PARENT SESSION
14 - Radioecology
2:10 PM to 5:20 PM, Monday, 13 May 2002
Session Chair: Gerzabek, Martin ¹, Kirchner, Gerald ², ^{1 2} .
Lehar B

(14-08) Balance of ¹³⁷Cs in an Austrian sewage treatment plant.

Bossew, Peter^*,1, Lettner, Herbert¹, Hubmer, Hugo¹, ¹ University of Salzburg, Salzburg, Austria

ABSTRACT- Activity concentrations of ¹³⁷Cs in sewage sludge of certain wate water treatment plants in the order of magnitude of several 100 Bq/kg d. s. by the mid 1990s motivated the Austrian authorities to investigate its origins and its balance in different product streams of the plants. The plant which was chosen for a detailed investigation is relatively small, serving several small towns and villages in Upper Austria with a total of 17,000 inhabitants, scattered in a predominantly agricultural region with hilly topography. During 1997, products of different flows of the plant (waste water, sewage sludge, sand, cleaned water) were sampled regularly in order to establish a balance of the ¹³⁷Cs throughput. Waste water input is due to domestic and, to a much minor extent, to (small) industrial activities, and to rainwater runoff through inlets and gullies along streets and roads, respectively. Out of the annual ¹³⁷Cs input of ca. 85 MBq, ca. 90 % could be contributed to runoff from unsealed areas (meadows, pastures, fields), ca. 7 % to the inhabitant's excretions, ca. 2 % to runoff of resuspended matter from sealed surfaces, and a very small fraction to input with tap water. Out of the total ¹³⁷Cs induced into the plant, 67 % left as compacted, fermented sewage sludge product, 18 % with coarse litter (predominantly sand) which is separated in the initial mechanical stage of the plant, and finally, 15 % with cleaned water, which is discharged into a river. During stationary dry weather ca. 83 % is discharged with cleaned water and most of the rest with the sludge. However, ¹³⁷Cs concentration in the ingoing waste water is higher by ca. 1 order of magnitude during rain. Out of the estimated ¹³⁷Cs inventory of the unsealed catchment area of the plant and the fraction of the annual radiocaesium throughput related to surface runoff an 'erosion rate' of (6.9 +/- 1.7) 10^-4 a^-1 could be determined. This may be compared with typically ca. 10^-4 a^-1 in forest and mountain catchments of Austria.

Key words: radiocesium, sewage sludge, soil erosion, surface water