The effect of peracetic acid (PAA) on indicator bacteria in secondary sewage effluent has shown PAA to be a viable alternative to other biocides. The synergy between UV/PAA and UV/H2O2 in laboratory experiments and at a pilot plant fed by the secondary effluent of two civil wastewater treatment plant has been investigated. This experiment is part of a larger research project on advanced treatment for municipal wastewater reuse in agriculture. A very high level of disinfection is necessary because of Italy's strict limits on unrestricted wastewater reuse in agriculture (2 CFU Total Coliform/100 ml). We found out that a combined treatment is satisfactory and that it is more efficient when chemical products such as PAA or H2O2 are introduced before UV irradiation, taking advantage of the hydroxyl radicals formation due to photolysis. At a cost parity PAA/UV advanced oxidation process shows the same disinfectant action as H2O2/UV while at a dose (amount) and cost parity, PAA shows a significantly better disinfectant action than H2O2.

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