PILOT STUDY OF PRE-TREATMENT TO MESOPHILIC ANAEROBIC DIGESTION FOR IMPROVED BIOSOLIDS STABILIZATION

Mesophilic anaerobic digestion of sludge pre-treated by thermal hydrolysis or thermophilic acidogenesis was very stable as measured by volatile acids-to-alkalinity ratio and pH. Volatile solids destruction and gas production were comparable or better than in the control digester. However, the organic loading of the digester fed with sludge treated by thermal hydrolysis was three to four times greater than the control. This is possible because of thermal hydrolysis partial transfer into solution of particulate organic matter in undigested sludge, thus making it more readily available for the active biomass in the reactor. Thermal hydrolysis meets the EPA temperature and time requirements to achieve Class A biosolids and, after anaerobic digestion, complete absence of fecal coliforms is observed. The two-phase system also achieved Class A biosolids as measured by fecal coliform concentrations. A progressive wash-out of fecal coliforms was observed in the mesophilic, methanogenic phase from initial numbers of the Class B biosolids used as seed. The enhanced mixing (more than one turnover per hour) and the draw-fill operation of the thermophilic digester are believed to have contributed to the superior fecal coliform inactivation.