Process
There are three approaches, ferric and lanthanum phosphate formation or struvite formation. An
insoluble precipitate is formed and then removed by decantation and / or AFM® filtration. AFM® is
perfect for this application because the ferric phosphate or struvite formed is efficiently removed by
AFM® without solidifying the filtration bed.
Molar ratios
1. Lanthanum addition is stoichiometric at a molar ration of 1:1
2. Ferric addition should be at a ratio of 2-4 to 1 molar Ferric to Phosphate. This will give a surplus
of ferric for coagulation and other flocculation reactions. 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 of ferric required.
3. 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.
4. The chemical reactions are rapid, 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. It is important to insure that the dissolved oxygen concentration is above 2mg/l or the
redox potential exceeds 300mv
5. Decantation may be required if the concentration of phosphate is above 5 mg/l as PO4-P, if not
proceed to AFM® filtration
6. AFM® filtration to remove the phosphate suspended solids; there will also be adsorption
reactions of phosphate PO 42- directly onto AFM®. An activated form of AFM® designed to be
even more selective for phosphate is in development.
AFM® filtration
• Mixing and contact time of >15 minutes not essential but recommended
• Precipitation and decantation of solids, required at high concentrations of solids and phosphate
• AFM® will provide physical filtration of more than 95% of all particles down to 1 micron. With no
flocculation, performance is confirmed at 95% down to 3 microns with ferric addition this will be
improved to less than 1 micron.
Dryden Aqua Ltd
Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland
Page 38 of 44
AFM® is a sustainable approach, there will be no requirement to replace AFM®, but the pre-treatment
and back-wash procedures are very important.