Over the last decades, membrane methods (especially reverse osmosis) have been increasingly used for treatment of liquid radioactive waste (LRW). The reverse osmosis methods are most efficiently used in combination with microfilters and ion exchange filters. Based on experience of using the NITI-designed modular sorption membrane system (MSMS) in LRW treatment and concentration process, an automated version of MSMS, MSMS-A, has been developed, which includes an automated reverse osmosis module. A drawback of the membrane technology in water treatment applications is that reverse osmosis elements are often clogged with low soluble calcium and manganese carbonates, iron hydroxides, and other residues. This reduces the efficiency of the reverse osmosis module and the quality of filtrate. 95–97% of quality reduction is determined by fouling of the membrane surface and only 3-5 % by densification of the membrane capillary porous structure. Moreover, contamination of membranes leads to an increase in gamma exposure rate from the reverse osmosis equipment, thus causing troubles to maintenance. Therefore, it is of current importance to improve the process of regeneration washing of reverse osmosis elements.
The authors investigated the efficiency of regeneration solutions prepared with phosphoric acid, ethylene diamine tetra-acetic acid (EDTA), citric acid, and oxalic acid. The phosphoric acid was shown to be poorly efficient in washing the reverse osmosis elements, as compared to organic acids. The choice among organic acids was based on the consideration that radioactive wastes generated from washing would be solidified by cementing. EDTA deteriorates the quality of cement compounds. The oxalic and citric acids form low-solubility compounds with calcium contained in cement clinker. In addition, they are commonly used as agents in preparation of decontamination solutions. Based on results of testing the citric acid solution as the most efficient and least toxic substance was chosen for chemical washing.
Testing of the spent washing solution has shown that iron compounds and other corrosion products make up to 85% of washed-off residues and the remaining are magnesium, calcium, and aluminum compounds. An automated module for washing the reverse osmosis elements has been developed with the purpose to ensure radiation safety of the process. Control of operation of the automated reverse osmosis module and module for washing reverse osmosis elements is provided from operator workstation. Spent washing solutions are drained to LRW tank. The portion of acid that has not reacted during washing process serves as a sludge inhibitor, mitigating fouling of reverse osmosis elements in next cycles of LRW treatment. This prevents sludging in the tank itself.