Anaerobic fermentation for the production of short chain organic acids: product concentration, yield and productivity in batch experiments at high feed concentration

With the general aim of maximising product concentration, yield and productivity in the anaerobic fermentation (AF) of biomass to make short-chain organic acids (SCOAs) with low-cost and environmentally sustainable processes, this study investigates the effect of high substrate concentration on the batch digestion of model biomass at uncontrolled temperature. Five substrate concentrations, between 24.7 and 394.6 gCOD l-1 (the latter is the highest substrate concentration reported in the literature for these types of studies), were investigated in batch reactors with no pH control. The highest substrate concentration led to a maximum product concentration of 61.5 g l-1, composed mainly of lactic acid (85 wt%). The lowest substrate concentration produced mainly acetic acid (54 wt%). The pH was acidic in all cases but was higher for the most diluted feed (6.2). Similar yields at the end of the experiments, between 15.7 and 22.0 % COD COD-1, were observed in all reactors. Over the length of the experiments, generally yields increased and productivities decreased, indicating the need of a compromise between yield and productivity. For the highest substrate concentration, a final productivity of 1.5 g l-1 d-1 was obtained.
Experiments with pH adjustment in the range 4.0-6.0 were performed, and results generally showed an increase in product concentration, yield and productivity as the target pH increased, with the highest values of concentration, yield and productivity (163 g l-1, 56 % COD COD-1 and 3.9 g l-1 d-1 respectively) obtained at pH 5.5.