Optimization of hydrogen production from pretreated rice straw waste in a mesophilic up-flow anaerobic staged reactor

This study was carried out to assess the efficiency of a mesophilic up-flow anaerobic staged reactor for continuous H2 production from pretreated rice straw waste. The reactor was operated at different hydraulic retention times (HRTs) of 20, 16, 12, 8 and 4 h. The organic loading rate and sludge residence time were kept constant at 30 g chemical oxygen demand (COD)/L/day, and 1.9 days, respectively. The results showed that increasing the HRT from 4 to 20 h increased the H2 production from 0.4 ± 0.1 to 3.6 ± 0.3 L H2/day, respectively. This corresponds to a H2 yield of 2.1 ± 0.2 mol H2/g CODremoved at an HRT of 20 h and 0.03 ± 0.002 mol H2/g CODremoved at an HRT of 4 h. Likewise, carbohydrate and COD removal efficiency was strongly dependant on HRT. The removal efficiency decreased from 76.5 ± 3.4% to 40 ± 2.2% for carbohydrate and from 77.7 ± 4.3% to 12.2 ± 2.1% for COD when the HRT is reduced from 20 to 4 h, respectively.The addition of presettled sewage sludge to pretreated rice straw at a mixing ratio of 1:4 (v/v) increased the volumetric H2 production from 3.6 ± 0.3 to 8.2 ± 2.5 L/day and the H2 yield from 2.1 ± 0.2 to 2.8 ± 0.3 mol H2/g CODremoved. Moreover, the removal efficiency of COD, volatile solids and carbohydrate was significantly improved.