The oxidation-ditch treatment plant described in Problem 12.50 has two aerobic digesters each with a liquid volume of 8000 ft3 and 200 cfm aeration capacity. The digesters are operated as a semibatch process. Each morning the air is turned off, and the digesting sludge solids are allowed to thicken. After several hours of quiescent settling, supernatant is withdrawn and returned to the plant inlet so waste sludge can be pumped into the digesters. Once a week, settled sludge is withdrawn from the hopper bottom of the digesters and hauled away for disposal by spreading on agricultural land. In order to maintain the MLSS concentration al 2500 mg/l in the oxidation ditches, an average of 2000 ft3/day of sludge containing 7600 mg/l of suspended solids (65% volatile) is pumped to the digesters, after withdrawal of supernatant. The average suspended-solids concentration in the digesting sludge is 11,000 mg/l, and the lowest temperatures measured in the aerating sludge are 8-10°C.The withdrawal of digested gravity-thickened sludge averages 700 ft3/day containing 14,000 mg/l of suspended solids (55% volatile). Calculate the volatile solids loading and air supply and compare the results with the recommended values given in Section 13.17. Calculate the sludge aeration period in degree-days. (The sludge age can be calculated by dividing the product of the volume of the digesters and the suspended-solids concentration in the digesting sludge by the average daily sludge withdrawal times its suspended-solids concentration.) How does the calculated aeration period in degree-days compare with the recommended value for adequate sludge stabilization?
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