What kind of sewer usually has the largest diameter sewer in the system?

• A. Interceptor sewer
• B. Branch sewer
• C. Main sewer
• D. Sub-main

Answer: (A)

The biggest-diameter sewer is the interceptor because it’s the main conduit that collects flow from a large area and carries it toward the treatment plant. It gathers all the smaller branches, branches into sub-mains, and main sewers, so it must be built large enough to handle the cumulative peak flow from the entire catchment. Branch sewers and sub-mains serve smaller areas and feed into larger lines, so they’re smaller. Main sewers are sizable too, but the interceptor is the downstream conduit designed to carry the combined flow from the whole system to treatment, requiring the largest diameter.