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Rules of discharging various categories noxious liquid substances at sea

Anti-pollution measures for seagoing chemical tankers

Operation of ships transporting noxious liquid chemicals in bulk , which when accidentally released into the sea from tank cleaning or deballasting operations may pose a serious threat to marine enviroment and therefore justify the application of special anti pollution measures that need to be considered.

The "International Convention for the Prevention of Pollution from Ships, 1973 " rules that a Cargo Record Book shall be kept, In certain cases, as detailed below, official National surveyors are required to make entries regarding tank washing etc. for certain cargoes. For each vessel shall be drawn up a "Procedures and Arrangements” book.

The convention divides noxious substances into three categories

Category A

Substances which if released into the sea from tank cleaning or deballasting operation would present a major hazard to either marine resources or human health or cause serious harm to amenities or other legitimate uses of the sea and therefore justify the application of special measures to prevent their escape into the marine environment.

Substances of Category A are bioaccumulated and liable to produce a hazard to aquatic life or human health; or are highly toxic to aquatic life.

Examples of Category A substances: acetone cyanohydrine, acrolein, carbon disulphide, creosote, cresols, dichlorbenzene, sodium pentachlorophenate, tetramethyl lead.


Category B

Substances of Category B are bio-accumulated with a short retention of the order of one week or less; or are liable to produce tainting of sea food; or are moderately toxic to aquatic life. Examples of Category B substances: acrylonitrile, allyl alcohol, carbon tetrachloride, chlorobenzene, ethylene dibromide, phenol, trichlor ethylene.


Category C

Substances which when released into the sea from tank washing and deballasting operations may present a minor hazard to either marine resources or human health or cause minor harm to amenities or other uses of the sea and therefore require special operational conditions.

Substances of Category C are slightly toxic to aquatic life and include additicrnally certain substances which are practically non-toxic to aquatic life.

Examples of Category C substances: acetaldehyde, acetic acid, allyl chloride, amyl acetate, benzene, chlorosulphonic acid, cumene, cyclo hexane, ethylbenzene, ethylene diamine, nonyl phenol, octanol, sodium hydroxide (caustic soda), styrene monomer, sulphuric acid, vinyl acetate, xylenes.





Category A discharges

: The convention rules that discharge into the sea of substances of Category A shall be prohibited. Tank washings are to be pumped to a receiving facility until the washings have a concentration of less than 0, 1 % in the Category A examples above (0, 01 % for carbon disulphide) and the tank is then empty . Thereafter the tank may be flushed to min 5 % of tank capacity and the water may then be discharged into the sea if all of the following conditions are satisfied (Category A):

a) Ship's speed greater than 7 knots
b) "Adequate dilution" in ship's wake to be proven to satisfaction to the Administration (reliable calculations or references to experiments)
c) Discharge below the waterline, taking into account the location of sea water intakes
d) Discharge not less than 12 NM from the nearest land at a depth of water not less than 25 metres.


Category B discharges

: Substances of Category B, contaminated ballast water or tank washings containing these substances shall be prohibited to be discharged into the sea. However, such mixtures may be discharged when the following conditions are fullfilled (Category B) :

a) Ship's speed greater than 7 knots
b) Concentration in ship's wake shall be proven by reliable calculations not to exceed 1 part per million
c) The maximum quantity of cargo discharged into the sea from each tank and ass. piping does not exceed 1 m 3 or 1/3000 of tank capacity, whichever is the greater.
d) As c) above
e) As d) above


Category C discharges

: Substances of Category C, contaminated ballast water or tank washings containing these substances are prohibited to discharge into the sea. However, such mixtures may be discharged when the following conditions are all fullfilled (Category C) : a) Ship's speed greater than 7 knots
b) Concentration in ships wake shall be proven by reliable calculations not to exceed 10 parts per million
c) The maximum quantity of cargo discharged from each tank and ass. piping not to exceed 3 m *3 or 1/10000 of tank capacity, whichever is the greater
d) As c) for Category A
e) As d) for Category A
The Convention further describes "Measures of Control" saying that each Government shall appoint or authorize surveyors whose duties shall include surveillance according to this Convention. Other measures of control:


Category A substances: The Master shall make entries regarding tank unloading and all cargo operations in the Cargo Record Book including concentrations of cargo in tank washings to be discharged; the latter also to be certified by the above surveyor. As further stated in the Convention the surveyor may satisfy himself that for Category A substances alternative procedures for pre-cleaning and direct calculation of maximum cargo remaining on board may be acceptable, provided no more pollution occurs other than ruled for Category A above. All such deviations from the strict Convention rulings to be entered in the Cargo Record Book together with all relevant calculations etc.

Category B and C substances: The Master shall enter in the Cargo Record Book when a tank is unloaded, piping systems drained and when he has ascertained that the amount of cargo left on board after unloading does not exceed the amounts allowed and necessary dilutions achieved etc (see above). Such observations and entries may be checked by the surveyor. The Master shall further record internal transfer of cargo, discharge of washings according to regulations as well as disposition of tank washings if cleaning is carried out in port, Residues from slop tanks and pump room bilges shall be treated equivalently with cargo from the cargo tanks.


The Cargo Record Book shall be completed on each occasion and for each tank whenever any of the following operations take place:

i) loading of cargo
ii) transfer of cargo during the voyage
iii) discharge of cargo
iv) transfer of cargo, cargo residues, etc to a slop tank
v) cleaning of cargo tanks
vi) discharge of slop tanks
vii) ballasting of cargo tanks
viii) discharge of ballast water discharge or escape of cargo or cargo mixtures

The Cargo Record Book shall be signed on each page by a responsible officer and the Master. The book shall be kept readily available on board.
A chemical tanker shall be surveyed by the relevant Administration with regard to this Convention and after compliance obtain a ''Certificate of Fitness for the Carriage of Noxious Chemicals in Bulk". The Certificate shall be endorsed with details of approved washing procedures and approved discharge arrangements for tank washings.

It should be borne in mind that individual ship has got own characteristics and limitations may involved handling various chemical cargoes . The master and all personnel in all cases must be aware of cargo/ship information that has been given and comply with relevant safety procedures.



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Disposal of tank washings, slops and dirty ballast - safe method



Following reference publications provide useful guidance and international regulations for carrying hazardous chemicals at sea.



Our detail pages contain somewhat larger lists of resources where you may find more useful information.

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