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**Design of sewer system Water Resources Engineering (Semester A -2017)**

M3H124594 (Semester A -2017) Design of sewer system

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**KEY LEARNING POINTS Good understanding of:**

the different types of sewerage systems the different types of methods used for sewer design the design of foul water sewer the design of storm water sewer

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**SEWERAGE is the water found in sewers.**

it can be a mixture of water which has been used for a variety of purposes at home, at work or in free time activities, and water used for business and industrial purposes.

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SEWERAGE SYSTEM The system of pipes used to collect and carry rain, waste water and trade waste away for treatment and disposal is called the sewerage or the waste water system

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**SOURCES OF WASTE WATER Domestic- 70-80% of the water supply**

Industrial (Permitted industries)- Depends upon the types of industry Commercial Storm water- flow from the precipitation Infiltration- Rain water likely to enter through leaky joints

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**TYPES OF SEWERAGE SYSTEMS**

There are three types of sewerage systems: 1- Foul sewers – carry waste water, for cooking and washing and waste from toilets to the wastewater treatment plants; 2- Surface water, or Storm sewers – carry rainwater from roofs, paved areas, pavements and roads, generally flow into streams, rivers or watercourses. 3- Combined sewer – this is a single pipe system carries both wastewater and surface water to the wastewater treatment plants, (in older town). All new sewerage systems should be designed on separate foul and surface water (storm sewers) systems.

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**DIFFERENCE BETWEEN COMBINED AND SEPARATE SEWERAGE SYSTEMS**

Sl. No. Combined System Separate System 1 Domestic sewage + Industrial flow in one pipe Flows are carried in separate pipes 2 Simpler in its arrangements and its operation There are two laterals in each street 3 Sewer size is big Main sewer is smaller 4 Possibility of settlement of solid in dry weather Flushed daily automatically 5 Suitable for the countries where rainfall is distributed all over the year. Suitable for arid countries 6 Because high volume, cost of treatment is high. Cost of treatment is less.

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**DESIGN PROCEDURE: foul sewer system**

Preliminary horizontal layout Preliminary breakdown in section Estimation of quantities of wastewater generated (Dry Weather Flow) Hydraulic design Determination of invert level and outlet level

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**Preliminary horizontal layout**

The sewerage pipe must be located in the surrounding area of the road. (public properties) The pipe to be located in private property due to certain topographies must apply for permission to certain individual before the construction can begin. Before any construction work can begin, the layout and overall existing building, pipe and electrical must first to be determine. During the construction work, the separation of sewer and main water should be requires of minimum horizontal separation of 3 m and a minimum vertical separation of 46 cm from water mains. The system must be drawn to show the location of the pipe and manholes location and flow of sewerage Sewer follow natural drainage ways to minimize excavation and pumping requirements. Location of pipe: place for easy connection for future user provide access for maintenance. Location of outfall: outfall point should be specified, near to the lowest point, next to receiving water body

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**2.Preliminary breakdown in section**

Manholes are structures designed to provide access to a sewer. Access is required for testing, visual inspection of sewers, and placement and maintenance of flow or water quality monitoring instruments. Manholes are usually provided at heads of runs, at locations where there is changes in direction, changes in gradient; changes in size, at major junctions with other sewers and at every 90 meter intervals depending on the size of the sewer pipes. The diameter of the manhole will depend on the size of sewer and the orientation and number of inlets.

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**Sewer Appurtenances: Manholes**

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**Sewer Appurtenances: Street inlets**

Street inlets called gutter are the openings through which storm water is admitted and conveyed to the storm sewer or combined sewer. The inlets are located by the sides of pavement with maximum spacing of 30 m.

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**3.Estimation of quantities of wastewater generated**

Estimation of quantities of flow considered for design is based on Dry weather flow for a separate sewer system

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**Estimation of future population**

Prospective population of the project area is calculated using the following formula (Growth rate method): Nfuture = Nactual . (1+a)b Nfuture : Number of inhabitant over design period of the project Nactuel : Number of inhabitant over the actual year a: population growth rate (in %) b : Design period of the project (in year) Design period: The length of the time up to which the capacity of a sewer will be adequate is called a design period. Normally design period for a sewerage system is considered as 30 years

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**DRY WEATHER FLOW (DWF) DWF = LP + I + E**

It is the quantity of water that flows through a sewer in dry weather when no storm water is in the sewer. The quantity of DWF primarily depends on: 1- Rate of water consumption 2- Population 3- Groundwater infiltration 4- Industrial discharge DWF = LP + I + E where: L = Domestic water consumption per head, P = Population connected to the sewer, I = Infiltration to porous pipes, E = Industrial discharge.

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**4.Hydraulic design of sewer**

Select pipe material Formula of Manning-Strickler Q = Ks S RH2/3 I1/2 Q : Discharge in m3/s ; S : Wetted area (m2) ; RH: Hydraulic radius(m); I : pipe slope ; Ks : rugosity coefficient (Ks=1/n with n Manning coefficient) ;

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**Pipe partially full Select diameter (start with minimum diameter)**

Sewer pipe are never designed to run full; there is always an empty space provided at the top. Select diameter (start with minimum diameter) Calculation of discharge for pipe running fully QFS Calculation of ratio Qmax/QFS, then determine Vact/Vmax

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It is very poor design to use less than 150 mm diameter foul pipes and some engineers specify a 225 mm minimum due to frequent blockages, which occur in 150 mm pipes. The design flow is checked for Maximum flow Q max = 6 DWF Full section flow for circular pipe made in concrete is QFS=23.97 * I1/2 * D8/3 Flow ratio Qmax/QFS is calculated, if ≥ 1 increase pipe size Partial flow chart is used to determine velocity ratio Vact/VFS

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Partial flow chart

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**Check of Velocity: self-cleansing condition**

Vact/VFS If 0.6 m/s ≤ Vact ≤ 2.5 to 3 m/s OK, keep pipe size If not The pipe size should be adjusted or the slope should be slightly increased

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EXAMPLE The proposed layout for a small sewerage scheme is shown below and the data relevant to this network is shown in the table below. Select suitable pipe size for the network. Branch L(m) Grad 1: Population E(m3/day) A 120 55 350 800 B 150 60 450 250 C 110 62 200 400 Water consumption = 400 liters per head per day Infiltration = 5% D.W.F.

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Design of Storm sewer

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**DESIGN PROCEDURE: Storm sewer system**

Preliminary horizontal layout Preliminary breakdown in section Estimation of quantities of storm water generated (Rational Method) Hydraulic design using Manning formula Check for velocity; if not in the range change the sewer diameter Determination of invert level and outlet level

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**Storm sewer design Two main methods used in sewer design in U.K.**

1-The Lloyd-Davies method (Rational Method) 2-The Modified Rational Method (MRM)

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**LLOYD-DAVIES METHOD (RATIONAL METHOD)**

This method uses: the rational formula to calculate runoff the Ministry of Health formula (MOH equations) to calculate rainfall.

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THE RATIONAL FORMULA A = Total area of the catchment, ha

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**THE MOH EQUATIONS TO CALCULATE RAINFALL**

where, i = Intensity of rainfall in mm/hr T = Duration of storm (is taken to be the time of concentration)

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TIME OF CONCENTRATION The duration of storm is taken to be the time of concentration of the area i.e. the time that any discharge of water takes to travel from one end of the catchment area to the outlet: Where, Te is the time of entering i.e. the time it takes for the droplet of rain to entry of the sewer. Tf is the time of flow i.e. the length of the pipe divided by the velocity Tf=L/V

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**ASSUMPTIONS OF LLOYD-DAVIES METHOD**

a) The rainfall intensity is constant throughout the storm duration and the frequency is once per year. b) The impermeable area is evenly distributed around the sewer. c) The impermeabilities of all surfaces remain constant throughout the storm and during subsequent periods of runoff. d) The velocities of flow in sewer are complete bore velocities.

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**STEPS IN LLOYD-DAVIES METHOD**

1- Find or assume a gradient for each pipe 2- Assume a pipe diameter 3- Estimate Te and find Tf =L/v 4- Find i (rainfall intensity) from MOH equations 5- Find Ap contributing to sewer 6- Find Q (runoff) from the rational formula 7- Check that diameter suits Q, if not go back to step no. 2

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**Hydraulic design of sewer**

Select pipe material Formula of Manning-Strickler Q = Ks S RH2/3 I1/2 Q : Discharge in m3/s ; S : Wetted area (m2) ; RH: Hydraulic radius(m); I : pipe slope ; Ks : rugosity coefficient (Ks=1/n with n Manning coefficient) ;

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Circular pipe Check that diameter select can transit maximum discharge generated by catchment area (Qp) Minimum commercial Diameter = 300mm Check self-cleansing conditions (Vmin = 0,6 m/s Vmax=4 m/s)

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Example 1 A storm sewer is proposed to drain a 12 hectares drainage area shown in the figure below. With given data in the table below determine the design discharge. Site Area (ha) C Time of concentration (min) A 4 0.8 10 B 8 0.5 30

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EXAMPLE The projected plan for a small rainstorm water drainage scheme is shown below. Assuming a worldwide time of entry of 6 minutes, design the pipe sizes using the rational (Lloyd-Davies) method. Sewer Pipe Impermeable Area In hectares, Ap(ha) Length of flow, L(m) Gradient 1: A 0.35 110 70 B 0.22 65 C 0.25 80 D 0.19 68 E 0.28 73 55

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Application Design storm sewer for the three pipes draining the above catchment area. Check self-cleansing condition.

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**5.Determination of invert level**

invert level : The pipe invert level is the level of the inside bottom of the pipe. Upper Invert Elevation=Ground surface – depth of cover – pipe wall thickness – pipe diameter. Lower Invert Elevation= Upper Invert Elevation-(Slope of sewer)x(Length of sewer) Minimum depth of cover is 1 to 2 m

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Example Upper Invert Elevation=Ground surface – depth of cover – pipe wall thickness – pipe dia = m m m m =17.5 m Lower Invert Elevation= Upper Invert Elevation-(Slope of sewer)x(Length of sewer) m ( m/m) x (707 m) = m Check: Depth of Cover Adequate/ Not adequate ? =19.00 m – (16.23 m m m) = 2.27 m OK If Depth of Cover Not adequate / too shallow Two alternatives: Repeat with a lower invert elevation, or A steeper slope

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**vertical profile of sewer line: Application**

The figure below present horizontal layout of foul sewer network. Four gravity pipes are lying starting from Manhole 1 to manhole 4. Manhole ground elevations for MH1 to MH4 are given respectively m, 237m, 236.6m and 235 m. Distances between manhole are respectively 50m, 20m and 50m. Pipe 1 is bounded by MH1 and MH2 have a diameter of 200 mm and a slope of 2%. Pipe 2 is bounded by MH2 and MH3 have a diameter of 200 mm and a slope of 2%. Pipe 3 is bounded by MH3 and MH4 have a diameter of 250 mm and a slope of 2%. Calculate inert elevation for sewer line from Manhole 1 to 4. Check cover then plot vertical profile of the same sewer line.

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