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Pump impellers: Type defines function

The impeller is the actual pumping device for waste water and therefore at the “heart” of the pump. However, waste water is very diverse in its composition: It may contain solids, wet wipes, sludge or faeces as well as stringy materials or substances forming foam. To achieve the best pumping results, the impeller should be ideally suited to the individual application. The objective is always the same: a clogging-free, durable and energy-efficient solution.

Pump impeller

First step: Dimensioning the pump impeller

Prior to selecting the impeller type the parameters of pump operation must be determined, such as the flow rate, pressure and piping diameter. This will help select the best suited pump type, size and capacity.

Another important factor is the operating mode: Will the pump be running continuously or will it be used for intermittent or short-time operation? This might also decide on the pump’s drive, for which the current Ecodesign Directive (2019/1781/EU) is to be considered.

Second step: Selecting the impeller type

The next step after defining the pump type and size, is the selection of an impeller type . KSB offers suitable impeller types for all requirements.
The main criterion for selection is the nature of the fluid handled, i.e. the solids content in the waste water. Is it stormwater, raw waste water or pre-treated waste water? Does the waste water contain faeces and wet wipes? 

These criteria can be “converted” into measurable values. The three measured variables of gas content, sand content and dry solids content, in particular, are decisive for the selection.

From a technical point of view, the size of the free passage and the efficiency are key factors when choosing an impeller.

Open or closed impeller type?

This is a brief overview of impeller types and their main applications. Primarily, a distinction is made between open and closed impellers. “Open” refers to a design without an outer (suction-side) shroud. Open impeller types include free-flow impellers, diagonal single-vane impellers and cutters. Examples of closed impeller types are single-vane impellers and multi-channel impellers.

Free-flow impeller

Centrifugal pumps with a free-flow impeller are preferably used for waste water containing solid substances and long fibres, coarse solids as well as entrapped gas or air. They offer the largest free passage of all available impeller types. This is why they are a good match for waste water with a large gas and sand content. The dry solids content (also dry substance or DS content) should not exceed 7 %. The highest efficiency is 58 %.

Free-flow impeller

Diagonal single-vane impeller

The diagonal single-vane impeller – another open impeller design – serves to handle fluids containing solids and long fibres, coarse fibres as well as entrapped gas or air. It is made to handle raw waste water, combined sewage, circulated and heating sludge as well as activated, raw and digested sludge with a solids content of up to 13 % as well as high-viscosity fluids. The best efficiency is 81 %. The free passage is large but slightly smaller than that of a free-flow impeller.

Diagonal single-vane impeller


Centrifugal pumps with a cutter are designed to handle waste water containing faeces when the discharge line diameter is relatively small (DN32 to DN 65). Their best efficiency is 58 %. The maximum dry solids content is 7 %.


Single-vane impeller

The single-vane impeller in closed design is used for raw waste water containing solids and long fibres as well as for recirculated and heating sludge and combined sewage. It is also suitable for handling raw, activated and digested sludge. This type can handle a higher gas content than a diagonal single-vane impeller. The maximum dry solids content is defined as 5 %. The best efficiency  is 78 %. 

Single-vane impeller

Multi-channel impeller

Opting for a multi-channel impeller makes sense when contaminated, solids-laden, non-gaseous fluids and sludges without any stringy material are to be transported. This impeller type is also suitable for pre-screened waste water, mechanically treated waste water, industrial effluents, landfill waste water as well as stormwater and activated sludge. The maximum dry solids content is 3 %. The best efficiency is 86 %. Depending on the size, free passages of up to 76 mm (3") are available.

The multi-channel impeller has the highest efficiency of the five impeller types featured. It is primarily used for waste water with a low contamination load or for pre-treated waste water – especially with a large sand content.

Multi-channel impeller

Grey cast iron or white cast iron?

Apart from the impeller type, the right material plays an important role. As a rule of thumb, grey cast iron is perfectly suitable for the majority of applications, given that waste water – unlike many other fluids – is not excessively abrasive. If, however, corrosive or chemically aggressive waste water is to be pumped, e.g. industrial waste water, special materials are required.

When pumping abrasive fluids, the usual choice are hard chrome/nickel alloys as they will make the impeller contours as durable as possible, maintaining pump efficiency and operating reliability in the long term.

Conclusion: The right impeller for every type of waste water

With its broad pump impeller range KSB can supply the ideal pump for every application.

Table KSB impellers