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

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The potential to use the results of biochemical acid potential (BAP) tests to predict the ultimate digestibility of raw and pretreated waste activated sludge (WAS) was investigated. The ultimate methane production from biochemical methane potential (BMP) tests that were conducted on raw and pretreated samples which spanned a range of biodegradability was found to be linearly related to the both volatile fatty acid (VFA) and soluble COD production in corresponding BAP tests. In addition, a linear relationship between NH4-N production in the BMP and BAP tests was also observed. Despite the linear nature of the relationships, the production of methane in the BMP tests was not directly proportional to the production of VFAs in the BAP tests. WAS samples with low digestibility had ultimate CH4 yields that were greater than the VFA yields in BAP tests while sludges with high digestibility had lower CH4 yields than the corresponding VFA yields. This trend contrasted with the NH4 results where the yields in the BAP tests were consistently less than those observed in the BMP tests. It was hypothesized that the varying relationship between CH4 and VFA yields was due to the inhibition of anaerobic oxidation of long chain fatty acids (LCFAs) in the BAP tests. LCFAs would be converted to CH4 in BMP tests but produced as digestion intermediates in the BAP tests and not measured as part of the VFA yield. Hydrogen and acetate were identified as the two most likely intermediates that would accumulate in the BAP tests which would cause inhibition. A stoichiometric model was assembled to facilitate an improved understanding of the biodegradation processes in the BAP and BMP tests. When the model was applied to the BAP tests the anaerobic oxidation of LCFAs and propionate and methanogenesis were excluded from the model. The model was employed to estimate the extent of degradation of lipids, carbohydrates and proteins in the batch tests as a function of the ultimate biodegradability of the sludges. The degradation of lipids was found to decrease while the degradation of carbohydrates and proteins was found to increase as the digestibility of the sludges increased. The varying ratio of lipid to protein and carbohydrate degradability with increasing digestibility of the sludges was found to describe the relationship between VFA production and CH4 production in the BAP and BMP tests respectively.
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Keywords: Anaerobic digestion; characterization tests; model; waste activated sludge pretreatment

Document Type: Research Article

Publication date: 2011-01-01

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