Sewerage systems—what are they?
The term “sewerage system” includes the entire infrastructure utilized for collecting, transporting, and disposing of sewage, but it does not incorporate the process of treating the sewage. This includes all appurtenances, devices, and equipment involved in the process such as inlets, catch basins, clean outs, manholes, lampholes, flushing tanks, grease and oil traps, inverted syphons, and storm regulators. These components play a crucial role in maintaining the functionality and efficiency of the sewerage system.
The sewerage system relies predominantly on gravity to facilitate the movement of wastewater and frequently also utilizes natural stormwater drainage.
Components of Sewerage System
The primary goal of the sewerage system is to ensure that the sewage discharged from the communities is appropriately collected, transported, and treated to the necessary degree before being finally disposed of so that it does not harm human health or the environment.
- House Sewer
The house sewer serves as a conduit for transferring wastewater from a building to the lateral or branch lines.
- Lateral and branch sewers
They are the upper portions of the municipal sewer system. When a lateral sewer line reaches its upstream end, it connects to a branch sewer, which is responsible for collecting wastewater from many lateral sewer lines.
- Sub-main sewers
They collect wastewater from numerous smaller sewers that serve a larger area, neighborhood, or housing complex. They transport waste to larger trunk sewer lines, lift stations, or water treatment facilities.
- Trunk/Main Sewers
The trunk/main sewers are the main pipes of the system that collects waste water. They are responsible for collecting and transporting waste water from many main sewage lines to either a water treatment plant or an intercepting sewer.
- Interception Sewers
Wastewater is collected from trunk sewers and transported to a water treatment facility. These lines have the largest diameter and run very far downstream in the sewer system.
- Lift or Pump Stations
Construction of pumping stations is necessary when sewage must be moved from a low elevation to a higher elevation or when the topography prevents gravity movement downhill. For the treatment of raw effluent, non-clogging pumps are available. They are installed in buildings known as lift stations. The amount of wastewater that needs to be treated and the required lifting height determine the size and design of the lift station.
Types of Sewers
Following are the different types of sewers according to various categories
Classification of Sewerage System According to flow
- Gravity Sewer System
Firstly, gravity sewer systems are the traditional method of sewage disposal.
They are an effective method for collecting and disposing of sewage from houses in regions where there is a low water table, the terrain slopes gently, and flooding does not occur frequently.
Gravity sewer systems use gradually sloping pipes to let sewage flow naturally away from your property and towards the collecting network. Ultimately, this network transports movements to a local or regional treatment facility.
However, if the terrain surrounding your property is extremely hilly, does not slope in the direction necessary for sewage to flow naturally downward, or is prone to flooding, a pressure sewer system may be the best option.
- Pressure Sewer System
Pressure sewer systems are a cost-effective and environmentally favourable method for collecting, transporting, and disposing of household wastewater.
They are often used in places where the land is either very hilly or very flat, where there is a lot of flooding or where the water table is high, or where other types of sewage systems are not practical.
A pressure sewer system consists of a series of sealed pipes that are interconnected and fed by individual pumping units.
The wastewater from the household is processed by the pumping unit, and then it is transferred through a short pipeline on the property to the pressure sewer that is located on the street.
The pressure sewer is a component of the overall piping network that transports wastewater to the closest sewerage treatment facility, which could be in the immediate vicinity or many kilometers away.
The only visible components of the pressure sewer system after installation are the storage tank lid, boundary valve kit, and control panel.
Classification of Sewerage System According to Shape
1. Circular Shape
It is the most popular. It is suitable for any type of waste.
2. Standard Egg Shape
It is best suited for combined sewers.
Its advantage over a circular shape is that it provides a greater flow velocity at a lower flow rate while maintaining the same capacity.
The construction of it is harder, and it is less stable.
3. New Egg Shaped Sewer
It is preferred for combined sewerage systems
4. Horseshoe Shaped Sewer
This is appropriate for large sewers that have high discharges, such as trunk and outfall sewers.
Such a sewer is appropriate when there is not enough headroom.
Its height is greater than its breadth; its wall is most inclined with a semicircular arch at the top; and its invert may be flat, parabolic, or circular.
5. Parabolic Shaped Sewer
The upper arch of the sewer is shaped like a parabola.
This can be used to transport relatively small amounts of sewage.
It is economical in construction.
6. Semi Elliptical Shaped Sewer
Its stability makes it suitable for soft soil.
It is only useful for transporting large quantities or numbers of effluent.
It is used when pipes are bigger than 1.8m in diameter.
7. Rectangular Shaped Sewer
It is typically used for covered storm water drains; however, because of its stability and simplicity of construction, it may also be utilised as a storage tank.
8. U Shaped Sewer
This type of sewer is used for combined sewers with maximal storm water flow.
It is used for longer sewers and particularly in open cuts.
The invert is formed in the shape of a semicircular arch.
9. Semi Circular Shaped Sewer
The bottom of this segment provides a wider base, making it suited for building huge sewers with less headroom.
It is obsolete.
10. Basket Handle Shaped Sewer
The upper portion of this type of sewer resembles a basket handle.
In this case, the upper part is wider than the lower part.
During the monsoon, it flows full through the narrow bottom portion.
Moreover, it is obsolete.
Classification of Sewerage System According to Material
- Asbestos Cement Sewer
They are made by combining cement and asbestos fibre.
They are appropriate for transporting domestic sewage.
Asbestos cement sewers work best as vertical pipes that take waste from the upper floors of multi-story buildings (in a two-pipe plumbing system).
- Brick Sewers
Brick sewers are constructed on-site and are used for large-scale sewer construction.
Additionally, brick Sewers are extremely beneficial for the construction of storm sewers and combined sewers.
These days, sewers are typically made of concrete rather than brick. Brick sewers are susceptible to deformation, which can result in leaks.
- Cement Sewers
PCC – for diameter < 60 cm
It is suitable for small storm drains and is not durable.
RCC – for diameters > 60 cm
They can be precast or on-site cast, and they can withstand high pressure, corrosion, and significant loads. These are heavy and hard to transport.
- Cast Iron Sewers
These types of sewer are watertight and of high strength and durability. Sewers made of cast iron are capable of withstanding high internal pressure and external loads. The following conditions are suitable for cast iron sewers.
- effluent is transported at high pressure
- the sewer line is subjected to significant external loads, such as beneath a railway, a foundation wall or below a highway.
- There is a significant temperature difference
- Steel Sewers
Steel sewers are ideal when the sewage is transported under pressure, as they are impervious, lightweight, pressure-resistant, and flexible. Generally, they are used for outfall and trunk sewers.
- Plastic Sewers
Sewage is now transported through PVC sewers. Plastic pipelines are corrosion-resistant. These types of sewers are bendable, lightweight, and smooth. However, these forms of sewers (Plastic sewers) have a high thermal expansion coefficient and cannot be used in extremely hot climates.
Types of Sewerage System
There are three different types of sewerage systems. Here is a list of them:
- Combined sewerage system
- Separate sewerage system
- Alternative sewerage system
Each is explained in more detail below:
Combined Sewerage System
Combined sewers are drainage networks that handle both regular household waste and excess water from storms. Due to the huge amounts of storm water that need to be transported during periods of wet weather, combined sewers often consist of pipes or tunnels that have a large diameter. While still often found in older urban areas, they are no longer incorporated into the planning or construction of new sewage systems. During periods of heavy rainfall, sewage must be discharged directly into the receiving water rather than processed at the wastewater treatment plants because these facilities are unable to process significant volumes of storm water. These combined sewer overflows, which contain untreated home sewage, produce periodic water contamination concerns and are extremely troublesome sources of pollution.
Separate Sewerage System
Both sewage and stormwater runoff are transported through separate pipes in this sewerage system. Stormwater can be discharged directly into a body of water because it is not as foul as sewage and no purification is generally required. Sewage collected from a city is properly treated before being released into a body of water or used for irrigation, ensuring that it complies with all applicable regulations. A separate system is beneficial and economical for big cities.
Partially Separate Sewerage System
In this system, stormwater, particularly that collected from roofs and paved courtyards, is discharged into the same drain as waste from residences, institutions, etc. Stormwater from other areas is collected separately using separate stormwater pipes. The sewage and storm water (or rain water) carried by sewers are often taken to a sewage treatment plant, whereas storm water (or rain water) carried by open drains is typically delivered to a natural stream or river for disposal.
Patterns of Sewage Collection
The patterns of collection systems are influenced by:
- The topography and hydrology of the area
- The location and disposal techniques employed.
- The type of sewage disposal system used
- The area to be served
The different patterns of sewage disposal system are as follows:
Perpendicular Pattern
Firstly, in this pattern, sewers carrying storm water are constructed so as to find the shortest route to natural water courses.
Thus, by laying the sewers perpendicular to the natural waterways, the shortest route to the waterways can be achieved.
Also, this design is appropriate for separate systems and partially separate systems in which storm water can be disposed of untreated.
Finally, it is not appropriate for a combined system because, in addition to making sewage treatment particularly challenging due to the high number of outlets, it also pollutes natural waterways.
Interception Pattern
Interception Pattern is an advancement over the perpendicular pattern. In this configuration, a large-diameter sewer is constructed along the water carrying sewage to a central location, where it can be disposed of with or without treatment.
Additionally, if the amount of storm water is substantial, overflows should be installed allowing excess sewage to escape into natural waterways through outlets that existed before the interception.
Radial Pattern
This pattern can be used if effluent is to be disposed of on land surrounding the city.
Also, this design provides a large number of outlets.
Sewers in a city are usually laid out in a radial layout, expanding outward from the city centre.
Additionally, this pattern allows the suburbs to be served by relatively tiny and short sewer lines, making it cost-effective.
Finally, this system includes several disposal works.
Fan Pattern
If the city is located near a river that is only on one side, the sewer can be laid so that all sewage flows to a central location where a single treatment facility is located.
In this pattern, a number of main sewers come together to make a fan-like shape, which is where its name comes from.
But, the only benefit of this system is that it only requires a single unit of treatment.
However, because the diameter of the sewer increases as the sewage travels, the diameter of the main trunk is very large, which raises the total cost of this design.
Zone Pattern
Zone Pattern: In the interceptor pattern, only one large intercepting sewer is used to collect and transport all effluent, causing it to be overloaded.
However, this overloading can be eliminated by increasing the number of interceptors in each zone.
Also, this pattern is better adapted for hilly terrain.
Which Sewerage System Should You Choose?
The discussion above suggests that a number of factors influence the sewage system that is chosen. Before deciding on a sewerage system, each type should be carefully studied with all of these factors in mind.
The stakeholders should gather, study, and analyze further information about the factors influencing the selection of the sewerage system to reach a final decision regarding the adoption of one system or the other. They should aim for a fair judgment that doesn’t favor any one system over another, which will ultimately result in the most cost-effective and beneficial solution for the community to be served.
The term “sewerage system” refers to the complete infrastructure that is used for the collection, transportation, and disposal of sewage, but does not include the process of treating the sewage.
This includes all appurtenances, devices, and equipment involved in the process such as inlets, catch basins, clean outs, manholes, lampholes, flushing tanks, grease and oil traps, inverted syphons, and storm regulators.