INFRASTRUCTURE PLANNING

SEWER SYSTEMS LAYOUT AND DESIGN

Swerage:

• The entire science of collecting and carrying of sewage by water carriage system through sewers is known as sewerage.

• SEWER SYSTEMS:

• 1.Combined system• 2.Separate system• 3.Partially separate system

1.Combined system

• When one set of sewers is laid carrying both the sanitary sewage and storm water, it is called a combined system.

• Merit: There is no need of flushing because self-cleaning velocity is easily available at every place due to more quantity of sewage.

• Demerit: During heavy rains, the over flow of sewers will endanger the public health.

2.Separate system

• When the domestic and industrial sewage are taken in one set of sewers where storm and surface water are taken in another set of sewers, it is called as separate system.

• Merit: The sewage flows in a separate sewer, therefore the quantity to be treated is small which results in economic design of treatment works.

• Demerit: There is always risk that storm water may enter the sanitary sewer and cause over flowing of sewer and heavy load on the treatment plant.

3.Partially separate system

• In separate system , if a portion of storm water is allowed to enter in the sewers carrying sewage , and the remaining storm water flows in separate set of sewers, it is called partially separate system.

• Merits: The work of house-plumbing is reduced because the rain water from roof, sullage from baths and kitchen etc can be taken in the same pipe carrying the discharge from the water closets. The water from all other places can be taken in separate sewer drains..

• Demerit: The cost of pumping is increased at disposal plants than separate system because the portion of storm water is mixed with sanitary sewage.

• .

• Pseudo separate or partially separate system

Network and layout of sanitary sewer systems

• The most common patterns of sewer collecting systems are:

• 1.Perpendicular pattern• 2.Intercepter pattern• 3.Radial pattern• 4.Fan pattern• 5.Zone pattern

Perpendicular pattern

• In this pattern sewers carrying storm water are laid in such a way as to seek the shortest possible path to the natural water courses, it can be obtained if the sewers are laid perpendicular to them.

• This pattern is suitable for separate system and partially separate system in which storm water can be directly disposed off without ant treatment.

• It is not suitable for combined system because • firstly it is very difficult to treat the sewage due to large

number of outlets and • secondly it pollutes the water of natural courses.

Interceptor pattern

• This pattern is an improvement over the perpendicular pattern.

• In this pattern sewers are intercepted by a large size sewer, which is laid all along the water carrying sewage to a common point, where it can be disposed of with or without treatment.

Radial pattern• This type of pattern can be employed if the sewage is

to be disposed off on land around the town.• In this pattern large number of outlets are provided.• Sewers are laid radially outwards from the center of

the city therefore it is called radial pattern.• In this pattern the suburbs can be served by the

relatively small and short lines of sewers which make it economical.

• The main disadvantage of this system being large number of disposal works.

FAN PATTERN• If the city is situated near the river, which is one one side of it

only, the sewer can be laid in such a way that the whole sewage flows to a common point where one treatment plant is located.

• In this pattern number of converging main sewers are laid, which form a fan like shape.

• The only advantage of this system being single unit of treatment work.

• the drawback of this pattern is the if outlying suburbs develop, it will increase the increase the load on the treatment works restricting the development.

Zone pattern

• In the intercepter pattern only one single large size intercepting sewer is used to collect and convey the entire sewage, due to which it is over-loaded.

• This over loading can be removed by providing more number of intercepters of each zone.

• This type of pattern is more suitable to sloppy areas as hills than flat areas.

PROCEDURE OF PLANNING

The combined system is most suited in areas having small rainfall which is evenly distributed throught the area, because at such places self cleaning velocity will be available in every season.

As only one sewer is laid in this system, therefore it can also can be used in crowded areas, where it is difficult to lay two sewers.

The combined system can also be suited in area having less sewage, to obtain the self cleaning velocity.

PROCEDURE OF PLANNING

If rain fall is heavy and it is for short time, it is better to provide separate system, because in combined system self cleaning velocity will not be available for most of the periods of years.

In rocky areas it is more expensive to lay one large sewer than two small sewers.

Separate systems should be provided if the sanitary sewage is to be pumped.

THANK YOU

• REFERENCES: Water Supply and Sanitary Engineering

• By G.S.Birdie J.S.Birdie