The temperature of waste water as well as river water is $20^o$C .The 5 day BOD of waste water at that temperature is 280 mg/lit and that of a river water is 2 mg/lit. The DO of content of waste water is zero and that of stream is 90% of the saturation of DO. If the minimum DO to be maintained in the stream is 4.0 mg/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($K_D$) as 0.4.