Increased biological hydrogen production with reduced organic loading

An experimental matrix consisting of reactor hydraulic retention time (HRT) and glucose loading rate was tested to understand the effect of organic loading on H₂ production in chemostat reactors. In order to vary the glucose loading rate over a range of 0.5-18.9 g/h, the glucose concentration in the feed was varied from 2.5 to 10 g COD/L under conditions where the HRT varied from 1 to 10 h (30°C, pH=5.5). Decreasing the glucose loading rate over this range increased the hydrogen yield from 1.7 to 2.8 mol-H₂/mol- glucose. High yields of hydrogen were consistent with a high molar acetate:butyrate ratio of 1.08:1 as more hydrogen is produced with acetate as a product (4 mol-H₂/mol-acetate) than with butyrate (2 mol-H ₂/mol-butyrate). It was thought that the decrease in yield with organic loading rate resulted from an overall decreased rate of hydrogen production. As the rate of gas production is reduced, H₂ supersaturation in the liquid phase is likely reduced, relieving inhibition due to H₂. Flocculation was also an important factor in the performance of the reactor. At the 5-10 g COD/L influent glucose concentrations, substantial flocculation was observed particularly as the feeding rate was increased due to a reduction in the HRT from 10 to 2.5 h. At the HRT of 2.5 h, biomass concentrations reached as much as 25 g/L. The flocculant nature of the biomass allowed reactor operation at low HRTs with steady H₂ production and >90% glucose removal. However, when the HRT was reduced to 1 h at a glucose feed concentration of 2.5 g COD/L, there was little flocculation evident resulting in wash-out of the culture. These results suggest that hydrogen yields will be optimized for more dilute feeds and lower organic loading rates than have typically been used in biohydrogen reactor studies.