Disinfection By-Product Challenges Associated with Converting from Chloramination to Free Chlorine Disinfection

摘要

Greenville Utilities Commission (GUC) operates a surface water treatment plant with an overall designrncapacity of 22.5 mgd. The treatment train consists of presedimentation, coagulation (alum), flocculation,rnsedimentation, intermediate ozonation for primary disinfection, chlorine addition on top of the filters forrnmanganese oxidation and chloramination for secondary disinfection. The water supply source is the TarrnRiver. Tar River water generally contains high TOC, color and manganese concentrations. Prior to plantrnexpansion in 2004, including the addition of ozone, primary and secondary disinfection was accomplishedrnusing free chlorine. The switch to chloramines was made to achieve compliance with the Stage 1 andrnStage 2 Disinfectant/Disinfection By-Product Rule (D/DBPR). In 2012, GUC conducted a water treatmentrnplant master planning effort to identify upgrade and expansion elements required for a 50-year planningrnperiod. A component of this analysis was to evaluate the feasibility of converting back to free chlorinernsecondary disinfection while still maintaining compliance with the Stage 2 D/DBPR. This change wasrnconsidered due to concerns related to the overall biological stability of the treated water and occurrencernof elevated HPC counts in the distribution system. Concerns related to potential future regulation ofrnorgano-nitrogen DBPs also drove the decision to evaluate free chlorine as a secondary disinfectant. Thisrnpresentation will focus on alternatives evaluated to achieve DBP compliance with free chlorinerndisinfection. The evaluation considered several options to achieve disinfection compliance including 1)rnoptimization of the coagulation process, 2) alternative oxidants such as chlorine dioxide, 3) powderedrnactivated carbon addition, 4) advanced TOC removal processes including MIEX and GAC, and 5)rnozone/biofiltration. The relative performance of each option and the projected capital and operating costsrnassociated with each option will also be presented.