Alternative Disinfection Mechanisms for Wastewaters Using Combined PAA/UV Processes.

As in the case with drinking water, the disinfection of municipal wastewaters has been driven both by the need to achieve certain target concentrations of microorganisms, as well as the desire to minimise the production of by-products. In recent years, UV disinfection has become the process of choice due to its ability to satisfy both goals in most cases. Another disinfectant, peracetic acid (PAA) has also found success in various applications, especially for wastewater reuse in agriculture, due to its ease of application, low capital cost, and absence of hazardous by-products. However both techniques have their limitations. For UV, these are the difficulty of disinfecting wastewaters with high particulate content (suspended solids), sometimes leading to an inability to achieve target microorganism concentrations at any dose (i.e. fluence), and the need for very high UV doses with highly-UV absorbing wastewaters (such as those with elevated iron concentrations). PAA is an oxidizing agent which is rapidly reduced in partially treated effluents or those with high chemical oxygen demand (COD), hence impractically high doses may be needed in order to achieve required inactivation levels.UV and PAA are believed to operate via different disinfection mechanisms: UV interferes with cell replication by creating dimers on the DNA strand, whereas PAA is believed to disrupt cell membranes, and block enzymatic and transport processes. Therefore it stands to reason that using the two processes in sequence should at least result in an additive effect, and possibly even a synergistic effect if additional mechanisms come into play. Indeed it has been proposed that UV radiation of PAA (which is, in fact, a quaternary mixture of peracetic acid, acetic acid, water and hydrogen peroxide) could create hydroxyl radicals (OH·) which would further aid the disinfection process.Accordingly, wastewater from the City of Montreal Wastewater Treatment Plant (a 7.6 × 106 m3/d physicochemical treatment facility) was disinfected in batch tests using PAA (2 and 4 mg/L; 30 minute contact time), UV (10 and 20 mJ/cm2), and a combination to assess whether additivity and/or synergism could be observed. In addition, UV was applied in four different ways to establish indirectly which mechanism was most likely responsible for the increased disinfection. Thus UV was applied immediately before PAA addition (Sequence 1), immediately after PAA addition (Sequence 2), following PAA addition after the contact period had expired and the PAA had been quenched (Sequence 3) or finally, towards the end of the PAA contact period, but before quenching (Sequence 4). Hence it was possible to track the mechanism whereby each disinfectant or combination of disinfectants acted on the cell. PAA residuals were also tracked in order to assess whether PAA was being consumed by normal oxidation reactions due to components in the wastewater itself, or whether it was further reduced by interaction with UV.When applied individually, doubling the PAA dose was more effective than doubling the UV fluence, probably due to the effects of particles in the latter case. When testing the two disinfectants under Sequence 2, inactivation efficiencies increased by approximately 1 log over PAA alone, and 2 logs over UV alone, and in most cases the interaction effect was statistically significant. Further tests showed that Sequence 4 would yield the highest disinfection efficiency, however statistical tests showed significance only for Sequence 2. These results would suggest that both mechanisms were operating, i.e. prior damage by PAA, as well as OH· production, and this leads to the most successful and efficient combination of both disinfectants. Further tests are underway to verify this last outcome.Although the combined disinfectants clearly improved the extent of inactivation of fecal coliforms, contrary to published literature even the results for the best combination of PAA and UV did not result in positive synergism, where:synergism = log [combination] – {log [PAA alone] + log [UV alone]}.This may be explained by the fact that in the current experiments, real wastewaters were being used, and the dose-response curve for each disinfectant was not linear. Hence the low doses applied individually would show steep slopes, whereas the combined disinfectants were operating in the recalcitrant range of inactivation. However, by redefining synergism as the difference between the alternative sequences and Sequence 1 (effectively the sum of both disinfectants applied to the same sample but without chemical interactions or wastewater effects), a more realistic measure of synergism was obtained which did show positive synergism.The results of these experiments show that by a careful choice of sequence and timing of the application of the two disinfectants (PAA and UV), very high log inactivations of fecal coliforms could be achieved. Furthermore, it appears that the success of this combination is a result of two mechanisms operating, i.e. prior damage of the organism by one disinfectant before applying the other, as well as the production of OH· or other radicals due to both disinfectants being present at the same time.