Proceedings of the First International Conference on Sustainable Construction, Tampa, Florida, 6-9 November 1994, 807-816.
SUSTAINABLE WATER RESOURCES AND URBAN REUSE
TECHNOLOGY
Kevin R. Grosskopf , M.E. Rinker, Sr. School of Building Construction, University of Florida,
Gainesville, Florida 32611 USA
Introduction
For centuries man's built environment and quality of life has been closely predicated on the
availability and sustainability of natural resources. As the common denominator in virtually every
ecosystem, water resources likewise serve as the cornerstone of human society and its sustainment.
The finite amount of water on the planet undergoes continuous reuse and regeneration while
traveling through the various stages of the hydrologic continuum. Yet the global demand for water
increasingly exceeds the limits of this slow moving cycle, compromising man's quality of life and
very existence. To meet the growing international demand for water, the pace of regeneration and
subsequent reuse must be accelerated. As a consequence, water reuse and resource sustainable
design are rapidly evolving within the current trends of environmentally conscious construction.
This concept of sustainability hinges on the idea of providing for the needs of the present without
detracting from the ability to serve the needs of the future. Overwhelming success in reclaiming
wastewater has since spawned interest in a new generation of resource optimization. Sustainable
water resources, greywater recycling, and other reuse technologies will play an increasing role in
water conservation, expanding potential reuse markets far beyond municipal reclamation alone.
Such advancements in green technology now provide cost effective recycling of wastewater
effluent, greywater, and other harvested resources for a myriad of urban non-potable applications
currently using potable water. The purpose of this study therefore, is to effectively communicate
and identify on-site water reuse alternatives that offer substantial economic and environmental
incentives for optimizing global water resources.
Social and Geoclimatic Influences
Increasing water demands spurred by the social and economic influences of population growth and
over development are seemingly inevitable. Geoclimatic and hydrologic forces such as drought and
saltwater intrusion are often unpredictable and virtually impervious to man's efforts to reverse them.
In an effort to concentrate understanding toward many of these international areas stricken by
intermittent drought, resource overdraft, population densities, and coastal development, the State of
Florida will serve as a model to evaluate urban water reuse and resource sustainable design for
reducing potable demand and environmental degradation. Similar to many urbanized areas
worldwide, Florida's population nearly doubled from 1960 to 1980, escalated 33% from 1980 to
1990, and is expected to increase an additional 19% from 1990 to 2000.1 Seven densely populated
regions represent 60% of the state's total population and nearly 70% of its domestic withdrawal
(Heath, 1981). Centered around tourism, agriculture, and industry, Florida continues to experience
a population increase of nearly 3.5% annually (State of Florida, 1990).
4000
18
3500
Withdrawal (MGD)
Population (millions)
21
15
12
9
6
3
3000
2500
2000
1500
1000
500
0
0
1950
1960
1970
1980
1990
2000
2010
2020
Years (decades)
Figure 1 Current and projected population increase
in Florida.
1950
1960
1970
1980
1990
2000
2010
2020
Years (decades)
Figure 2 Current and projected water demand
in Florida.
In spite of an average rainfall of 54 inches per year and limited efforts to optimize scarce water
resources, withdrawal rates in Florida continue to spiral upward. Use of potable water in Florida
has increased by a factor of 6 in the last ninety years, with 75% of the increase occurring in the last
twenty-five years. Furthermore, 79% of Florida's 13 million people reside near the coast (United
States, 1991). Such urban developments are primarily served by shallow aquifers further vulnerable
to saltwater intrusion, resource overdraft, and wastewater contamination. Figures 1 and 2 above
demonstrate the "mirrored" relationship that exists between population growth and subsequent
water demand.
Urban Water Recycling and Reuse
Urban reuse of wastewater has proven the most effective way to reduce water resource consumption
and the environmental dangers posed by the disposal of large quantities of insufficiently treated
wastewater. Water reuse can be classified as either non-potable (1) agricultural, (2) urban, (3)
industrial, or (4) indirect potable reuse as infiltrated aquifer recharge. Potable water is defined as
all water consumed for drinking, cooking, and personal hygiene. Potable water generally originates
from the highest purity source and is the most rigorously treated. The commercial and residential
structures that compose most urban development use in excess of 80% of their potable flow for
non-potable, or “non-drinking” quality consumption, resulting in a costly, inefficient use of a
limited resource. In select commercial applications, 75% or more of the domestic supply serves
toiletry fixtures alone. Conservatively, 70% of the current urban water demand could be
supplemented by reclaimed or reuse water technology.
The central core design of most commercial and residential structures coupled with a density of
occupants, would provide the greatest use of cost saving reuse water for the least "dual-plumbing”
investment. The Type A reuse system collects greywater, or wastewater consisting of little or no
organic matter into separate sanitary piping from non-fecal sources such as lavatories and sinks.
Once this clean effluent undergoes minimal but adequate treatment, it is dual-distributed to all
potential non-potable fixtures maintained separately of those requiring potable water. The
remaining wastewater is disposed of conventionally into municipal sanitary lines. Research has
concluded that Type A dual distribution/dual recovery will achieve maximum benefit in urban
residential structures, where the greywater supply (approximately 40-60% of total building flow)
from non-fecal fixtures balances the potential non-potable demand (40-60% of total building flow)
as demonstrated in Figure 3 on the following page.
10% Cooling &
HVAC
15% Lavs &
Irrigation
23% Laundry
34% Toilets
12% Cooling
& HVAC
6% Irrigation
25% Lavs &
Shower
75% Toilets &
Urinals
Figure 3. Wastewater flow characterization inFigure 4. Wastewater flow characterization in
typical residential structures.
typical commercial structures.
Wastewater collected from organic sources such as toilets and urinals, can likewise be recovered for
reuse. Dual distribution and single (conventional) wastewater piping systems may be used if the
volume of greywater alone is insufficient to properly maintain a reuse supply balance, or if
discharge flow is restricted. Commercial structures are commonly characterized by an unbalanced
flow of 10-25% greywater supply and 75-90% non-potable demand as demonstrated in Figure 4
above. Therefore, the total recovery of wastewater for treatment and reuse is desirable through dual
distribution/single recovery Type B greywater systems. Supply requirements for non-potable
fixtures currently using potable resources such as water closets, may very from 34% in residential
structures, to 75% or greater of total building flow in most commercial applications. Secondary
non-potable users such as mechanical makeup and irrigation, could further reduce potable demand.
Tertiary reuse applications such as fixture trap priming, aesthetic fixtures, and fire suppression
accounting for less than 10% of total flow could nevertheless provide an added incentive for
justifying urban reuse.
Table 1 on the
Sink
following page provides
Sink
Washer
a sample comparison
Washer
Lav.
between Type A and
Lav.
Lav.
Type
B
greywater
systems using a sample
Lav.
W.C.
250 unit test structure.
Greywater wet-vent
Although both systems
W.C.
Shower
resulted in substantial
Shower
water savings, Type B
Soil stack
dual
distribution
achieved superior flow
reduction by reclaiming
the total volume of
Non-fecal (greywater) to on-site treatment
wastewater for a greater
number of non-potable
Waste (blackwater) to sewer
applications.
Figure 5 Sample Type A greywater recovery diagram for multi-unit urban reuse.
Table 1. Conventional and reuse systems flow analysis
System
Piping
Treatment
Reuse
Applications
Water
Savings
Sewage
Reduction
Conventional
Base
None
N/A
0
0
Type A
Dual distribution
Dual sanitary
Filtration
Adsorption
Chlorination
Water closet
Irrigation
31,500GPD
(42% of total)
27,750GPD
(46% of total)
Type B
Dual distribution
Single sanitary
Biochemical
Filtration
Adsorption
Chlorination
Water closet
Irrigation
36,000GPD
(48% of total)
33,400GPD
(56% of total)
Generally, the greater the volume and cost of potable supply and subsequent wastewater discharge,
the more lucrative urban reuse becomes. Table 2 identifies a typical life-cycle approach for
determining the applicability for urban reuse in both residential and commercial buildings.
Hypothetically, both structures consist of 1000 tenants/occupants sharing 200 units/offices. By
amortizing the respective greywater investments using standard economic determinates over a 20
year period, the value engineering team can choose the most appropriate reuse option specific to
each building designation. Note that the Type B alternative may reduce nearly twice the
wastewater flow of the greywater only Type A system, yet is twice the initial investment. In
support of previous observations, the Type B system is most appropriate in commercial
environments where the total volume of wastewater is recycled to supplement the 75%-90% nonpotable demand. Conversely, Type A systems are seemingly most cost effective in residential
settings where the 40%-60% wastewater flow from greywater fixtures such as sinks, showers, and
lavatories, balances the 40-60% non-potable demand to flush tank toilets currently using potable
water.
Table 2. Conventional and reuse systems flow analysis
System specific costs
Conventional System
Type A System
Type B System
Initial cost increase
Operational & maintenance
Economic life
Interest rate
Amortized annual cost
Potable water cost
Wastewater treatment cost
n/a
$ 6,300.00
20 years
12%
n/a
$182,410.00
$ 56,700.00
$355,000.00
$ 20,700.00
20 years
12%
$ 47,526.00
$109,520.00
$ 34,125.00
$689,235.00
$ 27,384.00
20 years
12%
$ 92,273.00
$ 72,890.00
$ 22,650.00
Totals
$245.410.00
$211,871.00
$215,197.00
Water Reuse Alternatives
Primary non-potable consumers of domestic water are centered around toiletry activities, which use
between 34% to nearly 75% of total building flow for residential and commercial structures
respectively. Secondary non-potable applications such as HVAC make-up and irrigation will
further reduce potable demand. A few of the many non-potable reuse alternatives are provided
below:
Toilet and urinal flushing
As previously mentioned, toilet and urinal reuse will eliminate up to 75% of potable demand.
Commercial structures are typically provided 15gpm per urinal and nearly 40gpm to flush valve
toilets. Flush tank water closets most commonly used in residential structures, generally use 5-6
gal. per flush. Little or no fixture modifications are required prior to reuse.
Irrigation
The use of recycled wastewater for urban landscape irrigation is one of the fastest growing and
successful reuse options in the State of Florida. Exterior residential and commercial watering can
reduce nearly 40% of the total ground and potable water resources withdrawn annually. Irrigation
remains however, the reuse alternative with the highest potential of human contact and ingestion.
Direct contact with spray irrigation is among the most probable environmental pathway for
exposure to airborne pathogens and viruses. Reuse drip irrigation allows greywater to be applied
below the irrigated surface, providing a more efficient irrigation effort, reducing the exposure to
humans, and subsequently reducing the levels of treatment. This subsurface leaching technique can
also apply excess irrigated water to a slow rate land application medium below the irrigated strata
for indirect soil infiltration to groundwater reserves.
Mechanical heat exchange and make-up
Most urban structures use potable water within building HVAC systems as a medium to absorb and
expel heat. These cooling towers require 2.4-3.0gpm/ton of air handling capacity, amounting to
10% of total building water usage in some applications. HVAC use of recycled greywater within
continuously recirculating systems would require highly treated effluent consisting of high residual
disinfectants to prevent biogrowth, pH balancing to prevent scaling or corrosion, and ultra-filtration
to remove all suspended solids. Biogrowth can be controlled by the use of ammonia and nitrogen
reducing NaOCl and biological nitrification. Softened reuse water free of scale forming calcium
and magnesium, and acidic salts and precipitates can enhance systems performance.
Dry pipe fire suppression systems
Reuse water may be used for commercial and residential fire protection with little or no
modification to the existing system. Dry pipe fire suppression systems inhibit flow to the sprinkler
laterals when inactive, eliminating stagnant water which could provide a perfect environment for
residual breakdown, biogrowth, and scaling between flush cycles. Although dry pipe suppression
was originally designed for fire protection in unheated spaces, it will also serve to eliminate the
unavoidable settleable solids in greywater from collecting within the fine orifice plates of a typical
sprinkler.
Water Reuse Requirements
The overall concern for integrating the dual distribution of both potable and non-potable resources
is predicated on the economics and reliability of protecting building occupants from accidental
access, contact, or ingestion of greywater. Maximum obtainable separation distances for all
potable and non-potable distribution piping should be designed and constructed to provide at least
5’-0” o.c. or 3’-0” clearance outside to outside. Non-potable water pressures should be maintained
at 10psi or lower than the potable distribution pressure, and should never exceed 60 psi/8ft/sec in
any dual-plumbed structure. This method termed “pressure differential” ensures that non-potable
backflow will not occur in the event of an accidental cross-connection. All required potable makeup to a non-potable system can only be accomplished using an air gap separation, the distance of
which should be no less than twice the diameter of the potable supply pipe to the flood rim of the
non-potable receiving vessel. Backflow prevention devices and valves should additionally be
placed on all potable supply points which either provide make-up water to a non-potable system, or
supply potable fixtures within a non-potable zone.
All non-potable distribution lines should be labeled, color coded, and preferably constructed of
non-metallic materials wherever possible so as to be easily differentiated from the potable
distribution system. With the exception of the portions of piping connected directly to non-potable
fixtures, no other uncontrolled access to system should be available. Identification of all uses of
non-potable water within an occupied space or a space accessible to the public must be provided at
each prominent location of reuse. Prior to reuse systems certification and start-up, the on-site dual
distribution system must be filled, pressurized, and operated with potable water to ensure that no
potable fixture is operable or connected in any way to the non-potable distribution network. This
practice should be repeated immediately following any modification, repair, or servicing of either
potable or non-potable distribution systems. Finally, non-potable water should be dyed an
aesthetically pleasing, yet distinctive color to further reduce the potential of accidental misuse.
Water Quality Objectives and Standards
Treatment for recycled wastewater is typically categorized into (1) pre-treatment, (2) primary, (3)
biological, or secondary, and (4) advanced, or tertiary treatment. The level of treatment required is
predicated on the quality of grey or conventional wastewater to be treated and the water quality
objectives of the intended non-potable application. Primary treatment involves the screening,
filtration, and sedimentation of solids as well as and removal of mass organics in the form of
sludge. Further physical clarification is achieved by adding flocculents and coagulants to suspend
and bond finer particles for easier removal and total suspended solids (TSS) reduction. Primary
treatment is generally sufficient for direct greywater reuse and septic effluent for subsurface
irrigation and slow rate land application and infiltration. Secondary or “biological” treatment is
characterized by the removal of biodegradable organics, suspended solids, and pathogens passing
primary clarification. In this process, primary wastewater enters a resettling chamber where fine
solids are further flocculated and removed. Effluent then enters an oxygen rich, aerated chamber
where soluble organics are metabolized, resulting in a 90% reduction in biochemical oxygen
demand (BOD), or the amount of oxygen being used to degrade organic matter through biological
catalization. A lower BOD therefore indicates a reduced amount of organics present. Rigorous
secondary treatment and disinfection is generally accepted for the surface discharge and exterior
reuse of conventional wastewater.
Advanced Wastewater Treatment (AWT), approaching the quality and subsequent cost of potable
treatment, employs the use of a multiple barrier design. This tertiary process consisting of
redundant filtration and disinfection methods such as carbon adsorption, reverse osmosis, and
ozonation further removes trace pathogen and organic matter since the failure of one unit process to
remove a specific contaminant does not preclude effective overall treatment. Granular Activated
Carbon (GAC) is often associated with AWT. This unit process allows the adsorption of organic
and some inorganic compounds associated with undesirable water color and odor. The exceptional
“loading” capability of GAC is proportionate to its surface area relative to mass. It is estimated that
the extremely porous internal structure of GAC can provide tens of square meters of loading area
per gram of mass. Fifteen minute of true contact time in gravity fed or pressurized columns is
generally sufficient for fine particle removal.
Figure 6.
On-site recovery, treatment, and non-potable reuse cycling system for multiplex commercial and
residential structures.
Reverse osmosis is characterized by the use of spiral wound polymide membranes that act as a
physical barrier to the passage of pathogens, dissolved solids, nitrogen forms and other extremely
fine particles. Osmosis is a hydrodynamic force induced by the tendency of pure water to dilute a
contaminated source. The reverse osmotic process is initiated by forcing the pure water back
through a membrane composite with a force sufficient to overcome the natural osmotic pressure.
This unit process is also useful to remove chlorides for effective salinity and pH control. Ozonation
is a highly rigorous oxidation process which further alters organic compounds for disinfection.
Ozone, a highly reactive form of oxygen, is formed by passing pure oxygen through an
electromagnetic field, creating ozone along with very unstable oxygen radicals which readily
oxidize any organic or consumable matter present. UV and chemical enhanced ozonation form
hydroxyl free radicals and promoters which either destroy or condition contaminates to a state more
amenable to removal in subsequent multiple barrier processes. A final residual disinfectant process
concluding tertiary AWT is chlorination. Chlorine dioxide and other halogenated compounds such
as iodine form with water to form acids, destroying all remaining pathogens.
Reuse treatment achieves greatest benefit however, when the level of treatment is safely and
economically limited to the requirements of the wastewater source and the intended non-potable
application(s). Table 3 below identifies the recommended water quality objectives for both nonpotable and exterior wastewater reuse.
Table 3.
Recommended water quality objectives for non-potable interior and exterior wastewater reuse.
Condition
Biochemical Oxygen Demand, mg/L
Total Suspended Solids, mg/L
Chlorine Residual, mg/L
Fecal Coliform, per 100mL
Total Coliform, per 100mL
Turbidity, NTU
Nitrate, mg/L
Acceptable Limits
15-20
15
1
<2.2
3
2
<5
Condition
pH, LSI
Alkalinity, mg/L
Hardness, mg/L
Chlorides, mg/L
Chemical Oxygen Demand, mg/L
Ammonia, mg/L
Phosphate, mg/L
Acceptable Limits
5-8
250
150
<300
80-100
25
<5
In addition to the Type A system, other forms of greywater such as septic effluent and stormwater
can be harvested for limited treatment and non-potable reuse. The quality and quantity of available
greywater coupled the increasing costs of potable water and domestic sewer fees has made on-site
collection, treatment, and reuse of non-potable resources more attractive. Little or no treatment is
needed to reapply greywater to land where water is scarce. A typical residence of three can expect
to generate about 9,500cf of greywater a year, providing two inches of irrigation to a 1,000sf lawn
per week. If potable water use can be reduced by diminishing the use of potable water, and if
sewage loads are reduced 40-60% per residence, a substantial savings in municipal services and
treatment can be achieved. Figures 7 and 8 below illustrate water quality characteristics and
treatments for greywater reuse. A combination of plant and soil uptake, dilution, filtration, and
storage adequately accounts for all identified water quality objectives.
Figure 7. Water quality characteristics of domestic
greywater.
Figure 8. Water quality treatment of domestic
greywater.
Urban Water Conservation
There are currently a variety of products available on the market designed for water conservation.
Since the United States has the highest per capita use of water of any country in the world, the
variety of uses for water is not expected to drastically decrease as a result of continued overdevelopment and the myth of infinite water resources. Urban reuse coupled with water saving
fixtures may be more easily accepted by the public. Efficiency of water use has not previously been
the hallmark of fixture design. For example, the ratio of water to waste in a conventional flush
toilet is 80 to 1. It has been estimated that with the use of low cost, low water use fixtures, the
amount of water used in typical residential applications can be reduced by 19 to 44 percent. For
instance, alternative flush devices will reduce the amount of water used with each flush by as much
as 50%. Another category of water efficient toilet is the low volume toilet using 1.3 gallons per
flush. This is in compliance with the National Plumbing Efficiency Act now before congress which
would mandate all new construction to have such fixtures.
Flow rates of up to 4.5 gallons per minute are characteristic of conventionally engineered
showerheads whereas low-flow showerheads use 1.5 to 2.5 gallons per minute and do not lower
consumer preference in terms of acceptable performance. Low-flow showerheads are either aerated
or non-aerated. Non-aerated showerheads pulse the water while aerated showerheads mix air with
water while simultaneously maintaining pressure. It has been reported that a 16.4 % decrease in
water use occurred in a pilot program with the use of low-flow shower heads in a residential
development in Amherst, Massachusetts. In a Canadian study, it was found that using low-flow
heads in a 719 unit apartment building reduced water use by 53%. Low-flow faucet aerators can
reduce the water flow of the average kitchen or bathroom faucet's conventional rate of 3 gallons per
minute by 50 % or more.
Appliances such as washing machines and dishwashers can use more than 26 gallons per month
depending upon use. At the present time, there are not a great variety of water saving machines
available. Front loading washers, common in Europe, use up to one third less water than top
loading models favored by Americans. Maintaining adequate water pressure for residential areas is
also important for efficiency in the system but frequently water pressure is substantially higher than
is normally required. A pressure of between 50 to 60 psi for the mains and 40 psi inside a
residential unit is generally appropriate. Pressures twice the appropriate amount are not uncommon,
wasting water at every fixture within the distribution network. One method for conserving water is
the installation of a pressure reduction valve in the main water line, providing a equalized flow and
further reducing high pressure damage to fixtures and piping.
Conclusion
Urban water reuse and conservation are the primary alternatives that offer substantial economic and
environmental incentives for optimizing groundwater resources. Representative of many global
regions, Florida continues to experience a population increase of nearly 3.5% annually, primarily in
urbanized areas attributed to 60% of the state's population and nearly 70% of its groundwater
withdrawal. Use of groundwater resources will increase by a factor of 11 from 1950 to 2020, far
outpacing the rate of recharge provided by Florida's hydrologic cycle. In spite of an average rainfall
of 54 inches per year and profitable efforts to optimize limited water resources, withdrawal rates in
Florida will continue to spiral upward.
Of the commercial and residential structures that compose urban development, 70% use drinking
water for non-potable consumption. The 40%-60% potable resources dedicated for sanitary
functions in residential development, pales in comparison to the 90% figure identified in similar
commercial applications. 80% of the current urban water demand could be supplemented by
reclaimed or "greywater" technology. Type A and Type B greywater systems will achieve
maximum benefit in residential and commercial structures respectively, where the greywater supply
balances the non-potable demand for each. It has been estimated that the further use of low cost,
low water use fixtures, can reduce net flow 19 to 44 per cent. Finally, the common denominator for
environmentally sound water resource alternatives involves economics, which requires limiting
both potable demand and subsequent wastewater discharge.
Although constituents of
environmental concern, these factors pose intrinsic value to the water optimization scheme of reuse.
For if urban water reuse and conservation are to be considered a as viable alternative for water
resource restoration, it must be less expensive to reclaim an acre/ft of water than to import one.
Case Study: University of Florida Greywater Reuse System
The University of Florida is presently engaged in a pilot systems analysis using various reuse and
greywater systems. Storm water reclamation, greywater, and black water treatment are being
investigated as to their design and application in commercial and residential developments.
This work is being conducted in a residential dwelling complex that has been modified to test a dual
plumb system. Preliminary results show that this system will reduce potable flow and wastewater
discharge nearly 65% and will recover the initial capital cost increase in less than 2-3 years. These
systems will provide exceptional life-cycle water cost savings and water use surcharge waivers free
of use restrictions associated with lawn irrigation and aesthetic impoundments. These systems are
now in position for full-scale implementation and are anticipated to add tremendous marketability
for buildings and residential developments incorporating them.
References
Bouwer, Herman, 1978. Groundwater Hydrology. New York: McGraw-Hill Company.
Chansler, James M., 1991. "The Future for Effluent Reuse." Water Engineering and
Management. May: 14-19.
Heath, R.C. and C.S. Conover, 1981. Hydrological Almanac of Florida. Tallahassee: U.S.
Geological Survey.
National Association of Plumbing-Heating-Cooling Contractors, 1992. Assessment of Greywater
and Combined Wastewater Treatment and Recycling Systems. Falls Church: NAPHCC.
State of Florida, 1990. Florida Department of Environmental Regulation. Reuse of Reclaimed
Water. Tallahassee.
United States, 1991. Environmental Protection Agency. Municipal Wastewater Reuse. Washington,
D.C.