Question

The dilution factor D for an unseeded mixture of wastewater is 0.05. The DO of the mixture is initially 8.0 mg/L, and after 5 days it has dropped to 2.0 mg/L. The reaction rate constant K has been found to be 0.20 day? (i) What is the 5-day BOD of the waste? (ii) What would be the ultimate carbonaceous BOD?

Answer 1

Answer:

(i) 5-day BOD of the waste is 120 mg/l.

(ii) The ultimate carbonaceous BOD (Lo) is 20 mg/l.

Explanation:

The dilution factor D is 0.05.

The initial DO is 8.0 mg/L and the DO after 5 days is 2.0 mg/L.

The BOD of the waste for an unseeded mixture is

$BOD_5=(DO_5-DO_0)/D=(8-2)/0.05=6/0.05=120mg/l$

The ultimate carbonaceous BOD (Lo) can be calculated as

$L = L_o*10 ^{- k_1t} \\L_o=L/10^{- k_1t}=2/10^{- 0.2*5}=2/10^{- 1}=2*10=20mg/l$