The removal of radioactive cobalt, cesium, and iodine in a conventional municipal wastewater treatment plant

Abstract

ABSTRACT: A radiotracer study was conducted to determine removal efficiencies for cobalt, cesium, and iodine in a conventional municipal wastewater treatment plant. The study was conducted in a full‐scale facility that provides primary treatment through physical settling and secondary treatment using activated sludge. The radiotracers, 58 Co and 134 Cs as chlorides, and 131 I as Nal, were equilibrated in raw wastewater and introduced to the treatment system as a pulse in the incoming raw wastewater. Samples of wastewater and sludge were collected and analyzed to quantify primary treatment, secondary treatment, and overall removal efficiencies. Selected samples were filtered to determine the partitioning of the tracers between the solid and liquid phases of the wastewater and sludge throughout the treatment process. In the incoming wastewater, approximately 10% of the 58 Co, 7% of the 134 Cs, and 3% of the I31 I were associated with suspended solids. The suspended tracer fractions decreased across the treatment plant. Primary treatment resulted in the partial removal of the incoming suspended fraction with observed removal efficiencies of 8.1% and 3.0% for 58 Co and 134 Cs, respectively, and an estimated removal of 2% for 131 I. Approximately 80% of the radiotracers in the primary treatment sludge were associated with the sludge solids. The affinity of the radiotracers for the activated sludge solids was in the following order: 58 Co > 134 Cs > 131 I. Secondary treatment removal efficiencies of 25% and 9.4% were observed for 58 Co and 134 Cs, respectively. The secondary treatment removal efficiency for 131 I was estimated to be 2‐3%. Overall removal efficiencies were 31%, 12%, and 3‐5% for 58 Co, 134 Cs, and 131 I, respectively. The remaining fractions of the radiotracers remained within the soluble phase of the wastewater and were discharged in the treatment plant effluent.