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Greywater reuse can significantly reduce domestic water consumption. While the benefits are promising, risks are still under debate. Using a quantitative microbial risk-assessment model, we assessed the health risks associated with greywater reuse. The pathogens Salmonella enterica, Shigella spp., and Staphylococcus aureus were evaluated due to their possible prevalence in greywater and limited information regarding their potential risk with relation to greywater reuse for irrigation. Various exposure scenarios were investigated. Monte Carlo simulation was used and results were compared to the maximum "acceptable" limit of 10 −6 disability-adjusted life years (DALY) set by the World Health Organization. Safe reuse was met for all worst-case exposure scenarios for Staphylococcus aureus, Salmonella enterica and Shigella spp. If their concentrations were kept below 10,000, 50 and 5 cfu/100 mL, respectively. For the best-practice (more realistic) scenarios, safe reuse was met for Staphylococcus aureus if its concentration was kept below 10 6 cfu/100 mL. Salmonella enterica met the safe reuse requirements if a maximum concentration of 500 cfu/100 mL was maintained and Shigella spp. if a maximum concentration was lower than 5 cfu/100 mL. Based on reported concentrations of these bacteria in greywater, proper treatment and disinfection are recommended.

Increasing stress on urban water demand has led to the exploration of the potential of rainwater use and water recycling to promote sustainable water resources management. Rainwater harvesting (RWH) not only has the potential to reduce water demand but also contributes to other sustainable objectives, including reducing stormwater pollutant loads, reducing erosion, and inducing natural flow regimes by means of flood control, in urban streams. This research involved the design, construction, and field-testing of an RWH system used to irrigate greenhouses at the Macdonald Campus of McGill University in Quebec, Canada. The purpose of the RWH system was to collect rainwater from a roof area of ≈610 m2 (the Horticulture Services Building on the Macdonald Campus of McGill University) to meet the irrigation demands of the two Horticulture Research Center greenhouses on the campus (≈149 m2 each) from May to October. Over its two years of operation, it was found that the amount of rainwater collected did not only meet the peak irrigation demands of the greenhouses (which amounted to almost 700 gal of water per day), but that there was also enough water for the irrigation of the nearby student-run gardens. The harvested rainwater was clear and did not cause any harm to the plants. The major problem that was experienced during the operation of the RWH system was that of algae growth in one of the water collection tanks. This issue was resolved by covering the tank with metallic green wallpaper, thereby blocking most of the sunlight from entering the tank. The RWH system is currently being used for irrigation and as a demonstration project to promote the learning of sustainable technologies on campus and in the surrounding communities.

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