This study elaborates the rate-limiting steps of particle disintegration/hydrolysis of primary sludge using methane production rate (MPR) curves from multiple batch experiments. Anaerobic batch degradation of fresh primary sludge showed a complex MPR curve marked with two well-defined temporal peaks. The first immediate peak was associated with the degradation of relatively readily hydrolysable substrates, while the second delayed peak was associated with the degradation of large-sized particles. For simulating the second delayed peak, it was necessary to consider a more elaborate particle disintegration/hydrolysis model. Based on the anaerobic respirograms of 17 runs in four datasets and using a substrate characterisation approach similar to activated sludge models (ASMs), the primary sludge was classified into three biodegradable fractions having different kinetics. These are (1) a hydrolysable substrate (XSettle-I) showing a degradation typical to slowly biodegradable compounds, (2) a substrate fraction (XSettle-II) having a degradation similar to lysis of biomass fraction and (3) a substrate requiring disintegration before hydrolysis (XSettle-III) representing the large-sized particles in primary sludge. Based on these results, modifications in the model structure of anaerobic digestion model no. 1 (ADM1) are proposed to improve the modelling of primary sludge solid degradation in anaerobic digesters.
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