INFLUENCE OF STAGED OPERATION OF MESOPHILIC ANAEROBIC DIGESTION ON MICROBIAL REDUCTION

This study evaluated the efficiency of staged operation of mesophilic anaerobic digesters for pathogen removal from municipal sewage sludge. In this study two parallel three stage set ups (System I: 35°C-35°C-35°C and System II: 42°C-35°C-35°C), and one two stage setup (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) reductions 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 bacterial 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. First order decay coefficients were estimated for all of the indicator organisms in all of the digesters. Due to the variability in the data it was not possible to differentiate the values of decay coefficients between the different organisms and digester conditions. Excluding C. perfringens, the decay coefficients ranged from 2.1-14.6 d-1 and for these organisms an average decay coefficient of 5.4 d-1 was estimated. Decay coefficients in the digester operated at 42°C were consistently higher than those observed in the 35°C digesters and a correction factor ranging from 1.2-2.6 was estimated to account for the impact of the elevated temperature. C. perfringens generally had lower decay coefficients than the other organisms and they did not increase at elevated temperatures.