Reducing total trihalomethanes by removing total organic carbon and managing preoxidation and disinfection processes

Disinfection byproducts (DBPs) are a growing concern among water utilities. Not only are they well documented as a carcinogen but recent regulations established by the United States Environmental Protection Agency (USEPA) have strengthened the Disinfectant and Disinfection Byproducts Rule. Disinfection byproducts are formed when free chlorine reacts with natural organic matter found in all water. The most common and the only regulated disinfection byproducts are trihalomethanes (THMs) and haloacetic acids (HAAs).;With passage of the Stage 2 Disinfectants and Disinfection Byproducts Rule, the Truckee Meadows Water Authority (TMWA) in Reno, Nevada completed a study to ensure compliance with the new regulations. TMWA was also concerned with ensuring that the surrounding secondary systems (i.e., wholesale water customers) will also be in compliance. Sampling for total organic carbon (TOC) and total trihalomethanes (TTHMs) was conducted at the Chalk Bluff Water Treatment Facility, TMWA's primary water treatment plant. The results of sampling and laboratory testing showed that TMWA may need to alter some treatment processes to ensure that they are in compliance, as well as their wholesale customers. The concentration of TOC in the raw water was found to be closely related to the formation of TTHMs (R2 = 0.95). Proposed changes include better control and monitoring of the dose of sodium hypochlorite at the pretreatment ponds and the clearwell or using an alternative preoxidant such as chlorine dioxide in order to oxidize the precursors of DBPs thereby eliminating or reducing the formation of TTHMs. When chlorine dioxide was used as a preoxidant in the pretreatment ponds, the formation of TTHMs was reduced between 30% and 70% as opposed when sodium hypochlorite was used. Results from simulated distribution system (SDS) tests showed that a reduction in the initial TTHMs leaving the treatment plant would be amplified into large reductions of TTHMs at points in the distribution system with high residence times. As a result, small reductions in concentrations of TTHMs in the finished water leaving the plant can result in relatively large reductions in TTHMs within the distribution system.