Formation of Adsorbable Organic Halogens (AOX) from Chlorination and Chloramination in Full-scale Drinking Water Production

Abstract Formation of disinfection by-products (DBPs) in drinking water production constitutes a health hazard. The total amount of DBPs is usually measured as adsorbable organic halogens (AOX). We determined AOX in samples from four full-scale drinking water treatment plants (DWTPs) in Sweden using different types of raw waters and treatments. Raw water, water before and after chlorination or chloramination and tap water were sampled on five occasions during one year without applying quencher to stop chlorination reactions. Coagulation removed most (> 95%) of the AOX present in the raw water at one DWTP. Sand filtration used in all DWTPs had no clear effect on AOX levels already present in the intake water, while some AOX formation occurred during Ultraviolet (UV) treatment. AOX increased dramatically at all DWTPs during both chlorination and chloramination due to the formation of DBPs. When not using any quencher similar AOX formation was found in both chlorine and chloramine treated water, showing that substantial DBP formation occurred upon chloramination even if few regulated DBPs were detected in these DWTPs in previous assessments. Because the contribution of non-DBP AOX varies between sources and treatment processes, ∆AOX, i.e., AOX in final water subtracted by AOX prior to chemical disinfection, should be considered in DBP assessments.