Performance of a bench-scale membrane pilot plant for the upgrading of biogas in a wastewater treatment plant

A bench-scale membrane pilot plant for upgrading biogas generated at a municipal wastewater treatment plant was constructed and operated for extended periods of time. The raw biogas was available at 45–60 psia (3.1–4.1 bar) and contained 62.6 mol% CH₄, the balance being mainly CO₂ and a large number of organic impurities. The operation of the pilot plant was tested with two identical hollow-fiber modules for periods of over 1000 h (41 days) with each module. One of the hollow-fiber modules was tested at an average pressure of about 525 psia (36 bar) and at stage-cuts of 0.34–0.41, and the other module at about 423 psia (29 bar) and at stage-cuts of 0.36–0.39. The flow rates of the biogas feed were 30–36 ft³/h (2.4×10⁻⁴–2.8×10⁻⁴ m³/s) and 21–24 ft³/h (1.7×10⁻⁴–1.9×10⁻⁴ m³/s), respectively. The CH₄ concentration in the retentate stream (the upgraded biogas) was raised in these tests to 92–95 mol% CH₄. The performance of the pilot plant was stable over the entire test periods. An even higher CH₄ concentration of 97 mol% was reached in short-term tests at a stage-cut of 0.46. The raw biogas had to be pretreated to prevent the condensation of organic impurities which tended to dissolve the hollow fibers. Upgraded biogas containing over 90 mol% CH₄ produced in a large-scale membrane separation plant could be used for the generation of electricity. At the same time, the permeate (waste) stream would contain over 15 mol% CH₄ and could be used for heating applications.