D Question 25 1 pts Increasing the ultimate BOD in a stream

D Question 25 1 pts Increasing the ultimate BOD in a stream is likely to results in (..). (Mark all correct answers) A decrease in critical time An increase in critical time A decrease is kl ? A decrease in k2 An increase in Lo A decrease in the critical DO level A decrease in Lo An increase in the critical DO level No change in the DO deficit

Solution

Answer 25:

Since L₀ is the total amount of organic matter oxidised, so higher the BOD, higher the L₀. This implies that statement number 5 is correct and 7 is wrong.

If we differentiate the Streeter-Phelps equation and pt it equal to zero, we get equation for critical time (Time to reach minimum dissolved oxygen). From that equation we can see that higher the L₀ (or ultimate BOD), lesser the critical time. This implies that statement number 1 is correct and 2 is wrong.

k₁ and k₂ both depend only on temperature and not on BOD, so both the statements 3 &4 are wrong.

Since higher the L₀, higher the critical oxygen deficit, hence lower the critical DO level, statement number 6 is correct and 8 and 9 are wrong.

Answer 26:

The first statement that BOD is first order reaction is CORRECT, and that reaction is given mathematically as dC_t / dt = -kC_t or C_t = C₀e^-kt or ln(C_t/C₀) = -kt . So, Statements 1, 2, and 3 are CORRECT but statements 4, 6, 7, 8, and 9 are WRONG.

Half life is the time in which concentration becomes half for a first order reaction, i.e. C_t = 0.5C₀. Hence Statement number 5 is WRONG.