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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 the
surface of AFM. However the process can be improved by the the
addition of NoPhos 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 oxidised.
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.
NoPhos is supplied as a crystalline product, 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 catalyse 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 NoPhos complete will either 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. There benefit of this process
is that it is simple, and low cost, and there are no consumable except for
the NoPhos.
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