Predicting chlorine decay and THM formation in water supply systems

In 1976, the US Environmental Protection Agency (USEPA) published the results of a national survey that showed that chloroform and other trihalomethanes (THMs) were ubiquitous in chlorinated drinking water. Also in 1976, the US National Cancer Institute published results linking chloroform to cancer in laboratory animals, thus giving rise to an important public health issue. Although numerous disinfection by-products (DBPs) have been reported in the literature since that time, with more than 700 confirmed species to date, only a small number have been addressed in either quantitative or health effects studies. The DBPs that have been quantified in drinking water are generally present at low to mid μg/l levels or below. Approximately 50% of the total organic halide (TOX) formed during the chlorination of drinking water and more than 50% of the assimilable organic carbon (AOC) formed during ozonation of drinking water is still not accounted for and little is known about the potential toxicity of many of the vast number of DBPs present in drinking water. The presence of free chlorine is a prerequisite to THM formation. Therefore, a robust understanding of the mechanisms of both chlorine decay and THM formation are fundamental to the management of THMs in water supply systems. This paper presents a review of work undertaken to improve our understanding of these key phenomena and highlights areas of vulnerability in our knowledge and so recommends areas of future research.