Sewage, Septic and Sump Pumps
This post is about septic system pumps and making sense of the range of pump types and their specific uses. Broad categories of pumps used for water and wastewater management include sewage pumps, grinder pumps, septic pumps and sump pumps. These are NOT interchangeable terms, nor are they interchangeable devices. Each has a specific purpose and function within a septic system environment.
When Gravity Can’t Help, Pumps Take Over
Gravity is an important element of septic system design. It is the force that causes water to seek its lowest level whenever it experiences a change in elevation.
A straightforward, well-situated septic system begins with creating sufficient elevation changes between the main components of the system, so that gravity can direct household wastewater “downhill” from the toilets and drains to the septic tank, and from there effluent can flow further under the influence of gravity to the leaching field.
From the leaching field, gravity continues to drive the effluent downwards through the soil profile so that soil microbes and abiotic processes can perform the final cleansing before it returns to groundwater and nature’s water cycle.
Clearly, gravity’s effect on water is really useful. It can be quite spectacular, too, like when it is exploited to power a water fountain. Of course, we don’t intend for our wastewater to be gracefully arcing into the air! But the force of gravity can facilitate appropriate directional wastewater flow. Using gravity to drive the function of a septic system is far more prosaic than powering the Fountains of Versailles, but of much greater practical import!
Often, site situations or design specifics dictate that gravity can’t be exploited to drive flow through the system. These are instances where the septic tank, the leaching field or both are above the level of a building’s wastewater outlet. Similarly, buildings connected to municipal sewer systems may be located lower than the main sewer line.
In these situations, mechanical pumping is needed to assume the role of gravity and make wastewater flow to its proper destination. There are broad categories of pumps used for water and wastewater management in homes and commercial buildings of all sizes.
A Pump for Each Step in the Wastewater Journey
Sewage pumps, grinder pumps, septic or effluent pumps and sump pumps each have a specific purpose and function when included as a component of a septic system. Within each pump category, there will be a wide range of options regarding pump size, capacity, power and construction to meet the specific requirements of the system and the expected composition of the wastewater stream.
The easiest way to make sense of the different types of pumps is to view a septic system as a whole and where each type of pump comes into play. Let’s consider a septic system as fundamentally the movement of the wastewater from Point A to Point B to Point C.
Point A is the collection of wastewater sources – toilets and drains – in a home or a commercial or public building.
Point B is the septic tank, the collection point for the wastewater sources and the site of preliminary treatment. (For a municipal sewer system, point B would be the street sewer main).
Point C is the dispersal field that receives the treated septic effluent and provides final treatment before it reaches the environment at large.
If A is the highest point in the system, higher than B, and B is higher than C, gravity does all the necessary work of moving wastewater through the septic system or to the sewer main …
…and no pumps are needed. This represents a simple, ideally situated septic system.
Now, if Point B is higher in elevation than Point A or Point C is higher in elevation than Point B, then gravity can’t help …
…and some form up pumping is needed. There is a specific pump type for each transitional leg of the journey – from A (toilets and drains) to B (septic tank or sewer main), or B (septic tank) to C (effluent dispersal).
Although this greatly simplifies the septic system, the general principle holds even as we add complexity to the system, for example, secondary holding tanks, aerobic digesters, or mound dispersal. It also holds for situations in which only part of the septic system cannot be gravity-driven, for example, a basement toilet in an otherwise routine septic configuration, or for municipal sewer systems, in cases where wastewater may have to be pumped against pressure to a street sewer main that is higher than the house sewer outlet.
Moving Water is an Old Idea
So you might ask, what exactly is a “pump,” anyway, and why are there so many choices, and how do septic pumps work? I won’t go crazy with the historical background. Archimedes was the first to come up with a way of moving water. It worked well; it still does. Ok, enough history!
A modern centrifugal pump is an electric- or gas-powered machine that uses rotation to provide velocity to water, which causes it to flow. All the sewage and septic pumps we’ll be discussing are electric-powered centrifugal pumps.
There are three main components of any pump, and understanding these will help you distinguish the variety of pumps used in septic systems, and how their designs suit specific purposes.
The first component to consider is the impeller, the heart of the pump. It is the central, rotating part of the pump that transfers energy from the motor to the water being pumped by accelerating it away from the centre of rotation. A rotating disc with curved blades, or vanes, the impeller produces centrifugal force inside the pump that increases the pressure of the water, resulting inflow.
The second component is the casing, which captures the accelerating water and forces it into stable flow through a pipe. There are two types of casing, volute and diffusers, which differ slightly in how the internal design acts to build pressure in the rotating water. Sewage and septic pumps have volute casings.
The third component is the assembly that contains an intake port, the impeller, impeller shaft and bearings, an output port, a power supply, seals and structural components.
And that’s basically it. Pumps are simple in concept; the impeller spins, drawing water through the intake port and forcing it through the output port. However, they need to be carefully designed, engineered and manufactured to provide years of energy-efficient and reliable service.
Raw Wastewater Pumping:
Sewage Pumps, Solids-Handling Pumps, “No Clog” Pumps and Grinder Pumps
Let’s look first at pumps that handle the movement of the wastewater from toilets and drains into the septic tank or sewer main (the “A to B” journey in the diagrams above). Wastewater, of course, is comprised of water and human waste, dirt, grease, fats and oil, detergents and cleaning products, food scraps, and other solid or semi-solid material. A sewage pump system is designed to move wastewater and its component solids.
The most common, and most appropriate, pump for moving wastewater is the sewage pump. Remember, a pump uses rotation to accelerate water, causing it to flow. A sewage pump, and the related solids-handling pump, is specifically designed to pump wastewater that contains solid and semi-solid material.
The volute casing, impeller, and intake/output ports of a sewage pump are oversized compared to a standard pump, and typically designed to handle solids up to two inches in diameter, not coincidentally the typical dimensions of a toilet outlet. So, if it can be flushed, the sewage or solids-handling pump will be capable of pumping it.
A two-inch capacity should be a minimum solids handling capacity for any septic system, even though smaller (e.g., one and a half inch) capacity pumps are available. And of course, much larger capacity pumps are available for large commercial or municipal wastewater applications.
“Sewage pump” and “solids handling pump” are sometimes used interchangeably, but they are different machines. A sewage pump is submersible; it is placed directly into a drain basin or pit that will receive input from all the building’s wastewater sources. A solids-handling pump is slightly different, in that, although it can handle solids, it is not submersible and must be located in a dry basin plumbed to the wastewater receiving basin.
You may also hear or read the term “non-clog” pump used synonymously with sewage and solids handling pumps. Pump professionals consider this a misnomer because they know that any pump can get clogged if it’s not used appropriately. “Ejector pump” is another term sometimes used for raw wastewater pumping applications.
Sewage Pump Specifications
The number of available choices of sewage pumps can be a bit overwhelming, but knowing what criteria to focus on can narrow your range of choices substantially. Your pump choice should balance up-front costs, associated energy use and repair/maintenance costs with capacity, durability and expected operational life.
As with other aspects of design, installation and maintenance of a septic system, the first criteria is the expected demand on the system, meaning the number of toilets, sinks, showers and other drains, the expected composition of the wastewater input, whether it is a private or commercial system, and related factors. These will determine how frequently and for how long the pump needs to operate on a daily basis – the cycle frequency, which is based on pump capacity (measured in gallons per minute).
The Sump and Sewage Pump Manufacturers Association has a good education section at their website that details how to calculate expected system demand and choose the appropriate pump size.
Knowing the expected conditions of use will help you choose the design and construction of the right pump. For example, impeller design will balance solids handling capacity and water acceleration efficiency. Some impellers are constructed of thermoplastic and are lightweight and therefore energy-efficient, but are more subject to wear and may require replacement in time. Others are made of cast iron, bronze or steel, which are more durable but require more energy to spin.
A solids handling capacity of 2 inches is routine for residential and light commercial pump applications, though pumps with 2.5 inches or greater capacity are used in municipal or industrial applications.
Energy efficiency and cost of use figures in here, but so does the control of wastewater input. Feminine hygiene products, condoms, cigarette butts, syringes and similar items can wreak havoc on sewage pumps. Commercial applications (e.g., businesses and rental properties) may need to sacrifice energy efficiency for durability in light of this. Private residential applications, where what goes down the drain can be better controlled, can take advantage of lighter-duty, more energy-efficient pump designs.
Another issue is the “head” capacity of the pump, which is essentially the height that the wastewater needs to be pumped. A “typical” pump would have the capacity to pump wastewater 25 to 75 feet or more. Head capacity is a bit more than simply the height difference between the pump and the septic tank or sewer main. It is also a function of pipe diameter, the number of pipe junctions and the total length of pipe travelled.
The power needed for a certain head will indicate the horsepower needs. Higher horsepower means greater head and pumping capacity but also higher energy costs to run. Typical pumps range from ½ to 2 HP and run on 120V or 240V service.
What is a Grinder Pump?
Grinder pumps are often discussed in the context of sewage pumps, but they are different beasts. A grinder pump is NOT sewage or solids handling pump. The impeller and volute of a grinder pump are not designed to pump solids. Rather, the grinder pump is a liquid handling pump that incorporates a cutting ring and a rotating blade to grind or macerate solid material, creating a wastewater slurry that then enters the pump volute.
Which is the best choice? Again, it gets back to the specific needs of the system. Sewage pumps are by nature somewhat inefficient because of the large volume of the volute. In cases where the sewage needs to be pumped up a relatively high elevation (i.e., there is a large “head”), or, where sewage needs to be pumped against pressure into a sewer main, this may place excessive demand on the pump, creating a large energy draw and shortening the pump’s life. In cases like these, a grinder pump may be a better option for a more powerful and efficient operation.
But, (There’s always a “but”!) grinder pumps can be subject to clogging and failure. The cutting mechanism needs to completely macerate whatever solids enter it, because the pump itself, as we’ve seen, is a liquids-handling pump, not a solids-handling pump. The blades need to spin fast and powerfully to grind up solid material, that is, they need a lot of torque, which is provided by the motor. When the motor turns on, it sets the blades to spinning. But remember that pumps are not always running; they cycle on and off. If the blades are free of solid debris, the motor can get them spinning easily. When the pump cycles off, there may be solid debris left in the cutting mechanism, so that when the pump next tries to cycle on, the motor has to provide extra torque to overcome the debris in the blades.
Think of placing a thin stick between the blades of a fan, then turning the fan on. The fan may be able to break the stick, but it has to work extra hard to get the blades spinning before that happens. That extra torque requirement can wear on the motor, reducing its life while causing extra energy demand.
Blades can also dull too, of course. A grinder pump calls for good control of wastewater composition. Other disadvantages are that grinder pumps are more expensive initially and have higher use costs and typically the shortest life spans of all pump types used in septic systems.
Effluent Pumping: Effluent Pumps, Septic Tank Pumps
We’ve seen the options for moving wastewater into the septic tank (or a sewer main). Now let’s turn our attention to a septic tank with a pump system, and circumstances where the output of the tank requires a septic pump to the drain field or area of secondary treatment, most commonly a leaching or dispersal field. Septic tank pumps, or effluent pumps, are more typical liquid handling pumps. They are not designed to pump solids and need to be installed in a way that prevents any solid material from entering them. These come into play when the leaching field or secondary treatment tank is elevated or uphill of the septic tank. This is fairly common, for example in locations where topology (e.g., rocky or clayey ground) necessitates a “mound” type dispersal system.
Septic pumps are submersible, but of course, we can’t place them directly in the septic tank because pumping from here would disrupt the settling process as well as clogging the pump and possibly the drainage system too. In a two or three chamber septic tank, the pump can be installed in the final chamber. Even here, a “dirty water pump” or “trash pump” (same thing) is a prudent pump choice, because these can handle small solids that potentially enter the effluent stream. Even so, an effluent filter, installed before the pump to trap any solids, is always recommended.
Water Outside the Septic System: Sump Pumps
Note that I haven’t mentioned sump pumps yet, even though to most people this is probably the most familiar type of household pump. That’s because sump pumps have nothing to do with septic systems! They are an entirely different beast, with an entirely different function.
Think of two important characteristics of wastewater: 1) it contains a variety of liquid and solid components, and 2) it is wholly contained within the building’s plumbing system. The septic system is designed to handle wastewater.
Outside of the septic system, many buildings also collect uncontained water in their basements or crawlspaces, from groundwater or storm-water seepage. Removing this seepage water is the purpose of a sump pump. Sump pumps are positioned in a low point of the basement or crawlspace, commonly a pit dug specifically to create a low point, and will pump this seepage water through an external drain that carries the water away from the building.
Sump pumps are designed to handle water only. No solids, no chemicals, no greases or oils or detergents. Just water that seeps into a buildings lowest level.
Pump Control: Float Switches, Piston Switches, Electrical Requirements.
Obviously, we only want the pump to pump when there is something to pump! This minimizes energy use, and wear and tear on the pump itself. Sewage, septic and sump pumps are routinely controlled automatically by a float switch, ensuring that the pump operates only when there is sufficient wastewater, effluent or seepage water to pump. Manually controlled pumps are available but are not recommended for residential applications.
A sewage pump float switch or septic pump float switch rides on the surface of the wastewater or effluent that collects in the basin and is attached to the pump motor by a short length of the electric cord. It turns the pump on or off as it changes position with the level of the liquid. When the liquid rises, the switch activates the pump, and as the pump removes the liquid, the float lowers, switching the pump off.
The standard float switch contains a small ball of liquid mercury that actuates the switch. These are considered the most durable and reliable float switch design, but although they are sealed within the float, mercury is a highly toxic substance and has some manufacturers moving away from them. Mechanical switches replace the mercury with a metal ball that actuates the switch, though depending on the design and the manufacturer they can be subject to wear or fouling.
A Handy Table of Pump Synonyms
It’s clear that there are a lot of names for the pumps used in or near septic systems. Some are merely different terms for the same pump; others denote quite different functions. Here’s a handy table to keep it all straight!
|A Handy Table of Pump Synonyms
|· Sewage Pump
· Solids Handling Pump
· Ejector Pump
|Pumping wastewater and solids from source to septic tank or sewer main.
|· Grinder pump
|Pumping wastewater from source to septic tank or sewer main. Does not pump solids, grinds solids at intake to form a slurry, pumps the liquid slurry.
|· Effluent Pump
· Septic Tank Pump
· Dirty Water Pump
· Trash Pump
|Pumping solids-free septic tank effluent to leaching/dispersal field or other secondary treatment components.
|· Sump pump
|Removes building seepage water/stormwater/floodwater by pumping it to an exterior drain.
Not a component of a septic system.
Septic Pump Problems
Septic pump problems, or failure of any of the many unique pump types used to move wastewater and effluent, generally boils down to two issues; 1) ensuring that the installed pump is the right type and the right size for the job, and 2) proper care of what goes down the drain in the first place. A good rule of thumb here is if you can put it in the trash instead of down the drain, do so!
With proper sizing and proper use and maintenance, a septic pump can be expected to last 10 to 15 years, a sewage pump 7 to 10 years and a grinder pump 5 to 10 years. Of course, these are very general ranges; harsh conditions and careless use and maintenance can greatly reduce the life of a pump to just a few years, while careful use and yearly maintenance of the septic system as a whole can see pumps lasting 20 years or more.
We don’t always have gravity to help out with a septic system. Even some fountains have to make do without gravity. Fortunately, a variety of pumps are available to take gravity’s place when needed. If your septic system requires a pump, there is one for every situation, large or small. Knowing the details of pump types and their specific uses will help you choose and maintain the right pump for the job, for years of trouble-free service.