| By

The world’s commercial shipping fleet, already struggling with the cost of meeting environmental regulations, may be sitting on an even bigger time bomb. Analysis of hull anti-fouling coatings suggests they could be shedding a hitherto unsuspected one million tons of biocides into the oceans every year.

Anti-fouling coatings are intended to prevent the growth of slime, weeds and crustaceans on ship hulls. By nature they have to be toxic to marine life. However the coatings only last for a few years before becoming depleted through water abrasion, underwater cleaning and intentional ablative wear (termed “self-polishing” by the manufacturers). The biocidal constituents of the anti-fouling coatings are often long-lasting, and have nowhere to go but into the sea-bed where they accumulate in sediment and then marine flora and fauna.

Clean hulls travel more quickly through the water and need less fuel, so anti-fouling coatings are popular with ship operators. The biocides they contain have to be strong if they are to kill the notoriously tenacious marine growths, and operators generally want paint manufacturers to use the strongest biocides the law allows. The volume of biocides entering the oceans as a result has so far escaped broad attention.

The practice of underwater cleaning of ship hulls, in particular, releases point concentrations of an estimated two tons of toxic coating per cleaning. Such cleaning is banned by most ports, which has the effect of concentrating it in areas that are less scrupulous about environmental protection. Gibraltar and Algeciras, Piraeus, Singapore and some Turkish and Middle Eastern ports are known to have a laissez-faire attitude to cleaning operations, but there are also reports of cleaning being allowed in Britain’s Torbay, a popular holiday destination.

Underwater cleaning operations, to be effective, involve aggressive brushing by diver-controlled carts. In practice, they remove not just biofouling, but layers of anti-fouling chemicals and the epoxy paints beneath them. Because the material removed by cleaning is immediately lost to the water, the volume of biocides entering the marine environment is effectively impossible to measure. But information from industry sources at a recent event in Rotterdam gives an indication that the volume is far higher than previously thought.

Maersk Line, the giant of the commercial shipping world, are notoriously reticent about their operational and environmental practices. However, a former executive revealed at the Rotterdam event that it was accepted within the company that a single underwater cleaning operation will remove around one third of the thickness of anti-fouling paint on a ship hull. The cost of frequent repainting, he said, was considered to be a small price to pay for the fuel savings resulting from cleaner hulls. The company’s ships have frequently been observed undergoing underwater cleaning in the bay of Algeciras/Gibraltar and elsewhere.

But given that an average sized cargo vessel might be carrying as much as 10 tons of anti-fouling paint, the scale of biocidal pollution into the oceans becomes alarming. On these figures, around 3 tons of anti-fouling paint would be removed at each cleaning. Depending on their trading patterns, ships owned by operators who favour frequent cleaning might undertake the operation between once and four times a year.

Maersk Line’s practices tend to influence decisions by other operators who are trying to remain competitive with the Danish giant. By itself it operates some 750 container ships worldwide and favours anti-fouling paints over more environmentally friendly alternatives. If cleaned once a year, that number of ships might be releasing more than 2,000 tons of biocides into the oceans a year. For the world commercial fleet of 58,000 ships the figure becomes well over one million tons per year. Navies and yachts take the volume even higher.

According to the former Maersk staffer, the company’s anti-fouling policy had been set by the now deceased owner Mærsk Mc-Kinney Møller, who is reputed to have maintained a patriarchal hold over the family business until his death in 2012. The policy, according to the staffer, was to use the strongest anti-fouling possible and reapply it often. Maersk certainly sent shock waves through the industry five years ago when it became known they were paying shipyards to remove coatings considered more environmentally friendly and replace them with older-style biocides.

Heavy metals

Today’s anti-fouling coatings generally rely on copper compounds for their effectiveness, but ‘booster’ biocides are added, some of which are banned or heavily restricted for use on land. The copper content of anti-fouling paint is generally between 50 and 70%. Even so, copper anti-foulings are thought of as a second-best option by operators, since they are less effective and wear out more quickly than the now-banned Tributyl tin coatings. Tin was banned on ships in 2008 following the discovery of its effects on key species of marine flora and fauna.

Copper anti-fouling is often defended by paint manufacturers on the basis that the metal occurs naturally in the marine environment. However the quantities of copper which can accumulate around busy shipping lanes through ablation, and as a result of cleaning operations, is far beyond naturally occurring levels. A 2008 study reported in the China Science Bulletin determined there were serious effects from the copper and zinc (also used in anti-fouling) accumulating on the sea bed:

“Some samples of sea shell animals stuck and multiplied on the bottom (beneath the seawater) coated with antifouling paints were collected at some domestic maritime spaces, and the content of heavy metals was detected through Inductively Coupled Plasma-Mass Spectroscopy. Meanwhile, comparison with sea shell animals was made on market for edible use. It shows that the content of heavy metals in internal organs of these marine animals is very high due to the large amount of copper and zinc contained in the antifouling paints, and this also does severely harm to sea environment and ecology. To study and develop the novel antifouling paints without copper(I) oxide is an imperative task which brooks no delay.”

From ‘A survey analysis of heavy metals bio-accumulation in internal organs of sea shell animals affected by the sustainable pollution of antifouling paints used for ships anchored at some domestic maritime spaces’ WANG JunLian et al

Another recent study reported in Nature has discovered carcinogenic chemicals associated with anti-fouling coatings in marine creatures at depths up to 10,000 metres (see story here).