A membrane permeation system and process accommodates varying acid gas inlet concentrations over time while utilizing only the initially installed equipment and still maintaining the non-permeate gas specification. The system and process provide flexibility to operate efficiently over a wide range of inlet CO.sub.2 concentrations by adjustments to primary permeate, secondary permeate, and recycle gas operations. The glassy polymer membrane devices used in the system and process are selected so removal duty efficiency increases as acid gas concentration increase. Designing the system and process to handle about a 15% increase in acid gas concentrations over initial conditions effectively treats acid gas concentrations well above that 15% increase, thereby eliminating the need for additional equipment or for additional downstream amines and physical solvents.

A method for providing renewable natural gas (RNG) includes removing hydrogen sulfide and/or carbon dioxide from biogas to provide partially purified biogas, which may be stored in a mobile storage tank. The partially purified biogas is transported to a biogas upgrading facility, at least partially by truck, rail, or ship. At the biogas upgrading facility, the partially purified biogas is further purified to provide the RNG, which can be injected into a distribution system (e.g., natural gas grid) and/or provided for use in transportation.