The Navy uses sodium nitrite solution extensively as a rust preventing fluid in marine boiler maintenance operations which include the following: hydroblast boiler tube cleaning, hydro-static pressure testing, boiler lay-up, and rinsing. These maintenance processes generate millions of gallons of wastewater annually. Since nitrite oxidizes readily to nitrate, it will support surface water eutrophication and, therefore, is treated as a "de facto" hazardous waste regardless of whether contaminants are taken up in its usage. The Environmental Protection Agency standard threshold for nitrite-nitrogen in potable water is 1 mg/L, whereas naval boiler wash wastewater contains up to 250 mg/L. In addition to sodium nitrite, the wastewater stream also contains various heavy metals in ionic form.
The objective of this project was to develop an integrated, cost-effective process to recycle and denitrify hazardous sodium nitrate wastewater solutions to environmentally benign compounds that can be disposed of at a low cost while meeting National Pollutant Discharge Elimination System disposal limits with minimal nitrate conversion.
The technical approach consisted of the following: (1) investigation of an oxygen-exclusive conditioning process for bringing the used nitrite solutions back to specification quality, (2) development of a practical procedure for longterm storage of recyclable nitrate solutions without nitrate formation, and (3) full-scale design and demonstration testing of the integrated recycle/denitrification process.
It has been estimated that the total volume of sodium nitrite wastewater generated by all naval shipyards is about 3 million gallons annually, and that generated by Navy-wide boiler maintenance operations is 10 million gallons annually. The proposed chemical denitrification process has the potential of reducing the disposal cost by at least 95 percent (reduced from $3.50/gallon to $0.15/gallon operating cost). This equates $8 million savings per year for naval shipyards and $207 million savings per year for the Navy-wide boiler maintenanc e operations. The chemical process will not produce hazardous waste and the effluent produced can be discharged safely to the sanitary sewer.
The 5,000 gallon centralized sodium nitrite recycle/treatment system has been erected as an integral part of the existing industrial wastewater treatment plant facility located at the Naval Air Station, North Island, San Diego, CA. This nitrite denitrifrication system has capabilities for recycling and storing the nitrite solution for reuse, nitrite reduction using sulfamic acid, heavy metal precipitation, and neutralization for discharge. Additional tests were conducted during this reporting period with the assistance from Naval Surface Warfare Center, Naval Ship Systems Engineering Station (NAVSSES). System performance was found to be acceptable and the operation is monitored by a programmable logic controller using oxidation/reduction potential and pH inputs. A draft user data package was prepared by the contractor, Arthur D. Little, and was reviewed by the Naval Facilities Engineering Service Center and NAVSSES. An abstract entitled, "Full-Scale Demonstration of a Denitrification/Recycling System for Nitrite Wastewater." was submitted to the 7th (1997) International Offshore and Polar Engineering Conference for acceptance. This project was completed in FY 1996.