Effects of Ozone and Biological Degradation on the Removal and Transformation of Highly Hydrophilic DOC in a Conventional Water Treatment Process

This study examined the removal and transformation of total dissolved organic carbon (tDOC) and its hydrophilic fraction (HPI) in a full-scale water treatment plant utilizing coagulation/softening, ozonation, and biologically active filters (BAFs). The plant is supplied by high-DOC riverine water (9.9 ± 0.1 mg/L), with primarily hydrophilic DOC (HPI = 83%). The coagulation/softening units reduced HPI DOC by ~42%, leaving the hydrophobic DOC concentration unchanged. The treatment altered the characteristics of HPI fractions; HPI DOC displayed lower SUVA and higher percentages of low molecular weight compounds (LMWs < 1 kDa) and specific trihalomethane formation potential (STHMFP) than the raw water HPI. The plant’s 0.4 mg O₃/mg-C ozone dose decreased SUVA but did not alter DOC biodegradability. Increasing the ozone dose to 1 mg O₃/mg-C converted all DOC to HPI, enhanced biodegradability from 6.3% to 26%, and lowered STHMFP from 73.4 to 11 µg/mg-C. A 28-day biodegradation increased the percentage of LMWs while decreasing the proportion of compounds with MW > 1 kDa, so we could not confirm that LMWs were more biodegradable than larger compounds. The STHMFP of the Coag/Soft water treated by 1 mgO₃/mg-C increased to 60 ± 7 µg/mg-C after biodegradation due to the formation of LMWs.