Dear Chardrasekaran ...

Dear Chardrasekaran

Water Filters: One way of avoiding the fluoride from tap water is to purchase a water filter. Not all water filters, however, remove fluoride. The three types of filters that can remove fluoride are reverse osmosis, deionizers (which use ion-exchange resins), and activated alumina.

The technique of Fluoride removing is a combination of several unit operations and the process involves rapid mixing, chemical interaction, flocculation, sedimentation, filtration, disinfection and sludge concentration to recover waters and aluminium salts. Alum (hydrated aluminium salts) - a coagulant commonly used for water treatment is used to flocculate fluoride ions in the water. Since the process is best carried out under alkaline conditions, lime is added. For the disinfection purpose bleaching powder is added. After thorough stirring, the chemical elements coagulate into flocs and settle down in the bottom. The reaction occurs through the following equations

2 Al2 (SO4)3. 18H2 O + NaF + 9Na2CO3 → [5Al(OH)3.Al(OH)2F] + 9Na2SO4+NaHCO3 + 8 CO2 + 45 H2O 3 Al2 (SO4)3 . 18H2 O + NaF +17NaHCO3 → [5Al (OH) 3. A l (OH)2F] + 9Na2SO4+ 17 CO2 + 18 H2O

Precipitation methods

Method involving the addition in sequence, of an alkali, chlorine and aluminium sulphate or aluminium chloride or both was developed. It is cheap and is used extensively in many countries.

Though lime softening accomplishes fluoride removal, its high initial cost, large dosage and alkaline pH of the treated water renders it unsuitable for field application. Large dosage and alkaline pH of the treated water renders it unsuitable for field application.

Activated alumina Defluoridation Filter

Activated alumina is a granular, highly porous material consisting essentially of aluminum trihydrate. It is widely used activated alumina for fluoride uptake. An initial concentration of 5 mg/L was effectively brought down to 1.4 mg/L before regeneration and to 0.5 mg/L on regeneration with 2N HCl. The bed was regenerated with a solution of 2% Na OH,5% NaCl,2N HCl,5% NaCl and 2N HCl. The removal capacity of the medium was found to be about 800 mg/L of fluorid e/L of Alumina.

Activated Alumina can be regenerated with HCl, H2SO4, Alum or NaOH. The use of NaOH needs to be followed by a neutralization to remove residual NaOH from the bed. Fluoride removal by activated alumina is strongly pH dependent.

The ability of activated alumina to remove fluoride depends on other aspects of the chemistry of water as well. Such factors as hardness, silica and boron, etc., if present in water will interfere with fluoride removal and reduce the efficiency of the system.

The use of activated alumina in a continuous flow fluidized system is an economical and efficient method. The process could reduce the fluoride levels down to 0.1 mg/L.

Bone Char

The uptake of fluoride onto the surface of bone was one of the early methods suggested for defluoridation of water supplies. The process was reportedly one of the ion exchange in which carbonate radical of the apatite comprising bone, Ca(PO4)6.CaCO3, was replaced by fluoride to form an insoluble fluorapatite. Bone char produced by carbonizing bone at temperature of 1100-1600ºC had superior qualities than those of unprocessed bone and hence replaced bone as defluoridating agent averted in this method.

Contact Precipitation

It is a technique by which fluoride is removed from the water through the addition of calcium and phosphate compounds and then bringing the water in contact with an already saturated bone charcoal medium. 

Activated Carbon

High Fluoride removal capacities of various types of activated carbons had been reported.Alkali digested alum impregnated paddy husk carbon was an efficient defluoridating agent.

Lime

The fluorides in waters containing Magnesium, when treated with lime, are adsorbed on Magnesium hydroxide flocs enabling fluoride removal.In this case the water must be treated to a caustic alkalinity of 30 mg fluoride/L, a pH of 10.5 or above and as such recarbonation is necessary.Magnesia and calcined magnesite have also been used for fluoride removal from water and fluoride removal capacity was reported to be better at high temperature

Ion Exchange Resins

Strong base exchange resins remove fluorides either on hydroxyl cycle or chloride cycle along with anions. Since the proportional quantity of fluoride as compared to other anions is very small, the effective capacity of such resins works out quite low. Some inorganic ion exchangers.

Regards,

Prem Baboo