Flouride removal is by adsorption on APF-L followed by coagulation and flocculation.  The removal performance increases with water hardness and benefits from the addition of calcium or magnesium salts. The solids are then removed by AFM® filtration,  the surface of AFM® is an activated surface, similar to an activated alumina. AFM® will adsorb some aluminium flouride and calcium flouride particles from the water, as well as some flouride in solution, and then release the chemicals during a back-wash. The filter media needs to go through a conditioning process that takes up to 6 to 12 weeks.

Other approaches for the removal of flouride include reverse osmosis, or adsorption on to activated alumina.  With activated alumina,  once the media reaches capacity it needs to be replaced

Chemical parameterSoluble fractionInsolublePermitted concentration in drinking waterTypical performance at 5 - 10 m/hr filtration velocity
Flouride F- ion is soluble in water flourite
CaF2
MgF2
aluminium flouride
AlF3
1.5 mg/l >80 %
Aluminium     0.2 mg/l >95 %

Flouride is a naturally occurring halogen, Florine, Chlorine, Bromine and Iodine, the ionised halide form is flouride, chloride, iodide and bromide.  Flouride has a low toxicity and is occasionally added to some water supplies and dental products to help prevent tooth decay.  However exposure to flouride over a long period can result in displacement of calcium and the condition of  osteoporosis.

Flouride is common in water supplies, especially some ground water,  the consensus now is that Flouride should not be added to water supplies and should not exceed 1.5 mg/l in solution.

Process

  1. Aeration and mixing helps to convert  flouride, from an ionic salt to insoluble aluminium hydroxide / calcium / magnesium precipitate. The  pH should be around pH 7 and redox potential above 200 mv
  2. The raw water enters and aeration tank with a retention capacity of at least 60 minutes. The water is aerated with 1 times the amount of air. for example it water flow is 50m3/hr, the aeration capacity should be 50 m3/hr of air. APF-L is added to the water, via a pump and ZPM module. Application rate of APF-l is approx 50ml per 1 g of flouride. The quantity of APF-L is a variable, start off at a low concentration and gradually increase until the desired removal rate is achieved. It should be noted that as the performance of the filter will gradually improve the application rate of APF-L should decrease over the first 12 weeks.
  3. AFM® filtration to remove the suspended metal flouride complexes, in addition there will be some adsorption reactions. AFM®filters are back-washed to release all of the flouride. The AFM® filter bed contains, 15 % grade 3 and 2, 70 % grade 1.

Process options

Powdered activated carbon may be added to the aeration tank, this will reduce the amount of APF-L required. If it is not permitted to use APF or products such as PAC, then powdered activated carbon may be used. If there are no aluminium products then it is recommend that the AFM® filter bed contains  20 % grade 0 media on top of the grade 1. Bed composition is 15 % grade 3 and 2, 50 % grade 1 and 20 % grade 0.

Procedure

Aeration and mixing;

This is achieved through aeration of the water. The water is aerated for a period of no less than 60 minutes.  If water flow is 50 m3/hr the aeration level is 50 m3/hr of air and tank volume is 50m3 of water. Dryden Aqua manufacture fine bubble drop in air diffusers for this application.

pH;

The pH of the water should be between pH6.5 and pH 7.5,  but as close to pH 7 as possible.

Redox;

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 200 mv.

Operating parameters, AFM® filtration

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.

 RecommendedMaximumNotes
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 >45 m/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.

Note

APF-l is the same as standard APF  Dryden Aqua AllPolyFloc, but it does not contain NoPhos. All other components are in compliance to drinking water regulations. NoPhos is actually very good for removing Flouride, but it is not permitted in drinking water.

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