"In yet another example of public finance going to support
incineration, it is clear that the current lower rate of taxation for
incinerator bottom ash (IBA) represents a significant and unjustified
subsidy to incineration with no environmental benefits.
"There is increasing evidence that a significant proportion of
incinerator bottom ash should now be regulated as hazardous
waste in any case – largely due to the high levels of lead and zinc
compounds the ashes contain."
They actually recommend raising the landfill tax on IBA to at least the same as normal waste. By the time the Devonport incinerator comes on-line, that rate would be close to £100/tonne.
To dispose of 60,000 tonnes of IBA, MVV would have to pay over £5,000,000 in tax alone. They clearly have a big incentive to claim it is inert.