Aberdeen City, Aberdeenshire, and Moray Councils signed the final deal, which will see Acciona deliver the turnkey contract for the incinerator project.
The Ness Energy Project will be built at a former gas storage site on the East Tullos Industrial Estate in the south of the city.
Once commissioned in 2022, it will be capable of processing 150,000 tonnes of municipal solid waste every year to generate electricity.
The operation and maintenance of the facility will be in the hands of waste management specialist Indaver for a 20 year contract term.