The present invention provides a method of installing an environmental control system so as to allow for separate sizing of treatment and bypass capacity while also offering the ability to make or change either treatment or bypass capacities at different times. This is accomplished by containing the treatment and bypass functions in separate chambers, using screen, baffle, or coalescing media pack to further refine effectiveness and capacity of each structure independently. The control structure and interceptor structure may be pre-engineered to a variety of sizes, capacities, or other specifications. This allows simple selection of a specific control structure and a specific interceptor structure from a variety of combinations, eliminating the need for custom engineering for each installation.