TESTING OF TWO ULTRASONIC SLUDGE TREATMENT SYSTEMS FOR CONDITIONING OF THICKENED WASTE ACTIVATED SLUDGE

Thickened waste activated sludge (TWAS) from biological treatment processes is typically less amenable to anaerobic digestion than primary sludge. Recently, ultrasonic conditioning of TWAS has been receiving attention as a way to enhance gas production and volatile solids (VS) destruction. TWAS is composed of cellular material, extracellular polymeric substances (EPS) and some inert materials. The application of ultrasonic energy reportedly lyses cell walls and makes the cell material available for digestion. Consequently the overall digestion performance is enhanced. From 2003 to 2004, the Sanitation Districts of Los Angeles County (Sanitation Districts) evaluated ultrasonic conditioning equipment from two vendors, at the Sanitation Districts' Joint Water Pollution Control Plant (JWPCP) in Carson, California. The TWAS was conditioned prior to co-digestion with primary sludge in both full-scale and bench-scale mesophilic anaerobic digesters. The volumetric ratio of primary sludge to TWAS was three to one at the JWPCP. The results from this evaluation showed that ultrasonic equipment operating at 20 kilohertz (kHz) frequency and in the 0.135 - 6.7 kilowatts-seconds per gram (kW-sec/g) power range did not significantly enhance digestion performance. Possible reasons for the limited benefits of ultrasonic treatment include the high fraction of primary sludge in the combined sludge, insufficient ultrasonic energy applied to TWAS, and a high concentration of refractory EPS in the TWAS.