Near Real-Time Flow Cytometry Monitoring of Bacterial and Viral Removal Efficiencies during Water Reclamation Processes

Wastewater reuse has become an important part of the urban water supply portfolio in water stressed regions. Effective wastewater treatment processes are critical to protect public health during water reuse practices. However, the microbial removal efficiencies in wastewater reclamation plants are not routinely monitored due to the lack of a simple quantification method. This study applied a near real-time flow cytometry (FCM) technique to quantify the removal of total bacteria and viruses at three wastewater reclamation plants in Southern California. The results showed that the activated sludge process removed 1–2 log10 of bacteria but was not efficient at removing viruses. The membrane bioreactor process was capable of removing both bacteria and viruses with high efficiency. At the plant using chloramines as the main disinfectant, even though culturable total coliform bacteria were effectively reduced to the level meeting the California Title 22 Water Recycling Criteria (7-day median of 2.2 most probable number (MPN)/100 mL, and no more than one sample exceeds 23 MPN/100 mL), the disinfected final effluent still contained greater than 106 bacterial and 108 viral particles per mL in. In contrast, more than 4 log10 removal of both bacteria and viruses were observed at the plant using free chlorine as the main disinfectant. The results indicate that additional microbial indicators are needed and suggest the potential use of FCM as a rapid monitoring tool for evaluation of microbial removal.