Honolulu plans to use a deep-diving vessel to inspect a pipe that sends treated sewage into the ocean.

An engineering firm hired by the city will inspect the pipe belonging to the Sand Island Wastewater Treatment plant on Thursday.

They will use a deep-diving manned submersible belonging to the Hawaii Undersea Research Laboratory for the project. The engineers will assess the structural condition of the pipe’s flap gate.

The pipe transports primary treated effluent from the plant to a spot 240 feet underwater more than 2 miles offshore. The treated wastewater blends back into the environment.

The Sand Island plant serves urban Honolulu from Kuliouou to Salt Lake, including Waikiki and downtown Honolulu.