Characterization of Anaerobic Biodegradability of Raw and Pretreated WAS by BAP Tests

摘要

The potential to use the results of biochemical acid potential (BAP) tests to predict the ultimatedigestibility of raw and pretreated waste activated sludge (WAS) was investigated. The ultimatemethane production from biochemical methane potential (BMP) tests that were conducted onraw and pretreated samples which spanned a range of biodegradability was found to be linearlyrelated to the both volatile fatty acid (VFA) and soluble COD production in corresponding BAPtests. In addition, a linear relationship between NH4-N production in the BMP and BAP testswas also observed. Despite the linear nature of the relationships, the production of methane inthe BMP tests was not directly proportional to the production of VFAs in the BAP tests. WASsamples with low digestibility had ultimate CH_4 yields that were greater than the VFA yields inBAP tests while sludges with high digestibility had lower CH_4 yields than the correspondingVFA yields. This trend contrasted with the NH4 results where the yields in the BAP tests wereconsistently less than those observed in the BMP tests. It was hypothesized that the varyingrelationship between CH_4 and VFA yields was due to the inhibition of anaerobic oxidation oflong chain fatty acids (LCFAs) in the BAP tests. LCFAs would be converted to CH_4 in BMPtests but produced as digestion intermediates in the BAP tests and not measured as part of theVFA yield. Hydrogen and acetate were identified as the two most likely intermediates thatwould accumulate in the BAP tests which would cause inhibition. A stoichiometric model wasassembled to facilitate an improved understanding of the biodegradation processes in the BAPand BMP tests. When the model was applied to the BAP tests the anaerobic oxidation of LCFAsand propionate and methanogenesis were excluded from the model. The model was employed toestimate the extent of degradation of lipids, carbohydrates and proteins in the batch tests as afunction of the ultimate biodegradability of the sludges. The degradation of lipids was found todecrease while the degradation of carbohydrates and proteins was found to increase as thedigestibility of the sludges increased. The varying ratio of lipid to protein and carbohydratedegradability with increasing digestibility of the sludges was found to describe the relationshipbetween VFA production and CH_4 production in the BAP and BMP tests respectively.