A city discharges 1600 lit/sec of sewage into a stream whose minimum rate of flow is 6000 lit/sec. The temperature of sewage as well as water is 20$^{\circ}$C. The 5 day BOD at 20 0 for sewage is 200 mg/lit and that of river water is mg/lit. The DO content of sewage is zero and that of the stream is 90% of the saturation DO. If the minimum DO to be maintained in the stream is 4.5 mg/lit, find out degree of sewage treatment required. Assume the de-oxygenation coefficient as 0.1 and re-oxygenation coefficient as 0.3.

From the table, the value of saturation

D.O. at $20^{\circ}C$ = 9.17 mg/l (fix value assume)

D.O. content of the stream= 90% of saturation D.O.=$\left( \cfrac { 90 }{ 100 } \right) \times 9.17=8.25$mg/l

D.O. of mix at the start point (i.e at l=0)=$\left( \cfrac { 8.25\times 6000+0\times 1600 }{ …