Biological phosphorous reduction

A question of ecological selection

Bacterial biomass in wastewater treatment processes typically contains 1.5% phosphorus (P). If this phosphorus content could be increased to 5-7%, essentially all the phosphorus discharged to municipal wastewater treatment plants could be captured and removed in the wasted sludge. This P enrichment of the biomass is principle behind biological phosphorus (Bio-P) removal.

The wastewater treatment process environmental conditions are tailored to favor the preferential growth of certain P-bacteria that store large quantities of phosphorus as part of their survival strategy. These P bacteria become enriched due to exposure to alternating aerobic and anaerobic zones in the treatment process.

In practice, biological phosphorus removal is almost always used in combination with biological nitrogen removal, which also calls for distinct process zones that select for different kinds of bacteria that oxidize (nitrification) and reduce nitrogen (denitrification).

Wastewater Treatment Process description

The AnoxKaldnes biological nitrogen and phosphorus removal systems comprise of four process stages in series:

• Anaerobic activated sludge.
• An anoxic activated sludge stage where mixed liquor is recycled back to the first stage.
• A second anoxic activated sludge stage.
• An aerobic HYBAS™ hybrid stage with activated sludge and MBBR™ biocarriers together and where mixed liquor is recycled back to the third stage.

Nitrification takes place in the biofilm on the biocarriers inside the HYBAS™ hybrid stage giving a more stable and simple operation than one would experience with a conventional activated sludge biological nutrient removal approach.