Caproic acid production is linked to biomass activity during xylose fermentation

This study aims at addressing inconsistencies in literature regarding the organic loading rate (OLR) and hydraulic retention time (HRT) effect on caproic acid production, using biomass activity as an indicator. Xylose was fully consumed in the reference CSTR and SBR (HRT = 1-day; OLR = 12 gCOD/(L·d)), but different caproic acid yields (0.02 vs 0.11 Cmol/Cmol-s) were observed, which was linked to differences in biomass activity (12 vs 3.5 g/(g VSS·d)). At HRT 0.5 days, xylose conversion was incomplete and lactic acid and ethanol appeared, reducing caproic acid production. However, increasing xylose concentration in the feeding to 24 g COD/L did not change the caproic acid yield (0.12 Cmol/Cmol-s), which was explained by similar biomass activity as in the reference SBR (4.8 g/(g VSS·d)). These findings indicate that the SBR is the optimal configuration, since it allows maintaining a low biomass activity and therefore a high caproic acid yield.