Ozone-based water treatment (O3, O3/UV, O3/H2O2) for removal of organic micropollutants, bacteria inactivation and regrowth prevention

Ozone-based water treatments were investigated to simultaneously remove organic micropollutants (OMPs) and bacteria from surface water samples collected in a river reservoir supplying a drinking water treatment plant. Among the 21 environmentally relevant OMPs selected, 15 were detected at low (< 20) ng L-1 level. Surface water samples were then spiked with these 21 OMPs (100 ng L-1) and the ozone-based treatments were tested with excess of ozone and high hydraulic retention time (HRT: 41 min). Regardless of the treatment applied (O-3, O-3/UVA-LEDs, and O-3/H2O2), removal efficiencies higher than 85% were always obtained for 20 out of 21 OMPs (and lower than 50% for PFOS). Thus, single ozonation was considered the best treatment solution, being afterwards investigated in terms of water disinfection at milder operating conditions (3 mg O-3 L-1 and HRT: 10 min). Under these conditions, the removals of spiked OMPs were also higher than 85% (except for PFOS), whereas bacteria (i.e., total heterotrophs at 37 and 22 degrees C, E. coli and enterococci) decreased to values lower than the permissible levels for drinking water, even upon storage for 3-days. However, lower ozone doses (1.5 mg O-3 L-1) could not prevent the regrowth of total heterotrophs incubated at 37 degrees C, which reached levels five times higher than those recommended. Because drinking water treatments should not only consider OMPs abatement but also microbial inactivation for long periods of time, the results herein obtained point out the importance of using ozone doses capable of accomplish both targets when using single ozonation.