Abstract: A beltpress controller performs on-line measurements of output sludge cake thickness. A controller takes a product of the measured cake thickness with belt speed, moisture content and belt width to determine a rate of dry solid output per unit time. The calculated rate is compared with a desired rate and the sludge flow rate is adjusted by adjusting the speed of a sludge feed pump to obtain the desired dry solid output rate. In systems which mix polymer with the sludge to increase the dewatering efficiency, the sizing of a mixing orifice through which the sludge and polymer is provided is adjusted to maintain a substantially constant mixing energy. This prevents errors in the streaming current detector which measures the amount of excess polymer in the filtrate and the controller adjusts the speed of a polymer dosing pump accordingly.