Comparison of O3-BAC, UV/H2O2-BAC, and O3/H2O2-BAC treatments for limiting the formation of disinfection byproducts during drinking water treatment in India

Abstract

Natural organic matter (NOM) acts as a precursor to toxic disinfection byproducts (DBPs). Recently it has been established that a mixture of natural and effluent derived organic matter (NefOM) leads to very high concentration of DBPs. Both conventional and non-conventional/advanced ways of drinking water treatment do not remove NefOM sufficiently. Advanced oxidation processes (AOPs) have been shown to oxidize dissolved organic carbon (DOC) effectively but entail high energy inputs. Therefore, combining AOP as a pre-oxidation step with an economical system like biological activated carbon (BAC) filtration is a more economical solution. In this study, BAC was developed from virgin granular activated carbon (GAC) with river Yamuna water, which has the highest levels of DOC in the world, as influent water. Two columns were used, with one of them as a control. For the development of BAC from GAC, Empty bed contact time (EBCT) of 12 min was found to be effective. Steady-state was achieved after 90 days of continuous operation of the columns. After the steady-state was achieved, EBCT of 16 min was found to be optimum for the diffusion of organic molecules inside the biofilm and mineralization by the attached biomass on the surface of BAC.The percentage DO consumption and, DOC/UV254 reduction was found to be constant from t = 90 to t = 120 days, after which the column was assumed to be acting as a BAC column. To optimize the AOP dose for AOP-BAC experiments, biodegradable DOC (BDOC) was measured after various doses of individual AOP treatments. The maximum increase in biodegradability was observed in case of O3/H2O2 with BDOC of 3.43 mg/L at an ozone dose of 5.44 mg/L and 0.5 mM H2O2. All three AOP treated water were passed through BAC columns and change in BDOC, DOC, hydrophobic fraction of DOC and UV254 were observed. The maximum DOC, hydrophobic fraction of DOC and UV254 reduction was observed with O3/H2O2-BAC, as expected from BDOC results. The primary reason is enhanced biodegradability after optimum O3/H2O2 treatment and thus better utilization of simpler organic molecules by microbes in the BAC column. Maximum trihalomethane formation potential (THMFP) reduction was also observed in the case of O3/H2O2-BAC treatment.