Abstract:

The potential to use the results of biochemical acid potential (BAP) tests to predict theultimate digestibility of raw and pretreated waste activated sludges (WAS) wasinvestigated. The ultimate methane production from biochemical methane potential(BMP) tests that were conducted on raw and pretreated samples which spanned a range ofbiodegradability was found to be linearly related to both the volatile fatty acid (VFA) andsoluble COD production in corresponding BAP tests. In addition, a linear relationshipbetween NH4-N production in the BMP and BAP tests was also observed. Despite thelinear nature of the relationships, the ratio of the production of methane in the BMP teststo the production of VFAs in the BAP tests varied with the biodegradability of thesludges. WAS samples with low digestibility had ultimate CH4 yields that were greaterthan the VFA yields in BAP tests while sludges with high digestibility had lower CH4yields than the corresponding VFA yields. This trend contrasted with the NH4 resultswhere the yields in the BAP tests were consistently less than those observed in the BMPtests. It was hypothesized that the varying relationship between CH4 and VFA yields wasdue to the inhibition of anaerobic oxidation of long chain fatty acids (LCFAs) in the BAPtests. LCFAs would be converted to CH4 in BMP tests but produced as digestionintermediates in the BAP tests and not measured as part of the VFA yield. Hydrogen andacetate were identified as the two most likely intermediates that would accumulate in theBAP tests which would cause inhibition. A stoichiometric model was assembled tofacilitate the development of an improved understanding of the biodegradation processesin the BAP and BMP tests. When the model was applied to the BAP tests the anaerobicoxidation of LCFAs and propionate and methanogenesis were excluded from the model.The model was employed to estimate the extent of degradation of lipids, carbohydratesand proteins in the batch tests as a function of the ultimate biodegradability of thesludges. On the basis of model fitting, it was determined that the degradation of lipids inBMP tests decreased while the degradation of carbohydrates and proteins increased as thedigestibility of the sludges increased. The varying ratio of lipid to protein andcarbohydrate degradability with increasing digestibility of the sludges was found todescribe the relationship between VFA production and CH4 production in the BAP andBMP tests respectively.

Keywords: Anaerobic digestion, waste activated sludge pretreatment, characterizationtests, model, Biochemical Acid Potential (BAP), Biochemical Methane Potential (BMP).

Document Type:

DOI: http://dx.doi.org/10.2175/106143011X13233670703404

Appeared or available online: December 12, 2011

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