Pumps in wastewater treatment plant (WWTP) applications are typically equipped with sewage hydraulics (f.e. vortex, single or double channel impellers). Those impellers have a clogging preventing shape with large ball passing diameters, low blade numbers and depending on the impeller style, also special features (e.g. cutting functions). This clogging preventing shape has an influence on the resulting pump efficiency, which is in general lower than from clear water pumps.
This difference is caused by a closer gap design and the increased number of blades for clear water pumps. There are sewage applications, where those special impellers are not needed, but made by history, preferred.
However, in a WWTP the already screened wastewater doesn’t need special wastewater hydraulics. During the treatment process, organical and anorganical substances are removed from the wastewater. One of those sub-streams contains the removed organical load, like toilet paper and feces, which is mainly used in digesters for the degradation of the organic matter to methane gas.
Especially for the wastewater in this treatment step, a clear water pump is selected, which has a much better efficiency compared to the existing vortex pump.
By doing a field test in a sewage plant (> 500.000 population equivalent) the challenges from this organical loaded media and the effect on a clear water, open three-vane impeller are described. Different modifications of the three-vane impeller are made to still have the advantages of a high efficiency hydraulic in combination with an operational safety.
The right sampling as well as the description of the media pumped are very important for the hydraulic selection. Due to the different modifications, challenges, e.g. the trapping of hair in the impeller entrance, were able to overcome to achieve an efficiency advantage for the new selected open three-vane impeller compared to the previous vortex impeller.