Basic sewer design

The internal diameter of the sewer should not be less than 15 cm. Small sewers require more care than large sewers. The velocity due to which both floating and heavy solids are easily carried with the flow is called self-cleaning velocity. The recommended self-cleaning velocity for Indian conditions is 0.75 m/s to prevent sewage from accumulating at the bottom or on the sides of a large sewer. The slope of the sewer line should be in line with the flow velocity. Therefore, large sewers should be laid on flat slopes. The slope should be such that the flow velocity is not too high or too low and is within the permissible limits.

Basic sewer design



Methods of the sanitation

The sanitation of a town is done by the following two methods :

1. conservancy methods or dry type method.

2. water carriage system method.

A city's waste type products are collected and transported to be treated and disposed of by these methods. In the conservancy or dry line method, different types of refuse are collected, carried and disposed off separately. In the water carriage system, water is used as a medium to convey the sewage to the point of treatment or disposal. This method is more hygienic, but the initial cost for the construction of the system and the maintenance cost is high. There is no smell or sight nuisance in the water carriage method and the sewers being all underground do not impair the beauty of the place.


Sewerage System

We have already discussed that the process of collecting and delivering the sewage to the disposal point is called sewerage. The following are three systems of sewerage :

  1. the combined system.
  2. the separate system.
  3. Partially separate system.

1. The combined system : In a combined system of sewerage, only one set of sewers are laid for carrying sanitary sewage and storm water. This system is most suitable for the locality which has narrow streets. The maintenance cost of this system is very small. It also provides automatic flushing of sewers.

2. The separate system : In a separate system of sewerage, two sets of sewers are required. One for carrying the sanitary sewage and the other for carrying the storm water. This system is most suitable for rocky areas because laying of two small sewers is easy than one large sewer. It is also suitable when the topography is flat necessitating deep excavation for combined sewers.

3. partially separate sewerage system : In a partially separate system of sewerage, a small portion of storm water is allowed to enter in the sanitary sewage within permissible limits and the remaining storm water flows in separate set of sewers. This system is suitable by areas where storm water is small in quantity due to scarcity of rainfall. 

Classification of sewers

The sewers are classified as discussed below :

1. According to the construction material used, the sewers are classified as follows :

(a) Cast iron sewers

(b) Stone ware sewers

(c) Concrete or R.C.C. sewers

(d) Masonry sewers

(e) Asbestos concrete sewers

(f) Plastic sewer pipes.

2. According to the size of the sewers, sewers are classified as follows :

(a) Rectangular sewer

(b) Horse-shoe type

(c) semi-elliptical sewer

(d) Basical handle type 

(e) circular sewer 

(f) Egg-shaped sewer

Types of sewer joints

Joints are necessary to connect different pieces of sewer pipe to form a continuous length of sewer line. The sewer pipe joint are of the following types :

(a) Spigot and socket joint

(b) Collar joint

(c) Mechanical joint

(d) Bandage joint

(e) Flush joint

(f) Filled and poured type joints

For the sewers of diameters less than 600 mm, spigot and socket joint is preferred and for sewers of diameter more than 600 mm, a collar joint is used.