The possibility of using these ashes for construction purposes is limited by their often high contents of potentially toxic metals. The use of ashes may however also be environmentally advantageous since it reduces the need to run waste to tip, and also reduces the mining of rock which, when crushed, is the most common aggregate material at present.
The aim of the project was to improve the data available for the environmental assessment of incineraiton waste residues, especially bottom ash from the incineration of waste (" municipal solid waste incineration MSWI bottom ash") and from the burning of biofuels.
A method for life cycle analysis (LCA) was developed as a way of assessing the environmental impact of using ashes as aggregates in roads. This LCA method took into consideration both the leaching of metals and other types of emissions to air and water (e.g. CO2) and the use of resources in dealing with the materials. Case studies were made for the following scenarios: i) in asphalted roads with or without the use of MSWI bottom ash in the subbase, ii) three different ways of dealing with MSWI bottom ash; through road construction, running to tip or as drainage layers in tips; iii) three different ways of dealing with forest fuel ashes; through road construction, running to tip, or return to forest land. Since earlier research, especially in Holland, identified copper as a metal that can occur in elevated concentrations in leachate from MSWI bottom ash, the chemical forms of occurrence of copper in the ash were studied through x-ray absorption spectroscopy, and in the leachate by using ion selective electrodes. Column tests were also carried out to evaluate what effect salting of roads and the inflow of natural organic matter from the ambient environment can have for the leaching of metals from MSWI bottom ash in a road structure.
According to the results of LCA, the environmental impact of these scenarios is different, especially with regard to the use of resources and energy. Use of MSWI bottom ash in road construction and return of forest fuel ash to forest land resulted in less use of natural resources and energy than in the other options. There was a risk of leaching of potentially toxic metals regardless of how the ashes were dealt with. Leaching of metals, especially Cu, was identified as a relatively important aspect in the environmental assessment of the use of MSWI bottom ash.
Through x-ray spectroscopic studies of several weathered MSWI bottom ash samples, CuO (copper oxide) could be identified as the quantitatively most important copper mineral phase. In the leachate from MSWI bottom ash, copper was mostly bound as a complex to dissolved organic matter (DOM). The hydrophilic components of DOM were more important for copper complex formation than had previously been assumed.
On the basis of these results, the models for Cu complex formation to the organic matter in leachate could be improved. Addition of salt (e.g. through road salting) and organic matter from the surroundings influenced leaching of metals from MSWI bottom ash to only a small extent.
The results of this work will improve our estimates of the leaching of metals and the bioavailability of these metals. The work also demonstrates the importance of broadening system boundaries in conjunction with the environmental assessments of ashes, so that the environmental advantages and drawbacks of different management options may be elucidated. In conjunction with decisions regarding the use of incineration residues, the use of a life cycle assessment is recommended, in combination with more detailed risk assessments.
The doctoral student in this project was Susanna Toller (Olsson up to October 2008).
More details of this work can be found in several scientific articles and a doctoral thesis.
Literature:
Olsson, S., Kärrman, E. and Gustafsson, J.P. 2006. Environmental systems analysis of the use of bottom ash from incineration of municipal waste for road construction. Resources, Conservation and Recycling 48, 26-40.
Olsson, S., van Schalk, J.W.J., Gustafsson, J.P., Berggren Kleja, D., and van Hees, P.A.W. 2007. Copper(II) binding to dissolved organic matter fractions in municipal solid waste incinerator bottom ash leachate. Environmental Science and Technology 41, 4286-4291.
Olsson, S., Gustafsson, J.P., Kleja, D.B., Bendz, D., and Persson, I. 2009. Metal leaching from MSWI bottom ash as affected by salt or dissolved organic matter. Waste Management 29, 506-512.
Toller, S., Kärrman, E., Gustafsson, J.P. and Magnusson, Y. 2009. Environmental assessment of incinerator residue utilisation. Waste Management, accepted manuscript.