Comparative study on the efficiency of peracetic acid and chlorine dioxide at low doses in the disinfection of urban wastewaters

A comparison was made between the efficiency of low closes of peracetic acid (PAA: 1.5 mg/l) and chlorine dioxide (ClO2,: 1.5 and 2.0 mg/l) in the disinfection of secondary effluents of a wastewater treatment plant. Peracetic acid was seen to be more active than chlorine dioxide and less influenced by the organic content of the waste. Both PAA and ClO2, (2.0 mg/l) lead to a higher reduction in total and faecal coliforms and E. coli than in phages (somatic coliphages and F-specific RNA bacteriophages) and enterococci. Detection of faecal coliforms and E. coli should therefore be accompanied by a search for these more resistant microorganisms when assessing the conformity of wastewater for irrigation use, or for discharge into surface waters. Coliphages are also considered suitable indicators of the presence of enteric viruses. Although the application of low doses of both disinfectants offers advantages in terns of costs and produces not significant quantities of byproducts, it is not sufficient to obtain wastewater suitable for irrigation according to the Italian norms (E. coli < 10/100 ml in 80% of samples and <100/100 ml in the remaining samples). Around 65% of the samples, however, presented concentrations of E. coli lower than the limit of 5,000/100 ml established by Italian norms for discharge into surface waters.