Safeness of Greywater

There are many studies on the safety of reusing greywater. A consensus among studies is that greywater is not safe to ingest, and thus greywater systems should keep greywater in pipes and soaking into the ground to prevent accidental ingestion.

When examining the potential health risks associated with greywater it is important to look at those risks in a broader context. Risks associated with an insecure water future, climate change, and sewer overflows are all risks that could be alleviated by widespread use of greywater.

A few considerations to keep in mind when considering the relative safeness of greywater reuse are the following.

  1. Residential greywater systems generate greywater from people who already live together and eat together. Any risks associated with their greywater use should be any additional exposure route via the greywater system.
  2. There are no documented cases of illness transmitted from a greywater system in the United States.

Since greywater comes from showers and washing clothes, there can be small amounts of fecal matter in the water- which could potentially transmit diseases if a non-infected person ingested infected greywater. Many studies seek to identify how much potential fecal matter is in greywater by testing for an indicator bacteria called fecal coliform bacteria . Testing for fecal coliform is not actually testing the amount of feces in greywater, and this type of indicator bacteria is known to reproduce in greywater. A more accurate indicator that does not reproduce in greywater, but is more expensive to test for, is a chemical biomarker, Coprostanol, coming from cholesteral in the gut, is one such biomarker. One study comparing the result using both a fecal indicator bacteria and a chemical biomarker found that the fecal indicator bacteria overestimated the level of feces in the water by 100 to 1000 times. This suggests that many studies using fecal coliform indicator bacteria could be over estimating the amount of feces in the water by a very large amount.

Below is a short summary of several health documents


City of Los Angeles, Office of Water Reclamation Nov, 1992

Summary: This study tested greywater and greywatered soils from sites with in the city. They concluded that the soils in the test areas were already contaminated with animal feces so the addition of greywater may be irrelevant.  They also found that total coliform indicator bacteria increased with greywater application.

This study also tested for actual disease causing organisms (salmonella, shingella, and entamoeba histolyctia), which is uncommon in greywater studies, and they didn’t find any in any of the greywater.

Study Conclusions:

From the results presented above, including baseline data, it is clear that backyard soils are contaminated, whether they are in the control areas or in the gray-water-irrigated areas. If these findings can be generalized, the implication is that gray water irrigation- below the surface of the soil-does not by itself alleviate the health risks from handling the garden soil, as long as sanitary practices are followed.

It appears that the use of gray water at the pilot project sites does not pose a significant risk to the users or the community.  Since pilot projects were controlled, inspected, and repaired as needed, broad generalizations of this conclusion may be premature. However, certain more specific generalizations  appear inescapable, e.g.:

  • Indicator bacteria (total coliform) in the soil seem to increase with gray water application. However, the soil is already so heavily contaminated with animal fecal matter that the additional contribution of gray water may be irrelevant.
  • Disease organisms, normally capable of surviving in the soil for a few days, were not present in gray-water-irrigated areas. Neither have these organisms been detected in gray water in storage.  This may indicate either an entirely healthy test population (highly unlikely), or a mechanism for deactivation of pathogens. Either way, the results indicate that there may be minimal additional risk of exposure from use of gray water for irrigation of landscaping.

Faecal contamination of greywater and associated microbial risks

Jakob Ottoson and Thor Axel Stenström
a Department of Water and Environmental Microbiology, Swedish Institute for Infectious Disease Control, SMI, SE 171 82, Solna, Sweden

Summary: This study asserts that chemical biomarkers are much more accurate than biological indicators.

“Coprostanol was detected in all samples in amounts equivalent to a mean faecal load of 0.04 g person−1 day−1. The mean concentration in the greywater was 8.6 μg L−1, compared to an average of 10,000 μg L−1 expected in wastewater. The mean values of selected faecal indicators have been used to estimate the total faecal load in the greywater, giving a faecal load of between 0.04 and 65 g person−1 day−1 For the QMRA, the faecal load in greywater was based on coprostanol concentrations as a conservative biomarker, since coprostanol is not produced within the system.”

Even with a lower amount of estimated fecal matter in the greywater was not with considered safe enough to be publicly accessed, as in irrigating a sports field.

In conclusion we suggest that guidelines for greywater recirculation and reuse should not be based on thermotolerant coliforms as a hygienic parameter, because of the large input of non-faecal coliforms and/or growth of coliforms. The overestimation of the faecal load, and thus risk, that the indicator bacteria give is however to some degree compensated for by the higher susceptibility to treatment and environmental die-off. The risk model based on faecal enterococci densities correlated well to the risk from viruses, which is supposed to be the most prominent in a system without disinfection due to their high excretion figures, environmental persistence and low infectious doses. If guidelines should be based on bacterial densities, faecal enterococci are preferred.

Link: Faecal contamination of greywater and associated microbial risks

The Arizona Study: RESIDENTIAL GRAYWATER REUSE: The Good, The Bad, and The Healthy

Summary: This was the study that helped change the Arizona code, even though it found high levels of fecal coliform in some greywater systems. The first part of the study surveyed residents to find out how much greywater use is already happening “illegally”.

In the summary of the survey they found:

It appears that 20,000 to 30,000 households in Pima County might currently be reusing graywater. These households contain 50,000 to 80,000 persons. Graywater reuse is a major issue in terms of the number of persons involved. It appears that graywater reuse is more common in older homes and lower-income areas. Residents of manufactured homes may be particularly likely to reuse graywater because of the greater access to wastewater plumbing. Septic system preservation may be a factor in some graywater reuse. Whether most of these systems are being operated in a safe and sanitary manner is not known. However, the survey results also suggest that if graywater reuse is determined to not be a public health issue, and if permitting requirements are relaxed, graywater reuse might increase substantially. Whether and how ADWR and municipal water providers should encourage graywater reuse are issues that should be addressed.

The second part of the survey studied the quality of the greywater in several greywater systems. They used E. Coli as an indicator, citing a study in 1997 that says it’s most accurate (this was before the the 2003 Ottosom study that refutes this).  They didn’t include kitchen sink water based on this study because there were higher levels of E.Coli in the sink water.

Response to using fecal coliform bacteria as an indicator by Art Ludwig.

“A study which turned up 84,000 fecal coliform bacteria per 100 ml of kitchen sink water did not consider the possibility that indicators were multiplying and there wasn’t really that much feces (or pathogens) in the water. If they realized this equates to about a teaspoon a day of feces down the kitchen sink, they might have paused to consider if this much poop was really being dumped in the kitchen sink. But based on this study, the law did not allow kitchen sink water greywater systems, but it might be OK in reality.”

Link: The Arizona Study: Residential Graywater Reuse

Longterm Effects of Landscape Irrigation Using Household Graywater

by the Water Environment Research Foundation (WERF)

Summary: The two aspects of this report relating to environmental and public health are the issues with greywater chemistry- saying there is a wide array of chemicals found in greywater — citing a study on what chemicals are in household products- they say there could be up to thousands of chemicals in greywater. This shows the need for education for greywater users so they choose healthy products.

They bring up the controversy in the greywater studies with indicator organisms possibly not being an accurate measure of pathogens in greywater. A report from the Water, Environment, Research Foundation concludes:

“But there is controversy regarding whether the indicator organism counts are an accurate indicator of the actual health threat posed to the homeowner who comes into direct contact with graywater because fecal coliform concentrations have been observed to multiply in graywater, whereas pathogens have never been observed to grow in graywater and die off
rapidly. Therefore, a high graywater fecal coliform count may not indicate the same level of pathogen exposure risk as the same fecal coliform count found in treated wastewater.” (5-4 Long Term Affects of Landscape Irrigation Using Household Graywater)

They summarize other greywater studies, finding potential health risks from fecal coliform counts, pathogens associated with fecal mater entering the system, and pathogens on food/meat. They also discuss how numerous studies have inferred a level of fecal contamination that is higher than most likely accurate due to indicator bacteria multiplying in the greywater.

They discuss how people can be exposed to pathogens from eating plants or soil irrigated with greywater, coming into contact with ponded greywater or surface water contaminated by greywater runoff. They say little info is available for fate of greywater pathogens in soil and groundwater following irrigation.

Link: Longterm Effects of Landscape Irrigation Using Household Graywater

World Health Organization: Guidelines for the safe use of wastewater, excreta, and greywater

Summary: This document highlights how bacterial indicators overestimate the fecal load in greywater saying the use of E.Coli will lead to an overestimation of the fecal load by more than 3 orders of magnitude. They say the estimated fecal input to greywater is 0.04 +/- 0.02 grams feces per person per day, compared to 65grams and 5.2 grams per person per day using E.coli or enteroccoci as indicators.

Link: Guidelines for the safe use of greywater

EcoSanRes Introduction to Greywater Management

June 2004 Peter Ridderstolpe

Summary: The author discusses how greywater is considered harmless to public health by many types of people, the exception being public authorities. There is a discussion of the overestimation of fecal load in greywater due to studies that have tested indicator bacteria.

A more accurate measurements for fecal load in greywater estimated 0.04g feces per person per day, which if treated wastewater were to be treated to that level 99.97% of the pathogens must be removed, which doesn’t usually happen in practice. (so greywater is safe.) They conclude that untreated greywater can be expected to contain far lower densities of pathogens than effluent water from an advanced wastewater treatment plant.

Design and Performance of Ecological Sanitation Systems in Norway

by Professor Petter D. Jenssen, Department of Agricultural Engineering, Agricultural University of Norway

Summary: This study examines several existing ecological sanitation projects in Norway with the goal of finding ways become more sustainable. The author writes: In order to evolve towards a sustainable society we need to recycle nutrients, reduce the water consumption, and minimize the energy needed to operate waste treatment processes.

He concludes:

Experience from Norway shows that separate treatment of blackwater and greywater nearly achieves “zero emission” and almost complete recycling. Organic household waste can be treated in the same process as the blackwater and yield a fertilizer/soil amendment and energy. The water consumption can be reduced by up to 50%. Compact technically simple greywater treatment systems facilitates decentralized treatment even in urban areas, thus the need for a secondary piping and pumping system for transport of untreated wastewater is reduced.

Link: Design and Performance of Ecological Sanitation Systems in Norway

Microbial Risk Assessment Tool to Aid in the Selection of Sustainable Urban Water Systems

Summary: This study looks at how risk is assessed, and uses a type of assessment that considers more than one rink (eg. how many indicator bacteria are in the water).

This paper describes the development of a microbial risk assessment (MRA) tool, that allows integration with environmental, economic, socio-cultural and technical function in association with the Swedish Sustainable Urban Water Project (MISTRA,, and builds on what has been previously published by the consortium (Malmqvist et al., 2000). In taking a systems approach the MRA tool attempts to account for system failures and intrinsic variability of unit operations over their intended life-times, and in theory can be applied to a broad range of urban water systems.

Analyzing two types of water systems with this risk assessment method, the standard, centralized system was found to present a “minor” risk to the community, and the ‘sources separated system’ (separating greywater, urine, and feces) yeilded a lower risk, due to less impact from recreation swimming (due to accidental sewage overflows).