The Eastcroft incinerator has breached pollution limits three times in a week.
Emissions limits at the incinerator, off London Road in the city, were broken on December 28 and 30, and January 3, all for carbon-monoxide.
The breaches occurred when the plant's boiler was restarted, after a failure in a boiler tube caused the incinerator fire to go out.
The Environment Agency said it had given Eastcroft's owners, waste firm WRG, a written site warning over the breaches.
The agency has just granted Eastcroft a new operator's permit.
Campaigners opposing a £50m proposal to expand the incinerator to burn more rubbish say the breaches are worrying.
But
WRG said that, during the whole of 2005, the plant only breached emission levels for six hours out of a total 11,514 it was operating