Efficiency of sequential ozone and UV-based treatments for the treatment of secondary effluent

Abstract

Sequential ozone and UV-based processes (VUV irradiation and UV/H2O2) for the treatment of secondary effluent were investigated. The effectiveness of these sequential treatments was compared with the individual treatments in terms of reduction in DOC, COD, UV254 and colour and impact on BDOC. The greatest decrease in DOC, COD and UV254 was achieved for samples treated with ozone before UV/H2O2 (UVH), where a synergistic benefit was observed. By monitoring the formation of hydroxyl radicals (HO) during UVH and ozonation it was demonstrated that UVH treatments generated more hydroxyl radicals than ozonation, which explained the greater extent of mineralisation attained during UVH treatment. Sequential O3-UVH treatment (for 5 and 10min, respectively) enhanced the formation of biodegradable compounds such that when followed by biological treatment up to 73% of the DOC could be removed from the secondary effluent. Furthermore, application of the O3-UVH treatment achieved 2.5 times greater mineralisation than ozone alone. This improvement, however, required 4.5 times greater electrical energy input (EEI) for the same treatment time of 15min.