A city discharges 1500 liter per second of waste water into a river, whose minimum rate of flow is 3500 lit per second.

The temperature of waste water as well as river water is 20°C. The 5day BOD of waste water at that temperature is 300mg/lit and that of river water is 1 mg/lit.

The DO content of waste water is zero and that of the stream is 90% of the saturation D.O.

If the minimum D.O.to be maintained in the stream is 4.0mg/lit. Find out the degree of waste water treatment required. Assume the coefficient of de-oxygenation ($K_D$) as 0.1 and coefficient of re-oxygenation ($K_R$) as 04.

D.O at $20^0 C$ = $9.17 \ mg/l$ (assume fire value)

$\begin{aligned} \text{D.O. content of the stream} &= \text{90 % of saturation D.O.} \\ &= \frac{90}{100} \times 9.17 \\ &= 8.25 \ mg/l \end{aligned}$

$\begin{aligned} \text{Minimum D.O. to be maintained in the stream} \\ \text{Max permissible saturation deficit} \space (D_c) …