Abstract: The present invention is directed to a method and apparatus for maintaining steady state conditions in a separator. Steady state is maintained by controlling mass flow rates of influent to, and underflow from the separator. Specifically, an influent specific gravity and flow rate signals are monitored by an influent mass flow controller. That controller responds to variation in these signals to maintain a constant mass feed rate into the separator. A mass flow signal, comprising a composite of influent specific gravity and influent mass flow signals, is produced. That signal is fed to a thickener controller which also monitors mass flow of the underflow. The thickener controller adjusts the mass flow rate of the underflow in response to the influent mass flow signal. However, fluctuation of an interface level between clear and unclear areas of the thickener can effect flow rate adjustment.