Impact of using stored treated municipal wastewater for irrigation on the microbial quality and safety of vegetable crops

Climate change is increasing the demand for alternative water sources, with treated municipal wastewater from a wastewater treatment plant (WWTP) standing out as a potential alternative. A field study was performed to examine the impact of storage on the microbial quality of water from a WWTP and the possible microbial transmission to three vegetable crops irrigated with (stored) water from a WWTP. The analysis consisted of 13 microbial indicators, hygiene and pathogenic parameters. Results showed that storing water from a WWTP up to 32 days can result in a decrease of 4.8 log CFU/100 ml E. coli and the absence of Salmonella spp. and Listeria monocytogenes. Additionally, integrating waiting periods of 7 days post-irrigation resulted in very limited microbial transmission to the irrigated crops. However, continuous research and monitoring are essential to apply this method in practice, due to the variability of crop type and type of bacteria. As of now, in order to meet European legislation (2020/741/EC) and depending on the impact of post-harvest processing, further water treatment (e.g. disinfection) is still needed to safely use stored WWTP water as an alternative water source for irrigation.