Incineration and fixing of toxins

The Swedish Association of Waste Management Continuous effort is being made to improve waste incineration as a means of dealing with household waste and other combustible material, while also producing valuable energy. The means to this are further reduction of emissions to air, and ensuring effective long-term deposition of ashes and other residual products from the waste incineration process. The Swedish Association of Waste Management (Svenska RVF) has undertaken the biggest study to date into dioxins and waste incineration in Sweden. It shows that dioxins found in the residual waste from incineration are solidly fixed. This breaks the ecocycle of the dioxins in the waste. As a result, incineration and production of energy from waste can be considered a good way of dealing with combustible waste. The results are presented in this article. Modern society generates large amounts of waste. Over the years, incinerating waste to generate energy has become the most common method of dealing with combustible waste efficiently.In the mid-1980s, environmental problems resulting from the increased incineration of waste came to the fore. Regulations covering emissions to air were therefore tightened up dramatically. Waste incineration plants in Sweden were rebuilt to include highly advanced flue gas purification systems. The main aim of these measures was to reduce emissions of heavy metals, acidic substances and dioxins into the air. It is clear today that the construction of more environmentally friendly products, source separation of products such as batteries, and environmental protection initiatives made at the plants have together produced positive results. FIGURE 1. Treatment of household waste in Sweden, 1980-1999(thousands of tons/year)In autumn 1999 and spring 2000, Svenska Renhållningsverksföreningen (RVF) and waste incineration companies carried out an extensive investigation into dioxins in residues from waste incineration. Samples were taken at 21 of the 22 plants in operation in Sweden during 1999-2000. From the survey, it can be seen that discharges of mercury and cadmium in Sweden decreased by 99% between 1985 and 1999. Discharges of lead have been reduced from 25,000 kg a year to 35 kg a year, and zinc from 54,000 kg to 90 kg a year. At the same time, waste incineration has increased by 35%, and energy generation has more than doubled.Today, emissions of dioxins to air in Sweden are barely 3 g a year, compared to 90 g a year in 1985. Within a few years this figure will have been reduced to 1 g a year. FIGURE 2. Waste incinerated at the waste incineration plants in Sweden,1999 (tons) Where do dioxins come from? Dioxins have never been produced for commercial purposes, unlike some other environmentally harmful substances. Dioxins are formed in many different activities and processes in society, and are also formed in nature, both in natural and influenced conditions. For example, dioxins can be formed during incineration of organic material, from wood-burning stoves to the internal combustion engine. FIGURE 3. Waste incineration and energy production at Swedish waste incineration plants, 1985-1999 (tons and TWh)Bearing in mind that some of the dioxins are exceptionally harmful, stringent demands are in place to ensure that people are only exposed to extremely small doses of these substances. The rules set out by the National Swedish Food Administration, for instance, state that a person who weighs 70 kg should not be exposed to more than 350 pg (1 pg = 1x10-12 g) of dioxins a day.The main problem with dioxins and other stable organic environmental toxins - apart from their toxicity - is the fact that they are enriched in the food chains. Humans primarily encounter environmental toxins in the food they eat, and the same goes for dioxins. The amount of dioxins an individual is exposed to is determined by the dioxin content in their food, and by how much food they eat. The greatest source of exposure, despite very low dioxin content, is dairy products, as they represent such a large proportion of our diet. The dioxin content in air, water and vegetables, on the other hand, is so low in Sweden that they are not significant dioxin sources. FIGURE 4. Discharges of mercury and cadmium from waste incineration in Sweden, 1985-1999 (kg per year) Dioxin sources in society As dioxin is one of the most harmful environmental toxins, great efforts have been made to survey its occurrence. However, there is great uncertainty concerning the size of its sources and their significance from a health and environmental perspective.The RVF study contains a list of the dioxin content in various sources. It shows that, per annum,10-31 g of dioxins are emitted to the air from industry, 2.8-30 g from fires at rubbish tips, 3.7-22.5 g from energy production, 3 g from waste incineration and 0.8-2.9 g from traffic. Waste-to-energy plant at Malmö, Sweden. Photo: Sysav The quantity of dioxins given off in the waste incineration process has been greatly reduced, as emissions to air decreased from 90-100 g a year in the mid-1980s to 3 g a year in 1999. It is now estimated that the incineration of waste accounts for 5-6% of the total emissions to air. Other equally large, if not larger, sources of dioxins are the incineration of biofuel and emissions from uncontrolled fires at rubbish tips.A major advantage of incinerating waste is that the great majority of the dioxins are collected and deposited in ash and other residual waste from flue gas purification. The risk of the dioxins leaking out and polluting the environment is very low. Dioxins can be spread worldwide on air currents. Sweden is exposed to considerably greater levels of dioxins from other countries than it emits itself. Dioxins to air The efficient flue gas purification systems at the Swedish plants have in most cases already reduced the dioxin content in outgoing flue gases to below the EU's future emission limits, and in some cases, well below these limits. This means that, at most of the plants, over 99% of the dioxins are captured from the flue gases. All existing and new waste incineration plants must meet the EU's requirements by 2005. FIGURE 5. Emissions of dioxins to air from different areas of activity in Sweden, 1993 (grams) Dioxins in slag and residual products from flue gas purification Slag contains very small amounts of dioxins. Researchers in Sweden estimate that the 370,000 tons of slag separated at the waste incineration plants in 1999 contained 5-10 g of dioxin. These are solidly fixed to the particles, enabling the slag substance to be reused in road-building material, for example. FIGURE 6. Dioxin contents (Eadon and I-TEQ) in residues from flue gas cleaning at waste incineration plants in Sweden, 1999 (ng/gram)Over 90% of the dioxins are found in the residual waste from flue gas purification (ash and sludge). These remains are classified as hazardous waste and can therefore only be dumped at landfill sites with high safety requirements. This means for example that these residual products must be separated from other waste, and that special protective measures must be taken to prevent the leaching of dioxins, heavy metals and other pollutants.Dioxins in the residues from flue gas purification and slag are very solidly fixed and therefore do not leach out. Studies show that the background values for dioxins in rainwater and fall-out are at the same low level as the dioxin contents in leachate. FIGURE 7. Quantity of dioxins (Eadon and I-TEQ) in residues from flue gas cleaning at waste incineration plants in Sweden, 1999 (grams) Dealing with residual products Developments in waste incineration have led to considerable reductions in emissions of dioxins, heavy metals and other pollutants into the air. The pollutants that were previously emitted out through the factory chimneys with the flue gases are - to the extent that they are not broken down - stored in slag, ash and other residual products from flue gas purification. The focus in the dioxin issue has shifted to how the residual products are dealt with.As a stage in the Swedish Association of Waste Management's work to improve knowledge and the decision-making basis for measures to be taken, a study was carried out during 2000. The aim of the study was to establish whether dioxins in residual products leach out at the landfill site, and to find out the contents and quantities of dioxin in residual waste from flue gas purification. Samples were taken at all of Sweden's waste incineration plants, and the results reveal a relatively wide variation in the plants' dioxin content values.The relatively large variations between different plants could be due to differences in technical design, operating conditions, waste composition and other local differences. There is also considerable unreliability in the accuracy of the sampling and analysis methods, as the amounts to be measured are exceptionally small. What happens when the waste is incinerated? Waste generated by modern society is a reflection of production and consumption. The waste will therefore contain complex material including small amounts of mercury, cadmium, dioxin and other pollutants.The aim of waste incineration is to break the harmful ecocycle of heavy metals, dioxins etc. and to close favourable ecocycles of material and energy. To achieve these aims, modern waste management functions in the following way: source separation and special handling of waste not suitable for incineration the waste is collected, sorted, stored and fed into the incinerator incineration with energy recovery the flue gases are cleaned (several methods may be used) water used in wet flue gas purification is cleaned it is primarily district heating that is generated, though also electricity in some cases the slag is sorted and can be used in building materials ash and other residual products from the flue gas purification process are handled as hazardous waste. Taking this unreliability into consideration, analysts say that the average dioxin content found in remnants from flue gas purification following the waste incineration process could be 2-3 ng/g (1 ng = 1x10-9 g) for Sweden as a whole.This would mean that the total amount of dioxin fixed in the remnants from flue gas purification in Sweden is 110-120 g.However, the relatively great variations are of minor importance, as the study also shows that the dioxins are so solidly fixed in slag and residual products from flue gas purification that the risk of them leaching out is extremely low. The new Malmö waste-to-energy plant, Sweden is to begin operation this year, complementing the original facility. Photo: Sysav Swedish waste incineration - facts and figures In 1999 there were 22 plants that incinerated household waste and other combustible waste in Sweden. In the same year, 1.9 million tons of waste were handled and incinerated. Of this, 1.3 million tons were household waste, 0.5 million tons industrial waste and 0.1 million tons waste wood.All plants are fitted with equipment for dry purification of the flue gases and efficient separation of dust into different types of filter. Half of the plants are equipped with wet purification with flue gas condensation, and several more are being extended or are planned for extension. All plants also have equipment for reducing emissions of nitrogen oxide.The waste incineration plants produce a great deal of energy. The incineration process supplies 10% of the country's district heating needs. Energy production doubled between 1985 and 1999. The energy generated represents an important source for Sweden's energy supply. Future improvements A continuous effort is being made to further improve waste incineration with energy generation as a way of handling combustible waste while also producing valuable energy. The main aim of this is to reduce the content of dioxins and other hazardous substances in products that become waste after use, to reduce the formation of dioxins, to further reduce the already low emissions to air, and to ensure the effective long-term deposition of ashes and other residues from the waste incineration process.The construction and reconstruction of plants, using newer technology, will further reduce the dioxin content in residual products from flue gas purification, and reduce dioxin emissions into the air to 1 g a year, even though more waste will be incinerated. FIGURE 8. Dioxins in residues from flue gas cleaning at waste incinerationplants in Sweden, 1999 (µg per incinerated ton of waste, µg per MWh ofenergy produced)Much is known about dioxins and waste incineration. On the other hand, more reliable information about dioxin contents and amounts in waste is required as a basis for continued measures. RVF believes that Sweden's National Environmental Protection Agency should assume overall responsibility for future studies and research. The study on which this article is based was carried out by engineer NILS AHLGREN, an independent consultant in energy and the environment, and PROFESSOR STELLAN MARKLUND of the University of Umeå, Sweden, who has a doctorate in dioxins and conducts research into incineration technology and environmental effects.e-mail: info@rvf.se