Dioxin, contaminants from combustion

The results reported by Carrasco et al. (1998) revealed that nearly all studied metal emissions, measured at the exit of a cement kiln stack, were significantly higher when a blend of 80 wt% coal + 20 wt% TDF was combusted instead of pure coal. Especially notable are increased emissions in Cr, Mn, Cu, Zn, and Pb (Table 9). The exception to this trend is Hg, which exhibited a 30% reduction in its emission rate when the coal + TDF mixture was burned. The data further document reductions in NO and organic compound emissions, including PAHs, where the most drastic decrease was observed for dioxins and furans. On the other hand, emissions of CO, S02, and HC1 increased considerably with the addition of TDF (Table 9). The total particulate emissions from combustion of the blend were only slightly greater than those from pure coal. Carrasco et al. (1998) used their data to model atmospheric dispersion of the emitted contaminants in the vicinity of the... 

TCDD and other chlorinated dibenzo-J -di-oxins are released during the combustion of many polychlorophenols and also occur as contaminants in various chemicals such as the herbicide 2,4,5-trichlorophenoxyacetic acid. Most high-level exposure to 2,3,7,8-TCDD and other dioxins results from accidental releases or explosions in chemical plants or storage facilities for dioxin-containing chemicals. Because of the persistence of dioxin congeners in the environment and their potential for bioaccumulation, exposure may occur via the soil, air (especially when dioxins occur as combustion products), or water. When bound to components of the soil, the health hazard from 2,3,7,8-TCDD is reduced compared to ingestion of the pure compound. However, its bioavailability varies with the specific media in which it occurs. 

Concern about dioxin s effect on human health can be traced to a number of Industrial exposures. Its recognition as a byproduct In the production of large-scale quantities of chlorophenols such as 2,4,5- trichlorophenol (TCP) and pentachlorophenol (PCP), the finding of birth defects In animals exposed to 2,3,7,8- TCDD, and the recognition In the 1970 s of a more general environmental contamination due to emissions from combustion sources. 

TCDD and other dioxins which enter the atmosphere from combustion or from wind erosion of contaminated surface soils. 

It is now obvious that atmospheric transport of persistent toxic organic substances is the major pathway between ecosystems. For dioxin, volatilization of residues from contaminated soils was first noted as a concern at Seveso, Italy (4). The National Research Council of Canada reported that atmospheric emissions were the major source of chlorinated dioxins in the Canadian environment (5). A recent Ontario report estimates that from 8 10 kg of 2,3,7,8 -TCDD equivalents enter the Ontario environment annually from combustion of municipal refuse and sewage sludge and that all other combustion sources contribute from 20 -50 kg annually (6). The only other major source considered was from the use and disposal of chlorinated phenols. 

PCDD/Fs are emitted or spilled from many processes, but as they were found in the emissions from all combustion processes, thermal processes are recognized to be a main source, and a major public concern, especially waste incineration. But dioxins problem is attributed to all human activities, connected either with production or utilization of solid wastes and wastewaters. One of the crucial examples of the problem is sewage sludge contamination. 

Dioxins are formed as unwanted contaminants in a variety of combustion and manufacturing processes as well as through natural processes. The European Dioxin Inventory 1993-1995 showed that the contribution from the chemical industry was less than 1% of total dioxin emissions. 

On occasions, animal feed has been suspected of deliberate contamination. Incidents involving contamination of animal feed by industrial by-products such as polycylic aromatic hydrocarbons (PAHs) and combustion products such as dioxins are not uncommon. A problem with animal feed is that there is sometimes inadequate control over the provenance of feed constituents. For example, spent cooking oil from food-processing plants is a legitimate feed component. Unfortunately, the temptation for the unscrupulous to dispose of other unwanted oils in this way is too great for some. In many cases such adulterants are probably diluted to such an extent that they are undetectable by conventional chemical analyses. Nevertheless, they may still represent a longterm cumulative hazard to consumers of products from animals fed on such material. 

However several POPs, particularly the OCPs and dioxins, remain at low levels in the Australian environment and several remain persistent at low levels in body fats and fluids of Australians. The levels reflect the past use and persistence of OCPs in the Australian environment, contamination of the food chain and the capacity of the body to metabolise and store in body fats. The dioxins remain due to the ubiquitous nature of their sources with combustion as a major source and their persistence. Future trends are likely to mean very low-level residues in human fats of DDE, cyclodienes, HCB, HCHs and dioxins in the long term. Their rate of decline will probably depend on removing HCB from chlorinated industrial chemicals and OCPs from the environment (e.g. remediation of contaminated soils) by hazardous waste treatment methods (e.g. physical, chemical and biological degradation or fixation) or secure landfill. 

Ambient measurements confirm that environmental PCDD/F contamination is widespread and that virtually everyone - regardless of age, gender, or geographic location-is exposed to these compounds on a daily basis. We identify MSW incinerators as the major known source (48%) of environmental PCDD/Fs, followed by cement kilns, medical waste incinerators, wood combustion and pulp and paper mills. However, these known sources appear to account for only a fraction (10-30%) of the total annual atmospheric deposition of PCDD/Fs in the US. Thus, either significant unidentified sources of dioxin exist or most dioxin presently circulating in the atmosphere results from re-suspension and/or volatilization of historically deposited dioxin. 

In spite of some economical advantages, the idea of incineration is not accepted everywhere, the major problem being environmental hazards generated during combustion—the toxic gases and contamination of heavy metals (acid rain), dioxins and chlorine chemicals (mainly from PVC). Wherever incineration is used, plastics are not usually separated from the total waste, which reduces the energy recovery, and also creates more ash. Japan leads in the use of incineration of MSW at about 50% (including 67% of plastics wastes), compared to 30% in Europe and about 15% in the United States. 

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