Experimental disinfection by-product formation potential following rainfall events

Spring rainfall events can have deleterious impacts on raw and drinking water quality for water treatment plants that use surface waters. This study compares the influence of land use and climate on DBP precursors in two catchments supplying the region around the City of Québec, Canada, and assesses the variability of Disinfection By-Product (DBP) concentration and speciation following rainfall events. DBPs (trihalomethanes (THMs) and haloacetic acids (HAAs)) and their precursors in raw waters (pH, turbidity, specific ultraviolet absorbance (SUVA), total and dissolved organic carbon, bromides and chlorine dose) were monitored. Various experimental chlorination tests, DBP formation potential (DBPFP) and Simulated Distribution Systems (SDS), were also performed. Differences in pre-rainfall (baseflow) water quality were noted according to the different watershed land uses. Raw water quality patterns showed modifications between baseflow and rainfall periods, with a degradation of raw water quality according to turbidity and SUVA in both water sources. Rainfall events were also shown to alter organic matter reactivity with an increase in THM formation potential for both sites. A less noticeable impact on HAA formation potential was observed. However, no clear differences in DBPFP tests were observed between the sites. SDS tests showed that rainfall events lead to considerable rises in organic carbon reactivity of filtered waters, even after primary treatment, with a 2-fold increase in THM and HAA concentrations following rainfall for waters representing the end of one main distribution system (20 h contact time). These increases are linked mainly to a rise in non-brominated DBPs such as chloroform, trichloroacetic acid and dichloroacetic acid. This study confirms the importance of strictly controlling OM levels during drinking water treatment to ensure safe drinking water quality throughout the distribution system.