Influence of solids retention time on continuous H2 production using membrane bioreactor

The influence of solids retention time (SRT) on continuous H₂ production in a submerged membrane bioreactor (MBR) was investigated using mixed mesophilic microflora. The bioreactor was continuously operated at the four SRTs of 2, 4, 12.5 and 90 d on a glucose medium under the hydraulic retention time (HRT) of 9 h and the mesophilic condition of 35°C ± 0.5. Stable biogas production with H₂ content of 50.8%–60% was achieved at SRTs ranging from 2 to 12.5 d. No methane gas was observed in monitoring the experimental conditions. The H₂ production increased from 17.62 to 26.1 l-H₂/d when the SRT increased from 2 to 12.5 d, but decreased to 9.1 l-H₂/d at the 90 d SRT. The best H₂ yield, 1.19 mol-H₂/mol-glucose, was observed at the SRT of 2 d and the highest H₂ production rate, 5.8 l-H₂/l/d, was obtained at the SRT of 12.5 d. Stable H₂ production was achieved by maintaining the SRT in the range of 2 - 12.5 d, regardless of the fermentative pathway related to higher lactate production. The decrease in H₂ yield was observed at long SRTs due to the low volatile suspended solids/total suspended solids (VSS/TSS) as well as the high extracellular polymeric substances (EPS) concentrations. These results suggest that the SRT is the key factor enabling sustainable H₂ fermentation in MBR, and that an SLR value of around 1.6 kg-DOC/kg-VSS/d might be the specific condition for achieving optimum H₂ production.