THE CONTRIBUTION OF AMMONIA AND VOLATILE ACIDS TO PATHOGEN DESTRUCTION DURING MESOPHILIC AND THERMOPHILIC ANAEROBIC DIGESTION

Both total volatile acids (TVA) and ammonia are formed during anaerobic digestion of municipal sludges. Volatile acids can be toxic to microorganisms at low pH, while ammonia can be toxic at higher pH. The toxicity of these compounds on pathogens in anaerobic digestion has been shown in the literature. Still unclear, however, are the toxic impacts of TVA and ammonia on pathogens at suboptimal pH and concentrations. Results from two different studies are presented. The first study examines the impacts of increasing amounts of poultry blood on fecal coliforms and methanogens under both mesophilic and thermophilic anaerobic digestion. Poultry blood has a high organic nitrogen content (approx. 16,000 mg/L) that is readily converted to ammonia in anaerobic digesters. Where methanogens show increasing inhibition with increasing un-ionized ammonia, fecal coliforms show no impact until the un-ionized ammonia is high enough to cause complete or near complete destruction. Fecal coliforms may be impacted at lower un-ionized ammonia levels, as methanogens are, but these impacts may not be detected with fecal coliform enumeration tests. The second study investigated pathogen destruction efficiencies for various thermophilic reactor configurations. Regression analysis suggested that ammonia contributed to fecal coliform destruction in single-stage thermophilic anaerobic digesters, and other results suggested that un-ionized TVA also contributed to fecal coliform destruction in another thermophilic process configuration considered.