A Fully Operational Pilot Plant for Eliminating Radioactive Oils Mixed With Chlorinated Solvents

Abstract

Disposal of organic liquid waste has become an increasing issue for many nuclear sites. Existing disposal solutions such as incineration or super critical water techniques are not compatible with wastes containing chlorinated solvents or fluorine owing to corrosion problems. As an example several hundred cubic meters of lubricating oils mixed with trichloroethylene (TCE) or perchloroethylene (PCE) are stockpiled on several French nuclear sites. For several years Dewdrops has been developing an original combination of mineralization processes for waste oils and solvents particularly well suited to the nuclear field. The patented technology relies on the alternation of chemical and biological oxidation mechanisms. The oxidized organic material predominately forms carbon dioxide, water and inorganic salts. This paper details the procedure and the results obtained for a particular case at the Tricastin nuclear site of Areva NC (South France). The organic waste used in this study was a 85/15 v/v ratio mix of lubricating oil and TCE. The pilot plant build upon the technology has a daily treatment capacity of approximately 10 liters. In the first step the TCE is mineralized by the photo Fenton reaction. Using hydrogen peroxide with an ultraviolet regenerated iron catalyst, TCE is transformed to carbon dioxide, water and hydrochloric acid. After neutralizing with caustic soda, the next step is a multi-stage biodegradation process to eliminate the remaining lubricating oil. Carefully selected microorganisms use the organic waste as an energy source for their metabolism. During oil biodegradation over 75% of the carbon is released as carbon dioxide while the remaining is incorporated into the biomass. The aqueous phase is continuously separated from the biomass using cross flow filters. The output aqueous phase is treated with ozone and ultraviolet light to eliminate the remaining organic compounds. The final effluent obtained is in conformance with European water standards and can be disposed by normal means. It can also be adjusted to local requirements. The radioactive elements and heavy metals present in these lubricating oils are trapped by the biomass. The excess of biomass is recovered by centrifugation and mineralized by catalytic ozonation technology. The result of the tests was a radioactive waste reduction factor of 15. The mineral residue is a concentrate of inorganic salts with traces of radioactive elements as well as heavy metals. The radioactive elements thus recovered can be consigned to an official repository.