Impact of Nitrification-Denitrification on the CT Required for Reclaimed Water Disinfection

The California Title 22 regulations require a minimum chlorine concentration times contact time (CT) of 450 mg-min/L to achieve 4-log virus reduction (using poliovirus) and a 7-day median total coliform limit of 2.2 MPN/100 mL for disinfection of filtered secondary effluent. This paper presents the results of tests conducted with filtered nitrified or nitrifieddenitrified secondary effluent from various water reclamation plants (WRPs) to establish the CT required to achieve Title 22 disinfection requirements. In Title 22 use of a virus that is at least as resistant to disinfection as poliovirus is allowed for demonstration of virus reduction. Results from side-by-side disinfection studies with poliovirus and MS2 coliphage, using both free chlorine and chloramines, are also presented. Based on the test results, MS2 coliphage is substantially more resilient to both free and combined chlorine and can be used as a conservative surrogate to poliovirus.