UC Davis

Population growth, rapid urbanization, and climate change have been contributing to water scarcity in many regions in the world. Access to adequate and safe freshwater is one of the grand challenges of this time. Accounting for 70% of global freshwater withdrawals, agriculture is suffering the greatest impact from the water shortage. To relieve the pressure on water supplies, municipal-treated wastewater (referred to as recycled water here forth) has been recognized as an important alternative source for irrigation water and is increasingly being applied in arid and semi-arid regions. In California, about 46% of treated wastewater is recycled for agricultural use, while in Florida, the fraction accounts for 44%. In China, recycled water irrigation began in 1957 and the reclamation rate of treated wastewater increased to 62% in 2014 in the cities that pioneered the implementation of wastewater reclamation and reuse. Wastewater reuse has been long practiced in the Mediterranean basin, especially in the more water-scarce regions where the treated wastewater reuse is up to 5-12% of the total amount of treated wastewater effluent. Overall, GIS-based analysis has shown that the land area irrigated with recycled water increased from 20 million hectares in 2007 to 36 million hectares in 2017, which represents approximately 10% of the world irrigation area. The Water Research Foundation reuse of treated wastewater offers many potential benefits, such as 1) decreasing stress on freshwater supply; 2) reducing cost and energy consumption; 3) recycling nutrients and helping maintain soil fertility; 4) reducing discharge from sewage treatment plants into the environment; and 5) avoiding the impact of new water supply developments (e.g., dams, reservoirs). Key Findings: 1. New cultivars are needed, and development will be supported by further research into salt tolerant under field conditions and to better characterize plant response to salinity in heterogeneous soil conditions, particularly under microirrigation as salinity in the soil water (ECsw) is continuously changing over space and time. 2. Management practices such as blending, cycling, and sequential use should be adopted when saline-sodic recycled water is used for irrigation. 3. The quality of the recycled water can contribute to the number of heavy metals in agricultural soils affecting the microbiological balance of soils and reducing soil fertility. 4. When agricultural fields are irrigated with recycled water, constituents of emerging concern (CECs) are unlikely to significantly accumulate in the soil, as most CECs are susceptible to degradation in multiple pathways. However, due to the incapacity to evaluate the cocktail effect of CECs, as well as lack of knowledge regarding the toxicity of CEC transformation products, the actual risk may be underestimated. 5. To date, there is little evidence to suggest that adequately treated recycled water poses more risk in terms of waterborne microbial pathogens for produce-related illness or outbreaks than other sources of irrigation water, but epidemiological and quantitative risk assessment models suggest that guidelines for the use of recycled water should be regionally specific and consider overall population health.