Optimization of biohydrogen production inoculum development via hybrid pre-treatment techniques : semi pilot scale production assessment on agro waste (potato peels)

Abstract

The challenges of energy crisis and environmental pollution are vital issues hindering the global sustainable development as a result of over dependence on fossil fuels. These are driving the need to explore renewable and environmentally friendly energy sources. Biohydrogen has emerged as an eco-friendly renewable energy source and a suitable alternative to fossil fuels. However, the commercialization of biohydrogen energy is hindered by the high production cost and low yield which necessitates novel strategies for an economically feasible production. Some of these strategies include the development of stable inoculum, scale-up studies, and the utilization of renewable feedstock such as agro-food waste materials which are both abundant and sustainable. Inoculum pre-treatment is a vital aspect of hydrogen production technology as it contributes to the improvement of hydrogen yield. The inoculum pre-treatment method influences the community structure which in turn affects the microbial metabolism of hydrogen production. This study investigates novel inoculum development techniques and evaluates the feasibility of biohydrogen production from agro waste (potato peels). The linear and interactive effect of these techniques on inoculum efficiency as well as the key process parameters for hydrogen production from potato peels were modelled and optimized. Further assessment of the hydrogen production dynamics at the semi-pilot scale including the microbial community structure were investigated using the 16SrRNA gene clone library sequence analysis. A hybrid inoculum development technique of pH and Autoclave (PHA), pH and Heat shock (PHS) was modelled and optimized using the response surface methodology. The quadratic polynomial models had a coefficient of determination (R2) of 0.93 and 0.90 and the optimized pre-treatment conditions gave a 37.7% and 15.3% improvement on model predictions for PHA and PHS respectively. Maximum hydrogen yield of 1.19 mol H2/ mol glucose was obtained for PHA in a semi-pilot scale process. The interactive effect of a hybrid pH and microwave pre-treatment on mixed inoculum for biohydrogen production was investigated. The obtained model had a coefficient of determination (R2) of 0.87. Two semi pilot scale-up processes were carried out to assess the efficiency of the developed inoculum with and without pH control on biohydrogen production. A two fold increase in glucose utilization was obtained and a molar hydrogen yield of 2.07 mol H2/mol glucose under pH controlled fermentation compared to 1.78 mol H2/mol glucose without pH control. Methane production was not detected which suggests the effectiveness of the combined pre-treatment to enrich hydrogen producing bacteria. The developed inoculum was used to evaluate the feasibility of biohydrogen production from potato peels waste. The key process parameters of substrate concentration (g/L), nutrient supplementation (%), temperature (°C) and pH were modelled and optimized using the Artificial Neural Network (ANN) and Response surface methodology (RSM). The optimum conditions obtained were 50g/L of potato waste, 10% nutrients, 30°C and pH 6.5. A semi pilot production process under the optimized condition gave a hydrogen yield of 239.94mL/g TVS corresponding to a 28.5% improvement on hydrogen yield. Analysis of the microbial community structure showed the dominance of the genus Clostridium comprising of about 86% of the total microbial population including C. aminovalericum, C. intestinale, C. tertium, C. sartagofome, C. beijerinckii and C. butyricum in ascending order of predominance. Hydrogen consuming methanogens were not detected which further confirmed the efficiency of the hybrid inoculum pre-treatment. This study has highlighted the development of a novel hybrid inoculum pretreatment method to establish the requisite microbial community and to safeguard the stability of biohydrogen production. Furthermore, the potential of generating an economical feasible biohydrogen production process from potato waste was demonstrated in this work.