Effective monitoring of trihalomethanes in water supply systems : ensuring chemical and microbiological safety of chlorinated water

To ensure public health safety, water supplied to consumers should meet both microbiological and disinfection by-product (DBP) requirements. Water utilities are ensuring microbiological safety of water, but there is considerable variation in monitoring and reporting trihalomethane (THM) levels across Australia, for the obvious reasons of cost and lack of resources and skills to monitor total THMs (TTHMs). Such practices lead to neglecting the long-term health risks from DBPs, which are often exacerbated by overdosing chlorine. To overcome these problems, we suggest two simple methods, which calculate and record the maximum TTHM levels in a system from regular monitoring of chlorine. Both methods calculate the TTHM concentration by multiplying the chlorine demand by the yield. The yield is the mass of TTHMs formed per unit of chlorine demand (µg-TTHM/mgCl2). The chlorine demand is the sum of all chlorine doses minus chlorine measured at any point in the system. The simplest method uses a fixed yield derived from a wide range of waters. The second method estimates the system-specific yield from measurements of TTHMs and corresponding chlorine concentration at two locations. Both approaches can be used to estimate the maximum TTHMs based on dosed chlorine. Either approach will raise awareness among water utility operators and regulators of the long-term chemical risk to consumers’ health and identify any need to reduce it.