It is a complex process manifested by diverse groups of bacteria, involving a series of biochemical reactions using three steps similar to anaerobic conversion.
[citation needed] Utilization of wastewater as a potential substrate for biohydrogen production has been drawing considerable interest in recent years especially in the dark fermentation process.
The efficiency of the dark fermentative H2 production process was found to depend on pre-treatment of the mixed consortia used as a biocatalyst, operating pH, and organic loading rate apart from wastewater characteristics (Venkata Mohan, et al., 2007d, 2008c, d, Vijaya Bhaskar, et al., 2008d).
In spite of its advantages, the main challenge observed with fermentative H2 production processes is the relatively low energy conversion efficiency from the organic source.
Generation and accumulation of soluble acid metabolites causes a sharp drop in the system pH and inhibits the H2 production process.