Environmental Impacts Assessment of Recycling of Construction and Demolition Waste

Abstract

Construction and demolition waste (C&DW) is waste derived from the construction, demolition and renovation of buildings and civil infrastructure. With 900 million tons generated every year in Europe, it is the largest waste stream on the continent. C&DW is mainly constituted of mineral fractions, i.e. soil and stones, concrete, asphalt and masonry, and as such it has the potential to be used as aggregate in the construction sector. A typical application is in an unbound state as filler in road structures. This practice offers evident benefits in terms of resource savings, however it might lead to potential adverse impacts, especially related to the water-borne emission of pollutants, which need to be avoided. This requires first of all an estimation of their magnitude, and so the goal of this PhD is to provide an assessment of potential environmental impacts related to C&DW utilisation. C&DW is characterised by significant variability, especially in terms of leaching. Different levels of Ca, Ba, Cl-, Cr, K, Li, Mg, Na, Sr, Se, Si, SO4 and V are in evidence, depending on the ageing level (and therefore on the extent of carbonation) as well as the content of masonry. Both aspects may be optimised by appropriate measures within the C&DW waste management system, for instance by promoting source segregation of the concrete fraction stream or actively pursuing its carbonation. We found that leaching of Se, and to a lower extent Cr, Sb, SO4, Cl-, appears critical for C&DW in relation to existing national and European regulations. Cr mainly exists in C&DW leachates as anionic species, which may be interpreted as hexavalent species (mainly chromate). Despite being banned several decades ago, PCBs are still found in C&DW and in concrete raw materials, albeit in low, non-critical concentrations. This highlights their ubiquitous environmental presence. Several methods may be used to investigate leaching from granular C&DW, one of which is percolation tests. Compared to down-flow lysimeters with uncrushed C&DW, this study found that the use of standard up-flow columns, with materials below 4mm in particle size, may introduce differences especially in terms of pH, which in turn may affect the leaching of Al, As, Ba, Cu, DOC, Mg, Mn, P, Pb, Sb, Se, Si and Zn as a consequence of the crushing process, which results in the exposure of fresh, un-carbonated surfaces. However, when the scope involves quantifying cumulative release, standard up-flow columns may be considered appropriate, while for estimating early concentrations, relying on standard up-flow columns may be more problematic (e.g. Al, As, Cu, DOC, Mg, Mn, P, Pb, Sb, Se, Si and Zn), and the relationship between testing conditions and field conditions should be evaluated critically.Owing to its high toxicity and significant mobility, especially at high pH levels, Cr(VI) is one of the elements of concern found in C&DW leachates. Its fate in the sub-soil below road applications was assessed experimentally, and its vertical migration was then predicted through a model. Interactions with sub-soil particles, namely reduction to immobile Cr(III), are responsible for the retention of Cr(VI) in the first 70 cm of sub-soil below the C&DW sub-base. Temperate climates might inhibit the already slow reduction kinetics, resulting in Cr(VI) migration up to 2 m. The same case applies in situations characterised by high infiltration rates, such as unpaved roads, cracked asphalt cover or heavy rain events. By using holistic tools such as life cycle assessment (LCA) a general evaluation of the environmental consequences of C&DW utilisation system was provided. Although for most impact categories C&DW utilisation in road sub-bases does not provide environmental savings in absolute terms, it is generally less hazardous than when being landfilled (excluding toxicity impacts). On the other hand, landfilling appears better than C&DW utilisation when considering toxicity categories, owing to lower leaching in landfill scenarios over a 100-year time horizon. Oxyanions play a predominant role in leaching impacts, rather than cationic metals, and accurate modelling of Cr(VI) fate is essential to the results, while the heterogeneity of C&DW leachates does not play a crucial role in LCA results. C&DW carbonation leads to a trade-off between reducing global warming impacts and increasing toxic impacts related to the higher leaching of oxyanions. While leaching appears as the major problem relating to C&DW utilisation in LCA terms, uncertainties related to methodological aspects of leaching modelling in LCA should be acknowledged.