Assessment of management strategies for different types of waste thermal treatment bottom ash based on modelling of experimental leaching data

Thermal treatment of waste materials such as commingled municipal solid waste, selected fractions of municipal solid waste (e.g. refuse derived fuel, RDF or Solid Recovered Fuel, SRF) or special waste (e.g. hospital waste), in Waste to Energy (WtE) plants (e.g. combustion or gasification facilities), generates different types of solid residues, among which the most abundant are bottom ash (BA), which generally account for 10-20 wt.% of the input waste mass. This slag-like material is produced worldwide in large quantities. Over the past few decades, in some European countries (e.g. Denmark and The Nederland), BA from incineration of Municipal Solid Waste (MSWI) is increasingly being recycled in construction applications (e.g. in road foundations, as aggregate in concrete or embankments) due its suitable physical (e.g. bulk density, particle size) and mechanical properties (e.g. compressive strength). However, the main concern regarding the utilization of BA is related to possible contamination of the environment due to potential release of harmful compounds upon contact with water (i.e. its leaching behaviour). In fact, compared to natural aggregates, BA generally exhibits higher concentrations of contaminants (e.g. metals, metalloids and salts) that, depending on its specific characteristics and the environmental conditions to which it can be subjected to, may be more or less mobile. Thus, to ensure that the utilization or disposal of this type of residue is environmentally safe, its leaching behaviour in terms of the release of salts and toxic metals needs to be thoroughly investigated. Standardized laboratory leaching tests are frequently used tools to assess the potential release of contaminants from BA. Results from leaching tests alone might not be sufficient to predict the overall environmental impacts resulting from the reuse/disposal of BA; however, these may be used as input data for evaluation tools, such as risk assessment and life cycle assessment (LCA), which allow to quantify potential impacts on the environment and human health. The main objective of this doctoral thesis was to evaluate management strategies for bottom ash (BA) produced by three different types of waste thermal treatment plants, paying particular attention on the environmental consequences associated with the release of contaminants from these residues in the specific disposal/reuse scenarios selected. This evaluation was achieved by combining the most significant results obtained applying different types of leaching test (i.e. column percolation tests, batch tests as a function of L/S ratio, compliance leaching tests and pH-dependent leaching tests), with assessment methods such as LCA and risk assessment. Particularly, two different management scenarios, namely landfill disposal and reuse as unbound filler material in a road sub-base construction, were evaluated. BA samples generated from (i) a refuse derived fuel incineration plant (RDF-I BA), (ii) a refuse derived fuel gasification facility (RDF-G BA) and (iii) a hospital waste incineration plant (HW-I BA), were considered. These specific types of BA were selected since their leaching behaviour and potential alternative management options have been significantly less investigated up to now compared to MSWI BA. The results of the experimental activity indicated a fairly lower release of contaminants, as a function of both the L/S ratio and pH, for the RDF-G BA, which showed to comply with acceptance criteria for inert waste landfilling. While HW-I BA and RDF-I BA only met the limit values set for disposal in non-hazardous waste landfills. However, the latter type of BA (i.e. RDF-I BA) displayed a generally higher release of amphoteric metals (i.e. Pb, Zn and Cu) and chlorides. Moreover, a remarkable difference in the acid neutralization capacity (ANC) of the RDF-I BA compared to the other two types of BA, was observed. Indeed, while RDF-G BA and HW-I BA showed an almost negligible ANC, which was associated to their mineralogy mainly made up by amorphous phases, RDF-I BA displayed a significant ANC for pH values between 11 and 12, due to the abundance of hydrated phases detected in its solid matrix. In order to decrease the leaching of contaminants from the RDF-I BA the effect of (i) the removal of the fine particle size fraction (d<0.425mm), which showed a rapid and high release of contaminants, from the bulk sample of RDF-I BA and (ii) a natural weathering treatment, carried out at laboratory scale for a time period of 12 months, were also investigated. While the first treatment showed not to be effective, since the fine particle size fraction represented only 10% by weight of the total sample of the RDF-I BA; the weathering process has led on the one hand to a relevant decrease of the release of Ba, Cu and to a lower extent of Pb, but on the other hand showed to favour the mobilization of oxyanion-forming metalloids such as Cr and Mo. From the comparison between batch and column percolation tests as a function of the L/S ratio, carried out on each type of BA, a generally higher release (of up to two orders of magnitude) was observed for the batch tests. This finding was mainly attributed to the fact that, differently from the batch experiments, in the column tests the continuous renewal of the leachant solution can lead to non-equilibrium release conditions. This was also confirmed by the interpretation of the column test results with an analytical model that showed that most of the observed leaching trends could be described quite well assuming the contaminants release to be limited by mass-transfer. Finally, the results of the LCA indicated that when assessing possible management strategies for BA, its environmental properties should be surely included, since the leaching behaviour showed to significantly affect environmental impacts, especially with respect to toxicity-related categories, proving to vary case by case depending on the type of BA considered. This finding may have important implications for the management of the analysed types of BA, since up to now at least in Italy the same strategies (i.e. mainly disposal in landfills for non-hazardous waste) were applied for all kinds of bottom ash. However, it appears that, depending on the specific origin of the BA, alternative management options such as reuse in road as unbound material may also be viable. In particular, from an environmental perspective, the residues that showed the lowest impacts and hence may potentially be the most fit for reuse applications are the RDF-G BA, although as also found by LCA its negligible ANC could represent a limiting factor. Anyhow, from the risk assessment study, the obtained results highlighted that reuse in road may be a suitable alternative to landfilling also for the other two types of BA (i.e. RDF-I BA and HW-I BA). In fact, when the contaminants attenuation factors (i.e. LDF and SAM) were considered, the concentration values of metals and inorganic compounds estimated in the groundwater table, for a time period of 100 years, have shown not to exceed the limit values for groundwater protection set by the Italian legislation.