Canadian Guidelines for Household Reclaimed Water for Use in Toilet and Urinal Flushing

2007

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Note

At the request of the Federal-Provincial-Territorial Committee on Health and the Environment, a working group of government officers and invited experts has developed this consultation paper. This work was funded by the Canada Mortgage and Housing Corporation. The views and opinions expressed in this publication do not necessarily reflect those of any members or member government of the Committee on Health and the Environment. The contents of the paper should not be used for any purpose other than as a basis for providing comment to this Committee.

Table of contents

• Acknowledgements
• Executive summary
• 1.0 Introduction
  • 1.1 Scope of the document
  • 1.2 A risk-based approach
• 2.0 Guidelines for reclaimed water quality
• Part I: Potential Elements of a Management Framework for Reclaimed Water
• 3.0 Managing health risks in on-site and clustered reclaimed water systems
  • 3.1 Economic considerations
  • 3.2 Management models
  • 3.3 Technology validation and certification
  • 3.4 Installation and commissioning of new systems
  • 3.5 Operational oversight, inspections and monitoring
• Part II: Science and Technical Considerations
• 4.0 Risk assessment
  • 4.1 Hazard identification -- microbiological characteristics
    • 4.1.1 Significance of microorganisms in reclaimed water
    • 4.1.2 Viral reference pathogens
    • 4.1.3 Protozoan reference pathogens
    • 4.1.4 Bacterial reference pathogens
    • 4.1.5 Helminthic reference pathogens
  • 4.2 Hazard identification -- chemical characteristics
    • 4.2.1 Endocrine disrupting chemicals
    • 4.2.2 Pharmaceutically active compounds and personal care products (PPCPs)
    • 4.2.3 Complex mixtures
    • 4.2.4 Significance of chemicals in reclaimed water
  • 4.3 Exposure assessment
  • 4.4 Hazard characterization
  • 4.5 Risk characterization
• 5.0 Rationale
• References
• Appendix A: Abbreviations, acronyms and glossary
• Appendix B: Additional risk assessment information and calculations
• Appendix C: Technology verification and certification
• Appendix D: Treatment processes

Acknowledgements

The contributions of the following individuals to the development of these guidelines are acknowledged:

Working Group on Household Reclaimed Water

This work benefited greatly from the published reports of the Australian Water Conservation and Research Program. It also relied heavily on the information and concepts introduced in the draft National Guidelines for Water Recycling (October 2005) prepared by the Environment Protection and Heritage Council of Australia and the Natural Resource Management Ministerial Council of Australia.

Executive summary

The Canadian Guidelines for Household Reclaimed Water for Use in Toilet and Urinal Flushing have been developed as an option to reduce water consumption, in response to the growing interest for water conservation in Canada. The use of household reclaimed water can make significant contributions to reducing unnecessary water use. However, household reclaimed water must be treated and managed effectively as there is a potential health risk to users, particularly from pathogens that can be responsible for severe gastrointestinal illness. Although the long-term goal is to develop comprehensive guidelines to allow the safe use of reclaimed water for many beneficial purposes, the focus of this first version of the guidelines is limited to the specific end use of toilet or urinal flushing.

This document provides guidelines for reclaimed water quality, as well as guidance on potential elements of a management framework (Part I) and an overview of the scientific basis for the guidelines (Part II). It recommends possible elements of a management framework that are applicable to on-site or decentralized treatment of household water for reuse in residential or commercial toilet and urinal flushing. The design and installation requirements for such non-potable water systems are addressed in CSA Standard B128.1-06 (CSA 2006).

The objective of establishing Guidelines for reclaimed water is to ensure that the operation of water reclamation systems is protective of public health. Consequently, the guidelines include values for several water quality parameters that have been selected because they can demonstrate the effectiveness of treatment on an on-going basis.

Health effects

There are situations where the use of household reclaimed water to flush toilets (and urinals in commercial buildings) can make significant contributions to reducing unnecessary water use. However, the presence of microorganisms (bacteria, protozoa and viruses) and chemicals in household wastewater may pose a health risk to users. In particular, accidental ingestion of reclaimed water that contains human enteric pathogens could cause severe gastrointestinal illness. This is of particular concern for susceptible individuals, such as infants, the elderly, and those that have compromised immune systems, for whom the effects may be more severe, chronic (e.g., kidney damage), or even fatal.

Users of household reclaimed water for toilet and urinal flushing may accidentally ingest small volumes of water through aerosols or hand-to-mouth contact with droplets. There is also the potential for accidental cross-connection of a reclaimed water system with the drinking water system. Effective treatment can produce reclaimed water that is virtually free of disease-causing microorganisms. There are no negative health impacts expected from chemicals in household reclaimed water used only for toilet and urinal flushing.

Management framework

Management of on-site reclaimed water systems is a particular challenge. Such systems could include collection and treatment of water from single domestic dwellings or from clusters such as apartment buildings. While they will impact fewer people than large systems, from a process perspective, small systems have a complexity similar to that of larger systems. The potential health risks associated with decentralized reclaimed water treatment systems mean that there is a need for a high level of treatment reliability and oversight.

It is recommended that authorities develop and implement a management program for reclaimed water systems, giving due consideration to the protection of public health, local administrative and operational capacity, and economic considerations. A site-specific risk assessment should be conducted initially to determine the appropriate levels of microbiological reduction or inactivation needed for the specific system. Treatment technologies used should consistently achieve the guidelines levels established in this document. Operational oversight, inspections and compliance monitoring should form key components of a management program to ensure that treatment of reclaimed water is effective on a long term basis.

Introduction

Canadians are some of the highest per capita users of water in the world. According to Environment Canada's "Freshwater Website", simple changes to water use habits and household equipment can reduce water consumption in the home by up to 40%. There are many measures and strategies that can make a significant contribution to reducing water use. Some are quite common, simple and inexpensive, whereas others are relatively new or ground-breaking. One that fits into this latter category is using reclaimed water. There is a growing interest in using reclaimed water within the context of sustainable water management. Other factors that contribute to the interest in reclaimed water use include:

• the opportunity to provide reliable water services in remote or environmentally sensitive locations;
• overburdened traditional water sources;
• the rising costs of meeting drinking water treatment and wastewater discharge standards;
• the potential to reduce sewage discharges to water bodies;
• seasonal water shortages and droughts (potentially exacerbated by climate change); and
• population movement to large centres, resulting in changes to the spatial patterns of water demand (Anderson et al., 2001).

Despite the advantages of using reclaimed water, pathogens or chemicals in reclaimed water may be a risk to human health or the environment. Due to these risks and the low cost for water in Canada, pursuit of reclamation of water has been slow. At present, British Columbia is the only Canadian province to have enacted a reclaimed water standard for a variety of applications (Government of British Columbia, 1999). Alberta legislation allows the use of treated municipal wastewater for irrigation, and in support of the legislation, Alberta Environment (2000) has produced guidelines to aid in evaluating projects (Government of Alberta, 1993). The Atlantic Canada Standards and Guidelines Manual for the Collection, Treatment and Disposal of Sanitary Sewage includes a chapter on reclaimed water use, with a focus on irrigation (Environment Canada, 2000). Other provinces use a case-by-case approach to proposed water reclamation projects. In the absence of guidelines, some jurisdictions are using demonstration or test sites to explore water reclamation (CMHC, 1997; Ho et al., 2001).

Several reports have concluded that guidance and leadership from senior government on reclaimed water is needed to ensure that it is incorporated into future water management strategies (Brandes and Ferguson, 2004; Marsalek et al., 2002). It has been noted that two major barriers to the adoption of water reclamation as a strategy are: 1) the lack of standards for plumbing requirements for non-potable water systems, and; 2) the lack of national guidelines for reclaimed water quality (CMHC, 1997). The CSA International has developed Standard B128.01-06/B128.2-06 (CSA, 2006) to address the plumbing requirements. This current document addresses the first barrier and will contribute to the development of a consistent, national approach for the safe and sustainable use of household reclaimed water.

Scope of the document

This document provides guidelines for reclaimed water quality as well as guidance on potential elements of a management framework. Part I of the document provides guidance on management frameworks and models and Part II outlines the scientific basis of the water quality guidelines. The guidelines and management guidance presented in this document are applicable only to water reclamation where the water source is household greywater or wastewater and the end use is toilet and urinal flushing, either on-site or at a nearby residential or commercial location. Commercial applications are intended to be light commercial uses such as retail. This document does not cover rainwater harvesting, nor does it cover recycling of stormwater and treated municipal wastewater that may include commercial and industrial sources of contamination.

The limited scope of these guidelines is considered a first step towards broader uses of reclaimed water. The long-term objective is to provide the tools and guidance needed to allow the safe use of reclaimed water for many beneficial purposes, while minimizing the associated health and environmental risks. The design, installation and maintenance requirements for non-potable water systems are addressed in CSA Standards B128.1-06 and B128.2-06 (CSA, 2006).

These guidelines are intended for use by regulatory authorities, public health professionals, engineering consultants and others with a level of technical understanding of the subject area.

The guidelines take a conservative approach to establishing water quality parameters for reclaimed water. Even though exposure to reclaimed water used for toilet or urinal flushing is expected to be low, the potential health effects of coming into contact with microbiologically contaminated water are serious enough to warrant a precautionary approach.

A risk-based approach

This document adopts a risk-based approach in order to ensure appropriate quality and management of reclaimed water that is protective of public health over the long-term. The aim of a risk-based approach is to identify all of the potential hazards in a reclaimed water treatment system, assess their potential impact on water quality and on public health, and find ways to mitigate those risks rather than simply react when problems occur. Risk management considerations, including elements of a management framework and potential management models, are outlined in Part I.; The guidelines are based on risk assessment including the identification of hazards, assessment of exposure and characterization of risks as outlined in Part II.

Guidelines for reclaimed water quality

Table 1 recommends levels for several reclaimed water quality parameters. Within an overall management framework, the guideline values in Table 1 are intended to enhance treatment reliability and disinfection effectiveness, thus protecting public health. These guideline values could be used to ensure water quality conditions upon start-up of a reclaimed water system, for periodic verification of the system and as a safety precaution if operational parameters are not met.

All household reclaimed water should be disinfected. Primary disinfection may be accomplished by any chemical, physical or biological means that results in the destruction, inactivation or removal of microorganisms. Chlorination should be used at least as a secondary means of disinfection to maintain chlorine residual within the storage and distribution system. Box 1 provides the rationale for selecting these parameters. A management program, including treatment technologies in place, should consistently achieve the following reclaimed water quality criteria:

Table 1: Guideline values for reclaimed water used in toilet and urinal flushing^a

Parameter	Units	Water quality parameters
		Medianb	Maximum
aUnless otherwise noted, recommended quality limits apply to the reclaimed water at the point of discharge from the treatment facility or treatment unit. 5 = five-day biochemical oxygen demand; TSS = total suspended solids; NTU = nephelometric turbidity unit; CFU = colony-forming unit. bMedian of at least five samples collected over a 30-day period. cMeasured prior to disinfection point. dMeasured at the point where the treated effluent leaves the reservoir or storage.
BOD5	mg/L	≤10	≤20
TSSc	mg/L	≤10	≤20
Turbidityc	NTU	≤2 (alternative to TSS)	≤5 (alternative to TSS)
Escherichia coli	CFU/100 mL	Not detected	≤200
Thermotolerant coliforms	CFU/100 mL	Not detected	≤200
Total Chlorine residuald	mg/L	≥0.5

Box 1: Reclaimed water quality parameters

Parameter	Rationale for Selection
Biochemical oxygen demand (five-day) (BOD5)	Excessive BOD can lead to aesthetic and nuisance problems (odour and colour problems). Organics provide nutrients for microorganisms, adversely affect disinfection processes and consume oxygen. Maintaining BOD5at the levels in recommended in Table 1 will help ensure that aerobic treatment conditions are maintained.
Total suspended solids (TSS)	Organic contaminants and heavy metals are adsorbed on particulates, and this suspended matter can shield microorganisms from disinfectants; it is recommended that either TSS or turbidity be monitored (see Turbidity, below).
Turbidity	Turbidity is monitored for both health and aesthetic reasons. Turbidity can be organic in nature and may contain toxins and harbour pathogens. Excessive turbidity can lead to odour problems and will interfere with disinfection. It is a useful parameter for monitoring the performance of the treatment unit or facility. Maintaining levels are at or below those noted in Table 1 will help disinfection efficiency; it is recommended that either TSS or turbidity be monitored.
Escherichia coli	E. coli is a definitive indicator of recent faecal contamination of water. The goal of treatment is to reduce the presence and associated health risks from pathogens to an acceptable or safe level. One measure of the safety of the water is the absence of E. coli.  In this context, the presence of E. coli in reclaimed water leaving the treatment unit can be used to assess adequate disinfection.  A well-designed and operated treatment system should be capable of consistently reducing E. coli to undetectable levels. However, as even the most sophisticated treatment system cannot provide water that is absolutely free of disease-causing microorganisms all the time, the guideline value for Escherichia coli in household reclaimed water systems is none detectable per 100 mL as a median of at least five samples, with a maximum concentration of 200 CFU per 100mL. The maximum value of is considered conservative, as epidemiological studies of exposure in recreational waters to E. coli densities below 200 CFU per 100 ml have not shown an association with an increased risk of illness.
Thermotolerant coliforms	In some studies, high levels of thermotolerant coliforms were detected while no E. coli were found. In these cases, the absence of E. coli may not be sufficient to assess adequate disinfection. Thermotolerant coliforms are recommended as an additional indicator of effective disinfection processes.
Total chlorine residual	Disinfection is essential to this process and chlorine should be used as at least a secondary disinfectant in order to provide residual disinfection in the reclaimed water storage system. The total chlorine residual is a measure of all chemical species containing chlorine in an oxidized state. It is usually the sum of the free and combined chlorine concentrations present in water. A minimum measurable total chlorine residual of ³0.5 mg/L is an indication that the level of disinfection is adequate (e.g. exceeds the chlorine demand) and may prevent bacterial regrowth in the reservoir or storage tank.

Part I: Potential Elements of a Management Framework for Reclaimed Water

Managing health risks in on-site and clustered reclaimed water systems

Management of on-site reclaimed water systems is a particular challenge. Such systems could include collection and treatment of water from single domestic dwellings or from clusters such as apartment buildings. While they will impact fewer people than large systems, from a process perspective, small systems have a complexity similar to that of larger systems. The potential health risks (see Part II) associated with decentralized reclaimed water treatment systems mean that there is a need for a high level of treatment reliability and oversight.

Economic considerations

It is important to consider the costs and benefits of any water reclamation project. However, it is often difficult to get a true accounting of these costs (Law, 1996; Ni et al 2003; Radcliffe, 2004). It is recommended that any household water reclamation project be evaluated on a case-by-case basis to determine if it is economically feasible. The first step in this process should be to establish a water budget for all of the water uses in the building in question. Water efficiency measures, such as low-flow fixtures, should be adopted as a first step. If reclaiming water is still an attractive or necessary option after this analysis, proponents should consider the following costs: 1) capital costs of treatment system, storage and plumbing; 2) operation and maintenance costs, including electrical, repair and consumables; and 3) fees that may be applied for permits, inspections and maintenance. Other costs may also come to bear, while benefits will accrue from reduced water use and reduced need for wastewater treatment capacity. The  Canada Mortgage and Housing Corporation website includes several case studies of successful and economically feasible reclaimed water projects.

Management models

Experience with private wastewater treatment (e.g. conventional septic systems) has shown that most management programs rely on homeowners to assume full responsibility for the operation and maintenance of their individual systems. However, many of these programs experience problems for a variety of reasons, including:

• a lack of trained service providers;
• no legal authority to hold homeowners accountable for properly maintaining their system;
• little to no training for homeowners; and
• lack of inspections and monitoring once systems are in place.

Local and provincial governments will need a comprehensive strategy to effectively manage reclaimed water systems. The risk management principles outlined in the position paper From Source to Tap: The Multi-Barrier Approach to Safe Drinking Water (FPTCDW/CCME, 2004) can also be adjusted to apply to reclaimed water. The multi-barrier approach described in that paper recognizes that "the key to ensuring clean, safe and reliable drinking water is to implement multiple barriers which control microbiological pathogens and contaminants that may enter the water supply systems." This approach encompasses nine major interrelated elements, each of which is described in detail in the "From Source to Tap" document: these can be adapted and grouped together to address the safe management of reclaimed water systems, as shown in Box 2. Experience with reclaimed water systems has shown that most problems that arise can be attributed to more than one factor. Considering and addressing the elements in Box 2 together will support the effectiveness of the risk management strategies.

Box2: Elements for the safe management of on-site and clustered household reclaimed water systems

Commitment in overarching frameworks
The overarching legislative and policy frameworks outline who holds specific responsibilities for various aspects of a water system, and will vary across the country. These frameworks should be reviewed and revised to reflect responsibilities for reclaimed water systems. Policies should show a clear commitment to the responsible use of reclaimed water and to applying a preventive risk management approach. All policies should support public health goals.

Public involvement and awareness

It is essential to establish partnerships and communication among stakeholders and the public. The public should be aware that they can report concerns to the appropriate authority. Strategies to accomplish this goal may include:

• providing educational resources on health risks and practices such as water disinfection, guidelines, conservation issues and costs of providing service;
• making monitoring results or summaries available and relaying information about what the authority is doing to address the risks; and
• for cluster systems, issuing regular reports about the system, its operation and planned improvements or changes.

System analysis and management
Effective management of water reclamation systems is essential to ensure the protection of public health. Preventive risk management strategies or plans should be developed for all reclaimed water systems. There is a need for a high level of treatment reliability and oversight. Owners and operators of reclaimed water treatment systems need to understand, at a basic level, the entire recycled water system, the hazards and events that can compromise recycled water quality and the preventive measures and operational control necessary for ensuring safe and reliable use of recycled water. Providing additional information in an ongoing manner to owners so that they understand their responsibilities is very important. Owners should know what to do and whom to contact in case of treatment failure in their reuse systems, as well as how to maintain and operate their systems effectively. Authorities should implement an appropriate management program to suit the needs of the community.

Supporting requirements
These requirements include basic elements of good practice, such as employee (or owner, in the case of on-site systems) training, community involvement, research and development, validation of process efficacy, and systems for documentation and reporting.

Review
Evaluation and audit processes to ensure that the management system is functioning satisfactorily are included in this element. It also provides a basis for review and continual improvement.

There may be opportunities to integrate decentralized reclaimed water treatment considerations into other programs to manage systems more effectively; an example of this may be integrating a reclaimed water quality program with an existing program for decentralized wastewater treatment. The United States Environmental Protection Agency (U.S. EPA) has developed a handbook that outlines a useful process for developing a decentralized wastewater management program (U.S. EPA, 2005). The program elements shown in Figure 1 are applicable to management of reclaimed water systems.

Figure 1: Decentralized reclaimed water management program elements (reproduced from U.S. EPA, 2005)

There are several management models and approaches that can be adapted for decentralized reclaimed water quality systems. As these systems can be considered moderate to high risk, the appropriate management approach and program should respond to this level of risk. Table 2 adapts the U.S. EPA (2005) management models to apply to reclaimed water systems.

Table 2: Management models for decentralized reclaimed water systems

Typical application	Program description	Benefits	Limitations
1. Maintenance contract model
Systems serving a single-family home	System performance requirements Systems properly designed Installed according to CSA Standard B128.01-06/B128.2-06 Inspection prior to start-up Service contracts in place and maintained Inventory of all systems Contract tracking system	Lower risk of treatment malfunctions Homeowner's investment protected Less resource-intensive than other program options	Difficulty tracking and enforcing compliance due to reliance on the owner or contractor to report lapses in service No mechanism in place to assess the effectiveness of the maintenance program Requires contract tracking system
2. Operating permit model
Systems serving a single-family home Systems in a multi-unit residential or commercial building	System performance and monitoring requirements Engineered designs allowed, but may provide prescriptive designs for specific sites Regulatory oversight by issuing renewable permits that may be revoked for non-compliance Inventory of all systems Tracking of operating permit and compliance monitoring Minimum for larger-capacity systems	Regular compliance monitoring reports Non-compliant systems identified and corrective actions required	Higher level of expertise and resources for regulatory authority to implement Requires permit tracking system Requires enforcement powers for authorities
3. Responsible management entity (RME) operation model
Multi-unit residential or commercial buildings Cluster systems	System performance and monitoring requirements Professional operation and maintenance (O&M) services through RME Regulatory oversight by issuing operating permits to RME (system ownership remains with property owner) Inventory of all systems Tracking system for operating permit and compliance monitoring	O&M responsibility transferred to professional RME that holds the operating permit Problems identified before malfunctions occur	May require enabling legislation to allow RME to hold the permit for an individual system owner RME must have owner's approval for repairs Need for easement/right of entry Need for oversight of RME by the regulatory authority
4. Responsible management entity (RME) ownership model
Cluster systems serving multiple properties under different ownership	System performance and monitoring requirements Professional management of all aspects of decentralized systems Trained and licensed owners/operators Regulatory oversight through permits Inventory of all systems Tracking system for operating permit and compliance monitoring	High level of oversight Reduces risk of non-compliance Removes potential conflicts between owners and RME	May require enabling legislation or establishing of a management district May require significant financial investment from RME May limit competition /innovation

Across the country, the different institutions, arrangements and procedures involved in a management program will depend on many factors, including enabling legislation, available resources and the needs or desire of the individual or community to pursue water recycling. Because of this diversity, management programs and outcomes are also likely to be different from jurisdiction to jurisdiction. Management structures can range from an informal network of partners working under a coordinated framework to a highly structured responsible management entity (RME) that owns or maintains a set of treatment systems. Authorities in each jurisdiction will have to determine what type of management program will best suit the needs of their communities. Preventive risk management strategies or plans should be developed for all reclaimed water systems. The aim is to provide a measurable and ongoing assurance that performance requirements are met and that, as far as possible, faults are detected and corrective actions are taken before there is a negative health impact. While all risk management plans should be consistent with the principles described in the multi-barrier approach, the level of detail and demands of an individual plan should reflect the complexity and potential level of risk associated with the reclaimed water system in question, as well as the capabilities of the system owner/operator.

Technology validation and certification

The design requirements for decentralized treatment systems focus on the protection of public health and water resources. Yet systems must also be affordable. Prescriptive codes simplify design reviews, but limit development options and innovation. Experience has shown that equipment failures are at the root of many waterborne disease outbreaks. In the case of reclaimed water treatment systems, the potential health risks and the need for treatment reliability underscore the need to have system performance validated. Ideally, a technology verification program should be available to provide a reliable, third-party assessment and certification of treatment devices (see Appendix C). Protocols for testing processes or technologies should determine their performance under a variety of upset conditions. There is currently no technology verification program in Canada that targets reclaimed water treatment systems. The NSF International/American National Standards Institute (NSF/ANSI) Standard 40 and Bureau de Normalisation du Québec (BNQ) Standards NQ 3680-910/NQ 3680-915 are examples of standard and testing protocols intended for the certification of on-site wastewater treatment systems; these protocols could conceivably be adapted to meet the requirements for reclaimed water systems, particularly with regard to disinfection. They offer good starting points towards an appropriate reclaimed water technology verification protocol. A limited overview of applicable treatment technologies is provided in Appendix D.

Installation and commissioning of new systems

Authorities will need to ensure the proper installation and functioning of a system prior to commissioning and should adhere to the requirements of CSA Standard B128.01-06/B128.2-06, Design and installation of non-potable water systems/maintenance and field testing of non-potable water systems, for field-testing of a new system (CSA, 2006). Of particular importance is preventing cross-connections with potable water plumbing lines and the use of air gaps wherever possible (air gaps are preferred over backflow prevention devices) (NOWRA, 2004). In addition to the CSA Standard B128.01-06/B128.2-06 requirements, authorities should verify that sensors and monitoring instrumentation are functioning properly and that the treatment system is meeting the effluent water quality requirements (see Table 1) 30 days after start-up. Note that it may take up to three weeks for biological systems to reach equilibrium or steady-state operation following start-up or a significant process change. Additional specific requirements may be imposed to fit local conditions and capabilities.

As part of a management program, authorities should consider certification or licensing of installers, as well as appropriate training. These recommendations are not meant as a substitute for applicable legal requirements. Interested parties should ensure that they are aware of, and adhere to, any applicable legal requirements where a system is under consideration.

Operational oversight, inspections and monitoring

As previously noted, any management program should be developed with due consideration given to protection of public health, water quality guidelines, regulatory authority capacity, administrative and operational capacity, and the local political, social and economic climate. Once effluent water quality parameters are verified upon start-up as described in section 3.4, frequent sampling of decentralized and small-scale/on-site wastewater treatment systems may be too resource intensive and expensive to be practical. In addition, statistics such as median and average values have very little meaning when assessing small-system water quality, where samples may be collected only on an annual or biannual basis (NOWRA, 2004). For such systems, it is recommended that monitoring be based on robust secondary parameters, such as motor performance, fluid pressure, temperature and flow, in addition to real-time monitoring of chlorine residuals or turbidity with sensors that do not require frequent calibration. Verification of effluent water quality could be conducted on a periodic basis (e.g. biannually) and whenever the operational parameters show change in the system.

Once a treatment system has been shown to be capable of achieving the required water quality under specific operating conditions, verification of those operating conditions should be sufficient to verify continued performance. For example, once a specific chlorine dosage and residual concentration have been demonstrated to achieve the bacteriological water quality criteria, then verifying dosage and chlorine residual levels should be sufficient for routine monitoring. Periodic water quality sampling/analyses can be used to support this routine monitoring. Dosage can be verified by monitoring chlorine tank liquid levels, and chlorine residuals can be monitored using oxidation-reduction potential or other sensors on a real-time basis (as opposed to daily bacteriological verification sampling and testing).

Those parameters that can be measured with automated equipment and equipped with an alarm are most reliable when used on a continuous basis and can represent critical control points. Disinfection and power supply are two such critical control points. A disinfection system should be tested anywhere from daily to weekly, depending on the magnitude of the potential risk. For example, levels of chlorine residual can be used to monitor a chlorine disinfection system. A back-up power supply (e.g. battery or small generator) should be considered for short-term power loss. Consideration should be given to the use of telemetry where appropriate to allow better operational oversight.

Management programs should focus on proper operation and preventive maintenance to ensure long-term system performance. CSA Standard B128.01-06/B128.2-06 provides a maintenance schedule for various components of non-potable water systems, such as pumps, filter systems, storage and pressure tanks. This is in contrast to the more traditional "end-point" evaluation of water quality that focuses on system failure or malfunction. The elements described in this section provide a starting point for developing and implementing an effective management program.

Part II: Science and Technical Considerations

Risk assessment

The process of risk assessment includes four components:

1. Hazard identification -- Hazard identification is generally a qualitative process of identifying microorganisms, toxins or chemicals of concern in the water.
   
   
2. Exposure assessment -- The exposure assessment should provide an estimate (with associated uncertainty) of the occurrence and level of a contaminant in a specified volume of water at the time of the exposure event (ingestion, inhalation or dermal absorption).
   
   
3. Hazard characterization -- A hazard characterization will describe the adverse health effects that may result from ingestion of a microorganism, toxin or chemical. When data are available, the characterization should present quantitative information (dose-response relationship, probability of adverse outcomes).
   
   
4. Risk characterization -- The risk characterization is an integration of the three previous steps to derive a risk estimate -- that is, an estimate of the likelihood and severity of the adverse health effects that would occur in a given population, with associated uncertainties.

In the first step in the risk assessment process, hazard identification, it is necessary to establish, at least approximately, the quality and quantity of water that is produced from domestic activities (the household effluent) and that is available for treatment and beneficial reuse.

The terminology used in discussions of water reclamation often makes a distinction between "greywater" and "sewage." Sources of household greywater can include bath, shower, sink and laundry water, but not toilet water (Asano, 1998). Greywater does not generally include kitchen sink or dishwasher waste, as it is highly contaminated with fats and food waste. Domestic sewage includes the discharge from all household sources, including toilet and kitchen waste. Although greywater will contain less faecal matter than sewage, both sources of water can contain a wide range of agents that pose risks to human health, including chemicals and pathogenic microorganisms. Both types of water require similar levels of treatment. The type and use of household appliances, the number and age distribution of occupants, their personal habits and the total quantity of water used can all have a marked effect on the final composition of the untreated effluent. Constituents of untreated effluent may include:

• microorganisms, some of which may be pathogenic;
• chemical contaminants, such as dissolved salts (sodium, nitrogen, phosphates and chloride), soaps and detergents;
• a high organic content from fats and oils;
• particles from food, lint, grit, hair, etc.; and
• a variety of household, vehicle and garden chemicals.¹

Hazard identification -- microbiological characteristics

Microbiological hazards have been identified as the greatest source of risk to human health from the use of reclaimed water (Yates and Gerba, 1998; Toze, 2004; U.S. EPA, 2004; EPHC/NRMMC, 2005). Several factors contribute to the critical nature of microbiological contamination. These include the potentially high numbers of pathogens in effluent and the highly infectious nature of some organisms. The acute nature of disease in the exposed individual or community combined with the potential for person-to-person infection make microbiological threats of paramount importance (Devaux et al, 2003; FAO/WHO, 2003).

Human enteric pathogens can be found in water contaminated by human waste and may be washed into greywater during hand washing, bathing, showering and clothes laundering. In conditions of high levels of biodegradable carbon and warm temperatures, such as might be found in recycled water storage, opportunistic pathogens such as Pseudomonas aeruginosa and Aeromonas spp. could conceivably grow, while biofilms in water pipes have been shown to allow the growth of Legionella spp. and Mycobacterium avium. The growth and survival of total coliforms (indicator organisms) in household storage containers for potable water have also been reported (Trevett et al., 2005). Tables 3 and 4 demonstrate the wide range in the concentration of microorganisms that may be found in greywater and wastewater.

Table 3: Ranges of indicator bacteria reported in untreated greywater^a

Source of greywater	Concentrations (CFU/100 mL)
	Total coliforms	Thermotolerant coliforms	Escherichia coli	Faecal enterococci
a From Lazarova et al. (2003), Ottoson and Stenstrom (2003), Birks et al. (2004), Gardner (2003), and FBR (2005). b N/A = not available. c Wastewater from all domestic sources excluding the toilet and kitchen sink.
Hand basins	2.4 × 102 - >2.4 × 106	N/Ab	0-2.4 × 106	0-2 × 104
Bath/shower	N/A	N/A	N/A	6.3 × 104
Bath/shower and hand basins	2.5 × 102 - 1.8 × 108	0-5.0 × 103	10 to 105	10-105
Laundry, kitchen sink	7 × 105	7.3 × 102	N/A	N/A
Greywaterc	102-106	102-106	10-105	N/A

Table 4: Enteric pathogens and indicators reported in faeces and raw sewage^a

Organism	Numbers in faeces (per gram)	Numbers in sewage (per litre)
aFrom Chappell et al. (1996), Chauret et al. (1999), Haas et al. (1999), and EPHC/NRMMC (2005). bCell culture essays. cElectron microscopic observation of viral particles
Bacteria
Coliforms (indicator)	107-109
Escherichia coli (indicator)		105-1010
Pathogenic E. coli		Low
Enterococci (indicator)		105-107
Shigella	105-109	10 -104
Salmonella spp.	104-1011	103-105
Clostridium perfringens (pathogen and indicator)		104-106
Viruses
Enteroviruses	103-107b	102-106
Adenoviruses	1010c	10-104
Noroviruses	1012c	10-104
Rotaviruses		102-105
Somatic coliphages (indicators)		106-109
F-RNA coliphages (indicators)		105-107
Protozoa
Cryptosporidium	106-107	0-104
Giardia	105-107
Helminths
Helminth ova		0-104

Although several studies have shown that household greywater can contain high levels of indicator organisms (e.g. total coliforms or E. coli), it has been suggested that bacterial indicator densities overestimate the faecal load of greywater significantly when compared with chemical biomarkers (Ottoson 2002; Ottoson and Stenstrom 2003). Based on measured levels of coprostanol, Ottoson and Stenstrom (2003) estimated the faecal load in household greywater to be 0.04 g/day per person. Using counts of E. coli resulted in an estimated faecal load of 65 g for the same greywater. This illustrates that estimating the faecal load of domestic sewage at a household level is challenging, as there is great variability in colonic function, not only between individuals but also within the same individual. Wyman et al. (1978) studied bowel movements in healthy subjects and found the mean frequency of bowel movements to be approximately one in 24 hours, with a mean size of individual stools of 111.3 g (female, SD 32.5) to 142.4 g (male, SD 55.5). As seen in Table 4, a single gram of faeces can contain a very high number of pathogens if the individual has a gastrointestinal illness. In a sensitivity analysis of the results from a quantitative microbiological risk assessment of on-site sewage systems, it was found that the rate of production of viruses (defined by the excretion and infection rates) had a significant impact on risks associated with drinking water (Charles, 2004). The implication for reclaimed water is that a minor outbreak of disease in a household served by a small or on-site system could greatly increase the risk of disease.

Significance of microorganisms in reclaimed water

The diversity of microbiological pathogens that may be found in sewage and greywater makes it impractical to monitor all of the pathogens that could be present. In drinking water treatment, authorities rely on the detection of indicator organisms to provide information about either treatment performance or the potential presence or absence of pathogens. Traditionally, these indicators have been a bacterium (e.g. E. coli) or a group of bacteria (e.g. total coliforms or thermotolerant coliforms). However, it is now known that these bacterial indicators do not correlate with the presence of protozoan or viral pathogens. It is more difficult to remove or inactivate protozoa and enteric viruses than bacteria by standard drinking water and sewage treatment processes. Ingestion of low numbers of these organisms (compared with most enteric bacteria) can lead to illness. For these reasons, protozoa and enteric viruses are likely to be of greater concern than bacteria (Blumenthal et al., 2000; Dufour et al., 2003; Gerba and Rose, 2003).

As these groups of pathogens vary in their characteristics, behaviours and susceptibility to water treatment processes, leading health authorities² have recommended that reference pathogens be used to represent each of the major groups of pathogens (i.e. bacteria, protozoa and viruses) in a risk assessment. The reference pathogens described in this document have been well characterized in the literature. For this reason, only a brief description of these pathogens is provided, together with references for further reading.

Ideally, a reference pathogen will represent a worst-case combination of:

• high occurrence;
• high concentration in water to be reclaimed;
• high pathogenicity;
• low removal in treatment; and
• long survival in the environment.

Viral reference pathogens

There are numerous enteric viruses known to infect humans. Enteric viruses associated with human waterborne illness include noroviruses, hepatitis A virus, hepatitis E virus, rotaviruses and enteroviruses (polioviruses, coxsackieviruses A and B, echoviruses and four ungrouped enteroviruses). Enteric viruses are obligate parasites, depending entirely on other living cells for reproduction (Health Canada, 2004a; Krewski et al., 2004). Although they cannot multiply in the environment, viruses can survive longer in water than most intestinal bacteria and are highly infectious as well as resistant to disinfection. It has been well demonstrated that human enteric viruses can be recovered from domestic sewage and other sewage-contaminated waters, as well as recycled water distribution biofilms (Storey and Ashbolt, 2003). Infected individuals shed viruses through faeces, often for several weeks (Krikelis et al., 1984; Hovi et al., 1996; Cloete et al., 2004).

Rotaviruses have been used in several risk assessments that examine water quality (Havelaar and Melse, 2003; Westrell et al., 2003, 2004a; Howard et al., 2006). Rotaviruses have been identified as a significant cause of viral gastroenteritis worldwide and have a relatively high infectivity compared with other waterborne viruses (Havelaar and Melse, 2003; Cloete et al., 2004). Adenoviruses have also been suggested as a candidate reference virus because they cause a range of infections (including enteric and respiratory infections) that may be associated with use of reclaimed water (WHO, 2004). A recent study has confirmed that adenoviruses, in particular adenovirus 40, are the most resistant enteric viruses to inactivation by ultraviolet (UV) light (Gerba et al., 2002; Nwachuku et al., 2005). Noroviruses, although causing less severe disease than rotaviruses, have been shown to be a prevalent cause of gastrointestinal illness in developed regions (Lopman et al., 2003; Maunula et al., 2005). There is no published dose-response model for noroviruses at this time, but one study found that as few as 10 organisms may be sufficient to cause infection (Schaub and Oshiro, 2000). Humans are the only natural reservoir for noroviruses, enteroviruses and rotaviruses.

Owing to the prevalence of infection, the possibility of severe outcomes and the availability of a dose-response model, rotavirus has been selected as the reference pathogen for the viral risk assessment in these guidelines.

Protozoan reference pathogens

Protozoa are relatively large pathogenic microorganisms that multiply only in the gastrointestinal tract of their hosts. The enteric protozoa that are most often associated with waterborne disease include C ryptosporidium parvum and Giardia lamblia. Emerging protozoan pathogens include Cyclospora cayetanensis and many microsporidian species (Cloete et al., 2004). Cryptosporidium parvum has been identified as a good candidate for a protozoan reference organism. It is reasonably infective, although different genotypes appear to have unique virulence and infectious dose properties (Gale, 2001; Teunis et al., 2002; Health Canada, 2004b). This protozoan is resistant to chlorination and has emerged as one of the most important waterborne human pathogens in developed countries (NHMRC/NRMMC, 2004). Giardia lamblia is another protozoan pathogen that is highly resistant to environmental stresses. It is typically present at some 10-100 times the concentration of C. parvum (Yates and Gerba, 1998), and it may be marginally more infective than the latter (Rose et al., 1991). Giardia infections are believed to be endemic in both humans and animals. However, Giardia lamblia is more readily removed by water treatment processes and is more sensitive to most types of disinfection than Cryptosporidum spp.(Health Canada, 2004b; NHMRC/NRMMC, 2004; WHO, 2004).

As with rotavirus, the prevalence of C. parvum, the potential for widespread disease, the organism's resistance to treatment and the availability of a dose-response model make C. parvum a useful choice as the reference pathogen for protozoan hazards.

Bacterial reference pathogens

There are a number of candidates for bacterial reference organisms, including pathogenic E. coli, Campylobacter jejuni, Shigella spp. and Salmonella spp. Although E. coli is a normal component of the human faecal flora and a useful marker of faecal pollution, some strains are human pathogens. There are six main virulence types of pathogenic E. coli, which may be divided into non-enterohaemorrhagic and enterohaemorrhagic groups. The first group includes enteropathogenic, enteroinvasive and enterotoxigenic strains; approximately 2-8% of the E. coli found in water have been found to be pathogenic E. coli (Haas et al., 1999; Hunter, 2003). The enterohaemorrhagic strain E. coli O157:H7 has a higher disease burden per case than any of the other organisms noted above, owing in part to the potential for approximately 10% of children less than 10 years of age to develop haemolytic uraemic syndrome following exposure to this pathogen (Havelaar and Melse, 2003; Hunter, 2003). This organism has been of increasing concern in Canada since a devastating waterborne disease outbreak occurred in 2001 in Walkerton, Ontario. Together with Campylobacter jejuni, E. coli O157:H7 was identified as the aetiological agent in this outbreak, which resulted in 2300 illnesses and 7 deaths (O'Connor, 2002). This organism is prevalent in foods and appears to have a low median infectious dose (Haas et al., 1999). The severe illness caused by the O157:H7 strain of E. coli is a result of a pathogenic mechanism that produces shiga-like toxins. The dose-response relationship for Shigella dysenteriae and Shigella flexneri has been suggested as a reasonable approximation for E. coli O157:H7 (Cassin et al., 1998; IOM, 2002). This is supported by dose-response modelling work that incorporates data from E. coli O157:H7 outbreaks, which demonstrates a good fit to the Shigella model (Teunis et al., 2004; Strachan et al., 2005).

The availability of an acceptable dose-response model, data on levels of generic E. coli spp. in water and wastewater, the relatively low infectious dose and the severity of disease from E. coli O157:H7 make it an appropriate reference for bacterial pathogens.

Helminthic reference pathogens

Helminths are multi-organ worms that are more complex in structure than bacteria or protozoa. In general, helminth transmission by water is not a concern in developed nations such as Canada (Krewski et al., 2004). Addressing the health risk from the protozoan reference pathogen is expected to adequately address risks from helminths.

Hazard identification -- chemical characteristics

These guidelines focus on toilet and urinal flushing as an end use for household reclaimed water. As such, the health impacts from exposure to chemicals in the reclaimed water are expected to be minimal. Information on general physical and chemical characteristics is presented here, as these parameters may affect treatment requirements and system performance. The physical and chemical parameters most often measured in reclaimed water systems are shown in Table 5.

Table 5: Physical and chemical parameters measured in raw greywater and raw sewage^a

Parameter	Unit	Raw Greywater (range)	Raw Greywater (mean)	Raw sewage
aFrom WC/DHWA/DEWA (2005). bN/A = not available
Suspended solids	mg/L	45-330	115	100-500
Turbidity	NTU	22->200	100	N/Ab
BOD5	mg/L	90-290	160	100-500
Nitrite	mg/L	<0.1-0.8	0.3	1-10
Ammonia	mg/L	<1.0-25.4	5.3	10-30
Total Kjeldahl nitrogen	mg/L	2.1-31.5	12	20-80
Total phosphorus	mg/L	0.6-27.3	8	5-30
Sulphate	mg/L	7.9-110	35	20-100
pH		6.6-8.7	7.5	6.5-8.5
Conductivity	mS/cm	325-1140	600	300-800
Hardness (Ca and Mg)	mg/L	15-55	45	200-700
Sodium	mg/L	29-230	70	70-300

It is not yet possible to identify the complete mix of compounds present in wastewater (Crook, 1998; Eriksson et al, 2002), although these may include:

• endocrine disrupting chemicals;
• pharmaceutically active compounds (drug residuals) and personal care products; and
• complex mixtures.

As the long-term goal is to develop guidelines that will address many beneficial end uses of reclaimed water, it is useful to be aware of other chemical compounds that may be found in household effluent. These are discussed in the following sections.

Endocrine disrupting chemicals

Broad ranges of chemicals have been identified as having the potential to alter normal endocrine function in humans and wildlife; these chemicals are referred to as endocrine disrupting chemicals. Candidate endocrine disrupting chemicals include both synthetic and naturally occurring chemicals, such as surfactants, plasticizers, pesticides, polychlorinated biphenyls (PCBs), synthetic steroids, human and animal steroid hormones and phytoestrogens. The World Health Organization (WHO) and others have recently published reviews of endocrine disrupting chemicals in the context of both drinking water and reclaimed water (Damstra et al., 2002; CRCWQT, 2003; Ying et al., 2003).

Endocrine disrupting chemicals have been detected in reclaimed waters and in water bodies that receive reclaimed water discharges (Kolpin et al., 2002) and have been shown to affect aquatic biota. At this stage, there is no evidence that environmental exposure to low levels of potential endocrine disrupting chemicals affects human health. However, more research is needed on potential human health impacts of endocrine disrupting chemicals, their distribution in reclaimed waters and their removal by treatment processes (Asano and Cotruvo, 2004). There is very little information available on the presence of these chemicals in household wastewater.

Although comprehensive data are lacking, analyses of recycled water have generally found that levels of pesticides, PCBs and other organic chemicals identified as candidate endocrine disrupting chemicals are below limits of detection (EPHC/NRMMC, 2005).

Pharmaceutically active compounds and personal care products (PPCPs)

Pharmaceuticals are predominantly organic compounds formulated for therapeutic uses in humans and animals. Personal care products (PCPs) include the active ingredients found in cosmetics, fragrances, insect repellents, sunscreens and many other products. Hundreds of compounds are used in significant quantities. Although the fate of these compounds after wastewater treatment processes is still largely unknown, PPCPs are expected to be persistent in the environment, as they are designed to be highly soluble and not readily degradable in the gut. Some PPCPs are potential endocrine disrupters. The limited data available suggest that many of these chemicals survive treatment and that some others are returned to a biologically active form by deconjugation of metabolites (Wells et al., 2004; EPHC/NRMCC, 2005). Human use and excretion of these compounds are the primary source of PPCP residuals in sewage. The limits of detection for many compounds range from micrograms per litre to nanograms per litre.

The significance of trace organic compounds in wastewater is the subject of considerable debate (Fujita et al., 1996). Work by Ongerth and Khan (2004) demonstrates that residuals of pharmaceutical compounds will be present in wastewater effluents at concentrations that relate to use, excretion, degradability and other chemical characteristics. Residual concentrations reported to date are two or more orders of magnitude below those at which an effective therapeutic dose would result from ingesting water.

Complex mixtures

Complex mixtures of chemicals in drinking water and recycled water could have additive, synergistic or even antagonistic effects, even when the concentrations of the individual chemicals are very low or comply with water quality guideline values. Further research is required on the health effects of complex mixtures of chemicals.

Significance of chemicals in reclaimed water

Analyses of reclaimed water quality produced in American centralized treatment plants indicate that these facilities can consistently produce water that is of a chemical quality comparable to that of drinking water for most parameters, including heavy metals, organic chemicals, pesticides and disinfection by-products (Crook, 1998; U.S. EPA, 2004). A multi-year study of an advanced water recycling system in San Diego, California, characterized 138 organic compounds and 28 metals and inorganic compounds over a 1.5-year period. The study found no significant health risks from the non-carcinogenic health risk assessment. Carcinogenic risk associated with direct consumption of water from the advanced treatment facility was predicted to be approximately 1000 times less than that associated with consumption of the city's raw water supply (Olivieri et al., 1998). It has also been found that in centralized wastewater treatment systems, community-wide pretreatment and sewer use requirements effectively reduce the concentration of potential pollutants in the effluent (Chang et al., 2002). Smaller and on-site systems may have more difficulty in consistently achieving reductions in contaminant levels, and fewer data are available for these types of systems. In properly designed and managed recycled water systems where reclaimed water use is limited to toilet and urinal flushing, health impacts from these chemicals are not expected, because of the relatively low exposure (see Table 6).

Exposure assessment

The main focus of this exposure assessment is the consumer -- for example, a person who occupies a dwelling that is supplied with reclaimed water or where water is reclaimed on-site. In the case of centralized systems, occupational exposure can be managed by health and safety procedures in the workplace. A complete exposure assessment must consider both planned and unintended uses -- that is, intentional and accidental exposures. Unintended uses can be reduced by educating stakeholders (users, plumbers, etc.) and by management processes. However, it is difficult to completely eliminate all forms of misuse. The risk assessment in these guidelines does not address deliberate misuse, but does consider accidental misuse of reclaimed water, such as a cross-connection with the potable water supply. The exposure assessment is based upon the available information, but further research is required to provide more accurate estimates of volumes and frequencies of exposure.

Usually, the main route of exposure to microbiological and chemical hazards from various end uses of reclaimed water is ingestion. While this route is expected to be minimal in the particular case of reclaimed water used for toilet flushing, a cross-connection could lead to accidental ingestion.

Some uses of reclaimed water, including toilet flushing, can produce aerosols. There is a risk that, for example, microorganisms that cause respiratory illness (e.g. certain types of adenoviruses) may be present in aerosols and pose a hazard (Gerba et al., 1975). Aerosols and droplets may also deposit on surfaces that may in turn be touched by occupants, and subsequently ingested through hand-to-mouth contact. It is reasonable to assume that children will take less care to avoid hand-to-mouth contact after touching contaminated surfaces, but there is little information available to quantify this potential route of exposure (Trevett et al., 2005). The Australian draft National Guidelines for Water Recycling (EPHC/NRMMC, 2005) suggest an average exposure from toilet flushing of 11 mL per person per year from aerosols. Ottoson (2002) estimated water intake from inhalation of aerosols as a log-normal distribution (dependent on time and droplet size). York and Walker-Coleman (2000) suggested that for a residential irrigation scenario, "average" consumption can be based on accidental ingestion of 1 mL of reclaimed water per person per day on each of 365 days, while maximum limits can be based on accidental ingestion of 100 mL on one occasion per year.

The estimated exposure volumes and frequencies presented in Table 6 are the default values presented in the Australian government's draft National Guidelines for Water Recycling (EPHC/NRMMC, 2005). These guidelines note that the values are considered to be conservative.

Table 6: Exposures for recycled water

Source of exposure	Route of exposure	Exposure volume (mL)	Exposure frequency per person per year	Comments
aTwo recent reviews of drinking water consumption (Westrell et al., 2004b; Mons et al., 2005) calculated volumes of cold (e.g. unboiled) tap water consumption to be about 870 mL per person per day; therefore, 1 L is considered to be conservative.
Toilet flushing	Aerosol	0.01	1100	Frequency based on three uses of home toilet per day. Aerosol volumes are less than those produced by garden irrigation.
Cross-connection with drinking water supply	Ingestion	1000/day	1/1000 houses	Total consumption is estimated to be 1.5 L per day, of which 1 L is expected to be consumed cold (unboiled).a Affected individuals may consume water 365 days per year; however, only about 1/1000 houses is affected. This is likely to be a conservative estimate.

Hazard characterization

As previously noted, pathogens are likely to be the most significant health hazard in reclaimed household water used for toilet or urinal flushing, whereas chemical risks are expected to be minimal. For this reason, the hazard characterization focuses on the adverse health effects that may result from the ingestion of pathogenic microorganisms. The health outcomes associated with microbial infections are varied, ranging from asymptomatic illness to different levels of acute and chronic disease and potentially death. The relationships between doses of organisms and responses, in the form of incidence or likelihood of infection or illness, are obtained either from epidemiological investigations of outbreaks or from experimental human feeding studies (Rose et al., 1991; Haas et al., 1999; Haas, 2000; Teunis et al., 2004; WHO, 2004). In general, the doses associated with illness are much lower for viruses and protozoa than for bacteria. Ingestion of 1-10 virus particles or protozoan cysts can result in illness. In contrast, ingestion of 10³ to more than 10⁶ bacteria (depending on the type of bacterial pathogen) might be required to cause illness. Shigella spp., typhoid salmonellae and enterohaemorrhagic E. coli are notable exceptions to these, requiring fewer organisms to cause disease (Haas et al., 1999; Hunter, 2003; Teunis et al., 2004; WHO, 2004). An investigation of one outbreak found that average doses of E. coli O157:H7 in affected people were 30-35 organisms (Teunis et al., 2004). Other investigations have estimated a dose of 75 organisms ingested in a swimming-related outbreak in the United States and an average of 23 organisms consumed in a foodborne outbreak in the United States (Strachan et al., 2005). Dose-response can be influenced by host factors such as immune status, pre-existing health conditions and nutrition. The approach adopted in these guidelines is to conduct risk assessments for the general population, through the normal course of life. The dose-response models and calculations are presented in Appendix B. Separate risk assessments can be undertaken for specific subgroups with increased vulnerability, such as people with severe immunodeficiency. However, it may be challenging to identify appropriate dose-response relationships for these vulnerable subpopulations.

Risk characterization

Using a burden of disease approach, the risk characterization in these guidelines uses the information from the hazard identification, dose-response and exposure assessments to estimate the magnitude of risk. A sample risk characterization is shown in Appendix B, Table 3A, and summarized in Table 7, below. The example in Appendix B demonstrates that even with very conservative assumptions, effective water treatment should reduce the risk of illness and the associated disease burden to a very low level on an annual basis.

Table 7: Risk of illness and disease burden calculated for reference pathogens

	Cryptosporidium	Rotavirus	E. coli O157:H7
aThe disability adjusted life year (DALY) is a common unit of risk to compare different health effects that vary in severity (e.g. from mild diarrhoea to death). All of the health outcomes from a particular agent are summed to provide an estimate of the burden of disease attributable to the agent; see Appendix B for a more detailed explanation.
Risk of illness (per year, i.e. 1100 events)	7.2 × 10−7	4.5 × 10−5	3.5 × 10−6
DALY per yeara	1.1 × 10−9	3.5 × 10−8	1.7 × 10−8

Another approach is to calculate treatment goals to achieve a health target of 10−⁶ DALY³ for the specified uses of reclaimed water, based on the initial concentration of a reference pathogen in the untreated source water. The disease burden, in DALYs, is calculated from the estimated exposures to concentrations of pathogens in the recycled water. Because the reductions depend on the initial concentrations and the associated exposure, uses involving higher exposures will require greater reductions of pathogens from treatment.

Table 8: Required log reductions

Organism	Dose equivalenta	Required log reductions
		Based on aerosols from toilet flushing	Based on cross-connectionb
aDoses equivalent to 10−6 DALY. bBased on worst-case assumption of consuming 1 L/day for 365 days.
C. parvum	1.6 × 10−2	3.1	6.6
Rotavirus	2.5 × 10−3	4.5	9.0
E. coli O157:H7	7.1 × 10−3	5.3	9.8

It can be seen from Table 8 that relying on treatment technology to minimize the health risk from an accidental cross-connection with a reclaimed water system imposes extremely high treatment requirements. This example illustrates the need to implement a strong management program, with a particular focus on cross-connection control; the optimal choice of measures or combination of measures to be used will depend on an analysis of important factors in a particular situation (Blumenthal et al., 1989).

Given the scope of these guidelines and the associated low exposure, no health-based guidelines have been derived for chemicals in reclaimed water. However, the performance of small treatment plants and on-site recycled water treatment plants will be more susceptible than large plants to the impacts of unauthorized chemical discharges. Vigilance will be required to prevent or minimize any unauthorized discharges for on-site systems in particular. Preventive measures should include providing owners of systems with educational material about the need to avoid inappropriate dumping of household chemicals. The responsibilities of the owner in this regard will be similar to the need to protect, for example, a conventional septic system.

Rationale

The use of reclaimed water in residential or commercial locations can help reduce rates of water consumption in Canada. However, it is necessary to ensure that the use of reclaimed water does not pose a risk to the health of Canadians. To help address this issue, the Federal/Provincial/Territorial Committee on Health and the Environment (CHE) has asked Health Canada to lead the development of guidelines, working in collaboration with other federal departments, provinces and territories and other experts. The work has been funded by Canada Mortgage and Housing Corporation. Concurrently, CSA International has developed standards for the design and installation requirements for such non-potable water systems: CSA Standard B128.1-06/B128.2-06.

The guideline values for reclaimed water quality are based on a risk assessment approach and are established to protect public health from microbiological contaminants. There are no negative health impacts expected from chemicals in household reclaimed water used only for toilet and urinal flushing. In order to meet the reclaimed water quality guidelines, all household reclaimed water should be disinfected. Primary disinfection may be accomplished by any chemical, physical or biological means that results in the destruction, inactivation or removal of microorganisms. Chlorination should be used at least as a secondary means of disinfection to maintain chlorine residual within the storage and distribution system.

The installation of any system designed to use reclaimed water should include the development and implementation of a management program which gives due consideration to protection of public health, water quality guidelines, regulatory authority capacity, administrative and operational capacity, and the local political, social and economic climate. Once effluent water quality parameters are verified upon start-up, it may be appropriate for on-going monitoring to be based on robust secondary parameters, such as motor performance, fluid pressure, temperature and flow, in addition to real-time monitoring of chlorine residuals or turbidity with sensors that do not require frequent calibration. Verification of effluent water quality could be conducted on a periodic basis (e.g. biannually) and whenever the operational parameters show change in the system.

It is recommended that provinces and territories use this document as a basis for establishing their own requirements or options for the use of reclaimed water in their area of jurisdiction.

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Appendix A: Abbreviations, acronyms and glossary

Abbreviations and acronyms

Glossary

Appendix B: Additional risk assessment information and calculations

Health-based targets

Health-based targets are the "goal-posts" or "benchmarks" that have to be met to ensure the safe use of recycled water. In Canada, the most common form of health-based targets are numerical guideline values and/or performance targets for chemical and microbiological hazards. In relation to chemicals, a guideline value is generally the concentration or measure of a water quality characteristic that, based on present knowledge, does not pose any significant risk to the health of the consumer over a lifetime of consumption. Guideline values for microbiological hazards focus on reducing acute risks and generally rely on monitoring for indicator organisms. Performance targets describe the reduction in risk to be provided by measures such as treatment processes (aimed at reducing hazards) and on-site controls (aimed at reducing both hazards and exposure). The wide array of microbiological pathogens makes it impractical to measure for all of the potential hazards; thus, performance targets are generally framed in terms of categories of organisms (e.g. bacteria, viruses and protozoa) rather than individual pathogens.

Disability adjusted life years (DALYs)

The most recent edition of  the World Health Organization Guidelines for Drinking-water Quality (WHO, 2004) adopt 10−6 disability adjusted life year (DALY) as a reference level of risk. The draft Australian National Guidelines for Water Recycling (EPHC/NRMMC, 2005) also cite this level of risk. Havelaar and Melse (2003) note that the concept of the DALY has been introduced as a common unit of risk to compare different health effects that vary in severity -- for example, from mild diarrhoea to the most severe outcome, death. The basic principle of the DALY is to weigh each health effect for its severity, using standardized severity weights  provided within the Global Burden of Disease project (Murray and Lopez, 1996). This weight is multiplied with the duration of the health effect and the number of people affected by the particular outcome. When all of the health outcomes caused by a particular agent are summed, the result is an estimate of the burden of disease attributable to this agent. The key advantage of the DALY as a measure of public health is cited as its aggregate nature, combining years of life lost (quantity) with years lived with disability (quality). Other authorities use measures such as risk of infection or risk of illness. The U.S. Environmental Protection Agency (EPA) target is a risk of infection of 10⁻⁴ from pathogens in drinking water (one additional infection per 10 000 people) (U.S. EPA, 2004). The WHO reference level of 10⁻⁶ DALY is approximately equivalent to a lifetime additional risk of cancer of 10⁻⁵ (i.e. 1 case per 100 000 people) or, for a diarrhoea-causing pathogen with a low fatality rate, an annual risk of illness of 10⁻³ for an individual. To place this level of risk in a Canadian context,  there are approximately 1.3 cases of enteric disease annually per person in this country. The reported rate of diarrhoeal illness for specific pathogens (from all routes of exposure) in Canada (for the year 2004, rate per 100 000 population) is shown in Table 1B.

Table 1B: Rates of selected notifiable diseases in Canada, 2004^a

Notifiable disease	Rate per 100 000 population
	Age group: all ages	Age group: 1-4 years
a From PHAC (2005).
Campylobacteriosis	30.22	60.90
Cryptosporidiosis	1.85	11.56
Giardiasis	13.08	47.29
Shigellosis	2.35	5.55
Verotoxigenic E. coli (O157:H7)	3.36	13.15

Dose-response models

Risk assessments are commonly based on data and dose-response models developed from human feeding studies. Log-normal, beta-Poisson and exponential distributions (Table 2B) can be used to determine probabilities of infection following exposure to different doses of the pathogen (Haas et al., 1999). The dose from the use of reclaimed water used for flushing toilets is expected to be low, as the water is not intended for ingestion. The dose is derived from the potential for accidental ingestion and exposure as described in Section 4.3.

Table 2B: Dose-response relationships for reference organisms

Organism	Distribution	Model	Parametersa
a α and r are parameters describing probability of infection; d = dose; N50 = median infective dose; P = probability of infection. Model parameters are as described in Haas et al. (1999), except for C. parvum, where the value calculated in Health Canada (2004b) is used.
Enteric virus (rotavirus)	Beta-Poisson	P = 1 − (1 + d/N50(21/α−1))−α	α = 0.27 N50 = 5.60
Bacterium (E. coli 0157:H7)	Beta-Poisson	P = 1 − (1 + d/N50(21/α−1))−α	α = 0.2099 N50 = 1120
Protozoan (Cryptosporidium parvum)	Exponential;	P = 1 − exp(−rd)	r = 0.0047

Risk characterization

Using a burden of disease approach, the risk characterization in these guidelines uses the information from the hazard identification, dose-response and exposure assessments to estimate the magnitude of risk. A deterministic approach is used here to calculate a health-based target for the reference pathogens in the reclaimed water. A deterministic approach uses single estimates for exposure volumes and number of exposure events (e.g. point estimates). This has the disadvantage of neglecting to address variability and uncertainty and also tends to rely on conservative and even worst-case values. A stochastic analysis would help address these disadvantages, but would require more information than is currently available. A sample risk characterization is shown in Table 3B. Single estimates are used for exposure volumes and number of exposure events. The estimates used are believed to be conservative. Formulae used in the calculations are shown in Box 1B.

Table 3B: Potential disease burdens for aerosols from toilet flushing

	Cryptosporidium	Rotavirus	E. coli O157:H7
aConcentrations of Cryptosporidium and rotavirus in raw sewage are taken from EPHC/NRMMC (2005); numbers of adenovirus are used as an indication of rotaviruses because of the lack of enumeration methods for rotavirus. bConcentration of E. coli O157:H7 is calculated assuming that 2% of the maximum number of generic E. coli enumerated in raw wastewater samples from Canadian cities are pathogenic (6.2 × 106; Payment et al., 2001). More information is needed to refine this estimate c Based on log reductions shown in Table 4C; hazard concentrations reduced by secondary treatment, coagulation, filtration and disinfection. dConstants and models used to calculate risk of infection are shown in Table 2A. e Havelaar and Melse (2003). f DALYs per case based on Havelaar and Melse (2003). gThe proportion of the population susceptible to developing disease following infection. The figure of 6% for rotavirus is based on the fact that infection is common in very young children. The 6% equates to the percentage of population aged less than 5 years.
Organisms per litre in source watera,b	2000	8000	1.2 × 105
Log reduction provided by treatmentc	5	6	6
Exposure per event (L)	0.000 01	0.000 01	0.000 01
Dose per event (organisms)	2 × 10−7	8.0 × 10−8	1.2 × 10−6
Number of events per year	1100	1100	1100
Dose-response constantsd	r = 0.0047	a = 2.6 × 10−1	a = 2.1 × 10−1 N50 = 1120
Probability of infection per organism	4.7 × 10−3	2.7 × 10−1	4.8 × 10−3
Risk of infection (Pinf) (probability of infection per event)	9.4 × 10−10	4.6 × 10−6	6.0 × 10−9
Ratio of illness/infectione	0.70	0.88	0.53
Risk of illness (Pill) per event	6.6 × 10−10	4.1 × 10−8	3.2 × 10−9
Risk of illness (per year, i.e. 1100 events)	7.2 × 10−7	4.5 × 10−5	3.5 × 10−6
Disease burdenf (DALY per case)	1.5 × 10−3	1.3 × 10−2	5.5 × 10−2
Susceptibility fractiong	100%	6%	100%
DALY per year	1.1 × 10−9	3.5 × 10−8	1.7 × 10−8

Box 1B: Formulae used in Table 3B

Another approach is to calculate treatment goals to achieve a health target of 10−⁶ DALY⁴ for the specified uses of reclaimed water, based on the initial concentration of a reference pathogen in the untreated source water. The disease burden, in DALYs, is calculated from the estimated exposures to concentrations of pathogens in the recycled water. Because the reductions depend on the initial concentrations and the associated exposure, uses involving higher exposures will require greater reductions of pathogens from treatment.

The log reductions required to reach a target of 10−⁶ DALY per year in treated reclaimed water can be calculated. Dose equivalents to 10−⁶ DALY (dalyd) can be determined using the formulae given below:

DALY per year = P_inf per year * N * ratio of illness to infection * DALY per case * susceptibility fraction

Since the target DALY per year is 10−⁶ in this example, this equation can be written to solve for the dose equivalent:

Dose equivalent =	target DALY per year
	DALY per case * Pinf per organism * ratio of illness to infection * susceptibility fraction

Where concentrations of organisms in source water are known, required log reductions (Table 4B) can be calculated with the following formula:

Log reduction = log (concentration in source water × exposure (L) × N ÷ dalyd)

Table 4B: Required log reductions

Organism	Dose equivalenta	Required log reductions
		Based on aerosols from toilet flushing	Based on cross-connectionb
aDoses equivalent to 10−6 DALY (dalyd). aBased on worst-case assumption of consuming 1 L/day for 365 days.
C. parvum	1.6 × 10−2	3.1	6.6
Rotavirus	2.5 × 10−3	4.5	9.0
E. coli O157:H7	7.1 × 10−3	5.3	9.8

Appendix C: Technology verification and certification

Third-party verification is a well-recognized means of verifying manufacturers' claims for reuse effluent quality and equipment reliability. The most widely accepted standard for on-site wastewater treatment certification testing is the NSF/ANSI Standard 40 test. However, this standard is not designed to address the specific requirements of a reclaimed water treatment system. In response to this gap, there have been some initiatives to develop a testing standard for effluent disinfection and tertiary treatment -- for example, Washington State and NSF/ANSI Standard 46 disinfection and U.S. EPA tertiary treatment protocols. The Washington State protocol is summarized below.

Washington State Board of Health Disinfection Testing Protocol

In Washington State, manufacturers must certify and register their proprietary wastewater treatment products with the Department of Health before the local health officer can permit their use (Washington State Government, 2005a; 2005b).  In order to register bacteriological reduction processes, manufacturers have to meet the requirements for three treatment levels (A, B and C, as described in Table 1C) by sampling for thermotolerant coliform bacteria. The three levels are not related to water reuse criteria, but are intended for application to ground disposal systems with substandard soil conditions. This protocol effectively adds disinfection criteria to the existing ANSI/NSF Standard 40 testing protocol performance criteria. All test data submitted for product registration must be provided by an ANSI-accredited, third-party testing and certification organization whose accreditation is specific to on-site wastewater treatment products.

Table 1C: Washington State Department of Health product performance requirements for proprietary treatment products^a

Level	Parameters
	Carbonaceous BOD5 (mg/L)	TSS (mg/L)	Oil and grease	Faecal coliform (/100 mL)	Total nitrogen
aValues for Levels A-C are 30-day values (averages for carbonaceous BOD5 and TSS and geometric mean for faecal coliform). All 30-day averages throughout the test period must meet these values in order to be registered at these levels.
A	10	10	-	200	-
B	15	15	-	1000	-
C	25	30	-	50 000	-

During this testing, the following requirements apply:

• Collect both influent and effluent grab samples on three separate days each week, collected during a different daily NSF Standard 40 design loading period, to assess the overall treatment performance achieved by the full treatment and disinfection process.
• Ensure that influent characteristics fall within a range of 10⁶-10⁸ faecal coliform/100 mL calculated as 30-day geometric means during the test.
• Test the influent to any disinfection unit and report the following at each occasion of sampling performed: 1) flow rate; 2) pH; 3) temperature; 4) turbidity; and 5) colour.
• Obtain samples for faecal coliform analysis throughout the testing period, including both design loading and stress loading recovery periods, reporting the geometric mean of faecal coliform test results from all samples taken within 30-day or monthly calendar periods.
• Report all maintenance and servicing conducted during the testing period, including, for example, instances of cleaning a UV lamp or replenishment of chlorine chemicals.
• Manufacturers may register products in treatment levels A and B using disinfection, but they may not register products for treatment level C using disinfection. These treatment levels are not intended as field compliance standards; they are intended for establishing treatment product performance in a product testing setting under established protocols by qualified testing entities.

Manufacturers must register their proprietary treatment product(s) with the department by submitting a complete application, including:

• manufacturer and contact information;
• name, brand and model of the proprietary treatment product;
• description of the function of the proprietary treatment product along with any known limitations on the use of the product;
• product description and technical information, including process flow drawings and schematics; materials and characteristics; component design specifications; design capacity, volumes and flow assumptions and calculations; components; and dimensioned drawings and photos;
• daily capacity of the model or models in pounds per day of carbonaceous BOD₅;
• siting and installation requirements;
• detailed description, procedure and schedule of routine service and system maintenance events;
• estimated operational costs for the first five years of the treatment component's life, including both estimated annual electricity costs and routine maintenance costs, including replacement of parts;
• identification of information subject to protection from disclosure of trade secrets;
• copies of product brochures and manuals: sales and promotional; design; installation; operation and maintenance; and homeowner instructions;
• the most recently available product test protocol and results report;
• a signed and dated certification by the manufacturer's agent specifically including the following statement: "I certify that I represent (INSERT MANUFACTURING COMPANY NAME) and I am authorized to prepare or direct the preparation of this application for registration. I attest, under penalty of law, that this document and all attachments are true, accurate, and complete. I understand and accept that the product testing results reported with this application for registration are the parameters and values to be used for determining conformance with Treatment System Performance Testing Levels established in chapter 246-272A, WAC"; and
• a signed and dated certification from the testing entity including the statement: "I certify that I represent (INSERT TESTING ENTITY NAME) and that I am authorized to report the testing results for this proprietary treatment product. I attest, under penalty of law, that the report about the test protocol and results is true, accurate, and complete."

Products within a single series or model line (sharing distinct similarities in design, materials and capacities) may be registered under a single application, consistent with the provisions of their test protocol for the certification of other products within a product series. Products outside of the series or model line must be registered under separate applications.

Appendix D: Treatment processes

Water reclamation typically makes use of conventional wastewater treatment technologies that are widely used and readily available. The discussion of treatment for reclaimed water focuses largely on whether the treatment system is capable of consistently achieving an appropriate water quality. Most international examples of guidelines for the use of recycled water specify both general treatment processes and water quality limits for a particular group of applications (Bahri and Brissaud, 2003). For example, Title 22 of the California Code of Regulations states that to meet the "disinfected tertiary" reuse water quality standard, oxidation, coagulation, filtration and disinfection are required and must achieve a 5 log removal of viruses (CDHS, 2001). Operators monitor the performance of the system by measuring turbidity, chlorine residual and coliform bacteria, to indicate removal of:

• viruses (turbidity and chlorine residual);
• protozoa (turbidity); and
• bacteria (turbidity, chlorine residual and coliforms).

Title 22 does not require that reclaimed water be monitored for specific pathogens, except to validate non-conventional treatments used to supply water for impoundments. The State of Florida requires monitoring of Giardia and Cryptosporidium, with the sampling frequency based on the treatment plant capacity, but does not set limits on the numbers of these pathogens (FDEP, 1998).

Overview of wastewater treatment for reclaimed water

The terms widely used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, advanced secondary and tertiary. The definitions of these treatment levels vary. The definitions and descriptions provided in this appendix are for the purposes of these guidelines. Wastewater treatment levels considered suitable for the purposes of producing reclaimed water for toilet flushing use in residential and commercial buildings include secondary, advanced secondary and tertiary treatment systems. From a biological treatment perspective, these are typically characterized by the water quality produced in terms of biochemical oxygen demand (BOD) and total suspended solids (TSS) concentrations and the degree of nitrification achieved in converting ammonium (NH⁴) to nitrate (NO³). For example, MOEE (1994) lists effluent design objectives for mechanical secondary treatment as BOD₅ 25 mg/L and TSS 25 mg/L, whereas Environment Canada (1976) lists a minimum requirement of BOD₅ 20 mg/L and TSS 25 mg/L for effluent discharged to water.

Primary treatment

Primary treatment alone is not sufficient to generate reclaimed water of an acceptable quality. It is, however, an important pretreatment stage for most secondary and advanced secondary treatment processes. Primary treatment removes coarse organic and inorganic solids and grit by sedimentation and/or flotation. The organic contaminants removed can represent a significant portion of the overall BOD, TSS and fats, oils and grease in the raw wastewater. Some of the nitrogen and phosphorus may also be removed, but this is typically not an objective of primary treatment.

Secondary treatment

Secondary treatment includes an array of biological processes and requires an environment within the treatment system that is suitable for rapid microbial growth. Since aerobic (oxygen-consuming) bacteria treat wastewater more quickly and efficiently than anaerobic (no oxygen) bacteria, secondary treatment typically involves aerobic bacteria. This means that oxygen must be provided to the system either passively, through the diffusion of air through the system (as is the case with sand filters), or mechanically, introduced using blowers.

The principal purpose of secondary treatment is to remove the soluble organic components of the wastewater, in addition to colloidal or suspended forms, following primary treatment (or pretreatment) in a septic tank for smaller decentralized or on-site treatment systems. Treatment benefits include the removal of residual particulate material, inorganic contaminants and pathogens that are adsorbed (attached) to the biosolids within the system.

After secondary treatment, the effluent typically has BOD₅ and TSS concentrations less than 30 mg/L and can be effectively disinfected. Organic contaminants that are resistant to microbial breakdown, nutrients and residual solids may remain in the wastewater effluent after secondary treatment.

Advanced secondary treatment

In advanced secondary treatment, the same treatment processes and technologies described for secondary treatment are followed by filtration to remove residual and colloidal solids and some additional BOD.

Advanced secondary treatment refers to systems that can reliably achieve effluent quality approaching the detection limits for BOD₅, TSS and (with disinfection) thermotolerant coliforms. The effluent from advanced secondary treatment systems is expected to have BOD₅ and TSS concentrations less than 10 mg/L. Filtration is included in the treatment process when efficient disinfection is required; for example, the State of California (CDHS, 2001) requires that thermotolerant coliform levels be reduced to less than 2.2 CFU/100 mL. This latter level of treatment is a requirement often found internationally in standards or guidelines for "unrestricted public access" reclaimed water use. "Unrestricted public access" applications typically include recreational water uses, playing field irrigation, landscape impoundments, direct discharge to streams, vehicle washing, etc.

As noted above, advanced secondary treatment is defined as being capable of reliably and consistently achieving a BOD₅ and TSS effluent quality of less than 10 mg/L. Although secondary treatment technologies can be designed and operated to achieve this effluent quality, it is important to note that the quality of the effluent produced by commercially available manufactured products may vary widely between these products, even when based on the same technology. An appropriate strategy for ensuring that performance expectations are met is third-party verification of a treatment technology (see Appendix C).

Tertiary treatment

Tertiary treatment refers to further removal of colloidal and suspended solids, as well as nutrient (phosphorus and nitrogen) removal from the wastewater by either biological or chemical means. Excess nutrients can be a threat to the environment and, in some cases, directly to human health. In groundwater, elevated levels of nitrate can lead to methaemoglobinaemia in infants (Health Canada, 1987). Nitrogen released to surface water can be a factor in nuisance algal growth and, if released in the form of ammonia, can be toxic to aquatic organisms.

While nutrients are not generally of concern with respect to reclaimed water for toilet flushing, the presence of ammonia in the effluent can result in odour. This is primarily of concern for secondary treatment plant effluents, as treatment processes capable of reliably achieving an advanced secondary water quality will also convert the ammonia to nitrate, which has no associated foul odour.

Nutrient removal can be achieved in a number of ways, including biological and chemical treatment. Biological treatment is generally carried out using an activated sludge (suspended growth) treatment process, which has been compartmentalized into "environmental" zones, and in which bacteria can be conditioned to remove nitrogen or phosphorus.

Chemical treatment can also be used for phosphorus and ammonia removal. Phosphorus can be precipitated by adding specific chemicals to the wastewater or by adsorption through a special filter. Ammonia can be removed with ion exchange resins or with zeolite. However, chemical addition is not generally considered practical for small wastewater treatment applications. The simple conversion of ammonia to nitrate (i.e. nitrogen conversion but not removal) is also sometimes referred to as tertiary treatment. Although nitrogen is not effectively removed, the ammonia concentration in the effluent (and thus the potential aquatic toxicity) is reduced.

Treatment systems capable of removing nutrients biologically are more complex and require greater operator skill and attention than secondary treatment systems and require considerable engineering design input.

Disinfection

Disinfection is an essential treatment component for almost all wastewater reclamation applications. Disinfection destroys or inactivates the majority of microorganisms within the treated wastewater effluent, including those that are pathogenic to humans. There are three commonly applied methods of disinfection, each with its own advantages and disadvantages. These are: 1) chlorination (chlorine, chlorine dioxide, chloramines); 2) ozonation; and 3) ultraviolet (UV) irradiation. Many disinfection technologies are available and can be designed for treatment applications ranging in size from small on-site to large-scale treatment applications. Although there are exceptions, treated effluent intended for use as reclaimed water will generally require filtration in order to enhance the impact of disinfection processes.

Treatment processes

The following sections describe a number of aerobic treatment systems that have been developed to achieve secondary and advanced secondary wastewater quality levels. Wastewater treatment processes can be broken down into the following three generic categories:

1. attached film growth systems;
2. suspended growth systems; and
3. hybrid systems.

The pros and cons of treatment systems and various disinfectants are presented in the context of their usefulness for on-site or decentralized water reclamation. 

Attached growth systems

Attached growth wastewater treatment systems involve the mixing of wastewater, bacteria and oxygen to achieve treatment. However, unlike a suspended growth system in which the bacteria are vigorously mixed and rapidly removed from the system, the bacteria in an attached growth system are attached as a biofilm to support media and remain in the treatment system for long periods of time.

In fixed film processes (Figure 1D), the primary treated effluent is circulated past the support media and attached bacteria. The liquid is recirculated and sprayed on top of the support media, entraining oxygen for bacterial growth.

Rotating biological contactor

Attached growth systems can also vary with respect to how oxygen is provided to the bacteria. In some cases, such as rotating biological contactors (Figure 2D), the bacteria are lifted into the air from the wastewater by a rotating disk (to which they are attached).

Aerobic biofilters

Biofilter systems typically require primary pretreatment to prevent clogging of the filter media. Primary treated effluent is pumped on a timer-controlled basis to dose the top of the filter media (such as sand, peat or other media), and attached bacteria are bathed in wastewater and obtain oxygen from air. Examples of aerobic media filters include the single-pass sand filter (Figure 3D), single-pass media filter (Figure 4D) and intermittent sand filters, as well as textile biofilter systems (Figure 5D).

 

 

Subsurface flow constructed wetlands

Subsurface flow constructed wetland systems (Figure 6D) utilize the root system as a support medium for attached bacterial growth and as a source of oxygen for the bacteria. Primary effluent is discharged into a gravel bed in which the wetland plants are grown. As the primary effluent flows beneath the surface of the wetland, bacteria attached to the gravel and plant roots consume the soluble organic material and trap and digest organic particulate matter entering the system .

Wetland systems are very dependent upon site grade and size characteristics. Proper site evaluation is important to ensure the overall efficiency of the wetland system. In general, on small lots, careful placing and sizing are required, since the combined subsurface wetland and subsurface discharge field take up a large footprint. Most subsurface wetland treatment systems will achieve the efficiency of secondary treatment processes, however there may be concerns with the long-term efficiency of such systems (Hench et al 2003). Field size requirements may vary, depending upon local site soil conditions.

Sand filters

Sand filters usually consist of beds of sand that trap and adsorb contaminants as the wastewater flows through it. "Sand filters," depending on the design, can have two treatment functions, which are not necessarily inclusive: 1) physical filtration (separation) of particulate matter; and 2) biofiltration (i.e. intermittent or recirculating sand filters), which involves physical particulate separation and the adsorption and biodegradation of soluble and particulate organic contaminants from the wastewater (Hamoda et al., 2004).

If the sand filter is open at the surface and the flow rate is intermittent and/or low enough to maintain aerobic conditions (i.e. the supply of oxygen) for fixed film bacteria to treat the wastewater (i.e. intermittent or recirculating sand filter), the filter can provide biological treatment of the trapped organic material. Bacterial colonies attached to the sand particles adsorb organic contaminants and digest them. Other bacteria and microorganisms digest the bacteria growing on the organic material in the wastewater. As long as the sand filter is not overloaded, the filter will continue to treat wastewater effectively for many years before the sand needs to be replaced due to biosolids clogging at the surface. If the sand filter is flooded but open at the surface (i.e. slow sand filter), less oxygen is provided to attached growth bacteria and a lower level of biological treatment is expected.

If the sand filter is pressurized within a container or subject to high flows (i.e. a sand filter typically designed for physically removing sediment and particulates for potable water treatment) and no oxygen source is provided, then no biological treatment is expected. If adequate biological treatment is not achieved, bacteria in treatment stages beyond the sand filter will continue to consume the residual organic contaminants, and the effluent may become anaerobic (i.e. septic), resulting in downstream problems and odours.

The land area required for sand filtration depends on the degree of biological treatment required or expected. For example, intermittent sand filters or recirculating sand filters require significantly more land area than a pressure vessel style sand filter (note that the pressurized sand filter provides only physical filtration of particulates, not biological treatment). Similarly, intermittent sand filters may require up to 40 m² per household, whereas recirculating biofilters may require as little as 2 m² for the same level of biological treatment.

Before being treated in the sand filter, the wastewater needs to pass through a settling (septic) tank and possibly a grease trap or screen to reduce loading to the filter and avoid clogging. Properly designed sand filtration systems have the ability to treat wastewater to an advanced secondary standard, with relatively low maintenance and operating cost.

The advantages and disadvantages of attached growth systems are summarized in Box 1D.

Box 1D: Attached growth systems

Pros

Cons

Simple operation. Well-understood technology and process design. Less operator attention and maintenance than competing secondary treatment technologies. Low operating cost. Can be designed to nitrify (convert ammonia to nitrate). Effective at reducing the number of disease-causing microorganisms that may be present, but does not eliminate them. Produces less waste biosolids than suspended growth treatment systems.

No ability to adapt to varying wastewater characteristics (no adjustments). Typically has limited operational flexibility to adapt to changes in influent loading. The amount of media on which bacteria can grow is fixed, limiting the number of bacteria available for treatment. Greater capital and operating cost than conventional septic tank or mound disposal systems. High capital cost. High land area required in comparison to alternative mechanically based biological treatment systems. Not characteristically capable of tertiary treatment (nitrogen and phosphorus removal). In the case of media (sand, peat, etc.) filters, there is a potential for filter clogging to occur. Remedial activity can range from replacing the top layer of the filter, in the case of aerobic single-pass filters, to having to replace the entire filter bed, in the case of upflow sand filters.

Suspended growth systems

Suspended growth aerobic treatment plants use a high concentration of bacteria grown in an aerated tank to treat the wastewater. Raw or more often primary (septic tank) treated wastewater enters a tank, where it is thoroughly mixed with bacteria and oxygen, usually using compressed air. Adequate mixing of the wastewater is required to keep the bacteria in suspension and to transfer nutrients and oxygen to the microbes. The bacteria consume the organic matter in the wastewater as a food source for cell growth.

The treated wastewater and suspended bacteria are then transferred to a settling (clarification) chamber that separates the bacteria from the treated effluent. The design and operation of this separation stage are the most crucial factors in ensuring a high-quality effluent. If bacteria are not well separated, their presence contributes to both the BOD₅ and TSS effluent concentrations. In the warm season, and given adequate levels of bacteria, sufficient time in the aeration chamber to allow the bacteria to treat the wastewater and a well-designed settling chamber, this type of system is capable of achieving an advanced secondary effluent quality (i.e. BOD₅ and TSS less than 10 mg/L). However, at lower temperatures, suspended growth systems will not perform at the advanced secondary level.

In on-site and small-scale applications, aerated treatment units usually refer to activated sludge treatment processes in which air containing oxygen is bubbled into a tank containing wastewater and suspended bacteria. The air flow must keep the water well mixed to prevent bacteria from settling out on the bottom of the tank and to ensure that the bacteria are provided with adequate food and oxygen for growth. Bacteria present in the wastewater consume the dissolved oxygen and organic contaminants, reducing the concentration of the contaminants and, in turn, growing and producing more bacteria. Aerated treatment units typically include a clarification stage to remove the suspended bacteria and may be preceded by a septic tank to settle solids and remove oils and grease. The clarifiers are often the "weakest link" in the process, as it is difficult to design small-scale clarifiers that reliably remove suspended matter.

Figures 7D and 8D are two examples of suspended growth treatment systems, illustrating a multiple-tank configuration and a combined configuration (single tank with multiple partitions). Note the different settling chamber designs (central cone versus separate rectangular chamber).

 

Sequencing batch reactor

Although the sequencing batch reactor is also a suspended growth secondary treatment system, it deserves special mention due to its unique mode of automated operation and the absence of a separate settling chamber (Figure 9D).

Raw wastewater first receives preliminary treatment (typically using a septic tank) and then flows into an aeration tank that contains bacteria from the previous batch operation. Once the tank is filled, the bacteria and wastewater are vigorously mixed with air (providing mixing energy and oxygen), and sufficient time passes for BOD₅ removal to occur. A small amount of bacteria is removed (wasted) from the system to maintain a constant concentration for the next batch. The aeration is halted, and the remaining bacteria are allowed to settle to the bottom of the tank. The treated effluent is then drawn from the top of the tank, and the process is repeated. The treated effluent flows into a pump chamber, where it is discharged to the disposal field.

In addition to being capable of achieving advanced secondary treatment, this type of process can also be designed and operated to achieve tertiary treatment (biological nitrogen and phosphorus removal). Its key advantage over most other treatment processes is the small amount of land required, and its suitability for remote monitoring and control.

This type of treatment unit requires:

• ongoing inspection and maintenance to work efficiently;
• timers to be adjusted or changed periodically according to user habits; and
• sludge removal maintenance to prevent discharge of excessive amounts of suspended solids to the subsurface discharge field.

The advantages and disadvantages of suspended growth systems are summarized in Box 2D

Box 2D: Suspended growth systems

Pros

Cons

Well-understood technology and process design. Large number of commercially available products. Less land area required for treatment than with sand filter systems. Can achieve advanced secondary and tertiary effluent quality if appropriately designed and operated. High degree of operational flexibility to accommodate varying greywater/wastewater strengths and flows. For example, the concentration (number) of bacteria in suspended growth systems can be readily adjusted by increasing or decreasing the amount of bacteria wasted from the system. Suitable for remote monitoring and control.

High operating cost due to energy and operations and maintenance (O&M) labour. High capital cost. Requires potentially more operator attention than fixed film processes. Subject to process upsets due to high flows or chemicals present, resulting in poor effluent quality or discharge of large quantities of solids (sludge) that may block downstream irrigation pipe or create problems for reuse applications (e.g. sludge or sediment buildup in toilet tanks, reduced disinfection effectiveness, etc.). Greater amount of O&M required than for other treatment systems. Generates waste biosolids that must be stored and routinely removed from the site.

Hybrid systems

Hybrid systems incorporate both suspended growth and fixed film growth bacterial treatment processes. Examples of this include moving bed biofilm reactors (MBBR) and plant-based systems.

Moving bed biofilm reactors (MBBR)

A recent innovation is the development of fixed film support media that can be suspended (mixed) within a tank, referred to as the moving bed biofilm reactor (MBBR). The MBBR process involves discharging primary treated effluent into an aeration chamber containing up to 50% by volume of small, 7-10 mm, high-density polyethylene hollow media called "biofilm carriers." Bacteria become attached to the media, which are vigorously mixed like a suspended growth process, using compressed air to provide mixing energy and a source of oxygen. The advantage over suspended growth systems is that MBBR systems can sustain a significantly higher concentration of bacteria for treatment.

Plant-based systems

There are commercially available plant-based, typically greenhouse-enclosed, wastewater treatment systems. These systems are based on an extended aeration activated sludge process in which plant roots are suspended from the tops of the aerated tanks; bacteria grow both in suspension and attached to the plant roots. The plants serve primarily as support media for attached growth bacteria, and secondarily to extract nutrients from the wastewater.

Disinfection

Disinfection may be achieved using chlorine, chlorine dioxide, ozone or UV light. For comparative purposes, Tables 1D, 2D and 3D provide a comparison of the concentration (mg/L) and time (minutes) (CT) values for various degrees of virus and Giardia inactivation in water, for the methods of disinfection described in this section (chlorine, chlorine dioxide, ozone) as well as UV light dose. Note that the CT values shown for chlorine are based on having a free chlorine residual.

Table 1D: CT values for inactivation of viruses^a

	Inactivation (mg·min/L)
aFrom U.S. EPA (1999). CT values were obtained from AWWA (1991). bValues are based on a temperature of 10°C, pH range of 6-9 and a free chlorine residual of 0.2-0.5 mg/L. cValues are based on a temperature of 10°C and a pH of 8. dValues are based on a temperature of 10°C and a pH range of 6-9.
Disinfectant	2 log	3 log	4 log
Chlorineb	3	4	6
Chloraminec	643	1067	1491
Chlorine dioxided	4.2	12.8	25.1
Ozone	0.5	0.8	1.0

Table 2D: CT values for inactivation of Giardia cysts^a

	Inactivation (mg·min/L)
Disinfectant	0.5 log	1 log	1.5 log	2 log	2.5 log	3 log
aFrom U.S. EPA (1999). CT values were obtained from AWWA (1991). bValues are based on a free chlorine residual less than or equal to 0.4 mg/L, temperature of 10°C and a pH of 7. cValues are based on a temperature of 10°C and a pH in the range of 6-9.
Chlorineb	17	35	52	69	87	104
Chloraminec	310	615	930	1230	1540	1850
Chlorine dioxidec	4	7.7	12	15	19	23
Ozonec	0.23	0.48	0.72	0.95	1.2	1.43

Table 3D: UV dose (mJ/cm2) required for up to 3 log (99.9%) inactivation of various microorganisms^a

	Log inactivation
aFrom U.S. EPA (2003).
Microorganism	0.5	1.0	1.5	2.0	2.5	3.0	3.5	4.0
Cryptosporidium	1.6	2.5	3.9	5.8	8.5	12	-	-
Giardia	1.5	2.1	3.0	5.2	7.7	11	-	-
Virus	39	58	79	100	121	143	163	186

Chlorination

In small systems, chlorination is operationally the simplest method of disinfection. In individual household and very small cluster treatment system applications, this may involve using solid chemical (sodium or calcium hypochlorite) "pucks," which slowly dissolve in the effluent stream -- similar to those used in disinfecting swimming pools. These provide a simple means of disinfecting treated effluent over the wide range of flow rates that are common to residential systems. The tablets are conveniently dispensed using a tablet feeder or stacking tube. As the tablet at the bottom of the tablet feeder is dissolved into solution, it is replaced by the tablet stacked above it. Such systems will require regular inspection to ensure that the chlorine tablets are fully stocked.

In larger systems, chlorination may involve the operator having to mix a chemical solution, which is then pumped into the effluent stream. Contact time is important for effective destruction of pathogens, and dechlorination is sometimes required to destroy chlorine residuals that may be toxic to aquatic life. Because chlorinated organics are potential carcinogens and residual chlorine is toxic to organisms, the use of alternatives to chlorination (although more expensive to supply and operate) may be encouraged by both provincial and federal regulatory bodies.

The advantages and disadvantages of chlorination are summarized in Box 3D.

Box 3D: Chlorination

Pros

Cons

Easiest and least expensive disinfection method. Effective biocide. Colour and odour removal. Low operator skill requirement. Highly effective if properly designed and operated. Is the most widely used form of disinfection. Low capital cost. Typically lower operating cost for chemicals and operator O&M than ozone or UV technologies. Provides a residual disinfectant to ensure reuse water remains disinfected during prolonged storage.

Chlorine reacts with residual organic contaminants to form potentially dangerous by-products. Chemical handling requirements. Chlorine gas and sodium hypochlorite are corrosive. Sodium and calcium hypochlorite are more expensive than chlorine gas. High concentrations of hypochlorite solutions are unstable and will produce a chlorate by-product. Is less effective at high pH. Is less effective at cold temperatures. Forms oxygenated by-products that are biodegradable and can promote subsequent biological growth if a residual is not maintained. Chlorine is less effective in inactivating Giardia cysts, and Cryptosporidium oocysts are highly resistant to chlorine.

Chlorine dioxide

Chlorine dioxide is an intense yellow-green coloured gas that is generated on-site. Chlorine dioxide rapidly inactivates most microorganisms over a wide pH range and is considered to be more effective than chlorine for pathogens other than viruses. It cannot be compressed or stored commercially as a gas because it is explosive under pressure.

Chlorine dioxide disinfects by oxidation. Although chlorine dioxide is unstable as a gas (decomposing into chlorine gas, oxygen gas and heat), it is highly soluble in water and is stable as an aqueous solution in the absence of light. It is an effective disinfectant that rapidly inactivates bacteria, viruses and parasites such as Giardia and, to some extent, Cryptosporidium. However, the U.S. Environmental Protection Agency's Alternative Disinfectants and Oxidants Guidance Manual (U.S. EPA, 1999) notes that even under the most favourable conditions (i.e. pH 8.5), the doses required to achieve 2 log Cryptosporidium inactivation do not appear to be a feasible alternative. The dose is calculated to be more than 3.0 mg/L with a 60-minute detention time. At the neutral pH levels typical of reuse water, the required doses may be more than 20 mg/L. Similar to chlorine, the efficacy of chlorine dioxide as a disinfectant decreases as temperature decreases.

The advantages and disadvantages of chlorine dioxide disinfection are summarized in Box 4D.

Box 4D: Chlorine dioxide disinfection

Pros	Cons
More effective than chlorine and chloramines for inactivation of viruses, Cryptosporidium and Giardia (although still not considered practical for these protozoans). Colour and odour control. Easy to generate. Inactivates microorganisms over a wide range of pH. Provides residuals.	Chlorine dioxide gas is explosive and must be generated on-site. High cost for training, sampling and laboratory testing. Equipment is typically rented, and the cost of sodium chlorite is high. Decomposes in sunlight. Can produce noxious odours in some systems.

Ozonation

Ozone is typically generated on-site using a device that applies a high voltage potential to air, and the ozonated air is then bubbled through the treated effluent. Ozone is a very reactive form of oxygen and extremely strong oxidant that can destroy a wide variety of contaminants and microorganisms. Because of its high oxidizing power, ozone is one of the most effective biocides used in water treatment, efficiently destroying bacteria, viruses and protozoan cysts. Inactivation efficiency for bacteria and viruses is not affected by pH between 6 and 9.

Because ozone is chemically unstable, it decomposes to oxygen very rapidly after generation and must be generated on-site. Disinfection with ozone may result in the production of bromate (a potential carcinogen) in the water (Health Canada, 1998).

The advantages and disadvantages of ozone disinfection are summarized in Box 5D.

Box 5D: Ozonation

Pros

Cons

More effective than chlorine, chloramines and chlorine dioxide for inactivation of viruses, Cryptosporidium and Giardia. Controls odours and colour and precipitates residual contaminants. Requires a very short contact time. Biocidal activity is not influenced by pH. Limited operator skill level required, although a higher level of maintenance and operator skill is needed compared with chlorine systems. No chemical storage or handling requirements (ozone generated on-site). Typically less maintenance than with UV systems.

Disinfection efficiency adversely affected by variations in flows and in organic content of greywater/wastewater. Ozone is toxic, and off-gas must be destroyed. Results in a precipitate that must be subsequently removed. Higher operating cost than chlorination systems (operator attention and electricity). Higher capital cost in comparison with chlorination or UV systems. Generation requires high energy and should be done on-site. Highly corrosive and toxic. No disinfectant residual.

Ultraviolet (UV) light

In a UV disinfection system, high-intensity lamps are typically submerged in liquid and produce UV light that damages the genetic material of pathogens, so that replication cannot occur. UV disinfection technology is a proven solution for contamination by harmful microorganisms, including bacteria, viruses, spores and cysts. It is becoming increasingly popular for both potable and non-potable water applications, as no chemicals are required. UV systems transfer electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce.

UV disinfection is a physical process rather than a chemical one, and there is no residual effect that can be harmful to humans or aquatic life. UV technologies are relatively easy for homeowners to use, as there is no need to generate, handle, transport or store toxic/hazardous or corrosive chemicals. Care must be taken to keep the surface of the lamps clean, as surface deposits can reduce the intensity of the light radiation applied to the liquid. Turbidity and TSS in the greywater/wastewater adversely affect the effectiveness of UV disinfection. High turbidity or TSS can be the result of poor operator attention or servicing. Consequently, it is essential that turbidity levels be maintained below 2 NTU for effective UV disinfection. For this reason, it is advisable to filter the effluent before treatment.

The advantages and disadvantages of UV light disinfection are summarized in Box 6D.

Box 6D: Ultraviolet light disinfection

Pros

Cons

Low operator skill level required. No chemical storage or handling requirements (ozone generated on-site). No off-gas or chemicals to handle. Very effective against bacteria, protozoa and most viruses.

Disinfection efficiency adversely affected by variation in organic content of greywater/wastewater, flow and colour (UV absorbance). Adversely affected by particulates present in the treated water. Higher operating cost than chlorination systems (electricity, cleaning). Higher capital cost in comparison with chlorination systems. No disinfectant residual produced. UV lamp tubes are subject to biological growth and chemical coating phenomena that interfere with UV transmission and disinfection, requiring the lamp tubes to be regularly cleaned to ensure effective performance. Some viruses (e.g. adenovirus) are resistant to UV and require a higher dosage than other organisms (see Table 3C).

Biosolids and residuals treatment
Biosolids treatment involves the treatment of solids that settle out during either or both of the primary and secondary wastewater treatment process. Depending on the size of the treatment facility, the primary solids may be stored and hauled away (e.g. septic tank) or transferred to a digestion facility to be stabilized prior to disposal. Digestion may be carried out by bacteria aerobically (with oxygen) or anaerobically (without oxygen), the former being a faster stabilization process but requiring more power, and the latter being a slower process that can be used to generate methane gas (biogas) for power generation if at an appropriately large enough scale. Alternative means of organic solids stabilization include composting and incineration.

Selection of appropriate treatment levels or scale
Wastewater can be treated on-site, at the home or building where it is generated, or it can be transported via a sewer to a common wastewater treatment or water reuse treatment plant. Studies of centralized facilities have shown that wastewater treatment processes are capable of significantly reducing the numbers of pathogens or indicator organisms present in sewage, although removal efficiencies will vary with the treatment process type, retention time, oxygen concentration, temperature and the efficiency in removing suspended solids (Garcia et al., 2002; Koivunen et al., 2003; Scott et al., 2003; Rose et al., 2004). In one study, a full-scale municipal treatment plant using biological treatment, filtration and chlorination was shown to reduce total and faecal coliforms by >7 log and coliphages and enteric viruses by >5 log. Protozoan pathogens (Giardia and Cryptosporidium species) were reduced by more than 3 log (Rose et al., 1996). While filtration has been found to be the most effective treatment process (in a conventional treatment train) for removing protozoan cysts and oocysts, infectious Cryptosporidium oocysts are detected even in the final effluent from facilities that use filtration processes (Gennaccaro et al., 2003; Scott et al., 2003; Rose et al., 2004). Monitoring data from Florida facilities indicate that, in general, the facilities that have reported pathogen data have been well operated (based on TSS, turbidity and total chlorine residual measurements). Some of the Florida facilities reporting the highest concentrations of pathogens in treated water appeared to provide effective filtration and disinfection. The range of Giardia cysts reported as potentially viable was 10-90% (average 61%), while the viable fraction of Cryptosporidium ranged from 70% to 90% (average 77%) (York et al., 2003). These findings suggest that while effective treatment of wastewater will produce a high quality of effluent, it is likely that some risks from viable pathogens will remain.

Over the last 20 years, many of the processes found in centralized treatment systems have been incorporated into on-site systems. The result has been improved system performance and wider-scale acceptance of the on-site wastewater treatment concept. New technologies that are capable of advanced secondary treatment are becoming available for on-site applications suitable for water reuse consideration (Diaper, 2004, Chu et al, 2003). Table 4D provides an overview of the indicative removals of microbial hazards that can be achieved using various treatment processes. Ranges in treatment performance are shown, as even an optimized system will show some variability in treatment performance. The information in Table 4D can be used to characterize risk in a simple, deterministic process such as that described in Section 4.5 and Appendix B. However, to characterize risk more accurately, it is preferable to use information that is specific to a given system designed to address the local or unique conditions of the installation. As an example, membranes come with a relatively wide range of pore size, which will have different performance expectations.

There are relative advantages and disadvantages to every type of treatment technology, regardless of the scale of application. Some processes are better suited to on-site needs, while others are better suited to more centralized applications. Those technologies that are mechanically complex or require greater operator attention are better suited to centralized facilities where skilled personnel are available. Processes of this kind can be broadly referred to as intensive systems that offer high performance but require a high degree of inputs, such as power, process control and operator skill level. Alternatively, processes that have fewer operating controls or variables, or where few skills are required to operate and maintain the system, are generally better suited to on-site applications.

Table 4D: Indicative log removals of enteric pathogens and indicator organisms^a