Water Quality Guidelines for Wastewater Reuse | Wastewater Management

In this article we will discuss about the water quality guidelines for wastewater reuse.

Takashi Asano

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

Rafael Mujeriego

ETS de Ingenieros de Caminos, Canales y Puertos Universidad Politécnia de Cataluña. Gran Capitán D-1, 08034 Barcelona, Spain

Introduction:

As the demand for water increases, so does the importance of wastewater reclamation and reuse; that is, by allowing a water agency to supplement non-potable water needs with reclaimed wastewater. Water demands often exceed reliable water supplies, even in normal precipitation years, and new water resources development is increasingly costly and environmentally often prohibitive. Reclaimed wastewater is, after all, water resource existing right at the doorstep of the urban environment where water resources are needed most and priced the highest.

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

(1) Water pollution abatement due to no discharge or reduced discharged of pollutants in receiving waters such as in the touristic coastal waters,

(2) Increasingly stringent water pollution control requirements and, as a result, availability of high quality effluents for various beneficial uses,

(3) Public policy and ethics encouraging water conservation and reuse,

(4) Long-term reliable non-potable water supply guaranteed in nearby communities, and

(5) One of the water demand and drought management strategies in overall water resources management.

In the planning and implementation of wastewater reclamation and reuse, the in­tended wastewater reuse applications govern the required degree of wastewater treatment and the reliability of wastewater treatment processes and operations. Any effluent from a wastewater treatment step such as primary or secondary treatment can be reclaimed and reused for appropriate beneficial uses.

Seven categories of common municipal wastewater reuse are; agricultural irrigation, landscape irrigation, industrial recycling and reuse, groundwater recharge, recreational and environmental uses, non-potable urban uses such as toilet flushing, and potable reuse. Among them, large quantities of reclaimed municipal wastewater have been used in agricultural and landscape irrigation, industrial reuse, and ground-water recharge.

Technological Innovations for Safe Use of Reclaimed Wastewater:

The contaminants in reclaimed wastewater that are of public health significance may be classified as biological and chemical agents. Where reclaimed wastewater is used for irrigation, biological agents including bacterial pathogens, helminths, protozoa, and vi­ruses pose the greatest health risks.

To protect public health, considerable efforts have been made to establish conditions and regulations that would allow for safe use of reclaimed wastewater. Summaries of recommended microbiological quality guidelines by the World Health Organization (1989) and the State of California’s Wastewater Reclama­tion Criteria (1978) are presented in Table 1.

The WHO guidelines emphasize the use of a series of stabilization ponds for producing an acceptable microbial water quality; whereas, the California criteria stipulate conventional biological wastewater treatment, filtration, and chlorine disinfection.

Of the known waterborne pathogens, enteric viruses have been considered the most critical in wastewater reuse in the developed world because of the possibility of infection with low doses and the difficulty of routine examination for their presence.

Thus, essen­tially pathogen-free effluent via secondary treatment and tertiary treatment including chemical coagulation, flocculation, filtration, and disinfection is necessary for reclaimed wastewater applications with higher potential exposures; e.g., spray irrigation of food crops eaten uncooked, parks and playgrounds, and unrestricted recreational impound­ments.

Because of the stringent water quality requirements imposed upon such water reuse applications, the importance of granular-medium filtration, as a tertiary treatment step, has been demonstrated. The filtration removes substantial numbers of particles in wastewater; thus, promotes effective disinfection as well as esthetic enhancement of reclaimed water for beneficial uses. Figure 1 shows schematic diagram of full treatment (often referred to as the Title 22 process) as well as less costly alternatives, contact and direct filtration.

Two historical studies on enteric virus removal and inactivation in tertiary treatment (Pomona Virus Study and Monterey Wastewater Reclamation Study for Agriculture are reviewed in the following section. Both studies demonstrated that virtually pathogen-free effluents could be produced from municipal wastewater, via tertiary treatment and disin­fection by chlorine, for unrestricted water reuse applications.

Pomona Virus Study:

The need for research on the virus removal efficiency in tertiary treatment systems was recognised in mid-1970s. Periods of undiluted effluent discharges from the Los Angeles County wastewater treatment plants often correspond with warm periods when swimmers frequent the rivers.

Because of human contact, discharge to rivers must comply with the most stringent requirements specified in the California Wastewater Reclamation Criteria. The intent of these criteria was to protect swimmers from the transmission of waterborne diseases.

The stipulated ‘complete treatment’ system consisting of chemical coagulation, sedimentation, and filtration is costly as a wastewater reclamation system. Based on virus removal efficiencies, several tertiary treatment systems were investigated by the County Sanitation Districts of Los Angeles County on a pilot-scale facility (6.3 L/s) in Pomona, California (known as the Pomona Virus Study) as alternatives to the treatment require­ments specified in the Wastewater Reclamation Criteria.

The reference tertiary treatment system and the alternative systems evaluated included the following:

System A was the ‘complete treatment’ stimulated in the California Wastewater Reclamation Criteria (Fig. 1A) and consists of: alum coagulation (150 mg/L alum, 0.2 mg/L anionic polymer), flocculation (1-hr), sedimentation, filtration, and disinfection (2- hr chlorine contact or 18 min ozone contact). This system served as the reference system for comparison to the alternative systems described below.

The first of three alternative treatment systems, System B as shown in Fig. 1B, consisted of: low dose alum coagulation (5mg/L alum, 0.06mg/L anionic polymer), filtra­tion, and disinfection (2-hr chlorine contact time or 18-min ozone contact time). The second alternative system, System C, was comprised of: carbon adsorption (10-min empty bed contact time [EBCT]), disinfection (2-hr chlorine contact time or 18-min ozone contact time), and carbon adsorption (10-min EBCT).

The final alternative treat­ment system, System D, consisted of: low dose alum coagulation (5 mg/L alum, 0.06 mg/ L anionic polymer), filtration, and disinfection (2-hr free residual chlorination on a nitrified effluent).

Virus seeding experiments using poliovirus 1 were conducted in order to compare the virus removal efficiencies of the treatment systems described above. The un-chlorinated secondary effluent, containing a background level, expressed as the geometric mean, of naturally occurring viruses was 5 plaque forming unit per liter (pfu/L), was spiked to a virus concentration of 1.3 + 10⁵ pfu/L.

Figure 2 shows the overall virus removal efficiencies when a residual chlorine dosage of about 10mg/L (high chlorine residuals) and two-hour chlorine contact time were used. All three treatment systems produced virtually the same virus removal of 5.2 logs. System D removed 4.9 logs using 4 mg/L free residual chlorine (Note: Log removal can be calculated from the fraction remaining.

The log removal is the negative log of the fraction remaining. Therefore, if the fraction remaining is 0.10, the log removal is equivalent to one log removal. The 99.999 per cent removal means that the fraction remaining 0.00001 and is the equivalent of 5 log removal).

Fig 2. Poliovirus removal is seeding experiments with high chlorine residuals.

The overall virus inactivation and removal by the same treatment systems using an average combined chlorine residual of 5.0 mg/L (referred to as low chlorine residual) are shown in Fig. 3. With the low chlorine residual, differences in virus removal capability among the different tertiary treatment systems were apparent. Overall virus removals when ozone was used ranged from 5.2 to 5.4 logs, which was slightly greater removal than obtained in the high chlorine residual studies.

Fig 3. Poliovirus removal in seeding experiments with high chlorine residuals.

Based on the Pomona Virus Study, it was concluded that direct filtration (System B) or carbon adsorption (System C) achieved pathogen removal efficiency equivalent to the ‘complete treatment system’ (System A). Therefore, treatment of secondary effluent by direct filtration or carbon adsorption can be considered to be equivalent to the ‘complete treatment’ and, after disinfection, can be discharged into unrestricted recreational im­poundments.

Monterey Wastewater Reclamation Study for Agriculture:

The Monterey Wastewater Reclamation Study for Agriculture (MWRSA) was a ten- year, $7.2 million field-scale project designed to evaluate the safety and feasibility of irrigation food crops (that may be consumed raw) with reclaimed municipal wastewater.

The five-year field portion of the study began in late 1980 and continued through 1985. During these five years, a perennial crop of artichokes was grown along with rotating annual crops of celery, broccoli, lettuce, and cauliflower.

Extensive sampling of waters, soils, and plant tissues was conducted during the field studies. Since the virus removal capabilities of tertiary treatment systems were established by the Pomona Virus Study, investigations in MWRSA centered on the virus survival on crops and in soils in the field.

The following section summarizes the findings of the Monterey Wastewater Study for Agriculture conducted by Engineering-Science (1987). During the five-year field study period, no naturally-occurring animal viruses were recovered from chlorinated effluent of the two tertiary treatment systems (similar to System A and System B in the Pomona Virus Study) under investigation.

The cumulative sample volumes from these systems totalled 186,025 L and 159,402 L, respectively. The un-chlorinated secondary effluent contained measurable viruses in 80 per cent of the samples, with an average concentration of 22pfu/L with a range of 1 to 734 pfu/L. However, no animal viruses were recovered from any crop and soil samples.

Virus Seeding of Plants and Soils:

Monitoring for the presence of naturally occurring animal viruses showed that the influent to the two pilot processes (Castroville un-chlorinated secondary effluent) contained measurable viruses in 53 of the 67 samples taken. The median virus concentration was 2 plaque-forming units per liter (PFU/L): 90 per cent of the samples contained less that 28 PFU/L.

During the approximate five-year period, no in situ viruses were recovered from the chlorinated tertiary effluent of either process. No viruses were recovered from any of the crop samples, This was also the case for the soil irrigated with reclaimed water.

Although no in-situ viruses were recovered from irrigated plants and soil, it was important that an estimate be made of the ability of virus to survive under these condi­tions. Virus survival measurements were made in the laboratory and under field condi­tions. In the laboratory, the times required for a 99 per cent die-off in the viruses (T99) ranged from 7.8 days for broccoli to 15.1 days for lettuce. In field studies in Castroville, the T99 values were 5.4 days for artichokes, 5.9 days for romaine lettuce, 7.8 days for butter lettuce.

The survival of virus in Castroville soil was determined both under environmental chamber conditions and under field conditions. The T99 values for the decay of virus under environmental chamber conditions were respectively, 5.4, 9.7 and 20.8 days for 60, 70 and 80 per cent relative humidity.

In the field the T99s were 5.2 and 4.8 days for runs one and two, respectively. Thus, the rate of virus removal under chamber and field conditions was quite similar. No viruses were recovered from any soil section after 12 to 14 days of exposure.

Bacteria and Parasites:

During the five years of the study, the quality of irrigation waters improved because of the continued improvement in treatment plant operations and storage procedures. All three types of waters, including the well water control, periodi­cally exhibited high coliform levels. No salmonellae, shigellae, Ascaris lumbricoides, Entamoeba histolytica, or other parasites were ever detected in any of the irrigation waters.

The levels of total and fecal coliforms in soils and plant tissue irrigated with all three types of water were generally comparable. No significant difference attributable to water type was observed. No parasites were detected in soil samples.

Parasites were detected in plant tissue only in Year One, and there were no differences in level of contamination between effluent and well water-irrigated crops. Sampling of nearby fields detected no relationship between bacteriological levels and distance from the field site. The aerosol transmission of bacteria was thus deemed unlikely.

MWRSA Findings:

Engineering Science (1987) and Sheikh (1990) con­cluded the MWRSA findings as follows: Based on virological, bacteriological and chemi­cal results from sampled vegetable tissues, irrigation with filtered effluent appears to be as safe as with well water. After five years of field experimentation, results show few statistically significant differences in measured soil or plant parameters attributable to the different water types.

None of these differences has important implications for public health. Yield of annual crops is often significantly higher with reclaimed water.

Specific conclusions of the MWRSA study were:

1. No virus was detected in any of the reclaimed waters sampled although it is often detected in the secondary effluent.

2. The full treatment (Title 22) process is somewhat more efficient than the direct filtration process in removing viruses when influent is artificially inoculated (seeded) at extremely high rates. Both flow steams and remove more than five logs of virus (i.e., removal to below 1/100 000 of the seeded concentration).

3. Marketability of produce is not expected to be a problem.

4. The cost of producing filtered effluent (after secondary treatment) is estimated to be $0.06/m3, excluding conveyance and pumping costs.

Groundwater Recharge with Reclaimed Wastewater:

Groundwater recharge with reclaimed municipal wastewater is an approach to water reuse that results in the planned augmentation of groundwater for various beneficial uses. It is essential, therefore, that water extracted from a groundwater basin be of acceptable physical, chemical, microbiological and radiological quality.

Main concerns governing the acceptability of groundwater recharge projects are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is consumed by the public.

A source control program to limit potentially harmful constituents entering the sewer system must be an integral part of any groundwater recharge project. Extreme caution is warranted because of the difficulty in restoring a groundwater basin once it is contaminated.

Proposed Groundwater Recharge Regulations:

The State of California’s proposed regulations for groundwater recharge with re­claimed municipal wastewater rightly reflect cautious attitude toward such short-term health concerns. Proposed regulations are shown in Table 2. The regulations rely on a combination of controls intended to maintain a microbiologically and chemically safe groundwater recharge operation.

No single method of control would be effective in controlling the transmission and transport of contaminants of concern into and through the environment. Therefore, source control, wastewater treatment processes, treatment standards, recharge methods, recharge area, extraction well proximity, and monitoring well are all specified.

Table 2. Proposed requirements for groundwater recharge with reclaimed municipal wastewater.

The requirements in Table 2 are specified by ‘project category’ which identify a set of conditions that constitute an acceptable project. An equivalent level of perceived risk is inherent in each project category when all conditions are met and enforced.

Main con­cerns governing the acceptability of groundwater recharge projects with reclaimed mu­nicipal wastewater are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is eventually consumed by the public.

Microbiological Considerations:

Of the known waterborne pathogens, enteric viruses have been considered most critical in wastewater reuse in California because of the possibility of contracting disease with relatively low doses and difficulty of routine examination of reclaimed wastewater for their presence.

Thus, essentially virus-free effluent via the full treatment process (primary/secondary, coagulation/flocculation, clarification, filtration, and disinfection; see Figure 1) is deemed necessary by the California Department of Health Services for reclaimed wastewater applications with higher potential exposures, e.g., spray irrigation of food crops eaten raw, or most of groundwater recharge applications (Project Categories I, II, and IV in Table 2).

The wastewater treatment requirements in Table 2 are designed to provide assurance that reclaimed water is essentially pathogen-free prior to extraction from the groundwater. In addition to the treatment processes, passage through an unsaturated zone of significant depth (> 3 m) reduces organic constituents and pathogens in treated effluents.

At low infiltration rates of less than 5m/day in sands and sandy loams, the rates of virus removal are approximated by a semi-log plot (k = -0.007 log/cm) against infiltration rates, result­ing approximately 99.2 per cent or 2.1 logs removal for 3 m depth soils. The overall estimates for the removal of enteric viruses by the treatment processes, unsaturated zone, and horizontal separation (retention time in groundwater) as specified in the proposed criteria are in the range of 13 to 17 log removal.

Trace Organics Removal:

The regulations intend to control the concentration of organics of municipal wastewater origin as well as anthropogenic chemicals that have an impact on health when present in trace amounts. Thus, the dilution requirements and the organics removal specified in Project categories I and IV in Table 2 is too limit average concentration of unregulated organics in extracted groundwater affected by the groundwater recharge operation.

The concentration of unregulated and unidentified trace organics is of great concern since other constituents and specific organics are dealt with through the established maximum contaminant levels and action levels developed by the California Department of Health Services.

Approximately 90 per cent by weight of the organics comprising the total organic carbon (TOC) in treated municipal wastewater are unidentified. One of the health concerns related to the unidentified organics is that an unknown but small fraction of them are mutagenic.

Regulating the presence of trace amount of organics in reclaimed water can be accomplished by dilution using surface water or groundwater of less contaminated source. When reclaimed water makes up more than 20 per cent of the water reaching any extraction well for potable water supply, treatment to remove organics must be provided.

Because of lack of ideal measure for trace amount of organics in reclaimed water as well as in the affected groundwater, total organic carbon (TOC) was chosen, as a surrogate, to represent the unregulated organics of concern.

Although TOC is not a measure of specific organic compounds, it is considered to be suitable measure of gross organics content of reclaimed water as well as groundwater for the purpose of determining organics removal efficiency in practice.

However, there is insufficient basis for the establishment of a gross organics standard for the recharge water that protects public health. The proposed regula­tions shown in Table 2 require that the groundwater recharge projects by surface spread­ing resulting in a 20-50 per cent reclaimed wastewater contribution at any extraction well (Category 1), and the recharge project by direct injection resulting in a 0-50 per cent contribution (Category IV), must provide organic removal step sufficient to limit the TOC concentration of wastewater origin in extracted water to 1 mg/L.

Table 3 shows the maximum TOC concentration that may be allowed in the reclaimed wastewater, for a given per cent reclaimed wastewater contribution, to achieve no more than 1 mg/L TOC of wastewater origin in the extracted water.

Table 3. Maximum allowable TOC concentration in reclaimed wastewater (recharge water) where organics removal to achieve 1 mg/L TOC in extracted well water is required.

The numbers in Table 3 assume a 70 per cent reduction through the unsaturated zone and no TOC removal in the aquifer. The numbers associated with the direct injection were derived by dividing 1 mg/L TOC concentration by the fractional contribution of reclaimed water to native groundwater at the extraction point.

Thus, the numbers for the direct injection are 30 per cent of those for the groundwater recharge by surface spreading. In addition, direct injection projects would have to achieve a 70 per cent TOC reduction to compensate for the lack of unsaturated zone in the overall soil-aquifer treatment capability.

Inorganic Chemicals:

Inorganic chemicals, with the exception of nitrogen in its various forms, are ad­equately controlled by meeting all maximum contaminant limits (MCLs) in the reclaimed wastewater. By limiting the concentration of total nitrogen in the reclaimed water, detri­mental health effects such as methemoglobinemia can be controlled.

In those recharge operations where adequate nitrogen removal cannot be achieved by treatment processes or passage through unsaturated zone, the criteria provide the alternative method such as wellhead treatment to meet the allowable total nitrogen concentration of 10 mg/L as N.

How Safe is Wastewater Reuse?

Despite a long history of wastewater reclamation and reuse in many parts of the world, question of safety of wastewater reuse is still difficult to define and acceptable health risks have been hotly debated. In this section, a comparative assessment of the safety of wastewater reuse is discussed based on two recent publications on enteric virus risk assessment. Furthermore, some observations are presented pertaining to the proposed revision of the State of California’s Wastewater Reclamation.

When treated municipal wastewater effluents are used in the urban environment resulting possibility of direct contact with humans, considerable health concerns may be justified. These health concerns are specifically directed, in the industrialized countries with high health standard, to the presence of enteric viruses because of their low-dose infectivity, long-term survival in the environment, difficulty of monitoring, and low- removal efficiency in conventional wastewater treatment.

The potential health hazards inherent in the use of reclaimed municipal wastewater must always be guarded in such reuse applications.

Enteric Virus Concentration in Wastewater:

The enteric virus data assembled included 424 un-chlorinated secondary effluent samples in which 283 samples (67%) were virus positive and 814 chlorinated tertiary (filtered) effluent samples with 7 positive samples (1%). The data­base was obtained from the published reports from the water and wastewater agencies in California.

Quantifying the virus concentration (viral units, vu, per litre) in the treated effluent is the first step for estimating the risk of virus infection upon exposure to reclaimed municipal wastewater. The statistical model used was the lognormal distribu­tion and the goodness-of-fit of the hypothesized distribution was evaluated using the nonparametric Kolmogorov-Smirnov test.

The virus concentrations in un-chlorinated secondary effluents vary in a wide range. Furthermore, the virus concentrations among different treatment plants show distinctively different characteristics; e.g., the geometric means ranged four orders of magnitude (10 ^-4– 10° vu/L) and the spread factors ranged from 4 to 115.

The geometric mean values of un-chlorinated secondary effluent samples ranged from 0.0002 to 2.3 vu/L and 90 percen­tile concentrations ranged from 0.34 to 29 vu/L. The virus concentration of 0.01 vu/L is considered to be a reasonable estimate of the detection limit. Thus, characterizing the variability of enteric virus concentrations in un-chlorinated secondary effluents is ex­tremely important in virus risk assessment.

Two set of virus concentrations were used for the risk analysis: the data derived from the un-chlorinated activated sludge effluents and the chlorinated tertiary filtration effluents. For the first run, the geometric mean and the 90 percentile values for enteric viruses found in un-chlorinated 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 the reasonable estimate of limit of detection for enteric viruses and maximum concentration found in tertiary effluents.

Virus Risk Analysis:

To assess potential risks associated with the use of reclaimed wastewater, the fol­lowing exposure scenarios were developed for:

(1) Landscape irrigation,

(2) Spray irriga­tion of food crops,

(3) Recreational impoundments, and

(4) Groundwater recharge, and are summarized in Table 4.

The beta-distributed probability model based on Haas’ work (1983) was chosen for use in risk calculations because it best represented the frequency distribution of virus infection. Infectious models based on echovirus 12, and poliovirus 1 and 3 were used by Asano (1992) and the rotavirus model based on Rose and Gerba (1991) was used in the study by Tanaka (1993).

Table 4. Summary of Exposure Scenarios Used in Risk Analysis.

Results of the annual risk calculations are shown in Table 5, using the virus concen­trations of 0.01 vu/L and 1.11 vu/L from the chlorinated tertiary filtration effluents which are the reasonable estimate of the limit of detection for enteric viruses and the maximum concentration found in tertiary effluents in California.

The estimates of risk of infection presented in Table 5 show the range of risks associated with annual exposures encountered in 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. This reflects the differences in infectivity among different viruses. For groundwater recharge with reclaimed wastewater, with an effluent virus concentration of 0.011 vu/L, the annual risk of infection ranges from 5 × 10^-10 to 5 × 10^-11.

When virus concentration is increased to 1.11 vu/L which is the maximum virus concentration found in the chlorinated tertiary effluent, the risk of infection increased by 2 to 3 order of magnitude (6 × 10⁸ and 5 × 10⁹). Similar trends are noted in the other exposure scenarios.

Table 5. Annual Risk of contracting at least one Infection from Exposure to Reclaimed Wastewater based on Two Different Enteric Virus Concentrations.

Of the remaining three categories, the least restrictive use of reclaimed wastewater is for recreational impoundments where water contact sports such as swimming take place. In all cases, regardless of the starting virus concentration, the use of reclaimed wastewater for unrestricted recreational impoundments allows the most liberal exposure to enteric viruses according to the risk assessment.

The relatively high probability of infection is due to the fact that no dilution or virus die-off in the environment were included in the calculations, assuming the worst possible case.

Landscape irrigation for golf courses posed the second most exposure to reclaimed wastewater, and spray irrigation of food crops ranked third being two orders of magnitude lower in relative risk. The lower risks of infection in the cases of spray irrigation of food crops and groundwater recharge can be attributed to environmental factors such as use area controls.

In both cases, the exposure scenarios developed include virus die-off in the environment. These risk analyses, however, do not account for the variability of enteric viruses in the environment. Seasonal fluctuations in the endemic virus populations will affect the quantity and species present in the wastewater at any given time.

The lack of positive samples indicates that the chlorinated tertiary effluent is essen­tially virus free; however, none of the monitoring has produced enough positive samples to establish a measure of process reliability with respect to virus removal.

Since a wastewater treatment process rarely produces a constant quality effluent, due to daily and seasonal water quality variations, flow fluctuations, or process variability, the effluent produced should be expected to vary.

Indeed, the positive samples in tertiary effluents reported in Table 2 were associated with the design deficiencies and other operational difficulties. These could be detected by real time monitoring of effluent water quality with continuous turbidity measurement.

In the seven positive samples out of 841 samples analysed (roughly 1% positive), the virus concentration ranged from 2 to 111 vu/100 L. Therefore, it might be assumed that the wastewater treatment process produced a virus free effluent 99 per cent of the time. In this circumstance, the assumption of 5-log removal of viruses in tertiary treatment needs close examination in light of treatment process reliability.

If a treatment process for wastewater reuse has a reliability of 99 per cent, the process is expected to meet the performance requirements 99 per cent of the time. Thus, the permit limit of virus-free effluent is expected to be exceeded 1 per cent of the time, or three to four times a year.

The question in risk management in wastewater reclamation and reuse, then, becomes one of determining whether or not the presence of enteric viruses in the concentration range of 2 to 111 vu/100 L in approximately 1 per cent of the time in tertiary-treated reclaimed wastewater is significant to protect public health.

Further re­search is needed to characterize treatment process reliability that contribute to the overall reduction of infectious risks in wastewater reclamation and reuse.

The goal of essentially virus-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 5 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. The ‘safety’ of wastewater reclamation and reuse practice is defined by the acceptable level of risks developed by the regulatory agencies responsible for risk management and endorsed by the public.