A student began experiment for determination of 5-day, 20°

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A student began experiment for determination of 5-day, 20° C BOD on Monday. Since the 5^th day fell on Saturday, the final DO readings were taken on next Monday. On calculation, BOD (i.e. 7 day, 20° C) was found to be 150 mg/L. What would be the 5-day, 20°C BOD (in mg/L)? Assume value of BOD rate constant (k) at standard temperature of 20° C as 0.23/day (base e).

1. 129 mg/L
2. 127 mg/L
3. 128.0979 mg/L
4. 128 mg/L

Correct Option: C

128.0979 mg/L
k_D = 0.434 × 0.23 = 0.0998
BOD₇ = L[1 - 10^-kD^t]
∴ 150 = L[1 – 10^{(–0.0998 × 7)}]
⇒ L = 187.54 mg/L
∴ BOD₅ = L[1 - 10^-kD^t]
= 187.539 × [1 – 10^{–0.0998 ×5}]
= 128.0979 mg/L