The entrance leads to different kind of rooms including the sewer storeroom, septic tank and cesspool junction. The sign "SEPTIC POOL" can be seen on the wall not far shutter toward cesspool junction
Resident Evil 2
Upon exiting the septic tank, Leon will met Ada once again. With the help of Leon, she will enter the cesspool junction through a small ventilation shaft. She will throw a couple of items she can find while exploring Sewage Disposal Plant including Shotgun Shell and the Club Key. Unable to climb back onto ventilation shaft, she will try to find another way.