Design of Sewerage System and allied work

By Akshat Sudeshwar (2K13/EN/004)Siddhant Sunil Khaparde(2K13/EN/053)Swetank Sinha (2K13/EN/056)Vinay Prabhakar (2K13/EN/058)

Objectives

Review of design criteria of existing sewerage system

Design of sewerage systemCost estimation of sewerage system as

per Delhi Scheduled of Rates 2013 (DSR)

NECESSITY OF SEWERAGE SYSTEM

To transport water or waste water from one location to another by means of connecting pipes through gravity.

To provide a hygienic and healthy environment.

Increases the aesthesis and property value.

Types of Sewerage System• Combined System :- The drainage is taken

along with sewage

• Separate System :- The drainage and sewage are independently of each other through two different sets of conduits.

• Partially Separate System :- A part of drainage water is allowed to be admitted in sewers and similarly sometimes domestic sewage is allowed to admitted into the drains.

Classification of Sewage System

• Sanitary Sewers:-Designed to receive domestic sewage and industrial waste excluding storm water.

• Storm Sewers:-Designed to carry off storm water and ground water but excluding domestic sewage and industrial wastes.

• Combined Sewers:-Designed to receive sewage, industrial waste and storm water.

Why Combined System is normally not recommended?

• Sluggish flow during non-stormy days

• Leading to deposition of sewage

• Solids causing foul odours

• Increased cost of eventual sewage treatment or pumping cost, associated with disposal of sewage.

Design of Sewers

• Estimation of sanitary sewage

• Estimation of storm water runoff

• Hydraulics of sewers

• Design of sewer system

Estimation of Sanitary Sewage

• Per capita Sewage flow

• Flow Assumptions

• Ground water infiltration

• Effect of industrial waste

Per capita Sewage Flow

• Domestic sewage is a part of city sewage which means waste water that is discarded from households.

• There are 35 metropolitan cities (with more than 10 Lac Population) generates 15,644 Millions Litre per Day (MLD) of sewage. The treatment capacity exists for 8,040 MLD i.e. 51% is being created. In Madhya Pradesh Indore generates 204 MLD sewage, Bhopal generates 334.75 MLD, whereas Jabalpur contributes 143.34 MLD, but as far as treatment of sewage is concerned Indore treats 78 MLD (38%) and Bhopal treats 22 MLD (6%) of total sewage generation while Jabalpur did not have any facility of water

• Cities around Ganga basin are generating 2637.7 MLD sewage but are in a position to treat 1174.4 MLD i.e., 44.2%. The remaining sewage goes off in Ganga river without treatment, that pollutes the Ganga.

Sources of Water Quantity (MGD)

Yamuna River 310

Ganga River 240

Bhakra Storage 140

Expected Increase due to saving of losses via parallel lined canal and recycling backwashed water in filters of various WTPs

130

Sub Total 820

Ranney Wells/ Tube Wells (G.W.) 100

Total 920

Ground Water Infiltration

• Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier. It is related to the saturated hydraulic conductivity of the near-surface soil. The rate of infiltration can be measured using an infiltrometer.

Effect of Industrial Waste

Causes & Effects

Causes

• Effluents discharge into rivers

• Oil spills, garbage, sewage water

Effects

• Disturb ecosystem• Loss of aquatic life• Spread of Diseases

Estimation of Storm Water Runoff

• Rational method

Q=K*A*I*R Here Q= runoff or storm water flow K= constant A= catchment area in hectares R= intensity of rainfall in mm per hour I= impermeability factor.

Hydraulics of Sewers

1. Chezy’s Formula

V=c(rs)^0.5

Where, V= velocity of flow in channel in m/sec

r=hydraulic mean radius of channel

s=hydraulic gradient

c= Chezy’s constant

a. Kutter’s formula

c=(((23+(0.00155/s)+(1/n))/(1+(23+(0.00155/s)).(n/(r)^0.5)))

Where n=Rugosity coefficient depending upon the type of channel

surface.

2. Manning’s Formula

V=1/n*R^(2/3)*S^(1/2).

where, n = manning’s coefficient (the Value of manning’s coefficient n varies from 0.01 to 0.05 depending upon material of sewer line)

By comparing Chezy’s & Manning formula

C=1/n*(R^1/6)

3) 3) Hazen–William’s formulaHazen–William’s formula : : V=0.85*H*(R^0.63)*(S^0.50).V=0.85*H*(R^0.63)*(S^0.50).

where , H is Hazen- William’s coefficientwhere , H is Hazen- William’s coefficient

(the value of H varies from 100 to 150(the value of H varies from 100 to 150

depending upon sewer line’s material)depending upon sewer line’s material)

Minimum Velocity of Flow

• Minimum or self cleansing velocity is the

minimum velocity required to prevent

deposition & clogging of sediments.

• It depends upon the shape, size & specific

gravity of particles.

• Required at least once in a day.

Maximum Velocity of Flow

The maximum velocity at which no scouring

action or abrasion takes place is known as non

scouring velocity

Such a velocity depends upon the material

used for the construction of sewers

Material of sewerMaterial of sewer Non scouring velocity(cm/sec)Non scouring velocity(cm/sec)

Earth channelsEarth channels 60-12060-120

Ordinary brick-lined sewersOrdinary brick-lined sewers 150-250150-250

Cement concrete sewersCement concrete sewers 250-300250-300

Cast iron sewer pipesCast iron sewer pipes 350-450350-450

Vitrified tile and glazed bricksVitrified tile and glazed bricks 450-500450-500

Forms of Sewers

• Circular Sewers

• Non-Circular Sewers

Rectangular

U-Shaped

Horse-shoe

Parabolic

Sewer Appurtenances

Manhole Drop Manhole Flushing Tank Lamp hole Vent shaft Inverted Siphon Storm Water Inlets /Street Inlets Cover and Frames Catch Basin

Manholes

• A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber, sewer hole or confined space) is the top opening to an underground utility vault used to house an access point for making connections, inspection, valve adjustments or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains, district heating and gas.

About Manholes

• Manhole closings are protected by a manhole cover, a flat plug designed to prevent accidental or unauthorized access to the manhole. Those plugs are traditionally made of metal, but may be constructed from precast concrete, glass reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern).

• Manholes are usually outfitted with metal, polypropylene, or fiberglass steps installed in the inner side of the wall to allow easy descent into the utility space. Because of legislation restricting acceptable manual handling weights, Europe has seen a move toward lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.

Composite Manholes

• Composite (fiberglass) manholes are commonly used in applications where infiltration, exfiltration, or corrosion by hydrogen sulphide (from sewer gas) are a concern, or where structures need to be factory integrated into a manhole before placement.

• Structures commonly integrated into composite manholes include:– Flow inverts– Flumes

• Drop structures from higher elevation flows to lower elevation discharge pipes

• Occasionally, composite manholes will integrate:– Weirs– Storm water screening structures– Sewage grinders

• Energy absorbing structures to dissipate undesirable flow stream turbulence or velocity

Flushing Tank

• A flush tank is a tank which holds fluid in reserve for flushing. The classic example of a flush tank is the tank attached to a toilet. Flush tanks allow for the storage and release of a large volume of water. In addition to being used for toilets, they are sometimes attached to equipment which needs to be cleaned by flushing, and may be used to hold reservoirs of water for dishwashers and similar types of devices.

Sewer Materials

• Pipe is made from many different materials depending upon the properties of the media that is being transported. For example, pressure is used to transport potable water to it's end user.

• So, a pipe that has high pressure rating (typically greater than 150 psi) must be used. It would also be desirable for the pipe to have a smooth interior to promote good flow.

• While many different types of pipe have these qualities, the most common for use in potable water distribution are PVC Pipe, Ductile Iron Pipe and HDPE Pipe.

• For larger diameter applications, Precast Concrete Pressure Pipe (PCPP) and Spiral Welded Steel Pipe are practical. These are just typical materials that are used...when designing a project, a civil engineer will carefully review all of the practical options and make a selection that is then incorporated into his design.

Important Factors Considered for Selecting Material for Sewer

a. Resistance to corrosion

b. Resistance to abrasion

c. Strength and durability

d. Weight of the material

e. Imperviousness

f. Economy and cost

g. Hydraulically efficient

Design Considerations as per Manual on Sewage and Sewage treatment by CPHEEO

MINIMUM GRADIENT OF SEWER

• Minimum gradient for sewers shall be designed to fulfill the requirement of self cleansing velocity

Flow Variation

Design Calculation

Details about the system

Calculation of Quantities

Cost Estimation