Effects of temperature and total solid content on biohydrogen production from dark fermentation of rice straw:...

Semi-continuous experiments were carried out in lab-scale continuous stirred tank reactors to evaluate the effects of fermentation temperature (37 ± 1 °C and 55 ± 1 °C) and total solids (TS) contents (3 %, 6 %, and 12 %) on biohydrogen production from the dark fermentations (DF) of rice straw (RS) and the total operation duration was 105 days. The experimental results show that biohydrogen production (0.46-63.60 mL/g VS_added) from the thermophilic (55 ± 1 °C) DF (TDF) was higher than the mesophilic (37 ± 1 °C) DF (MDF) (0.19-2.13 mL/g VS_added) at the three TS contents, and achieved the highest of 63.60 ± 2.98 mL/g VS_added at TS = 6 % in TDF. The pH, NH₄⁺-N and total volatile fatty acid of fermentation liquids in the TDF were all higher than those in the MDF. The high abundance of lactic acid-producing bacteria resulted in low biohydrogen produced at TS = 3 %. Under the TDF with TS = 6 %, the highest abundance of hydrolytic bacteria (Ruminiclostridium 54.24 %) led to the highest biohydrogen production. The increase of TS content from 6 % to 12 % induced degradation pathway changes from biohydrogen production to methane production. This study demonstrated that butyric acid fermentation was the main pathway to produce biohydrogen from RS in both DFs.