⁷Be Levels in Sewage Sludge: A Baseline Study

Abstract

There is no serious information in the existing literature on the levels of radioactive ⁷Be in sewage sludge. Our research took the form of a baseline study to provide suitable information in this respect. In addition, the sludge is converted to fertilizer, which can lead to radioactive contamination of streams and underground water supplies by agricultural run‐off. According to the documented literature, the acceptable level of radioactivity in domestic water supplies is <0.1 Bq/L. The levels in the sludge were up to three orders of magnitude higher. We subsequently investigated the distribution of this radioactivity over a 3‐year period. Our study involved measurement, by gamma ray spectroscopy, of ⁷Be in sludge and corresponding waste water specimens, and evaluation of its distribution over a period extending from March 2000 to March 2003. The sludge and disinfected effluent samples were collected thrice monthly, and subjected to high‐resolution gamma‐ray measurement with a 28‐cm² Ge detector. Sludge sample masses of typically 700 g (dry weight) were prepared in suitable Marinelli beakers and counted for 12 hr. The liquid specimens (1 L) were treated in the same way. It was important for the disinfected water to be free of ⁷Be, as it was used for irrigation. Variations in ⁷Be activities were observed in all the analyzed sludge samples and studied as a function of time. The presence of ⁷Be was identified by the appearance of the characteristic 478 keV gamma ray and the computed half‐life of 53 days. The documented sources of environmental ⁷Be reveal that it generally arises from cosmogenic interactions. In the present investigation, ⁷Be activities up to 40 Bq/kg above the background were recorded and no appreciable activity was monitored in the liquid disinfected specimens. A suggestion to deplete the activity levels in the sludge prior to its use as a soil conditioner is outlined. Such preventative remediation is of considerable practical importance to environmental science, and to the future well‐being of the local biosphere. The study, therefore, formed an interesting contribution to research associated with the environment.

Keywords:

• Radioactivity
• Beryllium
• Sludge
• Waste water
• Gamma‐ray spectroscopy