Reclaimed Wastewater as a Resource | Wastewater Management

In this article we will discuss about:- 1. Introduction to Wastewater Reclamation 2. Categories of Wastewater Reuse 3. Elements of Wastewater Reuse Planning 4. Wastewater Reclamation Technologies 5. Health and Regulatory Considerations 6. Assessment of Safety of Wastewater Reuse Practices 7. Cost of Wastewater Reclamation and Reuse 8. The Future of Water Reuse.

Takashi Asano, Ph.D.

Department of Civil and Environmental Engineering University of California at Davis Davis, CA 95616, USA

Abstract:

For the last quarter century, a repeated thesis has been that advanced treatment of municipal and industrial wastewater provides a treated effluent of such high quality that it should not be wasted but put to beneficial use. This conviction in responsible engineering, coupled with the vexing problem of increasing water shortage and environmental pollution, provides a realistic framework for considering reclaimed wastewater as a water resource in many parts of the world.

In this paper, fundamental concepts of wastewater reclamation and reuse are developed that include categories of water reuse, planning methodologies, economics for water reuse, and technological innovations for the safe use of reclaimed wastewater. The paper emphasizes the integration of this alternative water supply into water resources planning.

In every wastewater reclamation and reuse operation, however, there is some risk of human exposure to infectious agents. Because of this concern, special attention is paid to tertiary/advanced wastewater treatment systems which produce essentially pathogen-free effluent for variety of beneficial uses such as toilet-flushing water in large commercial buildings; irrigation of parks, school yards, and golf courses; and groundwater recharge.

The paper also reviews the emergence of modern wastewater reclamation and reuse practices from wastewater to reclaimed water to re-purified water, and to discuss cost of wastewater reclamation and reuse.

Introduction to Wastewater Reclamation:

Wastewater reclamation and reuse is one element of water resources development and management which provides an innovative and alternative option for agriculture, munici­palities, and industries. The water pollution control efforts in many countries have made treated effluent from municipal wastewater available that may be an economical augmenta­tion to the existing water supply when compared to the increasingly expensive and environ­mentally destructive new water resources development.

However, wastewater reuse is only one alternative in planning to meet future water resources needs. Water conservation, water recycling, efficient management and use of existing water supplies, and new water re­sources development are examples of other alternatives.

Wastewater reclamation and reuse involves considerations of public health and also requires close examinations of infrastructure and facilities planning, wastewater treatment plant siting, treatment reliability, economic and financial analyses, and water utility man­agement involving effective integration of water and reclaimed wastewater.

Whether wastewater reuse will be appropriate depends upon careful economic considerations, poten­tial uses for the reclaimed water, stringency of waste discharge requirements, and public policy wherein the desire to conserve rather than develop available water resources may override economic and public health considerations.

Through integrated water resources planning, the use of reclaimed wastewater may provide sufficient flexibility to allow a water agency to respond to short-term needs as well as increase long-term water supply reliability without constructing additional storage or conveyance facilities at substantial economic and environmental expenditures. Thus, wastewater reuse has a rightful place and an important role in optimal planning and more efficient management and use of water resources in many countries.

Wastewater reclamation is the treatment or processing of wastewater to make it reusable, and water reuse is the use of treated wastewater for a beneficial use such as agricultural irrigation and industrial cooling. In addition, direct wastewater reuse requires existence of pipes or other conveyance facilities for delivering reclaimed water.

Indirect reuse, through discharge of an effluent to a receiving water for assimilation and withdraw­als downstream, is recognized to be important but does not constitute planned direct water reuse. In contrast to direct water reuse, water recycling normally involves only one use or user and the effluent from the user is captured and redirected back into that use scheme. In this context, water recycling is predominantly practiced in industry such as in pulp and paper industry.

Figure 1 shows, conceptually, the quality changes during municipal use of water in a time sequence. Through the process of water treatment, a drinking water is produced which has an elevated water quality meeting applicable standards for drinking water. The munici­pal and industrial uses degrade water quality, and the quality changes necessary to upgrade the wastewater then become a matter of concern of wastewater treatment.

Fig 1. Quality changes during municipal use of water and the concept of wastewater reclamation and reuse.

In the actual case, the treatment is carried out to the point required by regulatory agencies for protection of other beneficial uses. The dashed line in Fig. 1 represents an increase in treated wastewater quality as necessitated by wastewater reuse. Ultimately as the quality of treated wastewater approaches that of unpolluted natural water, the concept of wastewater recla­mation and reuse is generated.

Further advanced wastewater reclama­tion technologies, such as carbon adsorption, advanced oxidation, and reverse osmosis, will generate much higher quality water than conventional drinking water, and it is termed re-purified water. Today, technically proven wastewater reclamation or purification proc­esses exist to provide water of almost any quality desired.

Categories of Wastewater Reuse:

In the planning and implementation of wastewater reclamation and reuse, the intended wastewater reuse applications govern the degree of wastewater treatment required and the reliability of wastewater treatment processing and operation. In principle, wastewater or any marginal quality waters can be used for any purpose provided that they meet the water quality requirements for the intended use.

Seven categories of reuse of municipal wastewater are identified in Table 1, along with the potential constraints. Large quantities of reclaimed municipal wastewater have been used in four reuse categories; agricultural irrigation, landscape irrigation, industrial recycling and reuse, and groundwater recharge.

In California, where the largest number of wastewater reclamation and reuse facilities has been developed, at least 432 million cubic meters (10⁶ m³) of municipal wastewater are beneficially used annually. This figure corresponds to around 8 per cent of municipal wastewater generated in the State.

In Japan, there were 876 publicly owned treatment works (POTWs) operating in 1991, discharging approximately 110 × 10⁶ m³/year of secondary treated effluents. Of these, approximately 100 × 10⁶ m³/year of reclaimed wastewater from 99 POTWs are reused beneficially in such uses as industrial uses (41%), environmental water and flow augmen­tation (32%), agricultural irrigation (13%), non-potable urban use and toilet flushing (8%), and seasonal snow-melting and removal (4%).

Contrary to the arid or semi-arid regions of the world where agricultural and land­scape irrigation are the major beneficial use of reclaimed wastewater, wastewater reuse in Japan is dominated by the various non-potable, urban uses such as toilet flushing, indus­trial use, stream restoration and flow augmentation to create so-called “urban amenities.”

Figure 2 shows the comparative diagrams for Japan and California, depicting the various reclaimed water uses and the corresponding volumes per year. Recycling of industrial wastewater within factories, as well as wastewater reuse within the POTWs is not included in these diagrams.

Agricultural and landscape irrigation is the largest current and projected use of reclaimed wastewater in the United States. Irrigation uses can offer significant opportunity for wastewater reuse since, in many arid and semi-arid regions, 70 to 90 per cent of applied water is used in irrigation.

Much of the attention focused on reclaimed water over the last decade has been for its use in the urban environment, such as for landscape irrigation, and its potential for groundwater recharge. Nonetheless, the historical application for agricul­tural purposes continues to dominate in California, for example, amounting to 63 per cent of the total reclaimed water used in 1987. At least 20 different food crops were irrigated with reclaimed water, as well as at least 11 other crops and nursery products.

The largest industrial application of reclaimed water in California was for paper manufacturing. Other significant industrial uses were power plant cooling, watering of log decks, and cooling water in a steel manufacturing plant.

Elements of Wastewater Reuse Planning:

The trends and motivating factors in wastewater reclamation and reuse are character­ized as follows:

i. Water pollution abatement in receiving waters.

ii. Availability of highly treated effluents for various beneficial uses due to stringent water pollution control requirements.

iii. Providing long-term reliable water supply in nearby communities.

iv. Water demand and drought management in overall water resources planning.

v. Public policy encouraging water conservation and wastewater reuse.

A first step in water reuse planning is to determine whether a wastewater reuse project is for predominantly water supply or water pollution control. The role of reclaimed wastewater in water resources planning has become much more important in the last decade. Water demands often exceed reliable water supplies, even in normal precipitation years, and new water resources development is increasingly costly and environmentally often prohibiting.

Reclaimed water is, after all, a water resource existing right at the doorstep of the urban environment and a reliable source of water even in drought years that is capable of replacing potable water supplies for non-potable and sub-potable water uses.

Planning for water reuse normally evolves through three stages:

i. Conceptual planning.

ii. Feasibility investigation.

iii. Facilities planning.

During conceptual planning, a potential project is sketched out, rough costs are estimated, and a potential reclaimed water market is identified. If the concept appears worthwhile, a preliminary feasibility investigation takes place.

The preliminary feasibility investigation consists of:

i. Performing a market assess­ment for reclaimed wastewater,

ii. Assessing the existing water supply wastewater facilities and developing some preliminary alternatives,

iii. Developing or identifying the alternative non reclamation facilities, such as wastewater treatment for stream discharge or construct­ing dams and reservoirs for future water supply, with which to compare a proposed water reuse option, and

iv. Performing a preliminary screening of wastewater reuse alternatives to consider technical, economic and financial attractiveness, and other constraints such as public health protection.

Based on the preliminary feasibility investigation, if wastewater reclamation and reuse appear viable, then detailed actual planning can be pursued, refined facilities alterna­tives developed, and a final facilities plan proposed.

A key task in planning a wastewater reclamation project is to find potential customers who want and know how to use reclaimed wastewater for their applications. Whether a user is capable of using reclaimed wastewater depends on the quality of effluent available and its suitability for the type of use involved.

Although technical, environmental, and social factors are considered in project plan­ning, monetary factors usually override other issues when decisions are made about whether and how to implement a water reuse project. Monetary analyses fall into two categories: economic analysis and financial analysis.

Economic analysis focuses on the value of the resources invested in a project to construct and operate it, measured in monetary terms and computed in the present value. The basic result of the economic analysis is to answer the question: Should a water reuse project be constructed?

The financial analysis addresses whether a water reuse project is financially feasible. The project sponsor will need a source of capital and sources of revenue to pay for debt service and operation costs for both the proposed wastewater reuse project and any existing facilities. Thus, the equally important question to answer in the financial analysis is; can a water reuse project be constructed?

A common misconception in planning for wastewater reuse is that reclaimed wastewater represents a low-cost new water supply. This assumption is generally true only when wastewater reclamation facilities are conveniently located near large agricultural or indus­trial users and when no additional treatment is required beyond the water pollution control facilities from which reclaimed water is delivered.

The conveyance and distribution systems for reclaimed water represent the principal cost of most proposed water reuse projects. Recent experience in California indicates that approximately four million dollars in capital cost are required for each one million m³ per year of reclaimed water made available for reuse. Assuming a facility life of 20 years and a nine per cent interest rate, the amortized cost of this reclaimed water is $0.5/m³, excluding O & M costs.

Facilities Planning Report:

The results of the completed planning effort should be documented in a facilities planning report on wastewater reclamation and reuse. A suggested report outline is pre­sented in Table 2 which also serves as a checklist for the planning processes. Facilities design is the next logical step. Equally important, however, is securing users to take reclaimed water once it becomes available. Detailed considerations for the user contracts can be found in Asano and Mills, 1990.

Other planning factors of particular importance are engineering and public health. Engineering involves more than water distribution system design. A water reuse project is a relatively small-scale water supply project that includes matching water supply and demand, appropriate level of wastewater treatment, reclaimed water storage, and supple­ment or backup freshwater supply.

Wastewater Reclamation Technologies:

The importance of tertiary treatment consisting of chemical coagulation, flocculation, sedimentation, and filtration processes has been demonstrated as a conditioning step in wastewater reclamation by removing particles and turbidity for effective disinfection as well as an esthetic enhancement towards “sparkling clean” reclaimed wastewater (cf. Fig.1).

Presently, more than 50 tertiary treatment facilities are in operation in conjunction with wastewater reclamation and reuse in California to meet the most stringent water quality requirement of virtually pathogen-free reclaimed wastewater.

To illustrate the applicable technologies adopted in the wastewater reclamation and reuse, two landmark studies—the Pomona Virus Study and the Monterey Wastewater Reclamation Study for Agriculture—are reviewed in this section.

Reference Treatment Process—Full Treatment Process (Title 22 Process):

The most stringent treatment process specified in the California Wastewater Recla­mation Criteria (1978) is the full treatment process (Title 22 Process) shown in Fig. 3A. Although this process is economically feasible, it is costly due to the expenses associated with relatively high doses of coagulant chemicals (50-125 mg/L alum, 0.2 mg/L anionic polymer), sludge handling, and tertiary sedimentation tanks.

Thus, considerable efforts have been directed toward the development of less costly tertiary treatment alternatives that produce effluent quality comparable to that of the full treatment process.

Alternative Tertiary Treatment Process in the Pomona Virus Study:

The first effort to investigate alternative treatment processes was made at the County Sanitation Districts of Los Angeles County’s Pomona Research Facility during 1976-77; thus, known as the “Pomona Virus Study” (1977). The study compared the attenuated poliovirus inactivation and removal capabilities of an alternative tertiary process—contact filtration (see Fig. 3C)—to the specified full treatment process (Title 22 processes).

Figure 4 shows the results of poliovirus inactivation and removal in the tertiary treatment processes depicted in Fig. 3. When high chlorine residuals of approximately 10 mg/L were used, there was no difference in the overall removal or inactivation of the seeded poliovirus between the full treatment process and the contact filtration process.

Fig 4. Results of poliovirus inactivation and removal in tertiary processes.

When low chlorine residuals of approximately 5 mg/L were applied, a slight difference of 5.2 log removal vs. 4.7 log removal was observed. The log removal refers to the fraction of poliovirus remaining after treatment; thus, one log removal is equivalent to 90 per cent removal and five log removal is 99.999.

Comparative Treatment Studies in the Monterey Wastewater Reclamation Study for Agriculture (MWRSA):

The MWRSA was a six-year (1980-86), $7.2 million field-scale project designed to evaluate the safety and feasibility of irrigating food crops (many eaten un­cooked) with reclaimed municipal wastewater.

The two alternative tertiary treatment processes—direct filtration (see Fig. 3B) and contact filtration (see Fig. 3C)—were studied for the removal of enteric viruses. These direct and contact filtration processes are typically operated with a small quantity of alum addition in the range of 2-5 mg/L and chlorine disinfection with 5-10 mg/L chlorine dose and 1.5 hour contact time.

(a) Full Treatment (“Title 22”).

(b) Direct Filtration.

(c) Contact Filtration.

Enteric viruses were monitored in MWRSA for the presence of naturally occurring animal viruses in influents to and effluents from the full treatment process and the two alternative filtration processes. During the six year field study period, no enteric viruses were detected in the chlorinated effluent of either the full treatment or direct filtration processes.

A total of 186 m³ and 160 m³ were sampled from the full treatment process and direct filtration process, respectively. The unchlorinated secondary effluent (prior to tertiary treatment) contained measurable enteric viruses 80 per cent of the times sampled, averaging 2,200 viral units (vu) per 100 liters with a range of 100 to 73,400 vu/100 L.

As a result of the Pomona Virus Study and MWRSA, the California Department of Health Services has adopted direct or contact filtration as an acceptable alternative, provid­ing certain design criteria are met. Consequently, almost all of the tertiary treatment plants designed in recent years to meet the full treatment process requirements, specified in the Wastewater Reclamation Criteria, use direct or contact filtration process.

Health and Regulatory Considerations:

In every wastewater reclamation and reuse operation, there is some risk of human exposure to infectious agents. The contaminants in reclaimed wastewater that are of health significance may be classified as biological and chemical agents. For most of the uses of reclaimed wastewater, pathogenic organisms pose the greatest health risks, which include bacterial pathogens, helminths, protozoa, and viruses.

To protect public health, considerable efforts have been made to establish conditions and regulations that would allow for safe use of reclaimed wastewater. Although there is no uniform set of standards existing, several international, national, and state wastewater regulations have been available.

Although these wastewater reclamation guidelines and regulations lack explicit epidemiological evidence on which to base an assessment of health risks, they have been adopted, nonetheless, as the attainable and enforceable regulations in the plan­ning and implementation of wastewater reclamation and reuse projects.

For example, the State of California’s Wastewater Reclamation Criteria (1978) requires that reclaimed water used for landscape irrigation of areas with unlimited public access must be “adequately oxidized, filtered, and disinfected prior to use,” with median total coliform count of no more than 2.2/100 mL. To achieve these requirements, it requires the wastewater treatment processes consisting of biological secondary treatment and terti­ary treatment with filtration followed by disinfection.

Table 3 presents a summary of the California requirements for reclaimed wastewater used for irrigation and recreational impoundments. Note that there are many reuse applica­tions that do not require a high degree of wastewater treatment.

The California Wastewater Reclamation Criteria (commonly known as the Title 22 regulations) are basically health regulations, thus, the Criteria do not specifically address the treatment technology or the potential effect of reclaimed water on the crops or soil.

The median number of total coliform count and turbidity are used for the assessment of treatment reliability of wastewater reclamation plant. The criteria is being revised and expanded to accommodate detailed regulations on groundwater recharge, cooling towers, and in-building applications includ­ing toilet flushing.

Table 3. Summary of the California Waste water Reclamation Criteria.

Further safety measures for non-potable water reuse applications include:

(1) Installa­tion of separate storage and distribution systems of potable water,

(2) Use of colour-coded labels to distinguish potable and non-potable installation of the pipes,

(3) Cross-connection and backflow prevention devices,

(4) Periodic use of tracer dyes to detect the occurrence of cross contamination in potable supply lines, and

(5) Irrigation during off hours to further minimize the potential for human contact.

Assessment of Safety of Wastewater Reuse Practices:

Despite a long history of wastewater reclamation and reuse in many parts of the world, the question of safety of wastewater reuse is discussed based on two recent studies by Asano, (1992), and Tanaka, (1993) on the enteric virus risk assessment.

When treated municipal wastewater effluents are used in urban environments where there is a strong possibility of direct human contact, considerable health concerns may be justified. These health concerns are specifically directed, in the industrialized countries with high health standards, to control enteric viruses.

The risk of virus infection from exposure to reclaimed municipal wastewater was determined by applying risk assessment procedures to existing data on viral concentrations in treated wastewater.

A database was developed using published reports from water and wastewater agencies in California and included enteric virus data from 424 unchlorinated secondary effluent samples in which 283 samples (67%) were virus positive and 814 chlorinated tertiary (filtered) effluent samples with 7 positive samples (1%).

Quantifying the virus concentration (expressed as viral unit, vu, per litre) in the treated effluent was the first step for estimating the risk. Virus concentrations reported in unchlorinated activated sludge effluents and in chlorinated tertiary filtration effluents were evaluated in the risk analysis.

For the first risk analysis run, the geometric mean and the 90 percentile values for enteric viruses found in unchlorinated activated sludge effluents were used and 5-log removal (99.999%) of viruses was assumed in tertiary filtration and chlorine disinfection.

For the second run, two computer simulations used the virus concentrations of 0.01 vu/L and 1.11 vu/L from the chlorinated tertiary filtration effluents, which are reasonable estimates of the detection limit for enteric viruses and the maximum concentration found in tertiary effluents.

The estimates of risk of infection, expressed as annual risk, are shown in Table 4 for different wastewater reuse situations. The overall probability of infection due to ingestion of viruses is a combination of virus removal and inactivation by wastewater treatment, die- off in the environment, and dose-response.

For each exposure scenario presented, the range of risks covers 2-3 orders of magnitude depending on the degree of infectivity associated with individual groups of viruses.

To evaluate the safety of wastewater reclamation and reuse, the U.S. Environmental Protection Agency’s Surface Water Treatment Rule (EPA SWTR) was used as a point of reference. Acceptable risks for this evaluation were defined as meeting the 10^-4 infection risk criterion at least 95 per cent of the time, as well as by the expectation estimate using Monte Carlo methods.

For golf course and food crop irrigation, and groundwater recharge, the reliability of wastewater reclamation and reuse is such that more than 95 per cent of the time the criterion was met for all of the effluents examined. However, for recreational impoundments, the reliability of wastewater reclamation is not always as high as the use of drinking water supply specified in the EPA SWTR.

Table 4. Annual Risk of contracting at least. One Infection from Exposure to Reclaimed Wastewater at Two Different Enteric Virus Concentrations.

The goal of virtually pathogen-free reclaimed wastewater contained in California’s Wastewater Reclamation Criteria should not be interpreted to mean that the practice of using such water is risk-free. As Table 4 clearly shows, there is always some risk of infection due to exposure to reclaimed wastewater. However, this does not mean that the practice of wastewater reclamation and reuse is unsafe.

Cost of Wastewater Reclamation and Reuse:

To estimate the cost of the tertiary treatment system, several sources were used to determine costs from the published literature.

The costs for both the “Title 22 process” and “direct filtration process” were estimated and shown in Table 5.

The cost breakdown in one instance indicated that incremental tertiary treatment costs (chemical addition, filtration, solids treatment) were estimated to be only $0.06/m³ ($79/af) while distribution costs, administrative charges (accounting, monitoring, overhead), and replacement reserve fees were projected at $0.12/m³ ($ 142/af), $0.04/m³ ($54/af), and
$0.04/m³ ($50/af), respectively.

The critical importance represented by labour and energy costs in the water reuse system is noted. The ratios of tertiary treatment costs for the “Title 22” treatment train to the ‘direct filtration’ train ranges from 2.0 to 2.4 for capital cost, 3.9 to 5.6 for O & M cost, and 2.4 to 2.9 for life cycle cost for the treatment capacities ranging from 3,785 m³/d to 37,854 m³/d.

However, there is danger in comparing cost data from different studies and locations because of differing underlying assumptions, which often are not explicitly stated. As seen from the data shown in Table 5, costs are significantly affected by fraction of utilization of a facility over the course of a year.

Economic assumptions of useful lives and interest rates affect the amortization of capital costs embedded in unit costs. Reported costs may repre­sent current expenses for old facilities and do not reflect costs to construct those facilities at today’s prices as seen in the Irvine Ranch Water District in California.

One factor which appears to significantly affect costs is the degree of utilization of available capacity in the treatment plant.

Maximum utilization can be achieved by:

(1) Seasonal storage of effluent to compensate seasonal slack in water reuse demands,

(2) Obtaining a mix of reclaimed water uses to reduce seasonal demands, or

(3) Using alterna­tive water supplies for meeting peak demands.

The Future of Water Reuse:

Significant progress has been made with respect to developing sound technical ap­proaches to producing a quality and reliable water source from reclaimed wastewater. Continued research and demonstration efforts will result in additional progress in the development of water reuse applications.

Some key topics include assessment of health risks associated with trace contaminants in reclaimed water; improved monitoring ap­proaches to evaluate microbiological quality; optimization of treatment trains; improved removal of wastewater particles to increase disinfection effectiveness; the application of membrane processes in production of reclaimed water; the effect of reclaimed water storage systems on water quality; evaluation of the fate of microbiological, chemical, and organic contaminants in reclaimed water; and the long-term sustainability of soil-aquifer treatment systems.

A key to improving the implementation of water reuse is the continued development of cost-effective treatment systems.

To date the major emphasis on wastewater reclamation and reuse has been for non-potable applications such as agricultural and landscape irrigation, industrial cooling, and in-building applications such as toilet flushing. While direct potable reuse of reclaimed municipal wastewater is, at present, limited to extreme situations, it has been argued that there should be a single water quality standard for potable water.

If reclaimed water can meet this standard, it should be acceptable regardless of the source of water. While indirect potable reuse by groundwater recharge or surface water augmentation has gained support, some concerns still remain regarding trace organics, treatment and reuse reliability, and particularly, public acceptance.

A cautious and judicious approach is warranted to avoid potential health conse­quences that could result if a water reuse project is not successful. In addition, the importance of public confidence cannot be underestimated. The results of the studies reviewed in this paper provide strong evidence that reclaimed wastewater has the potential to serve as a viable source of water for potable water treatment.

Continued research and development efforts as outlined above are necessary to provide a sound scientific basis for crossing the threshold to direct potable reuse, when necessary.