The question in the title comes up in our technical department several times a week—from installers and wastewater pumping station designers alike. At first glance, it concerns the same type of device: both operate submerged in liquid, and both pump the medium under pressure. The difference, however, lies in how the device handles the solid fraction contained in the medium. In this study, as the IBO brand team at Dambat, we break down the subject technically: when a conventional submersible pump is sufficient and when a cutting system is absolutely necessary. We also discuss the key distinction for selection between greywater, domestic wastewater, and raw wastewater.
Submersible pumps versus grinder pumps—where is the dividing line?
The term “submersible pump” describes the design, not the intended application. It is a device with a hermetically sealed motor cooled by the surrounding medium, operating fully submerged. Among all types of pumps, this family includes both models for clean water and heavy-duty sewage units. Only the impeller type and the presence of a cutting mechanism determine what medium the device is suitable for. Therefore, proper pump selection begins with analyzing the liquid, not the motor power.
Submersible grinder pumps are a subset of submersible pumps in which a cutting system has been installed before the inlet to the hydraulic housing—a pair of blades, most commonly a stationary ring and a rotating blade made of stainless or hardened steel, which grinds the solid fraction into a suspension capable of passing through a narrow discharge outlet. The presence of this assembly distinguishes grinder pumps from an ordinary drainage unit. Where a drainage pump would become clogged by fiber or cloth, a unit with a blade cuts the material and pumps it onward through a small-diameter pipe. Among the various types of pumps, these models handle the most difficult media.
In practice, most selection errors result from confusing design with application. The way a drainage submersible pump works differs fundamentally from the way a sewage submersible pump works—and from how a sewage unit operates—despite the devices appearing identical at first glance. A vortex impeller in a drainage unit can pass contaminants up to several millimeters in size, but it should not be treated as a grinder pump—long fibers will wrap around the impeller and block the flow. The classification of grinder pumps is based precisely on the presence of a blade, not merely on the free passage. That is why only units with a cutting assembly are classified as grinder pumps. You can find a complete overview of the models in our sewage and septic tank pumps category.
Cutting system—how the grinding mechanism works
The heart of every blade-equipped unit is the cutting system. The geometry of the blade and ring, the hardness of the steel, and the rotational speed determine how effectively the device handles sanitary fibers, organic residues, and material that reaches a pumping station in real sewage systems. A well-designed grinding mechanism produces a suspension with particles fine enough to be pumped through DN32 or DN40 pipe instead of the DN80 or DN100 pipe required for free-passage pumps.
From an installer’s perspective, the cutting system has another advantage: it allows the discharge pipeline to run over greater distances and against a greater head without the risk of solids settling. Where a conventional free-passage unit would require a wide gravity collector, a properly selected grinding mechanism makes it possible to pump wastewater under pressure through a narrow pipe—often the only solution on plots located below the municipal sewer level. It is worth remembering that no cutting mechanism can handle sand or abrasive material—these fractions require separate units for sludge.
Greywater, domestic wastewater, and raw wastewater—the key to proper selection
From the perspective of pump selection, classifying the medium is more important than any catalog parameter. In design practice, we distinguish three basic categories.
Greywater is water discharged from sinks, showers, bathtubs, washing machines, and dishwashers—without fecal matter. It contains detergents, fats, and small organic contaminants, but is free of large solids. For greywater, a free-passage pump with a vortex impeller is sufficient in many domestic installations, without the need for blades.
Domestic wastewater is the medium from a complete sanitary system, containing fecal matter and hygiene products. This is a typical task for a unit with a cutting mechanism—the sanitary system generates exactly the fibrous contaminants that clog pumps without a blade. This group also includes septic tank pumps (fecal pumps), designed for the most difficult domestic medium.
Raw wastewater is untreated municipal medium entering network pumping stations and the inlet of wastewater treatment plants, with a variable and often high proportion of solids and fibrous material. For raw wastewater, heavy-duty sewage units are selected—either with an aggressive cutting mechanism or with an impeller offering a large free passage, depending on the characteristics of the network. This is a medium in which every design detail and corrosion-resistant material matters. We have collected practical guidance on the intended use of the individual series in the article types of grinder pumps.
Clean-water pumps versus dirty-water pumps
Clean-water pumps are designed for media without a solid fraction: tap water, rainwater, groundwater, or clean well water. They have narrow hydraulic channels, high efficiency, and often a high head. Introducing dirty water containing suspended solids into such a unit results in the impeller seizing—this is the most common cause of claims handled by our service department. Models for clean water and lightly contaminated water are collected in the lightly contaminated water pumps category.
Dirty-water pumps have a larger free passage and more durable materials. They can pump media containing mineral and organic suspended solids, although—without a cutting mechanism—they cannot handle long fibers. In this segment, cast-iron housings and stainless-steel impellers are standard because they determine durability when operating in dirty water. The key distinction is that dirty-water pumps are not the same as sewage units—the former handle suspended solids, but not fecal matter. Pumps for clean water and pumps for contaminated media use two completely different hydraulic systems.
A separate group consists of submersible sludge pumps for dense sludge with a high solids content. They require specific hydraulics and an appropriate motor power reserve, so we always select them individually. The same applies to pumps for clean water in technological circuits, where maintaining a stable operating point is important.
Submersible pump applications in installation practice
Submersible pump applications cover the entire spectrum—from domestic drainage to municipal infrastructure. Such broad pump applications require varied designs. The most common pump applications in our practice include: drainage applications (dewatering excavations, basements, and drainage wells), applications in pressurized and gravity sewer systems, wastewater treatment plant applications, and industrial applications in technological circuits. It is precisely these broad submersible pump applications that mean one catalog is not enough—each scenario requires a different design.
Drainage applications best illustrate the dewatering version: a unit with a float switch starts when the water level rises and switches off after the water has been discharged. These are classic construction applications—draining foundation excavations and securing construction sites. Unlike sewer applications, drainage applications around buildings protect foundations from groundwater pressure, while domestic applications allow water to be removed from flooded rooms. A wide selection of models for these tasks can be found among the pumps for irrigation and rainwater.
In wastewater treatment plant applications and network sewer applications, units with a cutting mechanism dominate—here, pumping wastewater requires certainty that no fiber will block the hydraulics. Wastewater treatment plant applications are an environment with the highest material requirements. Industrial applications, in turn, require materials resistant to chemically aggressive media and continuous submersible pump operation in 24/7 mode. This diversity of scenarios is precisely why the intended use of submersible pumps must be established before selecting the power, not afterward. Properly understanding the intended use of submersible pumps protects the investor from costly mistakes.
Professional septic tank pumps and tank emptying
Professional septic tank pumps are an area where the importance of the cutting mechanism is most apparent. Emptying septic tanks and emptying holding tanks requires a unit capable of handling a medium with varying consistency and contaminants not found in drainage water. Efficient septic tank emptying is an everyday task for these pumps. For septic service companies, high capacity and resistance to operation in difficult conditions are also important. Efficient septic tank emptying depends not only on the pump but also on a correctly selected discharge pipeline.
Where the discharge pipeline is narrow and long, transfer is carried out using a blade-equipped unit; where the priority is simply transferring a large volume over a short distance, a high-capacity free-passage impeller is the right choice. We have collected the complete range of units with cutting mechanisms in the category sewage and septic tank grinder pumps, while the selection context is described in the guide grinder pumps—when are they worth using? These professional pumps are the ones from this group most often supplied to septic service companies.
Submersible pump capacity and operating point selection
Submersible pump capacity is never a constant value—it is a point on the performance curve where the pump curve intersects the system resistance curve. The declared pump capacity in the catalog (maximum flow at zero head) is only an extreme point of this curve and says nothing about actual operation in an installation. Therefore, a properly selected submersible pump takes into account the required discharge head, the pipeline’s linear and local losses, and the characteristics of the medium.
When selecting a pump, we also consider how long the pump operates during each cycle and how often the device starts. A tank capacity that is too small in relation to the capacity leads to frequent starts and shortens motor life. This brings us back to the earlier point: the actual submersible pump capacity in a sewage application with a blade will be lower than in the catalog because some of the energy is absorbed by the cutting system. Multistage pumps are a separate family, used where a very high head is required at moderate flow—mainly in deep wells, not in sewer systems. A correctly selected submersible pump then operates close to its point of maximum efficiency.
Submersible pumps with float switches and pump protection
Submersible pumps with float switches are standard wherever unattended operation is required. The float switch turns the device on and off depending on the liquid level, protecting it from dry running. This is how a submersible pump works in automatic mode. A float-switch pump controls the cycle itself, which is why it is the default solution in drainage wells and residential pumping stations. It is worth remembering, however, that a float-switch pump needs room for the element to move freely—in a narrow well, an electronic sensor or external probe must be used instead. A conventional float-switch pump nevertheless remains the simplest and least expensive solution. Submersible pumps equipped with additional level probes also work well where a conventional float has no room.
Other pump protection features are just as important as the float switch itself. Thermal winding protection protects the motor from overheating, an overcurrent circuit breaker protects against overload and short circuits, and the mechanical seal in the oil chamber separates the water from the electrical components. Submersible pumps equipped with a complete protection package operate longer and more reliably. These pump protection features determine whether submersible pump operation in a difficult medium will remain trouble-free for years. We demonstrate a model with this equipment using the example of the IBO CTR 1500 with grinder.
Controls, variable-frequency drives, and accessories—the complete installation picture
The pump itself is only half of the installation. Controllers, check valves, filters, pressure gauges, and variable-frequency drives determine how steadily the entire system operates. A variable-frequency drive maintains constant pressure by regulating the motor’s rotational speed—in our range, it regulates the operating conditions of the water pump, not energy from PV panels. You can find the complete range of control equipment in the pump variable-frequency drives category. A check valve prevents the medium from flowing back through the discharge pipeline, while a pressure gauge allows you to monitor the pressure at the operating point—without these components, even the best selected submersible pump will not perform in the installation as it should.
Summary—how not to confuse design with intended use
Returning to the question in the title: a submersible pump is a broad family of designs, while grinder pumps are its specialized subset equipped with a cutting system. Submersible grinder pumps alone do not replace application analysis—the choice is determined not by the name but by the medium. We select one unit for greywater, another for domestic wastewater, and yet another for raw wastewater. The key is matching the design—free passage, impeller, and presence of a blade—to the actual characteristics of the liquid and the required discharge head and flow rate. The familiar types of pumps in our range are helpful here. The most frequently recommended submersible pumps in our portfolio are those correctly matched to the task, because pump selection begins with a question about the medium. It can also be helpful to make an informed choice of the best pump from several models with similar parameters, taking materials and accessories into account. The IBO and IPRO brands cover the full spectrum—from drainage units to professional sewage pumps—and the correct pump selection is always preceded by a discussion of the application. Such varied pump applications are best discussed individually. For every unusual project, we encourage you to contact our technical department so that together we can determine which recommended submersible pumps will work in your installation. A sample model that can serve as a starting point is the IBO WQV 7-16-1,5 from our range. It is a good example of how, among the many recommended submersible pumps, the right choice emerges by matching the pump to the medium.
FAQ—frequently asked questions
A submersible pump is a general type of design that operates while submerged. Grinder pumps are a version additionally equipped with a cutting system that grinds the solid fraction and allows it to be pumped through a narrow pipeline. The difference therefore concerns the presence of a cutting mechanism, not the principle of submerged operation itself.
Usually not. Wastewater from sinks, showers, and washing machines does not contain fecal matter or long fibers, so a free-passage pump is sufficient in many domestic installations. A cutting mechanism becomes essential only for domestic and raw wastewater.
Domestic wastewater comes from a household sanitary system and contains fecal matter and hygiene products. Untreated wastewater in a municipal network has a variable and often higher proportion of solids. The latter requires heavier-duty sewage units.
No. Units for water without contaminants have narrow hydraulic channels, and introducing suspended solids into them causes the impeller to seize. Dirty-water pumps with a larger free passage are used for contaminated media, while units with a cutting mechanism are used for fibrous matter.
It is the operating point on the performance curve—the intersection of the pump curve and the system resistance curve. It depends on the required discharge head, pipeline losses, and the characteristics of the medium. The catalog value is only an extreme point and does not reflect actual operation in the installation.
They operate unattended—the float switch turns the device on and off depending on the liquid level, protecting it from dry running. In confined wells where the float has no room to move, a version with an electronic sensor or external probe is used.
