Biodegradability of DBP precursors after drinking water ozonation

Abstract

Ozonation is known to generate biodegradable organic matter, which is typically reduced by biological filtration to avoid bacterial regrowth in distribution systems. Post-chlorination generates halogenated disinfection byproducts (DBPs) but little is known about the biodegradability of their precursors. This study determined the effect of ozonation and biofiltration conditions, specifically ozone exposure and empty bed contact time (EBCT), on the control of DBP formation potentials in drinking water. Ozone exposure was varied through addition of H₂O₂ during ozonation at 1 mgO₃/mgDOC followed by biological filtration using either activated carbon (BAC) or anthracite. Ozonation led to a 10% decrease in dissolved organic carbon (DOC), without further improvement from H₂O₂ addition. Raising H₂O₂ concentrations from 0 to 2 mmol/mmolO₃ resulted in increased DBP formation potentials during post-chlorination of the ozonated water (target Cl₂ residual after 24 h = 1-2 mg/L) as follows: 4 trihalomethanes (THM4, 37%), 8 haloacetic acids (HAA8, 44%), chloral hydrate (CH, 107%), 2 haloketones (HK2, 97%), 4 haloacetonitriles (HAN4, 33%), trichloroacetamide (TCAM, 43%), and adsorbable organic halogen (AOX, 27%), but a decrease in the concentrations of 2 trihalonitromethanes (THNM2, 43%). Coupling ozonation with biofiltration prior to chlorination effectively lowered the formation potentials of all DBPs including CH, HK2, and THNM2, all of which increased after ozonation. The dynamics of DBP formation potentials during BAC filtration at different EBCTs followed first-order reaction kinetics. Minimum steady-state concentrations were attained at an EBCT of about 10-20 min, depending on the DBP species. The rate of reduction in DBP formation potentials varied among individual species before reaching their minimum concentrations. CH, HK2, and THNM2 had the highest rate constants of between 0.5 and 0.6 min^-1 followed by HAN4 (0.4 min^-1), THM4 (0.3 min^-1), HAA8 (0.2 min^-1), and AOX (0.1 min^-1). At an EBCT of 15 min, the reduction in formation potential for most DBPs was less than 50% but was higher than 70% for CH, HK2, and THNM2. The formation of bromine-containing DBPs increased with increasing EBCT, most likely due to an increase in Br^-/DOC ratio. Overall, this study demonstrated that the combination of ozonation and biofiltration is an effective approach to mitigate DBP formation during drinking water treatment.

Keywords: Biofiltration; Disinfection byproducts; Empty bed contact time; Ozonation.