Choosing a waste water pump: the expert guide

First, a clarification: the pumps discussed here deliver no wastewater containing faecal matter. As soon as faecal matter or long-fibred organic matter is present, the application falls into the category of faecal and sewer impeller pumps, featuring the appropriate hydraulics and a double Floating ring seal.

Step 1: Analyse the pumped fluid

The medium determines almost everything else. Four characteristics are crucial:

• Degree of contamination and solids: Rainwater and slightly contaminated water place minimal demands on the system. As the solids content increases, the free ball passage becomes a selection criterion.

• Fibre content: Long-fibre materials are the most common cause of blockages and require a system capable of handling fibres.

• Chemical pollution: Salinity, pH values and corrosive additives determine the choice of material (see step 4).

• Temperature and abrasiveness: The temperature of the medium and the proportion of sand or sludge affect the materials and the seal.

Step 2: Determine the operating point

The head and pumped quantity determine the operating point. The geodetic height and the pipework losses are the decisive factors – not just the sheer difference in height. The determined operating point is plotted on the pump’s Characteristic curve; it should lie within the middle range, where efficiency is highest, rather than at the edge of the curve. A pump that is too small will not achieve the required head, whilst one that is significantly oversized will operate inefficiently and switch on and off too frequently.

Step 3: The right hydraulics

The impeller design determines fibre flow and suitability for handling solids:

• Multi-channel wheel: Open impeller for slightly dirty water containing small particles of solid matter; compact and robust.

• Single-channel wheel: For heavily contaminated, muddy media containing solids, offering good efficiency at higher pumped quantities.

• Vortex impeller (free-flow impeller): The impeller is recessed within the spiral casing, and solids are conveyed past the impeller – the design best suited to fibrous materials, with the lowest risk of blockages.

Step 4: Materials selection based on chemical resistance and abrasion resistance

The choice of material depends on the chemical load. Grey cast iron is suitable for wastewater subject to mechanical stress but with non-critical chemical composition. Standard stainless steel (1.4301) is suitable for slightly contaminated water without aggressive additives. In the presence of chlorides and at low pH values, higher-alloyed, molybdenum-containing stainless steels (e.g. 1.4436) are required; or – in the case of very high salt content – chemically inert plastics such as glass-fibre-reinforced plastic. Abrasive sand and sludge particles also require wear-resistant impellers and casings.

Step 5: Installation, operating mode and control

In the Type of installation A distinction is made between stationary wet installation with a coupling system (the Pump is lifted out for maintenance without having to enter the shaft), the simpler wet installation with a support foot for temporary installations, and mobile operation. The Operating mode distinguishes between continuous operation (S1, Motor submerged) and intermittent operation (S3 in 10-minute intervals; for example, S3 = 30% means 3 minutes of operation, 7 minutes of pause). The Level control This is achieved either via a fitted float switch (simple, robust) or via a control unit with a Pressure probe, which also offers monitoring and alarm functions.

Conclusion

The correct pump is determined by the media, the operating point and the installation – not by the Datasheet alone. If in doubt, comparing the calculated operating point with the pump performance curve will provide clarity; for chemically aggressive or abrasive media, it is worth checking the suitability of the materials used.