1. . 
A mud and solids-control system in which the only discarded waste is moist, drilled-up rock materials. Such systems are used for drilling wells in environmentally sensitive areas. No reserve-mud pit is used in a truly closed mud system. Mud is continually processed primarily by mechanical means, such as screening, hydrocycloning and centrifuging to remove solids initially. A second stage to remove colloidal solids is by wastewater cleanup techniques.