ONSITE GENERATION OF SODIUM HYPOCHLORITE – A KEY TO REDUCING TRIHALOMETHANE CONCENTRATIONS?

The use of gaseous chlorine for disinfection poses a number of potential regulatory and safety issues for drinking water utilities. In recent years, with the enactment of the Stage 1 Disinfection Byproducts Rule (Stage 1 DBPR), compliance issues with regard to TTHMs have increased. These difficulties will likely increase following the soon to be finalized Stage 2 Disinfection Byproducts Rule (Stage 2 DBPR), causing many utilities using gaseous chlorine for disinfection to look for alternate methods of disinfection. Combined chlorine, or chloramines, has widely been touted as the alternative of choice for DBP control; however, recent studies indicate that the disinfection byproducts (DBPs) formed from combined chlorine pose health risks that are at least as serious as chlorine.Inherent dangers in the transportation and storage of chlorine gas also exist, including lethal spills and explosions resulting in large areas of impact. Following the events of September 11, 2001, the consideration of other, less hazardous methods of disinfection became a priority for many facilities. In response to these mounting concerns, HSMM was commissioned by two drinking water utilities in the Carolinas to replace the gaseous chlorine disinfection system at their water plants with an onsite-generated sodium hypochlorite-type disinfection system. It was anticipated that not only would the new disinfection systems be far safer to use, but that the formation of trihalomethanes within the distribution would likely be reduced. It was likewise anticipated that the change in disinfection strategy would allow for the maintenance of a better disinfectant residual within the distribution system and a reduction in caustic usage. This paper presents the following key results from the work at the City of Hickory (NC) Water Treatment Plant and the Town of Cheraw (SC) Water Treatment Plant.At the two water treatment plants studied, onsite-generated sodium hypochlorite produces fewer TTHMs than gaseous chlorine at lower equivalent doses, while producing more TTHMs than gaseous chlorine at higher equivalent doses. The exact dose where this changeover occurs is site-specific.Based on compliance TTHM data, disinfection with onsite-generated sodium hypochlorite has reduced the annual average of TTHMs in the distribution system by approximately 25% for the City of Hickory.The Town of Cheraw does not currently have sufficient data to make a comparison based on annual averages. Based on a comparison of the one compliance result that has been received by the Town of Cheraw with the same quarter from the previous two years, onsite-generated sodium hypochlorite has reduced the TTHMs in the distribution systems by approximately 19%.Following the installation of the onsite-generated sodium hypochlorite system, the City of Hickory has seen a 29.6% decrease in the disinfectant dose and 28.7% decrease in causticusage.Following the installation of the onsite-generated sodium hypochlorite system, the Town of Cheraw has seen a 22.1% decrease in the amount of caustic used at the plant and a 34.1% increase in the amount of disinfectant applied.According to plant personnel interviewed during this study, the use of onsite-generated sodium hypochlorite allows for the maintenance of a better disinfectant residual than when using gaseous chlorine.