A system for controlling drilling mud density in drilling operations that combines a base fluid of lesser/greater density than the drilling fluid required at the drill bit to drill the borehole to produce a combination return mud in a riser. Concentric tubular members are disclosed, wherein one tubular member, such as the drill string, is utilized to inject the drilling fluid into the wellbore. Another tubular member carries the combination fluid to the surface for separation by a centrifuge system. By combining appropriate quantities of drilling mud with another fluid, the density of the combination fluid carrying drill cuttings to the surface can be regulated. During separation by the centrifuge, a weir plate having an adjustable height is utilized in the centrifuge to regulate the separation of the base fluid from the drilling fluid, producing a high density fluid and a low density fluid. The adjustable centrifuge permits the densities of the two fluids to be readily altered as desired for well drilling operations.