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

• Compared ozonation and UV/H2O2 for decolourisation of secondary effluent.
• Sequential ozone (5 min) and UV/H2O2 (10 min) most effective re water quality.
• For this sequence, achieved colour target then 73% DOC removed after biodegradation.
• Synergistic UVA254, COD and DOC, and faster colour removal demonstrated for sequence.

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 10 min, 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 15 min.