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Chapter:
1. An impervious area of `20` hectares is to be served by a sewer line. This area is subjected to three storms X, Y, Z with equal frequency of occurrence. The duration of three storms is `20` minutes, `30` minutes and `35` minutes respectively. Taking time of concentration equals to `30` minutes. Determine maximum runoff from each storm. [Ans : `0.938` `m^3`/s ,`1.134` `m^3`/s , `1.03` `m^3`/s]
Solution:
For impervious area, `C=1`
For area X,
`t=20 min`
`t_c=30 min`
Since `t
The contribution area is,
`=sum A**t/t_c`
`=20**20/30`
`=13.33 ha`
Now,
`i=760/(30)`
`=25.33` mm/hr
Thus, `Q=(CiA)/360`
`=(1**13.33**25.33)/360`
`=0.939 m^3/s`
For area Y,
`t=30 min`
`t_c=30 min`
In this case, contributing area is equal to entire area i.e, 20 ha.
Thus, `i=1020/(50)=20.4` mm/hr
Thus, `Q=(1**20.4**20)/360=1.134 m^3/s`
Again for area Z,
`t=35 min`
`t_c=30 min`
since `t>t_c`, contributing area is equal to entire area i.e, 20 ha.
Thus,
`i=1020/(35+20)`
`=18.55` mm/hr
Now,
`Q=(1**18.55*20)/360`
`=1.03\ m^3/s`
2. Explain factors affecting source of storm sewage.
The source of storm sewage is precipitation that reaches to the sewer depend upon the following factors:
- Area of the catchment
- Characteristics of catchment area
- Slope, size, imperviousness and shape of the catchment
- Initial state of the catchment with respect to wetness.
- Nature and number of ditches present in the catchment area.
- Intensity and duration of rainfall.
- Atmospheric condition.
- Obstruction in the flow of water.
- 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.
3. Assuming surface on which rainfalls in thickly built residential district as follows: `40 %` area consist of roofs and pavements having impermeability factor `C` as `0.85`, `40 %` of area consists of lawns and gardens having `C` as `0.20` and remaining area consists of ground surface having `C` as `0.1` .If total area draining to sewer is `5` hectares and maximum intensity of rainfall is `50` mm/hr, estimate probable runoff from area.
Solution:
Average impermeability factor is,
`C=(C_1A1+C_2A_2)/A`
`=(0.40**0.85+0.40**0.2+0.2**0.1)`
`=0.44`
Now,
The maximum probable runoff is given by,
`Q=(CIA)/360`
`=(0.44**50**5)/360`
`=0.3056\ m^3/s`
4. A certain district of a city has a projected population of 50,000 residing over an area of 40 hectares. Find design discharge for sewer line, for following data :
- Rate of water supply = 200 lpcd
- Average impermeability coefficient for entire area =0.3
- Time of concentration = 50 minutes
The sewer line is to be designed for a flow equivalent to wet wear flow (WWF) plus twice dry wear flow (DWF).Use British ministry of health formula. Assume that `70%` of water supply reaches in sewer as wastewater.(Ans: `0.648 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`
5. The catchment area of city is 500 hectares. Assuming that surface on which rainfalls is classified as follows : :
| S.NO | Types of surface | % area | C |
| 1 | Forest and wooden area | 10 | 0.15 |
| 2 | open ground + unpaved street | 10 | 0.20 |
| 3 | parks + lawns + gardens | 15 | 0.15 |
| 4 | Gravel roads | 20 | 0.25 |
| 5 | Asphalt pavements | 20 | 0.85 |
| 6 | Water tight roof surfaces | 25 | 0.9 |
Calculate run off coefficient of area. Also calculate storm water, if area to be served is 500 hectares, time of entry is 15 minutes and time of flow is 10 minutes.
[Ans: `0.5025`, `15.32 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`
6. Explain factors affecting Sanitary Sewage.
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.
7. A population 50,000 resides in a town covering an area of 75 hectares. Calculate design discharge for combined system of sewers. Given,
- Coefficient of runoff =0.7
- Time of entry = 5 minutes
- Time of flow = 20 minutes
- Water supply rate = 120 lpcd. [PU-2008]
Solution:
Here, time of concentration is, `t_c=20+5=25` min
Assuming `80%` of supplied water contributes to sewage and peck factor to be `2.5`.
Now, the sanitary discharge is,
`Q_(DWF)=(0.8**2.5**120**50000)/(1000**86400)`
`=0.139\ m^3/s`
Similarly,
`i=(1020)/(25+20)=22.67` mm/hr
Thus, the storm discharge is,
`Q_(WWF)=(0.7**22.67**75)/360`
`=3.306\ m^3/s`
Now, the design discharge for the combined system is,
`Q=3.306+0.139`
`=3.445\ m^3/s`
8. Explain sources of Sanitary sewage or dry wear flow.
The dry weather flow DWF or sanitary sewage basically includes the following sources.
- 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.
- Industrial sewage i.e, the sewage or wastewater obtained from manufacturing plants of industries.
- Ground water infiltrating into sewers through the pipe joints and other entry points.
- Illegal or unauthorized connection it includes entry or rain water and an unauthorized connection made by the people in the sewer.
- Private sources includes the wastewater generated from private sources like tube tube well, well etc.
9. Assuming surface on which rain falls in a thickly build up residential district as follows :
- `40 %` area consists of roofs and pavements (`C_1 = 0.80`)
- `60 %` of area consists of lawns and gardens (`C_2 = 0.2`)
Calculate coefficient of runoff. If area of district is 2 hectares and maximum intensity of rainfall is taken as 50 mm/hr, what is runoff of district? (Ans: `0.44, 0.122` `m^3`/sec)
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`
10. The surface of town on which rainfalls is classified as follows :
| Types of surface | % area | Runoff coefficient |
| Roof | 25 | 0.8 |
| Pavements and yards | 25 | 0.85 |
| Macadamized roads | 15 | 0.32 |
| Gravel roads | 10 | 0.2 |
| Unpaved streets | 20 | 0.15 |
| Lawns and gardens | 5 | 0.2 |
What is runoff of catchment if,
- Total area of catchment is 12 hectares
- Time of concentration for area is 15 minutes.
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`
All Chapters
Introduction
Quantity of Waste Water
Design of sewers Tutorial
Treatment of WasteWater
Topics
Sources of sanitary sewages
factors affecting sanitory sewage
determination of quantity of sanitary sewages
variation in the quantity of sanitary sewage
determination of quantity of sanitary sewages
factors affecting storm sewages
time of concentration
time area graph and many more illustrative examples
An impervious area of `20` hectares is to be served by a sewer line. This area is subjected to three storms X, Y, Z with equal frequency of occurrence. The duration of three storms is `20` minutes, `30` minutes and `35` minutes respectively. Taking time of concentration equals to `30` minutes. Determine maximum runoff from each storm. [Ans : `0.938` `m^3`/s ,`1.134` `m^3`/s , `1.03` `m^3`/s]
Explain factors affecting source of storm sewage.
Assuming surface on which rainfalls in thickly built residential district as follows: `40 %` area consist of roofs and pavements having impermeability factor `C` as `0.85`, `40 %` of area consists of lawns and gardens having `C` as `0.20` and remaining area consists of ground surface having `C` as `0.1` .If total area draining to sewer is `5` hectares and maximum intensity of rainfall is `50` mm/hr, estimate probable runoff from area.
A certain district of a city has a projected population of 50,000 residing over an area of 40 hectares. Find design discharge for sewer line, for following data :
- Rate of water supply = 200 lpcd
- Average impermeability coefficient for entire area =0.3
- Time of concentration = 50 minutes
The sewer line is to be designed for a flow equivalent to wet wear flow (WWF) plus twice dry wear flow (DWF).Use British ministry of health formula. Assume that `70%` of water supply reaches in sewer as wastewater.(Ans: `0.648 m^3/s`)
The catchment area of city is 500 hectares. Assuming that surface on which rainfalls is classified as follows : :
| S.NO | Types of surface | % area | C |
| 1 | Forest and wooden area | 10 | 0.15 |
| 2 | open ground + unpaved street | 10 | 0.20 |
| 3 | parks + lawns + gardens | 15 | 0.15 |
| 4 | Gravel roads | 20 | 0.25 |
| 5 | Asphalt pavements | 20 | 0.85 |
| 6 | Water tight roof surfaces | 25 | 0.9 |
Calculate run off coefficient of area. Also calculate storm water, if area to be served is 500 hectares, time of entry is 15 minutes and time of flow is 10 minutes.
[Ans: `0.5025`, `15.32 m^3/s`]
Explain factors affecting Sanitary Sewage.
A population 50,000 resides in a town covering an area of 75 hectares. Calculate design discharge for combined system of sewers. Given,
- Coefficient of runoff =0.7
- Time of entry = 5 minutes
- Time of flow = 20 minutes
- Water supply rate = 120 lpcd. [PU-2008]
Explain sources of Sanitary sewage or dry wear flow.
Assuming surface on which rain falls in a thickly build up residential district as follows :
- `40 %` area consists of roofs and pavements (`C_1 = 0.80`)
- `60 %` of area consists of lawns and gardens (`C_2 = 0.2`)
Calculate coefficient of runoff. If area of district is 2 hectares and maximum intensity of rainfall is taken as 50 mm/hr, what is runoff of district? (Ans: `0.44, 0.122` `m^3`/sec)
The surface of town on which rainfalls is classified as follows :
| Types of surface | % area | Runoff coefficient |
| Roof | 25 | 0.8 |
| Pavements and yards | 25 | 0.85 |
| Macadamized roads | 15 | 0.32 |
| Gravel roads | 10 | 0.2 |
| Unpaved streets | 20 | 0.15 |
| Lawns and gardens | 5 | 0.2 |
What is runoff of catchment if,
- Total area of catchment is 12 hectares
- Time of concentration for area is 15 minutes.
