Pathogen inactivation through organic acids in anaerobic digestion

Effective pathogen inactivation in wastewater sludge is one of the main goals of anaerobic digestion process. Current regulations limit achievement of Class A product only to thermophilic processes. The objective of this study was to show the ability of acid-phase digestion to effectively inactivate fecal coliforms at lower temperatures through antimicrobial properties of un-ionized VFA. Lab-scale acid-digesters were assembled and operated in a batch mode for 5 days at mesophilic (38°C) and low-mesophilic temperatures (21°C) and solids content (approximately 3.7%, 6.0% and 8.0% TS). Low-mesophilic digesters lowered fecal coliform levels to non detectable levels (Class A), while effluent from acid-digesters operated at mesophilic temperatures showed fecal coliforms at level of Class A and Class B limit. It was shown that the acidic stress conditions from un-ionized VFA forms contributed to effective inactivation of fecal coliforms in low-mesophilic conditions. Key factors that contributed to pathogen inactivation were un-ionized VFA concentrations, duration of harmful conditions and most importantly pH inside the digester that was below 6.0. The pH values higher than 6.0 in mesophilic temperatures did not offer conditions harmful enough to effectively inactivate fecal coliforms. An increase in organic loading due to sludge thickening resulted in elevated levels of u-VFA release in low-mesophilic conditions but it did not seem to affect pH.