Dioxins and Furans in Sewage Sludges - A review of their occurrence and sources in sludge and of their environmental fate, behaviour and significance in sludge-amended agricultural systems
Report No FR/D0025

Oct 1995


  1. Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are two groups of organic compounds which are ubiquitous in the environment at ultra-trace levels, but which have attracted considerable scientific and political concern because of their environmental persistence, tendency to bioaccumulate through the food chain and toxicity. In recent years they have attracted particular interest because of their presence in sewage sludges and they have been included on listings of the 'priority organic contaminants'.

  2. PCDD/Fs are not produced intentionally, but are released into the environment in ultra-trace amounts from various combustion processes and as a result of their occurrence as unwanted by-products in various chlorinated chemical formulations (eg. pentachlorophenol - PCP). There is continuing uncertainty over the relative importance of different sources of PCDD/Fs to the environment.

  3. The lowest and highest TEQ values for sewage sludge reported in the literature are 0.5 and 4100 ng/kg (DW). UK data reported in three surveys range between 9 and 206 ng/kg, with typical or representative values internationally in the range 20 to 100 ng/kg.

  4. There are many sources which have been suggested as potential contributors to the PCDD/F composition of sludges. Combustion-derived inputs will provide a 'baseline' input to the environment and hence sludge, which may be supplemented by trace impurities released from the manufacture and use of various chloroaromatics. PCP use in textiles has attracted attention as an important contributor in Germany. More recently, impurities in a dyestuff, chloranil, have attracted attention.

  5. Atmospheric deposition and sewage sludge would appear to supply roughly equal amounts of PCDD/F to UK soils each year, although deposition would appear to supply 30-times more TEQ.

  6. Based on the average composition of individual 2,3,7,8-substituted PCDD/Fs measured in UK sludges, OCDD has the greatest potential to be enriched in sludge-amended soil, followed by a range of Hp- and Hx-CCD/Fs and OCDF. These PCDD/Fs all have relatively low TEF values. Enrichment of the lower chlorinated tetra and penta-CCD/Fs which have higher TEFs is less marked.

  7. A simple pathways assessment procedure is presented, focusing on the transfers into milk and meat from pasture land and based on TEQs. This is designed for dairy and beef cattle separately. The scheme could accommodate differences in dietary composition, with animals eating a mixture of herbage, fodder/grain, soil and (potentially) sludge. Scenarios for unsludged rural and urban and typical and worst case sludge-amendment are considered. Increases in livestock PCDD/F intake due to most routine sludge additions in rural areas are shown to be minor compared to the differences projected between rural and urban grazing pasture. The only exceptions to this would result from unusually high rates of addition of soil or sludge ingestion.

  8. An assessment is made of the influence of unsludged rural and urban and worst case sludge scenarios on human TEQ exposure. Average UK dietary intake is 125 pg TEQ/day. Individuals living in rural and urban areas and consuming foods wholly derived from sludge-amended land are projected to have exposures of 103 and 151 pg TEQ/day, respectively (ie. -17 and +21% of the average exposure). The worst case sludge scenario was 204 pg TEQ/day (ie. +63% of the average exposure). The worst case sludge scenario, whilst higher than the typical rate at 3.4 pg/kg body weight/day, is still nearly a factor of three below the Tolerable Daily Intake (TDI).

  9. Predictions of future emission scenarios are made and the implications for sewage sludge composition examined. A series of assumptions is made, leading to a predicted decline in the sludge-derived inputs of TEQ to UK agricultural land from 25 g/year now (ie.1995) to 22 g/year in 2005, even though the quantity of sludge going to agriculture is expected to increase by 50%.

  10. Several areas of research are identified to resolve continuing uncertainties. These primarily focus on the sources of PCDD/Fs to sludge; the fate, behaviour and persistence of PCDD/Fs in soils; and the dynamics and modelling of PCDD/F transfers to, within and out of grazing cattle.

The project was funded by the Department of the Environment and managed jointly with UKWIR. In exchange, the Department and UKWIR jointly managed a separate project on the significance of organic contaminants in sewage sludge which was funded by UKWIR and also undertaken by Lancaster University.

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