Using ozonation and chloramination to reduce the formation of trihalomethanes and haloacetic acids in drinking water

Disinfection is a key treatment process for producing drinking water. However, it produces undesirable byproducts that may cause adverse health effects. Disinfection by-products (DBP) such as trihalomethanes (THMs) and haloacetic acids (HAAs) are considered potentially carcinogenic and have been recently associated with reproduction problems. In the province of Quebec (Canada), the regulation respecting the quality of drinking water (RRQDW) published in June 2001 establishes a maximum average level for total THMs of 80 µg/L. This standard is difficult to meet by small municipalities served by surface water and which apply a limited treatment before disinfection. The purpose of this research is to develop a protocol that would lead to the identification of alternative water treatment and disinfection strategies to reduce THMs and HAAs. The case under study is a small utility that currently does not comply with the Quebec RRQDW. This protocol involves pilot studies and laboratory assays that simulate the formation of THMs and HAAs in the distribution system. Different water treatment options were investigated. Those options involve ozone addition followed by slow sand filtration, and the introduction of chloramines and/or chlorine. The results of this research showed that the highest THM and HAA reduction is reached using ozone and chloramines as a disinfection strategy.

Conclusions of this research are as follows: • Concentrations of THMs and HAAs in the distribution system under study were the highest at the extremity. Laboratory-scale experiments showed similar behaviour of chlorinated DBPs for the highest contact times. • Two species of THMs (chloroform and CHBrCl2) and three species of HAAs (DCAA, TCAA and MCAA) were detected in the distribution system and during the laboratory-scale experiments. • Laboratory-scale experiments efficiently reproduced the levels of THMs and HAAs in the distribution system. They allowed one to compare of different treatment/disinfection strategies. • The laboratory-scale experiments on the pilot unit effluents demonstrated that the use of ozonation prior to sand filtration reduces significantly the concentrations of THMs and HAAs when chlorine is used as a disinfectant. However, the reduction of these compounds is much higher when chloramines are used as a secondary disinfectant. • The treatment/disinfection scenario consisting of ozonation/slow sand filtration + chlorine + chloramines and the scenario consisting of ozonation/slow sand filtration + chloramines would allow compliance with the RRQDW for THMs • The scenario consisting of ozonation/slow sand filtration + chloramines must be favoured in order to allow compliance for chlorinated DBPs and microbial inactivation efficiency standards. However, infrastructure and operational factors must also be considered. • In the future, when the water plant under study is updated, full-scale studies must be carried out to identify the optimal operational conditions that will simultaneously ensure adequate microbial inactivation levels, reduce chlorinated DBP occurrence and reduce operational costs.