Experimental investigation and mathematical modelling of batch and semi-continuous anaerobic digestion of cellulose at high concentrations and long residence times

In the context of the anaerobic digestion of slowly biodegradable substrates for energy and chemicals production, this study investigated the anaerobic digestion of cellulose without any chemical pre-treatments using open (undefined) mixed microbial cultures. The anaerobic conversion of cellulose was investigated in extended-length (run length in the range 518-734d) batch and semi-continuous runs (residence time 20-80 d), at high cellulose concentration(20-40 g L-1), at temperatures of 25 and 35 OC. The maximum cellulose removal was 77 % in batch (after 412 d) and 60 % (at 80 d residence time) in semi-continuous experiments. In semicontinuous experiments, cellulose removal increased as the residence time increased however the cellulose removal rate showed a maximum (0.17 g L -1 d-1) at residence time 40-60 d. Both cellulose removal and removal rate decreased when cellulose concentration in the feed was increased from 20 to 40 g L-1. Liquid-phase products (ethanol and short chain organic acids) were only observed under transient conditions but not at the steady state of semicontinuous runs. Most of the observed results were well described by a mathematical model which included cellulose hydrolysis and growth on the produced glucose. The model providedinsight into the physical phenomena behind the observed results.