Effects of temperature and total solid content on biohydrogen production from dark fermentation of rice straw: Performance and microbial community characteristics

Semi-continuous experiments were carried out in lab-scale continuous stirred tank reactors to evaluate the effects of fermentation temperature (37 +/- 1 degrees C and 55 +/- 1 degrees 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 VSadded) from the thermophilic (55 +/- 1 degrees C) DF (TDF) was higher than the mesophilic (37 +/- 1 degrees C) DF (MDF) (0.19-2.13 mL/g VSadded) at the three TS contents, and achieved the highest of 63.60 +/- 2.98 mL/g VSadded 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.

Automated summary

The study found that biohydrogen production was higher in the thermophilic dark fermentation (TDF) compared to the mesophilic dark fermentation (MDF), with the highest yield at TS = 6%. The fermentation liquids in TDF had higher pH, NH4+-N, and total volatile fatty acids than those in MDF, indicating differences in microbial activity. Lactic acid-producing bacteria were found to be abundant at low TS content, while the highest biohydrogen production was associated with the highest abundance of hydrolytic bacteria at TS = 6%.