Last Updated on October 8, 2022 by admin
A sewer force main is a pressurized pipeline that conveys wastewater from a sewage pump station to another pump station or a wastewater treatment plant. Force mains are used when gravity flow is not possible or practical. Instead, a force main may be used to convey wastewater uphill or over long distances. Wastewater in a force main is moved by pumps that push the wastewater through the pipe.
Sewer force mains can range in size from small to large-diameter pipelines. The typical sewer force main is 8 inches or larger. Larger force mains are often found in public sewer infrastructure. While older sewer force mains can be made of various materials, such as cast iron pipe; today sewer force mains are typically made of ductile iron pipe (DIP), polyvinyl chloride (PVC), or high-density polyethylene (HDPE). DIP and PVC are rigid materials susceptible to breaks due to ground settling or external forces such as vehicular traffic, while HDPE is flexible, making it resistant to breaks. However, other factors may make DIP or PVC a better choice. The material selection for a sewer force main is selected during the design process – specifically while developing the basis of design for the pipeline.
The Importance of Sewer Force Mains
Sewer force mains are essential to the overall wastewater collection and transmission system because they allow for the conveyance of large volumes of wastewater over long distances. Without sewer force mains, many areas would not have access to adequate wastewater treatment or proper sewer services.
Gravity sewers are often not possible or practical in certain situations – such as when an area to be served is hilly or too far away from a treatment plant. A force main may be the only way to provide adequate sewer service in these cases.
Installation of Sewer Force Mains
Sewer force mains are usually buried underground. However, they may also be located above ground if necessary. Above-ground force mains are typically found in locations where excavating would be difficult, such as waterway crossings on pylons or hanging from bridges.
The installation of a sewer force main can be a difficult and expensive task. First, the route for the pipeline must be determined. Once the route is selected, the trench or borehole for the pipeline must be excavated (depending on the construction methodology – open cut vs. trenchless.) The size of the trench or borehole will depend on the size of the pipeline being installed.
After the trench or borehole is excavated, the pipeline can be installed. When performing an open-cut installation (trenching), the pipeline is usually installed in sections, with each section being connected using fittings or a bell and spigot joint. When performing a horizontal directional drill (HDD), the pipeline is typically fused and slipped through the borehole. However, this is not always the case. Microtunneling and jack-and-bore, both trenchless construction methodologies, are installed in sections, while open cut pipe can also be fused.
Once the pipeline is installed, it must be tested to ensure it is functioning correctly. This is typically done by filling the pipeline with water and ensuring that there are no leaks. Once the pipeline passes the pressure test, it is ready to be placed into service.
Design Life of a Sewer Force Main
The lifespan of a sewer force main can vary. It is highly dependent on the material and site conditions. For example, if a DIP sewer force main is in a saline environment and does not have proper cathodic protection, the force main can last less than 20 years. However, if the force main is adequately designed and installed, there are instances that the life expectancy is 70+ years – this is often the case with directional drilling HDPE.
Factors such as corrosion, infiltration, and external forces can cause premature failure of a force main. Regular maintenance and inspection can help extend the life of a sewer force main.
Corrosion is often the most common cause of force main failure. The corrosive nature of sewage can cause internal corrosion of the pipeline – leading to leaks or breaks. External factors such as soil chemistry and water table can also lead to corrosion. This is particularly the case in a highly saline ground water table.
In low-lying coastal areas where the water table has saltwater intrusion, the fluctuation of the water table (its rise and fall) can create an extremely hostile environment for iron pipe (DIP or cast iron.) The saltwater promotes oxidation when it submerges the pipe and falls, exposing the pipe to oxygen. This environment predisposes an iron pipe to early failure.
Pipeline Analysis for Sewer Force Mains
The material selection for a sewer force main depends on the environment in which it will be installed. For example, if a force main is going to be installed in an area with a highly saline water table, an inert material such as HDPE or PVC might be preferrable due to its high corrosion resistance.
Material Lifespan (Years)
Below are some typical design life estimates for sewer force main materials. However, it should be noted that this is only a rule of thumb.
| Material | Design Life |
| Ductile Iron Pipe (DIP) | 20-70 years |
| HDPE | 70+ years |
| PVC | 50+ years |
| Concrete Pipe | 40-70 years |
The decision on what material to use for a sewer force main should be based on a comprehensive project site analysis. This analysis should consider the environment, geology, soil chemistry, and water table. In addition, the type of wastewater (sewage or treated effluent) should also be considered, as this will impact material selection.
Proper Operation and Maintenance of Sewer Force Mains
Force mains must be properly designed, installed, and maintained like all sewers. Poorly designed, installed, or maintained force mains can lead to a variety of problems, including:
– Overflows and sanitary sewer overflows (SSOs)
– Pump station flooding
– Reduced treatment plant performance
– Human contact with the sewage
– Odors
– Groundwater contamination
To avoid these problems, it is essential to have a comprehensive maintenance and inspection program in place. This program should include regular cleaning and televising of the force main and periodic testing of the pump station. In addition, the program should consist of a schedule for replacing pump station parts and force main segments.
A well-designed and maintained sewer force main can provide years of trouble-free service. However, if the system is not properly designed, installed, or maintained, it can lead to various problems. Regular cleaning and inspection are essential to ensuring the longevity of a force main.
Where Do Sewer Force Mains Discharge the Wastewater?
Sewer force mains can discharge to various wastewater system components, including other force mains, sewage pump station wet wells, gravity sewers, wastewater treatment plants, and even outfalls to running rivers and the oceans (in this case, the wastewater is typically treated effluent.)
If a force main discharges to another force main, this is called a manifold system. In a manifold system, each force main has its pump that pumps the wastewater to the next force main in the series.
A force main can also discharge to a sewage pump station wet well. In this case, the force main will connect to the wet well and discharge via free flow (essentially dumping the wastewater into the wet well), where a submersible or dry-pit pump will repressurize the wastewater and pump it into a new force main. This is called a repump arrangement and is sometimes called a piggyback pump station.
If a force main discharges to a gravity sewer, it will typically connect to the gravity sewer via a manhole. The wastewater will then flow via gravity through the gravity sewer to its destination (usually a sewage pump station.)
A force main can also discharge directly to a wastewater treatment plant. In this case, the force main will typically connect to the plant at the headworks. The wastewater will then undergo pretreatment, where heavy solids are removed before it goes to primary, secondary, and tertiary treatment.
Finally, a force main can discharge directly to an outfall. In this case, the wastewater is typically the treated effluent from the wastewater treatment plant. The wastewater will then be discharged directly into the receiving water (usually a river or the ocean.) It should be noted that if a force main discharges to an outfall, the effluent must meet discharge standards set by the EPA or state regulators.
It is important to note that a force main can discharge to more than one of these components. For example, a force main can discharge to both a sewage pump station and a wastewater treatment plant. In this case, the force main will typically have two connections, one for each destination. This happens when a larger force main splits into two smaller force mains.
Sewer force mains are an important part of the overall sewer system because they allow for the conveyance of large volumes of wastewater over long distances. Without sewer force mains, many areas would not have access to adequate wastewater treatment. Sewer force mains are usually buried underground but may also be located above ground if necessary. The typical lifespan of a sewer force main is 50 years or more, but regular maintenance and inspection can help extend the life of a sewer force main.
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