Process for controlling solids-liquid separation

A method of separating liquid from a solid-liquid mixture comprising:

• continuously separating liquid from a bed of said solid-liquid mixture moving through a liquid separating zone of a liquid separating device, said mixture having an upper surface,
• projecting light at a first angle with respect to said upper surface from a light source in a photodetector assembly onto a first illumination area on the upper surface of said mixture within said liquid separating zone,
• detecting light reflected from said first illumination area at a second angle with respect to said upper surface by a first photodetector,
• detecting light reflected from said first illumination area at a third angle with respect to said upper surface by a second photodetector, wherein said first and second photodetectors are positioned above said separation zone in said photodetector assembly,
• generating output signals from each of said first and second photodetectors corresponding to said detected light at said second and third angles, respectively,
• transmitting said output signals to a main control means and generating a control signal in response to said output signals, and transmitting said control signal to said liquid separating device and adjusting the rate of separation of liquid from said mixture.