# When and how in anaerobic digestion is water dissociated?

Anaerobic digestion and methanisation of organic feedstocks is a complex series of processes that result in biogas. One way to look at this is a very top-down, stochiometric approach as in the Buswell equation

$$\ce{C_aH_bO_c + \left(a - \frac{b}{4} - \frac{c}{2}\right)H2O -> \left(\frac{a}{2} + \frac{b}{8} - \frac{c}{4}\right)CH4 + \left(\frac{a}{2} - \frac{b}{8} + \frac{c}{4}\right)CO2}$$

There's also a Buswell-Boyle equation that takes nitrogen and sulfur into account, but right now I just want to understand one aspect.

During the process, at some point, water is dissociated. When does this happen (hydrolysis, acidification, ...), what role does the water play and what are sample reactions that illustrate this?

This is not a trivial question to answer, as many diagrams are often simplified and do not state the full chemical reactions that occur, such as from Aerobic and Anaerobic Digestion and Types of Decomposition education webpage from the Mountain Empire Community College.

However, according to this image (below) from the Ecocorp page - Metabolic Chain of the Anaerobic Process for the Utilization of Organic Wastes, water is a reactant in the acetogenesis stage and is removed as a product in the methanogenesis stage.

(Sample reactions are included in the image) (Just in case the image becomes a dead link), the chemical processes involving water from the diagram are:

acetogenesis

$$\ce{CH3CH2COOH + 2H2O -> CH3COOH + CO2 + 3H2}$$

methanogensis

$$\ce{4H2 + CO2 -> CH4 + 2H2O}$$

• $CH_3CH_2COOH$ is Propanoic acid? If you look at buswell boyle, water is only needed as an educt (for the whole methanization chain) for proteins and fats. So propanoic acid would only be created as a product of proteins and fats?
– mart
Jan 13 '14 at 16:30
• Oh, and the answer is great, I think! I'll wait a bit with the accept, see what the bounty brings, but this looks good!
– mart
Jan 13 '14 at 16:32
• scratch my first question, I found the answer on wikipedia.
– mart
Jan 13 '14 at 16:33