Hexavalent Chromium and Copper removal
|Chemical parameter||Soluble fraction||Insoluble||Permitted concentration in drinking water||
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. High concentrations of copper can be naturally occurring or as a results of industrial pollution. Chromium and copper are considered together because the same approach is used to remove both metals.
AFM® can adsorb hexavalent Cr6+ and copper Cu2+ directly from the water, but the capacity is quite low, so the best approach is to reduce metals to an insoluble form and remove by precipitation and filtration on AFM.
The Cr6+ is reduced to Cr3+ and copper is reduced from Cu2+ to CuO copper oxide by using a reducing agent; calcium polysulphide. The reaction takes time, so a buffer tank of more than 30 minutes is required. Another approach is to use zero valent iron, which is very effective at removing both chromium and copper.
- The pH of the water to be treated should pH7.0 to pH7.5, the pH may be increased to improve the performance of the process. MagpHlow may be added to the AFM or the initial reaction tank. Alternatively, caustic may be added to the reaction tank to raise the pH to 8.5
- Calcium polysulpide may be injected into the water via a ZPM cavitating static mixer. Application rate approx. 5:1 for the calcium polysulphide to chromium or copper
- GF50, Dryden glass sub 50 micron, may be added to the water along with the polysulphide. The powder acts as a nucleus upon which the metal ions precipitate. Application level of the GF50 should be 10 tomes the mass of metals.
- After injection the water enters a sedimentation decantation reaction tank for at least 30 minutes. Sludge should be removed, dried and reused or disposed of in a responsible manner.
- 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+ and Cu2+ ion on to AFM®
- Direct adsorption of chromium Cr3+ 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 and Copper
- Back-wash water sludge should be separated and disposed of responsibly
AFM® filtration performance will depend upon the operating parameters. It is important to use good quality filters; we recommend filters in compliance to the German DIN standard. The follow represents the optimum filter bed operating conditions.
|Bed depth AFM||1200mm||1400mm||Bulk bed density 1.25 to 1|
|Run phase water flow||5 m/hr||10m/hr||The slower the flow rate the better the performance|
|Running pressures (differential)||0.1 bar||0.4 bar||Do not exceed 0.4 bar differential|
|Back-wash water flow||>45m/hr||>45m/hr||Back-wash for 5 minutes, or until the water runs clear. Air purge not required|
|Rinse phase duration||5 minutes||5 minutes||It takes a few minutes for the bed to stabilize after a back-wash|
|Back-wash frequency / per week||1||14||Reduce back-washing to a minimum, frequency depends upon the concentration of solids and metals in solution|