Most homes built before 1970 are connected to a sewer through a gravity sewer system. A gravity sewer system collects wastewater from homes and transports it to a collection line by allowing gravity to force the flow. Collection lines are pipes that are installed at a slope to keep both water flowing and solids in suspension.
A gravity sewer system involves digging wide, downward-sloping trenches. This method works for downhill grades but is challenging when the terrain is rocky or slopes uphill. Other challenges include a high percentage of sandy soil, proximity to water or long piping runs. Lift stations are often combined with gravity sewers to “lift” the sewage flow and ensure that it keeps moving.
A pressure sewer system can offer a more effective and less costly method for sewage transport. First used in the early 1970s, a pressure sewer system uses a grinder pump located either outside the home or in a home’s lower level or basement. The grinder pump moves the home’s wastewater through small-diameter service lateral pipes to a larger collection system.
Today, pressure sewer systems provide daily service to more than half a million users worldwide and range from a single pump to thousands of pumps. A pressure sewer system provides excellent performance, high reliability, low upfront infrastructure costs, and affordable operation and maintenance costs.
Right Pump, Right System
A grinder pump is a submersible pump that includes a grinding mechanism. The pump grinds wastewater particulates to a slurry that can flow through small-diameter pipes. The grinder pump usually operates for less than 60 seconds as it moves wastewater through a home’s service collection lateral and into the pressure sewer collection system.
Grinder pumps in residential pressure sewer systems are typically installed in a simplex (one pump) configuration. Progressing cavity and centrifugal pump designs are available. Progressing cavity pump designs are usually 1-horsepower (HP) units, occasionally 2 HP. Centrifugal pump designs range from 2 HP to 15 HP. Some centrifugal pump designs use dual-stage technology to meet higher pressure demands.
Pressure sewer systems using grinder pumps are particularly effective in new construction, second-home communities and existing communities with aging septic tanks. Their smaller pipe diameters in the piping network reduce the solids size in the wastewater. Pressure sewers’ low operating and maintenance costs can economically solve the problems of aging septic tanks and unsatisfactory soil conditions.
Case Study: The Hideout
In 2010, the Roamingwood Sewer and Water Association (RS&W) and its engineering consultant, Cardno BCM, faced a failing gravity-based sewer system that included 29 duplex intermediate pumping stations. RS&W services The Hideout—a 3,200-home, gated community in the Pocono Mountains of northeastern Pennsylvania. This recreational community includes several lakes, a ski hill, a nine-hole golf course and centralized well-water supply and wastewater treatment facilities. The entire 2,700-acre community is built on a rocky foundation in hilly terrain.
The 29 existing pumping stations included the pumps of several manufacturers, which increased spare parts and maintenance costs. Pumps were running six to eight times per hour—instead of the planned six to eight times per day—as a result of extensive infiltration/inflow.
A grinder pump pressure sewer system at each home proved to be the most economical solution for RS&W. After a comprehensive hydraulic analysis and engineering design, the project was advertised for competitive bids.
RS&W selected a pressure sewer design using a combination of approximately 1,500 basin units and 500 indoor units, both outfitted with centrifugal grinder pumps.
Jack Lennox, RS&W’s executive director, says, “Replacing the existing gravity/pumped sewer system with an equivalent new system ‘in kind’ would have cost about 30 to 40 percent more than the pressure sewer system we are now constructing.”
Pressure sewers are typically buried at a fixed depth below the frost line and follow the contours of the land. They can deliver wastewater to a downstream collection manhole, to a force main or directly to a wastewater treatment plant. They can also move wastewater several thousand feet to a discharge point at a higher elevation.
Pressure sewers differ from gravity sewers, in which sewer pipes are necessarily larger (because they are not under pressure) and wastewater can only be conveyed downhill.
In the instance of RS&W’s Hideout community, the minimum sewer diameter in the failing gravity system was 8 inches, and ground elevation variations within the community exceeded 210 feet. In the upgrade to a pressure sewer system, new sewer pipe diameters range from 1.25 to 4 inches. These new pipes can convey wastewater across the full range of the community’s elevation changes.
A typical installation includes:
- A sewage basin or tank
- A grinder pump
- A lifting device to remove the pump for inspection and maintenance
- A check valve on the pump discharge line to prevent backflow when the pump switches
- A 4-inch basin inlet pipe from the home
- A small alarm panel on the side of the home
- A vent on the top of the tank
- A cover with optional access doors
- A curb box that will contain a shutoff valve to separate the pump from the system and a redundant check valve to maintain isolation between the tank, station and complete system
- Level sensors and an optional cable holder
- A junction box or electrical connection
5 Pressure Sewer Advantages
Pressure sewer systems have key advantages when compared with gravity sewer systems. These advantages can be categorized in five areas, which are described in this section.
1. Smaller Pipes
Smaller pipes requiring narrower and shallower trenches reduce material and installation costs. In some cases, piping can be installed using a technique called directional boring. Directional boring allows for installation of pipelines underground without an open trench. Directional boring involves an operator steering a small drill underground. The drill can turn corners, go uphill or downhill, or move around a tree with virtually no environmental impact.
Although directional boring was not employed in The Hideout, the smaller pressure sewer pipes represented a significant advantage for RS&W stakeholders. The design eliminated the need for the wider and deeper excavations with a gravity system.
2. Compatibility with Existing Collection Systems
Pressure sewers are compatible with other existing collection systems. Pressure and gravity sewer technologies can be mixed and matched into site-specific designs to provide a complete solution to wastewater collection challenges.
In RS&W’s case, modeling software was used to design a pressure sewer system that eliminated the need for the community’s 29 existing pumping stations. This system is expected to reduce annual power costs by $80,000. Lennox estimates that once the complete system is up and running, the community will see approximately $500,000 in annual savings. Using the monitoring software, the team was able to demonstrate exactly how the complete pressure sewer solution would work under both present and future conditions.
3. Freedom from Infiltration
Pressure sewers offer freedom from infiltration/inflow because the systems are sealed. With residential pressure sewers, the only extraneous water entry points in the system between the homes and the treatment plant are the homes.
With essentially no secondary leakage, wastewater flows are lower and wastewater characteristics are more consistent, which facilitates treatment and reduces energy costs. The life of an existing wastewater treatment plant can be extended, or the capacity of a new wastewater treatment plant can be reduced. The treatment process will not be upset by storm-related infiltration/inflow.
A gravity-based system requires manholes to be placed at set distances and at every pipe-turn. These manholes are potential entry points for infiltration/inflow. Higher infiltration/inflow complicates treatment and increases operating and maintenance costs. Infiltration/inflow had a severe adverse impact on the Hideout utility, according to Lennox.
“In RS&W’s situation prior to 2010, fully 75 percent of the flow entering our treatment plant was groundwater (inflow and infiltration), not sewage,” he said. “Influent wastewater strength is so weak and variable that it threatens the stability of our treatment process. By blocking infiltration/inflow, the new pressure sewer system should extend the life of our treatment plant very substantially. We will move from a plant that is near its design capacity to one that has capacity to spare.”
4. Reduced Capital Costs
Pressure sewer systems can reduce capital costs. Gravity systems require a significant upfront investment in excavation and piping before connecting any homes to the system. Upfront capital costs associated with pressure sewer systems are generally lower than other sewer systems.
5. Odor Control
Pressure sewer systems control odor more effectively than gravity systems. Odors can be an occasional problem in any wastewater system. Large-diameter, long-distance gravity mains can be a source of gas generation and odor. The short retention time and closed network of the small-diameter pipelines of a pressure sewer system are inherently less susceptible to these odor problems.