Abstract: A method for processing solid materials for disposal in an underground porous formation includes separating an initial slurry mixture into predefined components, and then continually classifying, shearing, recirculating and diluting the separated components until all solid particles in the initial slurry mixture are reduced to a desirable size. The reduced solid particles are then injected, as a slurry, into a porous underground formation. The desirable size is less than a size that has been found to cause problems with present methods of injecting waste products in underground porous formations.