Calcium effect on fermentative hydrogen production in an anaerobic up-flow sludge blanket system

The effects of calcium ions on a granular fermentative hydrogen production system were investigated in four lab-scale UASB reactors that fed on sucrose (20 g COD/L). The reactors were seeded with anaerobic sewage sludge microflora and operated at a temperature of 35 +/- 1 degrees, pH of 6.7 with hydraulic retention times (HRTs) of 24-6h. The experimental results indicated that calcium ion addition (75 - 150 mg/L) could enhance the granulation and elevate hydrogen production efficiency. However, an overly-high calcium concentration (300 mg-Ca(+2)/L) deteriorated the hydrogen productivity. A calcium concentration of 150 mg-Ca(+2)/L resulted in a peak HP of 3.6 mol H2/mol-sucrose and HPR of 807 mmol-H2/L-d at HRTs of 8 and 6 h, respectively. The EPS concentration of biohydrogenic biomass was higher than that of the aerobic or methanogenic biomass. The protein/carbon-ratio ranged from 0.17 to 0.26%. The multinomial regression analysis shows that the 75 - 150 mg-Ca(+2)/L calcium concentrations and HRT of 6 h were the optimal operating conditions to efficiently produce hydrogen.