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Fluoride removal from the water.


What is fluoride - Fluoride compounds are salts that form when the element, fluorine, combines with minerals in soil or rocks. Fluorine is a common element that does not occur in the elemental state in nature because of its high reactivity. It accounts for about 0.3 g/kg of the Earth’s crust and exists in the form of fluorides in a number of minerals.

Fluoride in water - Fluoride is found in all natural waters at some concentration. Seawater typically contains about 0.86 –1.4 mg/l while freshwater concentrations are in the range of 0.01 – 0.3 mg/l. In groundwater, however, low or high concentrations of fluoride can occur, depending on the nature of the rocks and the occurrence of fluoride-bearing minerals.

Guidelines and standards for Fluoride level in drinking water - Taking health effects into consideration, the World Health Organization (1996) has set a guideline value of 1.5 mg/1 as the maximum permissible level of fluoride in drinking waters. However, it is important to consider climatic conditions, volume of water intake, diet and other factors in setting national standards for fluoride. As the fluoride intake determines health effects, standards are bound to be different for countries with temperate climates and for tropical countries, where significantly more water is consumed. In an interview of Dr. Annette Johnson, described natural occurrence of fluoride, its adverse effects and the areas affected by fluoride. 

Removal of Fluoride from drinking water - The common methods used for the removal of fluoride from drinking water are divided in the following categories:

  1. Precipitation
  2. Adsorption and ion-exchange
  3. Membrane filtration 

1. Precipitation

Adding chemicals to remove fluoride as precipitates is called as the precipitation method. Among these are precipitations with calcium and aluminum salts.

A. Sources of calcium (Lime Vs Calcium Fluoride): Lime is a rich source of calcium however when compared to calcium chloride (CaCl2) it has lesser solubility (0.185 g/100 ml Vs 74.5 g/100ml). Consequently when lime is added as a removal agent the amount of calcium available for interaction with fluoride is significantly lower than when calcium chloride is added. When lime is added for treatment, the lower solubility of calcium would cause insoluble particles to form a sludge which requires high treatment costs for removal. Further, Lime also needs to be added in large amounts (30 mg/mg F) due to its lower solubility. When calcium fluoride is used, only small volume is enough per unit of fluoride removal (3 mg CaCl2/mg F), it has better rate compared to lime, it has ease of dispensing and monitoring of the additive. In addition to these, it has better control on the pH of the treated effluent. It is recommended as the most cost effective of all other additives. It has >90% efficiency in the removal of fluoride.

 Fig 1. Contact precipitation for fluoride removal Source - http://www.samsamwater.com/

B. Usage of other common additives: Alum (aluminum sulphate) is often used which has >90% removal efficiency. However a heavy dosage (150 mg/mg F) is needed. Another commonly used additive is alum + lime (Nalgonda method) this method also requires high dosage  (150 mg alum + 7 mg lime/mg F). Also, this method has the same disadvantages of using Lime and Alum which is that of the sludge rich in Al.

Fig. 2 - The fill and draw type Nalgonda technique for domestic and community Source http://bit.ly/1E1OLh1

2. Adsorption and ion-exchange - In the adsorption method, raw water is passed through a bed containing defluoridating material. The material retains fluoride either by physical, chemical or ion exchange mechanisms. The adsorbent gets saturated after a period of operation and requires regeneration. The different adsorbents used for fluoride removal include activated alumina, carbon, bone charcoal and synthetic ion exchange resins.

3. Membrane filtration process - Reverse osmosis and electrodialysis are two membrane filtration processes which can be used for removal of fluoride. Large scale electrodialysis plants are already used for making drinking water out of brackish water with high fluoride concentrations. Reverse osmosis requires between 2 and 4 gallons of water to produce 1 gallon of RO water. Source water with an abundance of contaminants (including hard water) can reduce the efficiency of an RO system and it can shorten the life of the membrane. Replacement of fouled membranes and lot of energy needed for this process are the major disadvantages.

Fig 3 - Schematic diagram of an electrodialysis plant http://bit.ly/1xDDZtK

 Adoption of appropriate method - International Groundwater Resources Assessment Centre (IGRAC) has created the following summary for the different options. The methods can be used by domestic well owners and communal water suppliers. Based on available information, a matrix is composed to give an idea about the applicability of the methods for some
given situations. 

Table 1. Summary of fluoride removal method source http://bit.ly/1HoDlQW

The colours in the matrix correspond with the appropriateness of the method for the given situation:
• Green colour means that the method is very suitable
• Orange colour means average suitability
• Red colour means that the method is unattractive or not applicable for the given situation. 

You can read some documents related to defluoridation methods here.

Fluoride in Drinking-water - WHO guidlines 

Review of Fluoride Drinking Water Standard by EPA

Fluoride in groundwater: Overview and evaluation of removal methods

Technologies for fluoride removal

Fluoride Treatment Methods - Some other methods for the defluoridation.


Leonardo Zanata
Thanks to TWN for this complet answer! Best Regards!
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