the sustainable development case for onsite ...

THE SUSTAINABLE DEVELOPMENT CASE FOR ONSITE WASTEWATER TREATMENT B. R. Bradley, G. T. Daigger, R. Rubin, G. Tchobanoglous*1

ABSTRACT The typical domestic wastewater treatment system is a centralized municipal-sized facility that treats wastewater to specified discharge limits, to protect human health and the environment. Yet 10 percent of wastewater generated in the U.S. is not treated in a centralized system, but rather in small systems receiving wastewater from single and multiple dwellings and small commercial establishments. Most of these small systems do not have discharge limits. Historically, and in the majority of cases today, onsite treatment employs technologies that represent a “lowest common denominator” approach. Frequently, the absolutely simplest, least cost, and least monitored systems are the only ones allowed. With so many recent technological advances in onsite treatment, it is appropriate to reassess its role. This paper presents a sustainable development context to define the value and role of onsite treatment. A sustainability assessment uses a broad range of criteria that accommodate changing demographics, values, and environmental resources. A framework of sustainability is developed to identify a reasonable set of social, economic, and environmental criteria for wastewater treatment. These criteria were applied to the conventional approach to onsite wastewater management, i.e. traditional septic systems permitted through health authorities, but without a monitoring or maintenance program. This evaluation produced defined social, economic, and environmental benefits and shortcomings. How shortcomings could be reduced or eliminated through technological advancements and changes to management of the systems was examined. One of the readily available advanced onsite systems, the textile filter pressure dosed dispersal system, was examined applying the same criteria. Keywords: sustainable development, sustainability analysis, textile filter, onsite wastewater treatment

INTRODUCTION Currently about 20 million households and businesses use septic systems in the United States. About 60 million people use some form of onsite wastewater treatment. These systems treat about 10 percent of the wastewater generated nationally. With populations growing in most areas of the country, the treatment percentage for onsite systems is also increasing. Most of these systems

*1 Barbara Bradley, Sustainable Development Engineering Manager, Nolte Associates, 15090 Avenue of Science, Suite 101, San Diego, CA 92128 (lead author, [email protected]). Dr. Glen T. Daigger, Senior Vice President and Chief Wastewater Engineer, CH2M HILL, Denver, CO. Dr. Robert Rubin, Professor, North Carolina State University, North Carolina. Dr. George Tchobanoglous, Professor Emeritus of University of California, Davis, CA.

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operate without performance monitoring. Generally these systems provide the intended health and environmental protection, but risks and impacts commonly occur. Concurrently, increases in natural resource consumption indicate that growing population and affluence negatively influence the availability of these resources. Biological resources are of particular concern worldwide because every major ecosystem in the world is in decline (Hawken, et al., 1999). In many countries, including the United States, this decline is occurring despite the soaring desire to preserve healthy ecosystems and wildlife populations. Evidence of the impacts by resource consumption varies geographically. For many, water resource limits pose real challenges that promise only to become more formidable with time. Future scenarios run the gamut, from depleting the water resources necessary to sustain ecosystems, to societies with diminished quality of life, to technological solutions and changes to consumption patterns that, with population restraint, deflect us from either of those dismal scenarios. The issue then is how society will develop to sustain the best possible outcomes. Within this context, the use of every natural resource merits examination. Technology, in all its forms, fuels the use of natural resources to improve the quality of human life. Technology affects the rate at which resources are consumed. Based on the current trend, improvements in technology inversely impact the availability of natural resources. Theoretically there is no reason why the impact could not reverse, i.e. technology would be used to improve – not deplete - the store of natural resources. In this paper, water quality and quantity are the natural resources in question with onsite wastewater treatment the technology examined. The value of any wastewater treatment system is measured by its ability to sustain the quality of human life, particularly human health, and the vitality of the environment.

SUSTAINABILITY CRITERIA FOR WASTEWATER TREATMENT SYSTEMS Sustainable development is a concept that has many definitions, which is sometimes confusing. Prior to evaluating onsite systems, a shared understanding of sustainable development is necessary. Definitions of Sustainable Development The following definitions and concepts are used to clarify what sustainable development is within the context of communities and onsite wastewater treatment. Sustain: to keep in existence without diminishing, to provide sustenance and nourishment. Development: to bring out the capabilities of and to bring to a more advanced or effective state. Development may be concurrent with growth, but the two are not synonymous. Carrying capacity: an ecological term used to mean the population that can be supported indefinitely by an ecosystem without destroying it. This term has been borrowed from the ecologists to explain the need to balance our development in a way that does not drain community capital or resources. Community capital: that legacy and evolving set of assets that comprises financial and built capital, human and social capital, and natural capital.


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Financial and built capital: includes our financial resources, economic systems, infrastructure, buildings, and manufactured goods. Human and social capital: includes our labor force, intellectual property, cultural wealth, political systems, and social virtues (e.g., public trust). Natural capital: includes the air, water, land, and web of living systems that supports people, including the functions that each provides, like the carbon dioxide sequestering function of the forest. Weak versus strong sustainability: Weak sustainability is an approach where the three categories of capital are considered interchangeable. For example, manufactured capital is considered of equal value to natural or intellectual capital and can replace either without changing a community’s total capital. Strong sustainability views the existing stocks of natural, human, and financial capital as necessary because the functions that each performs is unique and cannot be duplicated. Sustainable development is… “…development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” (World Commission on the Environment and Development, 1996) “…improving the quality of human life while living within the carrying capacity of supporting ecosystems.” (IUCN, et al., 1991 “…conservation-oriented social and economic development that emphasizes the protection and sustainable use of resources, while addressing both current and future needs, and present and future impacts of human actions.” (Border Environment Cooperation Commission, 1996) And, “Sustainable water resource systems are those designed and managed to fully contribute to the objectives of society, now and in the future, while maintaining their ecological, environmental, and hydrological integrity.” (American Society of Civil Engineers, 1998) In summary, sustainable development is about balancing our capital. It is a phrase used to describe a process for achieving development that simultaneously creates or preserves the vitality of society, economy, and the environment. The most fundamental definition of sustainable development for decision making is expressed by the mental model presented in fig. 1.


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Sustainable Solutions

Figure 1. Sustainable Solutions Address the Economy, Environment, and Social Equity Solutions in which social, environmental and economic needs are balanced occur in the intersecting area in the center. A corollary then is that an unsustainable community consumes resources faster than they can be renewed, produces more wastes than natural systems can process, and reduces the vitality and quantity of ecosystems. Unsustainable communities are also communities where sufficient numbers of people have an unacceptable quality of life, i.e. a lack of vitality that represents an unstabilizing force. The three community assets of the environment, economy, and social equity were used as the starting point for developing criteria that could be used to evaluate any type of wastewater treatment system. Each of these assets were interpreted as specific, applicable criteria to provide a meaningful basis for this evaluation. The proposed criteria are suitable for all wastewater treatment systems. The criteria should be quantifiable for use as long-term indicators of sustainability performance. As indicators, the criteria should accommodate the local context.

ONSITE SYSTEMS EVALUATED FOR SUSTAINABILITY The sustainable development criteria are used to evaluate two types of onsite wastewater treatment that are in operation today. The first is a conventional septic system, i.e. septic tank and leach field. The second system evaluated was a textile filter system with pressure dosed effluent dispersal. Conventional Septic System Conventional septic systems typically achieve the treatment levels shown in Table 1.


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Table 1. Comparison of Typical Wastewater in Collection Systems, Expected Residential Wastewater, and Expected Septic Tank Effluent Water Quality Parameter

Concentrations in Untreated Domestic Wastewater (mg/L), based on 454 L (120 . gal)/capita day

Concentrations in Untreated Residential Wastewater (mg/L), based on 189 L (50 . gal)/capita day

Septic System Effluent, Range and (Typical) Concentrations mg/L), based on 189 L (50 . gal)/capita day

Biochemical Oxygen Demand (BOD)

110-400 (250)

450

150-250 (180)

Chemical Oxygen /Demand (COD)

250-1,000 (500)

1,050

250-500 (345)

Total Suspended Solids (TSS)

100-350 (210)

503

40-140 (80)

20-85 (35)

nab

na

na

70

50-90 (68)

Organic Nitrogen as N

8-45 (13)

29

20-40 (28)

Ammonia (NH3)

12-50 (22)

41

30-50 (40)

4-15 (7)

17

12-20 (16)

50-150 (90)

164

20-50 (25)

Nitrogen (total as N) Total Kjeldahl Nitrogen as N

Total Phosphorus (P) Oil and Grease

a. Data compiled from Crites and Tchobanoglous, 1998. Effluent concentrations do not include treatment by the soil. b. na = not available

Textile Filter Treatment and Pressure Dosed Disposal System The second type of treatment system evaluated was a textile filter system with pressure-dosed dispersal. The system comprised a septic tank, effluent filter vault, textile filter, and shallow pressure-dosed soil absorption system. Either a single or two-compartment septic tank is used to remove settleable and flotable solids. The effluent filter vault retains residual solids that have not settled within the tank. Because the size of the solids in the effluent is limited, a multi-stage highhead turbine pump is feasible. Effluent from the septic tank is treated using a textile filter. The filter bed contains loosely placed textile material cut into two-inch squares. Effluent from the septic tank, applied to the surface of the textile filter, is treated biologically as it passes through the filter bed. A portion of the flow is recirculated for optimal treatment. Fig. 2 shows a schematic of the textile filter system. The effluent from the textile filter is applied to the soil using either a pressure-dosed shallow trench system or a drip irrigation system. Typically the distribution pipe is placed on the ground in a shallow excavated trench 15 to 30 cm (6 to 12 inches) deep. A half section of a larger pipe is


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placed over the distribution pipe and then covered with soil. If a drip irrigation system is used the distribution laterals are place approximately 15 cm (6 inches) below the ground. Figure 2 Schematic of Septic Tank with Textile Filter Installed at Habitat for Humanity 1, Lapine, Oregon. Source: Bounds, et al., 2000.

Textile Filter

Effluent Pumping Assembly in Filter Vault

2 Compartment 1500-gallon Septic Tank To Shallow Gravel-less Drain Field Drainfield Pump Basin

Table 2 shows effluent quality from six septic tanks with effluent screens and textile filters. Table 2. Typical Performance Data: Textile Filters Treating Screened Septic Tank Effluent Item

Unit

Range

Typical

Flow Rate

L (gal)/d

492-15,140 (130-4,000)

662 (175) b

BOD

mg/l

160-250

180

TSS

mg/l

0.4 – 3

2

TKN

mg/l

2–4

3

N03 –N

mg/l

3 – 25

< 10

TN

mg/l

5 – 29

< 20

FC

MPN/100 ml

100 - 1000

< 100

a. Adapted from data compiled by Bounds, et al., 2000 and Leverenz, 2000. b. For Single Family Residences


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WEIGHTING THE SUSTAINABILITY CRITERIA Sustainable development definitions vary according to the context in which it is applied. Even in the evaluation of onsite wastewater treatment systems presented in this paper, the relative weights for the sustainability criteria are affected by the values of the specific communities using the system (i.e., the social, economic, and environmental context). For example, environmental and climatic features, the neighborhood and other social factors, and the ability of the users to pay for the system and other economic factors affect the relative importance of each criterion. Therefore, it is necessary when applying these criteria to define the relative weight of each criterion. The three aspects of sustainability – social, environmental, and economic – are equally important, being non-exchangeable capital assets. However, it would be unlikely that the criteria developed within each category are equally important or of equal weight. Weights reflect the values and unique circumstances of the subject case, such as the local conditions described above. The criteria used to evaluate the treatment systems in this paper are shown in Table 3, along with the relative and normalized weights for each. These weights were assigned for illustrative purposes and would differ according to the specific application to reflect case-specific conditions. In this evaluation, the highest assigned weighting is ten and the lowest is one. The weights are normalized because the number of criteria per asset varies.

SUSTAINABILITY EVALUATION OF THE ONSITE TREATMENT AND DISPOSAL SYSTEMS The performance of the two systems was compared for every criterion. Each system was assigned a score, with five being the most desirable and one the least desirable. For this analysis, experience and judgment were used to establish the performance score. In other applications, more formalized practices can be used to assess performance. The final score per asset was normalized by dividing the score per asset by the number of assets. The performance and scores are provided in Table 3. The overall sustainability score for the conventional septic systems was 11.42 and 12.55 for the textile filter pressure-dosed dispersal system. These scores are relative to each other and are not meant to suggest an overall sustainability score for either of these systems as compared to some absolute score for sustainability (which does not exist), or as compared to other onsite systems or centralized collection and treatment systems. A detailed comparison of the two options suggests that a principal trade-off between the two systems is that the textile filter system increases initial installation and operations and maintenance costs, while producing a higher quality effluent that can be reused for subsurface landscape irrigation. Reuse of the effluent, in turn, produces the environmental benefits of reducing the discharge of pollutants (especially nutrients) to surface water and reusing the nutrients in the


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wastewater for the growth of landscape plants. Additionally, reuse of the effluent reduces the demand for water extractions from surface and ground water. For this particular example, the highest weighted social criteria are for protection of human health (weighted score of 10) and preservation of cultural traditions, ways of life, and physical heritage (weighted score of 9). Based on the analysis summarized in Table 3, it is easy to see that the application of the various criteria could result in tradeoffs when selecting a real system. However, that is a typical dilemma for treatment technologies and environmental infrastructure. The value of this type of decision making is that it is based on a balanced approach, providing equal importance to the three types of community capital. Given the long-lasting effects of environmental infrastructure, the sustainability analysis provides a basis for making credible tradeoffs.


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Table 3a

Sustainability Evaluation of the Septic System and Textile Filter-Pressure-Dosed Dispersal System Criteria

Crit. Weight

Performance

Rel.

Norm

Septic System

Score

Textile Filter-Pressure-Dosed Dispersal

Score

The treatment system protects public health.

10

.21

Under circumstances where coliform do not enter ground water used for consumption and if the septic tank is operating well and the leach field is not blinded, septic systems provide excellent treatment. Risk for non-performance may be high but is frequently unknown.

3.5

Textile filters produce much higher effluent quality and less likelihood of blinding the dispersal field. Risk is associated with reliance on homeowner to maintain the system.

4

Promotes societal virtues such as the public trust

6

.13

The consumer generally has little understanding of how the system works and is not supported by health agencies or others for assurance of longterm performance.

3

Consumer understanding generally low and is not supported by health agencies or others for assurance of long-term performance.

3

Preserves cultural traditions, ways of life, and physical heritage

9

.19

Allows for dispersed human settlement and opportunities for rural lifestyles and livelihoods. Promotes traditional dispersed land use pattern.

5

Allows dispersed human settlement, opportunities for rural lifestyles and livelihoods. But allowable smaller lot sizes may erode traditional way of life.

2.5

Community makes informed decisions. Actions reflect local values through a public process in which the public has a sense of ownership over the decision making.

7

.15

The consumer is the manager of the septic system and therefore has greatest sense of ownership for this type of treatment system. However, limits on allowable treatment systems and reuse of the treated effluent create limits act as limits to the sense of ownership.

4

The consumer is the manager of the textile system and therefore has greatest sense of ownership for this type of treatment system. Increased siting flexibility and reuse potential promote a sense of ownership.

5

Preserves aestheticallyvalued environments (beauty, open space, recreation, wildlife viewing pleasure). No olfactory or audible degradation

8

.17

Septic systems generally promote large lots and more open space. Unsuitable soils with high rainfall will produce odors. Degradation of aquatic environments may reduce aesthetic quality

4

Does not require as large a disposal area if the subsurface assimilative capacity is adequate. There is less likelihood that unsuitable soils with high rainfall will produce odors or degrade aesthetic quality of aquatic environment Increased housing density may reduce visual aesthetics.

4

Ability of all community members to attain highest potential as appropriate natural resource-based development

8

0.17

Traditional septic system promotes low density and rural lifestyles but cannot support high density development. System does not provide opportunities to reuse water for added advantage.

3

Textile systems promote low density and rural lifestyles, but can support higher density development. System provides reuse opportunities for landscaping.

Social Criteria

Overall weighted score


Performance

3.87


4.5

3.90

Table 3b

Sustainability Evaluation of the Septic System and Textile Filter-Pressure-Dosed Dispersal System Criteria

Crit. Weight

Performance

Rel.

Norm

Septic System

Ability of most community members to fund the costs for implementing the system.

8

0.19

The capacity of the community to finance the necessary capital improvement, considering initial and final population served.

10

The capacity of the community to finance the necessary system operation and maintenance, considering initial and final population and time varying demands.

Performance Score

Textile Filter-Pressure-Dosed Dispersal

Score

Conventional on-site systems are widely used, widely available, and of modest cost. Most cost goes to trenching and equipment installation.

5

Modest cost but more than conventional systems; higher costs for design & equipment are somewhat offset by lower trenching costs.

4

0.24

Conventional on-site systems are widely used, widely available, and of modest cost. Since individual systems are installed for each housing unit, no up-front capital investment in advance of population growth is required.

5

Textile systems are not used widely, because they have only been introduced over the past three years. Textile systems are of modest cost. With individual systems installed at each housing unit, no up-front capital investment in advance of population growth is required.

5

9

0.21

Principal operation is automatic, not requiring the resident’s involvement. Principal maintenance activity is to pump septic tank. Septic tank pumpers are generally in business in most localities. Principal challenge is septage disposal, which is either by discharge to a centralized wastewater treatment plant or to the land. In this case, septage is pumped and discharged to the centralized treatment plant.

4

Principal operation is automatic, not requiring the resident’s involvement. Principal maintenance activity is to annually rinse the biotube filter and to periodically pump the contents of the septic tank. In the event of a pump failure, alarm will sound and pump must be maintained, usually by a qualified repairman. Principal challenge is septage disposal, which in this case, septage is pumped and discharged to the centralized treatment plant.

3.5

The capacity of the community to finance the necessary long-term repair and replacement of the system.

7

0.17

Long-term repair /replacement is responsibility of home owner. Typical home owner does not plan for the expenditures, often has the financial resources. This cost usually associated with blinded disposal field or leaking tank.

3

Long-term repair/ replacement is the responsibility of the home owner. Typical home owners do not plan for such expenditures, but often has the financial resources. Blinding of disposal field is less likely so this cost would occur less often.

4.5

The system supports the explicitly stated community economic development objectives.

8

0.19

Septic systems are generally modest cost, but they produce no recreational or water resource benefits

Economic Criteria



2.5

3.98

Textile systems are generally modest cost, producing several potential financial benefits if water reused: (1) landscape improvements enhance property value, (2) reduce water bill, (3) irrigation of food crops offset food costs, and (4), irrigation of crops for economic gain. Overall weighted score

5

4.41

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Table 3b Criteria

Sustainability Evaluation of the Septic System and Textile Filter-Drip Dispersal System Crit. Weight

Performance

Environmental Criteria

Rel.

Norm.

Septic System

Surface water quality and quantity

9

0.16

Assumed adequate distance from surface water to attenuate water quality impacts, and the septic system works well. The septic system does not promote conservation and the permit conditions essentially eliminate reuse.

Ground water quality and quantity

8

0.14

With adequate distance to ground water to attenuate water quality impacts, the septic system works well. The septic system promotes recharge of ground water.

Aquatic ecosystems

10

0.18

Land-based ecosystems

10

Soil quality

Performance

Score

Textile Filter-Pressure-Dosed Dispersal System

Score

3

When limited distance to attenuate water quality impacts, the textile system works well because of the added treatment provided by the textile filter. The system promotes conservation through root-zone reuse in the shallow trenches.

5

4.5

Works well to attenuate water quality impacts. Promotes groundwater recharge, promotes water resource conservation through root-zone reuse.

5

With adequate distance from surface water, septic systems adequately protect aquatic ecosystems. However, septic systems typically do not promote conservation and the permits do not allow reuse, both of which would reduce impacts by water withdrawals.

3.5

With adequate distance from surface water, system provides significant protection of aquatic ecosystems. Textile systems promote conservation and reuse. Subsurface reuse is feasible within most permits

4.5

0.18

Typically, systems do not promote conservation, permits do not allow reuse, both of which would reduce impacts by water withdrawals. Systems allow for urban development, may promote urban sprawl.

1.5

Textile systems promote conservation through reuse which will reduce impacts by water withdrawals. Textile systems allow for urban development, which may promote urban sprawl.

3

7

0.13

Septic systems may promote salt accumulation soil. Leach lines may clog with bioslimes over time but the problem is localized. The pH is normally not altered unless greywater only is dispersed in the leach lines.

3.5

May promote accumulation of salts in soil. If drip lines used, salt buildup is less; treated effluent is dispersed over larger area. Drip lines may clog with bioslimes but can be designed to be self cleaning.

3.5

Air quality

6

0.11

Under normal operating conditions, septic systems do not cause odors. Absence of aerators and exposure to the atmosphere mean that toxic emissions and biosols are negligible.

5

Under normal operating conditions, textile systems do not cause odors. A fan is operated in the textile filter box but the low velocity and enclosed box provide little opportunity for biosol emissions. Some emissions of household toxics may occur intermittently and probably at low-risk levels.

5

Energy use

6

0.11

Normally no energy use except gravity.

5

Textile filter fan, pump consume energy.



3.57


3.5 4.24

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CONCLUSIONS Sustainable development incorporates social, economic, and environmental factors into the evaluation and selection of wastewater management options. An assessment approach was developed and applied to evaluate two systems using these factors. The approach can be applied in other settings by adjusting the weighting of the evaluation criteria to fit local conditions. The following are specific conclusions: •

A relatively uniform framework can be developed for evaluating the sustainability of wastewater management. The approach can be used to provide credible decisions and tradeoffs.

•

The sustainability of various wastewater management options can be compared when criteria are identified and weighted and performance measures selected that fit the specific conditions.

•

The criteria used to assess sustainability are unique to the specific application. A site-specific definition must be developed for each application. The site-specific definition for a particular application is achieved by adjusting the weighting factors for the individual criteria.

•

As new and improved onsite wastewater treatment technologies are developed, more wastewater management options will offer greater sustainability. These management options will improve the sustainability of onsite systems even further through increased reliability and flexibility.

•

Overall, advanced onsite wastewater systems, such as the textile filter with pressure-dosed disposal, offer a higher level of sustainability to users, the community, and the environment. At the same time, reductions in sustainability may occur because such systems will allow for higher housing densities in rural settings.

REFERENCES 1. American Society of Civil Engineers and UNESCO/IHP, 1998. Sustainability Criteria for Water Resource Systems, ASCE in Reston VA. p 44. 2. Border Environment Cooperation Commission, 1996. Project Certification Criteria published by the Border Environment Cooperation Commission, Ciudad Juarez, Mexico. November 9, 1996. 3. Bounds, et. al, 2000. Performance of Packed Bed Filters in Proceedings of Southwest OnSite Wastewater Management. Arizona Environmental Health Assn. Laughlin, NV. p 6-10. 4. Crites R., and G. Tchobanoglous, 1998. Small and Decentralized Wastewater Management Systems published by WCB McGraw-Hill, Boston, MA. p180-183. 5. Fodor, E., 1999. Better not Bigger, published by New Society Publishers, Gabriola Island, B.C., Canada. 6. Hawken, P., H. Lovin, and A. Lovin, 1999. Natural Capitalism, Little Brown and Company, Boston, MA. 7. Leverenz, H., 2000. Performance Evaluation of Textile Filter For Treatment of Septic Tank Effluent. Unpublished Masters Thesis, University of California, Davis, CA. 8. Parks, J., 2000. Personal communication with Jerry Parks of the Albermarle Regional Health District, Elizabeth City, NC. 9. Pinkney Brothers Engineering, 2000. Personal communication with Pinkney Brothers Engineering, Nashville, TN.


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10. Price, D., 2000. Personal communication with Dick Price, P.E., manager of Stevens County Public Utility District, PO Box 592 Loon Lake, WA. 11. Rocky Mountain Institute, 1998. Green Development: Integrating Ecology and Real Estate published by John Wiley and Sons, Inc. New York, NY. 12. US EPA, 1998. Clean Water Action Plan – Restoring and Protecting America’s Waters, Washington DC. 13. Water Environment Research Foundation, Northeast Consulting Resources, 1999. “Industry Endstates for WERF Future Mapping Workshop”, Chantilly, VA. Jan. 15 - 16, 1999.


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