Common separation methods such as pressurized water scrubbing, pressure swing adsorption, and amine scrubbing have considerable disadvantages: They need comparatively large amounts of energy as well as auxiliary materials and chemicals. Wastes and wastewater are generated that must be treated and disposed of. Further, the biogas after upgrading is usually at low pressure. Before it is fed into a medium-pressure grid, it needs to be compressed to 15-20 bar by, for example, an additional compressor. Conventional upgrading plants are therefore usually cost effective only for raw biogas quantities significantly in excess of 500 standard cubic meters per hour (Nm³/h). This usually makes them unsuitable for decentralized energy supply with a large number of relatively small plants.

In addition to those biogas upgrading technologies, membrane-based CO₂ removal from biogas stream has become increasingly established in recent years. Thanks to highly efficient and highly selective membranes, it is possible to separate CO₂ almost completely from methane in a compact 3-stage process. In some European countries such as France, Switzerland, Italy or Great Britain, membrane processes for biogas upgrading already have a market share of >60%. The advantage of membrane upgrading plants over conventional technologies lies, among other things, in the simplicity of the process and the low maintenance requirements. Of course, the spread of biogas upgrading plants is also influenced by local conditions and regulations as well as national incentives.

In the biogas upgrading process, the raw gas is compressed and fed into the interior of the hollow fibre membrane (feed side), the CO₂ is mainly directed to the outside (permeate side) due to the properties of the membrane, the methane collects at the end of the membrane (retentate side) in a highly concentrated form. By means of multi-stage interconnection it is possible to achieve a separation of the gas mixture of nearly 100%, usually the methane is available with a purity of >95% and CO₂ with a purity of >99%. Depending on the application, higher purities are also possible.

A small part of the biogas upgrading plants with SEPURAN® membranes realized overseas in the last years were equipped with a 2-stage membrane process. The desired product quality can be adjusted as desired, but due to the accompanying methane loss, this interconnection is not sufficient for European standards. On the European continent, the 3-stage treatment process has been used throughout due to the highest efficiency in the use of the SEPURAN® Green Membrane. Here, stage 1 and stage 2 are connected in series, but stage 3 is connected in parallel to stage 1. With this configuration, it was possible for the first time to obtain a very pure product with the highest methane yield using only one compressor.