Removal of chromium using AFM®
|Chemical parameter||Soluble fraction||Insoluble||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®.
- The pH of the water to be treated should be above pH 7.5
- Ferrous sulphate, and calcium polysulpite may be injected into the water via a ZPM cavitating static mixer. Application rate approx. 20:1, 20 ferrous to 1 Chromium, and 5:1 for the calcium polysulphite.
- After injection the water enters a sedimentation decantation reaction tank for at least 30 minutes.
- After decantation to water enters a second 30 minutes retention tank for aeration. The aeration level is the same as water flow, if water flow is 50m3/h air flow is 50m3/h applied via Dryden Aqua air diffusers. The aeration is not sufficient to oxidise the Cr3+ back to Cr6+ but it is sufficient to oxidise the Fe2+ to Fe3+ which helps to co-precipitate the Cr3+
- The water is pumped out of the aeration tank to an AFM®pressure filter.
Pretreatment of the water prior to filtration by AFM® is very important. AFM® will remove Cr6+ and Cr3+ by the following mechanisms:
- Direct adsorption of Cr6+ ion on to AFM®
- Direct adsorption of chromium Cr3+ : ferric particles onto the –ve charged surface of AFM®
- Physical filtration of most particles down to 1 micron
- Back-washing with water will completely remove the Chromium
- Back-wash water sludge should be separated and disposed of responsibly
|Bed depth AFM®||1200 mm||3000 mm||Bulk bed density 1.25 to 1|
|Run phase water flow||<5 m/hr||10 m/hr||The slower the flow rate the better the performance|
|Running pressures (differential)||<0.3 bar||<0.4 bar||Do not exceed 0.4 bar differential|
|Back-wash water flow||>45m/hr||60 m/hr||Back-wash for 5 minutes, or until the water runs clear. Air purge not required|
|Rinse phase duration||5 minutes||----||It takes a few minutes for the bed to stabilize after a back-wash|
|Back-wash frequency / per week||1||7||Reduce back-washing to a minimum.|