Secondary effluent disinfection by peracetic acid (PAA): microrganism inactivation and regrowth, preliminary results

The paper summarises - in terms of bacterial removal and re-growth - the results of a pilot scale study performed on a continuous flow pilot plant, using peracetic acid (PAA) as disinfecting agent. Trials were carried out with increasing doses and contact times (6, 12, 18, 36, 42, 54 min). The paper reports data deriving from the only doses (15 and 25 mg L^-1) which were able to ensure the respect of the limits for agricultural re-use, according to Italian law. Disinfection efficiency was comparable for E. coli, TC and FC, and lower for total heterotrophic bacteria (THB), and clearly grew with increasing Cát values. The effect of contact time was greater at the lower doses. Microbial counts, performed by both traditional methods and flow cytometry, immediately and 6 hours after the sample collection (PAA was previously inactivated by sodium thiosulphate and bovine catalase), showed that no appreciable re-growth took place after 6 hours for coliform group bacteria. THB formed a slightly lower number of colonies immediately after collection than after 6 hours. The slight variations observed after 6 hours were the result of the recovered ability of forming colonies and of the lysis of some of the residual bacteria, as it was shown by flow cytometry.