Sewage is defined as the type of waste water produced by the community of people mainly characterized by its physical condition, chemical and toxic constituents, and its bacteriologic status.

The sewage consists of the following two components:

• Sanitary Sewage or dry weather flow (DWF)
• Storm Sewage or wet weather flow (WWF)

The dry weather flow is the flow through the sewers that would normally be available during non-rainfall periods. The wet weather flow is the additional flow in the sewers that would occur in rainy season during rainfall.

Sewage is defined as the type of waste water produced by the community of people mainly characterized by its physical condition, chemical and toxic constituents, and its bacteriologic status.

The sewage consists of the following two components:

• Sanitary Sewage or dry weather flow (DWF)
• Storm Sewage or wet weather flow (WWF)

The dry weather flow is the flow through the sewers that would normally be available during non-rainfall periods. The wet weather flow is the additional flow in the sewers that would occur in rainy season during rainfall.

The dry weather flow DWF or sanitary sewage basically includes the following sources.

1. Domestic sewage i.e, the sewage or wastewater derived from residential buildings, individual houses, from commercial institutional and similar public buildings such as offices, schools cinemas, hotel etc.
2. Industrial sewage i.e,  the sewage or wastewater obtained from manufacturing plants of industries.
3. Ground water infiltrating into sewers through the pipe joints and other entry points.
4. Illegal or unauthorized connection it includes entry or rain water and an unauthorized connection made by the people  in the sewer.
5. Private sources includes the wastewater generated from private sources like tube tube well, well etc. 

The dry weather flow DWF or sanitary sewage basically includes the following sources.

1. Domestic sewage i.e, the sewage or wastewater derived from residential buildings, individual houses, from commercial institutional and similar public buildings such as offices, schools cinemas, hotel etc.
2. Industrial sewage i.e,  the sewage or wastewater obtained from manufacturing plants of industries.
3. Ground water infiltrating into sewers through the pipe joints and other entry points.
4. Illegal or unauthorized connection it includes entry or rain water and an unauthorized connection made by the people  in the sewer.
5. Private sources includes the wastewater generated from private sources like tube tube well, well etc. 

The quantity of dry weather flow or sanitary sewage is affected by the following factors.

• Rate of water supply
• Population growth
• Type of aria to be served ; and
• infiltration of ground water and ex-filtration of sewage ( addition and subtractions)

Rate of Water Supply:

Since a considerable part (generally 70 to 90 percent) of supplied water emerges as sanitary sewage, the quantity of domestic or sanitary sewage depends on the rate of water supply.

Population Growth:

Since the sewerage project is  designed to serve not only the present population but also for the predicted  future population within the design period also, the quantity of sanitary sewage increase with the increase in population.

Type of area to be served:

The quantity of sewage also depends upon the type of area to be served, which may be industrial or commercial or residential. The quantity of sewage produced by residential area depends upon the rate of water supply whereas the quantity of sewage produced by various industries depends on their industrial processes.

Infiltration and ex-filtration:

Infiltration is the entry of ground water into the sewers through leaking joints while ex-filtration is the leaking sewage from sewer line that percolates into the ground surrounding sewer. The infiltration increases the quantity of sanitary sewage while ex-filtration pollutes the ground water.

The quantity of dry weather flow or sanitary sewage is affected by the following factors.

• Rate of water supply
• Population growth
• Type of aria to be served ; and
• infiltration of ground water and ex-filtration of sewage ( addition and subtractions)

Rate of Water Supply:

Since a considerable part (generally 70 to 90 percent) of supplied water emerges as sanitary sewage, the quantity of domestic or sanitary sewage depends on the rate of water supply.

Population Growth:

Since the sewerage project is  designed to serve not only the present population but also for the predicted  future population within the design period also, the quantity of sanitary sewage increase with the increase in population.

Type of area to be served:

The quantity of sewage also depends upon the type of area to be served, which may be industrial or commercial or residential. The quantity of sewage produced by residential area depends upon the rate of water supply whereas the quantity of sewage produced by various industries depends on their industrial processes.

Infiltration and ex-filtration:

Infiltration is the entry of ground water into the sewers through leaking joints while ex-filtration is the leaking sewage from sewer line that percolates into the ground surrounding sewer. The infiltration increases the quantity of sanitary sewage while ex-filtration pollutes the ground water.

Generally, the rate of sanitary sewage should be equal to the quantity of water supplied but actually some additions due to private water supplies and infiltration of water and some subtractions due to leakages of water in the pipe lines and water consumption in drinking, cooking, sprinkling on roads, gardening of lawns and for various industrial purposes should be considered. Thus, the rate of sanitary sewage produced (in lpcd) may be assumed to be `70-90%` normally `80%` of the rate of  water supplied.

Average quantity of sanitary sewage (DWF) = 70 to 90 % of (population `**` rate of w/s) )

This average quantity of sanitary sewage cannot be used for design purpose because practically average sewage never flows in the sewers but varies from time to time. The design of sewers should be done for the maximum possible flow.

Maximum or peak quantity of sanitary sewage is equal to the peak factor times the average flow.

The peak factor of 2 to 4 is generally adopted in Nepal.

The minimum rate of sewage flow is generally taken as the one third of the average flow. 

Generally, the rate of sanitary sewage should be equal to the quantity of water supplied but actually some additions due to private water supplies and infiltration of water and some subtractions due to leakages of water in the pipe lines and water consumption in drinking, cooking, sprinkling on roads, gardening of lawns and for various industrial purposes should be considered. Thus, the rate of sanitary sewage produced (in lpcd) may be assumed to be `70-90%` normally `80%` of the rate of  water supplied.

Average quantity of sanitary sewage (DWF) = 70 to 90 % of (population `**` rate of w/s) )

This average quantity of sanitary sewage cannot be used for design purpose because practically average sewage never flows in the sewers but varies from time to time. The design of sewers should be done for the maximum possible flow.

Maximum or peak quantity of sanitary sewage is equal to the peak factor times the average flow.

The peak factor of 2 to 4 is generally adopted in Nepal.

The minimum rate of sewage flow is generally taken as the one third of the average flow. 

The source of storm sewage is precipitation that reaches to the sewer depend upon the following factors:

1. Area of the catchment
2. Characteristics of catchment area
3. Slope, size, imperviousness and shape of the catchment
4. Initial state of the catchment with respect to wetness.
5. Nature and number of ditches present in the catchment area.
6. Intensity and duration of rainfall.
7. Atmospheric condition.
8. Obstruction in the flow of water.
9. Time required for flow to reach the sewer.

The quantity of storm water that reaches to the sewer line depends upon the following factors:

• Area to be drained off
• Topography of the area and nature of the ground surface.
• Intensity of rainfall.

The source of storm sewage is precipitation that reaches to the sewer depend upon the following factors:

1. Area of the catchment
2. Characteristics of catchment area
3. Slope, size, imperviousness and shape of the catchment
4. Initial state of the catchment with respect to wetness.
5. Nature and number of ditches present in the catchment area.
6. Intensity and duration of rainfall.
7. Atmospheric condition.
8. Obstruction in the flow of water.
9. Time required for flow to reach the sewer.

The quantity of storm water that reaches to the sewer line depends upon the following factors:

• Area to be drained off
• Topography of the area and nature of the ground surface.
• Intensity of rainfall.

Solution:

Given,

Population = 50,000

Area = 40 ha.

Peak factor = 2

Rate of water supply = 200 lpcd

Avg. impermeability factor = 0.3

Time of concentration = 50 min

Now,

The discharge of DWF is,

`Q_(DWF)=(2**0.7**50000**200)/(1000**86400)`

`=0.1620 m^2/s`

Again,

Intensity of rainfall is given by,

`i=1020/(t+20)=1020/70=14.5714` mm/hr

Now, the storm water flow rate is,

`Q_(WWF)=(CiA)/360`

`=(0.3**14.5714**40)/360`

`=0.4857 m^3/s`

Thus, the combined design discharge is,

`Q=Q_(DWF)+Q_(WWF)`

`=0.1620 + 0.4857`

`=0.6477 m^3/s`

Solution:

Given,

Population = 50,000

Area = 40 ha.

Peak factor = 2

Rate of water supply = 200 lpcd

Avg. impermeability factor = 0.3

Time of concentration = 50 min

Now,

The discharge of DWF is,

`Q_(DWF)=(2**0.7**50000**200)/(1000**86400)`

`=0.1620 m^2/s`

Again,

Intensity of rainfall is given by,

`i=1020/(t+20)=1020/70=14.5714` mm/hr

Now, the storm water flow rate is,

`Q_(WWF)=(CiA)/360`

`=(0.3**14.5714**40)/360`

`=0.4857 m^3/s`

Thus, the combined design discharge is,

`Q=Q_(DWF)+Q_(WWF)`

`=0.1620 + 0.4857`

`=0.6477 m^3/s`

Solution:Average runoff coefficient is,

`C=(C_1A_1+C_2A_2)/(A_1+A_2)`

`=(0.4**0.8+0.2**0.6)/1`

`=0.44`

Again,

Runoff is given by,

`Q_(WWF)=(CiA)/360`

`=(0.44**50**2)/360`

`=0.122\ m^3/s`

Solution:Average runoff coefficient is,

`C=(C_1A_1+C_2A_2)/(A_1+A_2)`

`=(0.4**0.8+0.2**0.6)/1`

`=0.44`

Again,

Runoff is given by,

`Q_(WWF)=(CiA)/360`

`=(0.44**50**2)/360`

`=0.122\ m^3/s`

Solution:

Average impermeability factor is,

`C=(sum(C_1A_1))/A`

`=(0.8**0.25+0.85**0.25+0.32**0.15+0.2**0.1+0.15**0.2+0.2**0.05)`

`=0.5205`

Now, the intensity of rainfall is given by,

`I=760/(15+10)`

`=30.4` mm/hr

Thus, the runoff of the catchment is given by,

`Q=(CIA)/360`

`=(0.5205**30.4**12)/360`

`=0.527\ m^3/s`

Solution:

Average impermeability factor is,

`C=(sum(C_1A_1))/A`

`=(0.8**0.25+0.85**0.25+0.32**0.15+0.2**0.1+0.15**0.2+0.2**0.05)`

`=0.5205`

Now, the intensity of rainfall is given by,

`I=760/(15+10)`

`=30.4` mm/hr

Thus, the runoff of the catchment is given by,

`Q=(CIA)/360`

`=(0.5205**30.4**12)/360`

`=0.527\ m^3/s`

Solution:

Average runoff coefficient is given by,

`C=(C_1A_1+C_2A_2+C_3A_3+C_4A_4+C_5A_5+C_6A_6)/(A_1+A_2+A_3+A_4+A_5+A_6)`

`=(0.15**0.1+0.20**0.1+0.15**0.15+0.25**0.20+0.85**0.20+0.9**0.25)/1`

`=0.5025`

Now,

Time of concentration is,

`t=15+10=25\ mi n`

Now,

Storm water flow rate is,

`Q=(CiA)/360`

where,

`i=1020/(25+20)=22.67` mm/hr

Thus,

`Q=(0.5025**22.67**500)/360`

`=15.82 m^3/s`

Solution:

Average runoff coefficient is given by,

`C=(C_1A_1+C_2A_2+C_3A_3+C_4A_4+C_5A_5+C_6A_6)/(A_1+A_2+A_3+A_4+A_5+A_6)`

`=(0.15**0.1+0.20**0.1+0.15**0.15+0.25**0.20+0.85**0.20+0.9**0.25)/1`

`=0.5025`

Now,

Time of concentration is,

`t=15+10=25\ mi n`

Now,

Storm water flow rate is,

`Q=(CiA)/360`

where,

`i=1020/(25+20)=22.67` mm/hr

Thus,

`Q=(0.5025**22.67**500)/360`

`=15.82 m^3/s`

Solution:

Average impermeability factor is,

`C=(C_1**A_1+C_2**A_2)/(A_1+A_2)`

`=(0.8**0.5+0.1**0.5)/1`

`=0.45`

Now, the sanitary discharge is given by,

`Q_(DWF)=(3**700*5**150**0.80)/(1000**86400)`

`=0.0146\ m^3/s`

Again,

The intensity of rainfall is,

`i=760/(10+10)=38` mm/hr

Now, the storm discharge is,

`Q_(WWF)=(CiA)/360`

`=(0.45**38**5)/360`

`=0.2375\ m^3/s`

Now, the quantity of sewage for the combined system is given by,

`Q=Q_(DWF)+Q_(WWF)`

`=0.0146+0.2375`

`=0.2521\ m^3/s`

Solution:

Average impermeability factor is,

`C=(C_1**A_1+C_2**A_2)/(A_1+A_2)`

`=(0.8**0.5+0.1**0.5)/1`

`=0.45`

Now, the sanitary discharge is given by,

`Q_(DWF)=(3**700*5**150**0.80)/(1000**86400)`

`=0.0146\ m^3/s`

Again,

The intensity of rainfall is,

`i=760/(10+10)=38` mm/hr

Now, the storm discharge is,

`Q_(WWF)=(CiA)/360`

`=(0.45**38**5)/360`

`=0.2375\ m^3/s`

Now, the quantity of sewage for the combined system is given by,

`Q=Q_(DWF)+Q_(WWF)`

`=0.0146+0.2375`

`=0.2521\ m^3/s`