Disinfection of Membrane Bioreactor Permeate Using Free Chlorine: Virus Inactivation and Disinfection Byproducts Formation

The membrane bioreactor (MBR) process typically produces a high quality permeate that is essentially free of particulate matter and ammonia nitrogen. As a result, free chlorine may be an effective alternative to chloramines for disinfection of MBR permeate. California Water Recycling Criteria (Title 22) require all chlorine disinfection processes to meet a minimum CT value (product of total chlorine residual and modal contact time) of 450 mg Cl2-min/L, or to demonstrate at least 5-log inactivation of F-specific bacteriophage MS2 or poliovirus. This study used a pilot-scale chlorine contactor to investigate free chlorine disinfection of MBR permeate under conditions that simulated broken hollow-fiber membranes. Results confirmed conclusions from previous bench-scale experiments conducted by the Sanitation Districts of Los Angeles County; free chlorine disinfection of MBR permeate meets Title 22 requirements for viruses and bacteria at CT values significantly lower than 450 mg Cl2-min/L. In addition, free chlorine formed no significant amounts of the disinfection byproduct N-nitrosodimethylamine, and yielded levels of trihalomethanes and haloacetic acids well below drinking water standards.