Continuous biohydrogen production from starch with granulated mixed bacterial microflora

This work demonstrates a continuous-flow biohydrogen producing system able to produce H₂ from starch at a high volumetric rate of over 4 L h^-1 L^-1. Using phosphate-buffered medium containing cassava starch (15 g/L) as the feed, sludge granulation occurred within 15 days after start-up while operating at a hydraulic retention time (HRT) of 2.2 h, enabling efficient biomass retention in the bioreactor. Operation at a progressively decreasing HRT of 5.3-0.5 h gave rise to a H₂ content of nearly 50% (i.e., a CO₂/H₂ ratio of 1.0) and a H₂ yield of 0.97-1.43 mol of H₂/mol of hexose, which is 26-37% from the theoretical value. Operation at 0.5 h of HRT gave the highest H₂ production rate of 4.12 L h^-1 L^-1, respectively, while further shortening, of the HRT resulted in a washout of cells. The H₂ production rate obtained from this work appears to be much higher than those indicated in comparable studies using starch to produce H₂ via dark fermentation. The soluble metabolites were dominated by butyric acid (54-72% of total soluble products), followed by ethanol and acetic acid. The results indicate the feasibility of using starch as an inexpensive carbon substrate for high-rate and low-cost production of bio-H₂ via a granular-sludge-based continuous-flow bioreactor.