Thickened waste activated sludge (TWAS) from biological treatment processes is less amenable to anaerobic digestion than primary sludge. This is due to TWAS being composed mostly of cellular material and extracellular polymeric substances (EPS) instead of more easily digestible carbohydrates and fats that are typically found in primary sludge. In an attempt to improve TWAS digestion performance, a process for pretreating TWAS before digestion, known as the MicroSludgeTM process, was tested at the Los Angeles County Sanitation Districts’ (Districts) Joint Water Pollution Control Plant (JWPCP) in Carson, CA. The MicroSludge process is a chemical/mechanical system that uses caustic soaking to weaken cell walls and EPS, and a high-pressure shearing valve to lyse the cell walls and break down EPS to increase the overall solubility of the TWAS.
The MicroSludge process was evaluated using both full-scale and bench-scale mesophilic anaerobic digesters operated at various conditions. In most of the tests, the TWAS was digested with primary sludge. Pure TWAS was digested in some of the bench-scale tests. Digestion performance was determined by volatile solids destruction (VSD) and gas
production. In addition to digestion performance, the effects of the MicroSludge process on dewatering performance and odor generation were examined.
The results from the study indicate that the MicroSludge system significantly increased the solublization of TWAS. Under the operating conditions at JWPCP, VSD increased from 54 to 57%, and gas production increased by less than 5%. The MicroSludge process may have slightly improved dewatering performance with a small increase in cake solids content, and dewatered cake from the test digester was approximately 40% less odorous than cake from a control digester. Despite the minor improvement in digestion, odor control and dewatering performance, the Districts determined that the MicroSludge treatment process was not cost effective for treating TWAS at the JWPCP.