PAA disinfection: a feasible alternative to wastewater chlorination

The paper summarizes the results of a bench scale study to evaluate the feasibility of using peracetic acid (PAA) as substitute for sodium hypochlorite both for discharge into surface water and for agricultural reuse. Trials were carried out with increasing doses (1, 2, 3, 5, 10 and 15 mg/L) and contact times (6, 12, 18, 36, 42, 54 min) to study disinfectant decay and bacterial removal and re-growth, using F. coli and E. coli as process efficiency indicators. PAA decay kinetics was evaluated in tap water and wastewater: in both cases, PAA decays according to a first order kinetics with respect to time, and a correlation was found between PAA oxidative initial consumption and wastewater characteristics. PAA disinfection efficiency was correlated with operating parameters (active concentration and contact time), testing different kinetic models. Two data groups displaying a different behavior on the basis of initial active concentration’s ranges, respectively 1÷2 mg/L and 5÷15 mg/L, can be outlined. Both groups had a “tailing off” inactivation curve with respect to time, but the second one showed a greater inactivation rate. Moreover the effect of contact time was greater at the lower doses. Hom’s model, used separately for the two data groups, was found to best fit experimental data and the disinfectant active concentration appears to be the main factor affecting log-survival ratios. Moreover S-model allows to better explain initial resistance of E. coli, especially at low active concentrations (< 2 mg/L) and short contact times (< 12 min). Microbial counts, performed by both traditional methods and flow cytometry, immediately and 5 hours after the sample collection (both with or without residual PAA inactivation), showed that no appreciable re-growth took place after 5 hours neither for coliform group bacteria nor for total heterotrophic bacteria (THB)