Fermentative production of biofuels with entrapped anaerobic sludge using sequential HRT shifting operation in continuous cultures

In this work, a continuously stirred tank bioreactor (CSTBR) was used to produce gaseous (hydrogen) and liquid (ethanol) biofuels simultaneously from carbohydrate substrates using immobilized anaerobic sludge with polyethylene–octene–elastomer (POE) matrix. The continuous culture was operated by sequential decrease and increase of hydraulic retention time (HRT) to assess the effect of HRT shifting on the efficiency of H2 and ethanol production. The experimental results show that during HRT-decreasing operation, using sucrose as the carbon substrate resulted in increasing H2 and ethanol production with an increase in organic loading rate or a decrease in HRT. The best biofuel-producing performance occurred during HRT decrease, giving the highest H2 production rate and yield of 37.4 mmol/h/L and 1.18 mol H2/(mol(hexose)), respectively (at 4 h HRT), as well as a maximal ethanol production of 84.1 mmol/h/L and 0.9 mol ethanol/(mol(hexose)) (at 0.5 h HRT). For continuous cultures fed with glucose-based medium, the biofuel production rate and yield was lower than those from using sucrose, and the dependence of biofuels production on HRT shifting had a slightly different trend. Ethanol production with glucose-feeding cultures seemed to be optimal during a HRT increase (HRT = 1 h), but H2 production reached maximum during a decreasing HRT (HRT = 1 h). For all the continuous cultures, ethanol was the predominant soluble metabolite, accounting for 35–78% of total soluble microbial products, while production of acetate and butyrate was less significant. Calculation of total energy generation resulting from combination of the two biofuels shows that the best energy generation rate (116 kJ/h/L) and yield (1235 kJ/(mol(hexose))) was obtained during HRT decreasing (HRT = 0.5 h) while using sucrose was the carbon substrate.