AFM has been manufactured to have a  high negative zeta potential of around -70mv, Arsenic is present in a cationic form.  With no pre-treatment of the water,  a proportion of the Arsenic present in solution will be reversibly adsorbed directly onto the surface of AFM® .  However the process can be improved by the addition of NoPhos, preferably  under slightly acidic conditions.

The main active component of NoPhos is Lanthanum which reacts with the Arsenic to form insoluble lanthanum arsenate, LaAsO[4], which is then removed by the AFM® .  The best mode of operation is therefore to dissolve the NoPhos into water and dose at a controlled rate before the AFM® mechanical filters. The NoPhos will also cause flocculation of solids and precipitation of phosphate,  so sufficient needs to be added to the water to not only react with phosphate but also Arsenic.  If the NoPhos is over dosed then the AFM® will adsorb all or a proportion of the excess.

Basic calculation

1. Measure the dissolved phosphate concentration as PO4-P, in mg/l

2. Measure the Arsenic concentration as As, mg/l

3. If phosphate PO4-p is 0.5 mg/l,  then you need to add 1.5 mg/l of NoPhos  ( 3 times the level of PO4-P)

4. If Arsenic level is 0.2 mg/l then you need to add 1.2 mg/l of NoPhos ( 6 times the level of Arsenic)

Additional points

If the water supply is borehole water it needs to be vigorously aerated  and the oxygen level raised to at least 10 mg/l  ( when under 15 deg C). All volatile components needs to be gas stripped out of the water and all metals such as iron and manganese to be fully oxidized.  After gas stripping the water should enter a buffer tank in which it is gently aerated for a period of at least 10 minutes. The longer the aeration interval the better.

If the water contains any solids or dissolved organics, then we recommend the use of ferric chloride for the precipitation of the solids, the ferric also aids in arsenic removal.  The NoPhos is then applied after the ferric chloride injection immediately before the AFM® filters.

NoPhos is supplied as a crystalline product or in solution,  to use dissolve approx 2 kg into 25 litres of water and then use a dosing pump  to inject it into the water. The NoPhos should be added to the aeration tank.  The presence of iron can actually help catalyze the oxidation of Arsenic and the NoPhos forms an insoluble precipitate.

From the aeration tank the water may flow though a sedimentation tank, or a tank filled with plastic biofiltration media.  The arsenic ferric & NoPhos precipitate will drop out of solution in the tank, or form a coating on the biofilter media. In this situation the biofilter media is being used as a reaction surface, not as a biofilter.

After the above process there may still be some arsenic in suspension,  the AFM® filter can then polish the water and remove the residual arsenic. If arsenic is still found after the AFM® filter, then additional NoPhos may be required.  The benefit of this process is that it is simple, and low cost, and there are no consumables except for the NoPhos.