Within a pilot scale research comparing different disinfection techniques (NaClO, PAA, O3 and UV), a particular attention was given to microbiological indicators. Analyses were made for about 450 disinfection tests, carried out using a continuous flow plant fed with 1÷4.5 m3 h-1, and concerned total and faecal coliforms, Escherichia coli and total heterotrophic bacteria, enumerated by traditional plate count and by flow cytometry . For ozonation the removal of total and faecal coliforms and of E.coli were comparable, while for chlorination, at the same dose and contact time, the removal was about one order of magnitude greater for total coliforms than for faecal coliforms and the difference increased with increasing C·t values. PAA removed E.coli much more efficiently for doses and contact times up to 5 mg L-1 , while the removal trend of the three indicators was comparable for higher doses. In the reported experiments, UV radiation showed lower effect towards E.coli than towards total and faecal coliforms. However, the influent counts were very low (103÷104 CFU per 100 mL) and their removal was complete even at low doses (10÷20 mJ cm-2), while the inlet counts of the other indicators were higher and, thus, removal evaluation as log survival ratio was more realistic. No correlation was found between dose and contact time and log removal of total heterotrophic bacteria (THB), enumerated by plate count. Flow cytometry detected a number of THB 1÷2 orders of magnitude greater than plate count and showed the much stronger effect of chlorine than of PAA, even at low doses and short contact times (5 mg L-1, 12 minutes contact time). For PAA, cytometry could detect the effect of contact time. For UV, on the contrary, cytometry is not suitable because the treatment does not damage bacterial walls but genetic material: so, cytometry enumerates all cells as viable.
Effect of a preliminary flash-mixing step on the disinfection efficiency of peracetic acid
ANTONELLI, MANUELA;ROSSI, SABRINA;NURIZZO, COSTANTINO
2004-01-01
Abstract
Within a pilot scale research comparing different disinfection techniques (NaClO, PAA, O3 and UV), a particular attention was given to microbiological indicators. Analyses were made for about 450 disinfection tests, carried out using a continuous flow plant fed with 1÷4.5 m3 h-1, and concerned total and faecal coliforms, Escherichia coli and total heterotrophic bacteria, enumerated by traditional plate count and by flow cytometry . For ozonation the removal of total and faecal coliforms and of E.coli were comparable, while for chlorination, at the same dose and contact time, the removal was about one order of magnitude greater for total coliforms than for faecal coliforms and the difference increased with increasing C·t values. PAA removed E.coli much more efficiently for doses and contact times up to 5 mg L-1 , while the removal trend of the three indicators was comparable for higher doses. In the reported experiments, UV radiation showed lower effect towards E.coli than towards total and faecal coliforms. However, the influent counts were very low (103÷104 CFU per 100 mL) and their removal was complete even at low doses (10÷20 mJ cm-2), while the inlet counts of the other indicators were higher and, thus, removal evaluation as log survival ratio was more realistic. No correlation was found between dose and contact time and log removal of total heterotrophic bacteria (THB), enumerated by plate count. Flow cytometry detected a number of THB 1÷2 orders of magnitude greater than plate count and showed the much stronger effect of chlorine than of PAA, even at low doses and short contact times (5 mg L-1, 12 minutes contact time). For PAA, cytometry could detect the effect of contact time. For UV, on the contrary, cytometry is not suitable because the treatment does not damage bacterial walls but genetic material: so, cytometry enumerates all cells as viable.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
