Sewer systems are of different types. In this article, we will be looking at the types of sewer systems and how they operate.
A good sewage system is an indicator of a healthy community. The types of sewer systems adopted by anyone should be suitable for them and cost-effective. Sewer projects that can be completed within the shortest time frame possible and at the same time serve for as long as acceptable. Whatever be the types of sewer systems selected, they should be sanitary sewers.
Table of Contents
What is a Sewer System?
A sewer system is a set of pipes through which sewage flows. Aside from pipes, the sewage system consists of pumping stations, overflow facilities, retarding basins, connection facilities, inspection chambers, oil and sand traps, and treatment plants. All types of sewer systems should be designed and constructed to contain all sanitary wastes and exclude all forms of infiltration and inflow as much as possible.
Sewers collect wastewater from residential buildings, schools, shopping malls, old people’s homes, hospitals, motels, hotels, laundromats, lubes, swimming pools, event centers, factories, etc. into treatment plants.
Factors to be considered before choosing a sewer system are:
- Nature of the soil
- Construction, installation, and maintenance cost.
- Peak flow from domestic and industrial sewers
- Elevation of controlling service connections
- Groundwater infiltration and exfiltration
- Topography and depth of excavation
- Pumping requirements
- Location of the waste treatment plant
- Maintenance requirements
- Availability of existing sewers
Sewer pipes are usually inclined downward toward the central collection point so that sewage will flow to it naturally and finally into the treatment plants. Pumping stations may however be required In individual flatland areas and areas where watercourses are crossed, where gravity might not be strong enough to cause flow. In these pumping stations, wastewater must be re-pumped to the main reservoirs on higher grounds.
Sewer pipe must be strong enough to withstand the structural stresses to which it is subjected by being buried in the ground. In addition, the pipe itself and the joints between sections of the pipe must be capable of withstanding at least moderate water pressure.
Functions of Sewage System
The following are functions of the various types of sewer systems
- Sewer systems convey wastewater from generation points to treatment facilities.
- Sewer systems protect our water sources from contamination with untreated wastewater.
- Sewage systems create room for the reuse of wastewater after they have been treated.
- Sewage systems prevent the littering of soil environments with sullage.
- Sewage systems help improve water quality and general sanitation.
3 Types of Sewage Systems And How They Operate
Classification of sewage into types of sewer systems is done based on materials used, mode of construction, sanitary condition, and… Thus we have
- Types of sewer systems based on materials used
- Types of sewer systems according to the method of construction
- Types of sewer systems according to the source of sewage.
1. Types of Sewer Systems According to Method of Construction
When sewer systems are classified according to the method of construction, we have;
- Separate sewer systems
- Combined sewer systems
- Partially separate sewer systems.
Separate Sewer System
A separate sewerage system is one in which sewage and stormwater are collected into sewer systems. The sewage in the municipal sewer is collected into the waste treatment plant and the stormwater sewers are discharged into water bodies or reservoirs without any form of treatment. This reduces the volume of wastewater convened into the treatment facilities and the entire load on the treatment units.
The pipelines used for stormwater are usually placed in a way that allows downhill gravity flow to a nearby stream or a detention basin
The separate types of sewer systems require low capital, installation, and running costs. Sewers are more ventilated because they are of smaller sections. The size however makes the system susceptible to clogging and cleaning a difficult task. If set on a shallow gradient, flushing will be required for effective cleaning as self-cleansing velocity in the sewers cannot be assured.
Regular maintenance will also be needed as separate sewers are used. These maintenance activities such as repair on a damaged sewer can cause traffic congestion on highways. Care must be taken to ensure that people do not convert stormwater drains into solid waste dumpsites in seasons when there is no rainfall.
Combined Sewer System
As its name goes, combined systems are types of sewer systems where stormwater and wastewater are convened through the same set of sewers into the wastewater treatment facility. In this case, the cost of installation and maintenance will be less compared to that of a separate system.
Combined sewers are very common in old big cities but are no longer designed and built as part of new sewerage facilities in modern cities. They make use of large-diameter pipes or tunnels because of the volume of wastewater they carry especially, during the wet seasons.
The presence of stormwater reduces the concentration of the wastewater entering the treatment plant. The stormwater also provides automatic flushing in the system. However, the cost of installation and transportation of wastewater will be high, using this among other types of sewage systems. The combined sewer system is also prone to flooding during heavy rains.
This problem of overflow can be reduced by diverting the first flush of combined sewage into a large basin or underground tunnel temporarily. The wastewater can be treated thereby settling and disinfection or in a nearby wastewater treatment facility before final discharge into water bodies. Discharge into a nearby wastewater treatment facility should be done at a rate that will not overload the facility.
The use of swirl concentrators is another method that can be used in reducing the volume of wastewater in combined sewer systems. These swirl concentrators channel sewage through cylindrically shaped devices. This creates a vortex or whirlpool effect that helps concentrate impurities into smaller volumes of water for treatment.
Partially Separate Sewer System
These are types of sewer systems where sewage from homes and industries, in addition to stormwater from the backyard of homes are discharged into wastewater treatment plants. While stormwater from the front yards, streets, and roads are discharged into separate drains which are further discharged into natural water bodies.
2. Types of Sewer Systems According to Materials Used
Materials used for sewers can be asbestos, brick, cement, plastic, steel, or cast iron. Materials to be used are selected based on consideration of factors such as the volume of sewage to be conveyed, the source of sewage, etc. the type of sewer systems in this category are;
- Asbestos Cement (AC) Sewer Systems
- Brick Sewer Systems
- Cement Sewer Systems
- Cast iron (CT) Sewer Systems
- Steel Sewer Systems
- Plastic Sewer Systems
Asbestos Cement (AC) Sewer Systems
Asbestos Cement Sewers (AC Sewers) are types of sewer systems manufactured from a mixture of cement and asbestos fiber. Asbestos Cement. They are used in conveying domestic or sanitary sewage to treatment plants.
When the two-pipe system of plumbing is used in multi-story buildings, asbestos cement sewer is best used as a vertical pipe for carrying sullage from the upper floors of the building.
AC sewers are smooth, light in weight, durable, non-corrosive and can be easily cut, fitted, and drilled. They however cannot withstand heavy loads and are easily broken in handling and transport.
Brick Sewer Systems
These are types of sewer systems that are manufactured on-site. They are used for the construction of large sewage systems. They are also used in combined sewers.
Brick sewers are difficult to construct. They can crack easily causing leakage. For this reason, it is very important to plaster them.
Cement Sewer Systems
These days, cement sewers are replacing brick sewers. This is a result of cracks and leakage associated with brick sewers. Cement concrete sewers can be cast in situ or precast. They are resistant to heavy loads, corrosion, and high pressure. And are also heavy and difficult to transport.
Cast iron (CT) Sewer Systems
Cast iron sewer systems are superior to cement, asbestos, and brick sewers in durability. They are watertight and withstand high internal pressure and heavy loads. These types of sewer systems are used in places like below highways and railway lines. And in places where there is a considerable temperature difference.
Steel Sewer Systems
Steel sewers are light, impervious, flexible, and resistant to high pressure. They are used when sewage has to flow across a water body and underneath a water body or railway track. Steel sewers are also used for outfall and trunk sewers.
Plastic Sewer Systems
Plastic sewers are the types of sewer systems commonly in use. It the light-weighted, smooth, resistant to corrosion, and can be easily bent. However, they cannot be used in places with high temperatures.
3. Types of Sewer Systems According to the Source of Sewage.
The types of sewer systems in this category are;
- Domestic Sewage, Systems
- Industrial Sewage Systems
- Storm Sewage Systems
Domestic Sewage, Systems
Domestic sewage systems are also known as sanitary sewage systems. A sanitary sewerage system includes laterals, subdomains, and interceptors, underground pipes and manholes, pumping stations, and other appurtenances that convey sewage from homes to wastewater treatment plants.
Sanitary sewers have pipes that collect water from kitchen sinks, bathtubs, water cisterns, and laundry into the treatment plants. Wastewater carried includes greywater and blackwater or sullage. Graywater is liquid wastewater from the kitchen, laundry, and washrooms that do not contain human or animal waste. Blackwater is wastewater generated from toilets.
Industrial Sewage Systems
Industrial sewage systems carry wastewater from the point of generation to treatment plants. Industrial wastewater is not usually conveyed alongside domestic wastewater because industrial effluent contains special toxic substances.
Wastewater from industries consists of sullage and discharge from chemical processes which must be thoroughly treated before final discharge into waterways.
Storm Sewage Systems
Stormwater sewage systems collect runoff from precipitation ( rain and snow), into a system of pipes or open channels (manholes, ditches, swales,) and other conveyance methods from which they are discharged. In some places, water from runoff sewers does not undergo any form of treatment before discharge. They might be discharged directly into lakes, rivers, streams, and other water bodies, or into reservoirs where they are stored up for irrigation during the dry seasons.
What is the difference between sanitary sewer systems and others?
Sanitary sewers are sewers that convey wastewater generated in homes only.
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