The sewerage systems or water carriage systems are of the following three types: 1. Separate System 2. Combined System 3. Partially Separate System.
Type # 1. Separate System:
In this system two sets of sewers are provided-one for carrying domestic or sanitary sewage and industrial sewage, and the other for carrying storm water (or rain water). The sewage from the first set of sewers is carried to the treatment plant, and the storm water (or rain water) from the second set of sewers is directly discharged into a natural stream or river without any treatment.
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Advantages of Separate System:
The separate system has the following advantages:
(i) The quantity of sewage to be treated being small the treatment works of smaller size would be needed and also the load on the treatment units will be less.
(ii) The storm water (or rain water) is not unnecessarily polluted and hence it can be discharged into natural stream or river without any treatment.
(iii) If pumping is required for lifting of sewage at the treatment works, the system will prove to be economical both from the point of view of capital costs as well as from the point of view of running costs.
(iv) The sewers being of small size are economical. Further storm water (or rain water) may be carried through open or closed drains at or near the ground surface, consequently the cost of installation of the system would be low.
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(v) Sewers of smaller section can be easily ventilated as compared to those of larger section.
(vi) Sewage of more or less uniform characteristics flows through the sewers which will facilitate the treatment process.
Disadvantages of Separate System:
The separate system has the following disadvantages:
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(i) The sewers being of small size their cleaning is difficult.
(ii) The sewers are likely to get choked.
(iii) Unless laid at a steep gradient, self-cleansing velocity in the sewers cannot be assured and flushing shall have to be done. This may prove unsatisfactory and expensive.
(iv) The system requires two sets of sewers and hence it may prove to be costly.
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(v) Maintenance costs of two sets of sewers are greater than that for one.
(vi) The sewers or drains provided for carrying storm water (or rain water) come in use only during the rainy season. During other part of the year these may become the dumping places for garbage and may thus get choked.
(vii) Two sewers or drains in a street lead to greater obstruction to traffic while repairs of any one of them are being carried out.
(viii) In sewers of small size there being lesser air contact foul smell may be produced due to the formation of sewage gases.
(ix) Double house-plumbing would be required for making separate connections to two sets of sewers or drains. Moreover, there is a likelihood of wrong connections being made on account of which storm water (or rain water) may enter the sewer or drain meant for carrying sewage and thus cause overflow of sewage.
Conditions Favourable for Separate System:
The separate system is favoured under the following conditions:
(i) Uneven Rainfall:
When rainfall is uneven or it is concentrated for a short period during the year, it becomes economical to adopt separate system.
(ii) Separate Outlets for Sewage and Storm Water (or Rain Water):
The separate system can be justified when domestic or sanitary sewage and industrial sewage is to be collected and conveyed to a particular point for treatment and there is a separate outlet in the form of a natural stream or river for the disposal of storm water (or rain water).
(iii) Pumping Requirement:
When it is necessary to pump domestic or sanitary sewage and industrial sewage, adoption of this system will reduce the load on pumps.
(iv) Limitations of Available Funds:
If sufficient funds are not available in the beginning, sewers may be constructed to carry only domestic or sanitary sewage and industrial sewage, and the storm water (or rain water) may be conveyed through open drains. These drains can be converted into regular sewers later when additional funds are available.
(v) Flat Topography:
If the area is flat, the sewers will have to be laid at a certain depth below the ground surface in order to achieve the required gradient. In such cases separate sewers are more economical, because laying of a combined sewer of larger size at a greater depth below the ground surface will be costly.
(vi) Gradient of Sewers:
If it is not possible to lay sewers at suitable gradients, there is a danger of backing up of sewage into the houses. Under such circumstances, it is desirable to adopt separate system.
(vii) Subsoil Condition:
If the subsoil is hard, it would be difficult and costlier to lay combined sewer which is usually of large size. Thus in such cases separate system would be preferable.
(viii) Steep Topography:
If the area possesses steep slopes, it would be easier to convey storm water (or rain water) through open drains to the natural stream or river, and hence separate system may be adopted.
(ix) Time of Laying Sewers:
If sewers are to be laid before the area is developed, it is desirable to adopt separate system.
(x) Conversion of Existing Sewer:
It is not desirable to convert the existing sewer for carrying domestic or sanitary sewage and industrial sewage into a combined sewer because it becomes costly and it is also inconvenient. In such a case it is better to lay a separate sewer for storm water (or rain water).
Type # 2. Combined System:
In this system only one set of sewers is provided for carrying domestic or sanitary sewage and industrial sewage as well as storm water (or rain water). Thus in this case sewage and storm water (or rain water) are carried to the sewage treatment plant before its final disposal.
Advantages of Combined System:
The combined system has the following advantages:
(i) Since this system requires only one set of sewers the maintenance costs are reduced and hence it may prove to be economical.
(ii) The sewers being of larger size the chances of their choking are less and also it is easy to clean them.
(iii) The strength of the sewage is reduced by dilution due to storm water (or rain water). This helps to make the treatment process easier and more economical.
(iv) In this system automatic flushing is provided be the storm water (or rain water).
(v) This is a relatively simple system of collection of sewage and also in this system house plumbing is economical.
Disadvantages of Combined System:
The combined system has the following disadvantages:
(i) The cost of construction will be high because of large size sewers to be constructed at sufficient depth below the ground surface involving large excavation.
(ii) Because of large size of sewers their handling and transportation will be difficult.
(iii) Due to inclusion of storm water (or rain water) treatment works of larger size will be required and also the treatment units will be heavily loaded.
(iv) If pumping is required for lifting of sewage at the treatment works, the system will prove to be uneconomical both from the point of view of capital costs as well as from the point of view of running costs.
(v) Storm water (or rain water) is unnecessarily polluted.
(vi) During heavy rains the sewers may overflow and may thus create unhygienic conditions and cause pollution problem.
(vii) The large size sewers, if not properly designed, gets easily silted. Moreover, the dry weather flow being a small amount of the total flow, the large size sewer would often get silted up due to low velocity of flow during the dry part of the year.
(viii) Large sewers are more difficult to be ventilated than the smaller ones.
Conditions Favourable for Combined System:
The combined system is favoured under the following conditions:
(i) Even Rainfall:
If rainfall is evenly spread throughout the year the combined system can be adopted.
(ii) Pumping Requirement:
When it is necessary to pump domestic or sanitary sewage and industrial sewage as well as storm water (or rain water), the combined system may be adopted.
(iii) Restriction of Space:
When space available for laying sewers is restricted, it is desirable to lay a combined sewer.
(iv) Conversion of Existing Storm Water Sewer:
If existing storm water sewer may be converted into a combined sewer, then combined system may be preferred. However, such a conversion will be possible only when the quantity of domestic or sanitary sewage and industrial sewage is small as compared to that of storm water (or rain water).
It may, however, be mentioned that the combined system of sewerage has not been found quite suitable under tropical Indian conditions for reasons mentioned below:
(i) Heavy and concentrated rainfall occurs during the monsoon period, which at most places lasts for only 3 to 4 months in a year. Thus there is a large variation in the quantity of sewage flowing during the different months of the year.
(ii) The Dry Weather Flow (D.W.F) is generally a very small proportion of the total flow and hence sewer is likely to get silted up due to low velocity of flow.
(iii) Poor economy and limited funds being available.
(iv) There are difficulties in operation and maintenance of the system due to poor supervision of less qualified staff. Also the municipalities or local bodies in charge of the works normally do not pay much attention to keep trained and skilled staff.
Because of the aforesaid reasons the usual practice is to adopt either separate system of sewerage or partially separate system of sewerage.
Type # 3. Partially Separate System:
In this system domestic or sanitary sewage and industrial sewage, and the storm water (or rain water) which is drained from back yards and roofs of houses are carried in the same set of sewers, while the storm water (or rain water) drained from house fronts as well as from streets and roads is collected and conveyed in a separate set of open drains.
The sewage and storm water (or rain water) carried by the sewers is usually delivered to a sewage treatment plant, and the storm water (or rain water) carried by the open drains is delivered to a natural stream or river for disposal.
Advantages of Partially Separate System:
The partially separate system has the following advantages:
(i) It combines the advantages of both separate system and combined system.
(ii) The sewers to be provided are of reasonable size and hence their cleaning is not very difficult.
(iii) The storm water (or rain water) eliminates the chances of their choking. Moreover, the sewers are completely cleaned during rainy season.
(iv) The problem of drainage of storm water (or rain water) from houses is simplified. Disadvantages of partially separate system
The partially separate system has following disadvantages:
(i) The storm water (or rain water) admitted in sewers may increase the load on pumping and treatment units.
(ii) During dry weather when there is no rain water, the velocity of flow will be low. Thus self-cleansing velocity may not be achieved.
(iii) Storm water overflows may be required to be provided.
Choice of the Sewerage System:
The aforesaid discussion indicates that there are several factors which govern the choice of the sewerage system. Thus each type of sewerage system to be adopted should be carefully studied keeping in view the various factors before a final choice of the system is made.
Further sufficient data regarding the factors affecting the choice of the sewerage system should be collected, studied and analysed to arrive at the final decision of adopting one system or the other. A well balanced decision without prejudice to any particular system will prove to be economical and in the best interest of the community to be served by the system.