It's possible to remove fluoride

from drinking water,

but not every type of

water filter will work.

Method

 Process

Resources / Salient Features

Nalgonda Technique

The Nalogonda technique (named after the village in India where the method was pioneered) employs flocculation principle 1. Nalgonda technique is a combination of several unit operations and the process invloves rapid mixing, chemical interaction, floculation, 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.Al(OH)2F] + 9Na2SO4+ 17 CO2 + 18 H2O
 

 Salient features of Nalgonda technique

• No regeneration of media

• No handling of caustic acids and alkalis

• Readily available chemicals used in conventional municipal water treatment are only required

• Adaptable to domestic use

• Flexible up to several thousands m3 / d

• Applicable in batch as well as in continuous operation to suit needs simplicity of design, construction, operation and maintenance

• Local skills could be readily employed

• Higly efficient removal of fluorides from 1.5 to 20 mg/L to desirable levels

• Simultaneous removal of color, odor, turbidity, bacteria and organic contaminants

• Normally associated alkalinity ensures fluoride removal efficiency

• Sludge generated is convertible to alum for use elsewhere

• Little wastage of water and least disposal problem

• Needs minimum of mechanical and electrical equipment

• No energy except muscle power for domestic equipment

• Economical - annual cost of defluoridation (1991 basis) of water at 40 lpcd works out to Rs.20/- for domestic treatment and Rs.85/- for community treatment using fill and draw system based on 5000 population for water with 5 mg/L and 400 mg/L alkalinity which requires 600 mg/L alum dose.

• Provides defluoridated water of uniform acceptable quality

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 India.

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.

Alkali, chlorine;

Aluminium sulphate or aluminium chloride

Activated alumina

Activated alumina is a granular, highly porous material consisting essentially of aluminum trihydrate. It is widely used as a commercial desiccant and in many gas drying processes.

The studies, perhaps the earliest, have demonstrated the high potential of 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. Many modifications of process was suggested by subsequent workers, several patents based on the use of Aluminum oxide for fluoride removal were issued 1. Filter alum was used to regenerate activated alumina bed. The capacity of alumina to remove fluoride was reported to be proportional to the amount of filter alum used for regeneration up to a level of about 0.2kg of alum per litre of alumina. At this level the fluoride removal capacity was approximately 500 mg of fluoride per litre of alumina. Similar studies employing activated alumina was later conducted by many workers and all these works confirmed the ability of activated alumina for higher uptake of fluoride from water. Some researchers have concluded that removal was the result of ion exchange, but investigations by others have shown that the process is one of the adsorption and follows the Langmuir isotherm model.

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. Batch adsorption data14 showed very little removal at pH 11.0 and optimum removal at pH 5.0.Hence raw water pH & regenerated bed pH need to be ad justed accordingly.

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 for defluoridating water supplies15. The process could reduce the fluoride levels down to 0.1 mg/L. The operational, control and maintenance problems, mainly clogging of bed, may be averted in this method.

• Activated alumina

• Na OH,

• NaCl

• 2N HCl

• H2S04

• Filter alum

Advantages:

• It requires minimum contact time for maximum defluoridation.

• Percentage of regeneration is considerably high.

• There is very little attritional loss ( to a negligible extent) during the regeneration at the initial stage of operation

• It is indigenously available and cheap.

• Defluoridation capacity at neutral pH is appreciable, although it has greater defluoridation efficiency at low pH.

• Its defluoridation capacity is independent of temperature.

• The effect of other ions present in drinking water, like chlorides, sulphates and carbonates, over the defluoridation efficiency of activated alumina is minimum, eventhough the presence of bicarbonate ions show considerable influence in the process of defluoridation.

For cost and more details - see :

• http://www.iwtc.info/2008_pdf/2-4.PDF

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

The fluoride removal capacity of the product is 1000 mg/L
 

 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.
 

Degreased and alkali treated bones

Degreased and alkali treated bones are effective in the removal of fluoride from initial fluoride concentration ranging from 3.5 mg fluoride/L to 10 mg fluoride/L to less than 0.2 mg fluoride/L

Bone contain calcium phosphate and has a great affinity for fluoride. The bone is degreased, dried and powdered. The powder can be used as a contact bed for removal of fluoride in water. The exhausted bed is regenerated with sodium hydroxide solution
 

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Synthetic tri-calcium phosphate

The product is prepared by reacting phosphoric acid with lime(Bulusu). The medium is regenerated with 1% NaOH solution followed by a mild acid rinse

 It has a capacity to remove 700 mg fluoride/L

Florex
 

A mixture of tri-calcium phosphate and Hydroxy -apatite, commercially called Florex, showed a fluoride removal capacity of 600 mg of fluoride per liter and is regenerated with 1.5% sodium hydroxide solution. Owing to high attritional losses, Florex was not successful and the pilot plants using this material were abandoned

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Activated Carbon

Most of the carbons prepared from different carbonaceous sources showed fluoride removal capacity after alum impregnation. 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.

Investigations have shown that carbonized saw dust when quenched in 2% alum solution forms an excellent defluoridating carbon. The defluoridating process is stoichiometric and equilibrium is established between carbon & fluoride. On exhaustion (after continued use) the carbon can be regenerated by passing 0.2 to 0.5% alum solutions.

Activated carbon prepared by other workers from cotton waste, coffee waste, coconut waste etc., was tried for defluoridation but all these materials proved to be of academic interest only

Alkali digested alum impregnated paddy husk carbon

Alkali digested (1% KOH) & alum soaked (2% alum) carbon removed 320 mg fluoride per kg & showed maximum removal efficiency at pH 7.0.

Lime

The fluorides in waters containing Magnesium, when treated with lime, are adsorbed on Magnesium hydroxide flocs enabling fluoride removal12, 25,26.

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 necessary27.

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

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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, eg. complex metal chloride silicates, formed from barium or ferric chloride with silicic acid, also exchanged fluoride for chloride.

• Cation exchange resins impregnable with alum solution have been found to act as defluoridating agents. Alum treated cation exchange resins were used for defluoridation. ‘Avaram Bark’ based cation exchange resins, had been reported to work effectively in removing fluoride from water

• Polystyrene anion exchange resins in general and strongly basic quaternary ammonium type resins in particular are known to remove fluorides from water along with other anions. The fluoride removals by various anion exchange resins are given6 in the table

• Table 3 indicates that the resins studied yields 20 – 145 bed volume of defluoridated water per cycle. Subsequent experience showed that these resins lose their fluoride removal capacity on prolonged use (10 – 15 cycles) and a total replacement becomes necessary. A layer of white deposits was developed over the resin beds, and this may be the reason for this drop in the capacity.

Thus the anion exchange resins were found to be of relatively low capacity for fluoride removal. The cost of anion resins is Rs. 20 to 35 per litre. The results indicate that anion exchange resins are not economical for removing fluorides from water. Besides, the strong base anion exchange resins impart a taste to the treated water that may not be acceptable to the consumers.

Cation Exchange Resins

• Performance of Saw dust carbon (Defluoron–1), Carbion, Wasoresin – 14 and a polystyrene cation exchange resin for fluoride removal were compared35 and the results of the study are summarized in the table.4

• During the above studies the bed was regenerated with 200 ml of 1% alum solution and washed with tap water when the residual fluoride concentration reached 1.5 of fluoride

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Magnesia

Investigations were conducted to study the usefulness of magnesia in fluoride removal. Crystalline magnesium hydroxide was obtained by reacting a magnesium salt with milk of lime. The precipitate was filtered, washed and dried. The dried product was calcined at 1000°C for 3 hours to obtain magnesia. Varying quantities of magnesia were added to one litre aliquots of test water and stirred for 30 min using a jar test machine. Fluoride contents were estimated on one hour settled sample.

A typical groundwater containing 10 mg/L fluorides, 60 mg/I hardness, 500 mg/L alkalinity and 7.6 pH was studied using magnesia (MgO) concentrations of 10 - 1,500 mg/L. The treated water showed a pH above 9. The average fluoride concentration in the filtrate was 5.8 mg F/L where the dose was 1,000 mg/L. The fluoride at 100, 250 and 500 mg/L doses were 9.5, 8.9 and 8.4 mg F/L, respectively. A dose of 1,500 mg/L magnesia and a contact period of 3 hr was required to reduce the fluoride content in the water to 1 mg/L.

The high initial cost, large concentrations required, alkaline pH of the treated water and complexity of the preparation of magnesia are the inhibitive factors to render it acceptable in the field

The study established that magnesia removed the excess fluorides, but large doses were necessary. Moreover the pH of the treated water was beyond 10 and its correction by acidification or recarbonation was necessary.

All this adds to the cost and complexity of operations. The acid requirement can be to the extent of 300 mg/L expressed in terms of CaCO3/L

Serpentine

• Serpentine is a mineral name, which applies to the material containing one or both of the minerals, chrysotile and antigorite1. The composition of the mineral closely corresponds to the formula Mg6Si4O10 (OH). The material is green or yellow and is available in Andhra Pradesh. To test the capacity of serpentine to remove fluorides from waters, the green and yellow varieties were studied for their defluoridation capacity. Extensive laboratory investigations were conducted with a view to popularize the mineral, if found suitable as a defluoridating medium. A comparative evaluation was made using green and Yellow varieties of serpentine and the results are given in the table 5. It is concluded that cost of defluoridation is prohibitive with serpentine

Materials like clays, minerals, ion exchange resins, activated carbons, activated alumina, sulphonated coals and serpentine were tried for the removal of excess fluorides from water.

In-situ chemical treatment with lime, magnesium salts, iron and aluminum salts were also studied. Those that showed an encouraging trend on a bench scale were studied in detail.

These include ion exchange resins, saw dust carbon, coconut shell carbon defluoron-1 carbon, magnesia, serpentine and defluoron-2. Ion exchange resins, saw dust carbon, defluoron-1, magnesia and serpentine did not prove useful beyond bench –scale.

Lime stone, special soils and clay etc

• Recently limestone and heat-treated soil were tried for fluoride removal. Limestone was used in a two-column continuous flow system (limestone reactor) to reduce fluoride concentrations from wastewaters to below the MCL (Maximum contaminant level) of 4 mg/L. Calcite was forced to dissolve and fluorite to precipitate in the first column. The degassing condition in the second column caused the precipitation of the calcite dissolved in the first column, thus returning the treated water to its approximate initial composition.

• In laboratory experiments, the fluoride concentration of the effluent from all tested feed waters containing initial fluoride amounts from 10 to 100 mg/L. And a steady state of the system performance was quickly achieved, For instance, in an experiment when the input fluoride concentration was 100 mg/L, effluent concentrations from both columns were below 4 mg/L after only 8 pore volumes had passed. The proposed reactor has potential application to reduce concentrations from wastewaters of anionic elements similar in charge and size to carbonate ion, such as Selenate and arsenate and cations similar in size and charge to Ca2+ ,such as Cd2+.

• Pleistocene soil available locally in Xinzhou, China was able to remove fluoride from local ground water. X-ray diffraction analysis revealed that the soil is composed principally of quartz (50- 60%), Illite (30-40%), goethite (5-10%) and feldspar (5-10%). A substantial improvement in both permeability and the fluoride removal capacity of the soil was achieved by heating it in a Muffle furnace. A granular material can then be obtained by crushing the heated product

• The experimental results showed that heating at 400-500ºC has the optimal effect on the enhancement of the material’s fluoride removal capacity. A preliminary column experiment showed that 4.0 kg of 400ºC heat-treated soil can treat more than 300L of 5 mg/L fluoride feed water before the effluent fluoride concentration reaches 1.0 mg/L. Once the soil’s fluoride-sorption capacity had been reached, the material could be regenerated in a cost effective way: rinse the soil first with sodium carbonate solution, then with dilute HCl and finally with distilled water twice. After being air-dried the material is ready for reuse

• Attempts were made to use local Kenyan soil derived from volcanic ash (ex: Ando soils or soils with andic properties) as a fluoride sorbent37. The ability of Kenyan Ando soil to adsorb fluoride was determined experimentally. These results were extended to possible technical application using a one dimensional solute transport model. Based on the result it is concluded that the use of Ando soils appears to be an economical and efficient method for defluoridation of drinking water on a small scale in rural areas of Kenya and other regions along the Rift zone. Further research is warranted to evaluate its practical applications and social acceptance.

• Fluoride sorption studies were carried out on two clay minerals, montmorillonite KSF and kaolin, and a silty clay sediment series (SCSS, used in earthenware making) 38.The function of fluoride concentration, clay concentration and pH in clay-water suspensions was studied. Kaolinite, a dioctahedral two layered (Silica + alumina) Silicate(1:2 type),exhibited very little tendency for Fluoride sorption while montmorillonite,2:1 type material characterized by Octahedral sheet of alumina sandwiched between two tetrahedral sheets of silica, showed significant Fluoride sorption.
  The Fluoride sorption on montmorillonite KSF was found to be greatest at pH 1.9 ± 0.3,the natural pH of montmorillonite-water suspension. At pH 4.0 ± 0.36, the percentage fluoride sorption on montmorillonite decreased, followed by an increase around pH 5-6, after which the percentage decreased with increasing pH. The applicability of the Freundlich isotherm was also verified in case of montmorillonite KSF at low fluoride concentrations. As a result of fluoride adsorption, increased release of Fe2+, Cl-, NO3 - ions from montmorillonite matrix was observed. There was no effect on SO4 2- or PO4 2- solubility. Fluoride adsorption on SCSS was also significant and decreased regularly
  with increasing pH.

• On the basis of experimental data a plausible mechanism of fluoride sorption by clay minerals is suggested. Based on the results of fluoride sorption mentioned above, a pilot study on defluoridation of water employing clay (SCSS) as an adsorbent was als o undertaken which yielded promising results.

• Removal of fluoride by adsorption on to low-cost materials like kaolinite, bentonite, charfines, lignite and nirmali seeds was investigated

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Fly Ash

Retention of fluoride ion in dynamic experiments on columns packed with fly ash was studied40 at 20ºC with a series of aqueous solutions containing 1,5,10,20,50 and 100 mg fluoride/L/ The flow rate through a 450-g bed was £ 2ml/hr.

At the lowest fluoride concentration(1 mg/L), the fluoride level in the effluent initially increased and then gradually decreased down to 0 mg/L after 120 hours.

With higher fluoride concentrations in the feed solutions, the fluoride concentration in the effluent steadily decreased reaching 0 mg/L after 120-168 hours.

The fly ash was an effective sorbent especially at high concentrations.
 

Electro coagulation
Electrochemical methods

• Electro coagulation process with aluminum bipolar electrodes was used for defluoridation process41. The influence of parameters such as inter-electrode distance, fluoride concentration, temperature and pH of the solution were investigated and optimized with synthetic water in batch mode. The optimization process continued with Oued Souf water (South Algeria) where the influence of current density and area/volume ratio on the defluoridation process was evaluated. The electro coagulation process with aluminum bipolar electrodes permitted the defluoridation of Sahara water without adding salts to the treated water. The aluminum–fluoride weight ratio attained was 17/1.

• A technology of defluoridation through Electrochemical route has been developed42. The basic principle of the process is the adsorption of fluoride with freshly precipitated aluminum hydroxide, which is generated by the anodic dissolution of aluminum or its alloys, in an electrochemical cell.
   

• Constraints in the above technology: Electricity is the main raw material and hence wherever electricity is not available a suitable polar panel can be installed.

The process utilizes 0.3 to 0.6kwh of electricity per 1000 liters of water containing 5- 10 mg/L of fluoride.

The anode is continuously consumed and needs to be replenished. The process generates sludge at the rate of 80- 100 gm per 1000 liters (on dry basis).

Rare earth based materials

New water treatment processes have been developed for removal of hazardous anions such as Fluoride, Arsenic, Selenium species, and phosphate from water using rare earth based materials which have not been efficiently utilized by industry in spite of their abundance43. The state-of-the-art of rare earths in terms of cost, use and health effects and the environmental problems associated with hazardous anions in terms of treatment and toxicity are generally described. Solid lanthanum and Yttrium ions have been used as adsorbents for removing hazardous anions. Either lanthanum or Yttrium ions have been loaded on porous silica or alumina beads to improve economic and engineering performance; such rare earth impregnated materials have been successfully applied to the treatment of synthetic as well as industrial wastewaters.

A rare earth metal-based inorganic adsorbent, Cerium- Iron adsorbent (CFA), was developed and its performance for fluoride removal from water was evaluated44. The characteristics of the adsorbent were summarized. Experimental results show that rare earth metal adsorbents had a relatively high adsorption capacity and good kinetic property for fluoride ion removal. The highest capacity was obtained at pH 3, then it decreased with the increase of pH. The pH effect however, became inconspicuous when the pH was over 5.The results show that the adsorption of fluoride on CFA adsorption follows Freundlich isotherm in the tested range of fluoride concentrations. The adsorption capacity could almost be recovered by regenerating it with 1 molx1-1 NaOH solution

An adsorbent, which is a mixture of rare earth oxides was found to adsorb fluoride rapidly and effectively45. The effect of various parameters such as contact time, initial concentration, pH and adsorbent dose on adsorption efficiency was investigated. More than 90% of the adsorption occurred within the first 5-10 minutes. Adsorption was found to be dependent on the initial fluorid concentration and adsorption behavior followed Langmuir adsorption model. The optimum pH was found to be about 6.5. The presence of other ions such as nitrate and sulphate did not affect the adsorption of fluoride significantly (adsorption efficiency reduced from 85 to 79%) indicating the selective nature of the adsorbent. The adsorbed fluoride could be easily desorbed by washing the adsorbent with a pH 12 solutions. This study clearly shows the applicability of naturally occurring rare earth oxides as selective adsorbent for fluoride from solutions
 

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Tamarind Gel

The concentration of fluoride from solution of sodium fluoride of 10 mg/L could be brought down to 2 mg/L by the addition of tamarind gel alone and to 0.05 mg/L by the addition of small quantity of chloride with the tamarind gel.

tamarind gel

small quantity of chloride

Plant materials

The plant materials such as barks of Moringa olifera and Emblica officinalis , the roots of Vetiveria zizanoides and the leaves of Cyanodon tactylon were found to be good defluoridating agents

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