A process control apparatus and system for dewatering a solids ladened slurry of water and waste water embodying one or more of four primary process control techniques, depending upon the particular process application.The four process control techniques are (1) monitoring of the instantaneous solids concentration increase in the dewatering unit, (2) measurement of the rate of change of the solids concentration increase with respect to time, (3) controlled pumping to produce a substantially constant flow rate in the effluent from the dewatering unit until the terminal dewatering pressure is reached, and (4) variation of the amount of conditioning agents added to the influent slurry in response to one or more control signals derived from other components of the dewatering system and apparatus.