Solids control is a technique used in a drilling rig to separate the solids in the drilling fluids that are crushed by the drill bits and carried out of the well surface.
Normally, a solid control system contains five stages: the mud tank, shale shaker, vacuum degasser, desander, desilter, and centrifuge. The shale shaker is used to separate big solids with diameter above 75μm, the desander addresses solids from 45-74μm, and the desilter segregates solids between 15-44μm. Sometimes the desander and desilter are combined as one high efficiency mud cleaner. When air enters the drilling fluids, a vacuum degasser is used to separate the air. When there is no air in the mud, the degasser works as a big agitator. All these stages are mounted on the top of the mud tank. After separating the solids, the clean mud can be pumped into the borehole again.
Wood pellets are being used in place of fly ash as a less invasive solids control. Pellets are a green wood biomass. Pellet can be made of materials that would be otherwise land-filled.