TRIHALOMETHANE FORMATION AND CONTROL IS NOT JUST FOR WATER TREATMENT PLANTS

摘要

Do wastewater treatment plants have to be concerned about trihalomethane (THM) formation? After all, THMs are regulated under SDWA to an aggregate concentration of not more than 80 ppb. Ironically, 80 ppb or less is acceptable for drinking water but may not be good enough for wastewater treatment facilities that discharge to receiving streams that are not permitted to have a mixing zone. In some states the in-stream water quality standard for chlorodibromomethane and dichlorobromomethane is less than 1 ppb and for chloroform the limit is as low as 5.7 ppb. For wastewater treatment facilities that have to meet end-of-pipe water quality standards, disinfection with chlorine may cause a violation of the THM limit. Trihalomethanes are a group of compounds that have a halogen-substituted single carbon compound that is named as a derivative of methane. THMs are formed from a reaction of free chlorine with total organic carbon. There are ten typical trihalomethane compounds; however, only four of them are regulated by the Safe Drinking Water Act. Some states have proposed standards for three of the THM compounds that are more stringent than the standards set by EPA. Water quality based permitting is becoming common across the United States. States are slowly implementing all of EPA’s requirements for their water quality programs which include metals, conventional pollutants, organic chemicals, and toxicity. THMs are merely a subset of the organic chemicals regulated under the water quality programs. If your state is requiring you to monitor for a long list of pollutants, it is the beginning of water quality based permitting. If your permit does not define a mixing zone in the receiving stream, you may find that THMs are only part of a larger problem as you may have to meet water quality standard limits at end-of-pipe. This paper focuses on THMs and discusses several methods to minimize THM formation as well as several THM treatment approaches.