What Is Sewerage System?
A sewerage system, also known as a wastewater collection system, is a network of pipelines, pumping stations, and appurtenances that transport sewage from its source to a point of treatment and disposal. Different types of sewerage systems use sewers to transport sewage or surface runoﬀ (stormwater, meltwater, precipitation). Sewerage is terminated at the point of entry into a sewage treatment plant or at the point of outflow into the environment.
In simple words, sewerage system is a network of pipelines, chambers, manholes, and so on that transports sewage or stormwater.
Diﬀerent Types Of Sewerage System
There are three major types of sewerage systems. They are as follows:
- Combined sewerage system
- Separate sewerage system
- Alternative sewerage system
Each one of them is explained in detail below:
1. Combined Sewerage System
Combined sewers are systems that transport a combination of household and storm waste. Because of the huge volumes of stormwater that must be handled during wet weather, combined sewers are often made out of large-diameter pipes or tunnels.
They are ubiquitous in older cities, although they are no longer constructed or built as part of new sewerage facilities. Because wastewater treatment plants cannot manage significant amounts of stormwater, sewage must be discharged straight into receiving water during wet weather.
These combined sewer overflows, which contain untreated home sewage, produce repeated water pollution issues and are extremely bothersome pollution sources.
The combined sewer overflow problem has been alleviated in several large cities by directing the first flush of mixed sewage into a huge basin or underground tube. Swirl concentrators are another approach for regulating combined sewage.
These funnel sewage through cylindrically shaped devices that produce a vortex, or whirlpool, eﬀect. The vortex aids in the concentration of contaminants in a much smaller amount of water before treatment.
2. Separate Sewerage System
Two sets of sewers are supplied in this system: one for conveying domestic or sanitary sewage and industrial sewage, and the other for carrying stormwater (or rainwater).
The sewage from the first set of sewers is transported to a treatment plant, whereas stormwater (or rainwater) from the second set of sewers is dumped straight into a natural stream or river without treatment.
New wastewater collecting facilities are built as separate systems that can handle either domestic or storm sewage but not both. Storm sewers are often used to transport surface runoﬀ to a point of disposal in a stream or river.
Small detention basins can be installed as part of the system to temporarily store stormwater and reduce the magnitude of the peak flow rate. In contrast, sanitary sewers transport domestic wastewater to a sewage treatment plant. Pretreated industrial wastewater is permitted to enter municipal sanitary sewerage systems, while stormwater is not.
Storm sewers are typically constructed using reinforced concrete pipe sections. In some circumstances, corrugated metal pipes may be employed. Stormwater inlets or catch basins are placed at appropriate intervals in the right-of-way of a street or easements over private property.
Pipelines are typically placed to enable gravity flow downhill to a nearby stream or detention basin. Stormwater pumping stations are avoided whenever possible due to the extremely large pump capacities required to handle the intermittent flows.
Laterals, submains, and interceptors are components of a sanitary sewerage system. Aside from individual house connections, laterals are the tiniest sewers in the system.
The collector sewers connect to a major interceptor, also known as a trunk line, which transports sewage to a treatment plant. Precast lengths of reinforced concrete pipe up to 5 meters (15 feet) in diameter are typically used to construct interceptors.
Other sanitary sewer materials include vitrified clay, asbestos cement, plastic, steel, or ductile iron. Because of its lightweight and ease of installation, plastic is increasingly being used for laterals.
Iron and steel pipes are utilized in power lines and pumping facilities. When sewage must be pushed, force mains transport it under pressure.
3. Alternative Sewerage System
Because of low population densities or site factors such as a high water table or bedrock, the expense of conventional gravity sewers can be too high at times. Small-diameter gravity sewers, pressure sewers, and vacuum sewers are three alternate wastewater collection technologies that could be employed under these conditions.
Septic tanks are used in small-diameter gravity systems to remove settleable and floating materials from wastewater from each residence before it runs into a network of collector mains (usually 100 mm, or 4 inches) in diameter; these systems are best suited for small rural communities.
Because they do not carry oil, grit, or sewage solids, the pipes can be smaller in diameter and placed at lower slopes or gradients to reduce trench excavation expenses.
Pressure sewers are best employed in flat locations or when expensive rock extraction is necessary. Grinder pumps drain wastewater from each home into the main pressure sewer, which can follow the slope of the ground.
Sewage from one or more buildings flows by gravity into a sump or tank, from which it is drawn out by vacuum pumps placed at a central vacuum station and then flows into a collection tank in a vacuum sewerage system. Sewage is piped from the vacuum collection tank to a treatment plant.
Selecting A Sewerage System
According to the preceding discussion, there are various elements that influence the selection of a sewage system. Thus, before making a final decision on a sewerage system, each type of system should be carefully analyzed in light of the many aspects.
Furthermore, appropriate data on the factors influencing the choice of sewage system should be acquired, investigated, and analyzed before making the final decision to adopt one system or the other. A well-balanced judgment made without bias toward any particular system will prove to be cost-eﬀective and in the best interests of the community serviced by the system.
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