Optimizing treatment for reduction of disinfection by-product (DBP) formation

Abstract

Recent studies in communities in greater Cairo, Egypt have identified trihalomethanes (THMs) and haloacetic acids (HAAs) at levels that exceed regulatory limits depending upon the season. The objective of this study was to better understand the formation of DBPs in Nile River source water with a view toward optimizing conventional treatment, focusing on enhanced coagulation, to achieve reduction in DBP formation potential (FP) in a cost-effective scheme. To this end, characterisation of natural organic matter (NOM) in Nile raw water and after treatment by fractionating according to hydrophobic-hydrophilic properties was included in the analysis. Seasonal variations in raw water quality were found to be important for achieving optimum reduction of turbidity and DBP formation. In summer, alkalinity is lower; therefore, enhanced coagulation with 30 mg/L alum can be done at pH 6 without excessive additions of acid, and satisfactory reductions in turbidity and DBPs attained. During the remainder of the year, high alkalinity results in enhanced coagulation at about pH 6.5 and notably lower reduction of NOM, THMFP, and HAAs versus summer conditions. Supplementing enhanced coagulation with 10 mg/L PAC is highly recommended for these conditions as it improves removal of all DBP indicators. The transphilic fraction was the main contributor to DBP formation and the primary fraction removed by enhanced coagulation for achieving gains in THMFP reduction.