Drinking water suppliers in many arid regions are developing plans to purify municipal wastewater as a potable water supply (i.e., "potable reuse"). Potable reuse is considered a sustainable alternative to expand water supplies in drought-prone regions for several reasons:
- Sewers capture ~75% of the drinking water entering homes, so that municipal wastewaters can meet a significant fraction of municipal water supply needs.
- Municipalities control this water supply, avoiding conflicts over the rights to conventional imported water supplies.
- Potable reuse treatment trains are less energy-intensive than other alternative water supplies, such as seawater desalination.
- Instead of discharging municipal wastewater and associated contaminants to receiving waters, the additional treatment involved with municipal wastewater purification for potable reuse removes these contaminants, all while reducing the diversion of pristine waters from ecosystems associated with imported water.
However, concerns among regulators and the public regarding the occurrence of contaminants in potable reuse waters, driven by their ultimate origin in sewage, have slowed implementation. In our recent research published in Nature Sustainability, we directly compared potable reuse waters and conventional drinking waters serving the same areas using mammalian cell cytotoxicity assays. Cytotoxicity measures the reduction in the growth of mammalian cells relative to untreated controls. Because many toxicity pathways can reduce cell growth, cytotoxicity is considered a broad metric of toxicity. Our results indicate that the cytotoxicity of potable reuse waters is comparable or lower than that of conventional drinking waters. The cytotoxicity of potable reuse waters treated using reverse osmosis (RO) membranes was much lower than that of conventional drinking waters, reflecting the high degree of treatment employed for potable reuse.
Using RO membranes to produce high-quality potable reuse water requires a lot of energy and can be very expensive. As a result, many water utilities have opted to use alternative potable reuse treatment trains based on ozonation and biofiltration (O3/BAF). Even though previous research has found that O3/BAF does not remove as many chemical contaminants as RO membranes, our results show that O3/BAF also produce potable reuse water that is less cytotoxic than conventional drinking water. These findings indicate that, despite its sewage origin, the extensive treatment applied to purify potable reuse waters can produce very high quality water.
A second important finding related to the regulatory framework for potable reuse. Current regulatory approaches to chemical contaminants focus on a limited set of individual contaminant classes, including a subset of trihalogenated methanes (e.g., chloroform) and halogenated acetic acids. Our work indicated that these regulated classes accounted for only ~0.2% of the total cytotoxicity in either potable reuse or conventional drinking waters. Thus, a new regulatory approach is needed to better protect public health.