The plant supplies heating to more than 4,000 homes and businesses.
A public inquiry has begun over whether an incinerator in Nottingham should be allowed to expand.
Proposals to add a third furnace at the Eastcroft site have already been rejected once by city councillors.
The public inquiry was set in motion after councillors failed to consider revised proposals within an agreed 16-week time frame.
Protesters have said more waste should be recycled but the owners insist it is a better alternative than landfill.
The incinerator's owners, the Waste Recycling Group (WRG), have said the third incinerator would increase capacity from 150,000 to 250,000 tonnes of waste a year.
The plans also include building an extra gas filtration system and an energy recovery unit at the site close to Notts County's Meadow Lane football ground.
Waste energy
But Jon Beresford, from Nottingham Against Incineration and Landfill, said: "There is no need for the expansion. The amount of waste we are producing is decreasing, the amount we are recycling is increasing.
"One of the problems is if you build another incinerator like this at £50m, WRG will want long-term contracts.
"So straight away there will be 20-year contracts which will deter people from improving their recycling rates."
Robert Asquith, from WRG, said they were planning for the future: "We as a society will need more recycling - and we are developing those sites as are other companies.
"But you are still faced with the problem that a significant proportion of the waste stream is truly residual.
"And for this you have the choice of putting it in a landfill for which you get no benefit or extracting the energy from it in a waste energy plant."
The site supplies heat to more than 4,000 homes and businesses.
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