Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

The effect of effluent recycle on hydrogen production in an anaerobic continuous stirred tank reactor (CSTR) was investigated. The CSTR was fed on sucrose (20 g chemical oxygen demand (COD)/L) and at hydraulic retention time (HRT) of 12 h. The pH and temperature were regulated around 6.7 and 35 °C, respectively. The effluent was then recycled at recycle ratios of 0, 0.2, 0.4, 0.6, 0.8 and 1.0. When the recycle ratios increased, the volatile suspended solid (VSS) concentration also increased with a peak of 14.2% at the recycle ratio 0.4 compared with the VSS 3.52 g/L without effluent recycle. However, a drastic drop in hydrogen production performance was observed with increased recycle ratio. The strategy of increasing organic loading rate by increasing the substrate concentration and shorting the HRT was then applied keeping a recycle ratio of 0.2. VSS concentration increased with increasing substrate concentration but as the HRT was shortened from 12 h to 2 h stepwise, VSS concentration increased first and then decreased at HRT 8 h. The maximal hydrogen production was obtained at sucrose concentration of 20 g COD/L and HRT 6 h (organic loading rate (OLR) 66.7 g COD/L-d) with recycle ratio 0.2. The hydrogen yield, hydrogen production rate and specific hydrogen production rate values also peaked at the same conditions (sucrose 20 g COD/L and HRT 6 h) with 3.88 mol H2/mol sucrose, 807 mmol H2/L-d and 244.3 mmol H2/g VSS-d, respectively. This study showed peak hydrogen production performance at OLR 66.7 g COD/L-d in the recycled continuous anaerobic biohydrogen production system by controlling HRT. Thus, effluent recycling at the optimum ratio along with optimum HRT and substrate concentration can maximize the hydrogen production performance in an anaerobic biohydrogen production system.