Biostabilized waste from mechanical-biological treatment: environmental behaviour and recovery/reuse opportunities

In the last decades, the European Landfill Directive 1999/31/EC required Member States in changing their approach about waste management, in order to minimize, as far as possible, the amount of the landfilled wastes. In this regard, the organic fraction of Municipal Solid Waste (MSW) plays a key role, because of the huge global production of alimentary scraps from meals preparation and consumption, which is prevalent in the big cities and in areas that are densely populated. These large quantities represent a problem, not only because of the consumption of land where these waste are stored: the aerobic micro-organisms that naturally are in the organic fractions can easily degrade them in few days, leading significant negative environmental effects in terms of greenhouse gas emissions and pollution of surface waters and groundwater, soil and atmosphere. It is for this reason that it is mandatory for the Directive 1999/31/EC (and even more its subsequent modifications) not only the reduction of biodegradable waste but also their proper recovery and/or treatment before landfilling. The way the organic wastes are collected strongly influences all the next phases in their management, biological stabilization included. Organic waste that are collected separately from the rest of the urban waste can be aerobic/anaerobic biostabilized to produce compost and renewable fuel (biogas). Compost is an attractive option from the point of view of material recovery, as it can be used in agriculture as a soil improver; furthermore, several works investigated the possibility of using substances extracted from compost, in industrial activities or in soil remediation with positive feedbacks. Conversely, when the organic fractions are not separately collected, they need to be sorted from MSW stream and processed in a mechanical-biological treatment (MBT) plant. Here it is possible to recover ferrous and not-ferrous metals, that can be used by industries as secondary raw materials, and separate scraps and other dry materials (as plastics and papers), that can be easily burnt to recover thermal energy. The residues of these operations are finally aerobic/anaerobic treated, in order to break down the degradable substance content and ii Abstract fulfil the criteria imposed by the European Community. Focusing on the aerobic processes, several days are required for the proper treatment of the organic waste: the aerobic microorganisms quickly consume most of the organic carbon that is in the wastes in the firsts 4 weeks (under forced aerated conditions) and further 4-6 months (under natural aerated conditions) are needed to complete the biostabilization. Even though the first stage is required for both composting and MBT for their effectiveness, the secondary aerobic stage described above (named maturation) is performed only to improve the stability degree of compost. In this way, no further reductions in organic matter content occur after treatment, so that producers and users can be confident that potential contaminants (such as metals) have reached their maximum concentration. Conversely, since the MBT output is mainly employed for landfill cover due to the higher content of non-compostable materials and metals compared to compost, there is no need to reach so high levels of biological stability, and the maturation stage is not required. The PhD activity aimed to investigate the possibility of recovering/reusing the biostabilized wastes produced in MBT plants (in the following called BSW) and avoid, as much as possible, their utilization solely as landfill cover. To this aim, the followed steps were carried out: • A screening model has been developed to quickly evaluate the metal release from BSW to better evaluate their environmental behaviour; • The mechanical-biological treatment has been modified with the addition of a further aerobic stage, during which the main parameters related to environmental behaviour have been monitored, in order to evaluate possible improvements of the characteristics of the final output; • The possibility of recover BWS has been evaluated by extracting humic substances from this waste. First, considering that tests on percolation are often expensive and time-consuming, a simple method to anticipate the metal release from biostabilized waste that can be expected from percolation column tests is introduced. The method is based on the combination of total organic carbon (TOC) with a model that allows simulating the release of the organic carbon as a function of the liquid/solid ratio applied. Next, Abstract iii considering that in organic-rich wastes the leaching pattern of many metals is very similar to the one observed for the dissolved organic carbon (DOC), it is possible to derive specific correlation coefficients between DOC and metals concentrations. These correlation coefficients were derived by literature data of leaching tests (carried out on both composts and BSW) and integrated in the model for the cumulative release of DOC, allowing to describe the release of metals. The comparison of the results obtained with this method with the experimental data highlighted that this approach replicates quite well the leaching trend observed for most of the selected metals. Results also confirmed that bio-stabilized waste produced in MBT plants cannot achieve criteria imposed by Member States for reuse them as secondary material. However, BSW can be characterized by considerable percentage of organic fractions (sometimes higher than 20-30%) and that means that maybe a secondary aerobic stage, similar to the one expected in the composting plants, could affect its main physicochemical properties by modifying the structure of the organic matter and improve the environmental behaviour of the BSW. Namely, around 300-400 kg of BSW were collected at the end of their biological stabilization and they have been subjected to a further aerobic stage, lasting 6 months, in order to simulate the maturation that is typically applied in the last part of a composting treatment. During this period, a sample was collected each 30 days for monitoring relevant parameters, such as the volatile solids and the elemental composition (C, H, N, S content). Furthermore, a detailed characterization was performed on three samples collected throughout the simulated maturation (specifically at the beginning of the maturation and after 90 and 180 days), in order to evaluate changes in biological stability, total metal content and metal release. In addition, the metal speciation was assessed by a sequential extraction, in order to estimate the form in which they are available for increasing maturation times. The obtained results confirmed that the adoption of a prolonged maturation process could influence the characteristics of the BSW produced in the MBT plant. At the beginning of the simulated maturation, the BSW presented a Dynamic Respiration Index (DRI) value proper of a biologically unstable matrix. However, a higher stability degree was observed after at least 90 days (as shown by the DRI, the C/N ratio). The adopted treatment entailed also an improvement for the iv Abstract environmental behaviour of the material due to the reduction of the released metals and organic carbon, as also shown by the reduction in the mobile fraction of metals observed in the metal speciation analysis. However, these reductions resulted to be not enough noticeable to consider the reutilization of these matrices as recovery option. It is for this reason that attempts have been made to extract humic acids (HA) from this waste. Recently, the research interest is focused on the possibility of extract these substances from other organic wastes, such as compost. However, an interesting option could be to extract humic substances from wastes that differently from compost have no further use. The effect of the main operating conditions applied during the alkaline extraction of the humic acids was investigated (i.e. material particle size, liquid-solid ratio, NaOH concentration and extraction time), analysing the elemental composition, the main functional groups content and the optical proprieties for the extracted HA. The operating conditions that showed to improve the extraction extent were the NaOH concentration and the extraction time. Furthermore, the HA obtained in these specific conditions showed higher functional groups content and aromaticity, in agreement with the characteristics of the commercial HA, allowing to consider these substances for various industrial applications.