In the context of the biological treatment process of sewage, the task of submersible mixers is to provide optimum support with the microbiological processes. In the Wilo-EMU TRE, a low-speed propeller creates the necessary consistent current in the basin to keep the sewage sludge moving continuously and preventing deposits from forming at the bottom of the basin.

Aeration is required in order to maintain the biological processes. The airflow acts like a wall. Good interaction between aeration and the mixing process is needed for the sludge to be distributed evenly in the sewage collection basin even in the case of unfavourable inflow conditions.

The energy required per cubic metre of sludge moved in the basin is minimised through the use of a highly efficient asynchronous motor, a very efficient gear and balanced hydraulics in these special machines. Optimum dimensioning is extremely important here. Wilo introduced a dimensioning software to calculate the perfect number and position of the individual mixers in the basin as well as further important functions.

In 1996, the large wastewater treatment plants in Bottrop were equipped with EMU mixers from Hof. In 2016, the activated sludge treatment plant was completely retrofitted to highly efficient Wilo submersible mixers of the latest generation.

Given that mixers are in continuous operation, it was important to us to keep the required maintenance to a minimum and guarantee a long service life. To do this, we identified various technical solutions:• The motor and gear housings were made from cast iron with a specially patented Ceram coating designed to hinder corrosion caused by the constant contact between the submersible mixer and the water.

• The geometry of the plastic blades was designed such that they were energy efficient on the one hand but could also prevent clogging, that is the accumulation of fibrous material that sticks together in the sewage and forms strands that clog the mixer, thanks to their shape. In addition, smaller elements from the clarifier could not adhere to the blades so easily.

• Clogging was also to be prevented by switching the mixer on and off in a targeted manner.

• The mixers could also be delivered with a lowering device made of stainless steel that allowed the mixer to be lowered and lifted quickly and easily for maintenance purposes.

*current model name