How does a biogas digester work?

In the last article it was described what installations could be found at a biogas plant. As stated, the essential part of a biogas plant is the digester. In this article we will further explain how a biogas digester is build, and how it works.

In essence, a biogas digester works by creating an environment in which biomass can be fermented without the presence of oxygen. This is almost always done by entering biomass, like organic waste, manure, or sewage wastewater into large tanks. These tanks are air tight and the slurry is mixed. This means anaerobic fermentation will begin to take place and biogas is created. These so called biogas digesters capture these gases which can be used for different purposes. To capture the biogas, special roofs are needed. These roofs are often made of PVC or other types of foil. Some gastight roofs are made with a single membrane, or they can be made with a double membrane. Membrane Systems Europe specializes in these double membrane roofs and all the elements that such a roofing system needs.

Double membrane roofs

At MSE we produce and mount double membrane roofs. These consist of an inner and an outer membrane. The outer membrane Is always kept under pressure via air blowers. This means it is always blown up like a bulb. This outer membrane protects the inner membrane which expands following the production of the biogas.  If there is no gas production, or gas is being withdrawn from the digester, the inner membrane is lying flat on a support construction made of straps. Without an expanded outer membrane The foil is vulnerable to rain or snow, and might not expand when gas is produced due to the high pressure of the rain or snow. This is why most biogas digesters nowadays have a double membrane roof.

Mixers

A digester with a double membrane roof has multiple parts which it needs to function properly.  First of a digester needs mixers, in order to keep slurry flowing and keep it from stalling, which would hinder the production of biogas. These mixers can enter via the concrete walls of a digester or via the roof, through a so called service platform. A benefit of these service platforms is that when a mixer breaks down, it can be extracted from the digester without having to shut down the total production and open up the entire roof.

Air systems

As stated, to keep the outer membrane on a constant pressure level, air needs to be blown in, and of course, also let out.  For this, blowers, air inlets, and air outlets are added to the roofs. Depending on the amount of gas that the customer wants to withdraw at once, it can be calculated how strong the blower needs to be and how much air needs to be let out, not to disrupt the gas withdrawal, while keeping the outer membrane standing upright.

Over-pressure / under-pressure relieve valve

A digester also needs an over-pressure / under-pressure relieve valve (OUPV). This is necessary so the roof does not damage when there is too much gas production (the inner membrane might rupture), or when the gas withdrawal is too much (the roof will be sucked in a ‘vacuum’ between the straps of the support construction). The OUPV is always mounted to the wall of the digester, not the roof.

Measurement device

Also present in most digesters is a measurement device to assert the amount of gas in the digester. Here at MSE we use 3 different methods for measuring the gas level. These will be further explained in the following article.

The elements described in this article are some of the essential parts of a biogas digester. There can be many more elements such as a looking window in the outer membrane, or a snow rope atop of the outer membrane to remove excess snow. The parts mentioned above however are the most common. At Membrane Systems Europe B.V. we provide all the necessary parts which roofs of biogas digesters need. We are proud to help cover a greener future!