Fixtures with flood-level rims located below the elevation of the next upstream manhole cover must be protected from sewer backflow by…

Giải thích

Fixtures whose flood-level rims are below the next upstream manhole cover can flood when the public sewer surcharges, so they must discharge through an approved backwater valve. Fixtures above that elevation must not drain through the backwater valve, so they still drain by gravity if the valve closes. This protects low fixtures without impeding normal flow from higher ones.