Anaerobic digestion biogas upgrading using a two-stage membrane system under pilot-scale conditions

In the present work, the construction, and operation of a pilot-scale biogas upgrading system is presented, employing 2 commercial polyimide (PI) membranes. The Upgrading system treats biogas produced via anaerobic digestion of the sludge, produced from the treatment of municipal wastewater in the facilities of Thessaloniki's Wastewater Treatment Plant. The goal of the separation unit is the production of high purity biomethane (>95%) for potential reuse in terms of energy. The fabrication of the pilot scale system includes the scale up of a laboratory setup separating CO₂ from binary CH₄-CO₂ gas mixture. After the stability tests of the process, for the operation of 5 months (February to June 2023) the purity and recovery of CH₄ in the final gas product. The experimental results showed an average recovery of CH₄ of 95.7% for an average 55% feed composition, whereas the average purity in the final product was equal to 82.4%. The purity results were lower because of the N₂ presence in the product stream (average 17.5%). After normalization with the help of the lab-scale binary results, the expected results assuming N₂ absence would be 99.8% CH₄ purity and 67% CH₄ recovery. Finally, 3 different membrane configurations are compared in terms of their energy production, concluding to the efficiency of 2-stage configuration with recycling stream for the optimal combination of theoretical stage cut fractions.