An outline of the applicable systems of treatment to the drinking water containing Fluorides is reported both on the USEPA and AWWA where is listed the best available technology in synthesis:

1) Absorption on Activated Granular Alumina (pH 5,0–6,0) and following washing and regeneration
2) Reverse Osmosis
3) ED and EDR (Reverse Electrodialysis)
4) Absorption on Bone Char

These techniques, applicable for the elimination of Fluorides in low concentrations, have relevant costs of investment and a complex management, justifiable therefore to big water flow treatments. Companies and Organizations have studied systems of precipitation of the Fluoride ion in sedimentation plants, suitable for small and medium flow dimension waste water production.

A wide literature is available on methods of precipitation of the Fluorides with salts of Calcium, Aluminum and Iron, that exploit the lower Products of Solubility of the correspondents fluorinated salts.

The treatment of water containing Fluorides, has been traditionally realized with Lime in alkaline pH range.

The reaction is :

Ca(OH)2 + 2 HF = CaF2 + H2O

The addition of an anionic poly-acrylamide (High Molecular Weight) allows the agglomeration of the flocks and a quick clarification

The fundamental problem that exists using this technique, arises from the low solubility of the Calcium Hydroxide (around = 0,07%) that therefore requires an excess of reagent to get a complete precipitation. For against, the solubility of the Calcium Fluoride (Ksp = 4*10 exp-11) doesn't obtain a complete removal of the Fluorides as required by the discharge Limits.

Using Iron salts (II) (Sulfate) to get the formation of (FeF6)3 - , the results aren't totally satisfactory.

The study on the formation of Aluminum complexes with the Fluorine has been developed (Garrison Sposito: The Environmental Chemistry of Aluminium–CRC Press–1989).  Experimental trials confirmed the ability of absorption of Fluoride ions on the Aluminum Hydroxide matrix due to the dimension of the ion F - that is similar to the ion OH.

Experimental trials carried out with PAC or PASS and Alum not always obtained reproducible results

The method studied by Intecna, allows to carry out the reaction in more reduced timing and to obtain better removal performance higher than 98 %

The global treatment involves two steps :

1) Pre-treatment with Lime
2) Further selective treatment with Hycor Fl in a controlled pH range

The Intecna’s product Hycor Fl is a mixture of precipitated Aluminium Oxide and re-dissolution of a complex Aluminium salt at pH value < 1

The own product Hycor Fl, to be used for the Fluoride removal in water and wastewater, Patented 2003A02590, is suitable also for the use in potable water production

Technical Specifications guaranteed in the product are :
Aspect Liquid
Colour Yellow/Green
Density 20° C 1,35 +/- 0,1 gr/ml
Viscosity 20 ° C 30 +/- 10 cP
Freezing Point - 10 ° C
pH 1,2 – 2,7
Arsenic < 0,1 ppm
Cadmium < 0,1 ppm
Chromium < 0,1 ppm
Iron < 50 ppm
Manganese < 6 ppm
Mercury < 0,1 ppm
Nickel < 0,1 ppm
Zinc < 0,1 ppm

These parameters allow to meet the requirements of USEPA 816-F-02-013 July 2002 in terms of final contaminants in the produced potable water as :

MCL = Maximum Contaminant Level

ANALYTICAL METHODS

The Fluoride content in samples have been measured by the following methods :

a) FLUORIDE SELECTIVE ELECTRODE - WTW : Measure range 0,2 ppm – saturation – Reference Electrode : Ag/AgCl
b) SPECIFIC REAGENS : Zirconium salt - Sodium Arsenite : Method HACH DR 2500 – 8029 – Method EPA – range 0,02 – 2 ppm as F-

The samples have been tested by both the methods, to avoid interferences.

For further confirmation of the results, the more significant samples have been tested by ionic chromatography.

METHOD DESCRIPTION

The INTECNA’s method for Fluorides removal is suitable for potable water production and for wastewater treatment.

The method is an application of the Patented Product HYCOR FL and in glass industry, provides the following steps :

LIME PRETREATMENT

The usual treatment with lime in slurry allows to treat concentrated flows (Fluorides > 1500 ppm) rising from washing or process wastewater where Hydrofluoric Acid (or Ammonium Fluoride) has been used.

Such treatment obtains residual Fluoride content in the range of 35 – 60 ppm.


HYCOR FL TREATMENT

Limpid water from pre-treatment step has been treated as follows :
1) Mixing of HYCOR FL in a well stirred reactor for a reaction time of 3 – 5 minutes
2) pH adjustment in a second reactor with Lime slurry until pH value of 7,5 – 7,8
3) Anionic Polymer solution (HYCRAM A 760) addition to allow a formation of big flocks
4) Sedimentation in a final settling tank

Method is a conventional sedimentation process and HYCOR FL characteristic is his capacity to form complexes with Fluoride having Solubility Product (Ksp) 1000 times lower than Calcium Fluoride.

Required equipment is only a secondary sedimentation flocculating device, where reaction occurs.
Other chemicals (Lime – Anionic Polymer) are already available for the conventional lime process, so investment in plant reconditioning is negligible.

FINAL CONSIDERATIONS

1.The described method is suitable to treat wastewater containing Fluorides in the range of 50 – 100 ppm to allow a residual selective removal until final value < 2 ppm

2. The method is a simple sedimentation plant and does not affect water quality, because does not increases other parameters

3. The investment cost is very acceptable and the treatment cost is low if compared with other technologies