Dark fermentation

Dark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse groups of bacteria, involving a series of biochemical reactions using three steps similar to anaerobic conversion. Dark fermentation differs from photofermentation in that it proceeds without the presence of light. Fermentative/hydrolytic microorganisms hydrolyze complex organic polymers to monomers which are further converted to a mixture of lower-molecular-weight organic acids and alcohols by obligatory producing acidogenic bacteria. Utilization of wastewater as a potential substrate for biohydrogen production has been drawing considerable interest in recent years especially in the dark fermentation process. Industrial wastewater as a fermentative substrate for H2 production addresses most of the criteria required for substrate selection viz., availability, cost and biodegradability. Chemical wastewater (Venkata Mohan, et al., 2007a,b), cattle wastewater (Tang, et al., 2008), dairy process wastewater (Venkata Mohan, et al. 2007c, Rai et al. 2012), starch hydrolysate wastewater (Chen, et al., 2008) and designed synthetic wastewater (Venkata Mohan, et al., 2007a, 2008b) have been reported to produce biohydrogen apart from wastewater treatment from dark fermentation processes using selectively enriched mixed cultures under acidophilic conditions. Various wastewaters viz., paper mill wastewater (Idania, et al., 2005), starch effluent (Zhang, et al., 2003), food processing wastewater (Shin et al., 2004, van Ginkel, et al., 2005), domestic wastewater (Shin, et al., 2004, 2008e), rice winery wastewater (Yu et al., 2002), distillery and molasses based wastewater (Ren, et al., 2007, Venkata Mohan, et al., 2008a), wheat straw wastes (Fan, et al., 2006) and palm oil mill wastewater (Vijayaraghavan and Ahmed, 2006) have been studied as fermentable substrates for H2 production along with wastewater treatment. Using wastewater as a fermentable substrate facilitates both wastewater treatment apart from H2 production.