In a sewage treatment plan an ______________ is required to isolate an area inside the plant and a ___________ at the portable meter.

• A. Air gap, reduced-pressure zone backflow preventer
• B. Check valve, backflow preventer
• C. Backflow valve, air gap
• D. Isolation dam, relief valve

Answer: (A)

The key idea is preventing backflow contamination by using devices suited to where the potential risk exists. Inside the plant, a physical air gap provides a simple, reliable separation between the process side and any potable-water connections, so any backflow cannot reach the clean water supply. At the portable meter, a backflow prevention device is needed to guard the public water system from any backflow coming from the plant or its meters; the reduced‑pressure zone type offers robust protection against both backpressure and back-siphonage.

A check valve or backflow valve isn’t a complete solution for isolating an area inside a plant, and an isolation dam or a relief valve isn’t the standard equipment for backflow protection in this setting. The air gap for internal isolation plus a reduced-pressure zone backflow preventer at the meter best meets the goal of preventing contamination.