Q. For what capacity would you design a sanitary branch sewer to serve a section having a population of 12500 persons with a rate of supply of 135 Liters.

Ans.- Average flow equals - 12500 x 135 = 1692500 Liter / day.

                                              = 1692500 / 24 x 60 x 60 = 19.6 Liter / sec.

Making allowance for variation max 3 time of Qty for intensity of flow -

19.6 x 3 = 58.8 Liter / sec. = 0.06 cum / sec.

But this quantity represents only 2/3^rd fill there fore full capacity of sewer

= 1.5 x 0.06 = 0.09 cum / sec      for which the section is designed

Assuming a velocity of 1.00 m / sec.    ,     A = Q / V  = πD² / 4 V ,    D² = 4 Q / πV

D = √ 4 Q / πV = √ 4 x 0.09 / 3.14 x 1.00 =  0.34 m.

A sewer of 350 mm dia could be used.