Lime precipitation of raw wastewater is one method for removing phosphorus. Circular prima...

Lime precipitation of raw wastewater is one method for removing phosphorus. Circular primary clarifiers with aerated flocculation wells as illustrated in Figure were installed al a wastewater treatment plant for phosphorus precipitation and enhanced suspended-solids removal. The maximum hourly flow rate for each tank was 1 1 mgd.The lime dosage of 300 mg/l of CaO in a slurry was applied to the influent prior to entering the flocculation well. For this peak hydraulic loading, calculate the detention time based qn the entire tank vol lime, flocculation time based on the volume of the flocculation well, overflow rate, and weir loading. Compare these values with the values for flocculator-clarifiers given in Table 10.1

FIGURE: Illustration for Problem A circular primary clarifier with an aerated flocculation well that receives the wastewater after the addition of lime in a rapid-mix tank. The key dimensions are diameter of the tank 40 ft, diameter of the flocculation well 20 ft, side-water depth 10 ft, and submergence of flocculation well 6.0 ft.

Now, consider that the performance of the clarifiers was very poor. The overflow was continuously milky, carrying out approximately one-third of the applied lime. Subsequent recarbonation was not adequate to neutralize the pH below the range of 8.5-9.5; conse­quently, calcium carbonate precipitated in the following equalization tank and RBC tanks. The sludge production was significantly less than the anticipated quantity. Why was the operation of the clarifiers unsatisfactory? (Read the discussion of flocculator-clarifiers and examine Figure 10.9.)

Problem:

Lime precipitation of raw wastewater is one method for removing phosphorus. Circular primary clarifiers with aerated flocculation wells as illustrated in Figure were installed al a wastewater treatment plant for phosphorus precipitation and enhanced suspended-solids removal. The maximum hourly flow rate for each tank was 1 1 mgd.The lime dosage of 300 mg/l of CaO in a slurry was applied to the influent prior to entering the flocculation well. For this peak hydraulic loading, calculate the detention time based qn the entire tank vol lime, flocculation time based on the volume of the flocculation well, overflow rate, and weir loading. Compare these values with the values for flocculator-clarifiers given in Table 10.1