Fermentative hydrogen and methane productions using membrane bioreactors (original) (raw)

The role of energy as a stimulant for economic growth and environmental sustainability of any nation has made the focus on green fuels, including fermentative hydrogen (bioH 2) and methane (bioCH 4), to be a priority for the World's policy makers. Nigeria, as the most populous African country, with worsening energy crisis, can benefit from the introduction of the bioH 2 and bioCH 4 technologies into the country's energy mix, since such technologies have the potential of generating energy from organic wastes such as fruit waste. Fruit waste was studied in detail in this work because of its great economic and environmental potential, as large quantities of the wastes (10-65% of raw fruit) are generated from fruit consumption and processing. Meanwhile, bioH 2 and bioCH 4 productions involving anaerobic microorganisms in direct contact with organic wastes have been observed to result in substrate and product inhibitions, which reduce the gas yields and limit the application of the technologies on an industrial scale. For example, in this study, the first experimental work to determine the effects of hydraulic retention times and fruit mixing on bioH 2 production from single and mixed fruits revealed the highest cumulative bioH 2 yield to be equivalent to 30% of the theoretical yield. However, combining the fermentation process with the application of membrane encapsulated cells and membrane separation techniques, respectively, could reduce substrate and product inhibitions of the microorganisms. This study, therefore, focused on the application of membrane techniques to enhance the yields of bioH 2 and bioCH 4 productions from the organic wastes. The second experimental work which focused on reduction of substrate inhibition, involved the investigation of the effects of the PVDF membrane encapsulation techniques on the bioH 2 and bioCH 4 productions from nutrient media with limonene, myrcene, octanol and hexanal as fruit flavours. The results showed that membrane encapsulated cells produced bioCH 4 faster and lasted longer, compared to free cells in limonene. Also, about 60% membrane protective effect against myrcene, octanol and hexanal inhibitions was obtained. Regarding bioH 2 production, membrane encapsulated cells, compared to free cells, produced higher average daily yields of 94, 30 and 77% with hexanal, myrcene and octanol as flavours, respectively. The final part of the study, which was aimed at reducing product inhibition, involved the study of the effects of membrane permeation of volatile fatty acids (VFAs) on the bioreactor hydrodynamics in relation to bioH 2 production. The investigation revealed that low transmembrane pressure of 10 4 Pa was required to achieve a 3L h-1 m-2 critical flux with reversible fouling mainly due to cake layer formation, and bioH 2 production was also observed to restart after VFAs removal. The results from this study suggest that membrane-based techniques could improve bioH 2 and bioCH 4 productions from fermentation media with substrate and product inhibitions.