In the continuing management of Pherric’s coagulant treatment sludge ( Problems 11-12 an...

In the continuing management of Pherric’s coagulant treatment sludge ( Problems 11-12 and 11-17 ), your firm has been asked to perform a preliminary design of a sand drying bed. The sludge from a thickener at 2% solids is to be applied to the bed at a loading rate of 10 kg/m². A drained solids concentration of 6.3%, an initial residuals depth of 0.50 m, and a final solids concentration of 20% have been selected for the design. The following precipitation and pan evaporation data were obtained in the general geographic region of Pherric. For this preliminary design, estimate the area required for the sand drying beds. Assume the sludge will be stored on the beds even when there is no evaporation.

Problems 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 .

Problems 11-17

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.