Characterization of Ozone Impacts on WAS Properties and Digestibility

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 ozonation tests to evaluate the impact of ozonation on WAS properties and digestibility. 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. Particle size analysis did not provide insight into the extent of solublization of WAS. The extent of solublization of COD was higher for short SRT WAS streams. The COD and TKN 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 was generated as well as the ozone dose. The biodegradable fraction of COD was greater for short SRT WAS streams as compared to long SRT WAS. Ozonation marginally increased the ultimate digestibility of shorter SRT sludges while a high dose caused a substantial increase in the digestibility of the 15 day SRT sludge. The generation of NH4-N in BMP tests and volatile fatty acids (VFAs) in BAP tests revealed that ozonation substantially increased the rate of hydrolysis which is the rate limiting process in WAS digestion. The rates of NH₄-N generation were improved by about 77%, 24% and 46% for 1.95, 7 and 15 days SRT sludges. The BMP test was not a useful test for evaluating the rate of methane generation due to inhibition of methanogens in early stages of the BMP test for pretreated sludges. The BAP test as a shorter term test (10 days) than the BMP (55 to 60 days) test could provide information on the rates of hydrolysis and acidification/ammonification stages and is much more cost effective than performing long BMP tests.