How does the presence of natural organic matter (NOM) affect the formation of disinfection byproducts (DBPs) when using chlorine?

Natural organic matter (NOM) is a complex mixture of organic compounds that are naturally present in surface and ground waters. The presence of NOM significantly increases the formation of disinfection byproducts (DBPs) when chlorine is used as a disinfectant. Chlorine reacts with NOM to form a variety of DBPs, some of which are known or suspected carcinogens. The most common and regulated DBPs include trihalomethanes (THMs) and haloacetic acids (HAAs). THMs include chloroform, bromoform, dibromochloromethane, and bromodichloromethane. HAAs include monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. The amount and type of DBPs formed depend on several factors, including the concentration and characteristics of the NOM, the chlorine dose, the contact time, the pH, and the temperature. NOM contains various precursor compounds that react with chlorine to form DBPs. These precursors include humic acids, fulvic acids, and other organic molecules with reactive functional groups. The higher the concentration of NOM, the greater the potential for DBP formation. Therefore, removing NOM is a primary goal in drinking water treatment to minimize DBP formation. Treatment processes such as coagulation, flocculation, sedimentation, and filtration are used to remove NOM before chlorine disinfection. Alternative disinfectants, such as ozone and chlorine dioxide, can also be used to reduce DBP formation because they react differently with NOM than chlorine. For example, a water treatment plant that uses chlorine as a primary disinfectant and has high levels of NOM in its source water will likely have to implement enhanced coagulation or other NOM removal processes to comply with DBP regulations.