The activated-sludge treatment plant without primary sedimentation described in Problem is recommended for expansion to process a wastewater flow of 19,500 m3/d by adding one more aeration basin and clarifier. Using a waste activated sludge of 510 m3/d with 0.80% solids, determine the dimensions of three aerobic digesters using the same design data (except for waste-activated sludge volume and solids concentration) and criteria listed iii Example 13.14.
Problem:
An activated-sludge treatment plant without primary sedimentation processing a warm wastewater has two aeration basins with a total volume of 6730 m' and two clarifiers with scraper mechanisms to push the settled solids to a central hopper. Each aeration basin is prismoidal in shape, with the length twice the width and completely mixed by pedestal- mounted mechanical aerators. The waste-activated sludge is discharged to prismoidal thickening and holding tanks with a maximum liquid depth of 3.9 m and designed for fill- and-draw operation. Before withdrawing supernatant, the suspended solids thicken by hindered settling, and at least some compression, for several hours. The underflow of thickened sludge is discharged to open-air sand drying beds for dewatering and the supernatant is returned to the plant inlet.The loading on the plant is 13,000 nr/d with 260 g/nr' of BOD, is and the effluent contains 7 mg/l of BOD and 13 mg/l of suspended solids. The operating MLSS is 2300 mg/l, and the mixed liquor is normally 20°C or greater.The waste- activated sludge averages 340 m3/d with 0.80% solids. The designer assumed the waste sludge would thicken to 3.0% solids and drying time on the sand beds would be 32 d. In actual operation, the performance of the sludge processing system was a failure. The solids concentration in the thickened sludge averaged 1.8%, which is 67% greater in volume than a concentration of 3.0%. Furthermore, the wet sludge required two to three times longer than 32 d to dry sufficiently for removal from the sand beds. Since the cake remained moist on the bottom and did not shrink sufficiently to open cracks, the organic solids adhered to the sand grains, sealing the pores on the surface of the bed. Foul odors were emitted during drying. As a result, most of the settled sludge is hauled away in tank trucks for disposal in a dedicated landfill, which is a costly operation.
(a)How would you classify this activated-sludge process? Keep in mind the high temperature of the wastewater. (Refer to Sections 12.19 and 12.21.)
(b)solids concentration in the waste sludge? (Refer to Section 12.19.) of 0.80% in the waste-activated sludge have been improved by installing different clarifier mechanisms? (Refer to Section 10.11.) Could the return sludge rate in operation of the activated-sludge process influence the
(c)Why doesn’t the waste-activated sludge thicken to 3.0 Calculate the sludge age and multiply it by 20°C. Compare this to the aeration period of 200-300 degree-days given in Section 13.17. What does this value indicate?
(d)Could the solids concentration % by plain sedimentation? (Refer to Section 13.2.)
(e)Why is the thickened waste-activated sludge difficult to dewater on sand drying beds?
(f)What sludge processing and disposal system would you recommend for this treatment plant?
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