MSWI bottom ash used as basement at two pilot-scale roads: Comparison of leachate chemistry and reactive transport modeling

Abstract

The recycling of municipal solid waste incineration bottom ash as aggregates for road basement requires assessing the long-term evolution of leachate chemistry. The Dåva (Sweden) and Hérouville (France) pilot-scale roads were monitored during 6 and 10 years, respectively. Calculated saturation indices were combined to batch test modeling to set a simplified geochemical model of the bottom ash materials. A common reactive transport model was then applied to both sites. At Hérouville, pH and the concentration of most elements quickly drop during the first two years to reach a set of minimum values over 10 years. The decrease is less pronounced at Dåva. The evolutions of pH and major element concentrations are fairly well related to the following pH-buffering sequence: portlandite, C–S–H phases or pseudo-wollastonite and, finally, calcite in equilibrium with atmospheric CO₂. Al(OH)₃, barite, ettringite and monohydrocalcite may also control leachate chemistry. Cu release is correctly modeled by DOM complexation and tenorite equilibrium. Temperature has no significant effect on the modeling of leachate chemistry in the range 5–30 °C, except at high pH. Effects at road edges and roadside slopes are important for the release of the less reactive elements and, possibly, for carbonation processes.

Introduction

The recycling of municipal solid waste incineration bottom ash (MSWI BA), as aggregates for road and car-park construction, may impact the environment – both soil and water resources – by releasing salts and heavy metals into the leachates (e.g. Kosson et al., 1996). Therefore, characterizing the long-term evolution of the leachate chemistry is an important facet of the environmental impact assessment of such reuse scenarios. Recently, in complement to a large body of data obtained with batch and column tests, a few reliable field data have been acquired in large scale experiments for several years (Åberg et al., 2006, Flyhammar and Bendz, 2006, Hjelmar et al., 2007, Lidelöw and Lagerkvist, 2007, Dabo et al., 2009). The intrinsic mineralogical heterogeneity of MSWI BA, as well as the variability in the climatic events, especially the rainwater infiltration regime, complicate the interpretation of leachate chemistry.

Several geochemical modeling studies have been published to interpret leachate evolution from MSWI BA submitted to batch and column lab tests (e.g. Meima and Comans, 1997, Park and Batchelor, 2002, Astrup et al., 2006, Dijkstra et al., 2008, Hyks et al., 2009). By contrast, only a few modeling studies have been devoted to MSWI BA weathering occurring at pilot-scale applications, either in lysimeter cell (Guyonnet et al., 2008, Mostbauer and Lechner, 2006) or landfill (Johnson et al., 1999, Baranger et al., 2002). However, such situations substantially differ from a road basement configuration. Edge effects that are typical of road structures cannot be correctly reproduced for instance. To the authors’ knowledge, only Apul et al. (2007) have developed a reactive transport model relevant for road basements but the complexity of the chemical processes has been simplified to a set of K_d parameters.

This paper aims at presenting the results of modeling of leachate from two pilot-scale road basements containing MSWI BA: the Dåva site in Sweden (Lidelöw and Lagerkvist, 2007) and the Hérouville site in France (Dabo et al., 2009), monitored for 6 and 10 years, respectively. The comparison of these two sites that were developed independently helps to shed light on common processes, but also discrepancies, in view of rationalizing leachate emission. In the first part of the paper, calculated saturation indices and solubility diagrams are combined to batch test modeling to set a simplified geochemical model of the two bottom ash materials. The second part of this paper attempts to develop a reactive transport model applicable to field conditions. A reactive transport model can help to identify and discriminate between the main hydrodynamic and geochemical processes and can be an useful tool for environmental impact assessments. The pH-buffering processes, and their effect on the evolution of leachate chemistry (major elements and trace metals) over time, are more particularly investigated.