Report No FR/D0009

B Stuart, D Carter and R A Hoodless Laboratory of the Government Chemist, Queens Road, Teddington, Middlesex TW11 OLY

APR 1993


In 1989 a preliminary survey by the Department of the Environment of sewage sludges from locations across England confirmed the presence of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the 11 sludges sampled, even from plants serving essentially domestic drainage areas. Analysis of these materials is difficult and it was not possible to achieve complete results on all the sludges. The Department of the Environment Pollution Paper No. 27 "Dioxins in the Environment" recommended that further information is needed to determine how PCDDs and PCDFs enter the sewage system.

The main objectives of this study were:-

To determine the concentrations of PCDDs, PCDFs and PCBs in samples of sewage sludges representing a range of catchment types with emphasis on sewage sludge disposed to land.

To analyse for the seventeen PCDD and PCDF congeners containing chlorine substitution at the 2, 3, 7 and 8 positions so that International Toxicity Equivalents could be calculated based on the NATO/CCMS system.

To assess possible temporal variation in the concentrations of these compounds.

Sixteen sewage works were sampled at approximately three-monthly intervals and three of the sewage works were sampled on a monthly basis for ten months. Each sample, after suitable extraction and clean-up, was examined for PCBs using gas chromatography with electron-capture detection and for the seventeen 2, 3, 7, 8 substituted PCDD and PCDF congeners and for total tetra- to octa- CDD and CDF isomer groups using high resolution gas chromatography linked to high resolution mass spectrometry.

The results showed that the concentration of PCDDs and PCDFs increased with the degree of chlorination of the dibenzo-p-dioxins or dibenzofuran compound with heptachlorodibenzo-p-dioxins and octachlorodibenzo-p-dioxin predominating. The mean sum of tetr a-to-octa-CDDs and CDFs was about 20 g/kg dry sewage sludge. There was very little difference in the levels of residues in raw or digested sewage sludge except for heptachlorodibenzofurans and octachlorodibenzofuran where the residues were higher in the digested sludge. In general the mean concentrations were higher in sewage sludge from industrial catchments than from rural catchments. There was no major temporal variation in the residues detected from the three sites examined each month.

The exercise highlighted that there were analytical difficulties with the determination of these compounds in some samples due to the complexity and variability of sewage sludge.

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