The removal of Iron, Manganese and Arsenic from ground-water and surface water
|Chemical parameter||Soluble fraction||Insoluble||Permitted concentration in drinking water||Typical performance at 5 - 10m/hr filtration velocity|
Iron, manganese and arsenic are often found in borehole / tube wells and ground-water at varying concentrations depending upon the geology of the ground. The process used by Dryden Aqua to remove the chemicals is as follows;
- Oxidation reactions to convert metals from soluble ionic form to insoluble oxidized precipitate, pH correction and redox correction.
- Decantation may be required if the concentrations are above 5 mg/l, if not proceed to AFM® filtration
- AFM® filtration to remove the suspended metal oxide solids, there will also be adsorption reactions and surface oxidation reactions.
This is achieved through aeration of the water. The water is aerated for a period of no less than 30 minutes. If water flow is 50 m3/hr the aeration level is 50m3/hr of air and tank volume is 25m3 of water. Dryden Aqua manufacture fine bubble drop in air diffusers for this application.
The pH of the water should be between pH7 and pH8.5
The aeration system should increase the redox potential of the water. It is important to raise the potential to as high a value as possible. Certainly it should be over 300mv for ferric and arsenate. Manganese needs an additional oxidising agent to raise the potential to 500mv. A ZPM between the pump and AFM filters should also be fitted.
- Manganese oxidation requires aggressive aeration and an additional oxidising agent, such as hypochlorite, chlorine dioxide, hydrogen peroxide or ozone in order to raise the Redox potential to 500mv +/- 25mv.
- Ferric oxidation is simple, the aeration system will more than suffice.
- Arsenic oxidation also benefits from the addition of ferric to the water. Arsenic is removed by oxidation and co-precipitation with ferric, example 0.1 mg/l As requires up to 0.3mg/l of ferric (as ferric chloride). The ferric catalysis the oxidation of As5+ to As3+ and helps to co-precipiate the arsenic, which is then physically removed by the AFM filter. Any arsenic remianin as AS5+ in solution will be removed by adsorption on the the negative charged surface of AFM.
Pretreatment of the water prior to filtration by AFM® is very important. AFM® will remove the metals and metalloid by the following mechanisms;
- Oxidation and adsorption (similar to greensands and ferric media)
- Adsorption of sub-micron metal oxide particles
- Physical filtration of most particles down to 1 micron.
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 represent the optimum filter bed operating conditions.
|Bed depth AFM||>1200mm||>1200mm||Bulk bed density 1.25 to 1|
|Run phase water flow||5 m3/hr||10m/hr||The slower the flow rate the better the performance|
|Running pressures (differential)||0.2 bar||0.4 ba||Do not exceed 0.4 bar differential|
|Back-wash water flow||>45m/hr||60m/hr||Back-wash for 5 minutes, or until the water runs clear. Air purge not required|
|Rinse phase duration||5 minutes||or until water runs clear||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.|