Effects of sonication on bacteria viability in wastewater treatment plants evaluated by flow cytometry--fecal indicators, wastewater and activated sludge.

The application of sonication to wastewater or sludge contributes to the dispersion of aggregates, the solubilisation of particulate matter with an increase in its biodegradability, the damage of microorganisms due to the loss of cellular membrane integrity. This research is aimed at investigating the effects of sonication at 20kHz frequency on viability of microorganisms present in raw wastewater and activated sludge taken from a municipal wastewater treatment plant, as well as pure strains of Escherichia coli and E. faecalis. Flow cytometry was applied for the identification and quantification of viable and dead bacteria free in the bulk liquid, after the fluorescent staining of cellular nucleic acids. The main results showed that: (i) cells of E. coli were highly sensitive to sonication, even at low specific ultrasonic energy (E(s)), and disintegration of a large amount of cells was observed; (ii) on the contrary E. faecalis were more resistant than E. coli, even if high levels of E(s) were applied; (iii) bacteria in raw wastewater exhibited a dynamic of viable and dead bacteria similar to E. coli; (iv) in activated sludge samples, low levels of E(s) produced a prevalent disaggregation of flocs releasing single cells in the bulk liquid, while disruption of bacteria was induced only by very high levels of E(s).