Correct Answer - Option 4 : 2926 mg/L
Concept:
Bio-chemical oxygen demand:
(i) The amount of O2 required to carry out the decomposition of bio-degradable organic matter present in the system is termed as BOD.
(ii) BOD during 5 days at 20°C is taken as standard BOD and is approximately 68% of Ultimate BOD.
BOD at any time is calculated by,
BODt = BODu (1 - e-kt) or BODu (1 - 10-kDt)
Where k and kD represent deoxygentation constant at base 'e' and at base '10' respectively that signifies the rate of BOD reaction without affecting the ultimate BOD (BODu)
Calculation:
Given,
BOD5 = 2000 mg/l, k = 0.23
∵ we know that, BODt = BODu (1 - e-kt)
So BOD at 5 days, BOD5 = BODu (1 - e-kt)
2000 = BODu (1 - e-0.23 × 5)
⇒ 2000 = BODu × 0.6833
BODu = 2926 mg/l