ASSESSING THE FEASIBILITY OF WASTEWATER REUSE FOR A CARBON
SEQUESTRATION FOREST IN DAVIS, CA
A Project
Presented to the faculty of the Department of Civil Engineering
California State University, Sacramento
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
Civil Engineering
(Environmental Engineering)
by
Heta Shah
FALL
2013
© 2013
Heta Shah
ALL RIGHTS RESERVED
ii
ASSESSING THE FEASIBILITY OF WASTEWATER REUSE FOR A CARBON
SEQUESTRATION FOREST IN DAVIS, CA
A Project
by
Heta Shah
Approved by:
__________________________________, Committee Chair
John Johnston, Ph.D., P.E.
__________________________________, Second Reader
Ed Dammel, Ph.D.
__________________________
Date
iii
Student: Heta Shah
I certify that this student has met the requirements for format contained in the University format
manual, and that this project is suitable for shelving in the Library and credit is to be awarded for
the project.
__________________________, Graduate Coordinator
Matthew Salveson, P.E.
Department of Civil Engineering
iv
___________________
Date
Abstract
of
ASSESSING THE FEASIBILITY OF WASTEWATER REUSE FOR A CARBON
SEQUESTRATION FOREST IN DAVIS, CA
by
Heta Shah
The National Pollutant Discharge Elimination System (NPDES) permit for the
City of Davis was renewed in 2007with strict effluent quality requirements. These permit
requirements prompted a plan to upgrade the city’s existing wastewater system to tertiary
treatment. The city government is also working to meet greenhouse gas emission
reduction goals. This project is a feasibility study of using treated municipal wastewater
to irrigate long- lived trees on a city-owned 770-acre site to incorporate carbon from
atmospheric carbon dioxide (CO2) into biomass.
The major components of this study are: choosing a crop based on the amount of
carbon it can sequester per year and its climate and water quality requirements;
determining the degree of wastewater treatment needed under California law for this
application; performing a water balance based on the irrigation requirements of the tree
crop and potential threats to groundwater such as nitrogen or salinity loading; and
determining the mass of carbon that can be sequestered annually by this facility. Based
on the preliminary analysis contained herein, up to 569 acres of redwood trees could be
irrigated with the city’s wastewater flow. A forest of this size would capture 2236 Metric
v
Tons annually, which would more than offset the city’s CO2 emissions associated with its
water and wastewater utilities.
_______________________________, Committee Chair
John Johnston, Ph.D., P.E.
_______________________
Date
vi
ACKNOWLEDGEMENTS
The author would like to thank Dr. John Johnston, Dr. Ed Dammel and Dr. Ken Shackel
for their support, input, guidance and review of this project. Lastly, the author would like
to thank her family for their support throughout the program.
vii
TABLE OF CONTENTS
Acknowledgements………………………………………………………................ vii
List of Tables………………………………………………………………………...x
List of Figures……………………………………………………………………….xiii
Chapter
1. INTRODUCTION…………………………………………………………......... 1
2. BACKGROUND………………………………………………………………... 4
Carbon Sequestration…………………………………………………………... 4
Forest-Based Carbon Sequestration……………………………………………. 6
Irrigating Forest with Reclaimed Wastewater…………………………………. 12
City of Davis Wastewater Treatment Plant and Wetlands……………………...17
Future Reuse Opportunities……………………………………………………. 27
3. PROJECT CALCULATIONS...............................................................................31
Slow Rate (SR) Forest Treatment……………………………………………… 31
Site Characteristics and Climate………………………………………………...32
Carbon Sequestration Plan……………………………………………………... 36
Water Balance and Storage Requirement……………………………………… 47
Agro-Forestry Management Plan……………………………………………..... 78
4. DISCUSSION…………………………………………………………………… 80
Potential Benefits………………………………………………………………. 80
Uncertainties…………………………………………………………………….83
Unsettled Questions……………………………………………………………. 85
viii
5. CONCLUSION………………………………………………………………….. 86
References………………………………………………………………………….. 88
ix
LIST OF TABLES
Tables
Page
2.1
List of Available Carbon Accounting Tools…………………………………. 11
2.2
Forested Slow Rate Treatment Systems in the United States……………….. 13
2.3
Water Recycling Types and its Criteria……………………………………… 15
2.4
Approved Recycled Water Treatment and Uses……………………………... 16
2.5
Summary of Davis WPCP Secondary Treatment Ponds…………………….. 20
2.6
Average Annual WPCP Influent Flow Statistics for 2000-2004…………….. 21
2.7
WPCP Average Influent Water Quality Characteristics……………………... 22
2.8
Summary of 2007 NPDES Permit at Discharge Location-Willow Slough
Bypass…………………………………………………………………………23
2.9
Summary of Disposal/Reuse Alternatives Evaluate…………………………..28
3.1
Soil Type Classification of Study Area in Davis, California………………….35
3.2
Site Characteristics of Study Area in Davis, California……………………….35
3.3
List of Native Trees in the Area……………………………………………….37
3.4
Characteristics of Trees Selected for the Agro-Forestry Plan…………………38
3.5
Categorical Variables Available in COLE-EZ for the Filtering
Analysis………………………………………………………………………..40
3.6
Cumulative Carbon Storage in Different Pools (in Metric Tons of
Carbon per Tree) Generated using COLE-EZ for Douglas fir………………...42
3.7
Cumulative Carbon Storage in Different Pools (in Metric Tons of
Carbon per Tree) Generated using COLE-EZ for Ponderosa pine…………….43
3.8
Cumulative Carbon Storage in Different Pools (in Metric Tons of
Carbon per Tree) Generated using COLE-EZ for Redwood………………….44
3.9
Determination of Harvesting Age of each Selected Trees……………………..46
3.10 Average Monthly Pan Evapotranspiration, ETo in Inches for City
of Davis………………………………………………………………………...50
3.11 Average Monthly Precipitation, Pr in Inches for City of Davis………………..50
x
3.12 Average Monthly Pan Evaporation, Epan in Inches for City of Davis……….50
3.13 Threshold Salinity of Douglas fir, Ponderosa pine and Redwood…………..52
3.14 Leaching requirement of Douglas fir, Ponderosa pine and Redwood……….53
3.15 Nitrogen Uptake for Selected Forest Ecosystems with Whole Tree
Harvesting…………………………………………………………………… 58
3.16 Process Effluent Performance Criteria for Future Effluent
Requirements for the WPCP………………………………………………… 59
3.17 Nitrogen Loss Factor for Varying C:N Ratios………………………………. 60
3.18 Estimation of Monthly Hydraulic Loading Rate for Irrigating
Douglas fir and Redwood…………………………………………………… 62
3.19 Estimation of Monthly Hydraulic Loading Rate for Irrigating
Redwood…………………………………………………………………….. 63
3.20 Adjustment of Design Loading Rate to Meet the Nitrogen Requirement
of Douglas fir and Redwood to Achieve Maximum Tree Growth…………..66
3.21 Adjustment of Design Loading Rate to Meet the Nitrogen
Requirement of Ponderosa pine to Achieve Maximum Tree Growth………67
3.22 WPCP Improvements Project Design Flows………………………………... 69
3.23 WPCP Seasonal Average Influent Flow Statistics for 2000-2004…………...70
3.24
Flow Prediction of Average Monthly Inflows through 2030
at WPCP……………………………………………………………………..71
3.25 Determination of Acreage that could be Irrigated for
Growing Different Trees without Storage…………………………………... 73
3.26 Determination of Area Irrigated for Douglas fir, Ponderosa pine and
Redwood Using Wastewater Storage and Maximizing Nitrogen
Applications to the Tree Crop……………………………………………….. 77
4.1
Annual Mass of Carbon Captured in Biomass……………………………….80
4.2
2007-2010 CO2 Emissions of Davis, California……………………………...81
4.3
Percentage Community Carbon Emissions Potentially Offset by
This Project……………………………………………………………………81
xi
4.4 Comparison of Forest Alternative with Reuse Alternatives
Considered by City of Davis…………………………………………………82
5.1 Hydraulic Loading and Irrigated Area Results………………………………86
xii
LIST OF FIGURES
Figures
Page
1.1 Map showing City of Davis in California……………………………………..1
2.1 Carbon-Storing Components of Trees…………………………………………7
2.2 Water Pollution Control Plant Facilities and Proposed Forest,
City of Davis, CA……………………………………………………………..18
2.3 Existing WPCP Facilities Treatment Schematic……………………………....19
2.4 Proposed Process Flow Diagram For The City Of Davis Wastewater
Treatment Plant………………………………………………………………...25
3.1 Site Area Location of Proposed Forest Plan………………………………….. 33
3.2 Soil Map of Study Area in Davis, California…………………………………. 34
3.3 Sample Report Generated by COLE-EZ………………………………………41
3.4 Graph of Carbon Storage in Different Pools for Douglas fir…………………. 42
3.5 Graph of Carbon Storage in Different Pools for Ponderosa pine……………...44
3.6 Graph of Carbon Storage in Different Pools for Redwood…………………...45
3.7 Graph showing Average Annual Growth Rate for Selected Trees…………….47
3.8
Leaching Requirement as a Function of Applied Salinity and ECe
of Crop Salinity Threshold……………………………………………………52
3.9
Nitrogen Cycle in the soil……………………………………………………..55
3.10 Seasonal Prediction of Average Monthly WPCP Inflows through
2030…………………………………………………………………………...71
xiii
1
Chapter 1
INTRODUCTION
Carbon sequestration is a process of capturing and storing atmospheric carbon
dioxide (CO2) over the long term to mitigate global warming and avoid dangerous
climate change. This project is a feasibility study of using treated municipal wastewater
from the City of Davis, CA to irrigate a carbon sequestration forest which would
incorporate atmospheric CO2 into biomass.
Davis is a city in Yolo County, California 18 km (11 mi) west of Sacramento and
70 mi (113 km) northeast of San Francisco as shown in Figure 1.1. The city lies in the
Sacramento Valley at an elevation of about 16 m (52 ft.) above sea level. The local
topography is flat and the city is surrounded by rural land devoted to agriculture.
Figure 1.1- Map showing City of Davis in California (Google map)
The City of Davis provides wastewater treatment for all residents and businesses
within the city limits and two unincorporated areas - North Davis Meadows and El
2
Macero. The city’s Wastewater Pollution Control Plant (WPCP) currently serves a
population of approximately 65,000. The city’s National Pollutant Discharge Elimination
System (NPDES) permit was renewed in 2007 with strict effluent quality requirements
(SWRCB, 2007). These permit requirements prompted a search for options to upgrade
the existing wastewater system. The city government also has a plan to reduce
greenhouse gas emissions and as part of this effort it has conducted an inventory of
greenhouse gas emissions from various sources in community. The Davis Climate Action
and Adaption Plan (D-CAAP) contains the greenhouse gas emission reduction targets
adopted by the City Council in November 2008 (City of Davis, 2008). The City is
working with the University of California, Davis and other organizations to come up with
solutions for greenhouse gas reductions (City of Davis, 2010b). One strategy for reducing
greenhouse gas is carbon sequestration. Carbon sequestration is a process of capturing
and storing atmospheric carbon. The storage can be underground, in ocean beds, in soil,
or in the form of timber in trees. Using reclaimed water from the WPCP to irrigate trees
is a potential means of both offsetting greenhouse gas emissions and meeting NPDES
regulations.
The WPCP is located in a rural area outside the city. The effluent from secondary
treatment can be used to irrigate forest on 770 acres of land already owned by the city.
An additional potential benefit is that applying wastewater to land may reduce the
volume of water needing costly tertiary treatment for surface water discharge as specified
in the NPDES permit.
3
The major components of this study are:
(1)
Choosing a crop based on the amount of carbon it can sequester per year, and its
climate and water quality requirements,
(2)
Determining the degree of wastewater treatment needed under California law for
this application,
(3)
Performing a water balance based on the irrigation requirements of the tree crop
and potential threats to groundwater such as nitrogen or salinity loading, and
(4)
Determining the mass of carbon dioxide that can be sequestered annually by this
facility.
4
Chapter 2
BACKGROUND
This chapter contains background information on carbon sequestration by forests,
irrigating forests with wastewater, California water reuse regulations, plans for upgrading the
Davis WPCP, and other wastewater reuse options considered by city officials in past
planning activities.
2.1 Carbon Sequestration
There is a strong concern to reduce the concentration of atmospheric CO2 and other
greenhouse gases (GHGs) to mitigate the risks of global warming and ocean acidification.
Out of the few strategies listed below for mitigation of CO2 concentrations, one of the
strategies is to sequester CO2 from point sources or the atmosphere through natural and
engineered systems. There are several definitions used in the industry to define carbon
sequestration. One is “the process of transferring and securing storage of atmospheric CO2
that would otherwise be emitted or remain in the atmosphere” (Lal, 2008). Lal (2008)
described the following natural phenomena and engineering techniques to achieve carbon
sequestration:
1.
Oceanic Injection: Liquefied CO2 is injected at approximately 3000 m depth
under ocean beds. CO2 injection may, however, have adverse effects on deep-sea
biota. There are questions regarding the stability of injection, and the process is
expensive.
2.
Geologic Injection: Liquefied industrial CO2 is injected into deep geologic strata
such as coal seams, old oil wells, stable rock strata or saline aquifers.
5
3.
Scrubbing and Mineral Carbonation: This involves capturing industrial CO2 from
emissions (scrubbing) and converting it into geologically and thermodynamically
stable carbonate minerals (mineral carbonation).
4.
Oceanic Sequestration: This is a biological process in which phytoplankton
photosynthesis captures carbon dioxide in organic material which settles to the
ocean floor and is thus sequestered.
5.
Wetlands: Wetland restoration techniques, such as adding municipal secondary
effluent and managing hydrologic conditions to promote enhanced carbon
sequestration in aboveground vegetation and belowground organic soil.
6.
Agriculture: Carbon sequestration is achieved by modifying management
practices in farming applications such as avoiding tillage, using organic manure
and other techniques which promote carbon storage in soil layers.
7.
Terrestrial Sequestration: The forest ecosystem stores carbon in the forms of
harvestable timber, wood debris, wood products and other woody plants.
CO2 injection in the deep ocean, geological strata, old coal mines, oil wells, and
saline aquifers, are expensive and have issues with measurement and monitoring, leakage,
adverse ecological impacts and regulatory measures (Lal, 2008). These techniques are
still being developed and may be available for use in future. This project is focusing on
terrestrial sequestration as a strategy to reduce atmospheric CO2 concentrations.
6
2.2 Forest-Based Carbon Sequestration
Forest systems with enhanced woody growth are well-suited to long term carbon
sequestration. This implies transfer of atmospheric CO2 into long-lived carbon reservoirs
in tree biomass.
2.2.1 Forest vs. Agricultural Crop Carbon Sequestration
Agricultural crops have roots, stems, branches and leaves. Trees vertically expand
into roots, trunk, branches, stem and foliage such that each component is bigger in height,
width and depth than agricultural crops. Trees store larger magnitudes of carbon as lignin
and other relatively resistant polymeric carbon compounds than conventional agricultural
crops (IFPC, 2012).
Agricultural crops accumulate carbon mostly in the 0-30 cm effective depth of the
root zone. Deep-seated root system of trees can attach more carbon for up to several feet
in the ground. Figure 2.1 shows the different parts of the tree which can store carbon. The
amount of carbon stored trees is approximately 17% in roots, 50% in trunk, 30% in
branches and stems and 3% in foliage (Lal and Augustin, 2012). The large numbers of
leaves in the foliage or crown of evergreen trees, compared to crops, stores more carbon
in the trees’ biomass.
7
Live Tree - Carbon in crown, bole, roots of live trees.
Standing Dead Tree - Carbon in crown, bole, roots of standing dead trees.
Understory - Carbon in understory trees and shrubs. Diameter less than 1 inch.
Down Dead - Carbon in dead, downed wood more than 3 inches diameter.
Forest Floor - Biomass (dead) less than 3 inches diameter above mineral soil.
Soil (1m) - Organic carbon in the surface 1 meter of mineral soil, excluding coarse roots.
Figure 2.1 – Carbon-Storing Components of Trees (USDA Forest Service, 2013)
One of the main issues to consider in agricultural crop carbon sequestration is the
longevity of the product. Carbon captured in soils by crops can be lost due to tilling,
wind, fire, drought or pests. This carbon loss can be reduced by eliminating ploughing,
switching to no-till farming, growing cover crops in the rotation cycle and using organic
manure (Paustian et al., 2004). The amount of carbon stored in the soil due to agriculture
also depends on the net carbon left in the soil after decomposition of biomass. If
decomposition can be reduced, then amount of carbon stored in the soil can be increased.
8
This can be achieved by recycling crop residue and efficient use of water and nutrients
(Paustian et al., 2004).
In contrast, the role of forests, which act as a natural carbon sinks, is significant.
Long-term storage of carbon using forest can be realized by preserving trunk, stem and
branches either by long-lived tree growth or by putting them in productive use as timber.
It is estimated that about 12-19% of carbon from fossil fuel combustion is sequestered by
the existing forests of the United States (Ryan et al, 2010).
2.2.2 Carbon Pools Associated with Forests
Carbon pools associated with forests include the standing forest, forest products in
use, and wood products disposed in landfills (IFPC, 2012).
There are multiple ways to put parts of trees, including trunk, stem, branches and
dead leaves on the forest floor to commercial use without letting them decompose or burn
and release carbon into atmosphere. Modern forest products operations are efficient at
using each part of a tree. Trees are used to make lumber. Leftover chips and sawdust are
made into wood pulp for paper and other products. When wood is turned into pulp for
paper, heat and chemicals dissolve the lignin and release the cellulose fibers. Byproducts
of this process are used in asphalt, paint, chewing gum, detergents and turpentine. Other
refined cellulose products include rayon fabric, and nitrocellulose, which is used to make
nail polish, solid rocket fuel and industrial explosives (IFPC, 2012). The share of the
carbon contained in the salable portion of harvested timber that is sequestered in wood
and paper products (including recycled products) during their usable lives and afterward
9
in landfills is estimated to range from 20 percent to about 45 percent of tree biomass.
(Lubowski et al., 2006)
2.2.3 Enhancement of Carbon Sequestration through Management Practices
The amount of carbon that will be accumulated in the forest depends on location,
climate and plant species. Carbon sequestration can be increased by planting new trees on
lands previously used for other purposes (afforestation) or by planting additional or
replacement trees on existing forestland (reforestation). Sequestration is estimated to increase
from 2.2 to 9.5 metric tons of CO2 per acre over 120 years by afforestation (Birdsey, 1996)
and from 1.1 to 7.7 metric tons of CO2 per acre by reforestation (Row, 1996). Sequestration
can also be increased by management practices such as timely harvesting, choosing the right
tree species, and using wastewater effluent for afforestation.
Forests that are managed on a regular basis during the early growth stages can
remove significantly more atmospheric carbon by increasing the life span of a tree
compared to forests that are not intensively managed. Intensively managed redwood
forests, for example, support the greatest concentration of carbon storage (150 tons/acre)
among all California forest types (Nowak et al., 2002). Due to the tremendous increase in
carbon biomass during growth stages, net carbon storage per year in young trees is far
larger than in old, mature trees. To increase carbon sequestration in old trees, the focus
needs to be on ways to increase the foliage area in the trees and overall canopy growth.
This can be achieved by regular thinning of the trees and harvesting at the proper growth
age (Mader, 2007).
Management practices also involve selecting trees with appropriate life spans and
growth rates. Given the same life span and growth rate, larger trees at maturity will
10
sequester more carbon than smaller trees. Growth rates will affect net sequestration if the
tree is harvested before it reaches mature size. For example, two different species may
store 3 tons of carbon at maturity and live 100 years, but if one species reaches a mature
size after 10 years (fast growth) and the other after 90 years (slow growth), and if these
trees are harvested after only 50 years, the fast-growing tree will have sequestered more
carbon by the time of harvest. However, if both species live to maturity (100 years), there
is no difference in carbon storage. Large trees at maturity with long life spans and
moderate growth rates are useful to attain high net carbon storage.
2.2.4 Carbon Accounting Tools
The U.S. Department of Agriculture Forest Inventory and Analysis National
Program conducts a national-level strategic forest inventory which is a combination of
field-based measurements of carbon content and remotely-sensed observations. The data
collected provide estimates of forest age, cover types, disturbance that can be used for
modeling of components that are difficult to measure. A software toolbox has been
developed with basic calculation tools to help quantify forest carbon for planning or
reporting (USDA Forest Service, 2013). The tools listed in Table 2.1 are currently
available.
11
Table 2.1- List of Available Carbon Accounting Tools
Tool
Carbon OnLine
Estimator
(COLE-EZ)
U.S. Forest
Carbon Budget
Model
(FORCARB2)
U.S. Forest
Carbon
Calculation Tool
(CCT)
Forest
Vegetation
Simulation
(FVS)
i-Tree
Chicago Climate
Exchange (CCX)
Carbon
Accumulation
Table
Description
COLE is a planning tool that enables a planner to
estimate forest carbon characteristics of any county of
the continental United States for generating carbon
credits. COLE data are based on USDA Forest Service
Forest Inventory & Analysis and Resource Planning
Assessment data, enhanced by other ecological data. It
can calculate changes in net carbon stocks in each part
of a tree with an option for standing forests or managed
systems. These criteria are unique in the tool and
important for selection of trees to maximize annual
carbon sequestration.
FORCARB2, an updated version of the U.S. FORest
CARBon Budget Model (FORCARB), produces
estimates of carbon stocks and stock changes for forest
ecosystems and forest products at 5-year intervals.
The Carbon Calculation Tool 4.0 is a computer
application that reads publicly available forest inventory
data collected by the U.S. Forest Service’s Forest
Inventory and Analysis Program (FIA) and generates
state-level annualized estimates of carbon stocks on
forest land based on FORCARB2 estimators.
FVS is an individual-tree, distance-independent growth
model for forests that are in existence. The tool is used
in early stages of tree growth to predict changes in tree
diameter, height, crown ratio, and crown width, as well
as mortality over time. Two carbon reports can be
requested: the Stand Carbon Report and the Harvested
Carbon Report
i-Tree is a state-of-the-art, peer-reviewed software suite
from the US Forest Service that provides urban and
community forestry analysis and benefits assessment.
The i-Tree tool helps communities of all sizes to
strengthen their urban forest management and advocacy
efforts by quantifying the environmental services that
trees provide and assessing the structure of the urban
forest.
The CCX Carbon accumulation tables give an estimate
of CO2 /acre/year of tree growth for US regions and
species. If the species and region combinations do not
match a particular project, the carbon accumulation
values for the species in a similar climate can be
applied.
Citation
http://www.ncasi2.org/CO
LE
http://www.nrs.fs.fed.us/pu
bs/35613
http://www.nrs.fs.fed.us/pu
bs/2394
nrs.fs.fed.us/carbon/tools
www.itreetools.org
http://www.cinram.umn.ed
u/publications/landowners_
guide1.5-1.pdf
12
For this project, COLE-EZ was used as a planning tool for selecting a tree type
and calculating the net carbon storage annually. Other software tools such as FVS and iTree require real time data of existing forests which are not available in this project.
COLE-EZ allows estimation of carbon stocks for future planning of afforestation
(establishing a forest especially on land not previously forested) as well as reforestation
(planting new trees in areas where they have been removed) projects (USDA Forest
Service, 2013).
2.3 Irrigating Forest with Reclaimed Wastewater
The most common forest crops used in so-called “slow rate” wastewater irrigation
systems are mixed hardwoods and pines (Kemp et al., 1978). “Slow rate” (SR) means the
application of wastewater by spraying on permeable soil. Use of slow rate systems
reduces discharges to surface water and the potential for groundwater contamination.
Slow rate forested systems offer several advantages (Kemp et al., 1978):
1.
Forested systems can accommodate a longer wastewater application season than
agricultural crops, thus utilizing a higher annual volume of wastewater per acre.
2.
Forested systems allow higher hydraulic loadings than typical agricultural crops.
3.
During cold weather, soil temperatures are often higher in forestlands than in
agricultural lands.
A summary of representative slow rate systems and types of forest crops used is
presented in Table 2.2.
13
Table 2.2- Forested Slow Rate Treatment Systems in the United States (Crites et al.,
2000)
Location
Dalton, GA
Clayton, Co., GA
Helen, GA
St. Marys, GA
Mackinaw City, MI
State College, PA
West Dover, VT
Design Flow, MGD
30.0
19.5
0.02
0.3
0.2
3.0
0.55
Tree Types
Pines
Loblolly pines, hardwood
Mixed pine and hardwood
Slash pine
Aspen, birch, white pine
Mixed hardwood, pine
Hardwood balsam, hemlock, spruce
The use of treated wastewater for forest is less explored than agricultural and
landscape irrigation, which is widespread in various regions. For example, shade and
street trees and urban green areas are irrigated with treated sewage effluent transported by
tanker in some cities such as Cairo, Tehran and others in the Middle East, India and the
United States (El-Lakany, 1995). However, large-scale use of wastewater for the
irrigation of tree plantations or forests is still relatively limited. It is generally practiced
for waste disposal and treatment rather than for enhanced forestry production.
Pioneering studies on the application of treated municipal wastewater on forest
lands as a means of purification and groundwater recharge were carried out in central
Pennsylvania in the United States from 1963 to 1977 (FAO, 1978). Sewage effluent
which had undergone secondary treatment was sprayed on three different forest areas: a
mixed oak (Quercus spp.), a red pine (Pinus resinosa) and a sparse stand of white spruce
(Picea glauca) with different application rates. The results indicated that the controlled
irrigation of forests with up to 2.5 cm/ha (0.4 in/acre) of effluent per week over an year
can effectively remove nitrogen, phosphorus and other constituents, rendering water of
acceptable quality for drinking. 95 percent of the effluent applied was recharged to the
14
groundwater reservoir and nutrients in the effluent were responsible for an increase in
tree growth (measured by diameter) of 80 to 186 percent. A model of wastewater
purification similar to that tested in Pennsylvania has been adopted in parts of Spain,
Australia and India (FAO, 1978).
2.3.1 Water Quality Criteria for Wastewater Forest in California
Applying wastewater and sludge to land for remediation is recommended by the
Environmental Protection Agency (EPA) and others as a method to recycle nutrients and
organic matter and conserve water resources (Sopper and Kardos, 1973; Sopper et al.,
1982; Bastian and Ryan, 1986; Luecke and De La Parra, 1994). The level of sewage
treatment prior to land application can range from primary treatment using a simple
lagoon to tertiary treatment using a sophisticated mechanical wastewater treatment plant.
In California several state agencies, including the Department of Health Services
(DHS), the State Water Resources Control Board and the Regional Water Quality Control
Board, plus county and local authorities have regulatory authority over potential projects
using recycled water. A compilation of California laws related to recycled water,
including excerpts from the Health and Safety Code, Water Code, and Titles 17 and 22 of
the California Code of Regulations, are contained in a document referred to as the
“Purple Book,” last updated in June 2001 by California Department of Public Health
Services (CDPH, 2001).
The recycled water types, along with the corresponding water quality standards
and treatment processes required, are summarized in Table 2.3.
15
Table 2.3- Water Recycling Types and its Criteria (CDPH, 2001)
Total Coliform Bacteria (MPN/100 mL)
Code
Section3
60301.230
60301.220
60301.225
60301.900
Recycled Water
Type
Disinfected
Tertiary
Disinfected
Secondary – 2.2
Disinfected
Secondary-23
Un-disinfected
Secondary
Treatment
Process
Filtered1 and
Disinfected2
Oxidized and
Disinfected
Oxidized and
Disinfected
Oxidized
Median
Maximum allowed
in a 30-day period
2.2
2.2
23
240 (never exceed)
23
23
240
-
-
Notes:
1. “Filtered” means an oxidized wastewater that satisfies (A) or (B) below
(A) Has been coagulated and passed through natural soils or filter media with a specified
maximum flux rate, depending in the type of filtration system, and the turbidity does not
exceed any of the following:
(1) A daily average of 2 NTU
(2) 5 NTU more than 5 percent of the time within a 24-hour period
(3) 10 NTU at any time
(B) Has been passed through a microfiltration, ultrafiltration, nanofiltration or reverse osmosis
membrane so that the turbidity does not exceed any of the following:
(1) 0.2 NTU more than 5 percent of the time within a 24-hour period,
(2) 0.5 NTU at any time
2. Disinfected by either:
(A) A chlorine process with continuous CT of 450 mg-min/L with a modal contact time of 90
minutes (based on peak dry weather design flow).
(B) A combined process that inactivates and/or removes 99.999 percent (5-log removal) of Fspecific bacteriophage MS-2 or polio virus
(1) In the last 7 days for which analyses have been completed
(2) In no more than 1 sample in any 30-day period
(3) In no samples
3. The California Code of Regulations, Title 22, Division 4, Chapter 3
The four types of recycled water that are currently permitted by the CDPH and some
of the approved uses are summarized in Table 2.4.
16
Table 2.4: Approved Recycled Water Treatment and Uses (CDPH, 2001)
Treatment Level
Disinfected Tertiary Recycled
Water
Disinfected Secondary- 2.2
Recycled Water
Disinfected Secondary- 23
Recycled Water
Approved Uses










Spray Irrigation of Food Crops
Landscape Irrigation1
Non-restricted Recreational Impoundment
Surface Irrigation of Food Crops
Restricted Recreational Impoundment
Pasture for Milking Animals
Landscape Irrigation2
Landscape Impoundment
Surface Irrigation of Orchards and Vineyards3
Fodder, Fiber and Seed Crops
Un-disinfected Secondary
Recycled Water
Notes:
1. Includes unrestricted access golf courses, parks, playgrounds, school yards and other landscaped
areas with similar access
2. Includes restricted access golf courses, cemeteries, freeway landscapes, and landscapes with
similar public access.
3. No fruit is harvested that has come in contact with irrigating water or the ground.
Table 2.3 and Table 2.4 indicate that the option of forest irrigation proposed in
this project requires only Un-disinfected Secondary Recycled Water for irrigation as long
as care is taken that no irrigation with recycled water occurs for a period of 14 days prior
to harvesting or allowing access by the general public. Further, the Purple Book specifies
that, “No irrigation with, or impoundment of, un-disinfected secondary recycled water
can take place within 150 feet of any domestic water supply well. No spray irrigation of
any recycled water other than disinfected tertiary recycled water can take place within
100 feet of a residence or a place where public exposure could be similar to that of a
park, playground or schoolyard” (CDPH, 2001).
One concern that requires attention during the application of wastewater is
increasing the salinity of the soil. Light, frequent irrigation can increase surface soil
salinity that can limit crop production. On the other hand, over-irrigation can carry
contaminants to the groundwater. Wastewater provides an excellent source of nutrients
17
for vegetation, but care needs to be taken that wastewater leaching in groundwater is free
from excessive nitrate-nitrogen. If the nitrate-nitrogen level is higher than the maximum
contaminant limit (MCL) of 10 mg/L in groundwater that is used for drinking water, it
can be harmful to young infants and young livestock because it can restrict oxygen flow
in the blood stream (USEPA,2009; USDA-NRCS, 2011). Therefore, during the use of
wastewater for irrigation, a balance needs to be achieved through careful selection of
application rates that would prevent both increases in surface soil salinity and
contamination of groundwater.
Current regulations do not generally define the limits of specific constituents that
can be discharged in land application, although limits may be included in discharge
permits for individual facilities. Nevertheless, a limit of 10 mg/L of total nitrogen based
on the drinking water quality standard is recommended as the maximum allowable
percolating water concentration, as mentioned in Draft Groundwater Recharge Reuse
Regulations developed by the California Department of Public Health (CDPH, 2013).
2.4 City of Davis Wastewater Treatment Plant and Wetlands
The Davis Water Pollution Control Plant (WPCP) was constructed in 1970.
Figure 2.2 shows location of the WPCP with respect to the city and the site of the
proposed carbon sequestration forest.
18
Wastewater
Treatment Plant
Discharge Point
001 (Willow
Slough Bypass)
Wetland
Discharge
Point 002
(Toe Drain)
Proposed
Forest
Irrigation
Site
City of
Davis
I-80
Figure 2.2- Water Pollution Control Plant Facilities and Proposed Forest, City of Davis,
CA (Google maps)
The following description of WPCP is based on several documents published by
the City of Davis (City of Davis, 2005; City of Davis, 2010a; City of Davis, 2013). The
current plant processes, shown in Figure 2.3, consist of comminutors, grit removal,
primary sedimentation, oxidation ponds, aerated ponds, a Lemma (duckweed) pond,
overland flow terraces, disinfection and anaerobic solids treatment (City of Davis, 2005).
Figure 2.3- Existing WPCP Facilities Treatment Schematic (City of Davis, 2005)
19
20
During the summer months, primary effluent is pumped to three oxidation ponds
operated either in series or in parallel. During the winter months, a portion of the primary
effluent is sent directly to aerated ponds, always operated in series, to reduce the BOD
loading to the oxidation ponds. Depending on the season, the Lemna pond receives either
aerated pond effluent or oxidation pond effluent. The Lemna pond is operated yearround, primarily as a settling and polishing pond. The various pond designations, surface
areas, maximum depth, and available storage volumes are summarized in Table 2.5.
Table 2.5- Summary of Davis WPCP Secondary Treatment Ponds (City of Davis, 2013)
Designation
Maximum Depth(a), feet
Oxidation Pond 1
5.5
Oxidation pond 2
5.5
Oxidation Pond 3
5.5
West Aerated Pond
7.0
East Aerated pond
7.0
Lemma Pond
7.5
Totals
(a) Includes a freeboard of 2 feet
Surface Area, acres
Storage Volume, MG
33
35
35
5.3
5.3
5.0
119
57
61
61
12
12
12
215
Pond effluent is applied to the overland flow terraces through sprinklers, which
provide further biological treatment as the water flows over soil and through vegetation.
The overland flow terraces are divided into fifteen watering zones; the number of zones
in service at any given time depends on the season and operational and maintenance
needs. Effluent from the overland flow system is collected at the effluent pump station,
where it is pumped to the chlorine contact basin for disinfection prior to discharge.
The present treatment process involves oxidation and disinfection processes that
provide disinfected secondary-level treatment. Treated effluent is discharged either to
Willow Slough Bypass or to the Conaway Ranch Toe Drain via the Davis Restoration
21
Wetlands, subject to weather conditions and treatment performance. During winter, the
discharge is routed to the restoration wetland which has a volume of 430 million gallons
(MG). Disinfected secondary effluent is discharged into one of two wetland lagoons and
then flows into two tracts (out of seven) prior to discharge either to Willow Slough
Bypass or to the Conaway Ranch Toe Drain (City of Davis, 2005). Annual average
WPCP influent flows for years 2000-2004 are presented in Table 2.6.
Table 2.6- Average Annual WPCP Influent Flow Statistics for 2000-2004 (City of Davis,
2005)
Year
2000
2001
2002
2003
2004
Average
Annual Average Flow (MGD)
5.64
5.59
5.77
5.86
5.96
5.77
To understand the quality of influent entering into the system at different times of
the year, the City recently sampled WPCP influent for the following study periods (City
of Davis, 2013):

Special Monitoring Period Study No. 1: Daily data collected over a 14-day period
starting July 23, 2012 and ending August 6, 2012. These data represent the
influent conditions that occur when the student population in the city is at its
lowest.

Special Monitoring Period Study No. 2: Daily data collected over a second 14-day
period starting October 29, 2012 and ending November 11, 2012. These data
represent the influent conditions that occur when the student population in the city
is at its highest.
22
The average influent constituent concentrations measured during each of the two
monitoring periods are summarized in Table 2.7.
Table 2.7: WPCP Average Influent Water Quality Characteristics (City of Davis, 2013)
Parameters
Units
Special Monitoring Period
Study No. 1(a)
Study No. 2
Influent
Primary
Influent
Primary
Effluent
Effluent
Organics
BOD-total
mg/L
187
148
238
168
BOD-soluble
mg/L
63
79
82
84
COD-total
mg/L
491
355
605
367
COD-soluble
mg/L
136
166
156
157
Suspended Solids
TSS
mg/L
213
74
280
87
VSS
mg/L
174
63
259
82
ISS
mg/L
32
11
21
4
Nutrients
NH4-N
mg/L
31
30
36
34
NO3-N(b)
mg/L
<0.010
<0.010
<0.010
<0.010
NO2-N(b)
mg/L
<0.002
<0.003
<0.012
<0.012
TKN-total
mg/L
43
40
48
42
TKN-soluble
mg/L
31
31
36
36
Phosphorous-total
mg/L
5.2
5.4
5.5
5.5
Phosphorous-soluble
mg/L
3.1
3.8
4.0
4.2
Orthophosphate
mg/L
2.7
3.2
3.4
3.7
Other Parameters
Alkalinity (as CaCO3)
mg/L
427
440
408
409
Electrical Conductivity
uS/cm
1442
1500
1420
1410
Calcium
mg/L
31
32
34
33
Magnesium
mg/L
42
44
41
41
Total VFAs(d)
mg/L
80
74
70
77
(a) Influent data from the first week of sampling (July 23-30) is not included in the averages because
it was determined to be unrepresentative of normal WPCP influent. Influent data are shown for
July 31-August 6, 2012. Primary effluent data shown are for dates as indicated.
(b) “<” indicates average values that include non-detect results, with individual non-detect (ND)
data set at the method detection limit (MDL)
(c) Individual inert suspended solid (ISS) data points used in the average have been calculated as the
difference between paired TSS and VSS data points.
(d) Total VFAs has been calculated as the sum of paired results for the following acids, with ND
results ignored: acetic, butyric, DL-lactic, formic, isovaleric, propionic, pyruvic, valeric.
In 2007, the Regional Water Quality Control Board (RWQCB) issued new
National Pollution Discharge Elimination System (NPDES) permit requirements which
were much more stringent than previous requirements with respect to biological oxygen
23
demand (BOD), total suspended solids (TSS) and ammonia (City of Davis, 2013). Table
2.8 summarizes the 2007 NPDES permit effluent limits at the Willow Slough Bypass
discharge location. The permit also requires a compliance date of October 25, 2017 for
upgrading the current treatment system. To meet this requirement, the City needs to
construct a new tertiary treatment process.
Table 2.8- Summary of 2007 NPDES Permit at Discharge Location- Willow Slough
Bypass (SWRCB, 2007)
Parameter
BOD 5-day @
200 C1
Total Suspended
Solids1
pH
Settleable Solids1
Turbidity1
Total Coliform
Organisms1
Aluminum Total
Recoverable3
Ammonia (1
March – 31
October)
Ammonia (1
November – 29
February)
Cyanide
Iron, Total
Recoverable
Selenium, Total
Recoverable
1.
2.
3.
Units
Effluent Limitations
Maximum
Instantaneous
Daily
Minimum
Maximum
Average
Monthly
Average
Weekly
mg/L
Lbs/day2
mg/L
Lbs/day2
Standard values
mL/L
NTU
MPN/ 100 mL
10
630
10
630
15
940
15
940
µg/L
71
140
mg/L
Lbs/day2
0.43
26.9
1.04
65.1
mg/L
Lbs/day2
0.52
32.5
1.04
65.1
µg/L
mg/L
3.8
0.8
9.5
2
20
1300
20
1300
6.5
0.1
8.5
0.2
10
240
µg/L
4.4
7.1
Lbs/day2
0.28
0.44
Compliance is to be measured at Monitoring Location EFF-A as described in the attached MRP.
Based on an average dry weather flow of 7.5 MGD.
Compliance with the effluent limitations for aluminum can be demonstrated using either total or
acid-soluble (inductively coupled plasma/atomic emission spectrometry or inductively coupled
plasma (mass spectrometry) analysis methods, as supported by USEPA’s Ambient Water Quality
Criteria for Aluminum document (EPA 440-86-008), or other standard methods that exclude
aluminum silicate particles as approved by the Executive Officer.
24
The restoration wetlands downstream of the WPCP were put in service in 1999
with funding from the U.S. Army Corps of Engineers to restore wetland habitat in the
area (City of Davis, 2005). The land area of the wetlands is 400 acres, out of which 77
acres is used for wastewater treatment and the rest is used for stormwater detention. One
of the main issues with the current wetland configuration is that the long detention times
and lack of vegetated zones lead to algal growth and a rise in pH, making compliance
with existing discharge requirements difficult. In addition, test results over the last five
years show a steady rise of selenium in the wetland invertebrates and bird eggs,
indicating that continued use of the wetlands may be problematic as bioaccumulation of
selenium in the food chain can be toxic (City of Davis, 2005).
A Wastewater Planning Charrette was conducted on October 22-23, 2009 to
develop a cost-effective wastewater management plan for the City of Davis (City of
Davis, 2010a). The City has chosen the Charrette Plan as its preferred plan for
wastewater improvements. Figure 2.4 shows the proposed treatment plant processes.
Figure 2.4- Proposed Process Flow Diagram For The City of Davis Wastewater Treatment Plant. Dashed Lines Indicate
Alternative Flow Pathways or Intermittent Flow (City of Davis, 2010a)
25
26
Existing screening/grit removal, primary sedimentation, disinfection/
dechlorination facilities and anaerobic solids digestion are going to remain unchanged
(City of Davis, 2013). A secondary treatment process with biological reactors, secondary
clarifiers and associated facilities will replace the existing treatment ponds and overland
flow facility. For BOD removal, the improvement plan proposes a biological treatment
process that relies on either plug-flow activated sludge or oxidation ditch technology. It
should also provide biological nitrification and denitrification without an external carbon
source as long as the influent provides adequate carbon. Nitrification (oxidation of
ammonia to nitrate) can be achieved to a greater degree and more reliably with activated
sludge than with alternative processes such as trickling filters (City of Davis, 2010a).
Upon completion of the Wastewater Treatment Plant (WWTP) Improvements Project, the
existing treatment ponds will be used for long-term emergency and maintenance storage,
recycled water storage, stormwater storage, and flow equalization (City of Davis, 2013).
These ponds will, therefore, provide operational flexibility and reliability (City of Davis,
2013). For tertiary treatment, wastewater will be processed through chemical addition,
disk filtration, disinfection/dechlorination and post-aeration to achieve Title 22 defined
disinfected tertiary quality as described in Table 2.3.
The city currently relies entirely on groundwater wells for public water supply. A
combination of intermediate and deep wells extracts groundwater from underlying
aquifers. Studies (City of Davis, 2005 and 2013) have shown that if the city either
replaced intermediate aquifer wells with deep aquifer wells and/or developed a surface
water supply from the Sacramento River, TDS, EC, and selenium concentrations in the
27
wastewater would decrease. The City has proposed a surface water project to provide 12
million gallons per day (MGD) of surface water from the Sacramento River to Davis
water customers which should reduce concentrations of selenium, alkalinity, total
dissolved solids, and hardness (City of Davis, 2005 and 2010a). Compliance with
selenium requirements is expected to be possible even during periods when groundwater
would be used as part of the supply. The decrease in alkalinity will result in a decrease in
effluent pH. For this reason, the improved treatment plant will not have any salinity or
selenium removal processes. (City of Davis, 2010a)
2.5 Future Reuse Opportunities
Meeting the 2007 NPDES permit conditions will require construction of a tertiary
treatment system that could meet the Title 22 disinfected tertiary standards. This would
allow reuse of the city’s effluent in the future. The following disposal and reuse
alternatives were evaluated by the City (City of Davis, 2005):
1. Surface Water Discharge to Willow Slough/Conaway Ranch Toe Drain
(existing discharge) or Sacramento River
2. Export raw wastewater to Sacramento Regional County Sanitation District
(SRCSD)
3. Year-round land disposal/reuse to seasonal agricultural reuse/storage or yearround percolation basins
28
4. Seasonal Reuse by:
 Urban reuse from the main WPCP facility,
 Urban reuse through a satellite treatment facility separate from the
WPCP,
 Reuse at the Yolo Basin Foundation/Department of Fish and Game
(DFG) Wetlands, and
 Demonstration reuse on city-owned lands
The disposal/reuse alternatives evaluated by the City are compared in Table 2.9.
Table 2.9- Summary of Disposal/Reuse Alternatives Evaluation (City of Davis, 2005)
Reuse/Disposal
Alternative
Existing Discharge
Sacramento River
Export to SRCSD
Land Percolation
Storage/Reuse
Relative
Cost(1)
Level of Treatment
Assumed
Implementation
Risk
1.00
1.8
1.6
Tertiary
Tertiary
Secondary
(provided at SRCSD)
Secondary
Secondary
Low
High
Medium
1.1-1.3
2.0-3.0
High
Low
(1) Relative cost = cost of alternative divided by cost of existing discharge alternative.
As described in City of Davis (2005), the Sacramento River may offer dilution for
discharge but is expensive due to the need for a new pipeline 20 miles long from the
WPCP to the river. In addition, there may be opposition from downstream stakeholders
regarding the quality of water added as a new discharge to Sacramento River. Export to
SRCSD is also costly both because of the distance and because SRCSD is in the process
of adding tertiary treatment. The year-round land percolation option has number of
unknowns associated with it, such as the ability to maintain a sustained long-term
29
percolation rate due to heavy clays. Regulation of percolation operations is likely to be
changed as groundwater degradation continues to be evaluated. The storage and reuse
options were expected to be costly due to the large area of storage ponds required to hold
the water through the winter, and the large amount of land in addition to the existing City
of Davis parcel that must be purchased for reliable agricultural reuse (City of Davis,
2005). Agricultural use was considered the best option for reuse given the surrounding
land uses, but Conaway Ranch and other land owners who might use the water indicated
that it would either not meet their water quality needs (because it cannot be directly
applied to consumable crops) or would be more costly than simply drilling a shallow well
to produce irrigation water (City of Davis, 2013). Looking to lower costs and
implementation risks, the City decided to continue discharging at the existing locations
(City of Davis, 2013).
The goal of this project is to evaluate an option for wastewater remediation by
irrigating a carbon sequestration forest on land the city currently owns. The master plan
did not look at this option. The forest option requires only secondary-level treated
wastewater for irrigation. This will reduce the volume of wastewater that will require
tertiary treatment for surface water discharge per the NPDES permit. This could reduce
the overall cost of wastewater treatment. In addition, a carbon pool will be created in the
form of forest which will provide carbon sequestration. This alternative will also help
City achieve its greenhouse gas (GHG) reduction goals.
In the following chapters this option will be assessed further by choosing a crop
based on the amount of carbon it can sequester per year and its climate and water quality
30
requirements; performing a water balance based on the irrigation requirements of the tree
crop and the need to prevent potential threats to groundwater such as nitrogen or salinity
loading; and determining the mass of carbon that can be sequestered annually by this
facility.
31
Chapter 3
PROJECT CALCULATIONS
In this chapter, the feasibility of a carbon sequestration forest is assessed by
evaluating the suitability of the site for the forestry plan, choosing an agroforestry
management plan, and performing a water balance between forest water requirements and
available secondary treated wastewater effluent, based on hydraulic loading rates which
avoid leaching of nitrogen to groundwater and waterlogging.
3.1 Slow Rate (SR) Forest Treatment
Slow rate land treatment is the application of wastewater to a vegetated soil
surface. The applied wastewater receives significant treatment as it flows through the
plant root/soil matrix. Solids removal generally occurs at the soil surface and biological,
chemical and additional physical treatment occurs as the wastewater percolates
downward. Off-site runoff of any of the applied wastewater is prevented by the system
design.
Slow rate land treatment can be operated to achieve a number of objectives
including:

Further treatment of the applied wastewater,

Economic return from the use of water and nutrients to produce marketable crops,

Exchange of wastewater for potable water for irrigation purposes in arid climates’
and

Development and preservation of open space and greenbelts.
32
The selection of slow rate application for this project is based on water reuse
regulations and the limit of 10 mg/L nitrogen in the groundwater (CDPH, 2013). The
most cost-effective system applies the maximum possible amount of secondary treated
wastewater to the smallest possible land area for cultivating trees (i.e. increasing the
sequestered carbon biomass) at the highest growth rate possible. The principal limitation
to the use of wastewater for forested SR systems is that nitrogen removals are relatively
low unless young, developing forests are used or conditions conducive to denitrification
are present (USEPA, 1981).
The design of land treatment systems, wetlands, and similar processes is based on
the Limiting Design Parameter (LDP) concept (Crites et al., 2000). The LDP is the factor
or the parameter which controls the design by establishing the required size and loadings
for a particular system. If a system is designed for the LDP, it will then function
successfully for all other less-limiting parameters of concern. Experience has shown that
the LDP for SR systems depends significantly on the agronomic requirements of the crop,
the hydraulic capacity of the soil or the ability to remove nitrogen to the specified level
when typical municipal wastewaters are applied (USEPA, 1981). Whichever of these
parameters requires the largest treatment area for a given flow controls design as the
LDP.
3.2 Site Characteristics and Climate
As described in the introduction, this project would be implemented on the
approximately 770 acres of city-owned area located north of Interstate Highway 80
shown in Figure 3.1 (green areas). The site has the advantage that no land acquisition cost
33
will be involved to implement this project. An additional benefit would be relatively low
distribution costs due to its close proximity to wastewater treatment plant. Currently the
site is open land with no agricultural activities.
Figure 3.1- Site Area Location of Proposed Forest Plan (City of Davis, 2005)
Important site characteristics to be examined for the design of a forest system are
land use, type of soil, textural class, soil permeability and groundwater depth. A custom
soil resources report for the site was generated by the author from a USDA-NRCS
website (USDA-NRCS, 2013a). Types of soil in the area of study are shown in Figure 3.2
and listed in Table 3.1.
Figure 3.2- Soil Map of Study Area in Davis, California (USDA-NRCS, 2013a)
34
35
Table 3.1- Soil Type Classification of Study Area in Davis, California (USDA-NRCS,
2013a)
Area (Acres)
Soil Classification
(see Table 3.2 for names)
Ca, Rg, Mf
Ca, Sd
Ck, Sd
327.044
245.58
198
Total = 770
Soil Type
Silty clay, Silty clay loam
Silty clay, Clay
Clay
Other site characteristics that were considered in assessing the feasibility of a site
for slow- rate land applications are listed in Table 3.2.
Table 3.2- Site Characteristics of Study Area in Davis, California (USDA-NRCS, 2013a)
0 to 1
Mf –
Marvin
silty clay
loam
0 to 1
Rg –
Rincon
silty clay
loam
0 to 1
Sd –
Sacramento
clay,
drained
0 to 1
0.06 to
0.20
0.06 to
0.20
0.06 to
0.20
Groundwater
depth (ft)
> than
6.5 ft
> than 6.5
ft
Textural class
Silty clay
Silty clay
loam
Ca –
Capay
silty clay
Slope (%)
Soil
Permeability
(Ksat in in/hr)
CkClear
lake clay
Slow Rate
System
Requirement*
0 to 2
0 to 20
0.06 to 0.20
0.06 to
0.20
0.06 - 2.0
> than 6.5
ft
> than 6.5 ft
> than
6.5 ft
2 to 10
Silty clay
loam
Clay
Clay
Clay loam to
sandy loam
*USEPA, 2006
The custom soil resources report also provides land management interpretations
designed to guide in evaluating the soil response to forest management practices. The key
factor is that most of the soils under the study area have drainage limitations and flooding
which will reduce the choice of plants or require careful water management, or both.
Sunset Magazine climate zone descriptions provide guidance in choosing the right
tree that could be grown under the range of temperature differences in the area. The
climate zone for the City of Davis is number 14 (Sunset Magazine, 2013). Zone 14 is
36
comprised of milder-winter, marine-influenced areas and the cold-winter inland valley
areas. Over a 20-year period, this area has experienced temperature lows ranging from 26
to 16º F (–3 to –9º C) and 226 frost-free days (United States National Weather Service,
2013). Precipitation averages 16.83 inches per year, mainly between months of
November and March (CDEC, 2013).
3.3 Carbon Sequestration Plan
In this section, the method for systematically selecting forest trees and
determining the best harvesting ages to achieve maximum CO2 sequestration is presented.
Trees choices are based on following criteria:

Trees which produce marketable wood in the form of lumber or other longterm uses are preferred. To achieve increased carbon sequestration, trees
having trunk, stem and branches of superior strength-to-weight ratio are
desired because each part of tree can be put into long-term uses such as
construction lumber, plywood, furniture, paper and several wood pulping byproducts as described in Section 2.2.2. This implies that lumber trees are
preferred over ornamental trees.

Trees should be evergreen so that the needles take up more carbon throughout
the whole year.

Trees with fast growth rate and long maturity are preferred. As explained in
Section 2.2.3, tree build up carbon biomass through the growth of trunk, stem
and branches until maturity. After maturity is attained, the growth rate
37
declines as components lose biomass by withering or dying. A long maturity
period helps to gain positive net annual change in carbon.

Trees should survive in the temperatures (-3 to 34 oC) and soil conditions
(clay and loamy soil) of the study area.

Trees should be salt tolerant and survive in moist or saturated conditions.
Many native and California trees can be grown as forest. Native trees which are
common in the valley, but are not suited for carbon sequestration are listed in Table 3.3.
Table 3.3- List of Native Trees in the Area (Sunset Magazine, 2013)
Native Trees*
Attributes Not Suitable for Carbon
Sequestration
Different varieties of Willow
Small height, ornamental (no lumber)
California Sycamore
Ornamental (no lumber)
California white oak, White Oak, Valley Oak
Grows in Sand – Loamy soil, Slow growth rate
Different varieties of Cottonwood
Ornamental (no lumber)
*Personal communication with tour docent during site visit to City of Davis Wetlands (02/23/2013)
In this study, non-native trees such as Douglas fir, Ponderosa pine and redwood,
because they satisfy all the criteria listed above to maximize sequestration of carbon.
Specific characteristics considered in the selection of these trees are listed in Table 3.4.
Table 3.4 – Characteristics of Trees Selected for the Agro-Forestry Plan (Forest Foundation,2013; USDA-NRCS, 2013b)
No.
Trees
Scientific
Names
Forest Type Water
Needs
Soil
Type
Maturity
Attained
(Years)
Maximum
Height
(ft)
1
Douglas fir
Pseudotsuga
menziesii
Evergreen
2
Ponderosa
pine
Pinus
ponderosa
3
Redwood
Sequoia
sempervirens
Canopy Growth Longevity Climate
Uses
o
Width
Rate
(Years)
C
(Products)
(ft)
(Inches per
season)
20-30
24
150
- 33 to Structural
37
Lumber
Moist
to Dry
Soil
Clay,
Loam or
Sand
12-20
80-160
Evergreen
Moist
to Dry
Soil
Clay,
Loam or
Sand
19-250
50-100
25-30
24 to 36
151
Evergreen
Moist
Clay,
Loam or
Sand
10-50
70-100
15-30
36 or more
100
-40 to
43
Building
Timber
-9 to 38 Lumber for
decks,
fencing ,
outdoors
38
39
To assess the carbon pool storage in each part of a tree, which in turn helps to
select tree type to achieve maximum carbon storage, the COLE-EZ version of the COLE
software was used. The information regarding the selection of COLE-EZ over other tools
for this project is provided in Chapter 2. COLE originated as a collaborative project
between the National Council for Air and Stream Improvement Inc. (NCASI) and the
United States Department of Agriculture (USDA) Forest Service, Northern Research
Station (USDA Forest Service, 2013). The COLE-EZ 1605b Forest Carbon Reporting
Tool is a flexible web-based tool for forest carbon analysis that can be used to generate
forest carbon inventory estimates for any area of the continental United States.
Carbon estimates (Metric tons carbon per hectare) generated in COLE-EZ are
derived from a comprehensive public database of forest plot measurements from over
100,000 forested plots all over the continental United States maintained by the USDA
Service Forest Inventory and Analysis (FIA) program. COLE-EZ covers all trees species
that are encountered at different locations. The carbon analyses are established by fitting
equations to data through combinations of state, county, year and other filters categorized
in Table 3.5. The carbon calculations are based on methods described in Smith et al.
(2006).
40
Table 3.5- Categorical Variables Available in COLE-EZ for the Filtering Analysis
Variables
State / County
Owner / Owner Group
Forest Type/ Forest Group
Productivity Class/ Stand size
Land Clearing Status
Measurement Year
Stand Age Class
Stand Origin
Description
County where forest will be
located
Selection of land ownership
classes
Type of forest or kind of forest
trees
Having capacity to grow crops
of industrial wood. It classifies
diameter class of trees (large
diameter is >11.0 inches for
hardwoods and > 9.0 inches
diameter for softwoods; medium
diameter is > 5.0 inches and
small diameter is < 5.0 inches).
Afforestation is selected if this
stand is established on nonforest land, otherwise use the
reforestation option
Year at which data for trees
taken, if applicable.
Selection of reporting year
Planting or artificial seeding
Variables selected for
this study
Yolo, California
Private
Douglas fir/ Ponderosa
pine/ Redwood
Unstocked
Afforestation
Variable
Variable
Planted
Reports are generated in form of tables to reflect selection of filters giving a
breakdown of carbon stored into the component pools as explained in Section 2.2.1.
Figure 3.3 shows a sample report generated in COLE-EZ giving information on carbon
stocks (tons/acre or metric tons/hectare) in each part of the tree for base year (start year)
and reporting year (end year) and the change in carbon stored between base and reporting
years.
Figure 3.3: Sample Report Generated by COLE-EZ
41
42
Reports of carbon storage in each carbon pool at intervals of 5 years up to the end
of the growth cycle were generated for Douglas fir, Ponderosa pine and redwood as
summarized in Table 3.6, Table 3.7 and Table 3.8 respectively. Graphs in Figure 3.4,
Figure 3.5 and Figure 3.6 show cumulative carbon sequestered in metric tons/tree for
each pool over time in 5-year increments.
Table 3.6 - Cumulative Carbon Storage in Different Pools (in Metric Tons of carbon per
tree) Generated using COLE-EZ for Douglas fir
Year
Live Tree
5
10
15
20
25
30
35
40
1.8
9.2
20.5
32.9
44.5
54.5
62.6
69
Non-standing Tree
Biomass
21.5
19.4
18.4
17.8
13.4
17.3
17.1
17
Soil (1m)
Total Carbon
31.9
32.2
32.7
33.3
34.1
35
35.9
36.8
55.1
60.8
71.6
84
92
106.7
115.6
122.8
Carbon sequestration Metric Tons/tree
140
120
100
Live Tree
80
Non-standing Tree
Biomass
60
Soil (1m)
40
20
Total C
0
0
10
20
30
40
50
Years
Figure 3.4- Graph of Carbon Storage in Different Pools for Douglas fir
43
Table 3.7- Cumulative Carbon Storage in Different Pools Generated (in Metric Tons of
carbon per tree) using COLE-EZ for Ponderosa pine
Year
Live Tree
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
0.3
1.9
4.8
8.4
12.3
16.1
19.8
23
25.8
28.2
30.3
31.9
33.3
34.4
35.3
36.1
36.7
37.2
37.5
37.8
38.1
38.3
Non-standing
Tree Biomass
15.7
14.7
13.2
12.6
12.5
12.5
12.6
12.8
13
13.1
13.3
13.4
13.5
13.6
13.6
13.7
13.7
13.7
13.8
13.8
13.8
13.8
Soil (1m)
Total Carbon
14.1
14.2
14.4
14.7
15
15.4
15.8
16.2
16.6
17
17.3
17.6
17.8
18
18.2
18.3
18.4
18.5
18.6
18.6
18.6
18.7
30.1
30.8
32.4
35.7
39.8
44
48.2
52
55.4
58.3
60.9
62.9
64.6
66
67.1
68.1
68.8
69.4
69.9
70.2
70.5
70.8
44
Carbon sequestration Metric Tons/tree
80
70
60
Live Tree
50
40
Non-standing
Tree Biomass
30
Soil (1m)
20
Total C
10
0
0
50
100
150
Years
Figure 3.5- Graph of Carbon Storage in Different Pools for Ponderosa pine
Table 3.8- Cumulative Carbon Storage in Different Pools (in Metric Tons of carbon per
tree) Generated using COLE-EZ for Redwood
Year
Live Tree
5
10
15
20
25
30
35
40
45
50
55
60
5.4
25.3
51.8
77.3
98.3
114.4
126.2
134.5
140.2
144.2
146.9
148.7
Non-standing Tree
Biomass
10.4
18.2
24.3
29.1
32.9
35.7
37.8
39.42
40.6
41.536
42.33
42.8
Soil (1m)
Total Carbon
22.2
22.4
22.8
23.2
23.8
24.4
25.04
25.6
26.2
26.8
27.3
27.8
38
65.9
98.9
129.6
155
174.5
189
199.5
207
212.5
216.5
219.3
45
Carbon sequestration Metric Tons/Acre
250
200
Live Tree
150
Non-standing
Tree Biomass
100
Soil (1m)
50
Total C
0
0
20
40
60
80
Years
Figure 3.6- Graph of Carbon Storage in Different Pools for Redwood
The analysis shows that trees store more carbon per year while they are growing
as opposed to when they are mature. Proper harvesting ages for each tree were
determined by looking at the maximum average growth rates of carbon storage shown in
Table 3.9. The average growth rate is calculated by dividing the cumulative storage by
the growing time (e.g. for Douglas fir at 30 years the rate would be 54.5/30 = 1.82 metric
tons/acre/year). Harvesting at this time would utilize the full benefits of the growth stages
because average growth rate decreases after this time. Similarly, the harvesting ages of
Ponderosa pine and redwood should be at 40 years and 25 years respectively as shown in
Table 3.9 and Figure 3.7.
46
Table 3.9- Determination of Harvesting Age of each Selected Trees
Years
Douglas fir
Ponderosa pine
Redwood
Cumulative
Carbon
Storage in
Live Tree
(MT/acre)
Average
Carbon
Sequestration
Growth Rate
(MT/acre)
Cumulative
Carbon
Storage in
Live Tree
(MT/acre)
Average
Carbon
Sequestration
Growth Rate
(MT/acre)
Cumulative
Carbon
Storage in
Live Tree
(MT/acre)
Average
Carbon
Sequestration
Growth Rate
(MT/acre)
5
1.8
0.36
0.33
0.07
5.4
1.08
10
9.2
0.92
1.92
0.19
25.3
2.53
15
20.5
1.37
4.75
0.32
51.8
3.45
20
32.9
1.65
8.37
0.42
77.3
3.87
25
44.5
1.78
12.3
0.49
98.3
3.93
30
54.5
1.82
16.1
0.54
114.4
3.81
35
62.6
1.79
19.8
0.57
126.2
3.61
40
69.0
1.73
23
0.58
134.5
3.36
45
25.8
0.57
140.2
3.12
50
28.2
0.56
144.2
2.88
55
30.3
0.55
146.9
2.67
60
31.9
0.53
148.7
2.48
65
33.3
0.51
70
34.4
0.49
75
35.3
0.47
80
36.1
0.45
85
36.7
0.43
90
37.2
0.41
95
37.5
0.39
100
37.8
0.38
105
38.1
0.36
110
38.3
0.35
47
Average Annual Growth Rate
(tons/acre)
4.50
4.00
3.50
3.00
2.50
Douglas
2.00
Ponderosa
1.50
redwood
1.00
0.50
0.00
0
20
40
60
80
100
120
Years
Figure 3.7- Graph showing Average Annual Growth Rate for Selected Trees
3.4 Water Balance and Storage Requirement
The following sections describe slow rate wastewater treatment design factors that
would affect the hydraulic loading rate used to irrigate the forest under study, and
calculations of the acreage that could be irrigated with and without wastewater storage.
3.4.1 Slow Rate System Design Factors
As discussed in Section 3.1, this project is a slow rate (SR) land treatment system.
There are two basic types of SR systems (USEPA, 2006). Type I, Maximum Hydraulic
Loading, involves applying the maximum amount of water to the least possible land area,
a “treatment” system. In Type 2, Optimum Irrigation Potential, water is applied to match
the crop or vegetation requirements. In essence this is an irrigation or “water reuse”
system with treatment capacity being of secondary importance. In this study, Type 2 is
chosen so that trees receive water optimally to maximize carbon uptake. The water
48
balance equation which provides the hydraulic loading rate based on leaching
requirements is (Reed et al., 1988):
Lag = (ETc − Pr )(1 + LR) (
100
E
)
(3.1)
where
Lag = hydraulic loading due to tree requirements, in/month
Pr = design precipitation, in/month
ETc = crop evapotranspiration, in/month
LR = leaching requirement, fraction
E = efficiency of the irrigation system
Overall irrigation efficiency (E) is the fraction of applied water that becomes
available to the plants. Typically, 70 to 85% efficiency is commonly achieved using
sprinkler irrigation (USEPA, 2006). Crop evapotranspiration (ETc) is the sum of plant
transpiration and evaporation from plant and soil surfaces. The design ETc rate is an
important component in the water balance for both crop production and water quality
concerns. A high water loss due to ETc will tend to increase the concentration of
constituents in the percolate. Crop evapotranspiration (ETc) is commonly estimated based
on measured Pan evapotranspiration (ETo) and a crop coefficient (Kc) representing the
specific crop and growth stage.
ETc = ETo·Kc
where
ETo = Pan evapotranspiration, in/month
Kc = Crop coefficient
(3.2)
49
Historical data for monthly pan evapotranspiration (ETo) and monthly
precipitation (Pr) in inches/month were obtained from the Parameter-Elevation
Regressions on Independent Slopes Model (PRISM) (Orang et al., 2013). These were
used in the Simulation of ET of Applied Water (Cal-SIMETAW) application program,
which was developed as a cooperative effort between the University of California (UC)
and the Department of Water Resources (DWR) (Orang et al., 2013). The data are shown
in Table 3.10 and Table 3.11 respectively. Historical data for monthly average pan
evaporation (Epan) were obtained from the California Climate Date Archive as shown in
Table 3.12 (Western Regional Climate Center, 2008). The crop coefficient, Kc for
conifers is 1.0 for all seasons (FAO, 1988).
Table 3.10- Average Monthly Pan Evapotranspiration, ETo in Inches for City of Davis
Period of Record
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
1921-2010
1.49
2.34
4.54
7.13
10.19
12.17
12.77
11.28
9.08
6.35
2.89
1.45
81.68
Source- Data from the Cal-SIMETAW model using daily PRISM grid (4 X 4 Km) over Davis, UC Davis Biometrology Program
(Orang et al., 2013)
Table 3.11- Average Monthly Precipitation, Pr in Inches for City of Davis
Period of Record
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
1921-2010
3.92
3.87
2.77
1.17
0.56
0.2
0
0.05
0.26
0.9
2.36
3.54
19.6
Source- Data from the Cal-SIMETAW model using daily PRISM grid (4 X 4 Km) over Davis, UC Davis Biometrology Program
( Orang et al., 2013)
Table 3.12- Average Pan Evaporation, Epan in Inches for City of Davis
Period of Record
Jan
Feb
Mar
Apr
May
1917-2002
1.49
2.34
4.51
7.09
10.16
Jun
12.2
Jul
12.8
Aug
Sep
Oct
11.29
9.07
6.37
Nov
2.89
Dec
1.45
Annual
81.66
Source- Data from the California Climate Data Archive (United States Western Regional Climate Center, 2008)
50
51
The leaching requirement (LR) is the volume of deep percolation expressed as a
fraction of the applied water (USEPA, 2006). The leaching requirement is incorporated in
the study because if leaching is inadequate, harmful amounts of salt can accumulate in
the soil within a few cropping seasons. If the salt concentration exceeds the crop’s salt
tolerance threshold level, leaching is required to restore full productivity. Depending on
the degree of salinity control required, leaching may occur continuously or intermittently
at intervals of a few months to a few years. Where leaching is insufficient, crop losses
may become severe over time. Soil reclamation will be required before crops can be
grown economically again. The economical way to control soil salinity is to ensure that
the amount of salt added as irrigation water must equal the amount drained to maintain
salt balance.
Hoffman (1985) created a graph (Figure 3.8) relating Crop Salt Tolerance
Threshold Value (ECe), salinity of the applied water (Ca), and leaching requirement (LR).
Figure 3.8 is based on the results from several models run to calculate leaching
requirements (Tanji, 1990).
52
Figure 3.8- Leaching Requirement as a Function of Applied Salinity and ECe of Crop
Salinity Threshold (Tanji, 1990)
To find the leaching requirement, threshold salinity values (ECe) for the trees
under consideration were combined with the wastewater salinity. The ECe values used are
listed in Table 3.13.
Table 3.13- Threshold Salinity of Douglas fir, Ponderosa pine and Redwood (Tanji,
2000)
Trees
Douglas fir
Ponderosa pine
Redwood
Threshold Salinity (ECe)
2 dS/m
6 dS/m
2 dS/m
Slight tolerance
Moderately high tolerance
Slight tolerance
53
To be conservative, the salinity of applied water (Ca) was assumed to be 1500
µS/cm, i.e. 1.5 dS/m, as indicated in Table 2.7. Using threshold salinities given in Table
3.13 and 1.5 dS/m of salinity in the applied water, the leaching requirements shown in
Table 3.14 were estimated using Figure 3.8.
Table 3.14- Leaching requirement of Douglas fir, Ponderosa pine and Redwood
Trees
Leaching Requirement (LR)
Douglas fir
0.145
Ponderosa pine
0.04
Redwood
0.145
The limiting design parameter, hydraulic loading rate, is based on either (1) crop
water requirements, which include the leaching requirement and irrigation efficiency, or
(2) nitrogen limitations, as discussed next. The removal of nitrogen in land treatment
systems is complex and dynamic due to the many forms of nitrogen in wastewater
(organic-N, NH3, NH4, NO2, and NO3) and the relative ease of changing from one
oxidation state to another. The nitrogen present in typical municipal wastewater is usually
present as organic nitrogen (about 40 percent) and ammonia/ammonium ions (about 60
percent). Typically, only a portion of the ammonia nitrogen in municipal wastewater is
nitrified and the major nitrogen fraction in most effluents is in the ammonium form
(USEPA, 2006). Activated sludge and other high-rate biological processes can be
designed to convert the entire ammonia content to nitrate (nitrification). As discussed
earlier, the City of Davis is planning to adopt high-rate biological treatment for secondary
treatment to achieve nitrification.
54
Because all forms of nitrogen can be oxidized to nitrate and excessive nitrogen is
a health risk in groundwater, it is important in the design of slow rate land treatment
systems to identify the total concentration of nitrogen in the wastewater. Experience with
SR treatment processes demonstrates that the less oxidized the nitrogen is when entering
the land treatment system the more effective will be the retention and overall nitrogen
removal (USEPA, 2006).
The soil-plant system provides a number of interrelated responses to wastewater
nitrogen as shown in Figure 3.9 (USEPA, 2006). The organic nitrogen fraction, usually
associated with particulate matter, is entrapped or filtered out of the applied liquid
stream. The ammonia fraction can be lost by volatilization, taken up by the crop or
adsorbed by the clay minerals in the soil. Nitrate can be taken up by the vegetation, or
converted to nitrogen gas via denitrification in macro- or micro-anaerobic zones and lost
to the atmosphere. Nitrate can also be leached through the soil profile to the underlying
groundwater.
55
Figure 3.9- Nitrogen Cycle in the soil (USEPA, 2006)
The U.S. primary drinking water standard for nitrate is set at 10 mg/L as nitrogen
(USEPA, 2009). When potable aquifers, sole source aquifers, or wellhead protection
areas are involved, the current guidance requires that drinking water standards be met at
the land treatment project boundary (USEPA, 2006). As a result, nitrogen often becomes
the limiting design parameter for slow rate systems. There are safety factors inherent in
the following approach that ensures a conservative design. The procedure assumes that
all of the applied nitrogen will be converted to nitrate (i.e., complete nitrification) within
the same time period assumed for water application (no time lag or mineralization of
ammonia) and that there is no mixing or dispersion with the in-situ groundwater which
56
would lower the concentration before the groundwater crosses the project boundary
(USEPA, 2006).
Because nitrogen stored within the biomass of trees is not uniformly distributed
among the tree components, the amount of nitrogen that can actually be removed from
the site with a forest crop system will be substantially less than the estimated nitrogen
storage unless 100 percent of the aboveground biomass is harvested (whole-tree
harvesting). If only the merchantable stems are removed from the system, the net amount
of nitrogen removed by the system will be less than 30 percent of the amount stored in
the biomass (USEPA, 1981). The estimated annual nitrogen uptake (U) of forest
ecosystems in selected regions of the United States is presented in Table 3.15 (Crites et
al., 2000). Whole-tree harvesting is recommended to maximize nitrogen removal
provided it takes place in the summer when the needles are on the trees.
The role of the understory vegetation is particularly important in the early stages
of tree establishment. Forests take up and store nutrients and return a portion of those
nutrients back to the soil in the form of leaf fall and other debris such as dead trees. Upon
decomposition, the nutrients are released and re-absorbed by the trees. During the initial
stages of growth (1 to 2 years), tree seedlings are establishing a root system, and biomass
production and nutrient uptake are relatively slow. To prevent leaching of nitrogen to
groundwater during this period, nitrogen loading must be limited or understory vegetation
must be established that will take up and store applied nitrogen that is in excess of the
tree crop needs(USEPA, 2006).
57
The calculation to determine nitrogen limitations involves estimating an allowable
hydraulic loading rate based on an annual nitrogen balance (Ln), and then comparing that
to the previously calculated hydraulic loading rate based on crop needs and leaching rate
(Lag) to determine which value controls.
The USEPA Process Design Manual (USEPA, 1981) describes allowable annual
hydraulic loading rate based on nitrogen limits using the following equation,
Ln =
𝐶𝑝 (𝑃𝑟−𝐸𝑇𝑐)+(𝑈)(4.413)
𝐶𝑛 (1−𝑓)−𝐶𝑝
(3.3)
where
Ln = Allowable annual hydraulic loading rate based on nitrogen limits, in/month
Cp = Nitrogen concentration in percolating water, mg/L
Pr = Precipitation rate, in/month
ETc = Crop Evapotranspiration rate, in/month
U = Nitrogen uptake by crop, lbs/acre-year
Cn = Nitrogen concentration in reclaimed wastewater, mg/L
f = Fraction of applied nitrogen removed by denitrification and volatilization.
4.413 - unit conversion factor.
The limiting percolate nitrogen concentration, Cp is set at 10 mg/L which is the
nitrogen limit allowed in groundwater (CDPH, 2013). Monthly precipitation (Pr) and
monthly evapotranspiration rate ( ETc) are given in Tables 3.10 and 3.11 respectively.
58
Table 3.15- Nitrogen Uptake for Selected Forest Ecosystems with Whole Tree Harvesting
(USEPA, 2006)
Eastern forests:
Mixed hardwoods
Red pine
Old filed with white spruce plantation
Pioneer succession
Aspen sprouts
Tree Age,
Years
Average Annual Nitrogen Uptake,
lb/(acre-year)
40-60
25
200
100
15
5.15
-
200
200
100
Southern forests:
Mixed hardwoods
40-60
250
Loblolly pine with no understory
20
200
Loblolly pine with understory
20
250
Lake states forests:
Mixed hardwoods
50
100
Hybrid poplara
5
140
Western forests:
Hybrid poplara
4-5
270
Douglas fir plantation
15-25
200
a
Short-term rotation with harvesting at 4 to 5 years; represents first-growth cycle from planted seedlings
These rates are considered maximum estimates of net nitrogen uptake including
both the understory and overstory vegetation during the period of active tree growth.
Average annual nitrogen uptake (U) is assumed 200 lb/ (acre-year) for all the trees based
on value given for Douglas fir plantation in Table 3.15.
Monthly average nitrogen uptake is assumed to be proportional to growth as
indicated by evapotranspiration and is calculated by Equation 3.4.
𝑈𝑚𝑜𝑛𝑡ℎ = (
𝐸𝑇𝑚𝑜𝑛𝑡ℎ
𝐸𝑇𝑦𝑒𝑎𝑟
) 𝑈𝑦𝑒𝑎𝑟
(3.4)
As described previously, the City received a new NPDES permit for the WPCP
that requires compliance with a number of effluent requirements by October 25, 2017
(City of Davis, 2013). In response, the City has developed effluent performance criteria,
59
which are constituents and associated concentrations that shall be achieved when the
treatment plant is upgraded. These criteria are listed in Table 3.16.
Table 3.16- Process Effluent Performance Criteria for Future Effluent Requirements for
the WPCP (City of Davis, 2013)
Constituent
Units
NH4-N
mg/L
NO3-N plus NO2-N
mg/L
BOD
mg/L
TSS
mg/L
Design/Performance Criteria
Summer
Monthly Average: 1.3
Winter
Monthly Average: 1.7
Monthly Average:10
Monthly Average: 10
Weekly Average: 15
Monthly Average: 10
Weekly Average: 15
Process
Secondary
Treatment
Secondary
Treatment and
Tertiary
Filtration
Monthly average effluent concentrations of total nitrogen (Cn) which is sum of
NH4-N, NO3-N, and NO2-N from the upgraded secondary treatment process are 11.3 (10
+ 1.3) mg/l in summer and 11.7 (10 + 1.7) mg/l in winter as indicated in Table 3.16. The
organic nitrogen concentration is assumed to be negligible because it has been converted
to ammonium and then to nitrate (USEPA, 2006).
A nitrogen loss factor (f) has been developed to account for nitrogen lost to
denitrification, volatilization, and soil storage. When the wastewater contains a high
carbon-to-nitrogen (C:N) ratio, significant denitrification can occur (USEPA, 2006).
Because of the large influence of organic carbon on available nitrogen, the nitrogen loss
factor (f) varies as a function of C:N as shown in Table 3.17. Actual losses are dependent
on other factors including climate, forms of the nitrogen applied and application method.
60
Table 3.17- Nitrogen Loss Factor for Varying C:N Ratios (Reed et al., 1988)
C:N ratio
Example
f
>8
Food processing wastewater
0.5-0.8
1.2-8
Primary treated effluent
0.25-0.5
0.9-1.2
Secondary treated effluent
0.15-0.25
<0.9
Advanced treatment effluent
0.1
Given the amount of nitrification through the secondary process proposed by the
City, there will be little BOD left in the effluent and as shown in Table 3.17 the C:N ratio
is expected average about 1.05 (midpoint of 0.9- 1.2) and the corresponding nitrogen loss
factor (f) assumed in this study is 0.2 (midpoint of 0.15-0.25). As denitrification varies
with temperature, the nitrogen loss factor varies by about a factor of 2 for a 10 ºC
temperature change (Heinen, 2005). Accordingly, in this study f is assumed to be 0.2 for
spring and fall, 0.1 for winter reflecting a 10 ºC temperature drop and 0.3 for summer
representing a 10 ºC increase from average.
3.4.2
Hydraulic Loading Based on Slow Rate Wastewater Treatment
Using derived values for all the variables in the SR system design equations,
monthly values of Lag (Equation 3.1) were calculated and compared with Ln (Equation
3.3). The lower of the two values was used to determine the design monthly hydraulic
loading rates (Ld). Because the soil at the study area is mostly silty clay, a check was
performed to assure that the design hydraulic loading (Ld) does not exceed the
percolation rate (Pcalc) given by Equation 3.5 nor the maximum percolation rate of 10
in/month which is the field percolation rate measured near the study area (City of Davis,
2005).
61
Pcalc = Ld – ETc + Pr
(3.5)
Where
Pcalc= Percolation rate, in/month
Ld = Design hydraulic loading rate, in/month
ETc = Crop Evapotranspiration rate, in/month
Pr = Precipitation rate, in/month
Using the design hydraulic loading rate, the nitrogen concentration in percolating
water (Cpcalc) was calculated with Equation 3.6. The percentage of the tree nitrogen
requirement met by the wastewater influent after accounting for denitrification losses
(%Napplied) was calculated with Equation 3.7.
Cpcalc =
𝐿𝑑 𝐶𝑛 (1−𝑓)−(𝑈𝑚𝑜𝑛𝑡ℎ )(4.413)
𝑃𝑐𝑎𝑙𝑐
%Napplied =
𝐿𝑑 𝐶𝑛 (1−𝑓)
(𝑈𝑚𝑜𝑛𝑡ℎ )(4.413)
(3.6)
(3.7)
Tables 3.18 shows the monthly parameters for irrigating Douglas fir and redwood
and Table 3.19 shows the monthly parameters for irrigating Ponderosa pine, calculated
using this procedure.
Table 3.18- Estimation of Monthly Hydraulic Loading Rate for Irrigating Douglas fir and Redwood
1
2
3
4
Hydraulic
Loading Rate
Average Crop Evapo- Based on
Month
Precipitation Transpiration Tree Water
Requirements
LR=0.145
5
6
7
8
9
10
11
Fraction of
Hydraulic
Applied
Calculated
Potential Nitrogen
Loading Design
Nitrogen
Calculated Nitrogen
Nitrogen Concentration
Rate Hydraulic
Removed by
Percolation Concentration
Uptake in Applied
Based on Loading
Denitrification
Rate
in Percolating
by Tree
Water
Nitrogen
Rate
and
Water
Limit
Volatilization
Pr, (in)
ETc, (in)
Lag, (in)
Umonth,
(lbs/acre)
Cn,
(mg/L)
f
Ln, (in)
Ld, (in)
Pcalc, (in)
Jan
3.20
1.04
0.00
4.7
11.7
0.1
80.09
0.00
2.17
0.1
92.84
0.00
0.2
no limit
0.76
Cpcalc,
(mg/L)
12
Percentage of
Tree Nitrogen
Requirement
Met by Influent
(minus
Nitrogen Lost
to
Denitrification)
%Napplied
0
1.58
<0
<0
0.20
<0
14
3.91
1.01
<0
45
Feb
3.24
1.67
0.00
7.6
11.7
Mar
2.05
2.62
0.76
11.9
11.7
Apr
1.11
4.02
3.91
18.3
11.7
0.2
no limit
May
0.40
5.37
6.70
24.4
11.3
0.2
no limit
6.70
1.73
<0
56
Jun
0.11
6.43
8.50
29.2
11.3
0.3
no limit
8.50
2.19
<0
52
2.36
<0
53
2.07
<0
53
1.56
<0
59
46
0
Jul
Aug
Sep
0.01
0.02
0.13
6.80
5.99
4.64
9.15
8.04
6.07
30.9
27.2
21.1
11.3
11.3
11.3
Oct
0.79
2.88
2.81
13.1
11.7
Nov
1.83
1.58
0.00
7.2
11.7
Dec
2.86
0.99
0.00
4.5
11.7
0.3
0.3
0.2
no limit
no limit
no limit
9.15
8.04
6.07
0.2
no limit
2.81
0.72
<0
0.2
no limit
0.00
0.25
<0
72.68
0.00
1.86
<0
0
0.1
0
15.76
44.02
45.95
200.0
45.95
Column 8 - Equation 3.3. equals to infinity if Cn(1-f)<Cp
Column 9 - The smaller of columns 4 and 8.
Column 11 - Calculated Nitrogen concentration in percolating water after applying design hydraulic loading rate is negative indicating that more
nitrogen is removed by denitrification and volatilization (included in the f factor) than the amount that is applied.
62
Table 3.19- Estimation of Monthly Hydraulic Loading Rate for Irrigating Ponderosa pine
1
2
3
4
Hydraulic
Loading Rate
Average Crop Evapo- Based on
Month
Precipitation Transpiration Tree Water
Requirements
LR= 0.04
5
6
7
8
9
10
11
Fraction of
Hydraulic
Applied
Calculated
Potential Nitrogen
Loading Design
Nitrogen
Calculated Nitrogen
Nitrogen Concentration
Rate Hydraulic
Removed by
Percolation Concentration
Uptake in Applied
Based on Loading
Denitrification
Rate
in Percolating
by Tree
Water
Nitrogen
Rate
And
Water
Limit
Volatilization
Jan
3.20
1.04
0.00
Feb
3.24
1.67
0.00
7.6
11.7
0.1
92.84
Mar
2.05
2.62
0.69
11.9
11.7
0.2
no limit
0.69
0.13
<0
12
Apr
1.11
4.02
3.55
18.3
11.7
0.2
no limit
3.55
0.65
<0
41
1.11
<0
51
1.41
<0
47
1.52
<0
48
48
May
Jun
Jul
ETc, (in)
Lag, (in)
0.40
0.11
0.01
5.37
6.43
6.80
6.09
7.72
8.31
24.4
29.2
30.9
11.3
11.3
11.3
Aug
0.02
5.99
7.30
27.2
11.3
Sep
0.13
4.64
5.51
21.1
11.3
Oct
0.79
2.88
2.56
13.1
11.7
Nov
1.83
1.58
0.00
7.2
11.7
Dec
2.86
0.99
0.00
4.5
11.7
Cpcalc,
(mg/L)
Percentage of
Tree Nitrogen
Requirement
Met by Influent
(minus
Nitrogen Lost
to
Denitrification)
Umonth,
(lbs/acre)
4.7
Pr, (in)
Cn,
(mg/L)
11.7
12
f
Ln, (in)
Ld, (in)
Pcalc, (in)
0.1
80.09
0.00
2.17
1.58
<0
<0
0
0.00
0.2
0.3
0.3
no limit
no limit
no limit
6.09
7.72
8.31
%Napplied
0
0.3
no limit
7.30
1.33
<0
0.2
no limit
5.51
1.01
<0
54
0.2
no limit
2.56
0.47
<0
41
0.2
no limit
0.00
0.25
<0
0
0.1
72.68
0.00
1.86
<0
0
15.76
44.02
41.74
200.0
41.74
Column 8 - Equation 3.3. equals to infinity if Cn(1-f)<Cp
Column 9 - The smaller of columns 4 and 8.
Column 11 - Calculated Nitrogen concentration in percolating water after applying design hydraulic loading rate is negative indicating that more
nitrogen is removed by denitrification and volatilization (included in the f factor) than the amount that is applied.
63
64
Regardless of which tree is considered, it can be seen in Tables 3.18 and 3.19 that
calculated percolation rates (Pcalc) are far less than the measured percolation rate of 10
in/month. Negative values of calculated nitrogen concentration in percolating water
(Cpcalc) indicate that more nitrogen is removed by denitrification and volatilization
(included in the f factor) than the amount that is applied.
3.4.3 Hydraulic Loading based on Maximizing Tree Growth
As it can be seen in Section 3.4.2, using the standard SR design approach does not
fully satisfy the nitrogen requirement for tree growth as shown in column 12 (%Napplied)
of Tables 3.18 and 3.19. This shows that the design hydraulic loading rates do not
provide enough nitrogen to accommodate the full growth potential of the trees. To
maximize tree growth, the hydraulic loading rate should be increased to the point that the
nitrogen requirement of the tree is met, subject to prevention of groundwater
contamination. In this strategy, water is applied in excess of the water requirement of the
crop. The hydraulic loading rate is set to meet nitrogen requirement of the tree after
denitrification ( Lnreq) obtained by Equation 3.8 instead of the Ln that was used in
previous tables.
(𝑈𝑚𝑜𝑛𝑡ℎ )(4.413)
Lnreq =
𝐶𝑛 (1−𝑓)
(3.8)
To assure adequate drainage, the hydraulic loading rate is limited by percolation rate
calculated by Equation 3.9.
Lp = ETc – Pr – Pmax
where the maximum percolation rate, Pmax = 10 in/month
(3.9)
65
The smaller of the two hydraulic loading rates (Lnreq and Lp) values was used to
determine the design monthly hydraulic loading rate ( Ld). Nitrogen loading (minus
denitrification), Uloading is determined by Equation 3.10.
Uloading =
𝐿𝑑 𝐶𝑛 (1−𝑓)
4.413
(3.10)
Using the design hydraulic loading rate, the nitrogen concentration in percolating water
(Cpcalc) was calculated with Equation 3.6 except using Uloading in place of Umonth and
checked against the 10 mg/L limit(Cp) . Table 3.20 shows adjusted monthly hydraulic
loading rates based on meeting the nitrogen requirements of Douglas fir and redwood.
Table 3.21 shows adjusted monthly hydraulic loading rates based on meeting the nitrogen
requirements of Ponderosa pine. As can be seen, the calculated nitrogen concentration in
the percolate water was always zero because the nitrogen loading was based trees’
requirements.
Table 3.20- Adjustment of Design Hydraulic Loading Rate to Meet the Nitrogen Requirement of Douglas fir and Redwood to
Achieve Maximum Tree Growth
1
2
3
4
Hydraulic
loading rate
Average Crop EvapoMonth
based on
Precipitation Transpiration
crop
LR=0.145
5
6
7
8
Fraction Of
Hydraulic
Applied
Potential
Nitrogen
Loading To
Nitrogen
Nitrogen Concentration
Meet
Removed by
Uptake by in Applied
Nitrogen
Denitrification
the Tree
Water
Requirement
And
of Tree
Volatilization
9
10
11
Hydraulic
Design Percentage of
Loading
Hydraulic Nitrogen
Limited by
Loading Requirement
Percolation
Rate
Met
Rate
12
Nitrogen Loading
(Minus
Denitrification)
13
14
Calculated
Calculated
Nitrogen
Percolation Concentration
Rate
in Percolating
Water
Pr, (in)
ETc, (in)
Lag, (in)
Umonth,
(lbs/acre)
Cn, (mg/L)
f
Lnreq, (in)
Lp, (in)
Ld, (in)
%Napplied
Uloading, (lb/acre)
Pcalc, (in)
Cpcalc, (mg/L)
Jan
3.20
1.04
0.00
4.7
11.7
0.1
1.97
7.83
1.97
100
4.70
4.14
0.00
Feb
3.24
1.67
0.00
7.6
11.7
0.1
3.18
8.42
3.18
100
7.58
4.75
0.00
Mar
2.05
2.62
0.76
11.9
11.7
0.2
5.61
10.57
5.61
100
11.89
5.04
0.00
Apr
1.11
4.02
3.91
18.3
11.7
0.2
8.61
12.90
8.61
100
18.25
5.70
0.00
May
0.40
5.37
6.70
24.4
11.3
0.2
11.92
14.97
11.92
100
24.42
6.95
0.00
Jun
0.11
6.43
8.50
29.2
11.3
0.3
16.29
16.31
16.29
100
29.20
9.98
0.00
Jul
0.01
6.80
9.15
30.9
11.3
0.3
17.24
16.79
16.79
97
30.10
10.00
0.00
Aug
0.02
5.99
8.04
27.2
11.3
0.3
15.18
15.97
15.18
100
27.20
9.21
0.00
Sep
0.13
4.64
6.07
21.1
11.3
0.2
10.28
14.51
10.28
100
21.07
5.78
0.00
Oct
0.79
2.88
2.81
13.1
11.7
0.2
6.17
12.09
6.17
100
13.08
4.08
0.00
Nov
1.83
1.58
0.00
7.2
11.7
0.2
3.39
9.75
3.39
100
7.19
3.64
0.00
Dec
2.86
0.99
0.00
4.5
11.7
0.1
1.89
8.14
1.89
100
4.51
3.75
0.00
15.76
44.02
45.95
200.0
101.72
101.27
199.20
66
Table 3.21- Adjustment of Design Hydraulic Loading Rate to Meet the Nitrogen Requirement of Ponderosa pine to Achieve
Maximum Tree Growth
1
Month
2
3
4
5
6
7
8
9
10
11
12
13
14
Fraction Of
Hydraulic
Hydraulic
Applied
Hydraulic
Calculated
Potential
Nitrogen
Loading To
Design Percentage of
loading
Nitrogen
Loading
Nitrogen
Calculated
Nitrogen
Average
Crop EvapoNitrogen Concentration
Meet
Hydraulic Nitrogen
rate based
Removed by
Limited by
Loading (Minus Percolation Concentration
Precipitation Transpiration
Uptake by in Applied
Nitrogen
Loading Requirement
on crop
Denitrification
Percolation
Denitrification)
Rate
in Percolating
the Tree
Water
Requirement
Rate
Met
LR=0.04
And
Rate
Water
of Tree
Volatilization
Pr, (in)
ETc, (in)
Lag, (in)
Umonth,
(lbs/acre)
Cn, (mg/L)
f
Lnreq, (in)
Lp, (in)
Ld, (in)
%Napplied
Uloading, (lb/acre)
Pcalc, (in)
Cpcalc,
(mg/L)
Jan
3.20
1.04
0.00
4.7
11.7
0.1
1.97
7.83
1.97
100
4.70
4.14
0.00
Feb
3.24
1.67
0.00
7.6
11.7
0.1
3.18
8.42
3.18
100
7.58
4.75
0.00
Mar
2.05
2.62
0.69
11.9
11.7
0.2
5.61
10.57
5.61
100
11.89
5.04
0.00
Apr
1.11
4.02
3.55
18.3
11.7
0.2
8.61
12.90
8.61
100
18.25
5.70
0.00
May
0.40
5.37
6.09
24.4
11.3
0.2
11.92
14.97
11.92
100
24.42
6.95
0.00
Jun
0.11
6.43
7.72
29.2
11.3
0.3
16.29
16.31
16.29
100
29.20
9.98
0.00
Jul
0.01
6.80
8.31
30.9
11.3
0.3
17.24
16.79
16.79
97
30.10
10.00
0.00
Aug
0.02
5.99
7.30
27.2
11.3
0.3
15.18
15.97
15.18
100
27.20
9.21
0.00
Sep
0.13
4.64
5.51
21.1
11.3
0.2
10.28
14.51
10.28
100
21.07
5.78
0.00
Oct
0.79
2.88
2.56
13.1
11.7
0.2
6.17
12.09
6.17
100
13.08
4.08
0.00
Nov
1.83
1.58
0.00
7.2
11.7
0.2
3.39
9.75
3.39
100
7.19
3.64
0.00
Dec
2.86
0.99
0.00
4.5
11.7
0.1
1.89
8.14
1.89
100
4.51
3.75
0.00
15.76
44.02
41.74
200.0
101.72
101.27
199.20
67
68
The percentage of annual nitrogen demand (%Nsatisfied) is 99.6% for all the
trees, derived by Equation 3.11comparing with estimated average annual nitrogen uptake
shown in Table 3.16 that is satisfied by the application of wastewater.
%Nsatisfied =
∑ 𝑈𝑙𝑜𝑎𝑑𝑖𝑛𝑔
𝑈𝑦𝑒𝑎𝑟
x 100
(3.11)
3.4.4 Water Balance Determination
After determining the wastewater demand for tree irrigation, the next steps were
to match the loading to the amount of water available from the secondary treatment
system.
Two strategies for irrigating trees were investigated:
1.
Irrigating trees without the availability of any stored wastewater, meaning the area
to be irrigated is limited by the maximum monthly wastewater flow.
2.
Irrigating trees with winter storage so that the annual volume of wastewater can
be used to maximize the area that can be irrigated.
Population growth assumptions used in recent water supply planning studies were
used to develop future wastewater flow projections (City of Davis, 2013). Specifically, it
was assumed that WPCP influent flow and loads will increase at a rate of 0.5 percent per
year through 2019, and then at a rate of 1.0 percent per year for 2020 and beyond. It was
also assumed that the monthly pattern of wastewater influent flow for the period of 2009
through 2011 will be essentially the same as under current conditions. This assumption is
reasonable given the extremely slow rates of growth in the city in recent years. A WPCP
capacity based on 6.0 MGD average dry weather flow (ADWF) is expected to be
69
adequate through 2041 (i.e., providing three years for WPCP modification plus 25 years
of operation). The projection conforms to the recommendation of the Charrette Report to
construct a 6.0 MGD ADWF treatment process (City of Davis, 2013).
An adjustment factor to convert average dry weather flow to annual average flow
was developed based on historical influent flow data. The annual average flows (AAF)
are presented in Table 3.22. Average dry weather flow (ADWF) is defined as the average
of the three consecutive lowest monthly flows.
Table 3.22- WPCP Improvements Project Design Flows (City of Davis, 2013)
Flow Parameter
Average Flows
ADWF
Average Annual Flow (AAF)
Flow Value, MGD
6.0
6.6
The average annual flow (AAF) of 6.6 MGD was distributed by month based on
the seasonal flow variation for the years 2000-2004 shown in Table 3.23. Two dominant
factors affect seasonal flow variations: student population and rainfall. As indicated, the
City of Davis is unusual in that a significant portion of the service area population is
made up of students who attend the University of California at Davis (UC Davis). From
mid-June through mid-September and in November and December, a significant portion
of UC Davis students are on break and are absent from the community all or part of the
time. WPCP influent flows are impacted and reduced by this periodic decrease in
population. Influent flows to the WPCP are also influenced by the amount of infiltration
and inflow (I&I) present in the influent due to rainfall. I&I are generally elevated during
the wet season, which is defined as the period from November 1 through April 30 based
on typical rainfall patterns in the city. The four seasonal flow conditions of interest are:
70

Wet Season: Significant Student Population (January 1 to April 30),

Dry Season: Significant Student Population (May 1 to June 15 and September 21
to October 30),

Dry Season: Minimal Student Population (June 16 to September 20), and

Wet Season: Minimal Student Population (November 1 to December 31).
Table 3.23- WPCP Seasonal Average Influent Flow Statistics for 2000-2004 (City of
Davis, 2005)
Seasonal Flow
Condition, MGD
Jan-1 to Apr-30
2000
2001
2002
2003
2004
Average
5.90
5.84
6.12
6.20
6.38
6.09
May-1 to Jun-15 and
Sep-21 to Oct-30
5.83
5.72
5.84
6.08
6.07
5.91
Jun-16 to Sep-20
5.23
5.23
5.41
5.44
5.62
5.39
Nov-1 to Dec-31 (1)
5.51
5.50
5.56
5.53
5.54
5.53
Average
5.77
(1) Flows during the Thanksgiving and Winter Holiday break for UC Davis students are typically
the lowest daily flows recorded in a given year period.
The 6.6 MGD (AAF) is a 14% increase from historically average annual flow of
5.77 MGD based on 2000-2004 data. To predict the monthly flow pattern, the design
flow (AAF) of 6.6 MGD was prorated throughout year according to the seasonal pattern
in Table 3.23 distributed over a monthly time scale. The results are shown in Table 3.24
and Figure 3.10.
71
Table 3.24- Flow Prediction of Average Monthly Inflows through 2030 at WPCP
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sept
Oct
Nov
Dec
Average
Annual Flow
Influent Flow Conditions Historically
(Average of 2000 to 2004)
6.09
6.09
6.09
6.09
5.91
5.65
5.39
5.39
5.39
5.91
5.53
5.53
Flow Prediction of Average Monthly
Inflows Through 2030
6.9
6.9
6.9
6.9
6.7
6.4
6.1
6.1
6.1
6.7
6.3
6.3
5.77
6.6
Figure 3.10- Seasonal Prediction of Average Monthly WPCP Inflows through 2030
72
3.4.5
Water Balance without Wastewater Storage
This section presents the calculation of acreage that could be irrigated using
effluent available from secondary treatment without any storage. The design hydraulic
loading rate (Ld) that resulted from meeting the nitrogen requirements of the trees and
projected average monthly flows through 2030 were used to determine the area that could
be irrigated using the whole flow (Aw) each month according to Equation 3.12.
Aw =
𝑉𝑑 ×𝑑𝑎𝑦𝑠 ×12 𝑖𝑛𝑐ℎ𝑒𝑠×3.069
𝐿𝑑
(3.12)
Where
Aw = Area needed to accommodate whole flow, Acres
Vd = Design wastewater treated volume, MGD
3.069 is the conversion factor from million gallons to acre-feet.
As shown in Table 3.27, only 415 acres could be irrigated in July regardless of the
tree type chosen. Assuming 415 acres is irrigated throughout the year, the daily flow to
irrigation (Qi) and the daily flow that must be discharged to Willow Slough (Qd) is
calculated from Equation 3.13 and Equation 3.14. The results are shown in Table 3.25.
Qi =
415 𝐴𝑐𝑟𝑒𝑠 × 𝐿𝑑
𝑑𝑎𝑦𝑠 ×12 𝑖𝑛𝑐ℎ𝑒𝑠×3.069
Qd = Vd –Qi
(3.13)
(3.14)
Table 3.25- Determination of Acreage that could be Irrigated for Growing Different Trees without Storage
Douglas fir, Ponderosa pine and Redwood
Month
Design Hydraulic
Loading rate
Design wastewater Area needed to accommodate
Daily flow to irrigation*
treated volume
whole flow
Daily flow to discharge
Ld, (in)
Vd, (MGD)
Aw, (Acres)
Qi, (MGD)
Qd, (MGD)
Jan
1.97
6.90
3997
0.72
6.18
Feb
3.18
6.90
2240
1.28
5.62
Mar
5.61
6.90
1405
2.04
4.86
Apr
8.61
6.90
886
3.23
3.67
May
11.92
6.70
642
4.33
2.37
Jun
16.29
6.40
434
6.11
0.29
Jul
16.79
6.10
415
6.10
0.00
Aug
15.18
6.10
459
5.51
0.59
Sep
10.28
6.10
655
3.86
2.24
Oct
6.17
6.70
1240
2.24
4.46
Nov
3.39
6.30
2053
1.27
5.03
Dec
1.89
6.30
3809
0.69
5.61
Total
101.27
78.30
1140
1240
48%
52%
Annual Flow, (MG)
% of annual volume
* Values are based on irrigating 415 acres.
73
74
Under this strategy about 48% of the annual effluent volume would be utilized for
irrigation and 52% of treated wastewater effluent would be discharged into Willow
Slough, mainly in winter, regardless of tree type.
3.4.6
Water Balance with Wastewater Storage
This approach allows for excess water to be stored in the 215 million gallons
available in the existing wastewater treatment plant oxidation ponds, aeration ponds and
the Lemna pond. The ponds cover a total of 119 acres and have a minimum depth of 5.5
feet. Storage of excess water in the winter will allow more acreage to be irrigated.
Determining the irrigation area required an iterative approach. Initially the
irrigated area was assumed to be the whole 770 acres of city land. The daily wastewater
flow required to irrigate trees in 770 acres of land (Qi) using the design hydraulic loading
rate (Ld) to meet the nitrogen requirements of the trees (Section 3.4.3) was calculated
using Equation 3.15.
Qi =
𝐴 × 𝐿𝑑
𝑑𝑎𝑦𝑠 ×12 𝑖𝑛𝑐ℎ𝑒𝑠×3.069
(3.15)
Where
Qi = Daily flow to irrigation, MGD
A = Irrigated area, Acres
3.069 is the conversion factor from million gallons to acre-feet.
Wastewater in excess of that needed by the crop (Qd) on a monthly basis was
calculated using Equation 3.14. This volume was stored monthly as excess wastewater
volume (Vst). The cumulative volume stored at the start of the month (VS) is equal to the
75
cumulative volume stored at the end of the previous month (Ve). Some of the stored
wastewater would be lost to evaporation. To calculate this volume, the water surface in
the storage ponds was needed. In actuality, each pond with its different dimensions (area)
would fill up one after the other, i.e., not simultaneously but in series, and depths would
vary. Since only a rough estimate of water loss through direct evaporation of pond was
desired, the following approximate method was adopted.
The pond area (As), the so-called “storage area”, required to hold the cumulative
volume was calculated by Equation 3.16.
As=
Vi x 3.069
Dm
(3.16)
Where
As = Storage area to accumulate cumulative volume, acres
Vi = Cumulative volume stored, MG
= 0.5 x (Cumulative volume stored at the start of month + Cumulative volume stored
at the end of month without evaporation)
= 0.5 x (Vs + (Vs + Vst))
Dm = Minimum depth of pond (storage), feet = 5.5 feet (from Table 2.5)
The water surface area in the storage pond was estimated each month using
Equation 3.16 with the average volume stored, neglecting evaporation. The maximum
value allowed was 119 acres in accordance with existing facilities. Next the net pond
evaporation, Ep in MG was calculated using Equation 3.17.
((Kp x Epan )−P𝑟 ) x As
Ep =
12 inches x 3.069
(3.17)
76
where,
Kp = Pan coefficient = 0.7 which is applicable to Class A pan (Kohler et al., 1955)
Epan = Monthly average pan evaporation, inches (from Table 3.12)
Pr = Monthly average precipitation, inches (from Table 3.11)
Cumulative volume stored at the end of the month considering average evaporation (Ve)
is calculated by Equation 3.18
Ve = Vs + Vst - Ep
(3.18)
Wastewater in excess of what can be stored due to the storage capacity being
limited to 215 million gallon would be discharged or overflow (Vo) to Willow Slough.
These volumes were calculated using Equation 3.21.
Vo of current month = (Vs + Vst – Ep) in previous month – 215
(3.21)
where
215 MG is maximum storage available through existing ponds mentioned in Table 2.5.
Based on the initial water balance (area = 770 acres), it was seen that the pond
would start accumulating excess water in September for all trees. Accordingly, the
cumulative volume was set at zero at the beginning of this month and the water balance
calculations were carried out for the following year. In a properly balanced system, the
storage calculated at the end of August for all the trees should be zero. The ExcelTM
Solver function was applied to adjust the area being irrigated (A) until this requirement
was met. The calculations indicate that 569 acres of Douglas fir, Ponderosa pine and
redwood could be irrigated with the monthly storage volumes shown in Table 3.26.
Table 3.26- Determination of Area Irrigated for Douglas fir, Ponderosa pine and Redwood Using Wastewater Storage and
Maximizing Nitrogen Applications to the Tree Crop
1
2
3
4
5
6
7
8
9
10
11
Storage
Excess
Excess Cumulative
Area for
Design
Design
Daily Wastewater Wastewater Volume
end-ofIrrigated Wastewater Hydraulic
Cumulative
Net Pond
Month
Flow to (Not Sent Volume Stored at the
month
Area
Treated
Loading
Volume
Evaporation
Irrigation
to
(Storage Start of The
Volume
Volume
Rate*
Stored
Irrigation) In/Out)
Month
without
Evaporation
A,
Vd,
Ld,
Qi,
Qd,
Vst,
Vs,
Vi,
As,
Ep,
(Acres)
(MGD)
(in)
(MGD)
(MGD)
(MG)
(MG)
(MG)
Acres
(in)
Jan
569.0
6.9
1.97
0.98
5.92
183.46
215
307
119
-6.98
12
13
Cumulative
Volume
WasteStored at
water
end-of- discharged
month with (overflow)
Evaporation
Ve,
(MG)
215
Vo,
(MG)
172.08
Feb
569.0
6.9
3.18
1.75
5.15
144.15
215
287
119
-5.19
215
190.44
Mar
569.0
6.9
5.61
2.79
4.11
127.31
215
279
119
3.57
215
149.34
Apr
569.0
6.9
8.61
4.43
2.47
74.07
215
252
119
12.44
215
123.74
May
569.0
6.7
11.92
5.94
0.76
23.59
215
227
119
21.69
215
61.63
Jun
569.0
6.4
16.29
8.39
-1.99
-59.60
215
185
103
23.64
132
1.90
Jul
569.0
6.1
16.79
8.37
-2.27
-70.26
132
97
54
13.11
48
0.00
Aug
569.0
6.1
15.18
7.56
-1.46
-45.32
48
26
14
3.07
0
0.00
Sep
569.0
6.1
10.28
5.29
0.81
24.16
0
12
7
1.14
23
0.00
Oct
569.0
6.7
6.17
3.07
3.63
112.42
23
79
44
4.40
131
0.00
Nov
569.0
6.3
3.39
1.75
4.55
136.64
131
199
111
0.58
215
0.00
Dec
569.0
6.3
1.89
0.94
5.36
166.13
215
298
119
-5.95
215
52.10
78.3
101.27
816.74
Annual Excess Wastewater -Annual Evaporation losses-Annual Overflow
65.52
751.22
= 0.00 MG
Calculated Irrigated Area, A = 569.0 Acres
*Design hydraulic loading rate based on maximizing tree growth calculated in Section 3.4.3
77
The total volume of wastewater treated would be 2381 million gallons per year.
From Tables 3.28, it can be seen that for the Douglas fir, Ponderosa pine and redwood
water balance, 751 million gallons of wastewater, i.e. 31% of total volume would be
discharged to Willow Slough.
3.5 Agro-Forestry Management Plan
Secondary treated wastewater, especially if it meets the nitrogen effluent quality
planned by the City can be used to irrigate a carbon sequestration forest via a slow rate
application system. Trees such as Douglas fir, Ponderosa pine and redwood are
appropriate to cultivate for carbon storage.
As shown in Tables 3.9, the harvesting ages of Douglas fir, Ponderosa pine and
redwood should be 30, 40 and 25 years respectively. After harvest Douglas fir and
Ponderosa pine must be replanted. Redwood can reproduce by coppice (from stump) or
natural seeding (Birzer, 2010). Thinning at an intermediate age is recommended for
Ponderosa pine and redwood trees to stimulate growth of the trees and maintain
maximum biomass production. The purpose of thinning operation is to increase growing
space so that trees do not have to compete for light, moisture, and nutrients (Roth, 1989).
To achieve maximum nitrogen removal, whole tree harvesting must take place in the
summer. Wood from branches and trunk should be used for lumber. Leaves and root
systems should be sold for landscaping, although these uses do not create long-term
sequestration. No part of tree or debris should be burned. Trees should be spaced 8-10
feet (Reukema, 1979) and the strip method for harvesting should be used in which
78
alternative rows of tree are harvested. According to Row (1996), the advantages of
adopting this method are:
1.
Equipment can be more easily operated in a strip.
2.
Closer tree spacing may be used to achieve the desired nutrient uptake rates
during initial rotation.
3.
Uneven-aged trees will provide a consistent amount of carbon storage each year.
4.
The gaps created between rows will allow the trees to have more sunlight and
space which encourage growth.
To achieve optimum distribution and water-use efficiency, micro-irrigation or
sprinkler irrigation system could be used, but the decision would be based on economics.
(Bastian and Ryan, 1986)
79
Chapter 4
DISCUSSION
This Chapter discusses potential benefits of CO2 mitigation forest that was
assessed with the help of above chapters, uncertainties and unsettled questions that were
raised during this project and not covered due to scope of this project study. It is further
suggested to research and evaluate the unsettled issues indicated by the study in detail
before adopting CO2 sequestered forest as final reuse alternative.
4.1 Potential Benefits
Using on the average annual growth rates presented in Table 3.9 for harvest times
of 30, 40 and 25 years for Douglas fir, Ponderosa pine and redwood, respectively carbon
uptakes for each kind of tree were calculated. The results are shown in Table 4.1.
Redwood supports the highest carbon storage of 98.3 MT/Acre (3.93 x 25) with the
shortest harvesting cycle of 25 years. The 2236 MT/year captured would store in
approximately 279,500 board feet of marketable timber.
Table 4.1-Annual Mass of Carbon Captured in Biomass
Average
Annual Rate
Tree Type Harvest
of Carbon
Age
Capture
(MT/acre)
Douglas fir
Ponderosa
pine
Redwood
Area
Irrigated
Without
Storage
(Acre)
Annual Carbon
Capture Without
Winter Irrigation
Storage
(MT/yr.)
Area
Irrigated
With
Storage
(Acre)
Annual Carbon
Capture With
Winter Irrigation
Storage
(MT/yr.)
30
1.82
415
755
570
1036
40
0.58
415
241
570
330
25
3.93
415
1631
570
2236
80
For perspective, the total metric tons of carbon captured by this project (with
storage) were compared with the estimated carbon emissions summarized in Table 4.2.
Table 4.2- 2007-2010 CO2 Emissions of Davis, California (Sears, 2012)
Year
Emissions (Metric Tons CO2)
Total
Community
Community
Electricity
Stationary
Use
Combustion
Transportation
Solid
Water and
Additional
Waste
Wastewater
(Non-
Energy Use
Required)
Sources
2007
361,451
75,674
64,678
195,748
9,202
2,683
13,466
2008
360,550
75,570
64,872
196,772
8,510
2,840
11,987
2009
352,517
69,077
65,563
197,621
7,817
2,057
10,381
2010
335,187
52,234
65,142
197,527
8,094
1,367
10,821
In Table 4.3, the carbon captured by this forest proposal is expressed as a
percentage of the city’s 2010 carbon emissions in various sectors.
Table 4.3- Percentage Community Carbon Emissions Potentially Offset by this Project
2010 Community Emissions of
City of Davis
Emissions (Metric Tons
CO2)
% of Carbon Sequestered
through this Project
Community Electricity Use
Community Stationary Combustion
Transportation
Solid Waste
Water & Wastewater Energy Use
Additional (Non-Required) Sources
52,234
65,142
197,527
8,094
1,367
10,821
Total
335,185
4
3
1
28
164
21
0.7
As shown, the proposed forest has the potential to more than offset the city’s
current CO2 emissions due to water and wastewater energy use, but as a fraction of the
community’s total CO2 emissions, the offset by the forest is small. It should be noted that
cultivating lumber within the city saves the carbon that is currently used to cut and
81
transport lumber from forests far away. This savings is not accounted for in these
calculations. Neither are the CO2 emissions associated with forest management.
In Table 4.4, the irrigated carbon sequestration forest is compared with other
disposal/reuse options which the City examined the 2005 Master Plan discussed in
Section 2.5.
Table 4.4- Comparison of Forest Alternative with Reuse Alternatives Considered by City
of Davis
Level of Treatment Needed
Year-Round Land
Disposal/Reuse
Seasonal agricultural reuse/storage
Seasonal Reuse
Demonstration reuse on cityowned lands
Carbon Sequestration Forest
Comparative Criteria
Secondary
990 acres would be irrigated
with 100 acres of storage.
Significant purchase of land
(more the 200 acres) required.
Storage in existing ponds and
wetlands.
Marketable crop produced.
350 out of 770 acres would be
irrigated with 3 to 5 acres of
storage for demonstrating the
usage to agricultural users in
vicinity.
415 out of 770 acres could be
irrigated without any storage.
570 out of 770 acres could be
irrigated with storage in existing
ponds.
Marketable wood products
produced.
Tertiary
Un-disinfected Secondary
Based on the comparative characteristics listed in Table 4.4, the carbonsequestration alternative is more desirable for several reasons.
1.
The forest alternative requires un-disinfected secondary effluent as discussed in
Section 2.3.1. This is the lowest level of treatment of the three alternatives in
Table 4.4, and is less stringent than the treatment required under the City’s
82
NPDES permit. This aspect would be a large advantage to the City because
providing secondary treatment is less expensive than providing tertiary treatment.
2.
No additional land is required to dispose of the city’s whole wastewater flow
because a forest can accept more water than an agricultural crop.
3.
No additional storage beyond the existing ponds would be required.
4.
In addition of offsetting some of the city’s carbon emissions, the forest alternative
could generate marketable timber. Wood-pulp and mulch from the remaining
parts of the trees might also be marketable commodities. Another possible source
of revenue would be the sale of carbon credits (Lubowski et al., 2006).
4.2 Uncertainties
This study is a planning-level analysis and as such contains many uncertainties.
One set of uncertainties involves input parameters. The population projection for
determining the project design capacity was taken to be 1 percent per year as specified in
the Charrette plan (City of Davis, 2013). The irrigated acreage estimated in the water
balance will change if the population growth model is incorrect. Similarly, the
wastewater generation pattern which depends on rainfall and students residing in the city
could be incorrect since it is based on relatively few data. Changes in this parameter
would affect the influent flow characteristics which are the basis of the water balance
calculations. Another uncertainty is the performance of the proposed treatment plant
upgrades. Changes in effluent characteristics, particularly the nitrogen content and
salinity, would affect the calculation of allowable hydraulic loading rate, a key parameter
for the water balance.
83
Site characteristics are also uncertain. Recent data on percolation and vertical
transmissibility rates for the land proposed for this project are not available. Without
these parameters, it would be difficult to determine if the trees are receiving the required
irrigation depth and how much seepage is occurring beyond the irrigation depth under the
proposed irrigation schedule. A percolation rate of 10 in/month (City of Davis, 2005) was
applied to the water balance calculation. However, it is recommended to conduct
percolation rate testing at the proposed site.
Agronomic issues are a third set of uncertainties. One issue is the nitrogen
concentration limitation to prevent groundwater contamination. The carbon estimation
tool (COLE-EZ) and the nitrogen balance calculations used in this study do not model
nitrogen transformations in detail. How much nitrogen can be absorbed by each tree
(nitrogen requirement) and how much is removed in the soil (the “f” factor) were
assumed and required to be verified. In the water balance, the hydraulic loading rate of
each tree was increased above the water requirements to satisfy the trees’ nitrogen
requirements and maximize tree. A check was performed to ensure that excess water
would percolate beyond the irrigation depth of soil, but it is uncertain whether the trees
can tolerate the over-watering. If not, then the hydraulic loading rate would have to be
reduced and the area needed for irrigation increased. Another uncertainty is the growth
and carbon sequestration potential of the proposed trees which are not native to the
valley. The accuracy of potential carbon sequestration estimated by COLE-EZ needs to
be verified for the redwood trees recommended in this project. Further research or a pilot
study of irrigation techniques and scheduling, and planting, thinning and harvesting
84
operations are needed to determine how to obtain maximum carbon sequestration through
optimum growth of non-native lumber trees.
4.3 Unsettled Questions
Two issues not addressed in this study need to be settled before implementing this
proposal. First, costs have not been estimated for the installation, operation and
maintenance. Even though technically feasible, the City may not consider this to be a
cost-effective way to reduce its net carbon emissions. Second, the design assumes that
wastewater discharges to Willow Slough can be discontinued at certain periods of the
year when the whole wastewater flow would be diverted to irrigation. Further
investigation is recommended to determine whether this could pose a problem regarding
the maintenance of minimum flow standards established by the Regional Water Quality
Control Board.
85
Chapter 5
CONCLUSION
The option of growing a carbon sequestration forest for the City of Davis was
assessed in this project. Douglas fir, Ponderosa pine and redwood were determined to be
potential tree crops based on an evaluation of the climate, site characteristics and
potential ability of each tree for maximum long-term carbon sequestration. The proposed
forest irrigation would require only un-disinfected secondary recycled water under
California laws and regulations. Potential threats to groundwater such as nitrogen or
salinity could limit the land application of treated wastewater. For that reason, water
balances were performed using slow rate land treatment design procedures which take
into account the nitrogen balance and salinity (leaching requirement). The design was
controlled by the nitrogen requirements of the trees. Based on a 6.6 MGD average annual
design capacity of system, annual wastewater application rates, areas that could be
brought under irrigation with or without storage, and annual carbon sequestration masses
were calculated. These are shown in Table 5.1.
Table 5.1- Hydraulic Loading and Irrigated Area Results
Trees
Douglas
fir
Ponderosa
pine
Redwood
Annual
Hydraulic
Loading
Rate
Area
Irrigated
without
Storage
Carbon Sequestration
by Irrigating Land
Owned by City of
Davis without Storage
Area
Irrigated
with
Storage
Carbon
Sequestration by
Irrigating Land
Owned by City of
Davis with Storage
Lh
(inches/year)
Acres
MT/Year
Acres
MT/Year
101
415
755
570
1036
101
415
241
570
330
101
415
1631
570
2236
86
Redwood trees are the recommended species to be cultivated because they capture
greatest mass of carbon/acre/year. Based on the hydraulic loading assumptions and water
balance calculations, about 570 acres of city-owned land could be used for afforestation
with redwood trees and the available ponds at the wastewater treatment plant could be
used for storage. This project has the potential to offset the city’s CO2 emissions from its
water and wastewater energy use.
87
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