Modelling and optimization of microbial production of hydrogen on agro-municipal wastes.
The indiscriminate use of fossil fuels has led to global problems of greenhouse gas emissions, environmental degradation and energy security. Developments of alternative and sustainable energy resources have assumed paramount importance over the past decades to curb these challenges. Biohydrogen is emerging as an alternative renewable source of energy and has received considerable attention in recent years due to its social, economic and environmental benefits. It can be generated by dark fermentation on Organic Fraction of Solid Municipal Waste (OFSMW). These OFSMW exist abundantly and poses disposal challenges. This study models and optimizes the production of biohydrogen on a mixture of agro-municipal wastes; it examines a semi-pilot scale production on these substrates and the feasibility of generating bioelectricity from the process effluents and reviews the prospect of enhancing fermentative biohydrogen development using miniaturized parallel bioreactors. The fermentation process of biohydrogen production on agro-municipal wastes was modelled and optimized using a two-stage design. A mixture design was used for determination of optimum proportions of co-substrates of Bean Husk (BH), Corn Stalk (CS) and OFSMW for biohydrogen production. The effects of operational setpoint parameters of substrate concentration, pH, temperature and Hydraulic Retention Time (HRT) on hydrogen response using the mixed substrates were modelled and optimized using box-behnken design. The optimized mixtures were in the ratio of OFSMW: BH: CS = 30:0:0 and OFSMW: BH: CS = 15:15:0 with yields of 56.47 ml H2/g TVS and 41.16 ml H2/g TVS respectively. Optimization on physico-chemical parameters using the improved substrate suggested optimal setpoints of 40.45 g/l, 7.9, 30.29 oC and 86.28 h for substrate concentration, pH, temperature and HRT respectively and hydrogen yield of 57.73 ml H2/g TVS. The quadratic polynomial models from the mixture and box-behnken design had a coefficient of determination (R2) of 0.94 and 0.79 respectively, suggesting that the models were adequate to navigate the optimization space. The feasibility of a large-scale biohydrogen fermentation process was studied using the optimized operational setpoints. A semi-pilot scale biohydrogen fermentation process was carried out in 10 L bioreactor and the potential of generating bioelectricity from the process effluents was further assessed using a two-chambered Microbial Fuel Cell (MFC) process. The maximum hydrogen fraction of 46.7% and hydrogen yield of 246.93 ml H2/g TVS were obtained from the semi-pilot process. The maximum electrical power and current densities of 0.21 W/m2 and 0.74 A/m2 respectively were recorded at 500 Ω and the chemical oxygen demand (COD) removal efficiency of 50.1% was achieved from the MFC process. This study has highlighted the feasibility of applying agricultural and municipal wastes for large-scale microbial production of hydrogen, with a simultaneous generation of bioelectricity from the process effluents. Furthermore, the potential of generating an economical feasible biohydrogen production process from these waste materials was demonstrated in this work. Keywords: Biohydrogen production, Organic Fraction of Solid Municipal Waste (OFSMW), Modelling and optimization, Fermentation process, Renewable energy, Bioenergy