Multiple choice

A portion of wastewater sample was subjected to standard BOD test (5 days, 20 oC), yielding a value of 180 mg/l. The reaction rate constant (to the base ‘e’) at 20 oC was taken as 0.18 per day. The reaction rate constant at other temperature may be estimated by kr = k20 (1.047)T-20. The temperature at which the other portion of the sample should be tested, to exert the same BOD in 2.5 days, is

  1. 4.9 oC

  2. 24.9 oC

  3. 31.7 oC

  4. 35.0 oC
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Using the BOD equation BOD_t = L(1 - exp(-kt)), we set the BOD equal for two conditions. Since the BOD is the same, k1*t1 = k2*t2. Solving for k2 at 2.5 days gives k2 = 0.36. Using the temperature correction formula 0.36 = 0.18 * (1.047)^(T-20) leads to T = 35 degrees Celsius.

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