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Substance Flow Analysis for Dioxin 2002

1 Introduction

1.1 What are dioxins?

The phrase "dioxins" is typically used as a short designation of two groups of tri-cyclic, halogenated, organic compounds, of which some chlorinated compounds have turned out to be extremely toxic.

The first group covers the polychlorinated dibenzo-p-dioxins (PCDDs) and the polybrominated dibenzo-p-dioxins (PBDDs). As the number of halogen substituents may range from one to eight, the sub-group of chlorinated dioxins as well as the sub-group of brominated dioxins consist of 75 members or congeners, as they are named in this report.

The second group covers the dioxin-like "furans" or more precisely the polychlorinated dibenzofurans (PCDFs) and the polybrominated dibenzofurans (PBDFs). Again the number of halogens may range from one to eight bringing the number of congeners for the sub-group of PCDFs as well as for the sub-group of PBDFs up to 135.

To these groups of substances should be added the large groups of mixed brominated/chlorinated dibenzo-p-dioxins (PXDDs) and dibenzofurans (PXDFs) that consist of 1550 respectively 3050 different congeners /IPCS 1998/.

The chemical structure of dioxins and furans are shown in figure 1.1 below.

Figure 1.1
Chemical structure of 2,3,7,8-TCDD and 2,3,7,8-TCDF

1.2 Formation of dioxins

Formation of dioxins may take place at any combustion process based on natural organic materials including fossil fuels. This is due to the fact that chlorine and catalytic active substances such as copper are essential elements that will be present at least as traces in all kind of natural organic materials (but not necessarily in industrially manufactured chemical compounds). Larger quantities of chlorine, organic materials and catalyst should be expected to increase the amount of dioxins generated. Attention should be paid to a number of recycling processes involving metals, glass etc. that may lead to combustion of organic materials present like paint, plastic and dirt.

Other processes taking place above 250°C may develop dioxins in case precursors or organic matter as well as chlorine, oxygen and an appropriate catalyst are present. As organic matter could be present almost everywhere, at least as contamination, attention should be paid to all processes utilising halogenated compounds and taking place at relevant temperature levels. In reality only measuring may confirm or disconfirm formation of dioxins.

Whereas dioxins are likely to be decomposed at very high temperatures (above 800-1000°C) assuming adequate residence time at this temperature level, formation of dioxins may take place again at lower temperatures in the flue gas or on active surfaces by "De Novo synthesis". This sets the focus on all kinds of high temperature processes. The source of chlorine or bromine could be the material itself, assuming it contains such halogens that may be released to air during the process, or it could be the fuel. Attention should be paid to it that materials like clay and lime are sedimentary materials that naturally contain chlorine in the form of salts (chlorides), and that very small amounts of chlorine is needed to account for the content incorporated in dioxins.

For all thermal processes the presence of precursors may be anticipated to increase the probability of dioxin formation, and may reduce the need for catalytically active surfaces.

All processes involving chlorination of organic compounds or at which active chlorine is present together with organic matter may be regarded as potential sources of dioxin formation at temperatures below 250°C. Again only measurements may show whether dioxin formation actually takes place.

Photochemical and biological formation may be processes relevant to formation of dioxins in nature and by treatment of organic waste.

For all industrial and natural processes creating dioxins, it would be logical to expect dioxins to be present in all products or materials created by the process to the extent such products or materials actually contain organic matter. Accordingly, it would be logical to expect that all residues from combustion processes creating dioxins also contain dioxins. In case dioxins are created by the process of plastic manufacturing, also industrial products containing plastics should be expected to contain dioxins (has been confirmed for both brominated dioxins /IPCS 1998/ and chlorinated dioxins /Carroll et al 1999 quoted by Greenpeace 2000/). On the other hand glass and metals containing virtually no organic matter should not be expected to contain dioxins.

1.3 Toxicity equivalency factors for dioxins

Dioxins are always found in samples as a mixture of various congeners. The most toxic of the chlorinated dioxins is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The toxicity of other chlorinated dioxins is estimated relatively to 2,3,7,8-TCDD. Today only congeners with chlorine atoms in the 2,3,7,8-positions are considered to have toxic properties as TCDD.

Over the years a number of toxicity equivalency factor systems have been developed. The dominating system during the nineties has been the international system that was developed by a NATO-working group in the late eighties. This system replaced more or less the German UBA-system from 1985, the Nordic system from 1988 as well as older systems developed by USEPA.

Recently in 1998 a new system was developed and published by WHO /UNEP 1999/. This system gives in contrast to previous systems separate toxicity equivalency factors for humans/mammals, fish and birds. In table 1.1 the factors for the WHO, the Nordic, the German and the international system are listed.

It should be noted that the Nordic and the International systems are almost identical, whereas the German system as a very early system also assign toxicity to non-2,3,7,8-congeners. The new WHO-system differs strongly in the assessment of pentachloro- and octachlorodioxins.

The knowledge of brominated dioxins is less developed. On an interim basis WHO suggests that the current toxicity equivalency factors for chlorinated dioxins are also applied to brominated dioxins /IPCS 1998/.

In this report the international system will be used, mainly because most data reported follow this system.

Table 1.1
Important toxicity equivalency factor systems for dioxins

1.4 Properties and degradation of dioxins

However, all degradation processes apart from thermal degradation should be expected to be extremely slow. Preliminary estimates of degradation half-lives in nature indicate half lives in water and sediments ranging from around 30 years to around 200 years /Sinkkonen 1998/. In soil, it is generally accepted, that the half-life of 2,3,7,8-TCDD and other congeners is in the order of 10 years, which however may be due to physical loss processes like volatilisation, leaching of particles, oils and surfactants rather than degradation /Jones & Sewart 1997/. The fact, that dioxins have been detected in natural clay (reference is made to section 2.2.1) shows that dioxins have the ability under appropriate conditions to persist for thousands and maybe even millions of years.

A natural conclusion to be made based on this knowledge is that the fate of dioxins in industrial and residual products will depend strongly on the fate of the product itself. Logically it should be assumed that:

1. Dioxins integrated in products are likely not be degraded during the useful life of products.
2. A significant potential for circulation of dioxins between the technosphere and the environment exists.

Furthermore, attention should be paid to the risk that dioxins with a high number of chlorine or bromine atoms like octachlorodibenzo-p-dioxin that are relatively non-toxic, in nature or landfills might be degraded to the more toxic hepta-, hexa-, penta- or tetrachlorinated dioxins.

1.5 Sampling methods for measurements of dioxin emission

1.6 Basic assumptions for this investigation

Relation to the Danish paradigm on substance flow analysis (SFA)

This report is organised according to the Danish Paradigm for substance flow analysis (reference is made to /Lassen & Hansen 2000/). However, some adjustments to the paradigm have been necessary, as the paradigm is developed for substances used intendedly in products and causing emissions to the environment by manufacturing, use as well as disposal processes. The paradigm distinguishes between intended uses and non-intended used. Non-intended uses cover uses as a natural or anthropogenic contaminant.

By contrast, the use of dioxin can in no way - apart from laboratory purposes - be described as intended, and dioxins are by nature process related, although they may be present in products and materials contaminated by processes. Thus the choice has been made to organise the report according to processes and treat transport and fate by products as sub-items to the relevant processes.

Concepts and terminology

The SFA-methodology applied to dioxins in this report means that the following balance has to be considered:

Import + formation + extraction = export + destruction + emissions + stock building

The system considered is the Danish Society - or more precisely the technosphere within the Danish Society.

In geographical terms the system boundaries correspond to the national borders of Denmark. In temporal terms the boundary is 1 year taken as an average of 1998 and 1999, as most data utilised and in particular the statistical data describing activity levels relate to either 1998 or 1999. In case it has been necessary to use older or newer data, this is done based on the implicit assumption that such data are valid also to the period of 1998 to 1999.

Concerning the elements in the balance presented above, they should be taken as self-explaining perhaps with the exception of "stock building" that covers the change in the society’s stock of the substance in question that typically addresses the presence of the substance in products in use in the society. In case of dioxins the dominant type of product seems to be wood previously treated by pentachlorophenol.

Another concept normally used in SFA is the term "consumption" that covers the input into the society by end products. For substances used intendedly in products, the consumption is a key figure, as it indicates the total turnover of the substance in the society or by the product type in question. However, for dioxins formed un-intendedly by processes and to a significant extent emitted directly to the environment, the consumption by products is in itself not a relevant indication of the total turnover and needs to be supplemented by a calculation of the total formation of dioxin in Denmark.

Data reliability

It is recognised that dioxin formation is extremely process specific. This means that the local conditions of the individual manufacturing plants etc. with respect to actual raw materials and process conditions, flue gas cleaning and in particular temperature patterns in the flue gas cleaning system and chimneys have determining significance to the amount of dioxin created.

As a consequence, most confidence is generally placed with measurements from Danish plants, no matter whether they are few and may be regarded as spot tests rather than thorough investigations. Still they represent actual conditions in Denmark with respect to raw materials and process conditions.

Second most confidence is assigned to literature data available from comprehensive reviews like the European Dioxin Inventory, as these data build on many data from different countries thus reflecting the typical variation caused by different process conditions, besides that the data presented has been reviewed by competent persons.

The lowest level of confidence is assigned to individual literature data covering one situation or country only, as these data may in worst case deviate from the true Danish figures by several orders of magnitude due to different process conditions.

Whereas these considerations have guided the overall strategy for assessment of data reliability basically a case by case assessment has been performed. In some cases, it has not been possible to be critical, as only a few data – if any – were available. In the assessment attention has also been paid to the fact that the factors determining dioxin formation may be subject to variations at the individual plants (one will typically distinguish between "normal" and "deviating" process condition, where the deviating conditions may include start up and close down operations as well as other problems occurring during operation), and one should be prepared to expect significant variations in dioxin formation also for individual plants.

Although steps have been taken to improve the existing knowledge on dioxin formation and emission in Denmark by measurements, the number of analyses available to Danish plants is still limited, and the assessment presented in this report is primarily based on literature data originating from other countries. In adopting such figures for Danish conditions it has been considered more correct to rely on minimum and maximum figures than on average figures, as it is not known to what extent the data available is representative of Danish plants, and average figures would give a false impression of the accuracy of the estimates presented. A consequence of this approach is the very high intervals of uncertainty that typically have been accepted.

Dealing with uncertainty

In the update of the substance flow analysis the number of Danish measurements has grown and it has been considered practical to develop some general guidelines for assessment of data uncertainty. The aim of the guidelines is to obtain consistency in the treatment of data.

The guidelines appear in table 1.2. The measurements are evaluated in each case, which means that deviations from the guidelines might occur.

Table 1.2:
Guide lines for estimating uncertainty of measurements.

With only one Danish plant and one or two measurements from this plant the emission level is not satisfactorily covered, but it is assumed that the measurements can be used to indicate the emission level when an uncertainty interval of ± factor 3 is used. The uncertainty interval of ± factor 3 is a preliminary estimate of the range in which the emission is expected to be available. This estimate is primarily based on Danish experiences with waste incineration plants. It is considered that the interval limits can be regarded as a 90 % confidence interval around the true value.

If there exists more than one Danish plant and only one or two measurements are present, the emission level derived from the Danish data is considered to be too uncertain. The emission level will then be compared with the emission level stated in SFA 2000 /Hansen, 2000/.

With more than two measurements from the same plant the mean value, minimum and maximum is calculated on the basis of a 90 % confidence level. With more than two measurements and more than one plant the same statistics will be made. The calculated values will be compared to the emission level from SFA 2000 /Hansen, 2000/, as the uncertainty is larger in this situation because the measurements have not necessarily been made on all Danish plants.

Brominated dioxins

Measurements on brominated dioxins and in particular congener-specific measurements are relatively few, but in autumn 2002 the National Environmental Research Institute in Denmark has carried out such measurements on flue gas from Vestforbrænding, a major Danish waste incineration plant and from Kommunekemi, the central treatment plant for hazardous waste. These measurements are in this report used to estimate the total annual emission of brominated dioxins from waste incineration and from treatment of hazardous waste. Furthermore an attempt has been made to estimate the consumption of brominated dioxins present in electronic goods due to the use of brominated flame retardants. The estimate shows that the turnover of brominated dioxins most likely is significant, and it can only be recommended to devote efforts to improve the general knowledge of this issue, and in particular to develop analytical procedures that would allow quantification and evaluation of the relevant flows.

Destruction of dioxins

Destruction of dioxins is an issue normally not addressed by dioxin inventories, but anyhow relevant to include in a substance flow analysis. Destruction of dioxins in a modern society will primarily be related to incineration and other high temperature processes at which dioxins will be exposed to temperatures of 800-1000ºC or above for an adequate period of time. However, only little knowledge seems to be available on the exact rate of destruction by different processes.

For municipal waste incineration /UNEP 1999/ states that incomplete destruction or transformation of dioxins present in the incoming waste is not relevant to consider as a source for dioxin emission from modern incineration plants. Several studies quoted in /Dam-Johansen 1996/ indicate efficient destruction of dioxins by waste incineration although exact destruction rates are not stated apart from a few laboratory investigations resulting in complete destruction.

Based on this knowledge it is in this report assumed that dioxins directed to modern waste incineration plants and other high temperature plants operating at temperatures around 1000ºC or above like brickworks and cement manufacturing plants will be completely destructed. It is emphasised that the destruction rate will most likely never be exactly 100%, and so far it is not possible to say whether the destruction rate of the individual plant will be down to 99% or even further below, as the destruction rate should be expected to depend on plant design as well as conditions of operation. Thus, the assumption of complete dioxin destruction by high-temperature processes used in this report should be taken as an indication that such processes should generally be expected to result in a significant destruction of dioxins, but not as a documentation that complete dioxin destruction will always be obtained by any high-temperature process.

Finally it is emphazised that dioxin after destruction is often generated again later on in the chimney and the flue gas cleaning process either by "De Novo synthesis", formation from precursors or in some cases by chemical reactions below 300ºC.

1.7 Guidance for reading the report

The updating of the substance flow analysis for dioxin has been made through supplements and changes to the report from 2000 /Hansen, 2000/. This method is chosen because the original substance flow analysis from 2000 is still relatively new.

Table 1.3 summarises the sections in the report where new data have resulted in changes, meaning that the data presented in this report are different from the data in SFA 2000 /Hansen, 2000/. The amount of Danish investigations is divided into three categories: None, some and good.

Tabel 1.3
Changes in the updating of the Substance flow analysis for dioxin.