A blog from University of Borås

Sunday, February 9, 2014

Why big digesters for biogas? A PhD thesis

If you have seen biogas processes, you have probably noticed very big vessels for the biogas productions. They are normally 100-10,000 m3 in size. The reason of this big size is that the materials should stay in the reactors for about 30 days. The bugs that do the job and produce biogas are sensitive and need long time to grow. Just as example, you can compare E. coli bacteria that can duplicate itself in 20 minutes, while a methane producing bacteria (or archaea) need about 2 weeks to duplicate. It means if you don't want them to get away from the reactor, you should have big vessels, feed it slowly and let them to stay longer than two weeks.

In a PhD thesis that will be defended 27 Feb. 2014, Solmaz Aslanzadeh found a method to reduce this retention time from about 30 days to about 6 days! It means if her results can be applied commercially, the biogas digesters will squeeze by 80% in their size. You can read her thesis to see how she did it:

Thesis title: Pretreatment of cellulosic waste and high rate biogas production

Saturday, February 1, 2014

Chemical reactions in biogas fermentation

Biogas (or biomethane) production looks simple. Just put the materials and the digesting bacteria into a vessel, and let them take care of the rest and produce the biogas. They have done it for probably billions of years, and in industries today it is still the same.

However, if operators of a biogas plant do not follow the recommendations or do not know how to run a biogas plant, the chance of failure would be quite high. It is in fact one of the reasons that the numbers of biogas plants are not boosted globally. The point here is that we have many microorganisms (bacteria and archaea) that live perfectly together in a society. In a recent work, we have defined about chemical 50 reactions that occur in parallel and series and affect each other. These reactions were modeled using ASPEN Plus and checked against with several data available from experimental works and also industries. If you are investigating about biogas, this article and the original model (in ASPEN) could be interesting for you:

Rajendran, K., Kankanala, H.R., Lundin, M., Taherzadeh, M.J. (2014): A Novel Process Simulation Model (PSM) for Anaerobic Digestion Using Aspen Plus, Bioresource Technolology:
Available at:
and the model and prepublished article at: