The extended aeration system for the town of Nancy 1s described in problem Determine the aerobic digestion capacity for stabilizing the sludge from this plant in two diffused -air digesters equipped with supernatant draw-offs. Although sludge dewatering on open-air drying beds is being considered, the feasibility of dewatering by belt filler presses is also under consideration because or the long cold winter. The aerating sludge is expected to be as low as 8°C, and drying beds probably could not be cleaned for up to 3 months.
(a) Calculate the digester volume required based on the following:
(1) The daily waste-sol ids yield for design can be calculated from average flow and BOD, the waste sludge has a total solids content of 1.0%, and the solids are 70% volatile. :
(2) The volatile solids loading of the digester should not exceed 0.04 l b/ft3/day, and the aeration period should not be less than 200 degree-days, computed by multiplying the estimated sludge age by 8°C. :
(3) The thickened digested sludge has a solids concentration of 1.5%.:
(4) Volatile solids are reduced during digestion by 50%.:
(5) The average opera ting suspended-solids concentration in the digesters is 1.0%.:
(b) For the drying beds, assume a maximum allowable solids loading of 10 lb/ft 2/yr.
(c) For sizing belt filter presses, assume a maximum hydraulic sludge loading of 30 gpm per meter of belt width (Table 13.9) and a maximum opera ling period of 35 hr per week, 7 hr/day for 5 days. Two presses are required for reliability by the design-reviewing authority.
Problem:
A treatment plant has two oxidation-ditch activated-sludge systems, as illustrated in Figure 12.36. Each ditch has a liquid volume of 35.000 ft3 and is equipped with two horizontal-rotor aerators with capacity to transfer 1150 lb of oxygen per day at normal submergence. Each system has a clarifier of the type illustrated in Figure 10.12 with a diameter of 30 ft, a 9.0 ft side-water depth, and a single weir set on a diameter of 30 ft. The effluent chlorination tank has a volume of 2200 ft’. The design flow is 0.54 mgd: 0.35 domestic and commercial, 0.02 industrial, and 0.17 infiltration and inflow.The design BOD and suspended solids aie both 740 lb/day. During heavy rains, the peak hydraulic loading anticipated is 800 gpm for 2-3 hr. Calculate
(a) the BOD concentration at design flow and
(b)at design flow, the aeration period, volumetric BOD loading, and F/M at 2500 mg/l. How do these compare with values listed in Table 12.3?
(c) How many pounds of oxygen can be transferred to the mixed liquor per pound of BOD aeration tank loading?
(d) Calculate the overflow rate and weir loading at the peak hydraulic loading. How do these values compare with recommended design values?
(e) What is the detention in the chlorination tank at peak hydraulic flow? At twice the design flow? How do these values compare with the recommended detention time for chlorination of wastewater effluent?
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