Multiple process integrations for broad perspective analysis of fermentative H2 production from wastewater treatment: Technical and environmental considerations

The functional certainty associated with secondary or tertiary process integration towards enhancing the viability of fermentative biohydrogen (H2) production from wastewater stabilization was investigated with eleven diverse combinations of dark-fermentation (acidogenic, HA), photo-fermentation (HP for H2) and methanogenic (for CH4) processes. Based on the specificity of individual process, the biocatalyst and the feeding pH were selected and operated at uniform hydraulic retention time (48 h). Individually, HP operation showed higher H2 production (4.10 mmol H2) and yield (16.02 mol H2/kg CODR) than of HA (3.38 mmol H2, 11.33 mol H2/kg CODR) which was found contrary to the observed substrate degradation. Two-stage process integration showed marked improvement in both H2 production and substrate degradation. Integration of HP with HA showed maximum H2 production while HA with HP evidenced maximum H2 yield. Integration of methanogenic process with HP documented both higher biogas production and yield. Maximum substrate degradation was evidenced with three stage sequential integration of dark-fermentation, methanogenic and photo-fermentation processes. Three-stage integration contributed for higher substrate degradation rather than energy generation, especially with HP as the terminal process. Organic flux, energy efficiency and carbon footprint analyses were used to comprehensively delineate the practical consideration of the integrated processes.