l

Notes.

Iron & Manganese removal

1. The performance of any media bed, be it sand, brim or AFM follows the standard inverse square law,  it is therefore very important that the operational  water flow rates are reduced to the lowest possible rate. Ideally the water flow should be 5 cubm/hr/sqm of filter bed surface area,  however it should not exceed 15 cubm/hr/sqm.

2. When AFM is used for iron and manganese removal, the water is usually mechanically clean borehole water,  the back-wash frequency should be reduced to once per week, unless the pressure differential across the filter bed exceeds 0.5 bar, in which case the back-wash frequency should be increased.

3. When AFM is used for iron removal,  it is important to use a high water flow for the back-wash, but only for a short period of time. The back-wash interval may be as short as 2 minutes, too long a back-wash will remove all the iron from the filter and reduce the performance of the AFM. The presence of iron in the AFM filter actually catalyses the removal of more iron from the water.  It is therefore important not to back-wash all of the iron or manganese out of the AFM filter bed.

4. Iron & manganese removal is a mechanical as well as a physio-chemical reaction,  it is important that the oxygen content of the water entering the AFM filter is at least 80% saturated with a redox potential in excess of 250mv.

5. If the iron levels exceed 2 mg/l extra aeration or oxidation will be required in the oxidation tank

6. The oxidation tank also acts as a buffer tank and allows the crystal of ferric oxide or manganese dioxide to grow.  The longer the residence time, the larger the particles and the easier it is to remove them from the water, the minimum residence time is 5 minutes.  Occasionally ozone or hydrogen peroxide may be used to increase the oxidation potential of the water.  However we usually find that strong aeration using fine bubble air diffusers provides a better performance.

Iron & Manganese systems

1. Chiquita in Costa Rica, use borehole water from the tropical rain forest to wash the bananas.  Iron present in the water stains the bananas, in addition to iron the water is full of organics and latex from the plants, conventional iron removal system have been tried but they simply became choked with the high level of latex.  An AFM system solved problem and reduce the iron levels from a value greater than 10mg/l to less than 0.1mg/l.

2. Gerber Foods from Switzerland, also in Costa Rica use the bananas from Chiquita to produce baby food.  The company uses a large volume of water which was treated by AFM filters to remove the iron down to levels below 0.05mg/l

3. Palm Aquarium in Minorca Spain use 150 cubic meters per hour of ground water as a supply to the Aquarium.  Iron at levels above 0.1mg/l will damage the gills of fish and cause mass fish mortalities.  The iron levels in the feed water are greater than 0.5mg/l.  An AFM system removed the iron and gives crystal clear water.