HMIP moves to control TBT discharges from shipyards

Shipyards using tributyl tin (TBT) anti-fouling paints will be expected greatly to improve materials handling and install effluent treatment plant in order to minimise damaging discharges to the environment, under draft guidance issued by HM Inspectorate of Pollution (HMIP).1

The use of TBT paints on boats less than 25 metres in length was banned in the UK in 1987 after the chemical was found to be damaging shellfish populations. The UK is now leading calls for further curbs on TBT (ENDS Report 230, p 42 ).

Shipyards which apply or remove TBT or triphenyl tin (TPT) coatings on larger vessels are now the main source of organotins in coastal waters, and will come under integrated pollution control (IPC) by 31 January 1996. HMIP expects the coating processes to be upgraded to meet "achievable" levels of releases to the environment by 31 January 2000 - although it is showing increasing flexibility in the rate and degree of improvement expected under IPC (see above).

Some shipyards have a long way to go to meet the expected standards. In some instances, vessels are simply beached on the foreshore, stripped and repainted. In contrast, many Ministry of Defence docks operate to much more stringent standards. HMIP says that "it would be unlikely" to authorise a process unless it takes place in a dry dock.

HMIP expects some basic dockyard management practices to be implemented within 12 months of authorisation, including measures to contain washwaters, protect surfaces from paint overspray and avoid wind-blown particulate matter. A key measure is the prevention of particulate flushing from dry docks when vessels are refloated, as dispersion of such particles is a major source of TBT pollution.

In the longer term, HMIP expects existing shipyards to move towards a limit of 200ng/l of TBT or TPT in the final effluent. Where releases are intermittent, a higher concentration may be justified - but only following a mathematical study of the long-term environmental effect. However, HMIP will not necessarily require routine analysis for TBT in the discharge or receiving waters, and may accept "surrogate parameters".

The discharge limit compares with statutory environmental quality standards (EQSs) for TBT and TPT of 20ng/l for fresh water, and 2ng/l and 8ng/l, respectively, for salt water. The TBT standard is routinely exceeded in and around major ports. However, HMIP is barred from issuing an authorisation which will cause an EQS to be breached - raising questions over the expected rate of dilution of a discharge even at 200ng/l.

HMIP says that process water should be recycled as far as practicable. Effluent treatment plant that can meet the 200ng/l limit is now commercially available. An alternative may be to collect the effluent and send it to an external contractor for treatment - though "direct disposal of effluent to licensed landfill should not be encouraged".

At present, shipyards have little option but to continue using TBT, as the paint is usually chosen and supplied by the ship owner. The picture may change if efforts to negotiate tougher international controls, or even a ban, are successful.

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