Carbon dioxide is more soluble in water than methane. This phenomenon is employed to remove CO2 from biogas in water scrubbing technologies. Biogas is fed to a column where it is "washed" with counter-current water that is sprayed from the top of the column. The column is normally filled with some material to enhance the interface area promoting CO2 absorption. The CO2 is dissolved in the water that is then pumped to a "regeneration column" where CO2 is released. The regeneration of the water scrubbing process can be carried out at higher temperatures or at lower pressures. In this technology, H2S is removed with CO2. Also the purified CH4 stream (with purity up to 98%) should be dried after leaving the scrubber.

The solubility of CO2 in water strongly increases at lower temperatures. In order to reduce pumping energy, the water should be available at low temperatures. In fact, this technique is been employed in several countries with cold weather (Sweden, Switzerland, Germany, Austria, etc). Cooling down water may still be efficient for large facilities, but not for small applications.

Nowadays, water scrubbing is the most employed technique for upgrading biogas. Plants with processing capabilities from 80 to 10000 m3/hour are in operation. Main technology developers in this area are: Malmberg (www. malmberg. se), Flotech Inc. (www. flotech. com), Rosroca (www. rosroca. de), DMT (www. dirkse-milieutechniek. com), etc. In some of these webpages there is a video which actually explain graphically in detail how the process works.