Product Concentration, Yield and Productivity in Anaerobic Digestion to Produce Short Chain Organic Acids: A Critical Analysis of Literature Data

In order to make anaerobic digestion-based processes for short chain organic acids (SCOAs) production attractive, the key performance variables, i.e. concentration, yield and productivity of the produced SCOAs, need to be maximised. This study analyses recent literature looking for the effect of process operating parameters (feed concentration, pH, temperature and residence time) on the performance variables. Data from 551 experiments were analysed. Mean values of the SCOA concentration, yield and productivity were 10 g l-1, 32 % (COD COD-1) and 1.9 g l-1 d-1, respectively. Feed concentration and residence time had the most important effect. Higher feed concentration corresponded to higher product concentration and productivity, but to lower yield. The mean feed concentration was 109 gCOD l-1 and 19 gCOD l-1 in the experiments with the highest product concentrations and in the experiments with the highest yields, respectively. Shorter residence times corresponded to higher productivity. The mean HRT (hydraulic residence time) in the experiments with the highest productivities was 2.5 d. Sequencing Batch Reactors gave higher values of the performance variables (mean values 29 g l-1, 41 % COD COD-1 and 12 g l-1 d-1 for product concentration, yield and productivity, respectively) than processes without phase separation.