Hydrogen production by enrichment granules in an acidogenic fermentation process

Abstract

Energy supply and environmental protection are two crucial issues for sustainable development of today's world. Among renewable energies, hydrogen produced by biomass is a completely carbon-free fuel with a high energy yield (122 kJ/g), and considered a feasible alternative to fossil fuels. Harvesting hydrogen by fermentation process has attracted many researchers in recent years. This study demonstrated acidogenic sludge (pH 5.5) could produce hydrogen and also granulate in an anaerobic sequencing batch reactor (ASBR) fed with synthetic wastewater containing glucose with 6.7h hydraulic retention time (HRT).Optimization, enrichment, and stability of the acidogenic granular sludge have been investigated at a constant loading rate of 25 g-glucose/ (L.d). Results showed that hydrogen in the biogas increased from 15% to 48% by subjecting the biomass to a combination of heat-treatment, acidic pH, and carbon source limitation. The maximum hydrogen, yield, and production rate was 73%, 2.5 molH2/mol glucose, and 0.34 molH2/d, respectively. Microbial analysis indicated that enrichment by granulation was successful and microbial diversity changed significantly after treatments. The ASBR was operated for 445 days.The hydrogen producing granules were characterized. A typical matured granule was 1.7mm in diameter with an average of 43 m/h settling velocity.Moreover, as morphological analysis demonstrated, the inner and outer surface of the granules was comprised the same types of bacteria and hence had non-layered structure. A hydrogen producing granule had multiple cracks on the surface. The acidified effluent comprised volatile fatty acids (VFA) and alcohols. The VFA comprised acetate (73%), butyrate (23%), propionate (1.5%), caproate (0.69%), valerate (0.58%).