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 ft(3)/h (2.4x10(-4)-2.8 x 10(-4) m(3)/s) and 21-24 ft(3)/h (1.7 x 10(-4)-1.9 x 10(-4) m(3)/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.