A single-stage, high-rate trickling-filter plant treating wastewater with 200 mg/l of BOD cannot meet the effluent standard of less than 30 mg/l of BOD. Primary settling removes 35% of the raw-wastewater BOD and the 7-ft-dcep filters have an efficiency of 75% under a loading of 40 lb/1000 ft3/day of BOD and a hydraulic loading of 0.18gpm/ft2 without recirculation. This provides average effluent BODs of 30-35 mg/l during the summer and 40-45 mg/l during the winter. One proposal is to replace the stone media with the random packing described in Example 12.6, which has constants n - 0.39 and K20 = 0.120 min-1; cover the filters with fiber glass domes to prevent the wastewater from cooling below 16°C in winter; and modify the plant to provide both direct and indirect recirculation. Calculate the effluent BOD at 16°C for the fillers packed with the random plastic media at R = 1.0.
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