Sensor setpoints that ensure compliance with microbial water quality targets for membrane bioreactor and chlorination treatment in on-site water reuse systems

Eva Reynaert; Flavia Gretener; Timothy R Julian; Eberhard Morgenroth

doi:10.1016/j.wroa.2022.100164

Sensor setpoints that ensure compliance with microbial water quality targets for membrane bioreactor and chlorination treatment in on-site water reuse systems

Water Res X. 2022 Dec 27:18:100164. doi: 10.1016/j.wroa.2022.100164. eCollection 2023 Jan 1.

Authors

Eva Reynaert^{1

2}, Flavia Gretener^{1

2}, Timothy R Julian^{1

3

4}, Eberhard Morgenroth^{1

2}

Affiliations

¹ Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland.
² ETH Zürich, Institute of Environmental Engineering, 8093 Zürich, Switzerland.
³ Swiss Tropical and Public Health Institute, 4051 Basel, Switzerland.
⁴ University of Basel, 4055 Basel, Switzerland.

Abstract

Widespread implementation of on-site water reuse systems is hindered by the limited ability to ensure the level of treatment and protection of human health during operation. In this study, we tested the ability of five commercially available online sensors (free chlorine (FC), oxidation-reduction potential (ORP), pH, turbidity, UV absorbance at 254 nm) to predict the microbial water quality in membrane bioreactors followed by chlorination using logistic regression-based and mechanism-based models. The microbial water quality was assessed in terms of removal of enteric bacteria from the wastewater, removal of enteric viruses, and regrowth of bacteria in the treated water. We found that FC and ORP alone could predict the microbial water quality well, with ORP-based models generally performing better. We further observed that prediction accuracy did not increase when data from multiple sensors were integrated. We propose a methodology to link online sensor measurements to risk-based water quality targets, providing operation setpoints protective of human health for specific combinations of wastewaters and reuse applications. For instance, we recommend a minimum ORP of 705 mV to ensure a virus log-removal of 5, and an ORP of 765 mV for a log-removal of 6. These setpoints were selected to ensure that the percentage of events where the water is predicted to meet the quality target but it does not remains below 5%. Such a systematic approach to set sensor setpoints could be used in the development of water reuse guidelines and regulations that aim to cover a range of reuse applications with differential risks to human health.

Keywords: Bacterial regrowth; Chlorination; Online sensors; Risk-based monitoring; Virus removal; Water reuse.