RESIDENTIAL GRAYWATER REUSE STUDY

WATER QUALITY/SOIL QUALITY RISK ASSESSMENT

There is an absence of epidemiological data regarding the risks of gastrointestinal illness and the use of recycled household graywater for home irrigation purposes. Therefore, it is desirable to use a formal risk assessment framework to estimate the risks involved in the use of household graywater for irrigation. Quantitative risk assessment for microorganisms has been used for drinking water and reclaimed wastewater (Haas et. al, 1999). Since graywater is a form of wastewater, it is reasonable to apply a similar risk assessment methodology for the use of household graywater.

The risks involved in exposure to household graywater come from the enteric pathogens that could be present in the water. This research has shown that fecal indicators (fecal coliforms and streptococci) are present in graywater. This indicates that fecal contamination has made its way into graywater, which raises the possibility of human enteric pathogens, such as Salmonella and Shigella, as well as pathogenic enteric viruses, may have also made their way into the graywater supply.

It is known from this research that one very important human fecal indicator, Escherichia coli, has made its way into the graywater and graywater irrigated soil of several of the households participating in the study. E. coli is of significance because it is the only coliform exclusively fecal in origin (Gleeson and Gray, 1997). While not all strains are human pathogens (many are harmless mutalists in the human intestinal tract), the presence of E. coli suggests that pathogenic strains, if shed by members of the household, could end up in the graywater supply and also in the soil irrigated with it.

In order to estimate the risk of infection, a beta Poisson model was used:

P=1-(1+(N/b))-a

Where P is the probability of infection, N is the exposure, and a and b are values defined by the dose response curves specific to individual organisms (Rose).

For the purposes of this analysis, the main source of risk was assumed to be the ingestion of graywater irrigated soil by children playing in yards. Values for the average amount of soil ingested by a child during this activity are available (Haas, et. al, 1999). For a child under 6, the reported value is 200 mg/day, for a child over six, 100mg/day. Therefore, a single exposure for a child under 6 would be 200 mg, and for a child over 6 it would be 100 mg.

In estimating exposure for this model, there are two key elements: the amount of soil ingested and the number of microorganisms present in that amount of soil. With the data gathered in this study, there are different ways of estimating the amount of organisms ingested in a single exposure. One way is by using levels of fecal coliforms in soil. This can be modeled based on key assumptions:

1. That all fecal coliforms detected are E. coli
2. That they are all pathogenic strains

Using these assumptions, which constitute a worst- case scenario, exposure was calculated with numbers of fecal coliforms detected per gram of dry soil. Risk can be assessed household by household, using alpha and beta values for Escherichia coli of a=0.1705 and b=1.61´106 (Pepper, Gerba, and Brusseau, 1996).

Yearly exposure risk can be calculated by multiplying the risk for a single exposure by the number of exposures in a single year. Assuming exposure 350 days in a single year (Pepper, Gerba, and Brusseau, 1996), the risks become:

What do these estimates say about the level of risk? In the case of drinking water treatment, the U.S. Environmental Protection Agency recommends that treatment processes be designed so that a person is not subjected to a risk of infection of more than 1 per 10,000 per year (Pepper, Gerba, and Brusseau, 1996). If we set a threshold of acceptable risk of 1 in 10,000, it can be seen from Table 5 that some of these households are above the level of acceptable risks to varying degrees. Houses 1, 5, 6, 10, 13, and 14 all have risks of infection greater than 1 in 10,000 per year. In the cases of houses 6, 10, and 14, the risk is greatest. Looking at the characteristics of these households can give some idea as to the source of these risks. Sites 1, 6K, and 14 use kitchen sink graywater, indicating that the inclusion of kitchen sink water raises the risk of infection to unacceptable levels. Sites 5, 6, and 10 have children under the age of 5, which may contribute to fecal coliform levels. House 13 has neither children nor kitchen sink water, so the reasons for the elevated level of risk in this household are unclear. However, the general trend in these risk estimates is that risk is highest, surpassing the 1 in 10,000 per year threshold, in houses having small children and using kitchen sink graywater.

However, this analysis probably overestimates risk, since not all fecal coliforms are pathogenic, children are unlikely to play in exactly the same areas every day, and pathogens are not always present. Prevalence of enteric pathogens in humans ranges from about 1 to 5% in the United States (Haas, et. al, 1999). Thus, it is unlikely that any given pathogen would be present more than 5% of the time. In addition, not all infections result in disease. Finally, once applied to soil, pathogens will eventually die off, especially during the hot, dry summers in this region. This can be seen in studies with polioviruses and rotaviruses, which showed that these viruses do not persist more than 40 hours in secondary treated effluent applied to lawn grass during wintertime climate conditions in Pima County (Badawy, 1986). While bacterial pathogen risks may be overestimated, since protozoan parasites and enteric viruses have a higher infectivity, risks of infections from these pathogens might be underestimated (Haas, et. al, 1999).

CONCLUSIONS

Graywater Quality

Fecal coliforms

• Fecal coliforms were consistently detected in all samples from all sampling sites. The concentrations exhibit seasonal variation.
• Fecal coliform levels were significantly higher in households including the kitchen sink in their graywater than in houses excluding the kitchen sink, indicating that it is a major contamination source.
• Fecal coliform levels in graywater were also significantly higher in households with children, those with animals, and those using in ground storage tanks.

Escherichia coli

• E. coli was detected in samples from all sites.
• Levels of E. coli were higher in houses using in ground storage.
• Levels of E. coli were higher in houses using kitchen sink graywater
• The impact of children, animals, and storage in a household on E. coli levels in graywater appears to be small.

Protozoan Parasites

• Although samples were limited, no protozoan parasites were detected.

Coliphages

• There was only one occurrence of coliphages in graywater. This indicates that it was a random occurrence, and coliphages are not usually present in graywater. This suggests that coliphages are not a good indicator of graywater quality.

Irrigated Soil Quality

Fecal coliforms

• Fecal coliforms were detected in most samples of graywater irrigated soil, and exhibit seasonal variation.
• Levels of fecal coliforms in graywater irrigated soil were significantly higher than levels in background soil for most sites. Graywater irrigation does introduce fecal coliform contamination into the soil at levels above what is normally present.
• Fecal coliform levels were significantly higher in graywater irrigated soil at sites including the kitchen sink in their graywater.
• Levels of fecal coliforms in graywater irrigated soil differed significantly in houses with children under 12, those with animals, and those using above ground storage tanks. The impact, while significant, is small.

Escherichia coli

• E. coli and fecal streptococci were detected more frequently in graywater irrigated soil than in potable water irrigated soil. This indicates that irrigation with graywater does introduce E. coli and other organisms into the soil that would not otherwise be present.

Coliphages

• There were two occurrences of coliphages in soil, one in graywater irrigated and one in background soil. These occurrences were not correlated with any coliphages in the graywater at these sites, and thus appear to be random, again suggesting that coliphages are not a good indicator of soil quality after graywater application.

Recommendations Based on Risk

This analysis supports a recommendation that kitchen sink water should be excluded from graywater used for irrigation purposes, since it carries what is potentially the greatest risk of exposure to enteric pathogens (though not necessarily enteric viruses or protozoa). Some small additional risk may result from the presence of children, animals, and underground storage. For this reason, residents should be strongly encouraged to take into consideration the makeup of their particular household and the methods of irrigation (i.e., avoiding irrigation of entire lawns) before deciding how to recycle their graywater.

References

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13. Pepper, Ian, C.P. Gerba and M. Brusseau. (1996). Pollution Science. Academic Press, San Diego.
14. Rose, Joan. The Application of a Risk Assessment Model for Pathogenic Microorganisms in Sludge: A Case History of Risk from Sludge Treated Playing Fields (unpublished).
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