An activated-sludge system is to be used for secondary treatment of 15,000 m3/d of municipal wastewater. After primary clarification, the BOD is 130 mg/L, and it is desired to have not more than 5 mg/L of soluble BOD in the effluent. A completely mixed reactor
An activated-sludge system is to be used for secondary treatment of 15,000 m3/d of municipal wastewater. After primary clarification, the BOD is 130 mg/L, and it is desired to have not more than 5 mg/L of soluble BOD in the effluent. A completely mixed reactor is to be used, and pilot-plant analysis has established the following kinetic values: Y = 0.6 kg/kg, kd = 0.07 d-1. Assuming an MLSS concentration of 2,750 mg/L and an underflow concentration of 11,000 mg/L from the secondary clarifier, determine the followings.
[Use θc (MCRT or SRT) = 10 day.]
5.1. The volume of the reactor.
5.2. The mass and volume of solids that must be wasted each day.
5.3. The recycle ratio.
Hint: θc = VX/QwXu : MCRT (mean cell-residence time) [or SRT (solids retention time)]; average time microbes spend in reactor = ratio of total biomass in reactor to biomass wasted per given time; currently most commonly used design parameter
F/Mv: inversely proportional to SRT; ≈U (cell's specific substrate utilization rate)
θx = (YU – b)-1
U = Q(S0 – S)/VXa
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