Test: Environmental Engineering- 3 - Civil Engineering (CE) MCQ

The fluctuation in sewage discharge is found to be maximum in the lateral sewer.

The fluctuation in sewage discharge reduced from lateral sewer to branch sewer, from branch sewer to the main sewer and from the main sewer to outfall sewer. An example of the respective fluctuation values are given below:

Maximum velocity occurs at 0.81 times the diameter and maximum discharge occurs at 0.95 times the diameter.

Sewers are never designed to run at full depth. If the size of the Sewer is less than 400 mm, these are designed to run half full. If the Sewer size is in the range of 400 to 900 mm, these are designed to run 2/3rd full. When the size of Sewer is more than 900 mm, these are designed for 3/4th full.

(i) Maximum Daily discharge = 2 × Average Daily Discharge

(ii) Maximum Hourly discharge = 3 × Average Daily Discharge

(iii) Minimum Daily discharge = 2/3 × Average Daily Discharge

(iv) Minimum Hourly discharge = 1/3× Average Daily Discharge

In this system, two sets of vertical pipes are laid, i.e., one for draining night soil, and the other for draining sullage. The pipes of the first set carrying night soil are called soil pipes, and the pipes of the second set carrying sullage from baths etc., are called sullage pipes or waste pipes. The soil pipe as well as the waste pipe, are separately ventilated by providing separate vent pipes or antisiphonage pipes.

For,

Where d is depth of flow

v is flow velocity at depth d

D is diameter of sewer

V is flow velocity at full flow

∴ v = 1.0 m/s 

Peak flood by rational formula is given by Q_P = 136 KP_cA
Where, K = Co-efficient of run off; p_c = intensity (cm/hr); A = area in hectare

Overall, co-efficient of runoff (K or I)

 

Discharge, Q = AIR/360

A = Area in hectares

R = Intensity of rainfall in mm/hr

I = Co-efficient of run-off.

About 80% of the water supplied in the city enters in the Sewer.

Average discharge through the Sewer

Q_Avg = 0.25 m³/sec

Q_Design = 3 × Q_Avg

Q_Design = 3 × 0.25 = 0.75 m³/sec

Q_Design = 0.75 m³/sec

Where V_sc = self cleaning velocity or minimum velocity in m/sec

k = constant depend upon type of Particle present in the Sewage

f = friction factor

d = size of particle under consideration(m)

For inorganic solid i.e. Coarse sand

V_SC1 = 0.389 m/sec

For organic solid

 

V_SC2 = 0.523 m/s

Minimum velocity required will be maximum of V_SC1 and V_SC2

So, V_SC = 0.523 m/sec