A gravity sewer system collects wastewater from residences and transports it to a collection line. Gravity is the force behind the flow. Collection lines are pipes that are installed at a slope to keep both water flowing and solids
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. 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 residence'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 high reliability and low upfront costs.
Right Pump, Right System
Pressure sewer systems have key advantages to consider. 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 obstacles with virtually no environmental impact.
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 for wastewater collection challenges.
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 maintenance costs.
4. Reduced Capital Costs
Pressure sewer systems can reduce capital costs, especially initially. 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.
5. Odor Control
Pressure sewer systems often control odor more effectively. Odors can be an occasional problem in any wastewater system. Large-diameter, long-distance gravity mains can be a source of gas generation and, therefore, 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.
A version of this article appeared in the September 2014 issue of Pumps & Systems
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\up0 Walt Erndt
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is vice president and general manager, Municipal Group at CRANE Pumps & Systems.\
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\f8\i0\fs16 \cf0 \kerning1\expnd0\expndtw0 \up0 Pressure sewer system in a U.S. neighborhood\
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\f9\i\fs14 \cf0 (Image courtesy of CRANE Pumps & Systems)}