Pressure swing adsorption:
Pressure swing adsorption (PSA) is a discontinuous method. It uses the different adsorption behavior of carbon dioxide and methane on an adsorbent such as activated carbon or carbon molecular sieves. The gas mixture is separated under increased pressure (usually 8 to 10 bar), as this increases the load capacity of the absorber.
The easily adsorbable CO2 accumulates on the surface, while the methane flows through virtually unimpeded and exits at the head of the adsorber. When an adsorber is completely loaded with CO2, it must be regenerated. This can be carried out by either lowering the pressure or raising the temperature.
 At least two adsorbers are required for this reason. This ensures a
quasi-continuous supply of the product gas. However, several adsorbers
are generally connected in series to increase the purity of the product
gas.
Nevertheless, the methane loss for the plants already in operation is
approx. 2 -5 %.
Pressure water scrubbing:
The continuously operating process of pressure water scrubbing is based on the different solubilities of gases in water. Adsorption is dependent on pressure (usually 6-8 bar), temperature and the ratio of biogas to scrubbing water.
 The biogas is routed through the absorber column in the opposite
direction to that of the trickling water. The methane exits the column
at the head, while the scrubbing water laden with carbon dioxide
collects at the bottom of the column and is returned from there to a
regeneration unit. There it is relaxed, which reduces the solubility of
the carbon dioxide, and it bubbles up out of the scrubbing water.
Depending on the manufacturer, a stripping column can also be connected
downstream to remove CO2 residues from the water before it is then
reused. The methane losses with this method are approximately 2-4 %.
Selexol scrubbing:
Selexol scrubbing is a modified form of pressure water scrubbing. The difference lies in the fact that Selexol replaces water as a scrubbing solution. Methane dissolves in Selexol somewhat better than in water. However, the hygroscopic properties of Selexol make the process considerably more difficult. The methane loss for this method is also 1 – 4 %.
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Gas treatment 
