Because the settled water from Pherric ( Problem 11-12 ) has a very low percent solids,...

Because the settled water from Pherric ( Problem 11-12 ) has a very low percent solids, a thickener is being considered for treatment of the sludge before dewatering. Using the following batch settling curve data, design three alternative sludge thickeners with the following underflow solids concentrations: 0.30%, 0.40%, and 0.50%. An overflow rate of 2.5 m/h has been selected for the design.

A completed design will include the tank diameter, tank depth, and the running torque for the rake. Assume two day’s storage at an average P_s of 0.004.

Problem 11-12

The city of Pherric’s coagulation treatment plant sedimentation tank is being designed to treat a flow of 20,000 m³/d. Based on jar tests, the design dose of ferric chloride is estimated to be 18.0 mg/L and the polymer dose is estimated to be 0.1 mg/L. The design raw water suspended solids concentration is 19.4 mg/L. The design effluent suspended solids concentration from the settling tank is 3.0 mg/L. The following sludge characteristics have been selected for the design: sludge solids content = 0.010%, specific gravity of the sludge solids = 1.2. What volume of sludge from the settling tank must be disposed of each day? Assume that the ferric chloride is fed as FeCl₃ .