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Unformatted text preview: SOLUTIONS HOMEWORK NO. 6 CE 3400 Introduction of Environmental Engineering Material Covered: Chapter 11 (Wastewater Treatment) in Vesilind et al., 2010 1. This is Problem 11.11 in your textbook (reworded slightly for clarification) Mean cell residence time (MCRT or c ) is also called sludge age. Calculate the sludge age if an aeration tank in an activated sludge system has a hydraulic retention time of 2 hours and a suspended solids concentration of 2,000 mg/L. The flow rate of influent wastewater to the aeration basin is 1.5 m 3 /min, the return sludge solids concentration is 12,000 mg/L, and the flow rate of waste activated sludge (WAS) is 0.02 m 3 /min. Assume that Q w = Q r . Hint: You need to calculate values of c for sludge wasted from both the aeration basin and the recycle line. Start by calculating the volume of the reactor. It might help to draw a diagram and label it before starting. Answers : c (aeration basin) = ~6 days c (recycle line) = ~1 day Known: Hydraulic retention time = 2 hr X r = 12,000 mg/L X = 2,000 mg/L (= SS in aeration basin) X w = 2,000 mg/L Q = 1.5 m 3 /min Q w = 0.02 m 3 /min NOTE: X = X w but X X r and X w X r Step 1 : Calculate V of aeration basin because you know t and Q : t = V Q so V = t Q V = (2 hr ) (1.5 m 3 /min) = 120 min (1.5 m 3 /min) = 180 m 3 Step 2 : Calculate sludge age ( c ) for sludge wasting from (a) aeration basin and from (b) recycle line: ( a ) c = VX Q w X w = (180 m 3 )(2,000 mg / L )...
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This note was uploaded on 12/08/2010 for the course CE 2400 taught by Professor Colberg during the Spring '10 term at Wyoming.

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