Development of Protocols for Evaluating WAS Pre-Treatment

Waste activated sludge (WAS) samples that were generated over a range of solids residence times (SRTs) under controlled operating conditions were employed in bench scale sonication tests to evaluate the feasibility of developing protocols that might be employed for predicting the impact of WAS pretreatments on anaerobic digestion. Physical (particle size and suspended solids), chemical (chemical oxygen demand (COD) and biochemical ( readily biodegradable COD (rbCOD) and active heterotrophic biomass) analyses were conducted on raw and pretreated WAS samples and the results were compared to those obtained in biochemical methane potential (BMP) and biochemical acid potential (BAP) tests. Pretreatment resulted in a substantial reduction in particle size for sludges for all SRTs and sonication intensities examined. Particle size analysis did not provide insight into the extent of solublization of WAS. The volatile solids (VS) and COD responses indicated that a significant fraction of the WAS solids were solublized, and it appeared that the types of materials which were solublized was affected by the SRT at which the WAS generated as well as the level of sonication. The solublization of COD was greater for WAS streams that were generated at a longer SRT while a greater fraction of rbCOD was generated from the 1.95 day SRT WAS as compared to the 15 day SRT WAS. The generation of CH4 and NH4-N in BMP tests and volatile fatty acids (VFAs) in BAP tests revealed that sonication only marginally increased the yield of these products but substantially increased the rate of hydrolysis which is the rate limiting process in WAS digestion. The relative trends in the rates of NH4-N generation in the BMP tests and VFA generation in BAP tests versus sonication intensity differed for the sludges generated at differing SRTs.