Aeration tanks - second biological cleaning stage

“Much or little oxygen” is the motto for the microorganisms in the aeration tanks of the second biological cleaning stage. This allows to break down carbon and nitrogen in one single cleaning step. The tanks have alternating zones with either high oxygen (oxygen supply) or low oxygen concentration, resulting in the breakdown of ammonia nitrogen in the wastewater. Some microorganisms convert ammonia into nitrate (nitrification), while other microorganisms transform the oxygen bound in nitrate into gaseous nitrogen (denitrification). The phosphorus compounds dissolved in the wastewater are transformed into undissolved compounds by adding a precipitant (iron-III-sulphate) and are removed along with the sewage sludge.
Each aeration tank of the second biological cleaning stage consists of three cascades and one degassing zone, through which the wastewater flows sequentially. The mixture of wastewater and activated sludge enters the first cascade, which is the upstream denitrification zone. This non-aerated cascade is equipped with two vertical mixers which prevent activated sludge from settling down and blend water and sludge into a homogeneous mixture. The second and third cascades need varying amounts of oxygen, depending on the level of pollution and wastewater temperature, to support bacterial breakdown. To make cleaning efficient and economical, the amount of supply air needs to be continuously adjusted to the relevant conditions. Compressed air is blown in through 48,000 membrane diffusers, which are installed at the bottom of the 15 aeration tanks. Submersible mixers keep the wastewater-sludge mixture in motion to prevent activated sludge from settling out. From the third cascade the wastewater-sludge mixture flows into a degassing zone where large bubbles of air are blown in to remove any remaining gaseous nitrogen from the wastewater. The wastewater-sludge mixture is now on its way to the final clarifiers.

Technical parameters

  • 15 tanks
  • Length: 79 m
  • Width: 33 m each
  • Depth: approx. 5.5 m
  • Total volume: 171,000 m³

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