What Are Metabolic Processes?
Metabolic processes are the differing processes undergone by cells that are necessary for life; there are two types of metabolic processes: catabolic and anabolic. Catabolic processes break apart molecules to release energy. Alternatively, anabolic processes build molecules and fuse energy within them. Fermentation is a type of metabolic process that yields energy; fermentation takes place in the absence of oxygen and requires a sugar substrate. Fermentation is a lot like glycolysis with the exception that the H atoms from the NADH get transferred to other molecules rather than the electron transport chain in order to recycle NAD+.
Photo-Fermentation
Photo-fermentation is one way biohydrogen can be produced. This takes place with photosynthetic bacteria via the enzyme nitrogenase. This process uses light energy and bio mass (food waste). Light energy and the biomass go into a bacterial photosystem which produces two electrons and four ATP molecules. Then these products are then sent to the nitrogenase that are used to produce H2. Here is a figure that gives you a visual reference.
Dark Fermentation
Dark Fermentation begins with a biomass (food waste) acting as a substrate. This food waste undergoes fermentation but with the twist of lacking light energy (DARK fermentation). Unlike photo fermentation the products are H2 and CO2. The pH and partial pressure determines how effective dark fermentation is. It is proven that dark fermentation is the most space and time effective. The figure below illustrates the process of dark fermentation.