|Permitted concentration in
|Typical performance at
5 - 10m/hr filtration velocity
Chromium is occasionally found in ground water in trace amount from 1 to 10 ug/l
however the occurrence of chromium is usually associated with pollution from
industries such a leather tanning, wood preservation and textiles. Polluted ground
water may have much higher concentrations.
Hexavalent Chromium is very difficult to remove down to low concentrations,
however AFM® can adsorb Cr6+ directly from the water. If the concentration is low
then it may be possible to just filter the water through a deep AFM® filter bed
>1200 mm at a slow flowrate , 5m/hr. This may be all of the treatment required.
AFM® has a low capacity for chromium, so in order to improve performance water
pretreatment may be required The Cr6+ is reduced to Cr3+ by using a reducing agent
such as ferrous sulphate or calcium polysulphide. The reaction takes time, so a buffer
tank of more than 30 minutes is required.
After the 30 minute contact tank, the water overflows to a second contact tank which
is aerated, this oxidises any remaining ferrous to ferric which co-precipitates and
flocculates the Cr3+ to form a solid particle than can be more easily removed by AFM®.
Pretreatment of the water prior to filtration by AFM® is very important. AFM® will remove Cr6+ and Cr3+ by the following mechanisms:
|Bed depth AFM®
|Bulk bed density 1.25 to 1
|Run phase water flow
|The slower the flow rate the better the performance
|Running pressures (differential)
|Do not exceed 0.4 bar differential
|Back-wash water flow
|Back-wash for 5 minutes, or until the water runs clear. Air purge not required
|Rinse phase duration
|It takes a few minutes for the bed to stabilize after a back-wash
|Back-wash frequency / per week
|Reduce back-washing to a minimum.