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New Lease of Life for Hazardous Waste
October 1, 2010   
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A thermal method for turning sludge into a lightweight, environmentally safe synthetic aggregate is the latest eco-innovative technology that has brought Polish scientists international acclaim.

The technology and the aggregate produced with it won gold medals at the 57th Eureka World Exhibition of Innovation, Research and New Technologies in Brussels and at the Concours-Lepine international invention show in Paris. Another gold medal was granted at the 3rd International Warsaw Invention Show. The technology has also been named Innovation Leader 2009 in a nationwide innovation contest, and received an award from the Ministry of Science and Higher Education.

The technology was developed at the Institute of Mechanized Construction and Rock Mining (IMBiGS) in Warsaw, a research center with a long record of innovative ideas.

The institute’s director, Stefan Góralczyk, Ph.D., and Janusz Oleszczak, a senior executive responsible for development, received special awards at the Eureka exhibition in 2008 for their inventions and innovative research as well as promotion of innovation in Poland and abroad.

Eco-friendly recycling

The institute specializes in research on the eco-friendly recycling of waste. The method for making synthetic lightweight aggregate from waste treatment plant sludge is part of this trend. The new thing about the method is that it exclusively uses waste as the input raw material. The method’s inventors decided to use different kinds of waste and neutralize it in a single process, the result being a lightweight aggregate for a wide range of commercial applications, fulfilling all safety requirements—absolute non-toxicity of the product in particular.

The inventors chose sludge as their main raw material, to which they added mineral mining waste (coal mud) and municipal waste (small glass particles). Aggregate is obtained by means of thermal synthesis of waste, which yields a silicate sinter with the properties of lightweight aggregate.

The most important aspect of the method is that heavy metal compounds present in the input material are permanently built into the structure of the silicate, forming silicates just like those found in natural minerals. Thus, there is no danger that they will contaminate the environment during crushing or other mechanical processing. Furthermore, the method can even be used for waste containing large amounts of heavy metals and other hazardous substances, the inventors say.

Turning problems into profits

The method utilizes sludge in exactly the same form as it is found at dump sites, that is thickened exclusively using mechanical methods, without drying. Water, which accounts for about 80 percent of sludge mass, allows a granulate to be formed, turning it from a bothersome factor that needs to be removed in other thermal technologies into a desirable one. The method takes advantage of the special properties of individual waste fractions, allowing the thermal process to be conducted at temperatures of 900-1100 degrees Celsius, or about 400 degrees less than previously known methods of sintering materials to obtain aggregate. This makes production less energy-intensive, improving its profitability. Another upside of low-temperature synthesis is the smaller probability of dioxin emissions.

The lightweight aggregate obtained from municipal/industrial waste and mining sludge is an eco-friendly product that does not release any foreign chemical compounds into the environment, the inventors say. The product contains no substances that react in the natural environment, and its properties are the same as those of aggregates produced from natural raw materials, as shown by studies on hazardous substance leaching carried out at the Institute of Mechanized Construction and Rock Mining. The aggregate is suitable for use mainly in construction—for making lightweight concrete, structural concrete components, mortar, and bituminous mixtures. It can also be used in farming as a substrate for crops and in environmental protection as an insulation and drainage material.

Deluge of sludge

Neutralizing and utilizing sludge from waste treatment plants is a global problem. For many years, this waste has been used in agriculture. However, because municipal and industrial waste was discharged together, sludge was polluted with heavy metal compounds, organic substances, pathogenic bacteria, fungi and other hazardous materials. This means that biotechnologies processing such sludge do not meet present-day requirements for using them directly in agriculture. European Union environmental protection regulations practically preclude the use of such waste in farming; they also restrict the possibilities for storing the sludge. Meanwhile, with the growing number of waste treatment plants, the amount of sludge deposited there is growing exponentially. Estimates show that the amount of sludge will almost double over the next 10 years.

At the current state of knowledge, the safest way of neutralizing sludge is to utilize it with the use of thermal methods. Before incineration, however, the sludge has to be dehydrated; sludge drying and incineration uses up a lot of energy, making it a costly option. On the other hand, the calorific value of hydrated sludge is negative—after stabilization it reaches a calorific value of about 2-3 MJ/kg, whereas complete incineration, without adding extra fuel, is possible when the sludge’s calorific value is 8 MJ/kg. Moreover, incineration causes emissions of hazardous gases such as dioxins, so installations have to be fitted with waste gas after-combustion systems. A large amount of ash is left after the process, which cannot be used for anything due to the content of hazardous substances and can only be stored at dump sites.

Currently used technologies process the products of sludge incineration through hardening the ash in cement compositions, turning it into a solid that is later stored in the form of blocks at landfills. Cement compositions are used to build roads, deep foundations and enclosures for landfills. Such procedures have been used in practice for many years, but still raise controversy because of the danger related to the corrosion of concrete and the leaching of toxic substances from concrete containing sludge.
Ewa Dereń
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