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Mercury falls, thanks to Council controls

18 May 2005

Mercury falls, thanks to Council controls

The amount of potentially toxic mercury being washed down the drain from dental surgeries has been cut by two-thirds, thanks to tighter controls on its disposal.

In 2002, with the co-operation of the Wellington dental profession, all dental practices were required under the Trade Waste Bylaw to install, by July 2004, a mercury amalgam retention device in waste systems to capture over 95 percent of the mercury amalgam.

The main source of mercury in Wellington comes from dental practices as drillings or waste amalgam from tooth-repair work. In 1999, Wellington City Council began to monitor the amount of mercury in the waste system coming from dental practices.

Monitoring shows there has been a steady fall in the levels of mercury measured in sludge waste. The most recent checks, involving samples of sludge taken in January, February and March this year, show mercury levels are one-third of those originally measured in 1999.

“It is very gratifying that within a year of the last dental practice installing a mercury retention system we can see a significant result,” says Council Trade Waste Officer Ron Humphreys.

The mercury is now collected from surgery waste systems and recycled.

“We expect to see further reduction in mercury levels, but the project is a long-term one since it takes time for all mercury amalgam to be swept out of the sewerage system. It’s going to be hanging around in plumbing systems, in nooks and crannies,” Mr Humphreys says.

Environment portfolio spokesperson Cr Celia Wade-Brown says Wellington dentists have been great in reducing mercury contamination. “Now we need to reduce other pollution of stormwater and sewage.”

She urges Wellingtonians who care about the water quality of our streams and coast to have their say on the Council's Draft Liquid Waste Management Plan. Submissions close at 5pm on Friday (20 May).

Most mercury used is in a safe, inorganic form – in amalgam tooth fillings, where mercury is combined with silver, tin, copper and zinc. Over 80 percent of this is removed from the waste stream in the sludge produced at the city’s treatment plant at Moa Point. But mercury can act a cumulative poison when it is converted into a water-soluble form that can enter the food chain. In the sludge there is potential for micro-organisms to convert inorganic mercury into methyl mercury that readily dissolves in water. This is taken in by fish and from there into the food chain.

Sludge, mixed with green-waste from landfills, is turned into compost by Living Earth Limited. Levels of mercury in the compost have always met the resource consent limits set by Greater Wellington Regional Council, now they are well within these limits, Mr Humphreys says.

Living Earth Director Rob Fenwick says the drop in mercury is a great result. He says the new protocol for dentists was developed as a direct result of the Council deciding to compost its biosolids. He says there is now about two parts per million of mercury in the trade waste stream.


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