Anaerobic digesters make a lot of sense, and public policy should be encouraging them. They take waste products and manure and convert them to renewable energy.
Some of the energy is being used by the farm, undoubtedly, but this is energy that would be taken from the grid otherwise, so it is still helping. Not all whey is equal. Some can be used as a protein source, but not all whey is suitable for that purpose.
Digesters also can take manure, and pull the methane from it (a greenhouse gas), and convert it to energy, the substrate can then be used as bedding or land applied as fertilizer. It also reduces odor.
Digesters make a lot of sense in many ways and help solve multiple problems.