Continuous fermentative hydrogen production using a two-phase reactor system with recycle

The effects of effluent recycle were examined in a two-phase anaerobic system where the first phase was operated for fermentative hydrogen production and the second for methanogenesis. The hydrogen reactor was operated as a chemostat at 35 degrees C and pH 5.5 with a 10 h hydraulic retention time, and the methane reactor was operated as an up-flow reactor at 28 degrees C and pH between 6.9 and 7.2. Two recycle ratios were examined: 0 and 0.98. Effluent recycle reduced the required alkalinity for pH control by approximately 40%. The H-2 productivity metric, with a basis in electrons and incorporating both gaseous and dissolved H-2, was developed as a more fundamental reporting method than the molar H-2 yield. Without recycle, the H-2 productivity was 0.115 g of H-2 COD/g of feed COD, but decreased to 0.015 g of H-2 COD/g of feed COD with recycle (COD = chemical oxygen demand). Mass balances indicated the lower H-2 productivity during recycle was due to electrons being partitioned to methane and less-oxidized soluble constituents such as propionic acid, ethanol, and butanol. The results indicated that achieving high H-2 productivity with nonsterile wastewaters will be challenging and membrane filtration of the recycle liquid may be required to exclude the return of hydrogen-consuming organisms.