Calculation of radionuclide ground deposition by means of measurements on sewage sludge

The concentrations of various radionuclides have been measured in the incoming water, the outgoing water and the sludge from the sewage treatment plant serving the town of Lund in southern Sweden. The mean residence time for the water in the plant is 1 to 2 days, whereas for the sludge it is 3 to 4 weeks. Variations in the residence time, which may influence the efficiency of the plant, are related to the season of the year and the load on the plant. Measurements show that for ⁷Be, ⁵¹Cr, ¹³⁴Cs, and ¹³⁷Cs between 37 and 56% of the incoming activity leaves the plant with the sludge. For ¹⁰⁶Ru and ¹³¹I these figures lie between 6 and 14%. The deposition of ⁷Be has been measured at Lund and the relation between the deposition on the ground (Bq m^?2) and the activity concentration in the sludge (Bq kg^?1 dry weight) has been studied and found to by relatively constant at ?(0.8 ±0.2) kg m^?2. In measurements on sewage sludge, the detection limit for deposition of ⁷Be on the ground is around 16 Bq m^?2.