Case Studies of Trihalomethane Formation Within Filters and Distribution Systems
Creators: name: Fitzpatrick, Kevin M.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1996

description: Two distinct objectives were addressed in this study. The first objective was to examine the variability of trihalomethane (THM) formation in distribution systems which contain a combined chlorine residual as the secondary disinfectant. Historical data were gathered from the records of three utilities in North Carolina which use chloramines: Raleigh, Gary, and High Point. The Raleigh system, which recently switched from free chlorine to combined chlorine, was chosen as the main focus of this study. Cary and High Point data were used to supplement the findings from Raleigh. Seasonal and spatial variation in THM concentrations were compared when both free and combined chlorine residuals were used as the secondary disinfectant. Seasonal variation was found to be equally significant in Raleigh's distribution system for the times when both free and combined chlorine residuals were used. Spatial variation in THM concentrations as well as average THM concentrations decreased significantly, however, when Raleigh switched from free chlorine residuals to combined chlorine residuals. Similar observations were observed for the distribution systems of Cary and High Point. Linear relationships between THM concentrations and chlorine consumption in the distribution system were established for Raleigh during the period of time when they used free chlorine and when they used combined chlorine. These relationships showed that THM concentrations could be predicted in the distribution system from the amount of chlorine consumed. It was found that when Raleigh used combined chlorine, less THMs were formed per unit mass of chlorine consumed. A linear relationship between THM concentrations in the distribution system and chlorine consumption up to the point of entry (POE) to the distribution system was developed for Cary. The second objective of this study was to investigate THM production in the filters of a water treatment plant. The filters at the Orange Water and Sewer Authority's Jones Ferry Road Water Treatment Plant in Carrboro, NC, were used for this portion of the study. Three experiments were performed to determine if THM formation was greater in the filters due to reactions with organic matter deposited in the filters compared to only solution-phase reactions between chlorine and natural organic matter. These experiments involved collection of a sample of chlorinated filter influent and holding it in a bottle at the same temperature as the water in the filter. THM concentrations in the bottle were compared to THM concentrations in the filtered effluent. The results revealed that THMs are formed faster in the filters than in the solution phase alone, which is probably due to reactions with organic matter retained in the filter. Additionally, THM formation in filters was examined over the course of a filter run to see if THM production increased with increasing filter head loss, a reflection of increased deposits of organic matter within the filter bed. However, the findings of this were inconclusive.