Sunday, 05 February 2012

How does a digester work?

basic information priming

How does a digester work?

We know that biogas digesters produce biogas. But how? What are the feeding and operating parameters that determine the design and how well it works? 

Biogas systems use bacteria to break down wet organic matter like animal dung, human sewage or food waste. This produces biogas, which is a mixture of methane and carbon dioxide, and also a semi-solid residue. The biogas is used as a fuel for cooking, heating  , lighting or generating electricity.

There is  a lot of information about how a biogas digester works, and the information can be overwhelming and sometimes seems contradictory.

We link a few choice pieces of information that we believe to be quite important.

A simple biogas plant has a container to hold the decomposing organic matter and water (slurry), and another to collect the biogas. There must also be systems to feed in the organic matter (the feedstock), to take the gas to where it will be used, and to remove the residue.

In fixed dome biogas plants (the most common type), the slurry container and gas container are combined, so that the gas collects under a rigid dome over the slurry. As the slurry breaks down, the biogas which is produced pushes some of the slurry into a separate reservoir. When the biogas is taken off, the slurry flows back.

A biogas plant needs some methane-producing bacteria to get it started. Once the plant is producing biogas, the bacteria reproduce and keep the process going. Cattle dung contains suitable bacteria, and a small amount of cattle dung is often used as the ‘starter’ for a biogas plant, even when it is not the main feedstock.

But it's not limited to animal manures.  As an example, the SmartTop digester at my home, although started with cow manure, has been fed only grey (used) water and kitchen and garden scraps for a year.  Other feedstocks are sewage, algae, glycerine, grasses - pretty much everything that's organic, biodegradable and with low cellulose content. That means, wood is no good!

Take a look at our recent user survey, and note in the download available on that page the range of feedstocks that people are using to produce their clean biogas energy.

We will post additional information on the nutrients flowing from a biogas digester.

The link to the file for download below, is a great primer for understanding the key criteria and how they relate to a digester's functioning.  But, it is important to note that digester technology is very varied, and not everything stated is necessarily accurate for all digesters ... which takes me back to the opening comment on this posting.

Greg Austin