The organic loading rate (OLR) of the wastewater also influences the H2 production pattern, apart from other wastewater characteristics. H2 yields were inversely pro­portional to the glucose feeding rate, while the highest H2 yields were observed at lowest glucose loading rate [78]. Glucose concentrations exceeding 2 g/l (as co-substrate) showed suppression in H2 production [21]. A marked reduction

in H2 production rate was observed with an increase in OLR when chemical wastewater was used as the substrate [22]. H2 production was also found to decrease with an increase in OLR when dairy wastewater was used as substrate [31]. Similar observations with substrate loading have been reported in the literature [101, 102]. Decreased H2 production may also be due to end product inhibition by over­accumulated (supersaturated) soluble metabolites in the liquid phase at high OLRs [102]. However, each wastewater has its own threshold value, which relates to the system microenvironment and desired output [4,22,26, 31]. Feed consisting of only glucose as substrate showed a low H2 yield, while feed with chemical wastewater admixed either with glucose or domestic sewage as co-substrates showed a positive influence on the H2 generation rate [21, 71]. Domestic sewage addition showed a positive affect on the acidogenic fermentation process due to supplementation of additional micronutrients, organic matter and microbial biomass in the direction of enhance the process efficiency.