Removal of chromium

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