Environmental sustainability of two biological treatments for the organic wet fraction of municipal solid waste

The wet organic waste is an important fraction of municipal solid waste (MSW), which in European Countries can reach values as high as 30-40% of the total amount of generated MSW. Its management requires an appropriate household source separation, an efficient separate collection, and a final biological treatment of anaerobic or aerobic digestion. It has been demonstrated that, in a life cycle perspective, anaerobic digestion (AD) process has a higher environmental sustainability than aerobic composting process [1]. The AD of the wet organic fraction of MSW (OFMSW) is in fact able to minimize the greenhouse gas emissions and to produce a biogas for energy recovery. It is also characterized by the absence of emissions of bio-aerosols and bad odours, a limited utilization of land surface use, and a sufficient economic sustainability [2]. An interesting alternative to aerobic or anaerobic process units is that of an integrated anaerobic digestion plant, which includes a final aerobic treatment. This study aims to compare the environmental sustainability of this integrated solution with that of a “stand-alone” anaerobic process. The integrated solution taken as reference is made-up of three phases: pre-treatment, wet anaerobic digestion (with a Continuous-flow Stirred-Tank Reactor), and a final stage of post-composting in bio-cells. The pre-treatment is a mechanical sorting process that efficiently removes the out-of-target material, making the OFMSW a substrate suitable for AD. The thermophilic anaerobic phase generates two main products: a biogas, with a degradation rate of the volatile solids of 75%, and a solid digestate. The biogas, which is assumed to be composed by 60% of methane and 40% of carbon dioxide, is then burned in an internal combustion engine for electricity production, with a conversion efficiency of 34%. The digestate is dried and sent to the final aerobic phase to obtain a compost, which could be used as soil conditioner. The “stand-alone” anaerobic plant includes the same pre-treatment and AD phases, but it is not equipped with a final post-composting unit. The first configuration requires higher energy for the additional post-composting stage (which has been evaluated as equal to 20% of total electrical energy produced by biogas combustion), while the second configuration produces a lower quality stabilized material, then having a limited number of possible utilization. Data collected in two plants in operations in Italy have been utilized to estimate the environmental burdens for the development of an attributional Life Cycle Assessment (LCA) study. The functional unit coincides with the treatment of 200 t/d (then about 60 kt/y) of OFMSW. The system boundaries includes all the activities from the plant entry gate until the management of solid/liquid residues. The “system expansion” method was utilized to include the avoided burdens related to the recovery of energy and materials. It was assumed that the generated electrical energy replaces the production of electricity from the Italian grid. The compost produced in the integrated plant is assumed to substitute an amount of peat, which has been estimated assuming a carbon content of 20% for compost and 60% for peat. The digestate obtained from the “stand-alone” AD plant is not composted, and it is assumed to substitute inert materials for the operation of landfill capping, accordingly with Italian legislation. The LCA results indicate that the “stand-alone” AD plant has better environmental performances. In particular, its larger energy recovery leads to better results in terms of midpoint impact categories of “Global Warming”, “Non-Renewable Energy” and “Respiratory Inorganics” [3]. The integrated plant shows worst results also in terms of “Land Occupation”, due to the necessity to add a non-negligible amount of a bulking agent (i.e. straw) to the digestate in order to guarantee the utilization of compost as soil conditioner. A sensitivity analysis has been carried out assuming that the compost generated by the integrated plant could be used as substitute of a chemical fertilizer, highlighting the importance of compost quality in the comparison between the two configurations. [1] Yoshida, H., Gable, J.J., Park, J.K., 2012. Evaluation of organic waste diversion alternatives for greenhouse gas reduction. Resources, Conservation and Recycling, 60, 1–9. [2] Arena, U., and Di Gregorio, F., 2014. A waste management planning based on substance flow analysis, Resources, Conservation and Recycling, 85, 54-66. [3] Jolliet, O., Margni, M., Charles, R., Humbert, S., Payet, J., Rebitzer, G., Rosenbaum, R., 2003. IMPACT 2002+: a new life cycle impact assessment methodology. Int. J. LCA, 8 (6), 324–330

Technical and environmental performances of alternative treatments for challenging plastics waste

The recovery of resources from streams of mixed plastics waste is a technological and economic challenge since they contain various (and generally non-compatible) polymers, different (and often hazardous) additives, as well as multilayer structures and fiber-reinforced composites. Only a too limited part of these plastics - such as those coming from waste of electric and electronic equipment (WEEE), end-of-life vehicles (ELV) and construction and demolition waste (C&DW) - can be treated by mechanical techniques in the conventional recycling facilities, and a still smaller part is reintroduced into the market. Some innovative treatments have been recently proposed and appear suitable for these challenging waste streams. The paper describes technical characteristics of some of them, and compares their environmental performances with those of currently adopted management options. An environmental life cycle assessment was developed by taking into account the substitutability factor of obtained products and technological readiness level of the analyzed resource recovery processes. The focus is on new treatments of dissolution/precipitation, supercritical fluid extraction, catalytic pyrolysis, and waste-to-energy (WtE) equipped with carbon capture and storage unit (CCS). The results highlight the promising performances of some of these new options, quantify their potential environmental advantages, and suggest to take them into account in the definition of sustainable management schemes for the examined challenging plastics wastes. In particular, physical recycling by dissolution/precipitation process applied to one tonne of WEEE plastics, not treatable by mechanical recycling, can save up to about 2 tCO2,eq. with respect to landfill disposal and WtE with CCS, and more than 3 tCO2,eq. with respect to WtE without CCS. The performances of WtE with CCS appear of interest, particularly for WEEE and ELV mixed plastics, allowing to save up to 0.5 tCO2,eq. and 1.7 tCO2,eq., with respect to pyrolysis and WtE without CCS, respectively

Fluidized Bed Gasification of Mixed Plastic Wastes: A Material and a Substance Flow Analysis

Gasification as a reliable and convenient waste-to-energy process for the economic analysis of mixed-plastic waste (MPW) was investigated. To this end a pilot scale bubbling fluidized bed air gasifier was fired with two commercially available MPWs to obtain syngas composition and characterization of the bed material, cyclone collected fines and purge material from the scrubber. These results were then processed by means of Material and Substance Flow Analyses to evaluate the main process performance parameters for the two MPWs tested

Multi-Wall Carbon Nanotubes Obtained by Fluidized Bed Pyrolysis of Virgin or Recycled Plastics

A new technique for a continuous, mass production of high-quality multi-wall carbon nanotubes (MWCNTs), based on fluidized bed pyrolysis of polymers (virgin or recycled polyolefins and recycled polyethylene terephtalate), is described in detail. The study investigates the role of interactions between the bed material and the polymer particles injected into the reactor as well as that of the reactor temperature. Results are reported in terms of yield and quality of obtained MWCNTs, all characterized by thermogravimetrical analysis and SEM microscopy. The production of MWCNTs, in a relatively large quantity and at a low cost, is demonstrated as technically feasible

The role of activated carbon size in the catalytic cracking of naphthalene

Activated carbons are efficient catalysts for tar cracking, suitable for hot cleaning of the syngas produced during biomass- and waste-to-energy gasification processes. This study investigates the conversion of naphthalene, utilised as reference for tar compounds, when catalysed by a coal-derived activated carbon. The attention focuses on the influence of the operating temperature, in the range 750-900°C, and the size of selected activated carbon, which has been used under form of pellets, granules and powders. The conversion efficiency improves when the temperature raised from 750°C to 900°C (from 79% to 99%, for the pellets), and when the catalyst size reduced from pellets to powders (from 79% to 97%, at 750°C). The diffusional resistance in the catalyst particles has been then quantified in terms of Thiele modulus and internal effectiveness factor. A gradual reduction of catalyst surface area has been also observed for longer tests, due to the progressive deposition of soot from naphthalene decomposition over and inside the porous structure of the activated carbon. The carbon content of these deposits has been quantified, showing larger percentages on the surface of granules and powders.Activated carbons are efficient catalysts for tar cracking, suitable for hot cleaning of the syngas produced during biomass- and waste-to-energy gasification processes. This study investigates the conversion of naphthalene, utilised as reference for tar compounds, when catalysed by a coal-derived activated carbon. The attention focuses on the influence of the operating temperature, in the range 750–900 °C, and the size of selected activated carbon, which has been used under form of pellets, granules and powders. The conversion efficiency improves when the temperature raised from 750 °C to 900 °C (from 79% to 99%, for the pellets), and when the catalyst size reduced from pellets to powders (from 79% to 97%, at 750 °C). The diffusional resistance in the catalyst particles has been then quantified in terms of Thiele modulus and internal effectiveness factor. A gradual reduction of catalyst surface area has been also observed for longer tests, due to the progressive deposition of soot from naphthalene decomposition over and inside the porous structure of the activated carbon. The carbon content of these deposits has been quantified, showing larger percentages on the surface of granules and powders

A preliminary social assessment of innovative management options for mixed and hazardous plastics waste

Recyclability of plastics from waste of electric and electronic equipment (WEEE), end-of-life vehicles (ELV) and construction and demolition waste (C&DW) is a technological and economic challenge. It is complicated by their composition, which includes many polymers with high levels of contamination, as well as the large costs of treatments and the continuous evolution of the related legal framework (Ardolino et al., 2021; Cardamone et al. 2022). Innovative treatments able to remove contained contaminants, so generating secondary plastics of good quality and reducing adoption of improper strategies, were recently proposed in a H2020 project (Nontox, 2021). The good environmental performances of management schemes utilising these treatments for WEEE/ELV/C&DW plastics were quantified by means of Environmental Life Cycle Assessments (E-LCAs) as described by Ardolino et al. (2021) and Cardamone et al. (2022). The potential social impacts of these management schemes have been the focus of the preliminary Social Life Cycle Assessment (S-LCA) described in this study. It was developed in agreement with UNEP guidelines (2020) and ISO standards (ISO 14040-44). The analysis maintained the basic assumptions of the E-LCAs, in particular saving the management of WEEE/ELV/C&DW plastics annually collected in Europe as the functional unit. The current management scheme of each of them, including waste-to-energy (WtE) by combustion, sanitary landfilling and substandard options, were compared with the possible future scheme, implementing also innovative treatments of physical recycling (CreaSolv® and Extruclean), plastic upgrading and catalytic pyrolysis. Please click Additional Files below to see the full abstract

Environmental impacts of a material recovery facility in a Life Cycle Perspective

The study aims to evaluate the environmental performances of an integrated material recovery facility (MRF), which has a crucial role in the waste management plan of a region in the middle of Italy, characterized by a low level (less than 20%) of household source separation and separate collection [1]. The facility, which is able to treat about 30 kt/y of mixed waste, has three main units: a mechanical sorting platform, bio-cells for tunnel composting, and a landfill. The output streams of the sorting platform are the ferrous metals and mixed plastics, which are sent to the recycling processes, the solid recovered fuel (SRF), which is utilized in an external combustion-based waste-to-energy plant, and a low-quality organic fraction, which is treated in the on-site composting unit. The solid residues generated by these processes are about a half of the input stream, and are disposed in the annexed landfill. The bio-cells for tunnel composting are in operation since 2014, and so far produces just a low-quality compost, utilized for landfill capping, and a leachate, sent to an external wastewater treatment plant (WTP). The landfill produces a leachate, which is treated in the WTP, and a biogas, which is collected (with an efficiency of about 60%), and sent to a gas engine, having an electric energy conversion efficiency of 38%. Please click Additional Files below to see the full abstract

About the Environmental Sustainability of the European Management of WEEE Plastics

A huge increase of waste of electrical and electronic equipment (WEEE) is observing everywhere in the world. Plastic component in this waste is more than 20% of the total and allows important environmental advantages if well treated and recycled. The resource recovery from WEEE plastics is characterised by technical difficulties and environmental concerns, mainly related to the waste composition (several engineering polymers, most of which containing heavy metals, additives and brominated flame retardants) and the common utilisation of sub-standard treatments for exported waste. An attributional Life Cycle Assessment quantifies the environmental performances of available management processes for WEEE plastics, those in compliance with the European Directives and the so-called substandard treatments. The results highlight the awful negative contributions of waste exportation and associated improper treatments, and the poor sustainability of the current management scheme. The ideal scenario of complete compliance with European Directives is the only one with an almost negligible effect on the environment, but it is far away from the reality. The analysed real scenarios have strongly negative effects, which become dramatic when exportation outside Europe is included in the waste management scheme. The largely adopted options of uncontrolled open burning and illegal open dumping produce huge impacts in terms of carcinogens (3.5·10+7 and 3.6·10+4 person·year, respectively) and non-carcinogens (1.7·10+8 and 2.0·10+6 person·year) potentials, which overwhelm all the other potential impacts. The study quantifies the necessity of strong reductions of WEEE plastics exportation and accurate monitoring of the quality of extra-Europe infrastructures that receive the waste

An alternative management scheme for plastics from construction & demolition waste

Construction and Demolition Waste (C&DW) is a priority stream in the circular economy agenda, since it accounts for more than a third of all wastes generated in the European Union. About 1.8 Mt/y of these C&DW are plastics, whose valorisation has to overcome several obstacles: i) Current legislation recycling targets are established in terms of total recycled mass (Iodice et al., 2021), hence can be easier obtained by focusing on heavy fractions, i.e. metals and inert materials; ii) Plastics in buildings are often embedded behind walls, under floors and inside roofs: this complicates their gathering and separation (EC, 2021); iii) C&DW plastics often contain substances of concerns, allowed in the past but restricted by the current legislation (Wagner and Schlummer, 2020): the long lifetime of plastics in buildings - from about 15 years up to, sometimes, 100 years – it is thus a further technical obstacle for recycling; iv) Recycling entails high costs and needs specific policy actions to be implemented, such as landfill ban and the creation of a competitive market for secondary raw material (Pantini and Rigamonti, 2020). These constrains make collection and management schemes complex and variable from country to country. Moreover, the rare utilisation of a selective demolition as alternative to a conventional demolition further worsens the quality of recoverable materials. Please click Additional Files below to see the full abstract