Continuous biohydrogen production from starch with granulated mixed bacterial microflora

This work demonstrates a continuous-flow biohydrogen producing system able to produce H-2 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 H-2 content of nearly 50% (i.e., a CO2/H-2 ratio of 1.0) and a H-2 yield of 0.97 - 1.43 mol of H-2/mol of hexose, which is 26 - 37% from the theoretical value. Operation at 0.5 h of HRT gave the highest H-2 production rate of 4.12 L h(-1) L-1, respectively, while further shortening of the HRT resulted in a washout of cells. The H-2 production rate obtained from this work appears to be much higher than those indicated in comparable studies using starch to produce H-2 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-H-2 via a granular-sludge-based continuous-flow bioreactor.