FIELD OF INVENTION: This invention relates to a novel natural polymer (e.g. cellulose, chitosan) mediated self-sustained combustion synthesis of extremely small particles of magnesium oxide and its application in drinking water purification (e.g. fluoride removal). The proposed method of synthesis is superior to existing combustion based synthesis in many perspectives: small size of the nanoparticle, cost-effective recovery of the material, significant improvement in the fluoride adsorption capacity. This invention also relates to the development of a simple device for removing fluoride, heavy metals and Total Dissolved Solids (TDS) from contaminated drinking water. More specifically, the present invention relates to the removal of fluoride from drinking water using 3-7 nm particles of magnesium oxide. This invention also includes a method for removing total hardness, alkalinity and dissolved solids as well as maintaining pH as per the drinking water norms. The device includes a batch setup provided with an inlet followed by a dual media fixed bed unit, containing a sand bed followed by a manganese oxide-supported-activated alumina bed and an outlet at the end. The contaminated water is initially allowed to mix in the batch reactor for a definite period where fluoride is removed by nano-magnesia. The defluoridated water then passes through a dual media filter unit where pH, heavy metals, suspended solids and TDS are controlled and the decontaminated water flows out through the outlet provided. PRIOR ART: Water-related diseases constitute a major proportion of the public health problems in developing countries like India. Because of inadequate resources, availability of quality drinking water remains a dream for many people, especially in rural areas. Among diverse fresh water sources, groundwater is a major and often the preferred source of drinking water because it is expected to be free from microbial contamination and often requires little treatment. In rural part of India, a majority of water supplies (more than 80%) is obtained from groundwater (Census of India, quoted in The Times of India (Delhi edn.) Nov 7, 2003.). Sadly, now it is clear that the groundwater resources contain a number of dissolved toxic pollutants (organic, inorganic and biological) that cause adverse health effects. In our earlier work, we removed various contaminants from drinking water: organic contaminants such as pesticides (T. Pradeep, A. Sreekumar Nair, Indian Patent 200767 and PCT Application PCT/IN05/0002), inorganic contaminants such as mercury (T. Pradeep, Lisha K. P., Anshup, Indian Patent Application 169/CHE/2009) and biological contaminants such as E-coli (T. Pradeep, Prashant Jain, Indian Patent 20070608). Fluoride is a major inorganic contaminant present in many groundwater sources worldwide and needs immediate attention. The long-term consumption of fluoride contaminated water can cause skeletal and dental fluorosis, and can increase the risk of bone fracture, bone cancer (osteosarcoma cancer), and DNA damage (Ayoob, S., Gupta, A. K. Fluoride in drinking water: a review on the status and stress effects. Crit. Rev. Environ. Sci. Technol., 2006, 36, 433). Fluoride also has adverse effects on thyroid, immune system, brain and kidney as well. According to the recent information, fluorosis is endemic in atleast 25 countries around the world (http://wvwv.unicef.ora/wes/fluoride.pdf). including countries in Africa, Asia, and America. In India, fluorosis was first identified in Nellore district of Andhra Pradesh in 1937 (Shortt, W.E. Endemic fluorosis in Nellore District, South India. Ind. Med. Gazette, 1937, 72) and it is now prevalent in more than 17 states across the country (Susheela, A.K. Fluorosis management programme in India. Curr. Sci., 1999, 77, 1250) with its concentration varying from 2 to 30 mg/l. Statistics says more than 25 million persons in India are affected by fluorosis and approximately 66 million persons are at the risk of fluorosis. Unfortunately, there is no specific treatment for endemic fluorosis apart from drinking water free from fluoride. Apart from fluoride, the presence of heavy metals such as arsenic, mercury, lead, cadmium, etc. in drinking water continues to be a major concern because of their toxicity, accumulation in the food chain and persistent nature. Recent statistics show that various part of India and many other countries around the world are facing threat from one or more of these heavy metals (Acharyya SK, Chakraborty P, Lahiri S, Raymahashay BC, Guha S, Bhowmik A. Arsenic poisoning in the Ganges delta. Nature, 1999, 7, 401(6753), 545; Nickson R, McArthur J, Burgess W, Ahmed KM, Ravenscroft P, Rahman M. Arsenic poisoning of Bangladesh groundwater. Nature, 1998, 24, 395(6700), 338; http://www.indiaresource.org/documents/Plachimada Report Water Pollution.pdf). Chronic consumption of low concentration of these heavy metals can also cause various health problems, including cancerous and non cancerous diseases. Few prior arts unfolding the health issues arising from the chronic consumption of heavy metals are described in following reports: (a)Roberts, J.R. Metal toxicity in children. In: Training manual on pediatric environmental health. CA: Children's Environmental Health Netv\/ork. Emeryville, 1999 (http://www.cehn.org/cehn/trainingmanual/pdf/manual-full.pdf). (b) Agency for Toxic Substances and Disease Registry (ATSDR), ToxFAQs Lead http://www.atsdr.cdc.gov/tfacts13.html. (c)P.E. Marino, A. Franzblau, R. Lilis, Acute lead poisoning in construction workers: the failure of current protective standards. Arch. Environ. Health, 1989, 44, 140-145. (d)Agency for Toxic Substances and Disease Registry (ATSDR), ToxFAQs Arsenic http://www.atsdr.cdc.gov/tfacts2.pdf. (e) http://www.lef.org/protocols/prtcl-156.shtml Owing to the widespread occurrence and detrimental effects of these pollutants, various treatment methods have been developed over a period. A number of such methods are based on precipitation, ion-exchange, sorption and membrane processes. Among these, membrane technologies (nanofiltration, reverse osmosis) and ion exchange processes are not popular in developing countries like India because they are highly expensive. Sorption onto a solid surface is a preferred choice, especially in developing countries like India, because of its simplicity, ease of operation and lower cost. However, selection of the right material and its composition is most important for the effective removal of these contaminants from drinking water. As of now, varieties of sorbents were tested for the removal of heavy metals from aqueous medium. However, the potential and widely used sorbents are activated carbon or metal oxides, such as oxides of iron, aluminium, manganese or a combination thereof or Bentonite. Compared to granular activated carbon, oxides have higher affinities and adsorption capacities and are thought to be the most important scavengers of heavy metals in aqueous solution (H.J Fan, P.R. Anderson. Copper and cadmium removal by Mn oxide-coated granular activated carbon. Separation and Purification Technology, 2005, 45, 61). This is due to their relatively high surface area, microporous structure, and presence of OH functional groups capable of reacting with metals, phosphate and other specifically sorbing ions (M.S. Al-Sewailem, E.M. Khaled, A.S. Mashhady. Retention of copper by desert sands coated with ferric hydroxides. Geoderma, 1999, 89 249). Studies have also proved that metal oxides coated on suitable surfaces are more effective media because of their ease of use and ease in solid liquid separation (R. Han, W. Zou, Z. Zhang, J. Shi, J. Yang. Removal of copper(ll) and lead(ll) form aqueous solution by manganese oxide coated sand. I. Characterization and l10) of the treated water generated due to high pHzpc (12.7) of magnesium oxide. As far drinlPZC, oxide surface is negatively charged due to release of H+ ions in the water whereas at pH10) of the treated water. Hence, the treated water pH must maintained between 6.5 - 8.5. In this study, a user friendly method was proposed by making use of the OH" ion scavenging capacity of metal oxide sorbents. A double metal oxide media, namely, manganese oxide coated alumina was used as the sorbent in this study. Manganese oxide coating on activated alumina surface was done by pore volume impregnation of alumina with potassium permanganate and its subsequent reduction with ethanol. The pH control of fluoride treated water was carried out in a glass column packed with manganese oxide coated alumina at a flow rate of 3.7 ± 0.2 m^/m^.h. The characteristics of influent and effluent water are listed in Table 2. The effluent solution was collected at various intervals. The reuse potentials of this sorbents were also tested by regenerating the already exhausted sorbents with H2SO4 (0.25 N). The results of this study are showed in Fig. 12. The results revealed that manganese oxide coated alumina could reduce the pH of the fluoride treated water to an acceptable limit. The regeneration capacity of the sorbents was also found to be excellent and no significant reduction in performance was observed up to 2 cycles of adsorption process, which makes the sorbent more economical and viable option for field conditions. WE CLAIM: 1. A method for the production of metal oxide nanoparticles comprises of thermal combustion of a metal precursor salt in a metal holding template with a fuel, characterized in the size of produced metal oxide nanoparticles being less than 10 nm. 2. The method for production of metal oxide nanoparticles of claim 1, characterized in the fuel essentially comprising urea and glycine. 3. The fuel of claim 2, wherein it comprises a mixture of urea and glycine with fuel to oxidizer ratio more than 0.25 and less than 1. 4. The method for production of metal oxide nanoparticles of claim 1, characterized in the fuel essentially comprising of rice husk. 5. The method for production of metal oxide nanoparticles of claim 1-3, characterized in the fuel essentially comprising urea and glycine and wherein the molar ratio of glycine to metal precursor salt is more than 0.25 and less than 1. 6. The method for production of metal oxide nanoparticles as claimed in claim 1, characterized in the metal holding template being a natural cellulose fiber, chitosan or a combination thereof. 7. The method for production of metal oxide nanoparticles as claimed in claim 1, characterized in the metal holding template being activated carbon. 8. The method for production of metal oxide nanoparticles as claimed in claim 1, characterized in the metal holding template comprising of natural fibers such as silk, linen, jute, cotton, lignin, ramie, hemp or a combination thereof. 9. The method for production of metal oxide nanopartides as claimed in claim 1, characterized in the metal holding template comprising of artificial fibers such as rayon, nylon, polyester, latex, or a combination thereof. 10. The method for production of metal oxide nanopartides as daimed in daim 1, characterized in the metal holding template comprising of wood, bamboo, saw wood powder or a combination thereof. 11. The method for production of metal oxide nanopartides as claimed in claim 1, comprising of derivatives of cellulose, including but not limited to cellulose acetate, carboxymethyl cellulose or surface functionalized cellulose. 12. The method for production of metal oxide nanopartides as claimed in claim 1, characterized in the metal precursor salt selected from a group comprising of salts of magnesium, calcium, aluminum, zinc, manganese, iron, titanium, zirconium, lanthanum, silicon or a combination thereof. 13. The method for production of metal oxide nanopartides as claimed in claim 1, characterized in the metal precursor salt being a nitrate salt, chloride salt, acetate salt or a combination thereof. 14. A composition for water treatment comprising a plurality of treating media disposed in the path of water flow having at least: a. a fluoride adsorbent medium of a metal oxide nanopartides being less than 10 nm, and b. a pH controlling medium of a metal oxide 15. A composition for water treatment comprising a plurality of treating media in the path of water flow having at least: a. a fluoride adsorbent medium of a metal oxide nanopartides, and b. a pH controlling medium of a metal oxide wherein the pH controlling medium is disposed in the path of water flow after the fluoride adsorbent medium 16. A composition for water treatment comprising a plurality of treating media in the path of water flow having at least: a. a fluoride adsorbent medium of a metal oxide nanoparticles, and b. a pH controlling medium of a metal oxide wherein the pH controlling medium has metal oxide supported activated alumina. 17. The composition claimed in claim 14, wherein the fluoride adsorbent medium has magnesium oxide. 18. The composition claimed in claim 14, wherein the fluoride adsorbent medium includes magnesium oxide with other metals selected from a group of metals of aluminum, zinc, lanthanum, iron, silicon, calcium, titanium, zirconium, manganese or a combination thereof. 19. The composition claimed in claim 14, wherein the pH controlling medium has manganese oxide. 20. The composition claimed in claim 14, wherein the pH controlling medium includes manganese oxide with other metals selected from a group of metals of aluminum, zinc, lanthanum, iron, silicon, calcium, titanium, zirconium or a combination thereof. 21. The composition claimed in claim 14, wherein the fluoride adsorbent medium is loaded on a metal holding template which is a natural cellulose fiber, chitosan or a combination thereof. 22. The composition claimed in claim 14, wherein the pH controlling medium is loaded on a metal holding template which is a natural cellulose fiber, chitosan or a combination thereof. 23. The composition claimed in claim 14 includes further a third medium of a sand bed in the flow path. 24. The composition claimed in claim 14 includes further a third medium of a sand bed in the flow path, disposed after the pH controlling medium in the flow path. 25. A water filtering device for water treatment through removal of plurality of contaminants to obtain potable water comprising of plurality of treating media disposed in the path of water flow having at least a. a fluoride adsorbent medium of a metal oxide nanopartides, and b. a pH controlling medium of a metal oxide. 26. A water filtering device for water treatment to obtain potable water comprising of plurality of treating media disposed in the path of water flow having at least a. a fluoride adsorbent medium of a metal oxide nanopartides, and b. a pH controlling medium of a metal oxide wherein the pH controlling medium is disposed in the path of water flow after the fluoride adsorbent medium. 27. A water filtering device for water treatment to obtain potable water comprising of plurality of treating media disposed in the path of water flow having at least a. a fluoride adsorbent medium of a metal oxide nanopartides, and b. a pH controlling medium of a metal oxide wherein the pH controlling medium has metal oxide supported activated alumina. 28. The water filtering device as claimed in claim 25 for water treatment to obtain potable water wherein the adsorbent medium has magnesium oxide. 29. The water filtering device as claimed in claim 25 for water treatment to obtain potable water wherein the fluoride adsorbent medium includes magnesium oxide with other metals selected from a group of metals of aluminum, zinc, cerium, lanthanum, iron, silicon, calcium, titanium, zirconium, manganese or a combination thereof. 30. The water filtering device as claimed in claim 25 for water treatment to obtain potable water wherein the pH controlling medium has manganese oxide, said manganese oxide also capable of adsorbing heavy metals present in the water. 31. The water filtering device as claimed inclaim 25 for water treatment to obtain potable water wherein the pH controlling medium includes manganese oxide with other metals selected from a group of metals of aluminum, zinc, lanthanum, iron, silicon, calcium, titanium, zirconium or a combination thereof. 32. The water filtering device as claimed in claim 25 wherein the fluoride adsorbent medium is loaded on a metal holding template which is a natural cellulose fiber, chitosan or a combination thereof. 33. The water filtering device as claimed in claim 25 wherein the pH controlling medium is loaded on a metal holding template which is a natural cellulose fiber, chitosan or a combination thereof. 34. The water filtering device as claimed in claim 25 includes further a third medium of a sand bed in the flow path. 35. The water filtering device as claimed in claim 25 includes further a third medium of a sand bed in the flow path, disposed before the pH controlling medium in the flow path. 36. The water filtering device as claimed in claim 25 wherein the filtering device is a batch reaction setup. 37. The water filtering device as claimed in claim 25 wherein the filtering device has a filtering medium which is a candle, filter bed, column, packets and/or bags.