Fermentation to produce hydrogen is a dark, anaerobic process, with process similarities to the well-known anaerobic digestion process and to fermentation in the rumen. Carbohydrate-rich organic material is the preferred substrate, e.g. root crops, fodder grass or food industry co-products. The theoretical maximum hydrogen yield by fermentation is 4 moles hydrogen per mole of hexose sugar (e.g. glucose) fermented i.e approximately 0.5m3 hydrogen per kg glucose equivalent. Fermentation end products such as acetic and butyric acids result, which may be further fermented in the dark to methane by anaerobic digestion. These acids could also in principle be feedstock for photosynthetic bacteria to produce hydrogen, with a theoretical maximum of 12 moles hydrogen per mole of glucose equivalent. Fermentative bacteria producing hydrogen in the dark may be cultivated in pure culture or occur in uncharacterised mixed cultures selected from natural sources such as anaerobically digested sewage sludge or soil. Pure cultures of Clostridia, Enterobacter and Bacillus have been studied but their use would necessitate operation in sterile conditions, which is likely to make the process too costly. Pure cultures of extreme thermophiles such as Caldicellulosirupter saccharolyticus and Thermotoga can be grown to produce hydrogen at temperatures of 65ºC or above, which reduces contamination from other species. However this thermophilic process may be too energy demanding unless heat recovery in the overall process is efficient. Work on fermentative hydrogen production from mixed cultures in non-sterile, continuous reactors at temperatures around 30ºC (mesophilic) has been carried out in the Far East and in Wales in the University of Glamorgan. The bacteria are generally species of clostridia able to use a wide range of carbohydrate substrates, including cellulose. Hydrogen inhibits its production by hydrogenase enzymes for thermodynamic reasons, so procedures lowering the hydrogen concentration are favoured. The challenge is to gain good hydrogen yields and prevent shifts in microbial populations to bacteria consuming hydrogen rather than producing it. Once operating protocols have been established and optimised, operating costs are expected to resemble those for high-rate anaerobic digestion processes. In this task operating conditions for fermentation of sewage sludge giving maximal yields of H2 will be determined. Experimental work will be achieved using a pilot-scale (1m3) reactor working on a sewage treatment works site. On-site work is needed to avoid artifacts caused by feed storage, and to benefit from well-tried reliable technologies for feed delivery, gas recirculation etc. used at this scale. Operating conditions suppressing H2 consuming reactions (e.g. methane production, production of unfavourable fermentation end products (ethanol, lactate, propionate)) such as lowering H2 partial pressure, low pH, short hydraulic retention time) will be investigated. Control parameters allowing remote monitoring of process efficiency will be identified. To provide a gas stream to mix and lower the hydrogen partial pressure in the hydrogen-generating reactor, an anaerobic digester (5m3) will be operated on the effluent from the hydrogen generating reactor. This work is part-funded by the European Union in a project entitled REduction, MOdification & VALorisation of Sludge (REMOVALS). |
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