Enrichment of radioactive nuclides in sewage sludge

As a consequence of the Chernobyl reactor accident, precipitations in various parts of Europe carried radioactivity for a short period. This meant necessarily that the water received by sewage treatment plants was also polluted. Measurements in a sewage treatment plant having received sewage water which carried radioactivity showed the following: Active nuclides are removed from the sewage water and are to a high extent enriched in sewage sludge. On comparison of the values for the activity of sludges with the stages of the plant, an increase in specific activity (nCi/kg) is found in parallel to the increase in the share of dry substance. Centrifugation of sludges yields a supernatant of relatively low radioactivity, while the radioactive elements are enriched in the sediment. By far the highest activities are found in the ashes of burnt sludge (2860 nCi/kg) and in dewatered sludges. These results are confirmed by comparison measurements in six other plants. In the period between May 7th, 1986 and June 16th, 1986, activities in digester sludge were measured ranging from 395 to 58.6 nCi/l; in pressed sludge, 1717 to 290.4 nCi/l were measured. (orig./RB)