Coupling dark fermentation with biochemical or bioelectrochemical systems for enhanced bio-energy production: A review

Dark fermentation (DF) is one of the most commonly studied techniques for bio-hydrogen production from biomass and waste materials. However, the relatively low H2 yield has hampered its development on larger scale. As such, this study reviewed the potential of coupling the DF process with other biochemical techniques such as anaerobic digestion (AD) and photo fermentation (PF) or bioelectrochemical systems (BES) viz. microbial fuel cell (MFC) and microbial electrolysis cell (MEC) for maximising bio-energy production from DF effluent so as to enhance the potential of implementing the DF process on a larger scale. All the different systems reviewed assisted in enhanced bio-energy production with AD increasing the bio-energy recovery from 6.1% for single DF to 47.6% for coupled DF/AD while the two-stage DF/PF process enhanced bio-hydrogen production by 274% as opposed to single DF. Similarly, studies on integrating DF with MFC or MEC also resulted in additional bio-electricity and bio-hydrogen production respectively. Despite the obvious advantages of integrating the DF process with AD, PF, MFC or MEC, there are still some shortcomings as to the large-scale implementation of these coupled systems. These include the requirement of two or more bioreactors which represents an additional investment cost to the whole process. Other technical aspects include the use of pure H2-producing cultures or pre-treated mixed cultures on large-scale and light requirement for PF systems that still warranty further research. Notwithstanding the shortcomings, the coupling of the DF process with at least the AD process seems the most potential two-stage system for enhanced bio-energy recovery on large scale.