Most likely, when you picture sewage, you picture a foul sludge flowing through pipes outside, under the road. But there are various sewage systems, each serving a particular function for collecting trash and wastewater. Sanitary, storm, and mixed sewers are the three categories of sewers.
The three sewer systems mentioned above ensure the proper transportation and treatment of the trash we produce.
Table of Contents
What Are The Functionalities Of Sewer Systems?
The following are the roles played by the various sewer system types. From wastewater-producing locations to treatment facilities, sewer lines transport the waste. Our water sources are shielded from sewage contamination by sewer systems.
Wastewater that has been treated can be reused thanks to sewage systems. Sullage is kept from contaminating soil habitats by sewage systems. Improved water cleanliness and quality are made possible by sewage systems.
Types of the sewer system
- Separate system
- Combined system
- Partially separate system
1) Separate system:
- Sanitary and stormwater are carried separately in two sets of sewers
- Sewage conveyed to WWTP
- Stormwater discharged directly into the rivers without treatment
- The load on the treatment plant is less as only sewage is carried to the plant.
- The size of the sewer is small, thus economical
- When pumping is required, the system proves to be economical.
- Natural/stormwater is not unnecessarily polluted by sewage.
- Cleaning of sewers is difficult due to their small size. Specialized sewer maintenance equipment is needed to inspect and clean.
- The self-cleansing velocity is not easily obtained.
- The storm sewers come into operation in the rainy season only. They may be choked in the 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 a separate outlet for stormwater is available, and the topography is such that stormwater can be disposed of in natural drains.
2) Combined sewerage system:
In this system, the sewage and stormwater are carried combined in only one set of sewers to the wastewater treatment plant(WWTP) before disposal.
- Easy cleaning because of the larger diameter.
- Reasonable maintenance cost
- The strength of sewage is reduced due to the dilution of sewage by stormwater.
- This system requires only one set of sewers, making it economical.
- In storm season, the sewer may overflow and damage, causing serious health risks.
- The combined sewer gets silted and becomes foul on dry days.
- Load on treatment plant is more because stormwater is also carried there.
- The stormwater gets polluted unnecessarily
- The system becomes uneconomical when pumping is needed.
A 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 combined systems taking advantage of both systems.
- In this system, the sewage and stormwater of buildings are carried by one set of sewers.
- The stormwater from roads, streets, pavements etc, are carried by another system of sewers, usually open drains.
- It combines the good features of both systems
- The silting is avoided due to the entry of stormwater
- The stormwater from houses is easily disposed of
- The sewers are of reasonable size.
A very small fraction of the bad features of the combined system is there in a partially separate system.
Sewer 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 stormwater drains.
2) Horseshoe section
It may be used for large sewers with heavy discharges, such as for Trunk and outfall sewers
3) Egg-shaped section
It may be preferred for combined sewers(sewage+storm water)
4) U-shaped section
It 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 sizes 10 to 100 cm internal diameter and lengths up to 4.0 m.
- These pipes can be easily assembled without skilled labor with the help of special coupling, called ‘Ring Tie Coupling’ or a Simplex joint.
- The pipe and joints are resistant to corrosion and flexible, permitting 12o deflection for curved laying.
- These pipes are used for the vertical transport of water. For example, the transport of rainwater from roofs in multistoried buildings, sewage to grounds, and wastewater from kitchens and bathrooms.
1) Asbestos Cement Sewers
- Light in weight
- It can easily be cut, fitted, and drilled
- Durable against soil corrosion
- These pipes are light in weight and hence, easy to carry and transport.
- Easy to cut and assemble without skilled labor.
- The Interior is smooth (Manning’s n = 0.011) hence, it can make an excellent hydraulically efficient sewer.
- These pipes are structurally not very strong.
- These are susceptible to corrosion by sulphuric acid. When bacteria produce H2S in water, H2SO4 can be formed, leading to pipe material corrosion.
- Brittle and cant withstand heavy loads
- They are easily broken in handling and transport
2) Brick Sewers:
This material is used to construct large-size combined sewers, particularly stormwater drains. The pipes are plastered from outside to avoid entry of tree roots and groundwater through brick joints. These are lined from the inside with stoneware or ceramic block to make them smooth and hydraulically efficient. The lining also makes the pipe resistant to corrosion.
3) Cement Concrete Sewer:
- PCC- For dia up to 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 up to 0.45 m in diameter, and reinforcement cement pipes are up to 1.8 m in diameter.
- These pipes can be cast in situ or precast pipes. Precast pipes are better in quality than 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;
- 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 a wide range of sizes, and the trench can be opened and backfilled rapidly during the maintenance of sewers.
- 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 of withstanding greater tensile, compressive, and bending stresses. However, these are costly. Cast iron pipes are used for outfall sewers, rising mains of pumping stations, and inverted siphons, where pipes run under pressure.
These are suitable for sewers under heavy traffic loads, such as below railways and highways. They are used for carried-over piers in the case of low-lying areas. They form 100% leakproof sewer lines to avoid groundwater contamination. They are less resistant to corrosion.
5) Steel pipes
- These are used under the situations such as pressure main sewers, water crossings, bridge crossings, 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 loads and may collapse when negative pressure is developed. 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 a 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 made from a blend of clay and shale subjected to high temperature to achieve vitrification, resulting in a hard, inert ceramic. These pipes are used for house connections as well as lateral sewers. The pipe size available is 5 cm to 30 cm internal diameter with a length of 0.9 to 1.2 m. These pipes are rarely manufactured for diameters greater than 90 cm.
- Resistant to corrosion, hence fit for carrying polluted water such as sewage.
- The interior surface is smooth and hydraulically efficient.
- The pipes are highly impervious.
- Strong in compression, durable and economical for small diameters.
- 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 many 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. In contact with moisture, this gas can be converted into sulfuric acid. The formation of acids can lead to the corrosion of sewer pipes. Hence, the selection of corrosion-resistant material is a must for the long life of the pipe.
2) Resistance to abrasion
Sewage contains a 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 pipes internally. This abrasion can reduce the pipe’s thickness and the sewer’s hydraulic efficiency by roughing the interior surface.
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 many external backfill materials and traffic loads if any.
- They are not subjected to the internal pressure of water. To withstand external load safely without failure, the sufficient wall thickness of pipe or reinforcement is essential. In addition, the material selected should be durable and have sufficient resistance against natural weathering action to provide longer life to the pipe.
Weight of the material
- The material selected for the sewer should have less specific weight, which will make the pipe light in weight. The lightweight pipes are easy to handle and transport.
- The material selected for the pipe should be impervious to eliminate the chances of sewage seepage from the sewer to the surrounding.
Economy and cost
- The sewer should be less costly to make the sewerage scheme economical.
- The sewer shall have a smooth interior surface to have less frictional coefficient.
- The sewer shall have a smooth interior surface to have less frictional coefficient.
Types of sewers according to use
- House sewer: A sewer that carries sewage from the plumbing system of a building to the common sewer in the street. Its dia is 6 inches.
- Lateral sewer: The sewer which takes discharge from the house sewer. It is in the street. Its min dia is 9 inches
- Common sewer: A sewer in which all the adjacent properties have equal right of use e.g. Lateral sewer.
- Submain: A sewer that receives the discharge from several lateral sewers and conveys it to the main sewer.
- Main sewer/Trunk sewer: A sewer that receives the discharge from submain sewers and conveys it to the wastewater treatment plant(WWTP)
- Outfall sewer: A sewer that carries the discharge from the collecting system(WWTP) to the point of final disposal.
Relief sewer: A sewer constructed to relieve an existing sewer of inadequate capacity.
Intercepting sewer: A sewer that cuts a no of other sewers transversely to intercept the dry weather flow, but during wet weather flows, it carries a certain predetermined quantity of stormwater run-off.