Control of flocculant addition in sedimentation apparatus

Abstract

A process for settling suspended particles in a liquid medium comprises supplying a stream of liquor for clarification to a settlement vessel, taking off an overflow of substantially clear liquor from the top of the settlement vessel, periodically or continuously removing settled particles in the form of a flowable mud from the bottom of the settlement vessel, supplying flocculating agent for admixture with the liquor entering said settlement vessel and controlling the relative proportions of flocculating agent and supplied liquor in such manner as to maintain a substantial layer of cloudy liquor lying between said flowable mud and a supernatant clear liquor layer in said settlement vessel. The upper boundary of the cloudy liquor layer is continuously monitored by a probe device which draws liquor from within the cloudy layer and forwards it to an instrument which compares its turbidity with a known value. The level of the probe is automatically raised or lowered to cancel out any departure from the known value. The supply of flocculating agent may be automatically controlled in response to change of position of the probe.