Page 7: Canadian Guidelines for Domestic Reclaimed Water for Use in Toilet and Urinal Flushing

Part II: Science and Technical Considerations

4.0 Risk assessment

The process of risk assessment includes four components:

  1. Hazard identification-Hazard identification is generally a qualitative process of identifying microorganisms 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, inhalation or dermal absorption of a microorganism 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 of the risk assessment process for domestic reclaimed water, hazard identification, it is necessary to establish, at least approximately, the quality and quantity of water that is produced from domestic activities (the domestic 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 "wastewater." Sources of 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 these are highly contaminated with fats and food waste. Domestic wastewater includes the discharge from all domestic sources, including toilet and kitchen waste. Although greywater will contain less faecal matter than wastewater, both sources of water can contain a wide range of agents that pose risks to human health, including chemicals and pathogenic microorganisms.

Regardless of whether greywater or wastewater is being reclaimed, the finished water quality must meet the guideline values set out in Table 1. The treatment processes required to meet these guideline values may differ for wastewater and greywater; in most cases, there will be more than one treatment option available that is capable of producing reclaimed water of an acceptable quality. When selecting a reclaimed water treatment system, the disposal requirements for any by-products produced by the system need to be considered (e.g., biosolids, membrane concentrate). 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.Footnote 1

4.1 Hazard identification-microbiological characteristics

Microbiological hazards have been identified as the greatest source of risk to human health from the use of domestic reclaimed water (Yates and Gerba, 1998; Toze, 2004; U.S. EPA, 2004; NRMMC-EPHC, 2006). Several factors contribute to the critical nature of microbiological contamination. These include the potentially high numbers of pathogens in effluent, particularly in wastewater, 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., 2001; 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, whereas 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 indicator bacteria that may be found in greywater and wastewater (Table 3) as well as faeces and raw sewage (Table 4).

Table 3: Concentration ranges of indicator bacteria reported in untreated grey- and wastewater Table 3 Footnote a
Source of greywater Concentrations (CFU/100 mL)
Total coliforms Thermotolerant

Table 3 Footnotes

Table 3 Footnote 1

From Gardner (2003), Koivunen et al. (2003), Lazarova et al. (2003), Ottoson and Stenstrom (2003), Birks et al. (2004), FBR (2005) and NRMMC-EPHC (2006).

Return to Table 3 footnote a referrer

Table 3 Footnote 2

n.a. = not available.

Return to Table 3 footnote b referrer

Table 3 Footnote 3

Wastewater from all domestic sources, excluding the toilet and kitchen sink.

Return to Table 3 footnote c referrer

Hand basins 2.4 × 102 - > 2.4 × 106 n.a.Table 3 Footnote b 0-2.4 × 106 0-2 × 104
Bath/shower and hand basins 2.5 × 102 - 1.8 × 108 0-5.0 × 103 10-105 10-105
Laundry, kitchen sink 7 × 105 7.3 × 102 n.a. n.a.
GreywaterTable 3 Footnote c 102-106 102-106 10-105 n.a.
Wastewater 106-108 106-108 106-108 104 - 106

Table 4: Enteric pathogens and indicators reported in faeces and raw sewage Table 4 Footnote a
Organism Numbers in faeces
(per gram)
Numbers in sewage
(per litre)

Table 4 Footnotes

Table 4 Footnote 1

From Chappell et al. (1996), Chauret et al. (1999), Haas et al. (1999) and NRMMC-EPHC (2006).

Return to Table 4 footnote a referrer

Table 4 Footnote 2

Cell culture assays.

Return to Table 4 footnote b referrer

Table 4 Footnote 3

Electron microscopic observation of viral particles.

Return to Table 4 footnote c referrer

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
Enteroviruses 103-107 Table 4 Footnote b 102-106
Adenoviruses 1010 Table 4 Footnote c 10-104
Noroviruses 1012 Table 4 Footnote c 10-104
Rotaviruses   102-105
Somatic coliphages (indicators)   106-109
F-RNA coliphages (indicators)   105-107
Cryptosporidium 106-107 0-104
Giardia 105-107  
Helminth ova   0-104

Although several studies have shown that domestic greywater can contain high levels of indicator organisms (i.e., 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 of human faecal pollution (Ottoson, 2002; Ottoson and Stenstrom, 2003). Based on measured levels of the chemical biomarker coprostanol, Ottoson and Stenstrom (2003) estimated the faecal load in domestic 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 greywater at a domestic level is challenging.

Estimating the faecal load of wastewater is also a challenge. 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 ranging from 111.3 g (female, standard deviation [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. The implication for reclaimed water is that a minor outbreak of disease in a household served by a cluster or on-site system could increase the level of pathogens in the untreated water (Charles, 2004). If the treatment system cannot effectively deal with the increased pathogen loading, this could increase the risk of disease in the households receiving the reclaimed water.

4.1.1 Significance of microorganisms in reclaimed water

The diversity of microbiological pathogens that may be found in wastewater 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 to remove or inactivate bacteria by standard drinking water and wastewater 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 authoritiesFootnote 2 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.
4.1.2 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 and are more resistant to disinfection compared with most intestinal bacteria. They are also highly infectious. It has been well demonstrated that human enteric viruses can be recovered from domestic wastewater 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 enteric viruses most resistant 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 in children, 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.

4.1.3 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 Cryptosporidium 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 (at the dosage and contact times used for drinking water and wastewater treatment) 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, compared with Cryptosporidum spp., Giardia lamblia is more readily removed by water treatment processes and is more sensitive to most types of disinfection (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.

4.1.4 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 S. 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.

4.1.5 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.

4.2 Hazard identification-chemical characteristics

These guidelines focus on toilet and urinal flushing as an end use for domestic reclaimed water. As such, exposure to chemicals from the reclaimed water is expected to be minimal when compared with other domestic exposures. These guidelines also recommend that all domestic reclaimed water used for toilet and urinal flushing be disinfected. This may result in the formation of disinfection by-products (DBPs). However, the health impacts from exposure to chemicals, including DBPs, 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 wastewater Table 1 Footnote a
Parameter Unit Raw greywater (range) Raw greywater (mean) Raw wastewater

Table 5 Footnotes

Table 5 Footnote 1

From WC/DHWA/DEWA (2005)

Return to Table 5 footnote a referrer

Table 5 Footnote 2

n.a. = not available.

Return to Table 5 footnote b referrer

Suspended solids mg/L 45-330 115 100-500
Turbidity NTU 22-> 200 100 n.a.Table 1 Footnote b
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 (calcium and
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;
  • pharmaceuticals (drug residuals) and personal care products (PPCPs); 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 chemical compounds that may be found in domestic effluent, including DBPs that may be produced as the result of treatment. These are discussed in the following sections.

4.2.1 Disinfection by-products

Reclaimed domestic wastewater for use in toilet and urinal flushing should be disinfected prior to use to ensure that it does not pose an unacceptable risk to human health. DBPs are usually dissolved organohalogenated compounds formed from the oxidative breakdown of organic substances in water, as a result of the application of a disinfectant (Bellar et al., 1974; Rook, 1974; Rebhun et al., 1997). Chlorine is the most commonly used disinfectant for reclaimed water. Since high concentrations of DBP precursors can be found in reclaimed wastewater, chlorination of such water requires high chlorine dosage and long contact time- conditions especially conducive to the formation of DBPs (Cooper et al., 1983; Bauman and Stenstrom, 1990). In general, human exposure to DBPs is possible through multiple routes, including ingestion, dermal absorption and inhalation (Health Canada, 2006). In the case of domestic reclaimed water used for toilet and urinal flushing, ingestion or inhalation of or dermal contact with reclaimed water should be minimal, resulting in minimal overall exposure to DBPs.

4.2.2 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. 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; Snyder et al., 2007).

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 domestic 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 (NRMMC-EPHC, 2006).

4.2.3 Pharmaceuticals 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 consumer products. Hundreds of compounds are used in significant quantities. The fate of these compounds after wastewater treatment processes is still largely unknown. 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; NRMCC-EPHC, 2006; Snyder et al., 2007). Human use and excretion of these compounds are the primary sources 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.

4.2.4 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.

4.2.5 Significance of chemicals in reclaimed water

It has 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). Analyses of the quality of reclaimed water produced in U.S. 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 DBPs (Crook, 1998; U.S. EPA, 2004). A 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. The 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). 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 domestic 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 in the next section).

4.3 Exposure assessment

The main focus of the exposure assessment is the consumer-for example, a person who occupies a dwelling that is supplied with domestic 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. These guidelines take into consideration accidental misuse of reclaimed water, such as a crossconnection 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 Guidelines for Water Recycling (NRMMC-EPHC, 2006) suggests 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, whereas 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 Guidelines for Water Recycling (NRMMC-EPHC, 2006). 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

Table 6 Footnotes

Table 6 Footnote 1

Two 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.

Return to Table 6 footnote a referrer

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 365 for 1/1000
Total consumption is estimated to be 1.5 L/day, of which 1 L is expected to be consumed cold (unboiled).Table 6 Footnote a Affected individuals may consume water 365 days/year; however, only about 1/1000 houses is affected. This is likely to be a conservative estimate.

4.4 Hazard characterization

As previously noted, pathogens are likely to be the most significant health hazard in reclaimed domestic 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.

4.5 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 3B, and summarized in Table 7. The example in Appendix B demonstrates that even with very conservative assumptions, effective water treatment should reduce the annual risk of illness and the associated disease burden to a very low level.

Table 7: Risk of illness and disease burden calculated for reference pathogens
  Cryptosporidium Rotavirus E. coli O157:H7

Table 7 Footnotes

Table 7 Footnote 1

The 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.

Return to Table 7 footnote a referrer

Risk of illness (per year, i.e., 1100 events) 2.8 × 10−6 4.4 × 10-5 3.5 × 10−6
DALY per yearTable 7 Footnote a 4.2 × 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 DALYFootnote 4 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 pathogens in the recycled water. Because the reductions depend on the initial concentrations and the associated exposure, higher concentrations of pathogens in the wastewater or higher levels of exposure will require greater reductions of pathogens from treatment.

It can be seen from Table 8 that relying on treatment technology to minimize the health risk from an accidental cross-connection with a domestic reclaimed water system imposes higher treatment requirements. This 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). With a strong management program in place, treatment systems can be designed to meet the required log reductions based only on aerosols from toilet flushing.

Table 8: Required log reductions
Organism Dose
equivalentTable 8 Footnote a
Required log reductions
Based on aerosols from toilet flushing Based on cross-connectionTable 8 Footnote b

Table 8 Footnotes

Table 8 Footnote 1

Doses equivalent to 10-6 DALY.

Return to Table 8 footnote a referrer

Table 8 Footnote 2

Based on worst-case assumption of 1 person per 1000 (10-3) consuming 1 L/day for 365 days.

Return to Table 8 footnote b referrer

C. parvum 5.3 × 10−2 2.6 4.1
Rotavirus 5.5 × 10−3 4.2 5.7
E. coli O157:H7 7.1 × 10−3 5.3 6.8

Given the scope of these guidelines and the associated low exposure, no health-based guidelines have been derived for chemicals in domestic reclaimed water. However, the performance of small treatment plants and on-site recycled water treatment plants will be more susceptible than that of 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.

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