Biogas upgrading in municipal wastewater treatment plants

Renewables play a key role in clean energy transitions and the deployment of renewable power is one of the main enablers of keeping the rise in average global temperatures below 1.5°C. According to the International Energy Agency, the share of renewables in global electricity generation reached 28.7% in 2021. Still, renewable electricity needs to expand faster to reach the milestones in the Net Zero Emissions by 2050 Scenario. 

Beside wind, water and sun, biogas is becoming increasingly important in today's renewable energy mix - quite rightly, because it is a highly efficient energy source and an important component of decentralized supply structures.

Anaerobic digestion is a proven technology for sewage sludge treatment and which allows generation of renewable energy from the same process. In a paper published in 2021, the European Biogas Association EBA shows high potential of biogas production from industrial wastewater. When producing renewable gas from industrial wastewater, GHG emissions are saved in different ways. First, due to the reduced energy consumption in wastewater treatment installations. Second, by the replacement of fossil energy sources. Last, by bringing the wastewater in a closed and controlled environment, preventing methane emissions to be released into the atmosphere.

During anaerobic digestation, microorganisms break down the organic matter contained in the sludge and convert it into biogas. The benefits of anaerobic digestation of sewage sludge are widely recognised and the technology is well established in many countries. Sewage sludge is produced in wastewater treatment plants as part of the water cleaning process. The sludge contains the particles removed from the wastewater, which are rich in nutrients and organic matter, leaving the water clean for its release into nature. Growing population centres and expanding industry, which are increasingly well served by wastewater treatment facilities, result in rapid growth of sewage sludge production. As important consumers and generators of energy, municipal wastewater treatment plant operators are one of the numerous players influencing developments towards sustainable energy supply.

As previously mentioned, biogas is produced by the fermentation of biomass - an organic substance, for example, from wastewater, liquid manure and sewage sludge, or from renewable raw materials. However, in addition to the energy carrier biomethane, raw biogas also contains carbon dioxide as well as other trace gases.

Evonik's gas separation membrane SEPURAN® Green makes it possible to purify the biogas that accumulates during wastewater treatment processes into high-purity biomethane and return it to the circular economy. The CO₂-neutral gas can be fed directly into the municipal natural gas grid or used as a green fuel Bio-CNG (Compressed Natural Gas) to power municipal vehicles.

Evonik's SEPURAN® Green membrane has been especially developed for efficient upgrading of biogas into high purity biomethane. The market entry of Evonik into the Biogas upgrading industry in 2011 revolutionized the market and set new technology standards. The introduction of a highly selective SEPURAN® Green membrane in combination with a patent protected 3-membrane-stage process allowed for high methane yields and high product purities with an optimized energy consumption.

Various technologies are available for separating CO₂ from the raw biogas and upgrading it to biomethane. In addition to wellknown technologies such as pressurized water scrubbing, amine scrubbing or pressure swing absorption, membrane technology 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.

Polyimide-based membranes are the most widely used membrane treatment systems. 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.