AN EVALUATION OF STAGED DIGESTER OPERATION FOR ENHANCED PATHOGEN REDUCTION

This study evaluated the efficiency of staged operation of mesophilic anaerobic digester for pathogen removal from municipal sewage sludge. In this study two parallel three stage set ups each consisting three digesters in series (System I: 35°C-35°C-35°C and System II: 42°C-35°C-35°C), and one two stage setup consisting two reactors in series (System III: 35°C-35°C) were compared against a conventional single stage digester. The results showed that two and three stage operation were able to achieve Volatile Solids (VS) reduction of greater than 38%, which is required for vector attraction reduction. Staged operation improved VS reduction and biogas production as compared to single stage operation and 3 stage operation was superior to 2 stage operation. Stable operation in two and three stage configurations were indicated by low concentrations of total volatile fatty acids (TVFA) and substantial alkalinity generation. For all of the indicators examined staged operation resulted in superior inactivation as compared to single stage operation. With the exception of E. coli., all of the indicators were reduced to a greater extent in 3 stage digestion as compared to 2 stage digestion. With the exception of Salmonella spp. all of the indicators were removed to a greater extent when the first stage of the 3 stage system was operated at 42°C as compared to the 3 stage system that was operated at 35°C. Fecal streptococci and Clostridium perfringens were more resistant to inactivation in the digesters as compared to Fecal coliform, E. Coli. and Salmonella spp.