Sewerage System

Definition of some common terms

• Sewage

– Wastewater may be defined as a combination of the liquidor water-carried wastes removed from residences,institutions, and commercial and industrial establishmentstogether with groundwater, surface water, and storm water.

• Domestic sewage

– Liquid wastes coming from residential complex.

• Sewer

– A pipe or conduit which carries sewage.

• Sewerage

– Network of sewers used for the purpose of transportingsewage

Definition of some common terms

• Raw sewage

– Untreated sewage which is in fresh state

• Sullage

– Liquid wastes flowing from kitchen and bathrooms.

• Drainage

– The run-off resulting from the rain storms was called to bestorm sewage, but the modern approach is to call it stormdrainage or simply drainage.

Systems of sanitation • Conservancy system (dry system)

– Different types of wastes (dry or wet, foul or non-foul) are manuallycollected separately and transported over vehicles to the outskirts ofthe city and disposed off by composting method.

– Disadvantages

• Health hazards to the personnel handling the sewage

• Nuisance condition prevail while collecting and transporting

• It is inhuman practice and social injustice

• Water carriage system– The wastes from water-closets (WC) are collected by flushing with

water and this water is used to transport the wastes to treatment worksby a system of pipes.

– Advantages

• Hygienic but costly

• Waste strength is reduced by dilution and the waste becomes moreamenable for treatment.

Wastewater Treatment System

Wastewater Generation

Wastewater Collection

& Conveyance

Wastewater Treatment

Safe Disposal of

Treated Water

Systems of sewerage and their lay-outs

• Sewerage systems

– They are classified mainly into three types

• Separate system• Combined system• Partly separate system

Separate system

• Two separate sets of sewers are installed, one for sanitary sewage and other for

storm water.

• This system is desirable on the following conditions

– When it is required to treat only sanitary sewage and storm-water is not treated

– When the topography is flat which necessitates deep-cutting if combined sewers

are used

– Frequency and intensity of storms are not of high magnitude, where storm water

can be collected by surface drains

– Sewer laying requires rock-cutting, where cost of cutting is more for large

combined sewers

– When the financial position does not permit to have large combined sewers

– When there is a chance of combined sewers backing-up the flow into the house

sewers

Separate system …….

• Merits

– Size of sewer requires less

– Less harmless storm water can be disposed-off by dilution

– In a combined system, there is a chance of pollution of stream where overflow weirs are used. In a separate system, there is no such possibility

– Loading on treatment plant is less

– Capacity of pumps and their running cost is reduced

• Demerits

– Self-cleaning velocity cannot be attained during small quantity of dry weather flow (DWF)

– In the absence of favorable gradients for the sewer, flushing tanks are to be installed for cleaning

– Initial cost is high, when two separate sets are used

– Maintenance cost of sewer is high

Combined system

• When both sanitary sewage and storm water are carried in a single sewer, it is called a combined system.

• Combined system is used in the following circumstances

– When both the sanitary sewage and the storm water require pumping

– When density of population is so high, the space may not be available to have two separate sewers

– When the storm sewer exists already, sanitary sewage also can be admitted in the same sewer provided its quantity is small compared to storm water

– When the rainfall is evenly distributed during the whole year

Combined system ………..

• Merits

– Degree of complication and expense is reduced for house plumbing of the individual

– Both domestic sewage and storm water can be carried in a single sewer, so construction cost is less

– Sanitary sewage strength gets reduced because of dilution of storm water

• Demerits

– Size of sewer is large

– Excavation depth is more for self-cleaning velocities during DWF

– Pumping cost is more and treatment plant size becomes larger

– During heavy rains, there is a possibility of back-flow into the house plumbing fixtures.

Partly separate system

• In this system, in addition to the sanitary sewage, the initial washings from the drainage area due to rains is included in a common sewer.

• Merits

– House drainage connections becomes simplified

– Decomposition of sewage solids in sewer does not occur

– Sewers are of reasonable size.

• Demerits

– During DWF deposition takes pace in sewers

– As initial storm water is included, the cost of pumping and disposal units size increases

– It requires the construction of storm-water overflows.

Patterns of sewerage lay-out

• The network of sewers can be laid in different patterns, depending on the topography and development pattern of a city or town. The types of patterns are as follows

• Perpendicular pattern

• Interceptor pattern

• Zone pattern

• Fan pattern

• Radial pattern

• The factors which determine the pattern of sewerage are as follows

• Type of system (combined or separate)

• Street lay-out

• Topography and hydrological characteristics of the area

• Location of treatment works

Patterns of sewerage lay-out

Perpendicular pattern Interceptor pattern

Zone pattern

Fan pattern

Radial pattern

Patterns of sewerage lay-out

• If storm water drains are laid through shortest route to the existingsurface water courses, it is called perpendicular pattern.

• The natural water courses may be overflowed during flood season,which results back flow of wastewater in sewers, when sewers emptydirectly into a river. Then the sewers will be under pressure andsurrounding area may be flooded, when the sewers burst. So to avoidsuch back-flows, an interceptor sewer is laid to collect the sewagefrom all the main sewers and transport it either to a treatment plant ornatural water course at a downstream point. Also, when the flow inthe diluting water course is minimum, then entire sewage cannot beemptied directly into the river.

Patterns of sewerage lay-out

• If the area is of different elevations, then the entire area is divided into zones of different elevations.

• In fan pattern, the flow is collected from outward to inward, to a central trunk-sewer. This is used when the city is located in a valley.

• In radial pattern, the wastewater is collected from heart of the city to outside periphery by two or three trunk-sewers, where they are laid radially to the area to be served. This system is suitable when the city is developed in two or more directions in an elongated pattern from the heart of the city

Sewer materials

• Asbestos cement

• Plain or reinforced cement concrete

• Vitrified clay or stoneware

• Brick

• Cast iron

• Steel

• Plastic

Factors to be considered for selection of sewer material

• Hydraulic efficiency

• Resistance to abrasion

• Resistance to corrosion

• Strength

• Durability

• Cost

• Weight

• Imperviousness

Sewer sections

• Circular

• Egg shaped

• Horse shoe

• Parabolic section

• Semi elliptical section

• Semicircular section

• U-shaped section

• Basket handle section