Towards stormwater reuse risk management plans: Methodology and catchment scale evaluation of QMRA

The reuse of stormwater represents a potential option for meeting water demands in water stressed regions as well as preventing and mitigating diffuse pollution of receiving water bodies. Particularly, the elaboration of a risk management plan for stormwater reuse may help to understand associated environmental and public health risks and design fit-for-purpose water treatment processes. In this work, it is presented an innovative methodology to perform quantitative microbial risk assessment (QMRA) for stormwater reuse by using data simulated by SWMM software. Particularly, 210 rain events were simulated by SWMM after qualitative and quantitative calibration of the sewer network model of the city of Cupra Maritima (Italy) to identify sewer overflows. Obtained concentrations of pathogens (i.e., E. coli, Campylobacter) in overflows from each critical spillway were fitted by theoretical distribution curves. Hence, QMRA for Campylobacter was performed by Monte Carlo simulation and by linking observed overflows to the exposure events of stormwater reuse for the scenario of 1) municipal irrigation, 2) garden irrigation and 3) toilet flushing as defined by the Australian Guideline for water recycling. Furthermore, QMRA analysis was repeated after simulation of sewer overflow treatment by nature-based solution (NBS) with and without disinfection (UV and performic acid - PFA). Stormwater treatments were simulated by applying uniform distributions of expected range of bacteria log removals. Results showed that stormwater treatment by nature-based solution and disinfections (PFA dose of 2.5-5 mg/L) were able to reduce the risk of Campylobacter infection to acceptable level for most of spillways in the three investigated reuse scenarios. In addition, produced data were elaborated to identify critical overflows discharging in bathing water according to the indications of the EU bathing directive.