Fast phosphorus removal

The Japanese firm Asahi Kasei Chemicals claims to have developed the world’s fastest phosphorus adsorbent, together with an efficient water treatment system that uses it for the recovery of phosphorus from treated water.

Occurring in effluent water, phosphorus has been identified as a cause of eutrophication, in which an excess of nutrients in enclosed bodies of water such as lakes and harbours fosters the growth of an algal bloom, disrupting the local ecosystem and rendering the water unsuitable for many purposes.

But each of the conventional methods of large-scale dephosphorisation has its drawbacks. Using biological treatment, it is difficult to maintain consistently low phosphorus concentrations, and coagulation-sedimentation produces a large amount of sludge which requires costly treatment and disposal.

While it is possible to reduce phosphorus concentration to the order of 0.01ppm using adsorbents, the slow speed of adsorption to date has made large-scale application impractical.

Phosphorus is also an important resource, particularly for the production of fertiliser. With studies forecasting the depletion of phosphate mineral deposits in as little as a few decades, recovery from effluent water is potentially a valuable complementary source of the material. Conventional methods of dephosphorisation, however, generally recover phosphorus in the form of a mixture with many impurities.

Thus, there's a great demand for a practical technology to remove phosphorus from effluent water and recover it for use.

The phosphorus adsorbent developed by Asahi Kasei Chemicals comes in the form of small beads that feature a novel structure of surface micropores with an internal network of submicron pores, resulting in a large surface area. The structure gives the adsorbent a very high adsorption capacity and enables phosphate concentration to be reduced to 0.01ppm or less at a flow rate some ten times higher than with previously available adsorbents. The new adsorbent also readily desorbs phosphate ions in an alkaline solution, enabling repeated use.

The phosphorus removal and recovery system using the adsorbent comprises three stages. In the adsorption stage, water is fed through a column charged with the adsorbent and phosphorus is removed. In the desorption stage, an alkaline solution is passed through the column and the phosphate ions are desorbed. The adsorbent can then be used again in the adsorption stage. In the recovery stage, desorbed phosphate ions are separated as solid phosphate salt. The alkaline solution can then be used again in the desorption stage, while the recovered phosphate salt, of high purity, can be used as precursor for fertiliser.

The company is currently working with the Japan Sewage Works Agency to verify the performance of the adsorbent in the removal of phosphorus from secondary effluent from municipal wastewater treatment. The trial is scheduled for completion in fiscal 2008.