Two completely mixed facultative sewage ponds are in series, each HRT6 days. Overall BOD₅ reduction90%. Both have the same first-order rate constant. Find k in day⁻¹,to two decimals.
Each steady CSTR has C_out/C_in=1/(1+kθ). Two equal ponds give .10=(1+6k)⁻². Thus1+6k=√10 and $k=(\sqrt{10}-1)/6=.36038$ day⁻¹. Therefore **.36 day⁻¹**. Using a single12-day plug-flow exponential would model a different reactor system.
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