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Annex 4 Removal of Phosphate from water
Total phosphate includes three forms of phosphate;
1. Organic phosphate is found in plankton, algae and
bacterial cell biomass,
2. inorganic phosphate such as struvite, and
soluble reactive phosphate is referred to as ortho-
phosphate.
AFM® will mechanically filter the water down to less than
1 micron when coupled with pre-coagulation and
flocculation. The removal rate of organic and inorganic
particulate phosphate will be more than >95%.
AFM® will directly adsorb soluble reactive orthophosphate
PO 42- in the AFM® stern layer, the capacity for adsorption is low, but sufficient to make an impact on concentrations
remaining after coagulation with ferric, lanthanum or magnesium.
Water & Wastewater treatment to remove phosphate
AFM® provides a sustainable and efficient means of removing phosphate from wastewater.
There are three main approaches, all of which involve the precipitation of phosphate to form an insoluble salt by the addition
of;
(i)
ferric to form ferric phosphate
(ii)
magnesium to form struvite.
(iii)
lanthanum to form lanthanum phosphate
At Dryden Aqua we have been using (iii) Lanthanum salts (NoPhos) to remove phosphate in the aquarium and aquaculture
industry for over 20 years. Lanthanum is injected into the water at a 1:1 stoichiometric ratio to reduce organic phosphates
down to concentrations below 0.05 mg/l. NoPhos must be injected into the water before AFM® using an aggressive static
mixer such as our ZPM to ensure efficient use of NoPhos and removal of ortho-phosphate.
The process is simple, reliable and sustainable when Lanthanum chloride (NoPhos) is used. The performance of ferric is not
quite as good as lanthanum, in order to compensate for the reduced performance; typically, a 2: 4 excess molar ratio is
applied. More ferric may be required if there is a higher concentration of suspended solids or dissolved organics in the
water to be treated.
Ferric chloride is injected into the water via a ZPM or aggressive static mixer. Ideally there should be a 10-minute aerated
contact tank. The dissolved oxygen content must be kept above 2 mg/l or redox potential above 300mv. AFM® is highly
effective for the removal of ferric, arsenic and manganese and a good solution for the removal of the ferric phosphate salt.