Dioxins and furans in sewage sludges : A review of their occurrence and sources in sludge and of their environmental fate, behavior, and significance in sludge-amended agricultural systems

Jones, K. C. and Sewart, A. P. (1997) Dioxins and furans in sewage sludges : A review of their occurrence and sources in sludge and of their environmental fate, behavior, and significance in sludge-amended agricultural systems. Critical Reviews in Environmental Science and Technology, 27 (1). pp. 1-86. ISSN 1064-3389

Abstract

Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are two groups of organic compounds that are ubiquitous in the environment at ultratrace levels, but that have attracted considerable scientific and political concern because of their environmental persistence, tendency to bioaccumulate through the foodchain, 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'. PCDD/Fs are not produced intentionally, but are released into the environment in ultratrace amounts from various combustion processes and as a result of their occurrence as unwanted byproducts in various chlorinated chemical formulations (e.g., pentachlorophenol, PCP). There is continuing uncertainty over the relative importance of different sources of PCDD/Fs to the environment. The lowest and highest ΣTEQ values for sewage sludge reported in the literature are 0.5 and 4100 ng/kg (DW), U.K. data reported in three surveys range between 9 and 206 ng/kg, with typical or representative values internationally in the range of 20 to 100 ng/kg. Many sources 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 chloroanil have attracted attention. Atmospheric deposition and sewage sludge would appear to supply roughly equal amounts of ΣPCDD/F to U.K. soils each year, although deposition will supply a greater ΣTEQ. Based on the average composition of individual 2,3,7,8-substituted PCDD/Fs measured in U.K. sludges, OCDD has the greatest potential to be enriched in sludge-amended soil, followed by a range of Hp- and Hx-CDD/Fs and OCDF. These PCDD/Fs all have relatively low TEF values. Enrichment of the lower-chlorinated tetra- and penta-CDD/Fs that have higher TEFs is less marked. A simple pathways assessment procedure is presented, focusing on the transfers into milk and meat from pastureland and based on ΣTEQs. 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 with the differences projected between rural and urban grazing pasture. The only exceptions to this would result from unusually high rates of soil or sludge ingested. An assessment is made of the influence of unsludged rural and urban and worst-case sludge scenarios on human ΣTEQ exposure. Average U.K. dietary retake is 125 pg of ΣTEQ pez day. Individuals living in rural and urban areas are projected to have exposures of 103 and 151 pg of ΣTEQ per day, respectively (i.e., -17 and +21% of the average exposure). The worst-case sludge scenario was 204 of pg ΣTEQ per day, respectively (i.e., +63% of the average exposure). The worst-case sludge scenario, while higher than the typical rate at 3.4 pg/kg of body weight per day is still nearly a factor of three below the tolerable daily intake (TDI). Predictions of future emission scenarios in the U.K. are made and the implications for sewage sludge composition examined. A series of assumptions are made, leading to a predicted decline in the sludge-derived inputs of ΣTEQ to U.K. agricultural land from 25 g/year now (i.e., 1995) to 22 g/year in 2005, even though the quantity of sludge going to agriculture is expected to increase by 50%. Several areas of research are identified to resolve continuing uncertainties. These primarily focus on the sources of PCDD/Fs to sludge; the fate, behavior, persistence, and bioavailability of PCDD/Fs in soils; and the dynamics and modeling of PCDD/F transfers to, within, and out of grazing livestock.