## Design hydraulic detention time () or digestion time (days).

An aerobic digester is to be designed to stabilize 600 kg/d of primary sludge at 5% solids and 75% volatiles, along with 400 kg/d of waste-activated sludge at 1% solids and 75% volatiles. The volume of the combined sludge that will be fed to the digester is 104 m3/d. The minimum design operating temperature is 18∘C. A temperature correction coefficient () of 1.06 should be used to correct the detention or digestion time for temperature. The primary sludge contains 210 kg of BOD, with an oxygen demand of 1.9 kg of oxygen per kg of BOD, whereas use an oxygen demand of 2.0 kg of oxygen per lb of VS destroyed for the WAS. Assume that 90% of the BOD contained in the primary sludge is destroyed and that 50% of the volatiles solids in the WAS is destroyed. Determine the following:

a. Design hydraulic detention time () or digestion time (days).

b. Aerobic digester volume (m3).

c. Volatile solids loading rate (kg/d ⋅ m3) to the aerobic digester.

d. Quantity of oxygen required (kg/d) to stabilize the primary sludge.

e. Quantity of oxygen required (kg/d) to stabilize the secondary sludge (WAS).

f. Total quantity of oxygen required (kg/d) to stabilize the combined sludge.

g. Total air required (m3/min), if there are 1.2 kg of air per cubic meter of air and the atmosphere consists of 23% oxygen by weight (0.23 kg O2/kg air) and the diffusers have a transfer efficiency of 4.0%.

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