Biological treatment: removal of phosphorus

Principle

Phosphorus, usually present in the form of phosphates, plays an important role in the eutrophication of surface water, resulting in excessive algae growth and fish death due to low oxygen content. Effluent standards for P and PO₄^3- are imposed to combat eutrophication effects.

Phosphates can be removed chemically (coagulation/flocculation process) or biologically. In biological waste water treatment plants, phosphate is partly incorporated in new sludge biomass. The phosphate removal is primarily related to sludge production and sludge removal. The biological process can be modified to increase the phosphate removal. Specific bacterial species (eg. Acinetobacter sp.) allow to accumulate elevated amounts of phosphorus as polyphosphate granules. The biological dephosphatation is carried out through an alternating aerobic and anaerobic activated sludge system, in which phosphate uptake is optimized for Acinetobacter.

In comparison to physicochemical phosphate removal, the quantity of waste sludge is much lower. Monitoring of process parameters is crucial to ensure efficient operation. In some cases additional dosing of an organic carbon source (rapidly biodegradable COD) is necessary to achieve the desired COD / P ratio.

Applications

Various process configurations with aerobic, anoxic and anaerobic reactors allow simultaneous removal of phosphate.

In the Phostrip process, the settled sludge from the clarifier or secondary settling tank is pumped to a fermentation tank. In this anaerobic reactor, phosphates are released. With the aid of chemicals, the phosphates are precipitated in a separate clarifier. It has to be noted that the Phostrip process is unsuitable to treat waste waters with high nitrogen concentrations

The Phoredox process enables simultaneous nitrogen and phosphorus removal. The waste water is consecutively treated in an anaerobic, anoxic and aerobic reactor. In a final step sludge is separated from the treated waste water in a clarifier and recycled to the anaerobic reactor.

Biological waste water treatment facilitating nutrient removal is often used for domestic waste water and waste water from food industry. Biological dephosphatation requires monitoring of various parameters to run the process efficiently ( COD/BOD, dissolved oxygen, redox potential, nitrate concentration, …).

Operation costs

Apart from the investment costs of the infrastructure, additional operating costs  are mainly related to required follow-up of the process and post-treatment of the generated waste sludge.