The deal will see all operations other than its waste to energy business, which was de-merged earlier this year; pass to Fomento de Construcciones y Contratas.
WRG chief executive Jim Meredith said: WRG faces an exciting future as part of our Spanish colleagues ambitious plans to grow their involvement in the UK waste management sector. We are looking forward to the opportunities that this development represents to WRG, our employees and our customers.
The company also announced that its energy business that is responsible principally for the generation of electricity from landfill gas will remain in the ownership of Terra Firma and be called Infinis.
Lord Birt is to become chairman of the new company and is standing down from the same role with WRG, with the board expressing its appreciation for the valuable contribution he has made to the business since joining in January.