Meeting California Toxics Rule Limitations on Disinfection Byproducts by Use of Chlorine Dioxide for Effluent Disinfection

The California Toxics Rule (CTR) provisions of the City of San Luis Obispo Water Reclamation's Facilities National Pollutant Discharge Elimination System (NPDES) permit set stringent limits on trihalomethanes (THMs) in the treated effluent. These limits cannot be routinely met while using hypochlorite (HOCl) as the primary disinfectant, since chlorine compounds form THMs in the wastewater which exceed the permit limits. Chlorine dioxide (ClO2) was studied as an alternative disinfectant to determine its ability to achieve effective coliform inactivation without THM formation. A pilot plant was operated to demonstrate the effectiveness of ClO2 to achieve adequate levels of bacterial disinfection without forming THM byproducts. The ability of sodium bisulfite (NaHSO3) and activated carbon to remove any residual chlorite (ClO3−.) was also investigated. Results indicate substantial success in achieving these goals in pilot plant scale work and full-scale trials are currently underway.