Sewer Design

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TABLE OF CONTENTS Introduction 2 • Sewer 3 • Types of Sewer • Sewage • Sewage system 5 • Types of Sewer System 7 • Components of Sewer System 7–9 ○ Sewer 7 ○ Manhole 7 ○ Drop Manhole 8 ○ Pumping Station 9 • Design of Sewer System 12 - 15 • Design Procedure 16 - 17 • Design Data 18 • Conclusion 19 • Comments • Reference • Design of Wet Well 20-21 •

3-4 4

19 19

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INTRODUCTION We use water supply system in order to provide easy availability of water for drinking , washing etc. when this water is used it becomes waste water called "sewage". Proper system is required for the collection of waste water and conveying it to the point of disposal with or without treatment called as "sewerage system".

ABOUT SCHEME: On the east side it has Upper Chenab canal and Jhelum road. It has allocation of two parks, graveyard as well. In the east direction there is a treatment plant and disposal station. Maximum reduce level is 100.8 and minimum is 98.0. Name of the colony: FUTURE VISION HOUSING SOCIETY (UET LAHORE) Total # of residence covered: PLOTS

281

APPARTMENTS

3

FLATS

3

MISCELL.

SCHOOL,DISPENCERY,PARK

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SEWER: A pipe or system of pipes used to remove human waste and to provide drainage. OR A large, underground pipe or drain used for conveying waste water and sewage. The Local Authority is usually responsible for the sewers, which collect the effluent from various drains, the drains being the responsibility of the land owners.

TYPES OF SEWER: • A Sanitary Sewer (also called a foul sewer) is a type of underground carriage system, (the 'system of sewers' is called sewerage), for transporting sewage from houses or industry to treatment or disposal. In some areas, sanitary sewers are separate sewer systems specifically for the carrying of domestic and industrial wastewater, and are operated separately and independently of storm drains, which carry the runoff of rain and other water which wash into city streets. •

A Storm Sewer is designed to drain excess rain and ground water from paved streets,

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•

Ahsan Abid Kalim 06-CIVIL-103

parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems. A Combined Sewer is a type of sewer system that collects sanitary sewage and storm water runoff in a single pipe system. Combined sewers can cause serious water pollution problems due to combined sewer overflows, which are caused by large variations in flow between dry and wet weather.

SEWAGE: Sewage is water-carried wastes, in either solution or suspension that flow away from a community. Also known as wastewater flows; sewage is the used water supply of the community. It is more than 99.9% pure water and is characterized by its volume or rate of flow, its physical condition, its chemical constituents, and the bacteriological organisms that it contains. Depending on their origin, wastewater can be classed as sanitary, commercial, industrial, or surface runoff. The spent water from residences and institutions, carrying body wastes, washing water, food preparation wastes, laundry wastes, and other waste products of normal living, are classed as domestic or sanitary sewage. Liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes, are included in the sanitary or domestic sewage category if their characteristics are similar to household flows. Wastes that result from an industrial process or the production or manufacture of goods are classed as industrial wastes. Their flows and strengths are usually more varied, intense, and concentrated than those of sanitary sewage.

SEWAGE SYSTEM: -

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Sewage System transports sewage through cities and other inhabited areas to sewage treatment plants to protect public health and prevent disease. Sewage is treated to control water pollution before discharge to surface waters.

Collection: A sewage system may convey the wastewater by gravity to a sewage treatment plant. Where pipeline excavation is difficult because of rock or there is limited topographic relief (i.e., due to flat terrain), gravity collection systems may not be practical and the sewage must be pumped through a pipeline to the treatment plant. In low-lying communities, wastewater may be conveyed by vacuum. Pipelines range in size from pipes of six inches (150 mm) in diameter to concrete-lined tunnels of up to thirty feet (10 m) in diameter. Sewage can also be collected by low pressure pumps and vacuum systems. A low pressure system uses a small grinder pump located at each point of connection, typically a house or business. Vacuum sewer systems use differential atmospheric pressure to move the liquid to a central vacuum station. Typically a vacuum sewer station can service approximately 1,200 homes before it becomes more cost-effective to build another station.

Design and analysis of collection systems: Design and sizing of sewage collection systems considers population served, commercial and industrial flows, flow peaking characteristics and wet weather flows. Combined sewer systems are designed to transport both storm water runoff and sewage in

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the same pipe. Besides the projected sewage flow, the size and characteristics of the watershed are the overriding design considerations for combined sewers. Often, combined sewers cannot handle the volume of runoff, resulting in combined sewer overflows and causing water pollution problems in nearby water bodies. Separate sanitary sewer systems are designed to transport sewage alone. In communities served by separate sanitary sewers, another pipe system is constructed to convey storm water runoff directly to surface waters. Most municipal sewer systems constructed today are separate sewer systems. Although separate sewer systems are intended to transport only sewage, all sewer systems have some degree of inflow and infiltration of surface water and groundwater, which can lead to sanitary sewer overflows. Inflow and infiltration is highly affected by antecedent moisture conditions, which also represents an important design consideration in these systems. A sewer bed is a piece of land typically used by a municipality for the dumping of raw sewage. Usually raw sewage was brought by truck or drawn by horses to be dumped, but the practice stopped back in the 1940s.

Type of sewer systems: • Separate system: If the storm water is carried separately from domestic and industrial waste water. • Combined system: It is the system in which the sewer carries both sanitary and storm water. • Partially combined system:

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If some portion of storm water or surface runoff is allowed to be carried along with sanitary sewage. It is economical. We will design partially combined system.

Components of Sewage System: Sewer: A pipe or system of pipes used to remove human waste and to provide drainage.

Manhole: A manhole (alternatively utility hole, maintenance hole, inspection chamber or access chamber) is the top opening to an underground utility vault used to house an access point for making connections or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains and gas. It is protected by a manhole cover, also known as a 'biscuit', a plug designed to prevent accidental or unauthorized access to the manhole. They are vertical openings provided in sewerage system. Purpose: cleaning, inspection, house connection etc. Provision at:

change in sewer direction, diameter, slope, at the junction.

Manhole Spacing (WASA): Diameter Spacing

Drop Manhole:

225-375mm

Not >100m

450-750mm

Not>120m

>750mm

Not>150m

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When lateral or sub main join in a deeper sewer , excavation is saved by keeping the upper sewer at a reasonable grade and making the vertical drop at the manhole. It is constructed when the drop is more than 0.6m. Otherwise an ordinary manhole is built but the bottom is so arranged that the incoming sewage falls in a sloping channel without slashing.

Pumping stations: Pumping stations are facilities including pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems, such as the supply of water to canals, the drainage of low-lying land, and the removal of sewage to processing sites. A pumping station is, by definition, an integral part of a Pumped-storage hydroelectricity installation. They are used to elevate and transport waste water when: • • • • •

Continuation of gravity flow is no longer feasible. Basements are deep. Any obstacle lies in the path of sewer. Receiving stream is higher than the sewer. Sewage is to be delivered to an above ground treatment plant

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Infiltration: It is the waste water that enters sewers through poor joints, cracked pipes, walls and covers of man holes. Infiltration is almost nonexistent in dry weather but increases during rainy season. WASA uses the following infiltration rate for design of sewer system. Sewer dia 225mm to 600mm >600mm

infiltration 5% of average sewage flow 10% of ASF

INVERT LEVEL: The lowest inside level at any cross section of a sewer pipe is known as the invert level at that cross section. Sewer must be designed and laid at a specific slope to attain self cleansing velocities. The required slopes are achieved through calculation of invert levels of the sewer at various manholes. Invert level = NGSL/Road level-Depth of sewer-Thickness of sewer-Dia of sewer

Components of Sewage Pumping Stations: -

10 (i)

(ii)

(iii)

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Screens: used to screen out large floating matter that can damage pump. Dry Well: Used to house the pump. Wet Well: to receive wastewater.

General Design Consideration: (i) More than 1 pump should be provided to cope with available discharge, two pumps for small pumping stations and more than two for large pumping stations should be used out of which one is for min. flow, one is for avg. flow and one for max. Flow. (ii) Total pumping capacity of pumping station must be equal to the peak sewage flow. (iii) Stand by pump must be provided at the pumping station. Its capacity should be at least 50% of peak sewage flow. (iv) Alternate sources of power must be there at pumping station. (Either power from two different feeders or a diesel operated pumps). (v) Pumps should be of self priming type and should be of self priming type and should operate under positive suction head. (vi) Each pump should have individual intake. (vii) Screens with 50mm opening should be provided at pump station to avoid entrance of big particles in pumps. (viii) Size of dry well should be sufficient to house pumping machinery and for working.

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(ix) Dry wells are provided with sump pump which are usually reciprocating pumps to pump out sewage leaks in dry wells. (x) Sluice valve must be provided at suction and delivery side of pump and non-return valve at the delivery side (to reduce back hammer effect) (xi) Detention time in the wet well should not be more than 30min to avoid septic conditions.

DESIGN OF SEWER SYSTEM DESIGN CRITERIA: Design flow: Average sewage flow is calculated on the basis of water consumption and population. Average sewage flow Q (m3/c/d)

average water = 0.8 ×

consumption Qdesign = 2 peak factor Q + infiltration (10%) + storm water ×

×

(100% of peak flow) Design equation: Manning's formula is used for the sewer flowing under gravity. 2 1 V = × R 3 ×S n

Where; V = Velocity of flow , m/sec R = Hydraulic mean depth =

1 2

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When pipe is flowing or half full. S = Slope of the sewer

n = coefficient of roughness for pipes. (We use n=0.013 for RCC pipes)

Minimum self cleansing velocity: For partially combined sewer = 0.7 m/sec Maximum velocity: Not > 2.4 m/sec To avoid excessive abrasion, to avoid steep slope. Minimum sewer size: 225mm for lateral (WASA, PHED) To avoid choking of sewer with bigger size objects thrown through manholes. Minimum cover: 1 m earth covers on sewer crown. To avoid damage from live loads on sewer. Manholes: 1 manhole per 2 plots.

Plant Location: 1) General: - The following items shall be considered when selecting a plant site: a) Proximity to residential areas. b) Direction of prevailing winds. c) Necessary routing to provide accessibility by all weather roads. d) Area available for expansion.

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e) Local zoning requirements. f) Local soil characteristics, geology and topography available to minimize pumping. g) Access to receiving stream. h) Compatibility of treatment process with the present and planned future land use, including noise, potential odors, air quality, and anticipated sludge processing and disposal techniques. 2) Critical Sites: - Where a site must be used which is critical with respect to specific criterion appropriate measures shall be taken to minimize adverse impacts. 3) Flood Protection: - The treatment works structures, electrical and mechanical equipment shall be protected from physical damage by the maximum 100 year flood. Treatment works shall remain fully operational during the 25 year flood. This requirement applies to new construction and to existing facilities undergoing major modification. Flood plain regulations of State and Federal agencies shall be considered. 4) Plant Accessibility: - All plant facilities shall be accessible by all weather roads.

Quality of Effluent: The required degree of wastewater treatment shall be established by reference to applicable effluent criteria issued by the Division of Water Pollution Control for all projects involving new plants, new discharge locations or major upgrades.

Design: The goal of the preparers of this Design Criteria is to promote the simplest treatment scheme available that will meet the requirements of the permit while providing maximum ease of operation. While cost comparisons are important, long term operability and reliability should be an overriding influence in developing new sewerage collection and treatment works.

Type of Treatment: -

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1) As a minimum, the following items shall be considered in the selection of the type of treatment: a) Present and future effluent requirements. b) Location and local topography of the plant site. c) The effects of industrial wastes likely to be encountered. d) Ultimate disposal of sludge. e) System capital costs. f) System operating and maintenance costs and basic energy requirements. g) Existing unit process performance and capacity. h) Process complexity governing operating personnel requirements. i) Environmental impact on present and future adjacent land use. 2) The plant design shall provide the necessary flexibility to perform satisfactorily within the expected range of waste characteristics and volumes

DESIGN PROCEDURE: a)

Preparation of Hydraulic Statement: ➢ Find the present population of the project area. Then find the design population from the given design period. Afterwards find average sewage flow for the design population, select peak factor for you project area from given table. ➢ Draw the layout of the sewer system keeping in view the layout of the roads and streets (represent each sewer with a line and manhole with a dot). ➢ Number the manholes and identify each sewer line (like M2M2, M2M3 etc.). ➢ Allocate plots or area to each sewer line. ➢ Measure the length of each sewer line as per scale of your map. Also show direction of flow in sewer lines with an arrow. ➢ By adopting per capita sewage flow as 70% of water consumption, calculate average sewage flow and

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a)

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infiltration for each sewer line. For this design problem take infiltration as 10% of average sewage flow. ➢ Calculate peak sewage flow and finally the design flow for the sewer lines. ➢ Using the method of back calculation, find appropriate dia and slope for you sewer assuming that the sewer is flowing full. For back calculation choose a suitable design table with a suitable self cleansing velocity (0.6 m/sec). ➢ Use graph from the book of EW STEEL to find the depth of flow and actual velocity at design flow. ➢ If actual velocity and depth of flow are satisfactory then the dia and slope of the pipe are considered as final. If the velocity is less than self cleansing velocity then increase the slope of the sewer. ➢ In the end find the invert levels for the all the sewers and complete the table of calculations called “hydraulic statement”. (NOTE: a lot of care should be exercised in calculating the invert levels otherwise the whole scheme may fail due to incorrect levels) ➢ Draw the profiles or L-sections for all the sewer lines. Design of Pumping Station: Purpose: These are required to elevate and transport wastewater when: (i) Continuation of gravity flow is no more feasible and there is need to raise the HGL of sewer. (ii) Any obstacle lies in the path of sewer e.g. a river, canal etc. (iii) Receiving stream is higher than the sewer. Pumps for Sewage: Centrifugal, single suction non-clogging type pumps are normally used. These have impeller, having 2 or 3 vanes. Pump suction pipe is usually larger than the discharge pipe by about 25%. Smallest discharge pipe (3”)

=

75mm

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Smallest suction pipe

=

100mm (4”)

DESIGN DATA No. of Plots No. of Apartments No. of Flats Design Period

= 281 = 3 = 3 20 years

=

Present (2009)

At design (2029)

Persons/Plot

7

10

Persons/Apartment

400

600

Persons/Flat

200

400

Present Population = = 3767 Annual Population growth rate Density Population (2029) Pd

period

(7×28) + (400×3) + (200×3) = 1.98% (for Pakistan) = Pp [1 + 1.98/100 ]-20 = 5575

& from Table Pd = (10×281) + (600×3) + (400×3) Per Capita Water Consumption =

= 5810 (Use this value) Reg. No. + 300 lpcd = 403 lpcd

Average Sewage Flow = Pd × PCWC × 0.8/1000 m3/day = 0.02168 m3/sec Peak Factor = 4

=

1873

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CONCLUSION: • This sewerage system is designed keeping in view WASA criteria. • Minimum pipe size is 225mm. • Minimum cover is maintained as 1m throughout design. • We use RCC pipes. They are strong and long lasting. They are best to bear backfill loads. Giving high 3- edge bearing test strength value.

COMMENTS: • Velocity is taken as 0.7 m/s according to WASA criteria for partially combined sewer. • The diameter of pipes used in the design of the housing scheme should have been in the multiple of 75mm, but we did not use the diameters of pipes in multiples of 75mm, because these are not locally available. • In the Sewer Profile that we have drawn there is only one Drop Manhole i.e. M16

REFERENCE: • • •

Water Supply and Sewerage by EW Steel and McGhee. Design Aid (provide by the teacher). WASA design criteria.

Design of Pumping station • •

No of pumps = 2 Capacity of the Pump = 7492 m3/day

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Pump must run for at least 5-min Cycle time must not be less than 5 minutes but preferably 20 min. Peak Factor is 4

DESIGN OF WET WELL: Known data Avg. sewage flow, Qavg

= 1873 m3/day

Peak sewage flow, Qmax = P =

1.30 m3/min

=

7492 m3/day

=

5.20

m3/min V

=

tmin × P / 4

= 20 × 5.20 / 4

At Qavg, Running Time : t t t

= = =

t

=

26 m3

=

V/(P - Qavg)

V/ (P - Qavg) 26 / (5.20 – 1.30) 6.67 min

Cycle Time

>

5 min

=

(OK)

V/ (P-Qavg) + V/Qavg = 26 / (5.20-1.30) + 26 / (1.30)

=

26.7min > 20 min

Min. Cycle Time

= =

4 × V /P 4 × 26 / 5.20

= 20 min Detention Time

=

(OK)

t

(OK) = =

V/Q 26 / 1.3

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= Let Depth =

d

=

2.5 m

Consider wet well as circular, =>

=>

V

=

(π/4) × D2 x d

26

=

(π/4) × D2 ×

D

=

3.64 m

So, Depth of wet well = d = 2.5m Dia of wet well = D = 3.64m

2.5

20 min

(OK)