Statistical Comparison of Disinfection Efficacy of Ultraviolet Irradiation and Chlorination at Wastewater Treatment Plants in North Carolina
Creators: name: Healy, Timothy
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2007

description: Ultraviolet (UV) disinfection has replaced chlorine disinfection in many North Carolina wastewater treatment plants (WWTPs) mainly to avoid effluent toxicity. However, no studies have been conducted in North Carolina specifically to compare the efficacy of these processes and there are only a few comparisons with data from elsewhere in the U.S. The purpose of this study was to compare the effluent fecal coliform (FC) number concentrations achieved by UV irradiation and chlorination at 304 municipal WWTPs (60 utilizing UV irradiation and 244 utilizing chlorination) in North Carolina. Data were obtained from the NPDES monthly discharge monitoring reports from June 2004 to June 2006. The statistical analysis was performed using the generalized linear model (GENMOD) procedure in the SAS statistical package. WWTP were divided into three design flow rate categories (>1 mgd; 0.1-1 mgd; <O.lmgd). The other categorical independent variables included were total suspended solids (TSS), temperature and the ratio of actual flow to design flow rate (Q[act]/Q[D]). The regression equation produced by GENMOD with the independent variables selected was a poor predictor of effluent FC number concentration by either UV irradiation or chlorination. Using the model to account for the effects of the independent variables, no statistically significant difference was found between effluent FC number concentration at WWTPs using UV irradiation and chlorination. A predictor of effluent FC number concentration was developed specifically for the Triangle WWTP. The goal was to assist in the development of a compliance strategy for the Total Maximum Daily Loading (TMDL) to Northeast Creek. A multi-variable, step-wise linear regression analysis was performed using three years of discharge monitoring reports data for effluent TSS, effluent biochemical oxygen demand (BOD), temperature, Q[act]/Q[D], and rainfall (given its link to stormwater flow). Like the GENMOD results, the regression model for the Triangle WWTP was also a poor predictor of effluent FC number concentration. Unfortunately, neither modeling approach included the influent FC number concentrations because these data are not collected at WWTPs. Effluent FC number concentration would be directly proportional to the influent value if the disinfection rate is first order with respect to this independent variable, a widely accepted assumption. Thus, the lack of success in regression modeling may be largely explained by failure to include influent FC number concentration. Given that the statistical analysis showed that the same effluent FC concentration is produced by both processes, the choice between UV irradiation and chlorination then depends on cost. Capital cost and operating and maintenance costs for UV irradiation and chlorination were collected from the literature. UV irradiation was found to have a lower annualized cost than chlorination. In the event that a TMDL for Northeast Creek calls for higher removal of FC than can be achieved with either process, the membrane bioreactor (MBR) technology provides another option that gives essentially complete removal. MBR cost data from the literature showed this process to more expensive than either UV or chlorination; however, the cost basis includes the equivalent secondary, biological treatment as well as disinfection.