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Consider factors such as impeller type, input rate, head requirements, and dispersion distance when choosing and sizing a septic pump.
By Don Talend
As onsite treatment of wastewater becomes more common, conventional septic systems are becoming a little more powerful. In some areas where exurbia continues to push outward from municipalities equipped with sewer systems, terrain and soil may be less than ideal for traditional gravity-type septic systems and site managers are more typically investigating different pump types and capacities to disperse effluent to their leach fields.
Still, the most common type, gravity systems, relies on the most well-known law of physics to periodically dispense effluent to a leach field or distribution box located at a lower elevation than the septic tank. Raw sewage enters a two-chamber septic tank from the building on the site. Solids sink to the bottom of the first chamber via gravity, and liquids flow into a second effluent chamber. Bacterial action breaks down the solid waste into smaller particles. Liquids are also broken down by bacterial action and gradually dispensed to the leach field through lateral pipes that extend beneath the surface of the soil to the leach field. The effluent is gradually absorbed into the leach field, which is sometimes engineered with gravel-filled drainage trenches, through perforations in the pipes.
But as land becomes scarcer in some areas, the site manager might not have the luxury of terrain that accommodates a gravity system. In order to dispense effluent from the tank over elevations in the surrounding terrain, the system may require a pump. The choice of a type of pump depends upon manufacturers’ designs, and sizing a pump depends upon factors such as the input rate, head requirements (i.e., the vertical lift capacity), the distance of the leach field from the tank, the lateral pipe diameter, and the rate at which the soil in the leach field can absorb the effluent.
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Photo: Zoeller Co. |
| Pump choice depends on
manufacturers’ design. |
Submersible Centrifugals
The most popular types of septic pump are submersible centrifugal pumps. These are used either in enhanced-flow systems in which effluent is pumped up to a distribution box or distribution manifold and then gravity-dispersed to the leach field or in low-pressure pipe (dosing) systems in which lateral pipes equipped with small holes are pressurized to disperse the effluent—more common where soils are not particularly absorptive and suitable for gravity dispersal.
The reason that these kinds of pumps are submerged in the liquid in the septic tank is for durability. Although it would seem that this material would be destructive to a pump, the really harmful materials in the tank are the gases that rise to the top as the effluent is treated with bacterial action. “Preferably, we’re going to want them to be submerged,” says Mike Babrowski, vice president and marketing manager at Zoeller Co. “The typical application requires a variable-level switch—that enables you to keep the pump submerged. The reason for that is that the gases inside the tank are pretty nasty and so it’s preferable to keep your pump submerged. The typical effluent pump is cast iron or plastic with stainless steel hardware on it because of the corrosive environment.”
Submersibles are activated in one of two ways. With a variable-level switch, “When the water reaches a predetermined point, the switch will activate the pump and pump it down to a certain level and they can calculate how many gallons and give you the right dosage,” Babrowski explains. Alternatively, the effluent can be time-dosed from the tank at preset time intervals.
These pumps use centrifugal force to move the effluent. A rotating impeller generates the centrifugal force, which creates motion in the liquid. Technically speaking, the impeller creates kinetic energy in the liquid. Head pressure, which diverts the effluent through the lateral pipes, is generated by the pump volute—the stationary pump casing that converts the kinetic energy into pressure.
Manufacturers offer a wide variety of impeller designs. When deciding which type of pump to buy, the wastewater treatment manager should get an understanding of the operating principle behind the impeller design and a trade-off between clogging prevention and head pressure.
The Sump and Sewage Pump Manufacturers Association (SSPMA) divides pump designs into four categories. Open impellers are not equipped with backing plates and handle screened or unscreened solids in high-capacity, low- to medium-pressure applications. Semi-open impellers are designed with vanes attached to a solid backing plate. Enclosed impellers have backing plates both above and below the vanes. Vortex impellers use shortened vanes that create a vortex—a small whirlpool-like pocket of kinetic energy—in the liquid and move the liquid and solids along the edges of the vanes and the volute.
“The impeller teases water,” Babrowski says of the vortex impeller. “When you were a kid and you were in a tub of water, if you ever moved your hand in a rapid circular motion, when you pulled your hand out, what happened? The water kept traveling in a circular motion—that’s what a vortex does.” The idea behind generating a vortex is to minimize wear and tear on the impeller from solids. Babrowski notes that the vortex creates a gap between the impeller and any solids passing through, minimizing wear on the impeller.
Andrew Buuck, an applications engineer for Franklin Electric Co.’s wastewater products, notes that the choice of a vortex pump involves a trade-off. “The vortex is nice for being able to handle larger solids,” Buuck says. “Sometimes you lose a little bit of flow when you do that because you don’t have that power in the volute itself.”
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Photo by Zoeller Co. |
| Impeller design is a trade-off between clog prevention and head pressure. |
Some impellers are designed to prevent clogging even though the vanes come in direct contact with the effluent. One manufacturer has an impeller with two long vanes that are connected to a back plate, the presence; the presence of only two vanes minimizes the odds that the impeller will get clogged up with solid material.
Close-tolerance impellers have a very small space between the impeller and the bottom of the pump housing, which keeps larger solids from contacting the impeller. Like open, semi-open, and enclosed impellers, close-tolerance types do not have great solids-handling capacities. “The close tolerance will give you more head or [gallons per minute], but you’re trading off solids handling and non-clogging capabilities,” says Babrowski.
Manufacturers of vortex-type pumps do try to ensure that the equipment can disperse some solids. “They should have some solids handling capability,” notes Babrowski. “That varies from state to state—some states say three-eights, half-inch; some states allow as much as 2 inches. We prefer to have something in the neighborhood of half an inch to three-quarters of an inch; you don’t want the possibility of big solids going out to your field.”
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Pump Sizing Tutorial Available |
Turbine Pumps
Increasingly, manufacturers are recommending pumps with higher head capacities for customers. “The advent of the onsite wastewater treatment systems has been the thing that has brought the multi-stage pumps into use, where before they were always traditionally associated with clean water or water systems coming into the home,” says Charlie Utley, training manager for Franklin Water Transfer Systems. “We are seeing some higher head submersible centrifugals due to the requirements in some cases where you need higher pressure. Where typically a one-third-horsepower effluent motor pump might have a shutoff head of somewhere around 18 to 20 feet of head, now they’re designing those to operate closer to 30 feet of head to achieve the extra pressure they want for the effluent systems.”
In addition to recommending more powerful centrifugal pumps for septic systems, some manufacturers are recommending another type of pump that was previously used for clean-water applications. Turbine pumps are multi-stage centrifugal pumps that were previously used in wells, sprinkler systems, or subsurface drip irrigation systems. “It’s a 4-inch well pump that’s being used to pump effluent and that gives you high head capabilities,” says Babrowski. “We use that type of pump, but what we do is we put in a step filter system. Basically what we do is filter the water before it gets to the pump because these pumps can be clogged because they’re not designed for solids handling—they’re designed for clear water.
“I believe that the market is growing because there are more and more septic tanks being installed today,” Babrowski continues. “The areas they’re getting into are hilly and require a pump. Probably the biggest change is that an increasing number of these turbine pumps are being used today versus the standard centrifugal effluent pump, and that’s because they have requirements for more pressure to pump, more head, or more distance.”
“They’re usually filtered because of the construction of the pump; you don’t want any solids in there,” Utley adds of turbine pumps. “They’re fairly small flow compared to your submersible centrifugals, smaller flow and higher pressure, but that’s just to be able to produce the pressure you need at the dispersal point to operate a sprinkler head or to operate emitters in subsurface irrigation.”
Sizing the Pump Properly
The SSPMA recommends focusing on two main factors when sizing a pump: the flow requirement or pump capacity to meet system demands and the total dynamic head (TDH).
Generally speaking, a pump should be able to move effluent at a minimum flow rate of 2 feet per second; most pumps are rated to meet this requirement. The minimum acceptable flow rate is directly proportional to the lateral pipe diameter. In a low-pressure pipe system, the flow rate corresponds to the length and diameter of the lateral pipe, the number of pipes and fittings, and the number and size of holes in the perforated lateral pipes.
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Photo by Zoeller Co. |
| Friction head depends on flow rate and pipe diameter. |
TDH is the force required to move effluent from the lowest point in the system to its destination. In mathematical terms, it is the sum of static head, friction head, and operating head. Static head is the vertical height from the minimum water level (or pump shutoff level) in the septic tank to the distribution box or distribution manifold. The friction head depends on the flow rate and pipe diameter and is calculated according to the resistance in straight lateral pipes connected to the tank plus the resistance from fittings, valves, and other system components. These components must be converted to an equivalent length of straight pipe, and the SSPMA has worksheets for making these conversions per 100 feet of pipe component. Operating head is the pressure required to operate the system.
Pump manufacturers have various tools for sizing septic pumps to their applications, but several experts cited some common factors to consider. “It depends on the percolation of your soil, which is determined by the health department, which does a perc test,” says Babrowski. “They determine the percolation rate, which determines how often you’ve got to dose the field. You’ve got to have the soil determined because that’ll determine how much lateral line you need and how often it needs to be dosed. How high you’ve got to pump, how far you’ve got to pump, the diameter of the pipe, the size of the lateral fields, and your GPM requirements—do you need 40 gallons a minute or do you need 10 gallons a minute?—all are equally important.”
“It’s all dependent on where your pump is and where you’re pumping it as well as how many units and fixtures you have coming into that system,” says Buuck. Franklin Electric Co. has a method for calculating the right pump capacity with inputs such as how many fixtures come into the system, the distance to the leach field, pipe friction loss, and flow rate of material coming into the house.
When recommending a pump type and size, says Utley, “We always recommend a pump that’s designed for continuous duty, even though they’re rarely running continuously. That gives you a little added safety factor and they last longer. In order to do that, they have to run fairly cool.” Newer pumps are made of cast iron or newer plastics that dissipate heat from the motor. Also, “They’ve become more of a filtering situation, filtering the effluent before it enters the pump. They last a lot longer because they’re not dealing with the solids that they used to.”
Babrowski adds that filtering is becoming more common at the point where effluent is being dispensed. “Effluent filters are becoming more popular,” he says. “They’re installed in the outlet baffle; six or eight states require them by code now. What it does is it filters the effluent before it goes out to laterals. It helps prevent clogging up of lateral lines.”
Don Talend of Write Results in West Dundee, IL, is a communications and publicity consultant who has worked in print media for more than 15 years.
OW - September/October 2007 |
