Removal of chromium

Chemical parameter Soluble fraction Insoluble Permitted concentration in
drinking water
Typical performance at
5 - 10m/hr filtration velocity
Chromium Cr6+ Cr3+ 50 ug/l >80%

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®.


Procedure

  1. The pH of the water to be treated should be above pH 7.5
  2. 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.
  3. After injection the water enters a sedimentation decantation reaction tank for at least 30 minutes.
  4. 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+
  5. The water is pumped out of the aeration tank to an AFM® pressure filter.

AFM® filtration

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


Operating parameters

Chemical parameter range notes
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.