Effect of feed concentration and residence time on anaerobic fermentation in CSTR and SBR to produce short-chain organic acids

This study investigated the production of short-chain organic acids (SCOAs) and ethanol using anaerobic fermentation (AF) in semi-continuous CSTRs (continuously stirred tank reactors) with intermittent feed without solids separation and in SBRs(sequencing batch reactors) with solids separation. A model feedstock, which included the main classes of substances present in food waste, was used (24.7-394.6 gCOD L-1). To improve process sustainability, conditions of uncontrolled pH and room temperature were used. The
effect of feed concentration, HRT (hydraulic residence time), SRT (solids residence time) on product yield, concentration and productivity was investigated. In CSTRs (HRT=7.5-120 d), the highest product concentration was 113 g L-1, which is amongst the highest values reported for these processes. The product yield was in the range 15-43% g CODg COD-1 (as total COD of the products vs COD of the feed) and increased with the HRT.
Productivity strongly increased for lower HRTs, with maximum of 9.7 g L-1 d-1. SBR runs allowed to uncouple the HRT (2 d) from the SRT (2-20 d), improving process productivity for the most diluted feeds. For the most diluted feed, the productivity in SBR was over 5 times higher than the productivity in CSTR. Generally, the yield increased with the SRT. Lactic acid was the main product in all runs except in those with the lowest feed concentration. The analysis of the microbial community showed a strong and rapid selection towards the genus Lactobacillus.