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