Epidemiological study for the assessment of health risks associated with graywater reuse for irrigation in arid regions
Graphical abstract
Schematic representation of experimental design and main results suggesting that greywater use for garden irrigation posed no additional health risks to those that reuse it.
Introduction
Recently, there has been growing interest in the separation and reuse of graywater as a water-saving strategy, particularly in water-scarce regions. Approximately 60% of domestic effluent is graywater, including effluents from washing, bathing and laundry, but excluding toilet water (Gilboa and Friedler, 2008). Graywater is typically used to irrigate gardens and flush toilets and can save up to 50% of household freshwater demand (Maimon et al., 2010, Ottoson and Stenström, 2003). Furthermore, graywater reuse can be economically feasible on both a national and household scale (Gross et al., 2015).
Alongside its potential water-saving benefits, graywater must be handled responsibly to eliminate potential environmental and health risks (Gross et al., 2015). Graywater is often found to be contaminated with various pathogens associated with fecal contamination, as well as opportunistic pathogens (Table 1).
Fecal contamination of graywater is usually the result of activities such as washing fecally contaminated laundry (e.g. diapers and underwear) and showering and washing hands after contact with potentially contaminated surfaces and objects (Ottoson and Stenström, 2003). Fecal contamination, which is commonly estimated by fecal coliform concentration, exhibits high variability, ranging from non-detectable to as many as 106–107 CFU 100 ml− 1. The skin and mucus pathogens Pseudomonas aeruginosa and Staphylococcus aureus have been found in graywater at concentrations of 102 and 105 CFU 100 ml− 1, respectively (Gilboa and Friedler, 2008). Occasionally gastrointestinal bacteria, such as Salmonella enterica and Campylobacter, can be introduced by food-handling in the kitchen (Ottoson and Stenström, 2003, Gilboa and Friedler, 2008). On-site treatment systems and the subsequent localized discharge of treated effluent can lead to public health and environmental concerns through direct contact with the effluent and contamination of groundwater resources (Levett et al., 2010). Given the prevalence of fecal related bacteria found in graywater, the health concerns often associated with graywater reuse are mild to moderate gastrointestinal diseases brought on by the possible ingestion of minute to significant amounts of graywater via various exposure pathways (O'Toole et al., 2012, Maimon et al., 2010).
Epidemiology is an essential part of risk assessment. However, it may be limited by the sensitivity of the study, and seeking to associate very small risks to the background can prove to be challenging (O'Toole et al., 2012). Nevertheless, despite their limitations and difficulties, epidemiological studies can provide valuable information on the possible scale of water-related hazards, which complement the predictions obtained from quantitative microbial risk assessments (QMRAs). In addition, epidemiological studies are generally more readily understood and accepted by the public than theoretical models designed to predict disease risks (Sinclair et al., 2010). Interestingly, despite the increasing use of graywater worldwide, only a handful of studies have attempted to evaluate the health risks associated with graywater reuse by epidemiological methods (O'Toole et al., 2012, Sinclair et al., 2010). These studies were all performed in temperate regions where exposures are expected to differ from those in arid regions. Moreover, they were short-term, retrospective studies which were unable to account for seasonality and long-term effects.
In the study by O'Toole et al. (2012), members of an exposed (graywater-using) population in Australia were asked to respond to a questionnaire corresponding to the 2 weeks following graywater sampling in order to assess any link between the pathogens found in the graywater and cases of gastrointestinal illness within the household. Sinclair et al. (2010) performed an epidemiological study on a neighborhood in Australia with a dual reticulation system, where highly treated recycled wastewater was used for toilet flushing and other non-potable uses. The study compared the health status of residents from the dual reticulation area with that of residents of a nearby neighborhood with conventional water supply. Health-status determination was based on the reasons why the residents consulted local general physicians. There were no differences between the exposed group and the control group in either study.
The objectives of this study were, first, to determine whether long-term reuse of graywater for garden irrigation in an arid region leads to a higher incidence of gastrointestinal illnesses compared to a control group, and second, to compare the results of the epidemiological study to published results of health risks determined by QMRA, a common tool used by policy-makers to define risks.
Section snippets
Study population and climate
Exposed (graywater-using) and control (non-graywater-using) populations from the Ramat Negev regional council, Central Negev, Israel were asked about their health status on a weekly basis for a period of 1 year from December 2013 until December 2014. In Israel, graywater reuse is not prevalent due to legal issues. No official registry of graywater users exists, limiting the sample size in this study. The exposed group consisted of all known families recycling graywater in the region, i.e., 20
Graywater quality
In this study, the quality of the graywater subjected to various treatments was typical, with TSS ranging from below the detection limit to 50 mg l− 1, BOD from below the detection limit to 80 mg l− 1 and fecal coliforms from non-detectable to 105 CFU 100 ml− 1; all results were highly representative of graywater quality (Fig. 1). As expected, the range of results for all measured parameters was quite large as graywater quality changes between and within sources over time (Boyjoo et al., 2013, Gross et
Conclusions
No additional burden of disease was found among graywater users in this study, suggesting that graywater is not a major source of gastrointestinal disease. Moreover, based on the survey, the majority of the suspected exposures which occurred prior to the onset of illness included activities that were not related to graywater reuse. Previously reported QMRA results were more conservative than the findings from the current epidemiological survey. This implies that QMRA is a conservative tool and
Acknowledgments
This research was funded by the Rosenzweig–Coopersmith Foundation (RCF) and Zuck Macabi Foundation. The authors would like to thank the study participants, Ms. Rina Miaskovski for the laboratory assistance and Dr. Ruthi Berger for her assistance with the epidemiological questionnaire.
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