Chlorine Demand and Trihalomethane Formation during Chlorination of Wastewater in Hillsborough County, Florida: Effects of Temperature and Chlorine Dose

Abstract

The Northwest Regional Water Reclamation Facility (NWRWRF) in Hillsborough County, Florida, is considering whether to switch from disinfection via ultraviolet (UV) radiation to disinfection via chlorination. To aid in this decision, chlorine demand and trihalomethane (THM) formation potential were assessed following chlorination of NWRWRF effluent at different temperatures, chlorine doses, and contact times. A chlorine dose of 9 mg/L was adequate to meet the strict disinfection guidelines (C·t≥12 mg·min/L) at 16 or 23°C, but a dose of 12 mg/L was required at 3°C. Chloroform, bromodichloromethane, and dibromochloromethane were formed as disinfection by-products following chlorination; bromoform was not observed. Semiempirical equations were developed that can adequately predict the concentration of chloroform, bromodichloromethane, and total THMs as functions of temperature, chlorine dose, and contact time. THM formation in NWRWRF effluent followed the same general trends as had been observed previously in other studies on chlorination of wastewater, but specific details or dependencies are site specific. For instance, THM formation in NWRWRF effluent was much less sensitive to chlorine dose than had been observed previously. The authors conclude that changing from UV disinfection to chlorination may be a viable option for NWRWRF, especially under new regulatory limits (recently promulgated by the Florida Department of Environmental Protection) for THMs discharged to Class II or III receiving waters. Bromodichloromethane is the THM of principal concern because all other THMs were formed at concentrations considerably below prior and current regulatory limits.