Assessment of biogas production from MBT waste under different operating conditions

In this work, the influence of different operating conditions on the biogas production from mechanically-biologically treated (MBT) wastes is investigated. Specifically, different lab-scale anaerobic tests varying the water content (26-43% w/w up to 75% w/w), the temperature (from 20 to 25°C up to 55°C) and the amount of inoculum have been performed on waste samples collected from a full-scale Italian MBT plant. For each test, the gas generation yield and, where applicable, the first-order gas generation rates were determined. Nearly all tests were characterised by a quite long lag-phase. This result was mainly ascribed to the inhibition effects resulting from the high concentrations of volatile fatty acids (VFAs) and ammonia detected in the different stages of the experiments. Furthermore, water content was found as one of the key factor limiting the anaerobic biological process. Indeed, the experimental results showed that when the moisture was lower than 32% w/w, the methanogenic microbial activity was completely inhibited. For the higher water content tested (75% w/w), high values of accumulated gas volume (up to 150Nl/kgTS) and a relatively short time period to deplete the MBT waste gas generation capacity were observed. At these test conditions, the effect of temperature became evident, leading to gas generation rates of 0.007d(-1) at room temperature that increased to 0.03-0.05d(-1) at 37°C and to 0.04-0.11d(-1) at 55°C. Overall, the obtained results highlighted that the operative conditions can drastically affect the gas production from MBT wastes. This suggests that particular caution should be paid when using the results of lab-scale tests for the evaluation of long-term behaviour expected in the field where the boundary conditions change continuously and vary significantly depending on the climate, the landfill operative management strategies in place (e.g. leachate recirculation, waste disposal methods), the hydraulic characteristics of disposed waste, the presence and type of temporary and final cover systems

LCA of mechanical biological treatment of residual MSW in the city of Rome: current practices vs. alternative strategies

Mechanical Biological Treatment (MBT) is being increasingly adopted throughout the EU as one of the main elements of integrated waste management strategies with the aim of diverting waste from landfills and reducing the environmental impacts of waste landfilling, mainly related to leachate and biogas production and composition. Italy, in particular, has a long tradition in mechanical biological treatment of residual municipal solid waste (MSW), i.e. the unsorted waste remaining after at-source segregation and collection of recyclable fractions. According to the Italian Environmental Agency, in 2014 a total of 117 MBT plants were in operation in Italy, treating roughly 9 million ton of waste, corresponding to 32% of the yearly amount of produced MSW. Furthermore, in some Regions (e.g. Lazio Region), due to the establishment of more restrictive regional laws and regulations compared to National legislation, unsorted MSW waste has to be preliminarily treated through mechanical and biological processes before it can be landfilled or thermally treated, leading to the key role of the MBT technology within this context. The main environmental benefits of a MBT plant are generally associated to the recovery of recyclable materials (such as plastics, paper, metals and compost), the production of high calorific combustibles (i.e. Refuse Derived Fuel, RDF) for energy recovery and the biological stabilization of putrescible organic matter in waste in order to reduce methane and leachate emissions of treated waste when disposed of in landfills. Despite these advantages, potential environmental impacts may be generated also from MBT plant operations and, moreover, from downstream treatments and disposal of MBT output flows. Depending on the MBT facility configuration, the feedstock source, the biological treatment (e.g. aerobic, anaerobic or a combination of the two) and post-treatments (e.g. maturation), the quality of MBT produced wastes and, hence, potential emissions related to their disposal may significantly vary. The present study aims at evaluating the overall environmental burdens related to the current practices adopted for the management of unsorted MSW in the municipality of Rome (Lazio Region, Italy), which is essentially based on MBT plants, by Life Cycle Assessment (LCA) employing the waste-LCA model EASETECH using operational and experimental data. Previous LCA studies on MBT systems recognized the global environmental benefits of MBTs compared to direct landfilling but only few highlighted the need of carefully accounting for the downstream management of residuals and sorted fractions (e.g. the recovery of recyclables, RDF incineration, wastewater treatment, landfilling of stabilized waste and other residues), and of including uncertainties related to LCA modelling and inventory analysis (see e.g. Montejo et al., 2013; Beylot et al., 2015). For this reason, in the present study, emphasis was placed on the development of a robust and specific MBT inventory, through the collection of operational data from a MBT plant of the city of Rome, and characterization data of all input and output waste streams obtained through several sampling campaigns. The experimental data were interpreted and modelled in order to estimate the liquid and gaseous emissions that can be assumed as representative in a landfill disposal scenario for the analyzed system. Furthermore, experimental results were used to identify feasible alternative strategies for managing MBT output wastes. Up to now, only RDF (24.7% of the unsorted feed MSW) is sent to incineration for energy recovery, whereas all other residues (heavy scraps, stabilized MBT waste and stabilization scraps), corresponding to 55% of unsorted MSW, are landfilled. However, with the introduction of Solid Recovered Fuel (SRF) instead of RDF, and new standard values for SRF classification based on specific chemical and physical waste properties, it appears that thermal treatment may be applied also to MBT scrap flows such as heavy scraps and/or stabilization scraps. Incineration of these two flows was hence included in the LCA study and compared to the current practice of scraps landfilling in terms of total environmental impacts. References: Montejo et al., 2013. Journal of Environmental Management. doi: 10.1016/j.jenvman.2013.05.063 Beylot et al., 2015. Waste Management. doi: 10.1016/j.wasman.2015.01.

LCA of management strategies for RDF incineration and gasification bottom ash based on experimental leaching data

The main characteristics and environmental properties of the bottom ash (BA) generated from thermal treatment of waste may vary significantly depending on the type of waste and thermal technology employed. Thus, to ensure that the strategies selected for the management of these residues do not cause adverse environmental impacts, the specific properties of BA, in particular its leaching behavior, should be taken into account. This study focuses on the evaluation of potential environmental impacts associated with two different management options for BA from thermal treatment of Refuse Derived Fuel (RDF): landfilling and recycling as a filler for road sub bases. Two types of thermal treatment were considered: incineration and gasification. Potential environmental impacts were evaluated by life-cycle assessment (LCA) using the EASETECH model. Both non-toxicity related impact categories (i.e. global warming and mineral abiotic resource depletion) and toxic impact categories (i.e. human toxicity and ecotoxicity) were assessed. The system boundaries included BA transport from the incineration/gasification plants to the landfills and road construction sites, leaching of potentially toxic metals from the BA, the avoided extraction, crushing, transport and leaching of virgin raw materials for the road scenarios, and material and energy consumption for the construction of the landfills. To provide a quantitative assessment of the leaching properties of the two types of BA, experimental leaching data were used to estimate the potential release from each of the two types of residues. Specific attention was placed on the sensitivity of leaching properties and the determination of emissions by leaching, including: leaching data selection, material properties and assumptions related to emission modeling. The LCA results showed that for both types of BA, landfilling was associated with the highest environmental impacts in the non-toxicity related categories. For the toxicity related categories, the two types of residues behaved differently. For incineration BA the contribution of metal leaching to the total impacts had a dominant role, with the highest environmental loads resulting for the road scenario. For the gasification BA, the opposite result was obtained, due to the lower release of contaminants observed for this material compared to incineration BA. Based on the results of this study, it may be concluded that, depending on the type of BA considered, its leaching behavior may significantly affect the results of a LCA regarding its management strategies