow Volume
0 - 92% reduction
3 - 37 L
140 - 1700 L
Discharge Volume
0 - 89% reduction
4 - 38 L
190 - 1800 L
Methane Production
0.2 - 17 L
9 - 800 L
36
6. GREEN ROOF AND RAINWATER HARVESTING
A green roof and rainwater harvesting system for the Stanford Green Dorm can provide
both environmental benefits and valuable research opportunities. Through reducing stormwater
runoff, collecting and reusing rainwater, and improving the thermal performance of the building,
this system encourages sustainable environmental performance that can be measured with
various monitoring and testing technologies. LCC envisions the Stanford green roof and
rainwater harvesting system to be an integral component of the living laboratory, a component
that students and faculty alike can explore and research.
To keep the discussion of the green roof and rainwater harvesting system applicable to
various design layouts, LCC presents most of the data estimates on a per-square-foot basis.
These estimates include stormwater runoff volumes and thermal improvements. Though the
runoff volumes per square foot can be manipulated to apply to any specific layout, LCC presents
an example layout later in this section. This layout designates areas for different roof sections,
which LCC uses to estimate specific runoff volumes for rainwater harvesting. In doing so, LCC
not only gives an example of how much runoff the Green Dorm might produce, but also provides
a basis for estimating how the stored water can best be recycled. Throughout this discussion,
LCC highlights research opportunities that emphasize the value and learning potential of the
green roof and rainwater harvesting system.
6.1. Green Roof Background
Green roofs, which have vegetation grown directly onto a roof layer of substrate, are
becoming increasing popular on green buildings all over the world because of their sustainable
attributes. Some of their benefits include40:
-
stormwater retention and filtration
-
increased energy efficiency (from added insulation)
-
increased roof membrane life
-
sound insulation
-
reduction of urban heat island effect
-
aesthetic appeal
Particularly fitting for research at the Stanford Green Dorm are the first two benefits because of
their substantial environmental impacts and monitoring potential. The added substrate depth of
37
the green roof can filter and store rainwater during storm events and mitigate the overloading of
the sewers and municipal treatment plants. This stormwater control also provides the opportunity
for rainwater harvesting that can bring the building closer to its ultimate goal of closing the water
loop. Green roofs also add insulation that moderates the temperature flux within the building and
protect the roof from direct exposure to solar radiation, both of which reduce the energy demand
for space heating and cooling. This increased energy efficiency will contribute to the project’s
goal of an energy-neutral building. Additionally, while these two benefits have been tested at
other green roof research sites, there is significant room to expand research and quantifications at
the Stanford Green Dorm to contribute to the concept of the living laboratory.
The aforementioned benefits are the result of synthesizing the multiple layers of a green
roof, demonstrated in Figure 14. Descriptions of each layer are provided in Table 19. Within the
general layout of Figure 14, green roofs can be classified as intensive or extensive depending on
their specific characteristics. The two types are summarized in Table 20; intensive roofs have
thicker substrates and can support a wider variety of vegetation but require stronger structural
support, and extensive roofs have thinner substrates for greater flexibility of placement but can
support only limited types of vegetation.
Figure 1441. A typical cross-section of a green roof compared to a reference roof.
Table 1940. Explanation of Green Roof cross-section.
Layer
vegetation
growing medium
(substrate)
filter membrane
drainage layer
waterproofing
membrane
Description
specially-selected for irrigation characteristics, aesthetic appeal, etc.
can include a mix of natural and man-made substances; provides thermal insulation and
storage capacity for rainwater
contains the plant roots and growing medium while allowing water penetration
conveys the drained water to the collection system to prevent ponding
protects the underlying roof from leakage and damage
38
Table 2040. Comparison of Extensive and Intensive Green Roof Systems.
Both types of green roof provide benefits in terms of energy efficiency and stormwater
management. By implementing and testing both intensive and extensive green roofs, the
Stanford Green Dorm could help quantify the comparisons in Table 20 and contribute to a
broader understanding of the green roof’s primary functions.
6.2. Stanford Green Roof Layout
The Stanford Green Dorm Feasibility Study, prepared by EHDD Architecture, envisions
the green roof as a place for students to relax, study, and socialize1. This accessibility requires
that at least part of the green roof be an intensive green roof with sufficient load-bearing
capacity. The remaining roof surface is divided between regular rooftop and photovoltaic panels,
so there is opportunity to expand the green roof research capabilities by adding movable
extensive roof modules to parts of the regular rooftop. As the project is still in the early phases
of design, LCC will first provide general recommendations of the green roof characteristics and
will then later suggest a possible layout plan for the Green Dorm.
39
6.2.1. Intensive Roof Design
Intensive green roofs can have a substrate depth of 8-24 inches and can support a wide
variety of vegetation40. The deeper the substrate, the more noticeable the stormwater retention
and energy efficiency effects will be, and the stronger the required structural support. A likely
place for the intensive roof is above the first floor, where students can access it via the second
floor. This roof area is tentatively located above the entry porch, info center, and dining room1.
The indoor spaces can be monitored for temperature and energy fluxes, while the whole
intensive roof can provide helpful stormwater data.
6.2.2. Extensive Roof Design
Extensive green roofs have smaller substrate depths of up to 6 inches40, which allows for
a modular system for greater flexibility of research42. With movable modules, researchers can
vary the placement of the extensive sections to control for variables that might otherwise affect
the testing results. The extensive modules can also have varying vegetation and substrate types to
test the effects of those on the stormwater and energy effects. Such a system must be located in
areas that are inaccessible to students, such as the roof of the third floor. This area tentatively
covers student rooms1, so there are opportunities to observe the space-conditioning effects by
isolating individual rooms with extensive modules.
In order to quantify the benefits for both the extensive and intensive sections, however,
the remaining roof areas should be used as “reference roofs” for comparison. In particular, there
should be a specific reference roof for the intensive section of sufficient size and with equivalent
monitoring devices. This intensive reference roof should be located adjacent to the intensive
green roof in order to be exposed to similar weather characteristics and shading so that
variability between the roofs is minimized. Similarly, reference roofs between the extensive roof
modules can help with data comparison and act as buffer zones between different modules for
more accurate comparison of vegetation, substrate, etc. For this report, LCC uses the term
“reference roof” to refer to all non-vegetated roof surfaces.
6.3. Green Roof Stormwater Retention
On a regular roof, precipitation from a storm runs directly off the roof and surrounding
pavement and into the sewer system as stormwater, where it either flows directly to a free water
body (such as the San Francisco Bay) or is first treated in a wastewater treatment plant before
40
discharge. During heavy storm events, the stormwater can exceed the capacity of the sewer
and/or treatment plant in what is called a Combined Sewer Overflow (CSO), possibly resulting
in contamination of potable water sources and in the direct discharge of untreated wastewater43.
A substantial benefit of green roofs is their ability to decrease the volume and postpone the
discharge of stormwater into the sewers. Green roofs capture and hold water in the plant foliage,
absorb water in the root zone and substrate pores, and slow the velocity of the runoff by
extending the flow path through the vegetation and substrate44. This buffering prevents the
overloading of the sewer system during major storm events and creates a more natural
hydrologic cycle by allowing much of the precipitation to evaporate from where it was
deposited. To examine the potential benefits of the Stanford green roof, LCC will first present
case studies of existing green roofs and their stormwater retention capabilities. Then, LCC will
present a model for estimating the retention at the Stanford Green Dorm site and compare these
estimates to the retention observed at existing sites.
6.3.1. Green Roof Case Studies: Observed Stormwater Retention
Site 1: Wayne Community College, Goldsboro, North Carolina45
This extensive green roof covers 750 ft2 adjacent to an equal-sized reference roof, with
half of the green roof with a 2-inch substrate depth and half with a 4-inch substrate depth. The
climate is characterized by mild winters, warmer summers, and precipitation year-round, with
slightly higher precipitation between July and September.46 Stormwater retention over a ninemonth period was measured as 62% while individual-event retention figures ranged from 48% to
88%, depending on the saturation content of the substrate at the time of the event and the
evapotranspiration rates. The observed reduction in average peak flow was 78% (compared to
peak flow from an impervious surface).
Site 2: Neuseway Nature Center, Kinston, North Carolina45
This extensive green roof covers 290 ft2 with a 4-inch substrate depth, and is compared to
the 1820 ft2 of remaining rooftop. The climate is similar to that of Site 1. Stormwater retention
from four non-consecutive months was measured as 63%, with a range of 45% to 91%. The
observed reduction in average peak flow was 87%.
Site 3: Hamilton Apartments, Portland, Oregon47
This extensive roof covers 2620 ft2 with a substrate depth of 4-5 inches. The climate is
mild with steady precipitation in the winter and dry summers (June-September). A 16-month
41
period saw average stormwater retention of 69%, with close to 100% retention during the light
rainfall months of May-October. The average peak flow was reduced by 80-90%.
Site 4: Michigan State University Horticulture Teaching and Research Center, East Lansing,
Michigan48
These three extensive roofs of 60 ft2 each have a substrate depth of 1 inch. The climate is
characterized by cold winters and mild summers with moderate precipitation (1.5-3.5 inches per
month) throughout the year.49 The average annual stormwater retention for vegetated substrates
was 60%. Roof platforms with varying slope and substrate depth were also tested, and as
expected the deeper substrates and lower slopes saw higher stormwater retention.
In addition to these four sites, general retention data is available from Germany, the
world’s leader in green roof installations. The German industry standards specify 40-45%
retention in substrates of 0.8-1.6 inches, 50-55% retention in substrates of 2.4-3 inches, and 5560% retention in substrates of 4-5 inches45. Given these figures and the observed data from the
case study sites, LCC expects that the Stanford green roofs should retain approximately 50-70%
of annual precipitation.
6.3.2. Estimated Runoff from the Stanford Green Roof
The expected runoff from the Stanford green roof depends on a variety of factors,
including weather patterns, substrate depth, and type of vegetation. For these calculations,
precipitation and evapotranspiration data are taken from the California Irrigation Management
Information System (CIMIS) and the Western Regional Climate Center (WRCC). LCC has
assumed a standard warm-season turf grass for the intensive roof section and a typical green roof
ground cover for the extensive roof sections. For a discussion of assumptions and calculations,
please see Appendix F.
Figure 15 shows the regional precipitation and evapotranspiration data used to estimate
the green roof runoff. The figure demonstrates the typical climate of this area, with moderate
rainfall between late fall and early spring and close to no rainfall between June and August.
Evapotranspiration is highest during the warm months when there is more sun exposure and
higher temperatures.
42
8
Evapotranspiration
Precipitation
7
ETo/Precip (inches)
6
5
4
3
2
1
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 15. Average Monthly Evapotranspiration50 and Precipitation51 for Stanford Green Dorm.
From the above climate data and assumptions discussed in Appendix F, LCC has created
Table 21 below that summarizes the expected runoff characteristics of the two roof types.
Table 21. Estimated monthly runoff and retention from Stanford intensive and extensive green roofs.
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Runoff
(gal/sf)
Intensive
Extensive
1.5
1.8
1.1
1.5
0.2
0.9
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.4
0.8
1.2
1.5
Retention
(%)
Intensive
Extensive
28%
11%
38%
16%
89%
37%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
62%
26%
31%
13%
The data in Table 21 suggests an annual average retention rate of 55% for the intensive roof and
30% for the extensive roof. These estimates may be lower than observed retention rates because
the estimates account only for differences in vegetation type between the two roofs, and not
substrate depth. In reality, both the intensive and extensive roofs may have higher annual
retention rates because the substrates can hold water in the pores for eventual evapotranspiration.
Also, it is important to note that these calculations are very rough and assume each month is a
separate entity; in reality the weather patterns that carry over between months will have
significant effects on how much rainwater is stored for evapotranspiration.
For the purpose of sizing rainwater storage tanks (discussed later in this report), the data
in Table 21 is presented in Figure 16 below with estimated reference roof runoff as well. LCC
43
assumed a 10% retention rate for the reference roof that has been observed in other test locations
and accounts for light rain events when the precipitation evaporates before draining from the
2.0
100%
1.8
90%
1.6
80%
1.4
70%
1.2
60%
Intensive Roof
Extensive Roof
Reference Roof
Intensive Retention
Extensive Retention
1.0
0.8
0.6
50%
40%
% Retention
Estimated Runoff (gal/sf)
roof45.
30%
0.4
20%
0.2
10%
0.0
0%
Jan
Feb Mar
Apr May Jun
Jul
Aug Sep
Oct
Nov Dec
Figure 16. Estimated runoff and retention from Stanford intensive and extensive green roofs. Runoff from
reference roofs is also provided for comparison.
Unlike the reference roof, both green roof types suggest 100% retention between April and
October because of the light rainfall expected during these months. Green roofs typically retain
more water during scattered, light storms typical between April and October and retain less water
during regular, heavy storms when the substrates are continually partially saturated and have
lower additional capacity for water storage48. The estimates in Figure 9 are very rough, as they
also do not take into account varying saturation levels of the substrate that cannot be predicted
from year to year. Research at the Oregon site suggests that retention may be higher with uneven
rainfall distribution because the longer dry periods between storms increase potential
evapotranspiration47.
6.3.3. Living Laboratory Component
While stormwater retention has been studied somewhat in other green roof sites, there is
still large opportunity for research the Stanford Green Dorm. The substrate depth and type
44
should have noticeable effects on the retention volumes that are not included in the models
presented above. Testing different depths and types of substrate, both within the intensive roof
and between the different extensive roof modules, may help researchers develop a revised model
for estimating runoff quantities that includes the effects of the substrate. Faculty and student
researchers can test such variability through regular monitoring of the runoff volumes. LCC
presents suggestions for monitoring devices and techniques in the Living Laboratory section later
in this report.
6.4. Rainwater Harvesting for the Green Dorm
Rainwater harvesting is of particular interest for the Stanford Green Dorm because it will
bring the dorm closer to its goal of closing the water loop. Currently, rainwater harvesting is used
in many developing countries where there is a shortage of potable water52. Though there are
some locations within the US that have implemented rainwater harvesting, use of the technique
is far from widespread and there is tremendous opportunity for research and refinement.
Some benefits of a rainwater harvesting system include53:
-
providing water at no cost beyond that for treatment and storage
-
reducing stormwater runoff
-
providing water that is naturally soft and pH neutral (or slightly acidic)
-
providing water for non-potable indoor uses or outdoor irrigation
-
reducing the water demand from public utilities
In spite of these benefits, there are significant quality concerns with harvesting rainwater that are
difficult to anticipate because they depend on numerous factors, such as roof material, weather
conditions, and substrate material. However, LCC recommends utilizing rainwater harvesting for
both the water reuse benefits and for the research benefits that could help make rainwater
harvesting more widespread around the world.
In this section LCC will present estimates for the rainwater rooftop runoff volumes
available and use this to indicate possible storage tank sizes, then we will discuss the possible
quality concerns that will require further investigation when a rainwater harvesting system is
implemented.
45
6.4.1. Estimated Available Runoff
Runoff estimates per square foot are presented previously in Table 21 and Figure 16. At
this point LCC presents a possible layout for the roof systems in order to provide example runoff
volumes. The possible design schematic is shown in Figure 17.
Figure 17. Example layout for Stanford Green Dorm roof systems, including intensive, extensive, and
reference roof. Base diagram from Feasibility Study 1.
The intensive roof is accessible to students from the second floor while the inaccessible
extensive modules are located on the roof of the third floor. As discussed previously, specific
reference roofs are separated for monitoring purposes, although reference roof runoff refers to
runoff from all non-vegetated roof surfaces. In this schematic, the extensive modules can have
varying substrates and vegetation and can be moved around for research purposes. Reference
roofs intersperse the extensive modules to act as buffers between different modules.
Roof areas in the design schematic suggest monthly runoff volumes given in Table 22.
Table 22. Estimated runoff volumes (gallons) for Stanford Green Dorm based on example design schematic.
Roof Type
Area (sf)
Jan
Feb
Mar
Apr
May
Jun
Oct
Nov
Dec
Intensive
Extensive
Reference
1400
1500
7600
2000
2700
13700
1600
2300
12300
200
1400
9800
0
0
4400
0
0
1600
0
0
300
0
0
100
0
0
200
0
0
800
0
0
2900
600
1200
7700
1700
2300
11800
18400
16200
11400
4400
1600
300
100
200
800
2900
9500
15800
TOTAL
10500
Jul
Aug
Sep
46
Precipitation during June, July, and August is rare and the runoff during these months should not
be assumed available. Even if there is precipitation during these months, it is likely that the rain
will be extremely light and spread out so that close to all of it will evaporate before percolating
into a storage tank.
6.4.2. Quality Concerns
Runoff quality from the green roofs and reference roofs will vary significantly and
depend on many factors, including substrate and roof material, fertilizer and/or compost used on
green roofs, climate conditions (length of dry period before precipitation), and local rainwater
quality and atmospheric deposition. Here LCC discusses general trends observed in existing
rooftop collection sites to highlight the possible concerns for the Stanford Green Dorm.
Initial Rainwater Quality
The primary concern for initial rainwater quality before contact with roof surfaces is
acidity. For this area of California, acid rain is generally not a concern and the pH of local
rainwater falls between 5.0 and 5.554. This is within the expected range of 4.5-6.5 for natural
rainwater52.
Green Roof Runoff Quality
The green roof feature most in question for quality concerns is the substrate. Depending
on the contaminant in question and substrate design, the substrate can act as a source or sink for
contaminants. In general, the most important factor is the presence of compost and/or fertilizer
either in the substrate material or applied on the substrate surface. Substrates with higher
concentrations of compost in the media mix and more frequent application of compost and/or
fertilizer on the surface tend to have higher concentrations of phosphorus in the runoff45,47 ,55. For
example, a green roof in North Carolina produced runoff with concentrations of phosphorus
ranging from 0.6-1.4 mg/L while the reference roof runoff and rainwater had negligible
amounts45. Similarly, the concentration of phosphorus in runoff from a green roof in Sweden
was almost 7 times higher than that found in the rainwater55.
In addition to phosphorus, nitrogen in the form of ammonia or nitrate may be of concern
and may increase with the use of compost and fertilizer, although concentration differences at the
North Carolina green roof site were not statistically significant45. However, a study in Sweden
suggests that the substrate of green roofs may actually be a sink for nitrogen, with the observed
nitrogen concentration in the runoff being almost 60% lower than that found in the initial
47
rainwater55. Ultimately, it is difficult to anticipate whether the Stanford green roof will act as a
source or sink for nitrogen; monitoring and quality analysis can provide valuable insight into the
behavior of nitrogen in the green roof substrate.
High concentrations of phosphorus and nitrogen in runoff discharged to lakes and
reservoirs can have severe consequences because the overloading of nutrients can lead to algae
blooms, which deplete the oxygen in the water and endanger aquatic life56. While this may not
apply directly to a rainwater harvesting system, it highlights possible concerns over storing
rainwater with high concentrations of phosphorus and nitrogen because the stagnant stored water
may provide ideal conditions for algae growth and contamination. To minimize this risk, the
stored rainwater should not be exposed to light. In addition, minimizing the use of compost and
fertilizer may reduce the phosphorus and nitrogen concentrations, although complete avoidance
of compost and fertilizer may be infeasible. Extensive roofs are usually initially fertilized
regularly to stimulate plant growth and may require occasional fertilization after plant
development to maintain health55. Also, as mentioned previously in the blackwater system
discussion, the green roof provides an ideal location for testing blackwater-derived compost. If
this recycling is implemented, it is imperative that the green roof runoff quality be regularly
monitored and treated appropriately.
Heavy metals are less of a concern in green roof runoff, though their presence should still
be monitored. Studies in Germany and Sweden suggest that the substrates act as a sink for some
heavy metals present in the rainwater, including lead and cadmium55. It is unclear how much of
the heavy metals can be retained in the substrate, but at the very least green roof sites have seen
no change in the concentrations of heavy metals between the initial rainwater and the green roof
runoff55,57.
Reference Roof Runoff Quality
The quality of runoff from the non-vegetated roof surfaces is highly influenced by
weather conditions prior to precipitation and the roofing material. Rainwater can dissolve and
transport contaminants already present on the roof such as bacteria, molds, algae, protozoa, and
atmospherically deposited microbes and chemicals58. These contaminants are in higher
concentrations after a dry period, during which they have had time to settle and collect on the
roof surface52.
48
The roofing materials can act as a source of heavy metals, which can dissolve and
concentrate in the runoff52. Multiple rooftop rainwater harvesting systems around the world have
shown runoff concentrations of lead, copper, zinc, and cadmium in exceedence of freshwater or
drinking water standards52. Because of these heavy metal concerns, some roofing materials are
more appropriate for runoff collection; these recommended roofing materials are presented in
Table 23.
Table 2359. Potential Roofing Materials for Rainwater Harvesting.
Recommended
Metal
Slate
Clay
Tile
Not Recommended
Asphalt
Asbestos
Chemically Treated Wood
Some Painted Roofs
The recommended slate, clay, and tile roofing materials are most likely to aesthetically match
with Stanford’s red roofs. While some of the houses surrounding the planned Green Dorm site
have wooden shingle roofs (referred to in Table 23 as Chemically Treated Wood), research
suggests that wooden shingles are among the worst for contaminating runoff58.
Contaminants collected on the roof and originating from the roofing materials present
both primary and secondary health concerns: collected rainwater should be adequately filtered
and treated before indoor use, with less stringent standards for outdoor irrigation use; and
additional treatment may be required to minimize negative aesthetic effects such as color and
odor58. Reference roof runoff contaminants are often most concentrated during the “first flush”
of a storm, particularly after a long period of dry weather when contaminants have had ample
time to settle onto roof surfaces52. Atmospheric dissolution of contaminants also decreases as the
storm continues, so contaminant concentrations are high during the first minute of runoff and
tend to decrease to a constant value.
6.4.3. System Design and Treatment Suggestions
Because of the lack of standard runoff quality information and the research goals of the
Stanford Green Dorm, LCC recommends designing the rainwater harvesting system to maximize
the potential for monitoring and research.
Collection and Storage System
The most important task for accurately monitoring both the quantity and quality of runoff
will be to separate the runoff from the different types of rooftops. However, because of storage
tank cost and general layout feasibility, it may not be possible or recommended to have separate
49
storage tanks for each type of runoff. Instead, a possibility is to divert small volumes of each
type of runoff to the laboratory for analysis while sending the bulk of the flows to a common
rainwater storage tank, as suggested in Figure 18. The estimated annual runoff volumes from
each roof type are calculated from Table 22; the calculation for sizing the rainwater storage tank
is described in detail after the figure.
Figure 18. Suggested flows of harvested rainwater.
LCC recommends separate collection drains and gutters for the three roof types to maintain
water separation before the storage tank. Initial filtration and with gutter screens and roof
washers (see Appendix D) should occur at the entrance to the individual gutters to prevent large
debris from entering the collection system, while further settling can occur within the storage
tank itself.
Sizing the rainwater storage tank depends on what the stored water is to be used for. If
the water is to be used with greywater for indoor non-potable uses such as toilet flushing and
washing machines, then the water will be used throughout the year and a smaller tank is
acceptable. However, if the rainwater is to be stored for summer irrigation, a larger tank size is
required since precipitation primarily falls during the winter. It is important to note that storing
collected rainwater over a long period of time may increase the risk of water contamination and
bacterial growth in the stagnant water, although existing rainwater harvesting sites have been
inconclusive on this issue52. Quality monitoring of the runoff at the Stanford Green Dorm may
provide clearer information as to when to treat rainwater that will be stored for a long period of
time.
50
To size the storage tank, LCC used the runoff estimates explained above and a filter
efficiency of 80% to account for losses through leaks, filtration, and treatment58. The total
available annual runoff is estimated to be 65,000 gallons. Rather than attempting to store all of
the rainwater, LCC estimated the total average dry season demand for irrigation, which is the
volume that should be in storage for the dry season (roughly May through September)60.
Irrigation estimates from the California Academy of Sciences green roof, which includes many
common extensive green roof groundcovers, suggests ¼ gallon per cubic foot per week from
May to September61. Extrapolating this to the entire Stanford green roof and assuming an
average substrate depth of 1 foot yields a total dry season green roof irrigation demand of 15,000
gallons.
The remaining 50,000 gallons of the annual expected runoff will likely be available for
reuse between October and April. This averages out to 200-250 gallons of runoff per day during
the wet season, although the runoff will by no means be consistent from day to day. As
suggested in the Water Balance earlier in the report, there is already an expected surplus of
greywater available for indoor reuse, so depending on the circumstances of a given day the
collected rainwater may not be needed for use in the Green Dorm. A possible fate for the
rainwater may be temporary storage in the tank for any desired wet-season irrigation of the
landscaping. Alternatively, Stanford officials could look into “exporting” the excess greywater
and rainwater to the Stanford non-potable water supply pipelines to be reused for campus
irrigation or toilet flushing. It is also possible in the future that after testing and experimenting,
the collected rainwater and/or greywater can be reused for potable indoor uses such as
showering, which will increase the demand for recycled water in the Green Dorm.
Treatment Suggestions
In general collected rainwater is of high enough quality that treatment beyond screening
and settling (see Appendix G) is not required if the rainwater is to be used for non-potable
purposes58. If further treatment is desired to maximize user health and safety, LCC recommends
treating the rainwater in conjunction with the greywater flows in order to save on treatment costs
and system size. Many of the possible treatment methods recommended for rainwater coincide
with those explained in the greywater system discussion58.
51
6.4.4. Living Laboratory Component
Since the runoff quality varies significantly between green roof test sites, it will be very
beneficial to monitor the quality from the different components of the Stanford green roof. In
particular, quality must be monitored if compost from the blackwater system is used on the green
roof, and such monitoring can provide insight into the pros and cons of blackwater-derived
composting. For example, if the quality of the runoff decreases significantly with the compost
and more extensive treatment is required before the runoff can be reused, it may not be
environmentally beneficial to use the blackwater-derived compost. Also, because a major goal
for the project is to push further research, the Stanford Green roof can become a site that pushes
and establishes the regulations for reusing rainwater. With enough testing and monitoring over
the years, researchers may be able to develop methods for treating rainwater for safe potable uses
(such as use in showers, kitchen faucets, or even as drinking water). If this potential can be
realized, the dorm will be even closer to closing the water loop.
6.5. Temperature and Energy Effects of the Green Roof
In addition to the runoff water quantity and quality benefits, green roofs can moderate the
temperatures of roof membranes and indoor spaces mainly by providing thermal mass insulation
and shading. The thermal mass characteristic allows the roof to act as a buffer between the
indoor and outdoor environment, which shaves the peak energy loads by moderating the
temperature swings. The shading provided by the green roof allows plants to absorb solar
radiation before it hits the roof and prevents the transfer of heat from warm wind by blocking the
medium62. Multiple green roof test sites have demonstrated these benefits, so it is likely that the
Stanford Green Dorm will experience similar trends and will contribute to the quantification of
these benefits. Before extrapolating trends specific to the Stanford green roof, LCC will present
case studies and results of existing green roofs that can be used as models for the Stanford green
roof.
6.5.1. Green Roof Case Studies
Site 1: Fencing Academy of Philadelphia. Philadelphia, Pennsylvania63
This extensive green roof covers 3,000 ft2 of roof area with a substrate and vegetation
thickness of only 2.8 inches. The climate is characterized by short, high-intensity rainstorms in
the winter and hot, humid summers.
52
Site 2: Field Roofing Facility at the National Research Council. Ottawa, Canada64
This intensive green roof covers 400 ft2 with a substrate depth of 6 inches, adjacent to an
equivalent 400 ft2 of reference roof. There is substantial snowfall in the winter (constant layer of
snow in January-February) and mildly-warm temperatures in the summers.
Site 3. University of Central Florida. Florida.65
This intensive green roof covers 1,600 ft2 with a substrate depth of 6-8 inches, adjacent to
an equivalent 1,600 ft2 of light-colored reference roof. The climate is humid with hot summers,
warm springs and falls, and mild winters, and more rain falls during the summer months.
6.5.2. Observed Temperature Effects
In general, temperature and energy effects are more significant compared to reference
roofs during warmer weather, so LCC will focus on the warm-weather benefits of the green roof.
Roof temperatures were monitored at all three test sites throughout the year, and the warmweather results are given in Table 24.
Table 24. Comparison of maximum roof temperatures and roof temperature fluctuations at three green roof
test sites.
Test Site
Pennsylvania63
Canada64
Florida65
Maximum Average Daily
Temperature (F)
Reference Roof Green Roof
--------158
77
130
91
Difference
----81
39
Average Daily Membrane
Temperature Fluctuation (F)
Reference Roof Green Roof
90
18
83
22
59
7
% Change
-80%
-73%
-88%
As expected, the green roofs outperformed the reference roofs at each location, with important
implications. The reduction in the maximum average daily temperatures of the roofs reduces the
urban heat island effect of the building and also keeps the temperature of the inside of the
building lower during the warm months64. The reduction in the membrane temperature
fluctuation range has a similar moderating effect on the inside temperatures, but also increases
the lifespan of the roofing materials. Temperature swings of the roof membrane create thermal
stresses within the material that wear away at the roof’s protective capabilities and decrease its
ability to protect against water66. It is estimated that green roofs can extend the lifetime of a roof
membrane by more than 20 years63.
6.5.3. Observed Energy Effects
Reducing the roof temperatures has noticeable effects on the space-conditioning energy
demand of the indoor spaces. At Site 3 (Florida), the heat flux through the reference roof during
53
the hot months peaked in the middle of the day at 2.8 Btu/hr-ft2, while the heat flux through the
green roof was almost 80% lower, peaking during the night at 0.6 Btu/hr-ft2.65 Site 2 (Canada)
saw similar reductions in heat flow, with the average daily summer heat flux at 6-7.5 kWh (per
400ft2) for the reference roof and only 1.5 kWh for the green roof64. Converting these to Btu/hrft2 yields 2.1-2.7 Btu/hr-ft2 for the reference roof and 0.5 Btu/hr-ft2 for the green roof, which is
consistent with the data collected at the Florida site. The researchers at the Canada site assumed
that the heat flux values represent the energy required to maintain a constant temperature in the
building, so the energy demand can be extrapolated from heat flux data. The annual profile of the
energy demand at the Canada site is shown in Figure 19. The heat flux values in the figure do not
represent a direction of heat flow; they are the sum of the absolute values of the heat flow into
and out of the building. Because the space-conditioning energy refers to any energy required to
heat or cool the space (depending on the outdoor temperature), these absolute value sums are
representative of the energy required to maintain a constant temperature.
Figure 1964. Average daily energy demand due to heat flow through roof surfaces (Canada, Nov 22, 2000 –
Sept 31, 2001). Data is in kWh/day for the 400 ft 2 area of roof. Reference roof data is shown in blue (R) and
green roof data is shown in green (G).
As demonstrated by Figure 19 and by similar results at the other test sites, the insulation effects
of the green roof are more substantial in the warm months. For the Stanford Green Dorm, this
suggests that the air-conditioning load will be lower during the hot summer; or, if no cooling
system is used, the indoor environment will be more comfortable for the inhabitants.
54
6.5.4. Estimated Effects of Stanford Green Roof
To estimate the energy effects expected from the Stanford green roof, we can apply a
simple equation for heat flow through an insulation medium:
q = ΔT/R
where q is the heat transfer in Btu/hr-ft2, ΔT is the difference between the outdoor temperature
and indoor temperature, and R is the insulation value of the green roof (hr-ft2-ºF/Btu)67. A
Canadian study specifies green roof insulation of R-20 for an 8-inch substrate, which translates
to R-2.5 per inch40. A green roof company Roofscapes, Inc. gives a similar R-value of 2-5 per
inch44. The variation in R-value will depend heavily on the type of substrate used. For this
analysis, we assume an average R-value of 3 per inch, which can be modified later based on
substrate material.
Figure 20 demonstrates the results of this calculation for varying substrate depths.
88
86
1.4
84
Heat Flux q (Btu/hr-ft2)
1.2
82
4"
1.0
80
0.8
78
8"
76
0.6
12"
74
0.4
16"
72
0.2
24"
70
Average Maximum Ambient Temperature (F)
1.6
20"
0.0
68
May
Jun
Jul
Aug
Sep
Oct
Figure 20. Estimated heat flux through the Stanford green roof for varying substrate depths during warm
months. Indoor temperature is assumed to be 70ºF and the R-value is estimated as 3 per inch. The average
maximum ambient temperature is shown with a dashed line.
We have assumed an indoor temperature of 70ºF to reflect the expected comfort level within the
building from the passive solar design and cooling effects of the green roof, though this estimate
can be easily modified. Monthly average maximum ambient temperature is used to suggest peak
heat fluxes for each month51. As expected, the heat flux through the green roof decreases as the
55
substrate depth increases, though the benefits decrease incrementally as the substrate layer
thickens. The heat flux increases as the ambient temperature increases because the larger
difference in temperature between the inside and the outside drives more heat transfer through
the roof (assuming the indoor temperature is held constant).
The estimates in Figure 20 are within the range expected from the heat flux values for the
test sites in Florida and Canada (0.5-0.6 Btu/hr-ft2). Many of the Stanford estimates are slightly
lower because 1) the higher substrate depths provide more insulation, and 2) the Stanford values
use the average monthly maximum temperatures rather than a single-day peak temperature, so
the heat flux values in Figure 20 are average maximum heat flux rates. Any individual day with a
higher ambient temperature with drive up the peak heat flux rate for that day.
6.5.5. Living Laboratory Component
The Stanford green roof will provide opportunities to further examine these temperature
and energy benefits through comprehensive monitoring and data collection. Specific monitoring
devices are described later in this report; here we present broad ideas for future research. One
such idea is to examine the changes in energy and temperature effects with varying types of
substrates and vegetation. The extensive roof modules would be ideal for isolating these different
sections, as they can be easily altered to fit research needs. Varying substrate material will likely
affect the thermal mass properties of the roof, while varying the vegetation could alter the
amount of evaporative cooling from the modules. It would be more difficult, though not
impossible, to vary substrate materials in the intensive roof since the this would require partitions
separating the sections and the effects of one section could easily alter the effects of an adjacent
section. In addition, the higher depth of substrate in the intensive roof might overshadow any
effects of substrate type. However, the thicker layer of substrate in the intensive roof provides
more opportunities for research: a wider variety of plants may be tested for their thermal and
stormwater benefits while the effects of the deeper substrate can be compared to those of the
shallow-substrate extensive modules.
56
7. LIVING LABORATORY: FUTURE RESEARCH OPPORTUNITIES
Stanford’s Green Dorm will serve as a “living laboratory” to embody the ambitious
research goals of the university. The Green dorm will provide the opportunity for Stanford
faculty and students not only in the department of Civil and Environmental Engineering but
throughout the campus to research wastewater management and energy consumption in a reallife scenario. The students that live in the dorm can learn in great detail about the ecological
footprints of their daily habits by monitoring their water consumption and energy usage, and
perhaps research themselves how changes in their daily habits or new green technologies can
affect the efficiency of the entire dorm. In this section, LCC describes potential areas of research
in the green dorm specific to the three systems we have presented.
Greywater Research
Using bacterial indicator species (ie total coliform) can be problematic: although they are
easy to measure there is not always a clear relationship between the concentration of indicator
organisms and the actual pathogen present68. A positive coliform concentration only shows a
possible pathway for disease transmission, however not an actual risk of developing illness.68 A
possible area of research for the green dorm is to investigate the ability of indicator species to
identify the presence of pathogens in the greywater flow68. Possible indictor species to research
include the following: Escherichia coli, total coliform, fecal enterococci, and clostridium
perfringen.
The scientific literature points to the fact that greywater quality degrades rapidly over
time, however the state of California does not require treatment for greywater used for
subsurface irrigation6. Because of this a potential area of research could be to design two sets of
subsurface irrigation systems, one with treatment and one without treatment, for the green dorm,
and to measure the differences in the level of bacterial indicators present in the soil irrigated by
the two systems.
Components of the Living Laboratory

Investigate the ability of indicator species to identify the presence of pathogens in the
greywater flow

Evaluate if there is a difference in pathogen levels in soils irrigated by subsurface
irrigation with treatment or subsurface irrigation without treatment.
57
Classifying Greywater Flows High load vs. Low Load
An article by M.A. Lopez Zavala et al. separated greywater flows into two categories
lower load and higher load greywater68.
Low Load68
The lower load greywater was defined as having the main pollutants be COD and nitrates
such as soaps and shampoos, and there being very few pathogens present. These flows were
considered fine to be used for subsurface irrigation without treatment, to be broken down
naturally by the soil. However there are several factors listed in the article that could affect the
process decomposition. For example how the microorganisms in the soil behavior could play a
large role in the breakdown of the COD and organics that are present. For the living laboratory
component of the green dorm we are proposing that a study be done to investigate how these
variables listed below effect the breakdown of COD in the soil irrigated by greywater flows:
1. Soil microorganisms
2. Nutrients macro and micro
3. Environmental conditions(ie temperature, pH, infiltration, salt content)
High Load68
This type of greywater flow was defined as containing substance such as bleach and
bathroom cleaners. For these types of flow membrane bioreactor has a higher treatment
performance for water reuse in large buildings.
This article suggests that separating greywater flows into these two categories would
allow for safer and more effective greywater treatment.
Component of Living Laboratory

Separate the greywater flows and monitor the influent water quality and observe the final
level of treatment

Determine if there is a different in water quality after treatment between higher load and
lower load greywater flows
Blackwater Research
All of the blackwater technologies recommended for the Green Dorm are experimental,
particularly in the United States. While composting toilets and urine-diverting toilets are used
around the world, most are used in individual households. The Stanford Green Dorm will be one
of the first small-community-scale efforts to use these technologies, and careful monitoring of
58
volumetric flow information will advance research in the waste management field. Furthermore,
while general data exists on the nutrient content of human waste, very few case studies exist on
the exact nutrient content of the outputs of composting and urine-diverting toilets. The Stanford
Civil Engineering department and students have the expertise, time, and funding to collect muchneeded data on the quality of the inputs and outputs of these two toilet systems. Additionally,
while commonly used for kitchen waste digestion, anaerobic digesters and membrane bioreactors
are still in experimental stages for toilet waste digestion. Through monitoring and analysis, the
Green Dorm can provide cutting-edge research on these technologies.
Green Roof and Rainwater Harvesting Research
As mentioned previously in this report, there are many opportunities for research
regarding the green roof and rainwater harvesting system, including analyzing the effects of
substrate and vegetation variability on stormwater quantity, stormwater quality, and thermal
benefits. Effective monitoring of the system is important to these research goals and will also
provide opportunities for student involvement and interactive learning.
Stormwater Runoff Quantity and Quality
Existing green roof test sites suggest that stormwater runoff volumes decrease with
increasing substrate depth, however the quantifications are difficult to predict without an
effective model that incorporates the effects of substrate depth. Similarly, it is difficult to predict
the effects of different substrate materials and vegetation types on the quality of the runoff.
Careful measurement of runoff flows is necessary to improve such runoff predictions. Existing
green roof test sites use specifically-placed drains and gutters to keep water flows separate
between the different roof types, such as is shown in Figure 21:
Figure 2145. Schematic of runoff collection system from Wayne Community College Green Roof in North
Carolina.
59
A setup like that shown in Figure 21 is feasible for the Stanford Green Dorm: green roof
modules can slope towards a central drain with primary leaf screens, under-roof gutters can
convey the runoff to the side of the building where the first flush is diverted to roof-washers, and
from there most of the runoff can flow to the storage tank with some being diverted for testing in
the laboratory. Researchers can measure the flow quantities using various types of flow meters;
the set-up in Figure 21 uses weir boxes with depth sensors to measure the depth of water in the
runoff flow to the nearest ±1%45. Runoff that is diverted to the laboratory and that from the roof
washers can be analyzed for different contaminants in order to determine the optimum treatment
levels.
With multiple sections of intensive and extensive green roofs, researchers can vary the
substrate and vegetation to try to ascertain trends in the quantity and quality of the runoff. LCC
particularly recommends analyzing the effects of different types and levels of compost, as these
are very likely to influence runoff quality. Researchers can study the effects of compost from the
blackwater system in order to assess the effectiveness of such blackwater reuse.
Thermal Effects
The thermal benefits of the green roof depend heavily on the specific site location, so
monitoring these effects will allow Stanford to assess the green roof potential for energy
conservation in this urban area. Monitoring techniques at existing green roof test sites such as
those in Canada and Florida include the uses of thermocouples and heat flux transducers as
shown in Figure 22.
Figure 2264. Cross-section of green and reference roof with thermal monitoring devices.
60
Thermocouples, which measure the temperature profile through the roof, can be placed between
each layer to give researchers a sense of the insulation values of the different materials64. Heat
flux transducers can provide estimates of the energy flux through the roof – information that
researchers can use to estimate the energy savings from the green roof.
All of these research and monitoring ideas will not only enrich the academic lives of
faculty and students at Stanford, but will provide the knowledge of how to bring green
technologies and sustainable buildings to college campuses around the US. It is our aim that the
Green Dorm research opportunities will put Stanford in the forefront of this exciting age of
sustainable development.
61
8. CONCLUSIONS
LCC’s final design recommendations for the Stanford Green Dorm encompass the goals
of the project itself: a living laboratory for research, measurable environmental performance, the
most desirable housing on campus, and economic sustainability. By focusing on the water reuse
systems possible for the dorm, LCC’s design provides abundant research prospects for Stanford
students and faculty, brings the dorm closer to closing the water loop by reducing potable water
demand, and draws attention to the benefits of sustainable design. The greywater, blackwater,
and green roof and rainwater harvesting systems can work together in the cohesive design shown
in Figure 23.
| Inflow |
3000
gal/mo
(summer)
Treatment
Rainwater Storage
600 gpd
Showers
variable
Settling/
Sedimentation
Tank
|
TERTIARY
Biological
Treatment
600 gpd
Coarse
Filtration
Bathroom Faucet
200 gpd
Clothes Washer
180 gpd
Membrane
Filtration
Disinfection
180 gpd
Outflow
Green Roof Irrigation
variable
200 gpd
| Products |
80 gpd
Urine-diverting Toilet
40 gpd
Composting Toilet
20 gpd
TBD
Grey Storage
to inflow
Irrigation
Storage
to inflow
Urine Storage
to local farms
Compost
to Green Roof
80 gpd
460 gpd
Standard Toilet
470 gpd
50 gpd
50 gpd
Dish Washer
50 gpd
310 gpd
Kitchen Faucet
310 gpd
TBD
Subsurface Irrigation
130 gpd
Leaks
SEWER
with
optional
diversion
to
MBR lab
Figure 23. LCC Final Design Schematic.
The relationships in Figure 23 demonstrate the interconnectedness of various water system
designs and highlight the importance of continued research and design innovation. Through the
effective conservation of water and energy, the Stanford Green Dorm promises to be a leader in
sustainable development for campuses and buildings nationwide.
62
APPENDIX A: WATER USAGE SURVEY
Example Survey
Water Usage Survey for CEE 179C
Name:
Dorm:
How often do you do laundry and how many loads? ______________________
Example
Wednesday
Thursday
………….
Tuesday
Wednesday
Duration
of
shower
15 min
Duration of
faucet use
(washing face,
brushing
teeth, etc)
4 min
Duration
of
Drinking
Fountain
Use
2 min
Number of
Toilet Flushes
(include if
Duration
you flush
of Dish
twice)
Rinsing Other
4
1 min
2-May
3-May
……...
15-May
16-May
Summary of Survey Data
20 people participated in the water usage survey. The observation period varied with each
person and ranged from four days to two weeks. Data for toilet flushing were taken from
observation data, while data for laundry loads were taken from participants’ estimates of their
average laundry use. A summary of the survey statistics is shown in Table A-1.
Table A-1. Statistical Summary of Water Use Survey.
# people surveyed
minimum
maximum
mean
standard deviation
In-Dorm Toilet Flushes
(per day)
20
3
10
6.5
1.64
Laundry Loads (per
week)
20
0.8
2
1.1
0.27
There was more variation in the number of in-dorm toilet flushes than laundry loads, presumably
because the number of toilet flushes is much more dependent on personal lifestyles, including the
highly variable time spent outside of the dorm. For example, a student who works part-time
between classes or who studies outside the dorm will have fewer in-dorm toilet flushes than one
who studies in his/her dorm room or returns to the dorm frequently throughout the day.
63
APPENDIX B: REGULATIONS
Requirements in California Title 22 code of regulations for listed treatment methods
Section 60343. Primary treatment
All primary treatment unit processes shall be provided with one of the following reliability
features:
(a) Multiple primary treatment units capable of producing primary effluent with one unit not in
operation.
(b) Standby primary treatment unit process.
(c) Long-term storage or disposal provisions.
Section 60345. Biological treatment
All biological treatment unit processes shall be provided with one of the following reliability
features:
(a) Alarm and multiple biological treatment units capable of producing oxidized wastewater with
one unit not in operation.
(b) Alarm, short-term retention or disposal provisions, and standby replacement equipment.
(c) Alarm and long-term storage or disposal provisions.
(d) Automatically actuated long-term storage or disposal provisions.
Section 60347. Secondary sedimentation
All secondary sedimentation unit processes shall be provided with one of the following
reliability features:
(a) Multiple sedimentation units capable of treating the entire flow with one unit not in
operation.
(b) Standby sedimentation unit process.
(c) Long-term storage or disposal provisions.
Section 60349. Coagulation
(a) All coagulation unit processes shall be provided with the following mandatory
features for uninterrupted coagulant feed:
(1) Standby feeders,
(2) Adequate chemical stowage and conveyance facilities,
(3) Adequate reserve chemical supply, and
(4) Automatic dosage control.
(b) All coagulation unit processes shall be provided with one of the following
reliability features:
(1) Alarm and multiple coagulation units capable of treating the entire flow
with one unit not in operation;
(2) Alarm, short-term retention or disposal provisions, and standby
replacement equipment;
(3) Alarm and long-term storage or disposal provisions;
(4) Automatically actuated long-term storage or disposal provisions, or
(5) Alarm and standby coagulation process.
64
Section 60351. Filtration
All filtration unit processes shall be provided with one of the following reliability features:
(a) Alarm and multiple filter units capable of treating the entire flow with one unit not in
operation.
(b) Alarm, short-term retention or disposal provisions and standby replacement equipment.
(c) Alarm and long-term storage or disposal provisions.
(d) Automatically actuated long-term storage or disposal provisions.
(e) Alarm and standby filtration unit process.
Section 60353. Disinfection
(a) All disinfection unit processes where chlorine is used as the disinfectant shall be
provided with the following features for uninterrupted chlorine feed:
(1) Standby chlorine supply,
(2) Manifold systems to connect chlorine cylinders,
(3) Chlorine scales, and
(4) Automatic devices for switching to full chlorine cylinders.
Automatic residual control of chlorine dosage, automatic measuring and recording of
chlorine residual, and hydraulic performance studies may also be required.
(b) All disinfection unit processes where chlorine is used as the disinfectant shall be
provided with one of the following reliability features:
(1) Alarm and standby chlorinator;
(2) Alarm, short-term retention or disposal provisions, and standby
replacement equipment;
(3) Alarm and long-term storage or disposal provisions;
(4) Automatically actuated long-term storage or disposal provisions; or
(5) Alarm and multiple point chlorination, each with independent power
source, separate chlorinator, and separate chlorine supply.
65
APPENDIX C: CONTEXT OF WASTE MANAGEMENT SYSTEMS
Origins of Waste Management Systems
The United States and much of the developed world today uses large, centralized
methods of treating human waste. Human waste is flushed by water through the sewer to
municipal treatment plants; these plants filter and settle out large particles in the sewage, remove
potentially harmful nutrients using microorganisms, and remove pathogens and toxins using
chlorine and other chemical processes. The treated wastewater is then released into nearby water
bodies69.
It is easy to regard water-flushed sewage systems as the only reasonable method of
managing human wastes. However, we present a bit of history here to show that this system of
waste management is very much a product of historical events and not as much an analytical
choice.
The origin of municipal treatment plants dates back to 19th century London, where
diseases such as cholera were very common. Through observation of the conditions of the poor
in the slums of London, a young lawyer named Edwin Chadwick noticed a link between filth
levels and disease prevalence. He suggested that civil engineers also be trained in “sanitary
science” so that public works systems could be designed to best minimize disease propagation.
He said that water should be used to move wastes from homes for the first time, that pure water
should be available for drinking, and that adequate drainage should exist in the streets.
Chadwick wrote these ideas in a report called On the Sanitary Conditions of the Labouring
Population of Great Britain70,71.
In response to Chadwick’s report, the cities of New York and Boston began public works
programs to remove human waste from homes using water through sewage pipes. In the early
stages, wastewater was simply released into water bodies, untreated. However, adverse health
effects and, later, environmental effects influenced American policymakers to draft legislation
regulating the release of sewage and requiring its treatment. In response to the legislation, largescale treatment facilities were built to treat the large quantities of sewage conveyed by the public
works system. Thus, the modern system of municipal wastewater treatment was born70.
66
A Shift in Thinking
Today, the municipal wastewater treatment system removes the majority of nutrients in
wastewater. However, changing social and economic factors in the past decade have led people
to question the current system and have motivated them to consider alternatives.
First, more people have become aware of the negative environmental impacts associated
with nutrient releases into water bodies. For example, nitrogen and phosphorus are two nutrients
common in human waste that can plants and bacteria use to grow. The chemical reactions the
plants and bacteria use to grow also require oxygen, which the organisms find dissolved in the
surrounding water. As the plants and bacteria grow, the level of dissolved oxygen in the
surrounding water falls. Fish and other aquatic species rely on certain levels of dissolved oxygen
and can die if the levels get too low.
Second, whether because of high fertilizer costs or the desire to find more sustainable
ways to dispose of wastes, people have begun to regard the large nutrient content of human
waste as a resource. While disruptive if released into natural ecosystems, the nitrogen and
phosphorus in human waste can be beneficial if used to fertilize cultivated area. Technologies
such as urine-diverting toilets and composting toilets capture the nutrient potential of human
waste.
Lastly, the costs of municipal treatment have risen as legislative requirements have
become more stringent and population density has increased. Economic analyses of municipal
treatment plants suggest ways that alternative systems could decrease the cost of wastewater
treatment. For example, an estimated 70-90% of annual municipal treatment costs are transport
costs; less centralized technologies may be more cost-effective72.
67
APPENDIX D: URINE REUSEABILITY BASED ON STORAGE
TEMPERATURE AND TIME
Höglund recommends the following storage conditions to ensure the safety of urine for
fertilizer use. Table D-1 is taken directly from page 51 of her report, “Evaluation of microbial
health risks associated with the reuse of source-separated human urine,” published by the Royal
Institute of Technology and the Swedish Institute for Infectious Disease Control31.
“Table D-1. Relationship between storage conditions, pathogen content a of the urine mixture and
recommended crop for larger systemsb. It is assumed that the urine mixture has at least pH 8.8 and a
nitrogen concentration of at least 1 g/l
Possible pathogens in
Storage Temperature
Storage Time the urine mixture
Recommended crops
food and fodder crops that are to be
4ºC
≥1 month
viruses, protozoa
processed
food crops that are to be processed, fodder
4ºC
≥6 months
viruses
crops
food crops that are to be processed, fodder
20ºC
≥1 month
viruses
crops
20ºC
≥6 months
probably none
all crops
a
Gram-positive bacteria and spore-forming bacteria are not included.
b
A larger system in this case is a system where the urine mixture is used to fertilize crops that will be consumed by
individuals other than members of the household from which the urine was collected.
c
Not grasslands for production of fodder. Use of straw is also discouraged, further discussed below.
d
For food crops that are consumed raw it is recommended that the urine be applied at least one month before
harvesting and that it be incorporated into the ground if the edible parts grow above the soil surface.”
68
PLEASE SEE HANDWRITTEN SHEETS FOR APPENDIX E.
69
APPENDIX F: GREEN ROOF RUNOFF ASSUMPTIONS AND
CALCULATIONS
Background and Assumptions
The estimates of green roof runoff are based on a simple mass balance that assumes two
possible fates for precipitation: evapotranspiration or runoff. Evapotranspiration is a regional
measurement of the water loss through both plant transpiration and evaporation from soil and
plant surfaces. Reference Crop Evapotranspiration (ET0) measures the water loss from
thoroughly-studied and controlled grass surface. A crop coefficient (Kc) is used with different
vegetation to relate the expected evapotranspiration (ETc) to that of the reference crop (a
standard grass or alfalfa surface73):
Equation F-1. ETc = ET0 * Kc
Crop coefficients typically range from 0.1 to 1.2 (10-120% of reference crop evapotranspiration)
and vary based on the specific characteristics of the vegetation73.
LCC has assumed a typical warm-season turf grass for the intensive roof section since
this simple ground cover will be most inviting for students to relax and socialize. Stanford is
located in a warm/arid region for turf grass specifications and possible types of appropriate turf
grass include Bermudagrass, Buffalograss, and Zoysiagrass74. These warm-season turf grasses
are assigned a crop coefficient value of 0.675.
For the extensive roof surfaces, LCC has assumed a typical green roof cover, the sedum
acre. This perennial ground cover crop has been successful on existing green roofs because of its
drought-resistence and ability to grow in shallow substrates45,47 ,48. Sedum acre is assigned a
crop coefficient value of 0.2575.
Precipitation data is taken from the Western Regional Climate Center (WRCC) database,
which includes monthly average precipitation data for Palo Alto from the period of record, 1953
to 200651. Evapotranspiration data is not available through this source, so data is taken from the
California Irrigation Management Information System (CIMIS) database. The nearest
evapotranspiration location is in Morgan Hill, slightly south of San Jose, so data is taken from
this station from the period of record, 1997 to 200750. LCC chose to use data from the two
different sources rather than taking precipitation data from the CIMIS database because of the
longer period of record available through the WRCC. The ten-year period of record from the
CIMIS database includes a large anomaly from July 1999 in which 1.83 inches of rain fell (most
70
other July volumes are close to 0 inches), and this significantly increases the average July
precipitation value for the ten years. The 53-year period of record from the WRCC database
provides more accurate average monthly precipitation values because anomalies are
overshadowed by more typical rainfall patterns.
Mass Balance
The model used to create runoff estimates assumes that precipitation leaves the green
roof through either runoff or evapotranspiration. Using the evapotranspiration data from the
CIMIS database, LCC estimated evapotranspiration volumes and subtracted them from
precipitation volumes to suggest runoff volumes.
First, LCC multiplied ET0 data for each month by the crop coefficient chosen for each
roof type to yield the expected green roof ETc in inches (following equation C-1). A simple
conversion translates these values and the precipitation data into gallons per square foot. Next, to
conserve the mass balance, LCC compared the precipitation and ETc data of each month: during
warmer months when ETc exceeded precipitation, ETc is reset to be equal to the precipitation
value for that month since ETc loss cannot be greater than the amount of water available. The
difference between the resulting ETc values and the precipitation values (P) is the estimated
runoff for each month. Percent retention is the ratio of ETc to precipitation.
Equation F-2. Estimated Runoff = P - ETc
Equation F-3. % Retention = ETc / P * 100%
The monthly results of these calculations are presented in Tables F-1 (intensive roof) and F-2
(extensive roof).
Table F-1. Intensive roof mass balance calculations.
Month
Precip (in)
ETo (in)
Eq. F-1: ETc (in)
convert ETc (gal/sf)
ETc actual (gal/sf)
convert Precip (gal/sf)
Jan
3.22
1.48
0.89
0.55
0.55
2.01
Feb
2.88
1.83
1.10
0.68
0.68
1.80
Mar
2.31
3.43
2.06
1.28
1.28
1.44
Apr
1.04
4.59
2.75
1.71
0.65
0.65
May
0.38
6.28
3.77
2.35
0.24
0.24
Jun
0.08
7.02
4.21
2.63
0.05
0.05
Jul
0.02
7.11
4.27
2.66
0.01
0.01
Aug
0.05
5.95
3.57
2.23
0.03
0.03
Sep
0.18
5.15
3.09
1.93
0.11
0.11
Oct
0.69
3.77
2.26
1.41
0.43
0.43
Nov
1.80
1.87
1.12
0.70
0.70
1.12
Dec
2.77
1.45
0.87
0.54
0.54
1.73
Eq. F-2: Runoff (gal/sf)
Eq. F-3: % Retention
1.5
28%
1.1
38%
0.2
89%
0.0
100%
0.0
100%
0.0
100%
0.0
100%
0.0
100%
0.0
100%
0.0
100%
0.4
62%
1.2
31%
71
Table F-2. Extensive roof mass balance calculations.
extensive crop coefficient
Month
Precip (in)
ETo (in)
Eq. F-1: ETc (in)
convert ETc (gal/sf)
ETc actual (gal/sf)
convert Precip (gal/sf)
0.25
Jan
3.22
1.48
0.37
0.23
0.23
2.01
Feb
2.88
1.83
0.46
0.28
0.28
1.80
Mar
2.31
3.43
0.86
0.53
0.53
1.44
Apr
1.04
4.59
1.15
0.71
0.65
0.65
May
0.38
6.28
1.57
0.98
0.24
0.24
Jun
0.08
7.02
1.76
1.09
0.05
0.05
Jul
0.02
7.11
1.78
1.11
0.01
0.01
Aug
0.05
5.95
1.49
0.93
0.03
0.03
Sep
0.18
5.15
1.29
0.80
0.11
0.11
Oct
0.69
3.77
0.94
0.59
0.43
0.43
Nov
1.80
1.87
0.47
0.29
0.29
1.12
Dec
2.77
1.45
0.36
0.23
0.23
1.73
Eq. F-2: Runoff (gal/sf)
Eq. F-3: % Retention
1.78
11%
1.51
16%
0.91
37%
0.00
100%
0.00
100%
0.00
100%
0.00
100%
0.00
100%
0.00
100%
0.00
100%
0.83
26%
1.50
13%
72
APPENDIX G: RECOMMENDED MINIMUM RAINWATER
TREATMENT58
Screening
Screening occurs through the use of gutter screens and roof washers. Gutter screens are
typically composed of ¼-inch wire mesh that is installed along the open length of the gutter to
keep leaves and other large debris out of the collection system, with further protection from wire
baskets at the head of the gutter downspout. Roof washers separate the “first flush” volumes that
typically contain higher concentrations of contaminants. They are installed between the head of
the gutter downspout and the conveyance system to the storage tank. Runoff water first enters the
roof washer until the roof washer is full, then the subsequent runoff flows directly to the storage
system. The water collected in the roof washer can be analyzed for research purposes, treated
and combined with the other storage water, and/or discharged to the sewer.
Settling
Settling occurs within the storage tank as particles are allowed to settle to the bottom of
the tank for later removal. Generally no specific device is required as gravity forces the
settlement.
73
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