Quantity of Sewage

1.      Write a short note on Domestic and Industrial sewages.

-          Distinguish between Domestic and Industrial sewages.

-          Write a note on sanitary sewage.

Ans. The total sewage from the domestic and industries are termed as sanitary sewage. The following are the bifurcation of sanitary sewage,

·         Domestic sewage: The sewage collected from water closets, urinals, lavatory basins of homes, offices and institutions. It is highly foul in its nature because of night soil (excremental matter). This sewage contains large number of pathogenic bacteria. Domestic sewage require great care while handling and disposal.

·         Industrial sewage: The sewage collected from industrial and commercial establishment is termed as industrial sewage. The quality and the foul smell depends upon the nature of the industry. Sometimes it needs treatment before disposal into the public sewers.

-          The wastes from manufacturing processes are also called as trade effluents or trade waste. Ex: Wastes from slaughter-houses, mills, laundries, chemical plants.

2.      What are the factors considered while determining the quantity of dry weather flow?

-          What is dry weather flow? Enlist the factors to be considered while calculating quantity of D.W.F.

-          List the factors affecting the dry weather flow.

-          List the factors affecting the sanitary sewage.

Ans. The sanitary sewage or dry weather flow (D.W.F) depends upon the following four factors,

a)      Population: The quantity of sewage to be removed will depend upon the population and contribution of sewage per capita, therefore, the D.W.F increases with the increase in population served.  

The following design periods are desirable for the various parts of the sewerage system.

Laterals	About 50 years
Sub-main of branches	More than 50 years
Main and trunk sewers	30 to 50 years
Pumping plant	5 to 10 years
Treatment units	10 to 30 years

b)      Rate of water supply: The quantity of water entering the sewer is slightly less than the quantity of water supplied due to leakage from pipes, lawn sprinkling, etc. The rate of sewage will be assumed to be about 60 to 70 % of the water supply.

c)      Type of area served: The quantity of sewage depends on the type of area such as residential, industrial, or commercial.

-          In residential area, the quantity of sewage produced depends directly on the quantity of water supply. It is expressed as litres per capita per day.

-          In industrial area, the quantity of sewage produced depends on their various industrial processes.

-          In commercial area, the quantity of sewage produced depends on the development of the area. The rate of flow of sewage is expressed in litres per day per square metre of area.

d)     Ground water infiltration:

-          It means the leakage of water from the ground surrounding the sewer.

-          In case of sewers which are laid below ground water table and water logged areas, an allowance should be made for the water entering the sewers through leaky joints.

-          Infiltration of rain water into the sewers must be prevented to reduce the excess flow of sewage into the treatment plant.

3.      Differentiate between infiltration and exfiltration.

Ans. Infiltration is used to indicate the leakage of water into the sewer, whereas exfiltration indicates the leakage of sewage from sewers to ground.

Infiltration	Exfiltration
Leakage of water from the ground surrounding the sewers	Leakage of sewage into the ground surrounding the sewer
The exfiltration pollutes the underground water	The infiltration unnecessarily increases the quality of sewage.

4.      What is infiltration? List the factors on which infiltration depends.

-          List the factors affecting the infiltration.

Ans. The term infiltration means is used to indicate the leakage of water from the ground surrounding the sewer. In case of sewers which are laid below ground water table and water logged areas, an allowance should be made for the water entering the sewers through leaky joints. The quantity of ground water infiltration depends upon the following factors.

Ø  The subsoil water

Ø  The nature of the soil

Ø  The type of the soil

Ø  The length of the sewer

Ø  The nature and size of the sewer

Ø  The materials of sewers

Ø  The total number of sewer joints

Ø  The depth of the sewage below ground water table.

5.      What is meant by variations in rate of sewage?

-          Discuss in brief about the variation in rate of flow of sewage.

-          List the factors that affect the rate of flow of sewage.

Ans. Variation in flow of sewage varies with the season, the day, the hour and other conditions like the following,

·         Climatic condition

·         Habits of the people

·         Type of industries and their activities, etc.

The variation in the rate of sewage affects the design of a sewer. The sewers are designed for the maximum flow in the worst possible case. The maximum and minimum rate of flow are usually expressed as percentage of the average rate of flow.  

6.      What is quantity of sewage? List the factors affecting the quantity of sewage.

-          List the factors affecting the quantity of sewage.

Ans. The quantity of sanitary sewage should be equal to the quantity of water supplied by the water works. But actually it is not so and some addition and subtractions are done in it to determine approximately correct quantity of sewage.

Additions: Private water supplies, infiltration of water.

Subtraction: Due to leakage of water pipe lines, and water consumption for drinking, cooking, gardening of lawns, etc. 

The following are the factors affecting quantity of sewage:

a)      The design of sewerage scheme

b)      Sewage disposal work

c)      The shape, size, gradient and depth of sewers

d)     Sizes of pumping installation units

e)      The sizes of the various units in the sewage disposal works.

7.      What are the factors to be considered in designing a sewer?

Ans. After the estimation of quantity of sewage, variation in its quantity the next step is to design the sewer section. The following important factors are considered while designing sewer section.

a)      The design period to be taken as 30 years, according to the population forecast.

b)      The non-silting and non-scouring velocities of flow should be maintained by providing suitable gradient.

c)      Gradient should be such that sewage flows under gravitational force only.

d)     The sewers should be capable of receiving the maximum expected discharge at the end of the design period.

e)      In case of combined system of sewerage, the section of the sewer is to be designed in such a way that self-cleansing velocity must develop at minimum discharge.

f)       The minimum size of public sewer should not be less than 150 mm. In practice 200 mm size sewer is provided in plain area and 100 mm in hilly area.

8.      Explain minimum and maximum velocity of sewer.

-          Write note on self-cleansing velocity (limiting velocity) and non-scouring velocity.

Ans. Minimum velocity: Minimum velocity at which zero silt deposition takes place is called self-cleansing velocity. The velocity in a sewer should be such that there is no deposition of silt, organic and inorganic matter, and any other solid matter of the sewage. The self-cleansing velocity depends on the nature of suspended matter in sewage and size of sewer.

-          In case of separate sewer a minimum of 0.6 m/s should be maintained.

-          A velocity of 0.9 m/s would be very acceptable.

-          The velocity of flow should not be less than 0.75 m/s in case of combined sewer.

Self-Cleansing Velocities

Diameter of Sewer (mm)	Velocity (m/s)
150 to 250	1.00
300 to 600	0.75
Above 600	0.60

Maximum Velocity: The maximum permissible velocity at which no scouring action will occur is known as Non-scouring velocity. It mainly depend on the material used in the construction of sewers.

-          Higher velocities will cause turbulent flow, thus reducing the capacity and life of the sewer.

-          Higher velocities can be controlled by limiting grades and providing drop manholes, at suitable places.

Non-Scouring Velocities

Type of material of sewer	Non-scouring velocity (m/s)
Brick lined	1.5 to 2.4
Concrete	2.4 to 3.0
Stoneware	3.0 to 4.5
Cast iron	3.5 to 4.5
Earthen channel	0.6 to 1.2

9.      What is limiting velocity (self-cleansing velocity)? Explain its role in sewers.

Ans. Minimum velocity at which zero silt deposition takes place is called self-cleansing velocity. The velocity in a sewer should be such that there is no deposition of silt, organic and inorganic matter, and any other solid matter of the sewage. The self-cleansing velocity depends on the nature of suspended matter in sewage and size of sewer.

-          In case of separate sewer a minimum of 0.6 m/s should be maintained.

-          A velocity of 0.9 m/s would be very acceptable.

-          The velocity of flow should not be less than 0.75 m/s in case of combined sewer.

Self-Cleansing Velocities

Diameter of Sewer (mm)	Velocity (m/s)
150 to 250	1.00
300 to 600	0.75
Above 600	0.60

10.  Define sewer grades.

Ans. The sewage flows in the sewers under gravitational force only, which is obtained by laying the sewers on slope or grade. The designer has to determine the optimum slope or grade at which the sewers are to be laid, so self-cleansing velocity is developed in the sewage flow. Usually the pipe sewer for house should be from 1 in 40 to 1 in 80.

The gradient depends on the following factors:

o   Nature of ground

o   Diameter of sewer

o   Allowable maximum and minimum velocities

o   Quantity of sewage.

11.  Enumerate about minimum size of a sewer.

Ans. The minimum size of a public sewer should not be less than 150 mm diameter. Generally, 200 mm diameter is provided, but in hilly areas for a maximum slope it may be 100 mm diameter. For domestic sewers, the smaller the diameter the greater the slope. Hence larger diameter sewers are preferred.

Problem: Refer text books and previous question paper 

Reference:

·         Water supply and Sanitary Engineering, by Rangwala, New Delhi, India.

·         Environmental Engineering II, by B.C. Punmia, Pande, B.B. Lal. Lakshmi Publications, New Delhi, India.

·         Water supply and Sanitary Engineering, by Birde.