History
Even with load on top there is still some oil in the discharged water from the slop tank. Starting in the 1970s, equipment capable of using crude oil itself for washing began to replace the water-based washing, leading to the current technique of crude oil washing. This reduces the remaining deliberate discharge of oil-contaminated water and increases the amount of cargo discharged, providing a further benefit to the cargo owner.
Crude oil washing equipment became mandatory for new tankers of 20,000 tons or more deadweight with the 1978 Protocol to the 1973 MARPOL Convention. Revised specifications for the equipment were introduced in 1999.
Modern tankers also use segregated ballast tanks and these remove the problem of discharge of ballast water.
Read more about this topic: Crude Oil Washing
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