What are the differences between gravity, vacuum, and low-pressure sewer systems?
Wastewater collection systems are the infrastructure constructed to transport wastewater and solids from source such as residential, commercial and industrial areas to a treatment location prior to final discharge. There are three main types of sewer collection systems: gravity, vacuum, and low-pressure systems. This article provides a summary of each, including details why one may be selected over another.
Gravity sewer construction
Gravity sewer systems explained
The traditional gravity sewer system is the most common type designed to transport wastewater. This type of system has pipes laid with a downhill slope that use gravity to convey wastewater from homes and businesses towards a treatment plant. Gravity sewer systems have no power requirement from the source to the lift (pump) station and are cost-effective to install.
The design challenge occurs as the pipes must be constructed to match the local land contours and in flatter areas become progressively deeper throughout the network to maintain a continuing downhill slope. Engineers are familiar with designing this type of network and it results in discrete sewer catchments with gravity mains feeding to a wastewater lift station and forced main that transport the collected wastewater to a higher level and closer to the treatment plant.
Why design a different system?
The cost of construction and ongoing maintenance increases in areas that have a high-water table, hard/rocky ground, or aggressive acid sulphate soil. When engineers are looking to sewer new areas that have flat topography; gravity sewer networks are not always the optimal design after completing a triple-bottom line analysis.
How does a low-pressure sewer system work?
A low-pressure sewer (LPS) system uses a grinder pump to macerate and pump wastewater from homes and businesses. The wastewater is transported through small-diameter pipes under low pressure to a collection tank or treatment plant. Low-pressure sewer systems are ideal for areas with challenging topography, such as very flat areas, steep hills, or rocky terrain.
However, they require regular maintenance and are more expensive to install than gravity sewer systems. LPS systems require additional infrastructure on the customer’s property including the underground tank and pump, and need a control panel to alert if high levels or other faults occur. Electrical costs to power the pump are generally paid by the landowner.
Typical low-pressure grinder pump and storage tank
What are the advantages of a low-pressure sewer system?
Cost-effective installation: Low-pressure sewer systems can be installed at a lower cost compared to traditional gravity sewer systems. This is because they require smaller pipes and shallower excavation depths.
Flexible layout: Low-pressure sewer systems can be designed to follow the contour of the land, which means that they can be installed in areas with challenging terrain or where gravity sewer systems would be difficult to install.
Reduced inflow and infiltration: with a sealed system and no manholes, there are only very limited situations where wet weather would increase flow.
Reduced risk of pipe blockage: Low-pressure sewer systems are less likely to experience backups because they use individual grinder pumps to transport wastewater from each property to the main sewer line.
Reduced odours: The sealed nature of the low-pressure sewer system means that there is less opportunity for odours to escape from the system, leading to a more pleasant environment for residents.
Easy to expand: Low-pressure sewer systems can be easily expanded to accommodate new developments or additions, making them a flexible option for growing communities.
How does a vacuum sewer system work?
A vacuum sewer system uses a vacuum pump and differential pressure to remove wastewater from homes and businesses. Each property drains to an on-property collection pit, which is connected through the vacuum pipe system to a large vacuum pump station. When the collection pit valve opens, there is a pressure differential that sucks the wastewater from the pit into the connected pipe system, the pit outlet valve closes when the pit is emptied.
The pipe system is laid in a sawtooth profile that maintains a pressure differential and transports slugs of wastewater towards the vacuum station as valves in the system open with a blast of atmospheric pressure. The large vacuum pump at the end of the line will turn on and off intermittently to maintain the correct vacuum pressure range at the downstream end of the system.
Vacuum Sewer Schematic (Srstevens3, CC BY-SA 4.0)
What are the advantages of vacuum sewer system?
Cost-effective installation: Vacuum sewer systems only require shallow and narrow excavation compared to traditional gravity sewer systems resulting in a faster and cheaper construction cost.
Flexible layout: Vacuum sewer systems pipe routes are less affected by local topography and can be designed to follow the contour of the land, which means that they can avoid running through private property and be installed in areas with challenging terrain or where gravity sewer systems would be difficult to construct.
One pump station: Pumping is achieved by a single pump station at the end of the system.
No exfiltration: The vacuum nature of the system means that exfiltration should not occur.
Reduced odours: The sealed nature of the vacuum sewer system means that there is less opportunity for odours to escape from the system, leading to a more pleasant environment for residents.
About the Author
Mark Lee is the Business Development Manager (AUS/NZ) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.