THM formation in surface waters

Abstract

Chlorine is the most popular secondary disinfectant used around the world and has a long history of use to achieve sufficient disinfectant level. It is economical and an effective disinfectant. Despite the benefits, the chlorinated distribution system water has been shown to contain harmful by-products in 1974. Since then, water utilities not only have to maintain sufficient chlorine residual but also ensure the customers are not exposed to high amounts of harmful by-products. In the presence of dissolved organic matter (DOM), chlorine reacts with organic carbon and forms disinfection by-products (DBPs). Many of these compounds are considered toxic, and their concentration in drinking water needs to be limited to comply with the respective regulations. There are around 700 DBPs identified currently, but trihalomethanes (THM) are in the highest concentrations and is the regulated compound. This work focuses on THM, which include chloroform, bromoform, bromodichloromethane, and dibromochloromethane. Higher concentrations of these by-products can potentially cause many illnesses including cancer and birth defects. Therefore, keeping a low concentration of these products in drinking water is important. Despite much research, one of the obstacles to adopting an appropriate method to control THM is the lack of predictive tools. At best, the most current methods measure the maximum THM formed - termed as the THM formation potential or THMFP - by artificially chlorinating the water at a very high initial chlorine concentration and incubate at 25oC for 72 hr. Water utilities experience varying retention times, temperature and thus have to dose varying chlorine levels depending on treated water quality and temperature. The THMFP does not represent the actual scenario where lower concentrations of chlorine are dosed, and various temperatures are experienced with various retention times. One of the methods suitable to translate into the actual scenario is to calculate the yield of THM formed per reacted chlorine. With the availability of an accurate chlorine prediction models in water supply systems, the yield can be easily used to calculate the THM concentration at any given point of a distribution system. To apply the yield concept for water distribution systems, it is important to establish the variability of the yield within the usual operating range, i.e., pH 7.5-8.5, different DOC levels, different water sources, bromide concentrations less than 500 I±gL-1. Within the tested range, initial pH or DOC did not affect the yield, but initial bromide concentration highly affects the yield and the relative concentrations of each halogenated species. More interestingly the DOC or bromide did not affect the molar yield of TTHM, but the fraction of each THM species remained constant after chlorination indicating that not only TTHM but also the individual species can be deterministically predicted at any point of a distribution system. The variable one has to know are the total amount of chlorine dosed into the water and the chlorine concentration at a given point and what was the bromide concentration. With the availability of accurate chlorine decay models, chlorine at any point in the water supply system can be predicted, and thus one can predict the concentration of THM. This approach also allows for modelling and prediction of THM under different operating regimes. Furthermore, bromide accelerated the chlorine decay rate, but at lower levels (Br

Date of Award	2018
Original language	English