Treatment of sodium nitrite-containing boiler wastewater

Abstract

An integrated process for treatment of wastewater from steam boiler clean operations, containing sodium nitrite and also usually heavy metal ions such as Cu, Fe, Pb, Cr, Cd, Zn and Ni, as well as suspended solids and sludge, to remove the nitrite, substantially reduce the amount of such metal ions and remove suspended solids and sludge in the treated wastewater. The process comprises adding sulfamic acid to the wastewater, particularly in about stoichiometric proportions with respect to sodium nitrite to form gaseous nitrogen. The reaction takes place at ambient temperature and atmospheric pressure. An alkali such as sodium hydroxide is added to the resulting solution in amount sufficient to raise the pH thereof to about 10, and precipitating the metal ions. The precipitates and suspended solids and sludge are removed from the resulting wastewater by settling and filtration, and the wastewater free of nitrite and of substantially reduced metal ion content and free of suspended solids as well as sludge, is in condition for discharge to sewers.

What is claimed is: 
     
       1. A process for treatment of wastewater from steam boiler cleaning operations, said wastewater containing sodium nitrite, which comprises adding sulfamic acid to said wastewater in a proportion of 1 to about 1.4 moles per mole of sodium nitrite and   converting substantially all of the nitrite ion to nitrogen.   
     
     
       2. The process of claim 1, using substantially stoichiometric proportions of sulfamic acid with respect to sodium nitrite. 
     
     
       3. An integrated process for treating wastewater from steam boiler cleaning operations, said wastewater containing sodium nitrite, heavy metal ions, suspended solids, and sludge, which comprises adding sulfamic acid to said wastewater in substantially stoichiometric proportions with respect to sodium nitrite,   converting substantially all of the nitrite ion from the sodium nitrite into nitrogen while maintaining the reaction mixture at a pH ranging from about 1.5 to about 4, the reaction taking place at ambient temperature and at atmospheric pressure,   adding sodium hydroxide or calcium hydroxide to the resulting treated wastewater in an amount sufficient to raise the pH of the solution to about 10, and precipitating said metal ions as insoluble hydroxides, and   removing said insoluble hydroxides, said suspended solids and said sludge from the treated wastewater.   
     
     
       4. The process of claim 2, and after the reaction is completed for converting nitrite ion to nitrogen, adding alkali to the resulting solution in an amount sufficient to raise the pH of the wastewater to approximately 7, to permit sewer discharge of the treated wastewater. 
     
     
       5. The process of claim 2, said wastewater also containing heavy metal ions capable of forming insoluble hydroxides, and after the reaction is completed for converting nitrite ion to nitrogen, adding an alkali to the resulting solution in an amount sufficient to raise the pH of the solution to about 9 to 10, and precipitating said metal ions. 
     
     
       6. The process of claim 5, wherein said alkali is selected from the group consisting of alkali metal hydroxide and alkaline earth metal hydroxides. 
     
     
       7. The process of claim 5, wherein said alkali is sodium hydroxide or calcium hydroxide. 
     
     
       8. The process of claim 5, said wastewater also containing suspended solids, and including removing the precipitated heavy metal ions and said suspended solids from the treated wastewater. 
     
     
       9. The process of claim 3, said removing said insoluble hydroxides and said suspended solids including settling said insoluble hydroxides, said suspended solids and said sludge in the treated wastewater, filtering the resulting mixture and discharging the resulting final treated wastewater to sewer. 
     
     
       10. The process of claim 3 wherein said metal ions are selected from the group consisting of Cu, Fe, Pb, Cr, Cd, Zn and Ni ions.