Biological Phosphorous Elimination

Phosphorus is an eutrophificating substance and is biologically and/or chemically removed from the wastewater. Since the biological phosphorus elimination alone cannot reliably meet the monitoring value for phosphorus, it is usually used in combination with the chemical phosphorus precipitation, which means by adding precipitants.

Biological phosphorus elimination counteracts the disadvantages of chemical phosphorus precipitation: The cost of precipitant and the salination of the receiving water are reduced.

The aim of biological phosphorus elimination is the storage of phosphorus in the biomass. Phosphorus is required by all organisms and makes up about 3% of their dry cell mass. In addition to the growth-related P uptake of the microorganisms, insoluble phosphate compounds can also accumulate on the sludge flakes. In this way, approximately 20-30% of phosphorus can be removed from the water.

An additional biological P-elimination can be achieved if the activated sludge is exposed to anaerobic, anoxic and aerobic conditions with a suitable operational management. For this purpose, an anaerobic mixing tank is installed in front of the aeration tank. The anaerobic zone forces a stress situation, which is crucial for the additional biological P-elimination: The microorganisms cannot breathe and in order not to die, they release phosphates stored in their cells. Thereby energy is released, the microorganisms survive and, as a result, absorb significantly higher amounts of phosphates in the aerobic zone (see phosphate accumulating organisms, PAO). In this way, approximately 70-90% of the phosphorus can be removed from the water.

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