(20)4. A BOD test is run using 50 mL of treated wastewater mixed with 250 mL of

Question

(20)4. A BOD test is run using 50 mL of treated wastewater mixed with 250 mL of pure water. The initial DO of the mix is 8.8 mg/L. After 5 days, the DO is 3.2 mg/L After a long period of time, the DO is 1.0 mg/L, and it no longer seems to be dropping. Assume the only BOD measured is carbonaceous. (a) (b) (c) (d) What is the five-day BOD of the wastewater? Estimate the ultimate carbonaceous BOD. What would be the remaining BOD after five days have elapsed? Estimate the reaction rate constant k (day1).

Explanation / Answer

Five day BOD = Initial DO - Final DO / P

P = 50/(50+250) = 1/6

(BOD)5 = (8.8 - 3.2 )/ (1/6) = 33.6 mg/L

Ultimate carbonaceous BOD = (8.8 - 1 )/ (1/6)

= 46.8 mg/L

For first order decay

Lt = L0 exp (-kt)

33.6 = 46.8 exp(-5k)

0.7179 = exp(-5k)

k = 0.0662 day-1

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