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3.1.2 Nalgonda Technique

The Nalogonda technique employs flocculation principle 1. Nalgonda technique is a combination of several unit operations and the process invloves 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 [6].

3.1.3 Precipitation

   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 [6].

3.1.4 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 [6].

3.1.6. 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 [6].

3.1.7 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. [6].

3.1.8 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 [6].

3.1.9 Membrane filtration

Of advanced water treatment technologies, Membrane filtration processes are the most advanced of the reported technologies that have been applied to in treatment of pure and ultra pure water. Reverse osmosis(RO), one of the best defluoridation technologies, is recommended by US EPA in 2003[3]. RO and nano-filtration (NF) are the well known membrane technologies that can remove a large spectrum of pollutants from water such as turbidity, salinity, pathogens, heavy metals, natural and synthetic organics, and hardness[4]. These processes are reported to be effective in in water defluoridation and produce high quality water during water treatment. NF membranes runs at lower pressure and have lower capacity as compared to RO membranes. electrodialysis is another membrane technology that is applied to big scale plant water treatment of high fluoride brackish water for drinking water supply[5]. Electrodialysis is same as RO, except that it uses an applied direct current potential instead of pressure, to separate ionic contaminants from water. Water does not physically go through the membrane in the electrodialysis process as such a particulate matter is not eliminated. Thus, the ED membranes are not technically regarded as filters. Comparing to RO, the water quality from electrodialysis treatment may require post-treatment stabilization. The process tends to be most economical for source water with TDS levels in excess of 4,000 mg/L. It is established that RO and electrodialysis have very high defluoridation capacities (85 – 95 %) and function effectively in any pH range. However the water loss is high (20 - 30 % for electrodialysis, 40 – 60 % for RO), have high capital cost and are energy intensive [5]. For membrane technologies, It often require special instrument, electrical energy and specialized training for operators as such the capital and operation costs are high. Low applicability is therefore envisaged for rural sectors of the developing countries where energy and trained human resource are often deficient [6].

References

1. Groundwater Quality: Tanzania, WaterAid, 2001

2. Perspectives in Water Pollution; Chapter 4 Ground Water Contamination with Fluoride and Potential Fluoride Removal Technologies for East and

Southern Africa, InTech, 2013

3. US EPA. Water Treatment Technology Feasibility Support Document for Chemical contaminants. EPA-815-R-03-004, EPA 2003.

4. Dysart A. Investigation of Defluoridation Options for Rural and Remote Communities. Research Report No 41, The Cooperative Research Centre for Water Quality and Treatment, Salisbury SA 5108, AUSTRALIA 2008.

5. Zakia A, Bernard B, Nabil M, Mohamed T, Stephan N, Azzedine E. Fluoride removal from brackish water by electrodialysis. Desalination 2001; 133, 215 - 233.

6. http://www.bibliotecapleyades.net/salud/salud_fluor23.htm

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