Introduction

The Fluoride removal in drinking water and in wastewater has been subject of many publications and studies, that have progressively developed the aspects of toxicity on the man and on the environment.

The better known Internet web-sites that developed the thematic related to the content of Fluorides in drinking waters and to the impact on the man are www.who.int, www.nutritionfoundationofindia.res.in and also www.icoh.org. The web site www.fluoridealert.org outlines a wide collection of data and bibliography on the subject, and an evaluation on the entity of the emissions in air and water related to many Countries.

The manufacturing firms of fertilizers, of aluminum and glass, traditionally have to tackle this problem and other activities as the electronic production, the surface treatments of copper and aluminum and stainless steel produce wastewater with a strong presence of Fluoride ion.

Treatment Methods

An outline of the applicable systems of treatment to the drinking water containing Fluorides is reported both on the USEPA and AWWA sites www.awwa.org where the Best Available Technology are listed 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.

This method 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 treatment with HYCOR FL in a controlled pH range

The product HYCOR FL is a mixture of precipitated Aluminium Oxide and re-dissolution of a complex Aluminium salt at pH value < 1, can be used for the Fluoride removal in water and wastewater, Patented, and is suitable for the use in potable water production.

Technical Specifications
Aspect	Liquid
Colour	Yellow
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

Contaminant	MCL (mg/L)
Antimony	0,006
Arsenic	0,01
Barium	2
Beryllium	0,004
Cadmium	0,005
Chromium (total)	0,1
Mercury	0,002

• The product does not contain Organic Substances
• The product does not contains Disinfectants or Disinfection by-products or Radionuclides
• The product does not contains Microorganisms

Furthermore, the use of the product HYCOR FL allows to meet the National Secondary Drinking Water Regulations (NSDWR) reported in the same USEPA current Recommendations.

Analytical Methods

The Fluoride content in samples was measured by the following methods:

1. FLUORIDE SELECTIVE ELECTRODE - WTW : Measure range 0,2 ppm - saturation - Reference Electrode : Ag/AgCl
2. 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, being the possibility of big interferences.
For further confirmation of the results, the more significant samples have been tested by ionic chromatography.

In the following, the results of Fluoride Removal are reported, which were obtained in several HYCOR FL applications around the World:

Case 1 – Stainless Steel Pipe Manufacturing

Case 2 – Glass Manufacturing

Case 3 – Aluminium Surface Treatment

Case 4 – Aluminium Surface Treatment

Case 5 – Drinking Water Treatment

Principles of the Method

The method is an application of the patented product HYCOR FL, which is suitable for the removal of Fluorides in potable water production and in wastewater treatment through the following steps:

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

The product allows the removal of Fluorides below 0,5 ppm

Example

In the following example, we assume a treatment of 5 liter/sec equivalent to 18 m³/hour of a water containing 5 ppm of Fluorides

Storage of HYCOR FL

The storage of HYCOR FL is not required because the feeding is realised directly by the 1300 Kg containers. For a flow of 18 m³/hour the consumption is in the range of 400 - 500 ppm equivalent to 9 Kg/hour. One container allows a treatment of about 144 hour (6 days).
By maintaining in the plant 5 containers there is an autonomy approximately of one month of treatment.

Dosing Pump

The dosing pump is in moplen (Milton Roy or equivalent). The product is strong acidic and the dosing pump must be acid proof.
The dosing capacity is in the range of 0 - 20 Kg /hour.

Piping Valves

From the storage to the reaction vessel the material suggested is moplen.

Reaction Vessel

The reaction vessel is realised in Moplen and the suggested volume is 1,5 cubic meter.
The reaction vessel must be equipped with a mechanical stirrer (90 - 140 rpm).
In the reaction vessel the pH is 4,5 - 5,5.
To avoid Iron contamination or corrosion phenomena all the material in contact with the water must be acid proof (MOPLEN - PVC - POLIETHYLENE)
During the HYCOR FL addition there is a formation of a slightly opalescent suspension.
The transfer from the reaction vessel to the second reactor (where pH will be adjusted until 7,5 - 7,8) can be realised by overflow (from the top of the reaction vessel) or by transfer pump.
The more suitable (and economic) method is the overflow.

pH - Correction Vessel

The pH - correction vessel is realised in Plastic or epoxy coated carbon steel.
The suggested volume is 3 m³ to allow a pH correction with a better volume compensation.
The vessel must be equipped with a pH-meter with an electrical command on the Lime slurry.

Lime Slurry Preparation Vessel

The vessel where a slurry of 8 % Lime will be prepared, is realised in plastic.
The suggested volume is 1 m³.
The vessel must be equipped with a mechanical stirrer to avoid Lime settling
The slurry will be pumped to pH correction vessel by a dosing pump (membrane).

Sedimentation Vessel

After the pH correction there is the formation of flocks and the transfer from the pH correction vessel to the sedimentation vessel can be realised by a centrifugal pump.
On the pump DELIVERY it will be injected the Anionic Polymer solution (A 760 supplied in liquid form ready to be used). After the injection the flow must load the sedimentation vessel without other pumping to avoid the flocks breaking.
The sedimentation vessel volume is 30 m³.
It´s suggestible the use of lamellar sedimentation equipment to reduce the required space and to improve the sedimentation phase.

Final Security Filter

To avoid small particle escape. It´s suggested a security sand/activated carbon small filter.
Such component is a standard package suitable to treat 20 m³/hour.
A suggestible package is :
- One sand filter in work
- One sand filter in back wash
- One Activated carbon always in work.

Sludge Discharge

The sludge collected at the bottom of the sedimentation vessel must be discharged every 3 - 4 hours.
The sludge in such plant could be filtered by press filter or transferred to filtration beds, depending on the local rules.

Final Considerations

1. The described method is suitable to treat a water containing Fluorides in the range of 5 - 15 ppm
2. The method is a simple sedimentation plant and does not affect the potable water quality
3. The investment cost is very acceptable and the treatment cost is low if compared with other technologies
4. The result will be the production of a potable water with a Fluoride content less than 1 ppm

Bibliography

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