Sewer System | Types, Shapes, Materials

Types of sewer system

  1. Separate system
  2. Combined system
  3. Partially separate system

1) Separate system:

  • Sanitary and storm water are carried separately in two sets of sewers
  • Sewage conveyed to WWTP
  • Storm water discharged directly into the rivers without treatment

Advantages:

  • The load on the treatment plant is less as only sewage is carried to the plant.
  • The size of sewer is small, thus economical
  • When pumping is required, the system proves to be economical.
  • Natural/storm water is not unnecessarily polluted by sewage.

Disadvantages:

  • Cleaning of sewer is difficult due their small size.
  • The self cleansing velocity is not easily obtained.
  • The storm sewers come in operation in rainy season only. They may be chocked in dry season by garbage.
  • Maintenance cost is high.
  • Sewage sewers are provided below storm sewers which cause greater depth and pumping at WWTP

The separate system is suitable when separate outlet for storm water is available and the topography is such that storm water can be disposed of in natural drains.

2) Combined sewerage system:

In this system the sewage and storm water are carried combinely in only one set of sewers to the waste water treatment plant(WWTP) before disposal.

Advantages:

  • Easy cleaning because of larger diameter.
  • Reasonable maintenance cost
  • Strength of sewage is reduced due to dilution of sewage by storm water.
  • This system requires only one set of sewer making it economical.

Disadvantages:

  • In storm season sewer may overflow and the sewer may damage causing serous health risks.
  • The combine sewer gets silted and becomes foul in dry days.
  • Load on treatment plant is more because storm water is also carried there.
  • The storm water gets polluted unnecessarily
  • The system becomes uneconomical when pumping is needed.

Combined sewerage system is suitable when the space available for laying two sets of sewers is less and when pumping is not required.

3) Partially separate sewerage system:

  • This system is the compromise between separate and combine system taking the advantages of both systems.
  • In this system the sewage and storm water of buildings are carried by one set of sewers.
  • The storm water from roads, streets, pavements etc are carried by other system of sewers usually open drains.

Advantages:

  1. It combines the good features of both systems
  2. The silting is avoided due to entry of storm water
  3. The storm water from houses is easily disposed of
  4. The sewers are of reasonable size.

Disadvantages:

Very small fraction of bad features of combined system are there in partially separate system.

Sewers pumping stations and water reclamation centers

Shapes of sewers

According to Circular Sections

According to Non-Circular Sections

1) Rectangular Section

Generally used for covered storm water drains.

2) Horse shoe section

May be used for large sewers with heavy discharges, such as for Trunk and outfall sewers

3) Egg-shaped section

May be preferred for combined sewers(sewage+storm water)

4) U shaped section

May be used for large sewers

Materials for Sewers

  • These are manufactured from a mixture of asbestos fibers, silica and cement. Asbestos fibers are thoroughly mixed with cement to act as reinforcement.
  • These pipes are available in size 10 to 100 cm internal diameter and length up to 4.0 m.
  • These pipes can be easily assembled without skilled labour with the help of special coupling, called ‘Ring Tie Coupling’ or Simplex joint.
  • The pipe and joints are resistant to corrosion and the joints are flexible to permit 12o deflection for curved laying.
  • These pipes are used for vertical transport of water. For example, transport of rainwater from roofs in multistoried buildings, for transport of sewage to grounds, and for transport of wastewater from kitchen and bathroom.

1) Asbestos Cement Sewers

Advantages

  1. Smooth
  2. Light in weight
  3. Can easily by cut fitted and drilled
  4. Durable against soil corrosion
  5. These pipes are light in weight and hence, easy to carry and transport.
  6. Easy to cut and assemble without skilled labour.
  7. Interior is smooth (Manning’s n = 0.011) hence, can make excellent hydraulically efficient sewer.

Disadvantages

  1. These pipes are structurally not very strong.
  2. These are susceptible to corrosion by sulphuric acid. When bacteria produce H2S, in presence of water, H2SO4 can be formed leading to corrosion of pipe material.
  3. Brittle and cant withstand heavy loads
  4. They are easily broken in handling and transport

2) Brick Sewers:

This material is used for construction of large size combined sewer or particularly for storm water drains. The pipes are plastered from outside to avoid entry of tree roots and groundwater through brick joints. These are lined from inside with stone ware or ceramic block to make them smooth and hydraulically efficient. Lining also makes the pipe resistant to corrosion.

3) Cement Concrete Sewer:

  • PCC- For dia upto 60 cm. Suitable for small storm drains. Not durable
  • RCC- For dia > 60 cm. Resistant to heavy loads, corrosion and high pressure. These are very heavy and difficult to transport.
  • Plain cement concrete (1: 1.5: 3) pipes are available up to 0.45 m diameter and reinforcement cement pipes are available up to 1.8 m diameter.
  • These pipes can be cast in situ or precast pipes. Precast pipes are better in quality than the cast in situ pipes.
  • The reinforcement in these pipes can be different such as single cage reinforced pipes, used for internal pressure less than 0.8 m; double cage reinforced pipes used for both internal and external pressure greater than 0.8 m;

Advantages

  • Strong in tension as well as compression.
  • Resistant to erosion and abrasion.
  • They can be made of any desired strength.
  • Easily molded, and can be in situ or precast pipes.
  • Economical for medium and large sizes.
  • These pipes are available in wide range of size and the trench can be opened and backfilled rapidly during maintenance of sewers.

Disadvantages

  • These pipes can get corroded and pitted by the action of H2SO4.
  • The carrying capacity of the pipe reduces with time because of corrosion.
  • The pipes are susceptible to erosion by sewage containing silt and grit.

4) Cast Iron Sewers

These pipes are stronger and capable to withstand greater tensile, compressive, as well as bending stresses. However, these are costly. Cast iron pipes are used for outfall sewers, rising mains of pumping stations, and inverted siphons, where pipes are running under pressure.
These are also suitable for sewers under heavy traffic load, such as sewers below railways and highways. They are used for carried over piers in case of low lying areas. They form 100% leak proof sewer line to avoid groundwater contamination. They are less resistant to corrosion.

5) Steel pipes

  • These are used under the situations such as pressure main sewers, under water crossing, bridge crossing, necessary connections for pumping stations, laying pipes over self supporting spans, railway crossings, etc.
  • They can withstand internal pressure, impact load and vibrations much better than CI pipes. They are more ductile and can withstand water hammer pressure better. These pipes cannot withstand high external load and these pipes may collapse when negative pressure is developed in the pipe. They are susceptible to corrosion and are not generally used for partially flowing sewers.

6) Plastic sewer

  • PVC sewers are used for carrying sewage. They are resistant to corrosion. They are light in weight, smooth and can be bent easily. However they have high coefficient of thermal expansion and can’t be used in very hot areas.
  • Other sewers : Wooden and stoneware sewers (rare now)

7) Vitrified Clay or Stoneware Sewers

Vitrified clay pipe (VCP) is pipe made from a blend of clay and shale that has been subjected to high temperature to achieve vitrification, a process which results in a hard, inert ceramic. These pipes are used for house connections as well as lateral sewers. The size of the pipe available is 5 cm to 30 cm internal diameter with length 0.9 to 1.2 m. These pipes are rarely manufactured for diameter greater than 90 cm.

Advantages:

  • Resistant to corrosion, hence fit for carrying polluted water such as sewage.
  • Interior surface is smooth and is hydraulically efficient.
  • The pipes are highly impervious.
  • Strong in compression, durable and economical for small diameters.

Disadvantages:

  • Heavy, bulky and brittle and hence, difficult to transport.
  • These pipes cannot be used as pressure pipes, because they are weak in tension.
  • These require large number of joints as the individual pipe length is small.

Selecting Material for Sewer

1) Resistance to corrosion

Sewer carries wastewater that releases gases such as H2S. This gas in contact with moisture can be converted into sulfuric acid. The formation of acids can lead to the corrosion of sewer pipe. Hence, selection of corrosion resistance material is must for long life of pipe.

2) Resistance to abrasion

Sewage contain considerable amount of suspended solids, part of which are inorganic solids such as sand or grit. These particles moving at high velocity can cause wear and tear of sewer pipe internally. This abrasion can reduce thickness of pipe and reduces hydraulic efficiency of the sewer by making the interior surface rough.

Strength and durability

  • The sewer pipe should have sufficient strength to withstand all the forces that are likely to come on them. Sewers are subjected to considerable external loads of backfill material and traffic load, if any.
  • They are not subjected to internal pressure of water. To withstand external load safely without failure, sufficient wall thickness of pipe or reinforcement is essential. In addition, the material selected should be durable and should have sufficient resistance against natural weathering action to provide longer life to the pipe.

Weight of the material

  • The material selected for sewer should have less specific weight, which will make pipe light in weight. The lightweight pipes are easy for handling and transport.

Imperviousness

  • To eliminate chances of sewage seepage from sewer to surrounding, the material selected for pipe should be impervious.

Economy and cost

  • Sewer should be less costly to make the sewerage scheme economical.
    Hydraulically efficient
  • The sewer shall have smooth interior surface to have less frictional coefficient.

Hydraulically efficient

  • The sewer shall have smooth interior surface to have less frictional coefficient.

Types of sewers according to use

  1. House sewer: A sewer which carries sewage from plumbing system of a building to the common sewer in the street.Its dia is 6 inches.
  2. Lateral sewer: The sewer which takes discharge from the house sewer. It is in the street. Its min dia is 9 inches
  3. Common sewer: A sewer in which all the adjacent properties has equal right of use e.g. Lateral sewer.
  4. Submain: A sewer which recieves discharge from several lateral sewers and convey it to the main sewer.
  5. Main sewer/Trunk sewer: A sewer which receives discharge from submain sewers and convey it to the waste water treatment plant(WWTP)
  6. Outfall sewer: A sewer which carries discharge from collecting system(WWTP) to the point of final dispoal.

Relief sewer: A sewer which is constructed to relieve an existing sewer of inedequate capacity.

Intercepting sewer: A sewer which cuts transversely a no of other sewers to intercept the dry weather flow but during wet weather flow it carries certain predetermined quantity of storm water run-off.

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