There are various patterns of collecting systems, but the following are most common: 1. Perpendicular Pattern 2. Intercepter Pattern 3. Radial Pattern 4. Fan Pattern 5. Zone Pattern.
1. 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. The shortest possible path to the natural water courses 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 of without any 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.
2. Intercepter 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.
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If the quantity of storm water is very large, overflows should be provided as shown in Fig. 2.2 allowing the excessive sewage to spill over into natural water courses through outlets, which were existing before the interception.
3. 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. The sewers are laid radially outwards from the centre of the city; therefore, this is called as radial pattern. In this pattern the suburbs can be served by relatively small and short lines of sewers which make it economical. The main disadvantage of this system being large number of disposal works.
4. Fan Pattern:
If the city is situated near the river, which is on 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 fanlike shape, from which it derives its name.
The only advantage of this system being single unit of treatment work. But the diameter of the sewer increases as the sewage moves, due to which the diameter of the main trunk is very large, increasing the over-all cost of this pattern. The second drawback of this pattern is that if the outlying suburbs develop, it will increase the load on the treatment works, restricting the development.
5. 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 as shown in Fig. 2.5. This type of pattern is most suited to sloppy areas as hills than flat areas.
