More than 72,000 merchant ships enrolled on the Lloyd’s Register,
register of maritime navigation, and several tens of thousands of other
vessels constantly plough through the seas. They transport varying quantities
of oil and fuel for their own use, and up to several thousand tonnes for
the largest passenger boats and container ships.
All these vessels are equipped with oil residue recovery systems, generated by filtering heavy fuel oils before using them as propulsion fuel. They also have waste water drain systems and separators onboard. The residues from settling and filtering accumulated during journeys need to be disposed of for treatment in ports. However, the lack of facilities in some places, the time spent and the cost generated by these operations in a deballasting station in port can mean that vessels dispose of all or part of the contents of their recovery tanks at sea.
This discharge constitutes the main source of pollution by non-tankers.
Degassing and deballasting of their fuel tanks and accidents, causing several
tens of litres to several thousand cubic metres to be spilled, can also
add to this pollution, on a more exceptional basis.
According to the US National Academy of Sciences, these spills totalled 400,000 tonnes in 1973, double the amount spilled by oil tanker accidents.
The reinforcement of surveillance and the installation of separators onboard new vessels meant that the quantities fell to 320,000 tonnes in 1981, then 260,000 tonnes in 1989. The situation seems to have deteriorated since, with an estimated 555,000 tonnes in 2000, making non-tankers the number one source of oil discharge by maritime transportation.
These figures should however be handled with care. Due
to a lack of statistics, they are simply estimates for each category of
vessel. The fluctuation from one estimate to another may be more a reflection
of the inaccuracy of the data than of a real change. In reality, the actual
quantities may in fact be greater than these estimates, which only take
into account the merchant ships listed in Lloyd’s Register.
Possible modifications to the Marpol Convention and to regional agreements are currently being investigated, to continue to reduce these volumes by introducing rules for non-tankers similar to those which apply to tankers. Thus their operational discharge must now no longer have a hydrocarbon content of over 15 ppm. It is unfortunately not always easy to impose restrictive and expensive measures upon thousands of independent shipowners, who may be more concerned by financial and practical implications than environmental protection.