A wastewater pump’s throughlet size is frequently used to specify clog resistance, despite data that demonstrates the irrelevance of this measurement. Clogging is a highly undesirable operational problem in wastewater pumping that drastically reduces pump efficiency and can cause hard clogging, unplanned callouts or even sewage overflows.
This white paper describes the importance of a pump’s hydraulic design for achieving clog-free operation. It also establishes how a pump’s throughlet size is a misleading parameter in specifying clog resistance.
Modern wastewater
Investigations and studies of modern wastewater have shown that it rarely contains hard, solid, spherical objects with a diameter as large as the inner diameter of the piping system. Objects that are truly solid and hard, such as stone, brick, or steel, are also rare, and these items rarely reach the pump because they will be trapped on a flat horizontal surface where the liquid is stagnant or the carrying velocity is low. By far, the most common solids found in municipal wastewater are organic and often consist of long and stringy shapes, such as fibers.
Modern wastewater also contains a higher amount of synthetic cloth and artificial fibers than ever before. The vast array of household cleaning products, such as tissues, wipes, and dishcloths, are to blame. These products should be disposed of in the trash or compost, but many consumers flush them down the toilet, thus adding synthetic fibers to the wastewater stream.
How traditional hydraulic designs are affected by modern wastewater
Stringy objects tend to get caught in traditional impeller types even if the throughlet size is large. As shown below, the problem point is the leading edge of the impeller vanes. All impeller designs have one or more leading edges.
Accumulation in a single-vane impeller (left) and vortex impeller (right)
Soft, stringy, and elongated objects in wastewater are continuously fed into the pump; some of these will meet a leading edge on one of the impeller vanes. The fibers tend to wrap around the edge and fold over on both sides of the vane. On straight and moderately curved leading edges, the debris will not dislodge; instead debris will continue to build up. These accumulations will create big lumps or bundles of solid organic material (sometimes called rag balls).
As these objects accumulate in a traditional impeller design, the following become likely:
- The flow rate of the pump decreases as the solid objects start to constrict the free passage of liquid. This usually leads to decreased efficiency. This phenomenon is called soft or partial clogging because the pump continues to operate. It will take longer to pump down the sump with a constricted impeller than with a non-constricted impeller.
- The input power increases when the accumulated objects make contact with the volute and create drag. Drag leads to lower efficiency and to the risk of a stop due to motor overloading. The solids act as a brake which increases the required input power. Once the running current exceeds the trip current, the pump is shut off due to hard clogging.
With decreased pump efficiency, the operational cost for the end user is increased because the pump must operate for a longer time and consume more energy to handle the inflow. A motor overload or pump trip also adds cost for the end user because it requires a service technician to visit the pumping station to clean and restart the pump.
Self-cleaning N-technology hydraulic design
Today there are better and more advanced hydraulic designs available to increase a wastewater pump’s clog resistance and to maintain pump efficiency over time. A state-of-the-art self-cleaning design, with substantially backswept leading edges and a relief groove, has proven to be the answer to most clogging problems.
This self-cleaning hydraulic design does not accumulate the typical contaminants present in modern wastewater. Solids that land on the leading edges of the impeller are continuously pushed towards the periphery and out through the pump discharge via the relief groove located in the insert ring.
Modern, self-cleaning N-technology hydraulic design
Clogging and pump lifecycle costs
A clog-resistant pump results in a lower total cost of ownership. The main variables in the lifecycle cost are initial investment, energy costs, and maintenance costs.
The following bar charts illustrate lifecycle costs for different pump sizes depending on the number of clogging events. It is fairly common for a small wastewater pump station to have two unplanned call-outs due to pump clogging. The cost of these call-outs will equal or exceed the initial cost of the pumps. For a troublesome pump station with 10 call-outs per year, unplanned maintenance costs will far exceed installation costs, and in smaller pumps, exceed the energy costs.
Cost distribution for a pumping station with no call-outs (left), two call-outs (center), 10 call-outs (right)
Because energy and maintenance costs represent the vast majority of the total cost of ownership, the best solution for the end user is a well-designed pump station with modern pumps that are clog-free, reliable, and energy efficient.
Download the white paper to explore the effect of hydraulic design on clog resistance in more detail.