Method and apparatus for controlling sludge dewatering

A sludge dewatering control system for use in a sludge dewatering plant having a coagulant addition means for adding a polyelectrolyte coagulant to sludge and a dewatering means for dewatering the sludge which has been subjected to coagulation reactions by addition of the coagulant, which system comprises:

• a sludge concentration measuring device for successively measuring sludge concentration in a sludge feed pipe upstream from a path leading from the coagulant addition means, said concentration being calculated from any one of the following combinations:
• (a) temperature and capillary suction time of the sludge, (b) temperature, capillary suction time and electrical conductivity of the sludge, (c) temperature, viscosity and electrical conductivity of the sludge, and (d) temperature, viscosity, electrical conductivity and the capillary suction time of the sludge,
• a measuring means for successively measuring the capillary suction time of the sludge,
• a measuring means for successively measuring filtrate discharged from the dewatering means,
• a coagulant feed measuring device for successively measuring flow rate of the coagulant to be introduced to the sludge feed pipe downstream from the sludge concentration measuring device, and
• a control means for controlling the coagulant addition means to control the dosage level of the coagulant based on output from an arithmetic device, said arithmetic device successively determining a coagulant dosage, at which a decrease ratio defined below is maximized, based on changes in the decrease ratio responsive to changes in the coagulant dosage ratio calculated from the sludge concentration and the coagulant flow:
• Decrease ratio (%) of capillary suction time (CST)=((the CST of the sludge-the CST of the filtrate)/(the CST of the sludge)).times.100