Flame Retardants

Persistent pollutants in land-applied sludges 

Nature v.412 12jul01

Disposal of sewage sludge by application to agricultural and other land is widely practised and is presumed to be environmentally beneficial, but we have found high concentrations of an environmentally persistent class of organic pollutants, brominated diphenyl ethers (BDEs), in 'biosolids' from four different regions of the United States. These compounds are widely used as flame retardants, and their presence suggests that the environmental consequences of land application of biosolids need further investigation. We also frequently detected BDEs in wildcaught fish, indicating another pathway for human exposure.

Over half of the sewage sludge produced annually in the United States is applied to land, amounting to roughly 4 million tons in 1998 (ref. 1). Sludges are treated before application to reduce odour and pathogen content and their metal burden is regulated. But attention has focused less on persistent organic pollutants since usage of the most notorious (for example, polychlorinated biphenyls) has decreased and pretreatment of industrial waste water has improved^1,2. 

We analysed 11 biosolid samples before land application from Virginia, Maryland, New York state and California, and found that they all contained high concentrations of BDEs. These flame-retardant polymers are structurally similar to polybrominated biphenyls, the use of which was curtailed after a significant contamination incident in 1973 involving livestock feed in Michigan³.

However, global consumption of BDEs continues to increase, reaching 67,125 metric tonnes in 1999 ^4,5. The most bio-accumulative and toxic BDEs (those containing 4-6 bromine atoms) are being increasingly detected in humans and wildlife from both developed and remote areas5-7. These were present in significant amounts in the biosolids we examined and their relative contributions matched those in 'Penta', the commercial formulation used as a flame retardant in polyurethane foam (Fig. 1). North America accounts for about 98% of global demand for Penta, estimated at 8,290 tonnes in 1999⁴.

Figure 1 Brominated diphenyl ethers (BDEs) are produced commercially and occur in the environment as mixtures of compounds of varying bromination. Relative contributions of tetra- (BDE-47), penta- (BDE-100 and -99) and hexa- (BDE-154 and -53) brominated versions were similar in 11 biosolids obtained from four different regions of the United States, in the Penta commercial product (used as a flame retardant in polyurethane foam), in treated foam and in wild-caught fish (data shown are for 15 composite samples of channel catfish, Ictalurus punctatus, an omnivorous bottom-dwelling species) collected from Virginia lakes and rivers (error bars represent standard deviation). BDE-209 was not detected in fish but was present in biosolids.

How BDEs are released from polymers has been uncertain, as these applications are considered to be non-dispersive⁷. However, breakdown of discarded polyurethane foam, which may contain up to 30% Penta by weight⁵, may contribute to this. We found that the surface of foam became brittle and sloughed off after 4 weeks of exposure to ambient summer conditions. The particles generated are easily transported and the polymer matrix preserves the formulation's original BDE composition.

The total concentration of Penta-like BDEs in these biosolids was 1,100-2,290 _g per kg dry weight, suggesting that input was high and consistent, regardless of the region of origin and irrespective of preapplication treatment (see supplementary information). Concentrations exceed those in European sludges by 10- to 100-fold8, which is commensurate with the greater demand for Penta in the United States. The European Commission recently proposed a ban on the use of Penta, on the basis of its reported exponential increase in human breast milk and perceived health risks⁹.

The fully brominated Deca product constitutes 82% of the total global BDE market⁴. It is rarely reported in wildlife, perhaps because of its low bioavailability. Deca consists principally of a single BDE (BDE-209) and is used to curtail fires in textiles and in relatively stable, rigid polymers, such as those used in television and computer casings⁵. Unlike those of Penta constituents, BDE-209 concentrations varied widely among the biosolids we analysed (84.8-4,890 µg kg^-1; see supplementary information). Although there is little evidence for the degradation of Deca to Penta-like compounds, some photolysis of Deca to less brominated diphenyl ethers is possible^5,7.

We also detected BDEs in 87% of fish sampled from Virginia waters (quantification limit in fillets, 5 µg per kg lipid; n=334). The principal Penta constituents (BDE-47, -100 and -99) predominated in these samples (Fig. 1). This finding indicates that significant environmental release of these pollutants is occurring in the United States and that humans may be exposed to them through their diet. Carp from one Virginia stream contained 47,900 µg kg^-1 of total BDEs, rivaling the highest fillet burdens reported in the world so far⁴. These compounds are also detectable in urban and rural air¹⁰, indicating the potential for long-distance atmospheric transport. It seems that BDEs are an important - but generally unrecognized - persistent organic pollutant in the United States. Extensive use of Penta and the high burden of BDEs in land-applied biosolids may facilitate environmental dissemination of less-brominated BDEs both locally and globally.

Robert C. Hale, Mark J. La Guardia, Ellen P. Harvey, Michael O. Gaylor, T. Matteson Mainor, William H. Duff

Department of Environmental Science, Virginia Institute of Marine Science, PO Box 1346, Gloucester Point, Virginia 23062, USA e-mail: hale@vims.edu 

1. US Environmental Protection Agency. Biosolids Generation, Use and Disposal in the United States. EPA530-R-99-009 (Washington DC, 1999).

2. National Academy of Sciences. Use of Reclaimed Water and Sludge in Food Crop Production (Washington DC, 1996).

3. Di Carlo, F. J. et al. Environ. Health Perspect. 23, 351-365 (1978).

4. Renner, R. Environ. Sci. Technol. 34, 452A-453A (2000).

5. World Health Organization. Environmental Health Criteria 162: Brominated Diphenyl Ethers (Geneva, 1994).

6. de Boer, J., Wester, P. G., Klamer, H. J. C., Lewis, W. E. & Boon, J. P. Nature 394, 28-29 (1998).

7. Renner, R. Environ. Sci. Technol. 34, 223A-226A (2000).

8. Sellstrom, U. et al. Organohalogen Compounds 40, 383-386 (1999).

9. European Commission press release, 30 January 2001.

10. Strandberg, B. et al. Environ. Sci. Technol. 35, 1078-1083 (2001).

Supplementary information 

Table 1. Concentrations of BDEs (ug/kg dry weight basis) detected in biosolids receiving various pre-application treatments and originating from different regions of the US. Class A biosolids are treated to eliminate pathogens.