Stirred or Shaken...?

In the academic wastewater treatment society, there is often contentious controversy about which mixing principle is the most suitable for biological wastewater treatment. This is especially true for activated sludge plants in the Sequence Batch Reactors known as SBR plants. In SBR plants, in particular, the type of the state of art used for targeted mixing is of great importance. This is because oxygen deprived, anoxic or anaerobic phases can be used to achieve high denitrification rates within an SBR cycle. Using a vertical flow unit, it could be seen that, high immiscible effects with a high surface area of the sludge flocs can be established by the local energy input in the form of a primary flow directed downwards.

Based on fluid dynamics simulations carried out by the Technical University of Dresden, it was possible to prove under the condition of identical energy input, complete mixing is ensured by the vertical flow. Whereby, in the case of the horizontal flow it was occasionally found that only the water volumes move along their horizontal axis as a result of the teacup flow effect, which is a rather undesirable clogging flow with dead water zones and no thorough mixing could be determined.

In a traditional aeration tank, where mixing takes place 24/7, this problem does not arise, as there is sufficient activated sludge stabilization time for mixing/denitrification. In an SBR reactor on the other hand, where mixing takes place for a maximum of 1-1.5 hours per cycle and the denitrification performance is the measure of success, the importance of the mixing technology is crucial. It should be made clear at this point that there is no preference for a specific mixing approach, but rather a different process engineering requirements must be ensured depending on the application.

Furthermore, every nitrifying SBR system shows floating sludge on the water surface in the inactive phase caused by denitrification. It is known that the water surface is not circulated in SBR plants. Depending on the process conditions (change of season), floating sludge or floating sludge patches appear on the water surface, which can impair the function of technical equipment such as measuring technology, probes or other similar devices. This can also have a considerable negative influence on the clear water discharge.

Additionally, during vertical mixing, the concentration of the surface water flow gravitates to the point of highest energy density, which is located just at the vertical mixer. A floating sludge extracting device similar to that used in secondary clarifiers in the immediate proximity of the highest density point of the floating sludge to be removed from the process. In particular, duckweed or water lentils, which are frequently observed on SBR plants and proliferate on the water surface of an SBR reactor and thus impair the clear water flow, can be eliminated by utilising the vertical flow effect at the water surface and using a floating sludge extractor.