Removal of Phosphate from water

Chemical parameter Soluble fraction Insoluble Permitted concentration in drinking water Typical performance at 5 - 10m/hr filtration velocity  

Phosphate

Soluble reactive

PO42-

(Fe3+)2(PO42-)3

Mg2+NH4+PO42-

No limit >90%  
The details below provides a list of phosphate minerals that will form insoluble precipitates
 
  • Mitridatite group:

Phosphate is often added to municipal water to form an insoluble phosphate layer on the inside of pipework and tanks.  The presence of phosphate in municipal wastewater, or wastewater discharge to canals, rivers and lakes is of more concern because of the eutrophication impact and potential to fertilise algae and cyanobacteria blooms.

AFM is perfect for the tertiary treatment of waste water because if does not become a biofilter or suffer from bio-dynamic instability and transient wormhole channelling experienced by sand filters.

 The process used by Dryden Aqua to remove phosphate is as follows;

Process

There are two approaches, ferric phosphate formation or struvite formation. Ferric or magnesium will react with phosphate to form an insoluble precipitate, which is then removed by decantation and / or AFM filtration. AFM is perfect for this application because the ferric phosphate or struvite that is formed will be removed by AFM, but AFM will not become blocked when back-washed properly.

Molar ratios

  1. Ferric addition should be at a ratio of 2 to 1 molar Ferric to Phosphate.  This will give a surplus of phosphate. The optimum concentration should be determined on a cases to case basis because water with a high concentration of suspended solids, or other chemicals will influence the concentration required.
  2. Struvite. The molar ration NH3:Mg:PO4 equates with 1:8:3, this is not stoichiometric but it has been found in different water types to give good results.  It will be a requirement to adjust the injection of magnesium to determine the optimum ratio.
  3. The chemical reactions are rapid, and a period of 15 minutes is sufficient.  Dryden Aqua air diffusers are perfect to perform the mixing action because they are easy to remove for cleaning and descaling
  4. Decantation may be required if the concentrations are above 2 mg/l, if not proceed to AFM® filtration
  5. AFM® filtration to remove the phosphate suspended solids, there will also be adsorption reactions and phosphate PO42- will be adsorbed directly onto AFM.  An activated form of AFM designed to be more selective for phosphate is in development.

 

AFM® filtration

Pre-treatment of the water prior to filtration by AFM® is very important. AFM® will sustainably remove phosphate  There will be no requirement to replace the AFM,  but the pre-treatment and back-wash procedure is very important.

  • Mixing and contact time of >15 minutes
  • Precipitation and decantation of solids
  • Physical filtration of most particles down to 1 micron by AFM

Operating parameters

AFM® filtration performance will depend upon the operating parameters.  It is important to use good quality filters,  we recommend filters in compliance to the German DIN standard.  The follow represent the optimum filter bed operating conditions.

range notes
Bed depth AFM 1200mm   Bulk bed density 1.25 to 1
Run phase water flow 5 m/hr 10 m/hr The slower the flow rate the better the performance
Running pressures (differential) 0.2 0.4 Do not exceed 0.4 bar differential
Back-wash water flow >45m/hr 60m/hr Back-wash for 5 minutes, or until the water runs clear. Air purge not required
Rinse phase duration 5 minutes Or until water runs clear It takes a few minutes for the bed to stabilize after a back-wash
Back-wash frequency / per week 1 7 Air purge is not required, but may be used to reduce back-wash water consumption.

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