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Disposal of tank washings ,slops and dirty ballast - chemical tankers guideline

During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges.

Mandatory pre-wash water

Mandatory pre-wash procedures should be conducted strictly in accordance with the ship's P&A Manual, and the resulting contaminated wash water should always be discharged to shore. The intention of MARPOL is that this should happen immediately following the cargo discharge operations, and in the same port. However, occasions do arise when adequate shore reception facilities for the washings are not provided, and the ship must retain the washings on board until arrival at another port. MARPOL addresses this matter, and the P&A Manual will provide guidance on the correct procedures for a particular ship. During such a voyage, the slops and tank washings should be given the same safety and environmental care as the original cargo.

Dirty ballast

Dirty ballast, caused by ballasting into a cargo tank before the tank is cleaned, should be treated as slops, and must be disposed of in accordance with MARPOL and the ship's P&A Manual.

Safety precautions during discharge of cargo slops into the sea

When discharge overboard is permitted, it should only be undertaken when the ship is at sea normally be below the waterline through an underwater discharge outlet on the side of the ship away from essential water inlet valves. In the interests of safety, this procedure should be adopted even when it is not a mandatory requirement.

When any discharges are made above the waterline, care should be taken to avoid cargo vapour or liquid blowing back on board. If such a risk exists, discharge should be made below the waterline: if this is not possible, consideration should be given to altering the ship's course or speed to reduce the risk, and personnel on deck should wear appropriate protective clothing.

Management of slop tanks

Compatibility of various cargo and cleaning chemicals should be considered just as carefully when handling slops as when handling the cargoes themselves. Particular care is needed when washing several tanks which have contained dissimilar cargoes, and compatibility should be taken into account when selecting the destination tank for stripped wash water. The following should be avoided:

Mixing of slops from Annex I (oil) cargoes with slops from Annex 11 (chemical) cargoes.

Mixing of slops from incompatible cargoes.

Mixing of slops from vegetable oils or fats with chemical slops or petroleum oil slops.

If the ship's cargo tanks are used as slop tanks, care should be taken to avoid introducing slops from cargoes which are incompatible with the tank coating. In this regard, some cargoes which are themselves compatible may, when mixed with water, form acids and thus damage a coating, e.g. slops from hydrolytic cargoes in a zinc coated tank.


Desloping of any kind of slops to a shore reception facility is a costly affair. Therefore it is highly recommended that in the event of any slops generation following pre agreement / written communication is made with charterers Operator.

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