Slop Tank
A Slop Tank is a cargo tank set aside on a tanker to collect the oily mixtures produced by tank cleaning and crude oil washing — the water washings, oily residues and clingage. The mixture is allowed to settle so the oil separates and rises to the top; the water beneath can then be decanted off within the pollution limits, leaving the oil (the slops) retained on board.
Slop tanks are central to the load-on-top and retention procedures required by MARPOL that keep oil out of the sea. The retained slops are either kept for the next cargo, delivered ashore, or dealt with at a reception facility.
The slop tank is what lets a tanker clean its cargo tanks without pumping oil into the sea: by collecting, settling and retaining the oily residues, it turns tank cleaning from a pollution source into a controlled, MARPOL-compliant operation. It is fundamental to clean tanker operations.
What is kept in a slop tank?
The oily water and residues from tank cleaning and crude oil washing. After the oil settles out, the separated water may be discharged within limits and the oil (slops) is retained on board or landed ashore.