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Hydrogen production by dark fermentation of biomass resources

Final Activity Report Summary - BIOHYP (Hydrogen production by dark fermentation of biomass resources)

Hydrogen production via dark fermentation is an ever-present phenomenon that occurs under anoxic or anaerobic conditions in nature. A wide variety of bacteria is able to convert organic matter to H2, CO2 and metabolites like organic acids and ethanol. In general, these H2 producers live in close vicinity of other microorganisms which consume these metabolites and H2 to produce their own products. In this way a stable ecosystem is created that prevents the end product inhibition of H2 producers, because the H2 produced is immediately consumed by other organisms. In the engineered dark fermentation process, carbohydrate-rich biomass is used as substrate using pure or mixed cultures of bacteria under the appropriate circumstances to produce H2 in the gas phase and organics acids in liquid phase. Using waste or residual biomass instead of disposing it, to produce a clean and renewable energy source is a gainful operation. The simultaneous waste treatment and sustainable energy production as alternative to energy consuming conventional treatment processes makes dark fermentative H2 production from biomass sources more attractive even the yields are lower than the theoretical potentials.

Although a considerable amount of literature has been published on dark fermentative H2 production at mesophilic temperatures, studies under thermophilic conditions are very limited. In this project, dark fermentative H2 production under thermophilic conditions from different carbohydrate sources such as glucose, xylose, lactose and glycerol was investigated using natural mixed culture inocula. Moreover, the influence of pH, batch and continuous feeding, removal of dissolved H2 in order to lower H2 partial pressure and organic loading rate on fermentation type and H2 yield was assessed comprehensively.

Briefly the following findings were obtained:
- Yard waste and food waste composts are appropriate to be used as inoculum for H2 production via thermophilic fermentation of sugars glucose, xylose and lactose.
- Fermentation pH below 6 and hydraulic retention time of 1 day suppress methanogenic H2 consumption. To overcome non-methanogenic H2 utilisation, pH has to be below 5.5.
- H2 consumption via sulfate reduction is minimised by using low sulfur content feed and avoiding use of sulfuric acid in pH adjustment.
- As H2 partial pressure increase the metabolic pathways shift to production of more reduced substrates such as lactate and subsequently H2 yield drops.
- Thermophilic conditions provide more stable H2 production through energetically more favorable butyrate fermentation under moderately high H2 partial pressure (45-55kPa).
- Higher organic loading rates (5 g/ favour H2 production by lowering redox potential and avoiding unfavourable propionate fermentation. However it should be not so high to avoid free volatile fatty acid inhibition.
- The nutrients and trace elements not available in the biomass source has to be supplied.
- Fermenters with low hydraulic retention time need to be configured to maximise the bacterial mass
- Although some saccharolytic clostridia are known to produce H2 from glycerol, a considerable H2 production was not accomplished with yard waste and food waste composts used.