The sewage systems are classified as follows: 1. Combined System 2. Separate System 3. Partially Separate System.
When only one set of sewers is laid, carrying both the sanitary sewage and storm water, it is called combined system.
When the domestic and industrial sewage are taken in one set of sewers, whereas storm and surface water are taken in another set of sewers, it is called separate system.
In the 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.
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The combined system is most suited in areas having small rainfall which is evenly distributed throughout 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 be used in crowded areas, where it is very difficult to lay two sewers. The combined system can also be suited in area having less sewage, to obtain the self-cleaning velocity.
If rainfall 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 period of the 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.
Merits and Demerits of Separate System:
Following are the merits of separate system:
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(i) The sewage flows in separate sewer; therefore the quantity to be treated is small which results in economical design of treatment works.
(ii) Separate system is cheaper than combined system, because only sanitary sewage flows in closed sewers and the storm water which is unfoul in nature can be taken through open gutters or drains, whereas both types of sewage is to be carried in closed sewer in case of combined system.
(iii) During disposal if the sewage is to be pumped, the separate system is cheaper,
(iv) There is no fear of stream pollution.
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Following are the demerits of separate system:
(i) Generally self-cleaning velocity is not available, due to small quantity of sewage; therefore flushing is required at various points.
(ii) 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.
(iii) As two sets of sewers are laid, therefore its maintenance cost is more,
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(iv) In busy lanes laying of two sewers is difficult which also causes great inconvenience to the traffic during repairs.
Merits and Demerits of Combined System:
The following are the merits of combined system:
(i) There is no need of flushing, because self-cleansing velocity is easily available at every place due to more quantity of sewage.
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(ii) Rain water dilutes the sewage; therefore it can be easily and economically treated.
(iii) House plumbing can be done easily because only one set of pipes will be required.
The following are the demerits of combined system:
(i) Initial cost is high as compared with separate system.
(ii) It is not suitable for areas having rainfall for smaller period of the year, because the dry-weather flow will be small due to which self-cleaning velocity will not be available resulting in the silting up of the sewers.
(iii) If the whole sewage is to be disposed off by pumping, it is uneconomical,
(iv) During heavy rains, the overflowing of sewers will endanger the public health,
(v) In congested areas, it is easy to lay one large sewer than two smaller Sewers.
Comparison of separate and combined systems:
Separate System:
1. The quantity of sewage to be treated is less, because no treatment of storm water is to be done.
2. In the cities having more intensity of rainfall this system is most suitable.
3. If due to topographical feature the sewage pumping is required, this system is more suitable.
4. As two sets of sewer lines are laid, this system is cheaper, because sewage is taken in underground sewer and the storm water in open drains.
5. Less degree of sanitation is achieved in this system, as storm water is directly disposed of without treatment.
6. In narrow streets, it is difficult to use this system.
7. If the rainfall is throughout the year and intensity is less, this system is not suitable.
Combined System:
1. As the treatment of sewage as well as storm water is to be done, the treatment cost is high.
2. In the cities having less intensity of rainfall this system is more suitable.
3. In this case the cost of pumping is high, as storm water is to be pumped along with the sewage.
4. Over all construction cost is more than separate system.
5. High degree of sanitation is achieved in this system.
6. It is more suitable in narrow streets.
7. In such cases this system is most suitable.
Merits and Demerits of Partially Separate System:
Following are the merits of partially separate system:
(i) As it is an improvement over separate system, economical and reasonable size sewers are required.
(ii) The work of house-plumbling is reduced, because the rain water from roof, sullage from baths and kitchens 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 or drains.
(iii) No flushing is required, because the quantity of sewage is increased as small portion of storm water is allowed to enter in sanitary sewage.
Following are demerits of partially separate system:
(i) The cost of pumping is increased at disposal plants than separate system because a portion of storm water is mixed with sanitary sewage.
(ii) There are possibilities of over-flow, requiring storm-overflows.
(iii) In dry weather, the self-cleaning velocity may not develop.