Aspects of THM formation in drinking-water

The formation of trihalomethanes (THMs) in drinking water is the result of reaction between chlorine and organic matter, mainly of natural origin (NOM). This paper presents some of the findings of a laboratory study of THM formation in 12 Irish surface waters that are important water supply sources. At the production plants these waters are treated by conventional chemical coagulation, rapid gravity filtration and chlorination, using aluminium sulphate as primary coagulant. The same processes were applied at laboratory scale. A standard chlorination was used throughout, based on the chlorine dose required to produce a residual free chlorine (RFC) concentration of 0.5 mg/L after a 30-min contact time. It was found that the RFC decay rate could be approximately modelled as a first order reaction using the 2 h RFC concentration to quantify the reaction rate constant. The applied standard chlorination dose, which is that recommended by World Health Organisation (WHO), gave a wide spread of Ct values. The results indicate that a chlorination norm, expressed as RFC at a contact time of 2 h, would provide a more consistent set of Ct values for the waters tested in this study. It was found that the rate of total THM (TTHM) formation could be modelled as a hyperbolic growth function, defined by two parameters, t₅₀ and TTHM_max, where I₅₀ is the reaction time required for the TTHM concentration to reach half TTHM_max, its ultimate value. For the set of waters studied, the t₅₀ values varied in the range 1.06-2.48 h, while the TTHM_max values varied in the range 22-56 μg/L. The THM species formed in the test set of waters included chloroform (CHCl₃), bromodichloromethane (CHBrCl₂) and dibromochloromethane (CHBr₂Cl). Bromoform (CHBr₃) was not detected in any of the samples. In the case of the high alkalinity waters of the test set, a downward adjustment of the coagulation pH to the region 5.5-6.5 resulted in lower coagulant and chlorine demands, improved TOC removal and reduced THM formation.