Types of Sewerage System | Combined | Separate | Partially Separate

What is Sewerage System?

Sewerage is the infrastructure that conveys sewerage, surface runoff using Sewer. It encompasses components such as receiving drains, manholes, pumping stations, stored overflows and screening chambers of the combined sewer or sanitary sewer. Sewerage enters the entry sewerage treatment plant or at the point of discharge into the environment. It is the system of pipes, Chambers, manholes etc. that convey the sewerage and stormwater.

Types of Sewerage System
  1. Combined System
  2. Separate System
  3. Partially Separate System
Combined Sewerage System

In a combined system, the same sewer is intended to carry both the domestic sewage, industrial wastes as well as the surface and the stormwater flow.

Types of Sewerage System

Situation for Adoption
  1. Rainfall is even throughout the year.
  2. Both the sanitary sewage and the stormwater have to be pumped.
  3. The area to be sewered is heavily built-up and space for laying pipes is not enough.
  4. Effective or quicker flows have to be provided.
  5. If the sewers are laid along with the overall development of the area, a combined system is preferred.
Advantages
  1. Rainwater keeps sewage fresh making it easier and more economical for treatment purpose.
  2. Dilution also helps, this being in itself a method of treatment.
  3. Automatic flushing is provided by water.
  4. This is the simplest method of collection and house plumbing economies.
Disadvantages
  1. The bigger size of the sewer would involve larger excavation.
  2. Overflowing under worst conditions may endanger public health.
  3. Cost of pumping and treatment would increase due to the large quantity of sewage to be handled.
  4. The dry weather flow is a small amount of the total flow, the large size of the sewer would often result in causing silting up due to low velocity of flow during the dry period of the year.
Separate System 

In a separate system, the domestic sewage and industrial wastes are carried in one set of sewers whereas the storm and surface water are carried in another set of sewer.


Situation for Adoption
  1. Where rainfall is uneven.
  2. Sanitary sewage is to have one outlet and other outlets for store or surface water are available.
  3. Sanitary sewage is to be pumped.
  4. Separate sewer must be placed deeper and the stormwater drains nearer the surface to economies excavation.
  5. If the ground has steep slopes, it is easier to convert stormwater through an open drain to the natural stream.
  6. Finance available are small but sanitary drainage is imperative.
  7. If the subsoil is hard, it is difficult and costlier to lay combined sewer of large size.
  8. If the sewers are laid before the area is developed, it is preferable to adopt a separate system.
Advantages
  1. Being smaller in size, the sewers are economical. The surface water may be taken in open or closed conduit or drains at or near the surface and discharged at suitable outlets, thus greatly simplifying the design of sewers of stormwater drains.
  2. There is no risk of stream pollution as no storm overflows are to be provided.
  3. The quantity of sewage to be treated is small, the disposal or the treatment works can be economically designed.
  4. If the pumping of sewage at the treatment works is necessary, pumping cost would be much less as there is no need to pump the stormwater.
Disadvantages
  1. Unless laid at a steep gradient, self-cleaning velocity in the sewer cannot be assured and flushing shall have to be done. This may prove unsatisfactory and expensive.
  2. Risk of encroachment by unauthorized rainwater collection and consequent overflows of sewage maybe there.
  3. Double house plumbing is another disadvantage. Two sewers or drains in a street leading to greater obstruction of traffic which repairs to any one of them are being carried out.
  4. Maintenance costs of two systems are greater than that for one.
Partially Separate System

A partially separate system is a modification of the separate system in which the separate sewer discharging domestic sewage and industrial wastes also contain a portion of the surface drained from back-paved yards and roofs of the house.

Advantages
  1. It simplifies the drainage of the houses.
  2. It provides reasonable sizes of sewers and is economical.
  3. The rainwater provides some safeguard against silting in the sewer.
Disadvantages 
  1. Low velocity during the dry period.
  2. Storm overflows may be found necessary
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Also, read - 
1. Common Impurities In Water
2. Water Pollution and Sources of Water Pollution

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