Title of Invention

A PROCESS FOR TREATMENT OF WASTE WATER CONTAINING NITRATE IONS

Abstract

Diposal of wastewater containing high nitrates is a serious and global problem. Most manufacturing industries generate nitrates in wastewater. Nitrate contamination in drinking water causes methemoglobinema, a disease in which oxygen-bearing capacity of blood is reduced, and which is called Blue-Baby-Syndrome, which mostly affects infants and nursing mothers. this invention relates to a process for treatment of wastewater containing nitrate ions. In this process the NO3 ions present in the liquid waste are converted to harmless N2 (or N2 and CO2)gas by first reducing it to nitrite ions by nascent hydrogen and then by reactin g it with sulphamic acid (or with urea) at ambient temperature and pressure. The treated solution has nitrate ions are4 below 10 mg/lit (for sulphamic acid and below 40mg/lit for urea) and zinc/magnesium below 2mg/lit, which is considered as free from nitrate and magnesium or zinc being below their permissible discharge limits. The metals magnesium or zinc added for generating nascent hydrogen is removed from the treated waste water to meets the norms for disposable wastes. The process is very useful to the chemical industries which have nitrate ions in their effluents.

Full Text

FORM 2 THE PATENTS ACT 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10; rule 13) 1. A PROCESS FOR TREATMENT OF WASTE WATER CONTAINING NITRATE IONS 2. Secretary, Department of Atomic Energy, Government of India, Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg, Mumbai 400 039, Maharashtra, India. GRANTED 27-1-2005 The following specification particularly describes the nature of the invention and the manner in which it is to be performed. - 764/MUM/2003 04/08/2003 Field of invention The present invention relates to a process for treatment of wastewater containing nitrate ions so as to reduce thenitcateionconcentration in the wastewater. The process of the present invention is particularly useful for chemical industries which discharge wastewater containing nitrate ions above prescribed disposal limits, for example, 44.3 mg/lit N03 " (=10 mg/lit Nitrate-nitrogen) for inland surface water discharge and 88.6 mg/lit (=20mg/lit Nitrate-nitrogen) for the marine coastal areas discharge (as per the Central Pollution Control Board: Pollution Control Acts & Rules, Report No. PCU2/1992, Schedule II inserted vide G.S.R. 919 (E) dt 12-9-88. Published in the Gazette No. 488 dt. 12-9-88). Background of the invention Most manufacturing industries generate and discharge nitrates in wastewater. The industries that usually discharge nitrate ions in wastewater include nuclear, metallurgical, fertilizer industries, organic synthesis plants involved in nitration of aromatic compounds e.g., benzene to nitrobenzene, phenol to nitro-phenol etc. Disposal of liquid waste containing high nitrates is a serious and global problem. Nitrate contamination in drinking water causes methemoglobinema, a disease in which oxygen-bearing capacity of blood is reduced, and which is called Blue-Baby-Syndrome, which mostly affects infants and nursing mothers. Attempts have been made in the past to remove nitrates by converting the same to nitrites. However, nitrite ions is also harmful. Nitrite ions in the stomach form N-nitrosoamine, which causes stomach cancer. (Mirvish S; Nature, 315, 461-462,1985). Also under certain conditions nitrite salts get converted to nitrate as follows: NaN02 -» NaN03 +Na20+2NO (A1) 5NaN02 ->3NaN03 +Na20+N2 (A2) For these reasons it is necessary to convert the harmful nitrates and nitrites to nitrogen. Presently, denitration of wastewater containing nitrates is carried out either by chemical, biological or thermal methods. All these, but the biological processes produce NOx gases, while the biological process produces N2 and NOx gases. Processes generating NOx (except biological) have separate treatment for NO*. The different known denitration methods are described below: Chemical methods: (i) Andrew P. Murphy {Nature, 350, 223-225, 1991) has developed a process to reduce nitrate to ammonia, nitrogen and nitrite with aluminum powder. In a pH range of 9 to 10.5 nitrates is reduced to ammonia with aluminum powder. Probable reactions are as follows: 3N03-+ 2AI+3H20 -> 3N02" + 2AI(OH)3 (A3) N02"+2AI+4H2 -> NH3 +2AI(OH) 3 +20H" (A4) N02"+2AI+4H2 -> N2+2AI(OH)3+20H" (A5) We have found that 60-95% of the product is ammonia and the rest is a combination of nitrogen gas. Ammonia can be removed either by air stripping or other technique e.g., precipitating as MgNhUPCU. The average residual nitrate concentration in the treated waste is about 50 mg/lit. This method requires less time, there is no heating involved and it is comparatively a low cost method. However, it requires consumable chemicals, produces NH3 along with NOxand the conversion efficiency is less than 95%. (ii) Horold, S; Vorlop, K.D Tacke; T; Sell; M. {Catalyst Today, 17, 21-30, 1993) have developed a process where hydrogen is used for the reduction of nitrate ions in the presence of a catalyst. The prior art teaches use of palladium as catalyst and it is found that 99.9% of nitrate is reduced to nitrogen gas and only 0.1% formed ammonia. The drawback is that the catalyst is sensitive to poisoning in presence of a heavy metal and the process is applicable only for a very low concentration of nitrate. The probable reactions are as follows: N03" + H2 -» N2 (A6) NO3-+ H2 -► NH3 (A7) Like the above method, this method requires less time, there is no heating involved and it is comparatively a low cost method. However, it produces NH3 along with NOx and the conversion efficiency is less than 95%, and the main problem is the catalyst is sensitive to other metal ions. (iii) US Patent No.6383400 teaches denitration process with formic acid in presence of Pt/SiC>2 catalyst; Os, Ra etc catalysts. Formic acid reduces nitric acid concentration as follows: 2HNO3 + HCOOH -> 2N02 +C02 +2 H20 (A8) {[HN03] >8M}} 2HN03 + 2HCOOH ^ NO + N02 +2CO2 +3 H20 (A9) {0.5 2HN03 + 3 HCOOH ^ 2 NO +3 C02 + 4 H20 (A10) 2HN03 + 4HC00H -* N20 + 4C02+5H2 (A11) {Moderate excess of formic acid} 2 HNO3 +5 HCOOH ^ N2 + 5C02 +6 H20 (A12) {Large excess of formic acid} However, this method is applicable only for nitric acid and not for metal-nitrates. All these existing chemical processes reduce the nitrate ions to No*, NH3, and or N2 gases. [N2 gas in reactions A5, A6 & A12, NOx (nitrogen oxides, e.g., N02, NO, N20 etc.) in reactions A3, A8 to A11 and NH3 in reactions A4 & A7] Biological methods: Biological denitration process reduces nitrates to gaseous nitrogen in presence of denitrifying bacteria. Broad ranges of bacteria are used; these include Pseudomonas, Microcous, Archromobactor, Thiobacillus and Bacillus etc. The gaseous product is mainly nitrogen gas containing nitrous oxide and nitric oxide (NOx). Biomass is generated as solid waste. Natural biological denitrification occurs slowly in aquifers in which a sufficient source of reducing organic carbon is present. Water treatment processes stimulate denitration by injection of organic nutrients, such as methanol, glucose and starch or mixture of these. For low nitrate (10gm/lit) liquid cannot be treated directly by this method. It needs to be diluted before denitration. Denitrifying bacteria are sensitive to heavy metal ions. Moreover, the process itself is very slow. It takes about 35 hours to reduce nitrate concentration from 10 gm/lit to 100 mg/lit. Since processing time is high, a huge storage space is required. Though it is a low cost method and has simple operation, the main disadvantage of this process is that it is not suitable for NO3" concentration >10gm/l. Thermal denitration process: In this process, thermal energy is used to decompose nitrates present in the aqueous waste. The aqueous waste is fed into a fluidized bed calciner. After evaporating the aqueous portion, the solid nitrate residue is converted into granular solid oxides by calcination/decomposition. The calcination process heats a substance to adequate temperature but below the melting or fusion point. Thermal methods of decomposition can use either the nitrate decomposition reaction or the reduction of nitrate by adding reducing agent US 5118447 teaches a thermal method of denitration for the denitration of drinking water, at a temperature of 200°C to about 600°C. US 444723 teaches a system for denitration of nitric acid solution by microwave-irradiation. See also Thermal Denitration, DOE/EM-0616: US Dept of Energy, and Heckner, H. N. J. Electroanal Chem.; 83, 51-63; (1977). The thermal methods require less time and are suitable for high N03" ion concentrations. However, these methods produce NOx gases requiring further treatment, and need high temperature and energy consumption. Objects of the invention The principle object of the present invention is to provide a novel chemical denitration method to reduce the nitrate ion concentration of wastewater; below the permissible limit as approved by CPCB by using a non-thermal and non-chemical process, and to obviate the drawbacks of the prior art methods. Summary of the Invention Accordingly the present invention provides an improved process for reducing the nitrate and other metals and sulphate ion concentration of wastewater containing such ions at levels above the discharge norms so as to meet the discharge norms for disposal comprising the steps of: i. determining the pH of said wastewater and, if required, adjusting the pH of said wastewater between 2 and 4; ii. adding at least one metal element selected from magnesium in amount of 60% by wt. of total nitrate, zinc in an amount of 150 to 160% wt of the total nitrate, to said acidified wastewater to generate nascent hydrogen; iii. adding a nitrogen releasing agent selected from sulphamic acid in amount of 150-200% by wt. of nitrate, urea in amount of 0.707 to 0.8 kg per kg. of total nitrate and ferrous sulphate; iv. adding an alkali to precipitate hydroxide of said metal of step ii such that the zinc ion concentration in the treated water is within a permissible limit, wherein said alkali is added slowly until the pH of the wastewater reaches between 7 and 7.5 when said metal element in step ii is magnesium and wherein the process comprises pretreatment of said wastewater comprising diluting the concentration of initial nitrate ion (N03") of said wastewater to reduce the concentration of nitrate ions when the concentration is above 100gm/lit to an extent such that the concentration of sulphate ions produced in the process is equal or below the prescribed discharge limit set by CPCB for inland surface water storage. The concentration of nitrate ions in the diluted wastewater should is equal or below 100 gm/lit for marine discharge and to 505.3 mg/lit for inland surface water discharge so that the sulphate ion concentration in the water after treatment by the process of the present invention would not cross the permissible discharge limit 1000 mg/lit set by CPCB for inland surface water discharge. The adjustment of pH in step i is carried out by either adding an acid or a base depending on the initial pH of the wastewater. The acid is selected from dilute sulfuric acid or hydrochloric and the base is preferably NaOH or KOH solution or lime [Ca(OH)2]. The pH is preferably maintained in the range of 2 to 4. The generation of nascent hydrogen is carried out by addition of a metal element preferably magnesium in the form of granules in an amount 60% wt of the total nitrate or zinc dust at 150 to 160% by wt of the total nitrate to the liquid waste. Other metals like iron in powder form may also be used. The nitrogen-releasing agent is preferably sulphamic acid powder in an amount 150 - 200% by wt of the total nitrate. Another nitrogen-releasing agent may be urea in an amount of 0.717 to 0.800 Kg/ Kg NO3". Ferrous sulphate may also be used for the purpose. The nitrogen-releasing agent is preferably added a period of time after the addition of metal in step iii. The period of time is preferably 15 minutes to 45 minutes, most preferably half an hour. The nitrogen-releasing agent is added slowly for a duration of around 3 hours adding a small quantity at a time until the NCVion concentration reaches below 10 mg/lit. The alkali of step v is selected from NaOH, KOH and Ca(OH)2 which is added slowly until the pH of the treated wastewater reaches in a range of 7 to 7.5 if magnesium is added in step iii to 4 to 4.5 if Zn is added. The magnesium or zinc is precipitated out as hydroxide in step vi. If desired, the precipitated hydroxides may be separated from the liquid waste water by conventional means such as filtration. Detail description of the Invention: It is desirable that the initial concentration of nitrate ions (NO3") in the wastewater is in the range of 44 mg/lit to 100 gm/lit for inland surface water discharge and in range of 44 mg/lit to 505.3 mg/lit for marine coastal area discharge. If the starting nitrate ion level is higher then, after the treatment by the present of invention, the sulphate ion concentration would cross the permissible discharge limit 1000 mg/lit set by CPCB for inland surface water discharge. In the process of the present invention the nitrates (NO3") present in the liquid waste is converted to N2 gas in presence of a metal preferably magnesium or zinc metal and a nitrogen releasing agent preferably sulphamic acid (NH2SO3H) in acidic media. Four reactions take place in this process of denitration: Initially an acid or base is added in order to bring the pH of the solution in the range of 2 to 4, such that the first reaction takes place at that pH range. In the first reaction (Reaction 1 or 1" or 1"), the metal (Mg or Zn) reacts with dilute sulphuric acid (or hydrochloric acid) and produces nascent hydrogen [H] and metal dissolves in the solution by forming sulphate (MgS04 or ZnS04) or metal chloride (MgC2 or ZnC2) in case of HCI. Both Zn and Mg powders are pyrophoric in nature but it is safe to use them in large scale, only precaution should be taken in storing them in flame free area. Between the two, if the recovery of metal is envisaged then Zn it better than Mg. Since Zn is recoverable form the solution, by applying electrolysis. For low nitrate concentration of feed solution Mg is preferred because, solubility of magnesium-sulphate is less compared to zinc-sulphate and Mg is required only 60% wt of the total nitrate, whereas Zn is required 150 to160% wt of total nitrate. All reactions are carried out at ambient temperature and pressure. Mg +H2SO4 -> 2[H] + MgSC4 (1) Zn + H2SO4 -> 2[H] + ZnS04 (1) The metal element of step iii may also be Fe powder in which case N2O3 gas is produced along with nitrogen gas as per the following reaction: Fe + H2S04 -» 2[H] + FeS04 (1") In the second reaction (Reaction 2), nascent hydrogen [H] reduces the NO3" to N02". N03" + [H] -> N02" (2) In the third reaction, the nitrogen-releasing agent produces nitrogen gas by reacting with N02". Reaction 3 shows the reaction when sulphamic acid is used as the nitrogen-releasing agent. If urea is used as the nitrogen-releasing agent, it reacts with NO2" ion and produces nitrogen and carbon dioxide (Reaction 3"). NC2 + NH2SO3H -» N2 t +H++S04"2+H20 (3) 2 H+ + 2N02" + CO (NH2) 2 -> 2N21 + C021 +3 H20 (3) When urea is used as the nitrogen releasing agent, the pH of the solution has to be maintained between 2 to 4 by adjusting the concentration of sulphuric or hydrochloric acid and/or by adding a base such as sodium hydroxide or magnesium oxide or calcium hydroxide if necessary. Ferrous sulphate can also be used as the nitrogen-releasing agent. In the last stage of the process (Reaction-4), metal sulphate or chloride (MgS04 or ZnS04 or MgCI2 or ZnCI2) present in the liquid is precipitated out as hydroxide by adding NaOH solution slowly, up to pH: 7 to 7.5 for Mg or pH 4 to 4.5 for Zn. MgS04+2 NaOH -> Mg(OH) 2 + Na2S04 (4) OR ZnS04 +2NaOH -> Zn(OH) 2 + Na2S04 (4") If reuse of zinc is envisaged, it is possible to recover it by electrolysis and reused. For this, preferably sulfuric acid is used to maintain pH of the wastewater. Zinc is converted into zinc sulphate and it is recovered in metallic form, from the acidic denitrated solution containing, zinc-sulphate and the recovered zinc can be reused in the process. The above process has the advantage that the reactions are reasonably fast and the total process takes around 4 hours to reach completion. Furthermore, the conversion efficiency of the overall reaction is very high and is greater than 99%. [Conversion efficiency, E = {(N03" ions concentration in feed solution - N03" ions concentration in treated solution) X 100 / (N03" ions concentration in feed solution)}] EXAMPLES The invention will now be illustrated by way of examples and drawing where figure 1 shows a flow diagram of the process of the present invention. The examples are by way of illustration only and in no way restrict the scope of the invention. Samples of waste water containing nitrate ions of different concentrations were prepared by adding sodium nitrate, ammonium nitrate and dilute nitric acid, in process water with nitrate ion concentration in the range of 24 -10Ogm/lit. EXAMPLE -I A sample was prepared by adding nitric acid in process water to make 0.5 N solution. Nitrate ion concentration was 31.5 gm/lit. To 500 ml of the prepared solution, dilute H2SO4 was added slowly so that the pH s maintained between 3 and_4. Then, zinc dust (24gm) was added slowly while constant stirring. Sulphamic acid (30gm) is then added slowly. The addition was continued for 4 hrs. The NO3" ion concentration was measured and found to be below 10 mg/lit. Zn++ was precipitated as Zn(OH)2 by adding NaOH (2M) solution till the pH reached between 4 to 4.5. The precipitate was filtered out. The experiment was repeated with 500 ml of the prepared solution and with Mg-granules (19gm) fnstead of Zn-dust and keeping other steps, same as for Zn-dust. The final nitrate ion concentration was estimated as 8mg/l. Mg++ precipitated as Mg(OH)2 by adding NaOH (2M) solution up to pH of 7 to 7.5 and filtered. The treated wastewater was analyzed by standard methods of analysis and the findings are given in Table-1. It has been observed that the treated wastewater meets the discharge norms for inland surface water discharge (=10 mg/l Nitrate-nitrogen) as well as for the marine coastal areas discharge (=20mg/l Nitrate-nitrogen) and Mg++or Zn++ ion concentration was below 2 mg/l. EXAMPLE-II The procedure of the Example I was repeated with another sample of waste water [500 ml] prepared by adding ammonium nitrate [18.677gm] and nitrate ion concentration was [29.5gm/lit]. Dilute H2SO4 was added to the solution to bring the pH to 3.5. Mg-granules (8.685 gm) were added and the solution was continuously stirred. Dilute H2S04 was added drop-wise to maintain the pH in range 3 to 4. After 30 minutes, sulphamic acid powder (28gm) was added for a period of 3 hours adding a small quantity at a time. The nitrate ion concentration estimated below 10mg/lit. NaOH solution (5M) was added slowly in stirred condition up to pH of 7 to 7.5 and the precipitated magnesium hydroxide was filtered. EXAMPLE-HI The procedure of the Example-ll was repeated with another sample of waste water [1000 ml] prepared by adding sodium nitrate [47.38gm] and nitrate ion concentration was [34.56gm/lit]. Dilute H2SO4 was added to the solution to bring the pH to 3.5. Then Zn-dust (51.84gm) was added and the solution continuously stirred. Dilute H2S04 was added drop-wise to maintain pH between 3 and 4. After 30 minutes, sulphamic acid powder (90gm) was added for a period of 3 hours, adding a small quantity at a time. The nitrate ion concentration estimated below 10mg/l. NaOH solution (2M) added slowly in stirred condition till pH in the range of 7 and 7.5 was reached and the precipitated magnesium hydroxide was separated by filtration. EXAMPLE-IV The procedure of the Example-ll was repeated with another sample of waste water [500 ml] prepared by adding potassium nitrate [58.165gm] and nitrate ion concentration was [35.67gm/lit]. Dilute H2SO4 was added to the solution to bring the pH to 3.5. Mg-granules (10.7 gm) were added and the solution was continuously stirred. Dilute H2SO4 was added drop-wise to maintain pH between 3 and 4. After 30 minutes, sulphamic acid powder (36.00g) was added for a period of 3 hours, adding a small quantity at a time. The nitrate ion concentration was estimated to be below 10mg/l. NaOH solution (5M) was then added slowly with constant stirring till pH of 7 to 7.5 is reached. The precipitated magnesium hydroxide was separated by filtration. EXAMPLE-V The procedure of the Example-ll was repeated with another sample of waste water [1000 ml] prepared by adding sodium nitrate [137.10gm] and nitrate ion concentration was [100gm/lit], Dilute H2SO4 was added to the solution to bring the pH to 3.5. Mg-granules (60gm) were added and the solution continuously stirred. Dilute H2S04 was added drop-wise to maintain pH between 3 and 4. After 30 minutes, sulphamic acid powder (200gm) was added for a period of 3 hours, with a small quantity at a time. The nitrate ion concentration estimated below 10 mg/lit. NaOH solution (2M) was then added slowly with constant stirring up to pH of 7 to 7.5 and the precipitated magnesium hydroxide was separated by filtration. EXAMPLE-VI. The procedure of the Example-ll was repeated with actual liquid waste collected from a metal/slag dissolution plant containing nitrate ion concentration of 26.56-gm/lit. 10 liter of the solution was taken for the denization. Other ions present in the solution were Na+ (concentration: 40gm/lit) K+ (concentration: «30gm/lit), Fe+++ (concentration: «10gm/lit), Mg++ (concentration: *10g/l) etc. and these ions have no effect on the process. Dilute H2SO4 was added to the solution to bring the pH to 3.5. Zn-dust (400gm) was added slowly and the solution was continuously stirred. Dilute H2SO4 added drop-wise to maintain pH: 3-4. After 1 hour, sulphamic acid powder (532gm) was added for a period of 3 hours, with a small quantity at a time. The nitrate ion concentration was estimated below 10mg/l. NaOH solution (2M) was added slowly in stirred condition up to pH of 7 and 7.5. The precipitated magnesium hydroxide was separated by filtration. The nitrate ion concentrations of all the solutions in the experiments have been measured, and all are below 10 mg/lit. Table-1 shows the experimental results. The experiments were conducted with nitrate ion concentrations from 24 gm/lit to 100 gm/lit and in all the experiments nitrate ion concentration had been reduced below 10 mg/lit. The process is feasible for a wide range of initial nitrate ion concentrations. Conversion efficiency is higher than 99.96 % in all the cases. All the above experiments repeated with Zn-dust instead of Mg-granules and in all cases nitrate ion concentration reduced below 10 mg/l. TABLE-1: Experimental results of denitration of nitrate solution by chemical methods. » Exp. No. Type of waste water solution Solution volume N03" concentration before denitration (In gm/lit) NO3" concentration after denitration (In mg/lit) Conversion Efficiency (In %) 1 Dill. HN03 (0.5 N) 1000 ml 24.00 7.890 99.96 2 NH4NO3 500ml 28.95 9.870 99.96 3 NaN03 1000 ml 34.56 7.870 99.97 4 KNO3 500 ml 35.67 6.980 99.98 5 NaN03 500 ml 100.00 7.900 99.99 6 Plant waste 10 liters 26.56 8.870 99.96 Advantages of the Invention The main advantages of the present invention are i) high conversion (denitration) efficiency, ii) less processing time. The other advantages of the invention are: ■ The process is very simple and safe. ■ The process operates at ambient temperature and pressure, which means less capital cost, and no heating is involved. ■ The process of denitration is comparably fast and requires less time compared to biological processes. ■ The process eliminates nitrate pollutants in the waste water produced by various industries e.g. fertilizer, metallurgical etc., which use nitric acid and/or nitrate salts as intermediate chemicals. ■ Produced N2 gas or/ and CO2 (only N2 when sulphaamic acid is used, N2 and & CO2 when urea is used) are harmless. ■ Chemicals required are common and readily available in the market. We Claim: 1. An improved process for reducing the nitrate and other metals and sulphate ion concentration of wastewater containing such ions at levels above the discharge norms so as to meet the discharge norms for disposal comprising the steps of: i. determining the pH of said wastewater and, if required, adjusting the pH of said wastewater between 2 and 4; ii. adding at least one metal element selected from magnesium in amount of 60% by wt. of total nitrate, zinc in an amount of 150 to 160% wt of the total nitrate, to said acidified wastewater to generate nascent hydrogen; iii. adding a nitrogen releasing agent selected from sulphamic acid in amount of 150-200% by wt. of nitrate, urea in amount of 0.707 to 0.8 kg per kg. of total nitrate and ferrous sulphate; iv. adding an alkali to precipitate hydroxide of said metal of step ii such that the zinc ion concentration in the treated water is within a permissible limit, wherein said alkali is added slowly until the pH of the wastewater reaches between 7 and 7.5 when said metal element in step ii is magnesium and wherein the process comprises pretreatment of said wastewater comprising diluting the concentration of initial nitrate ion (N03~) of said wastewater to reduce the concentration of nitrate ions when the concentration is above 100gm/lit to an extent such that the concentration of sulphate ions produced in the process is equal or below the prescribed discharge limit set by CPCB for inland surface water storage. 2. A process as claimed in claim 1, wherein said precipitated hydroxide in step iv is separated from the liquid waste water by conventional means such as filtration. 3. A process as claimed in claim 1, wherein said adjusting of pH in step i is carried out by adding an acid or a base depending on the initial pH of the said wastewater. 4. A process as claimed in claim 3, wherein said acid is selected from sulfuric acid and hydrochloric acid. 5. A process as claimed in claim 3, wherein said base is selected from NaOH and KOH 6. A process as claimed in claim 1, wherein said magnesium is in granular form. 7. A process as claimed in claim 1, wherein said zinc is zinc dust. 8. A process as claimed in claim 1, wherein the nitrogen-releasing agent is urea and the pH of the mixture is maintained between 2 and 4 by maintaining the concentration of either sulphuric or hydrochloric acid. 9. A process as claimed in any of preceding claims, wherein said nitrogen-releasing agent is added a period of time after the addition of metal in step ii. 10. A process as claimed in claim 9, wherein said period of time is between 15 to 45 minutes preferably half an hour. 11. A process as claimed in any preceding claim, wherein said nitrogen-releasing agent is added slowly for a duration of around 3 hours adding a small quantity at a time until the said nitrate ion concentration reaches below 10 mg/lit. 12. A process as claimed in claim 1, wherein said alkali in step iv is selected from NaOH, KOH and Ca(OH)2. 13. A process as claimed in claim 1, wherein said alkali in step iv is added slowly until the pH of the wastewater reaches between 4 and 4.5 when said metal element in step ii is zinc. 14. A process to reduce the nitrate concentration of wastewater to meet the discharge norms for disposal substantially as herein described particularly with reference to examples and the accompanying drawing. Dated this 1st day of August 2003.

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