When the system of sewers is laid separately to carry domestic and industrial sewage in one set of sewers and the storm water in another set of sewers, it is called separate system.
When one set of sewers provided to carry domestic and industrial sewage is also allowed to carry a portion of storm water is carried in a separate set of sewers, this system is called partially separate system.
It is said to be a combined system of sewerage when only one set of sewers is laid to carry both both the domestic and industrial sewage and also the storm water. The combined system is used in areas which have a small rainfall and that too scattered evenly throughout the year. In such places self cleaning velocity will be available almost in all the sections. This system is also employed in the towns where quantity of the sewage is small.
In the areas where there is heavy rainfall during the rainy season, it is advisable to provide the separate system because the self cleaning velocity shall not be available for most of the period of the year.
Sewerage is the system by which sewage is carried. It may be classified into the following three types:
1. Separate System
Merits and Demerits of separate system are as follows:
As two separate sewers are provided to carry sewage and storm water, the quantity of sewage to be treated at treatment plant is small due to which design of treatment works can be made economical.
The separate system is generally cheaper due to the fact that only sanitary sewage is required to be taken in closed sewer. The storm water can be taken in open drains which are constructed with much less cost.
There is no fear of stream pollution in the separate system.
If only the sewage is to be pumped during disposal, this system is cheaper.
As separate set of sewers is laid for sewage and storm water, therefore its maintenance cost is more.
Due to small quantity of sewage in the sewer, it is difficult to get self cleaning velocity. Therefore flushing of sewer is required for its cleaning.
There is always a possibility of storm water to get mixed with sewage sewer which may cause overflowing of sewer and making heavy load on the treatment plant.
The two sewers laid in a lane may create a great inconvenience to the public at the time of its maintenance.
2. Partially Separate System
Merits and Demerits of the partially separate system:
It is an improved storm of separate system. The size of sewer can be designed as per requirements and economically.
The work of house plumbing is reduced because rain water from roofs, sullage from bathrooms and kitchen is allowed to mix with sewage in the same sewer which carries the discharge from water closes but the water from all other places can be taken in separate sewer or drain.
As the small portion of storm water is taken in the same sewer which carries sewage due to which self cleaning velocity is obtained and hence no flushing of the sewer is required.
As the portion of storm water is taken with the sewage, therefore the cost of pumping is increased than separate system.
In dry weather, there is every possibility that self cleaning velocity may not develop in the sewer.
The possibility of overflow is also not ruled out in partially separate system.
3. Combined System
Merits and Demerits of combined system:
In combined system self cleaning velocity is easily available at all places in sewer. So there is no need of flushing the sewer.
The sewer is diluted by rain water due to which it can be treated easily and economically.
House plumbing can be easily done because only one set of pipes is required to be laid.
In congested areas, it is easy to lay one large sewer than two separate sewers.
Combined system has a high initial cost.
As the quantity of sewage is increased in this case, therefore pumping of it is not economical.
The areas which have a rainfall for the smaller period of the year, the combined system is not suitable at such places because it shall not be possible to develop self cleaning velocity during dry weather as the flow will be small which shall result in silting of the sewer and hence flushing of the sewer shall be necessary.
During the period of heavy rains, the overflowing of sewers may endanger the public health.