Disinfection assessment in full-scale chlorine contact chambers

Abstract

In 1998, the City of Brantford Water Purification Plant was retrofitted with an ultra-high-rate upflow clarification process and two post-clarification chlorine contact chambers. The objective of the work described herein was to assess the theoretical inactivation of Giardia lamblia cysts by free chlorine disinfection in the new chlorine contact chambers. At the time this research was conducted, Ontario drinking water disinfection requirements were not as comprehensive as they are now, following the Walkerton outbreak. In an effort to remain proactive with respect to potential new disinfection regulations in Canada, the 1989 United States Environmental Protection Agency Surface Water Treatment Rule (SWTR) was used to assess the disinfection capability of the City of Brantford chlorine contact chambers. The Ontario Ministry of the Environment promulgated the Ontario Drinking Water Protection Regulation (DWPR) shortly after this project was completed (using the SWTR as a model), reaffirming the relevance of the approach described herein. This study showed that the new chlorine contactors did not provide sufficient contact time to ensure the minimum 0.5-log inactivation of G. lamblia cysts under all potential adverse scenarios. Contactor modifications were made to correct design deficiencies. The comprehensive assessment of the disinfection strategies presented here may be useful to other utilities in Ontario that must now comply with newly implemented DWPRs as well as utilities in other provinces that wish to be proactive with respect to potential new regulations in Canada regarding pathogen control. Key words: surface water treatment rule (SWTR), disinfection, chlorine, Giardia lamblia, CT concept, tracer studies, T10 contact time, log inactivation.