Gravity sewer systems are the primary method used by municipalities to transport residential sewage waste to a collection or treatment facility. Gravity sewers use the force of gravity to push fluid through networks of large sloping pipes to the sewer main. For all sewer technology, fluids require a minimum flow velocity to keep solids suspended. When solids settle in a wastewater system bad things happen, such as blockages or odor problems.
States and municipalities publish design criteria which dictates the standards for the required minimum flow as well as the minimum pipe diameters and minimum slopes to accomplish these flows. What this means for contractors is that a sewer main must be installed deeper than the discharge lines from each source connected to it. Lift stations can be used to pump fluid to a higher elevation, which allows the sewer main to be located closer to the surface; however, lift stations can be expensive to construct and more expensive to maintain. Overall, the construction of sewer infrastructure requires extensive digging of trenches, which is costly and has an environmental impact. These challenges are more difficult in areas where water tables are high or the landscape is hilly, rocky or flat.
Pressure sewer systems were designed in the early 1970s as an alternative for applications where gravity sewer systems were less feasible or economical. It provided a means to combat difficult terrain and reduce the upfront cost of slow buildout projects or developments. The primary differences between conventional gravity sewer systems and pressure sewer systems are in the piping network and the reduction of solids size in wastewater at each residence. Pressure sewer systems utilize a network of grinder pumps to transport wastewater through small diameter pipes, directly to a sewer main, lift station or treatment system. A grinder pump is a submersible pump designed to reduce particulates in a waste stream into a fine slurry using a grinding mechanism. In order to overcome challenging terrain, pipes are installed just below the frost line and follow the natural landscape. The pumps must operate with high pressure and limited flow when compared to a gravity system.
There are two basic forms of grinder pumps that can be used in pressure sewer applications—progressive cavity or centrifugal. With a progressive cavity grinder, pumping is performed by a specially shaped, stainless-steel rotor that turns within a multilobed rubber stator. The friction developed between the two components develops the pressure used to transport fluid. Centrifugal grinder pumps use a vortex impeller to create pressure and flow. As the impeller rotates, it creates a vortex in the pump casing that carries the slurry outward from the volute chamber to the piping system.
History of Progressive Cavity Grinder Pumps in Pressure Sewers
Originally, progressive cavity grinders were the pump of choice for pressure sewer applications due to their ability to pump with a narrow range of flow (10 to 15 gallons per minute [gpm]) at high head pressures (up to 150 feet). Progressive cavity pumps are not self-limiting with respect to pressure; as the system volume increases, so does the pressure. If the system pressure exceeds the shutoff head pressure for the pump, the pump will continue to try to overcome back pressure until something breaks down. The breakdown could be as simple as a tripped thermal sensor or as serious as a broken line or damaged pump. At the least, the increased pressure leads to increased wear. Since these pumps operate by way of friction, the components of the pump are experiencing continuous wear.
Wear is accelerated with high pressure, high flow, long run times or the addition of grainy particulates in the waste stream, such as sand and dirt. The pump stator of a progressive cavity grinder is both the highest wear item and one of the more expensive components to replace, making repair or premature failure costly.
Dual-Stage Pumps
For a long time, centrifugal grinder pump manufacturers were offering the standard single-stage grinder in pressure sewer applications. These pumps could make do in some applications and could offer more flow than a progressive cavity grinder (55-60 gpm vs 15 gpm, respectively) but were limited to about 100 feet head (vs 150 feet). The benefit of a centrifugal pump was the characteristic of running along a pump curve. The pump can safely operate by idling at shut-off head without causing any damage. Unlike the progressive cavity pump, the centrifugal pump will simply wait for the system pressure to stabilize and will return to pumping as normal.
Due to these benefits, the single-stage centrifugal grinder pump was accepted as a good alternate in the market compared to the progressive cavity grinder pumps in lower head applications; however, that left a gap in the higher head applications. To address the market need for a higher head centrifugal grinder, dual-stage, centrifugal grinder pumps were developed specifically for pressure sewer applications. By sacrificing some flow, the addition of a second-stage impeller allowed double the head pressure. The result of this innovation was a centrifugal grinder pump that can operate at 200 feet head pressure and that, in low pressure situations, can pump nearly 30 gpm. Because of their higher flows, one centrifugal grinder pump operating under normal conditions can scour a two-inch line, opposed to progressive cavity grinder units, which require two to three pumps running at the same time to scour a 2-inch line. Keeping lines scoured is important for minimizing odor and preventing blockages. The cost to operate a dual-stage centrifugal grinder is often less expensive due to the shorter run times required because of higher flow.
The beauty of pressure sewer systems is their interchangeability and flexibility. Grinder pumps are designed for compatibility, so as long as there is enough flow entering a system, that system will perform properly, regardless of the grinder pump technology used. As a result, there is the ability to use either centrifugal, progressive cavity grinder technologies or both technologies together in a single system. These residential grinder stations can be used to develop full pressure sewer systems, or used with gravity sewer systems as a tool for hard-to-reach locations or one-off upgrades, like replacing a septic system.
Manufacturers of both centrifugal and progressive cavity grinder pumps offer upgrade units, which allow users to easily integrate their product into an existing station to improve pump or system performance with minimal material and labor costs.
Systems are constantly changing; capacity will fluctuate over time as inflows can increase or decrease based on factors of the installation’s environment. In some cases, upgrading the pump technology can solve existing problems in a system, such as increasing fluid velocity. This can help to keep a line scoured and decrease wastewater fluid retention time, assisting in odor control and preventing corrosion.