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Available online xxxKeywords: Biohydrogen Homoacetogenesis Methanogenesis Hydraulic retention time Dark fermentation a b s t r a c tMethanogenesis and homoacetogenesis are two notorious hydrogen-consuming reactions during dark fermentation for biohydrogen production. The focus of this study was on the role of hydraulic retention time (HRT) to control methanogenesis and homoacetogenesis in an upflow anaerobic sludge blanket (UASB) reactor and a packed bed reactor (PBR). The HRT was changed from 24 to 4 h and 24 to 2 h in the UASB and PBR, respectively. A maximal hydrogen yield of 1.47 mol/mol glucose added with a high hydrogen production rate of 4.38 L/ L/d was achieved at 8 h HRT in UASB. In comparison, a maximal hydrogen yield of 0.89 mol/ mol glucose added with a high hydrogen production rate of 10.66 L/L/d was achieved at 2 h in PBR. With the reduction of the HRT, the volumic hydrogen consumption due to methanogenesis in the UASB was decreased from 12.1 to 3.1%. As for PBR, the value was reduced from 66.9 to 31.4%. Homoacetogenesis in the UASB and PBR was dramatically suppressed when the HRT was decreased to 8 and 4 h, respectively. However, these hydrogenconsuming microbes cannot be completely removed. Microbial diversity analysis using Illumina MiSeq sequencing revealed the existence of Clostridium ljungdahlii, a homoacetogen, in UASB and PBR at low HRT. In addition, the low HRT reduced relative abundance of Clostridiaceae and accelerated the proliferation of lactic acid producers and ethanol producers in the UASB and PBR, which were mainly from the families Ruminococcaceae and Leuconostocaceae. ScienceDirect j o urn al h om epa ge: www.elsev ier.com/locate/he i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e8 http://dx.Please cite this article in press as: Si B, et al., The role of hydraulic retention time on controlling methanogenesis and homoacetogenesis in biohydrogen production using upflow anaerobic sludge blanket (UASB) reactor and packed bed reactor (PBR), International Journal of Hydrogen Energy (2015), http://dx.

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