(WO9419287) What Is Claimed Is: 1. A method of reducing the amount of hydrogen sulfide which may be present in an aqueous system comprised of water and hydrogen sulfide and preventing the formation of hydrogen sulfide in the aqueous system, comprising the step of bringing nitrate and nitrite ions, or compounds which yield these ions, into contact with the aqueous system in a concentration sufficient to establish and enhance the growth of denitrifying bacteria, wherein denitrifying bacteria are present in the aqueous system prior to said step or added concurrently with or after said step, and wherein the denitrifying bacteria in the system outcompete sulfate reducing bacteria which may be present in the system for organic nutrients which may be present in the system, such that the sulfate reducing bacteria are deprived of the organic nutrients they need to produce hydrogen sulfide. 2. The method of claim 1, wherein the aqueous system which is contacted with the nitrate and nitrite ions or compounds yielding these ions, contains sulfate- reducing bacteria. 3. The method of claim 1, wherein the aqueous system which is contacted with the nitrate and nitrite ions or compounds yielding these ions, contains denitrifying microorganisms. 4. The method of claim 1, further comprising adding denitrifying microorganisms to the aqueous system before, concomitant with, or after the ions or ion yielding compounds are added to the aqueous system. 5. The method of claim 1 which is a batch method, whereby the aqueous system is contacted once with the nitrate and nitrite ions or ion-yielding compounds. 6. The method of claim 1 which is a continuous method, whereby the aqueous system is repeatedly treated with the nitrate and nitrite ions or ion-yielding compounds. 7. The method of claim 1, further comprising adding to the aqueous system a water-soluble carbon-source nutrient for the denitrifying microorganisms before, concomitant with, or after the addition of the ions or ion-yielding compounds. 8. The method of claim 7, wherein the carbon source comprises an acetate. 9. The method of claim 1, wherein the aqueous system to be contacted indigenously comprises a carbon-source nutrient for the denitrifying microorganisms. 10. The method of claim 1 , further comprising adding a molybdate or molybdate yielding material, to the aqueous system either before, concurrently, or after the addition of the nitrate and nitrite ions or ion-yielding compound. 11. The method of claim 10, wherein about 5 to about 250 ppm of molybdate are added to the aqueous system. 12. The method of claim 1, wherein the aqueous system to be contacted, at the time of nitrate and nitrite ion contact, does not contain sulfate-reducing bacteria which are generating hydrogen sulfide. 13. The method of claim 1, wherein the aqueous system is contacted with less than about 3000 ppm of the total of nitrate and nitrite ions. 14. The method of claim 13, wherein the aqueous system is contacted with between about 25 and about 500 ppm of the total of nitrate and nitrite ions. 15. The method of claim 13, wherein the aqueous system is contacted with between about 1 and about 500 parts of nitrite ions and between about 1 and 500 parts of nitrate ions. 16. The method of claim 15, wherein the ratio of nitrate to nitrite ions is between about 50:1 to about 1:50. 17. A method of recovering oil from oil wells comprising injecting an aqueous system which has been treated in accordance with claim 1, into a subterranean oil-bearing formation. 18. A method of reducing the amount of hydrogen sulfide which may be present in an aqueous system comprised of water and preventing the formation of hydrogen sulfide in the aqueous system, comprising the step of bringing molybdate and nitrite ions or compounds which yield these ions, into contact with the aqueous system in a concentration sufficient to inhibit or kill sulfate-reducing bacteria which may be present and in a concentration sufficient to establish and enhance the growth of denitrifying bacteria. 19. A method of claim 18, wherein about 1 to about 250 ppm of nitrite ion and about 5 to about 200 ppm of molybdate ion are contacted with the aqueous system. 20. A method of reducing the amount of hydrogen sulfide which may be present and preventing the formation of hydrogen sulfide in an aqueous system, comprising the step of bringing nitrite ions or compounds which yield these ions, and at least one of nitrate and molybdate ions or compounds which yield these ions into contact with an aqueous system in a concentration sufficient to establish and enhance the growth of denitrifying bacteria, wherein said denitrifying bacteria are present in the aqueous system prior to bringing the ions into contact with the system, and/or added concurrently with or after the ions are brought into contact with the system, and wherein the denitrifying bacteria in the system outcompete sulfate-reducing bacteria which may be present in the system for organic nutrients which may be present in the system, such that the sulfate reducing bacteria are deprived of the organic nutrients they need to produce hydrogen sulfide. 21. A process according to claim 1, wherein the denitrifying bacteria consume as a nutrient hydrogen sulfide present in the aqueous system thereby further reducing the amount of hydrogen sulfide in the aqueous system. 22. A composition useful for reducing the amount of hydrogen sulfide which may be present in an aqueous system and preventing the formation of hydrogen sulfide in the aqueous system, comprising (a) nitrite ions or compounds which yield these ions, and (b) at least one of nitrate and molybdate ions or compounds which yield these ions, wherein the ions are present in a concentration sufficient to establish and enhance the growth of denitrifying bacteria and in a concentration sufficient to inhibit the growth of sulfate reducing bacteria in the aqueous system. 23. The composition of claim 22, which comprises each of nitrite, nitrate, and molybdate ions or compounds which yield these ions. 24. The composition of claim 23, wherein t h e composition further comprises water. 25. The composition of claim 23, wherein t h e composition further comprises a carbon source. 26. The composition of claim 24, which comprises sodium nitrite, sodium nitrate, and sodium molybdate. 27. A process according to claim 1, wherein the aqueous system contacted does not contain hydrogen sulfide. 28. A process according to claim 1, wherein the aqueous system contacted contains hydrogen sulfide.
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