Final answer:
The ultimate CBOD (Chemical Biochemical Oxygen Demand) of a river 40 km downstream is approximately 7.39 mg/L.
Step-by-step explanation:
The ultimate CBOD can be calculated using the decay coefficient and the velocity of the river.
The decay coefficient represents the rate at which the organic matter in the river decays.
To calculate the ultimate CBOD, we can use the formula:
CBOD(d) = CBOD1 * e^(-K * d/v)
Where CBOD(d) is the ultimate CBOD at distance d, CBOD1 is the initial CBOD, K is the decay coefficient, d is the distance, and v is the velocity of the river.
In this case, CBOD1 is 20 mg/L, K is 0.2 /day, d is 40 km (which can be converted to 40,000 m), and v is 20 km/day (which can be converted to 20,000 m/day).
Using these values in the formula, we can calculate the ultimate CBOD 40 km downstream:
CBOD(40,000 m) = 20 mg/L * e^(-0.2 /day * 40,000 m / 20,000 m/day)
CBOD(40,000 m) = 20 mg/L * e^(-2)
CBOD(40,000 m) ≈ 7.39 mg/L