Dear Mr. Rezania,

Photo, dark and combined photo/dark fermentation are processes used for bio-hydrogen production. However, the anaerobic digestion process produces a biogas consisting of mainly methane and carbon-dioxide. While some hydrogen is produced in the initial phases of the anaerobic digestion process, these are consumed by methanogens to produce methane (hydrogenotrophic methanogenesis). As such, I would not include anaerobic digestion as a method to produce hydrogen.

Besides, the dark fermentation/anaerobic digestion processes are now being coupled whereby hydrogen is initially produced in the dark fermentation process and the resulting "broth" (consisting of soluble metabolites such as acetic acid) from the dark fermentation process is then subjected to the anaerobic digestion process for methane production. The biogas from both processes is often referred as hythane (hydrogen+methane).

You may compare H2 yield from the photo vs. dark fermentation process individually. You may also do a combined dark/photo fermentation process and compare the total H2 yield with the sum of the H2 yields from the single dark and photo fermentation processes.

Kind regards,

Bundhoo M.A. Zumar

Dear Shahab

In process of hydrogen production via fermentation, the pathway of organisms would be acting  very important role. Based on identification of microorganism we decide what kind of process should be followed.  Assume you have substrates which is abundant in your region and you have decided to convert the source to clean fuel like hydrogen. At first you think what would be the right species of organisms to utilize the substrate; how fast the organism grows? What would be your product yield? What kind of pathway of metabolism the organism follow? Then you think about the process and how would you perform the biochemical reactions.  Since you which to obtain valuable product; you have no choice to follow anaerobic process since bioconversion can occur in absence of oxygen. In past 2 days I have wrote several pages to respond you; due to internet speeds, have lost them! Then I decided to wrote short paragraphs. If you are interested on my work. I would recommend to look for a few papers I have published:

World Journal of Microbiology and Biotechnology

February 2007, Volume 23, Issue 2, pp 275–284

Bioconversion of synthesis gas to hydrogen using a light-dependent photosynthetic bacterium, Rhodospirillum rubrum

G. D. NajafpourEmail authorH. Younesi

Regards

Dear Dr. Shahabaldin Rezania

Please read the article of

"Mr Kaushik Nath et al DOI 10.1007 / s00253-004-1644-0" and

"Mr. Liam Jed Thompson et al DOI 10.1007 / s10529-007-9527-y"

on biohydrogen production via anaerobic digestion

regards