Mechanical Biological Treatment Research Articles

Regional management options for floating marine litter in coastal waters from a life cycle assessment perspective

PurposeDespite the increasing number of publications focusing on the management of marine plastic debris, most research is carried out using an upstream perspective, focused on sources and pathways of marine litter accumulation, as well as actions to prevent plastics from entering the environment. The aim of this paper is therefore to investigate a combination of waste management technologies for marine litter in order to inform decision-making on environmental impact hotspots at regional level.MethodA study of the North Sea German Bight identified existing technologies suitable for the collection and treatment of floating marine debris including, recycling of plastics, mechanical biological treatment (MBT), and landfilling. Processing data for reported marine litter compositions in the region are used to inform the modelling of a waste management strategic plan (WMSP) aimed at valorising various fractions. Hence, collected floating marine litter is separated into derelict fishing gear (DFG) and mixed marine litter (MML) which are sent respectively to a recycling plant for plastic granulates production and to an MBT plant for recovery of metals and electricity generation. Environmental impacts of the WMSP are evaluated using the Life Cycle Assessment methodology and compared with incineration considered as the prevalent waste scenario.Results and discussionAs partly expected, the LCA results reveal higher environmental performance in all impact categories for the incineration scenario. In particular, the WMSP contributes to Global Warming Potential (GWP) more than 10 orders of magnitude less than the incineration scenario. However, the breakdown of results related to the WMSP indicates the highest contribution to environmental impacts attributed to electricity and heat generation from refused-derived fuel and emissions at the combined heat and power plant, as well as electricity and diesel consumption. Lowest contributions are attributed to the recycling plant. The sensitivity analysis revealed low contributions to GWP if plastic debris such as DFG is diverted to recycling while toxicity-related categories are improved by efficient metal and energy recovery at the MBT plant.ConclusionFindings of this study show that no single treatment method is enough rather a combination of different treatment pathways should be designed considering the composition and properties of accumulated marine litter in a specific area. However, recovering plastic litter and diverting useful materials from waste-to-energy to recycling improve the environmental performance. Reviews suggest inclusion of valorisation treatment options in future WMSPs of marine litter such as plastic-to-fuel technologies.

Changes in Main Parameters of Biological Stabilisation of Municipal Waste from a Full-Scale Mechanical-Biological Treatment (MBT) Installation

Abstract Poland's mixed municipal waste management system has been based on mechanical-biological treatment (MBT). One of the MBT processes is the biological stabilisation of fractions of less than 80 mm. The final product after biological stabilisation is a stabiliser that, once the conditions required by law are met, can be landfilled. The stabilizer must meet the following requirements: the loss of ignition (LOI) of the stabiliser is less than 35% DM, the organic carbon content is less than 20% DM and the AT4 value (aerobic 4-day respiration test) value is less than 10 mgO2/g DM. The study aimed to analyse the changes that occurred during the first 4 weeks of biological processing of the fraction of less than 80 mm of mixed municipal waste. The bioreactor feedstock consisted of 41.7% organic fraction (OF), the LOI was 32.3%, and the AT4 value was 45.7 mgO2/g DM. During biological processing, at 4 weeks, the loss of OF in the total weight of the waste was found to be 22.6%, the losses after roasting decreased by almost 90% and the value of AT4 was 6.6 mgO2/g DM. This means that the stabiliser already after 4 weeks, according to the law, could be sent to a landfill.

An assessment of the current situation and recommendation of management scenarios for municipal solid waste management in Ho Chi Minh city

Ho Chi Minh city (HCMC) is currently facing a critical problem of the uncontrollable municipal solid waste (MSW). The most common waste disposal practice in the city is landfilling, but this is not a sustainable solution. From 2010 to 2022, the amount of waste generated has increased annually, from 6,000 tons/day to 9,200 tons/day, with an average growth rate of 6-8% per year. This study calculated the average emission factor of various districts in the city such as districts 1, 3, etc. at 0.83 kg/capita/day and district Binh Chanh, Cu Chi, etc. at 0.73 kg/capita/day and forecasted that the amount of waste will reach approximately 16,100 tons/day by 2050. Currently, 80-90% of collected MSW is landfilled, while only 10% is recycled. Composting and energy recovery practices are rarely used. This paper reviewed the current solid waste practice in HCMC and proposed four management scenarios to develop a sustainable solid waste management system. The first scenario is landfilling, which involves non-segregated waste disposal at the source. The second scenario is recycling, which involves segregating waste at the source for recycling, reuse, and waste reduction. The third scenario is composting, which requires using a mechanical-biological treatment plant for sorting, composting, and producing fuel from waste. The fourth scenario is waste-to-energy, which uses a waste incineration plant to generate energy from waste.

Emissions associated with the management of household organic waste, from collection to recovery and disposal: A bottom-up approach for Sydney and surrounding areas, Australia

Recent advances in waste management policies and initiatives world-wide have placed greater focus on the potential greenhouse gas emissions-avoidance that can be achieved via sustainable waste management practices. In New South Wales, Australia, household organic waste has been identified as a problematic stream that through better management, could lead to better environmental outcomes and reduced emissions. However, emissions associated with waste management are poorly characterised in Australia, obscuring decision making around optimal pathways that can maximise landfill diversion and minimise emissions. This study addresses this data limitation by estimating and analysing the emissions associated with household organic waste management. The approach applied route optimisation to estimate emissions from points of collection (weekly and fortnightly) to recovery at composting facilities and disposal at landfills, as well as mass balance modelling to estimate direct and indirect emissions associated with windrow composting and mechanical biological treatment. Landfill gas modelling was also used to estimate lifetime emissions from disposal, as well as potential avoided emissions through recovery. Results for the Greater Sydney and surrounding area show total gross greenhouse gas emissions of approximately 390,100 tCO2-e, and balanced by approximately 145,600 tCO2-e of emissions avoidance through landfill diversion. The average net emissions intensity of jurisdictions in the study area was approximately 133 kgCO2-e/t of waste diverted; or 423 kgCO2-e/t of waste managed. Emissions intensity was highest for the mixed waste collection stream, where diversion pathways are limited; and lowest where separate food collection and mechanical biological treatment was employed. Findings indicate that greater separation of food waste from the mixed stream to dedicated food waste collection will result in the greatest improvements in both emissions intensity and landfill diversion.

Towards fuels production by a catalytic pyrolysis of a real mixture of post-consumer plastic waste

The contribution provides a valorization alternative for rejected plastic wastes from mechanical-biological treatment (non-recyclable material) via an in-situ catalytic pyrolysis process focused on the production of a liquid fraction with similar properties to traditional fuels (i.e., gasoline, kerosine, and diesel). According to the ASTM recommendations, on small samples without prior physical separation, fuel fraction identification was carried out by Simulated Distillation along with a hydrocarbon types analysis and complemented with CHNS-O analysis and Fourier Transform Infrared Spectroscopy. Two catalytic structures were employed named Sepiolite and Montmorillonites, both K10 and K30, which after simple heat treatment to stabilize the structure, were characterized to analyze the main properties affecting the catalytic activity and product yields (i.e., morphological and acidity properties). A whole screaming of the products by analogy with hydrocarbon of the petroleum industry is presented. Such an approach allows a real evaluation of the studied technology in the current energy scenario.

Modification of the Aeration-Supplied Configuration in the Biodrying Process for Refuse-Derived Fuel (RDF) Production

Biodrying is an essential part of the mechanical–biological treatment process that minimizes moisture content and simultaneously maximizes heating value for refuse-derived fuel (RDF) production. Although the mechanical separation process operates effectively in Thailand’s RDF production, high organic content levels and their degradation cause moisture contamination in RDF, producing wet RDF. Aeration is essential for an effective biodrying process, and can reduce RDF’s moisture content as well as increase its heating value. To maximize the biodrying effect, aeration should be optimized based on the waste conditions. This study proposes a modified aeration-supplied configuration for wet RDF biodrying. The aeration rate was modified based on the period within the biodrying operation; the first period is from the beginning until day 2.5, and the second period is from day 2.5 to day 5. The optimal aeration supply configuration was 0.5 m3/kg/day in the first period and then 0.3 m3/kg/day until the end of the process; this configuration yielded the greatest moisture content decrease of 35% and increased the low heating value of the biodried product by 11%. The final moisture content and low heating value were 24.07% and 4787 kcal/kg, respectively. Therefore, this optimal aeration-supplied configuration could be applied to meet the moisture content and low heating value requirements of the RDF production standard for Thailand’s local cement industry.

Proposal for Implementation of Extraction Mechanism of Raw Materials during Landfill Mining and Its Application in Alternative Fuel Production

New approaches to waste management and the demands of the circular economy have changed the management of landfills. Over time, the decomposition of buried waste primarily determines the amount of recyclable and combustible materials. This pilot study attempted to assess the feasibility of extracting and recovering energy-intensive raw materials from landfills by developing a waste extraction mechanism and creating a solid recovered fuel (SRF) production line for use as a replacement fuel in the cement industry. The proposed mechanism consisted of two stages. The first stage was recommended to be carried out on the landfill territory by screening out the fine fraction and extracting inert materials and bulky waste. The second stage should be on the mechanical biological treatment (MBT) plant’s territory by adding additional technological equipment to the MBT line. The productivity of the SRF production line was calculated and was 4.9 t/h. The mechanism proposed in the work was tested at the operating test site in Lithuania. The composition of Landfill Mined Residues (LMRs) was studied, and the energy potential of the studied part of the landfill was calculated, which was 196,700 GJ. It has been found that the SRF produced complies with the European Union (EU) standard and, according to its classification characteristics, belongs to class III and can be used as a replacement fuel in the clinker firing process. An environmental and economic efficiency assessment was conducted using SRF in the cement kiln. The calculation result showed that using 10% SRF as a replacement fuel for coal used in clinker firing at 2.51 t/h would save 1274 USD/h in coal costs. This use of SRF will emit 3.64 t/h CO2 and achieve a net savings of 1355 USD/h. The mechanism proposed in this work aimed at reducing waste in landfills by converting materials into energy resources will help achieve the circular economy’s goals.

A mini review on the main challenges of implementing mechanical biological treatment plants for municipal solid waste in the Latin America region: Learning from the experiences of developed countries.

Most of the municipal solid waste (MSW) generated in the Latin America (LATAM) region is currently disposed of in landfills and dumpsites, which results in many negative environmental impacts. Mechanical biological treatment (MBT) is a strategy that combines the mechanical separation of recoverable materials with the biological stabilization of organic matter. MBT plants have proven to be a good alternative for the management of MSW and have been successfully implemented in developed countries for more than 30 years. However, the efficient introduction of these facilities in developing countries, such as those in the LATAM region, requires further feasibility studies. Therefore, this mini review seeks to offer significant insights into the main challenges that must be overcome to facilitate the implementation and operation of MBT plants, considering the unique technological, sociocultural, economic and political context of the LATAM region, through a comparison of the reported experiences of MBT plants in more developed countries with those in the LATAM region. The analysis herein indicates that key actions for the successful operation of MBT plants in the LATAM region include both the implementation of source separation as well as selective collection practices. Moreover, this work shows that other factors, like the establishment of valorization markets with safe working conditions for informal collectors, the development of intermunicipal cooperation schemes and the enforcement of strong regulatory frameworks for waste disposal specifications, are important contextual factors that have allowed the efficient operation of MBT plants in developed countries. Although the implementation of many of these practices will be challenging, such measures are necessary to increase the sustainability of MSW treatment practices in the LATAM region.

A general model for air gasification of heterogenous municipal solid waste

Sustainable management of municipal solid waste (MSW) is crucial due to its huge volume and cash flow. Among the different waste-to-energy solutions, gasification is considered an interesting option compared to incineration, which is the most common technology to valorise MSW, because of higher efficiency, lower environmental impacts, and producing hydrogen or other biofuel and biochemical products. However, mechanical–biological treatment of MSW results in heterogenous materials mainly composed of plastics, paper, and textiles, which troubles a reliable study of gasification. Hence, a flexible model of a fluidized bed air gasifier is developed in Aspen Plus in this work based on the experimental data from the literature to estimate the syngas properties from residual MSW of composting facilities.The developed model gives a low heating value (LHV) of the syngas in the range 4.2–7.3 MJ/Nm3 considering the characteristics of the residual MSW from a real composting facility, which is in line with the experimental values reported in the literature. Furthermore, a sensitivity analysis is performed varying the composition of the residual waste from the mechanical–biological treatment to support preliminary feasibility analyses of energy production from heterogeneous materials. The results showed a good capability of the model in predicting the energy performance of the fluidized bed gasifiers varying feedstock materials and equivalence ratios.

Morphological composition of bio-waste collected selectively in towns and villages during autumn and winter

It is generally believed that bio-waste is a suitable substrate for valorizations through fermentation with biogas production. An increasing number of municipalities in Poland implement selective collection of bio-waste. Limited qualitative and quantitative data are available on the composition of household bio-waste, including the contaminant content. This paper presents the adopted research method and the analysis results of the composition of household bio-waste segregated at source from 4 rural and 4 urban communes, from districts with single-family and multi-family housing. Household bio-waste is mostly food waste (43.9 to 56.0%), with a smaller percentage of garden waste (27.4 to 46.3%). According to data from other countries, in Poland, ‘fruit and vegetable’ waste constitutes the main part of household bio-waste (from 40.0 to 53.8%). The share of “contaminants” is relatively high. The most contaminated waste is that collected in containers in city districts with multi-family housing (16.6%). The share of contaminants in bio-waste collected in bags (rural areas and cities with single-family housing exceeded 10.0%). Implications: This article presents the results of an empirical analysis of the morphological composition, occurring contaminants and seasonal fluctuations in the stream of biodegradable waste delivered to the mechanical-biological treatment (MBT) installation in Marszow (Poland, Lubuskie province). This installation serves the area of 22 municipalities with more than 200.000 residents. Nearly 98% of households and housing estates are covered by regular waste collection. The study covered 12 streams of bio-waste delivered to the installation, divided into rural and urban areas with single-family and multi-family housing during the 2 seasons of autumn and winter. The importance of the conducted research is emphasized by the fact that the analysis of the morphological composition of bio-waste generated in households is rare, which is probably due to its labor- and time-consuming nature. The data obtained are a source of valuable information for both national and local governments, as well as can help meet the ambitious targets set for EU member states in terms of preparing for reuse and recycling of municipal waste, which are unlikely to be achieved without proper management of bio-waste. Another important argument is to reduce food waste in line with the sustainable development goal of reducing food waste by 50% by 2030.

Use of a novel water separation process for the recovery of the mineral fraction from the stabilized organic fraction found in municipal waste.

Currently, the Stabilized Organic Fraction (SOF) 0-80 mm separated from the municipal waste stream is sent in its entirety to landfill in the mechanical-biological treatment plants operating in Poland. Such action does not align with the adopted EU strategy focused on waste management in the circular economy. This study aimed to assess the technical feasibility of the process of purification of mineral fractions with a grain size of 10-80 mm separated from SOF on a technological installation designed for glass recovery, built at the Marszów Plant. In terms of material, mineral fractions are a mixture of stones, rubble and glass, contaminated with various components, mainly organic. In 2020, the glass recovery installation was expanded with an innovative process of gravity separation to separate mineral fraction from SOF with the use of a jigging machine. A pulse jig used in the mining industry for coal enrichment processes was used for the first time in municipal waste management. The installation allows for the recovery of between 71.1 and 96.6% of mineral fractions contained in the input (84.8 ± 9.5% on average). The proportion of organics, plastics and other impurities in the waste is 4.1 ± 2.1% of the fraction mass. The process consumed water at an average rate of 0.34 ± 0.10 m3 tonnes-1 of treated waste and generated 0.26 ± 0.09 m3 tonnes-1 of wastewater. Electricity consumption averaged 1.20 kWh tonne-1 input.

Characterization of screenings from urban wastewater treatment plants: Alternative approaches to landfill disposal

Increasingly, wastewater treatment plants are laying the foundation for a circular economy. A novel approach to operating these plants is to redefine them as biorefineries to produce energy and move toward zero waste. To this end, an analysis of wastewater screening was carried out, as this is the only waste that does not yet have a valorization route, being generally disposed of by landfill. This process causes economic and environmental costs and will be restricted in the coming years, so it is necessary to search for alternatives. Sampling was carried out at the South Biofactory (Granada, Spain). The screening composition was mainly composed of sanitary textiles (52.1%) and other fractions such as paper and cardboard, vegetables, plastics, and other fine particles, similar to the rejects from mechanical biological treatment. The samples showed a high lower calorific value (24.29 MJ/kg dry sample), which was reduced by the moisture content of the waste (77.3%). The lower calorific value results, and the chlorine and mercury contents, met the requirements for the potential production of solid fuel recovered from the screening. Consequently, this alternative could be developed for energy recovery in thermochemical processes.

Energy recovery options for the management of cellulose-based bio-plastics and mixed municipal solid waste

The incomplete degradation of bio-plastics waste is undermining the suitability of current anaerobic digestion (AD) and composting, opening up the exploration of new treatment routes, including mixed waste. Bio-plastics are treated with mixed waste in a mechanical-biological treatment (MBT) plant with the production of a dry fraction for incineration and a biologically stable one for landfill. An alternative post-treatment of the bio-stabilised waste based on AD is possible. It follows that the main goal of this study is to verify the feasibility of managing cellulose-based bio-plastics with mixed municipal waste, comparing the AD of the MBT plant outputs with the possible solution of incineration. Experimental lab tests have been carried out to describe the two processes, whose results were the basis for an economic analysis. The values of biochemical methane potential (BMP) and higher heating value (HHV) are used as an index for AD and incineration, respectively. Samples with and without bio-plastics were created; fresh and processed (7 and 14 days of biostabilization) samples were, thus, analysed. For AD, the fresh waste averaged 177 NmL CH4/gVS and a 22% and 33% decrease in BMP value was achieved after 7 and 14 days of biostabilization, respectively. Samples with and without bio-plastics revealed a similar trend. 7 and 14 days of biostabilization increased the HHV of fresh waste (18.01 J/kg) of 16.1% and 19.7%, respectively. Bio-plastics did not significantly change the HHV. The economic analysis revealed the suitability of both AD and incineration, irrespective of the presence of bioplastics.

A Comprehensive Review for Removal of Non-Steroidal Anti-Inflammatory Drugs Attained from Wastewater Observations Using Carbon-Based Anodic Oxidation Process.

Non-steroidal anti-inflammatory drugs (NSAIDs) (concentration <µg/L) are globally acknowledged as hazardous emerging pollutants that pass via various routes in the environment and ultimately enter aquatic food chains. In this context, the article reviews the occurrence, transport, fate, and electrochemical removal of some selected NSAIDs (diclofenac (DIC), ketoprofen (KTP), ibuprofen (IBU), and naproxen (NPX)) using carbon-based anodes in the aquatic environment. However, no specific protocol has been developed to date, and various approaches have been adopted for the sampling and elimination processes of NSAIDs from wastewater samples. The mean concentration of selected NSAIDs from different countries varies considerably, ranging between 3992–27,061 µg/L (influent wastewater) and 1208–7943 µg/L (effluent wastewater). An assessment of NSAIDs removal efficiency across different treatment stages in various wastewater treatment plants (WWTPs) has been performed. Overall, NSAIDs removal efficiency in wastewater treatment plants has been reported to be around 4–89%, 8–100%, 16–100%, and 17–98% for DIC, KTP, NPX, and IBU, respectively. A microbiological reactor (MBR) has been proclaimed to be the most reliable treatment technique for NSAIDs removal (complete removal). Chlorination (81–95%) followed by conventional mechanical biological treatment (CMBT) (94–98%) treatment has been demonstrated to be the most efficient in removing NSAIDs. Further, the present review explains that the electrochemical oxidation process is an alternative process for the treatment of NSAIDs using a carbon-based anode. Different carbon-based carbon anodes have been searched for electrochemical removal of selected NSAIDs. However, boron-doped diamond and graphite have presented reliable applications for the complete removal of NSAIDs from wastewater samples or their aqueous solution.

Effect of MBT on landfill behavior: an Italian case study

Bad choices in municipal waste (MW) management cause negative effects on sustainability. Evolving regulation has identified prevention and recycling as the best strategies; nevertheless, disposal in landfilling sites plays an essential role since a complete zero-waste scenario is not realistic, currently. Nowadays, policies require a preliminary waste stabilization to decrease the putrescible content. Therefore, mechanical biological treatment (MBT) has replaced the previous crushing, aimed at simple volume reduction. Literature has proved the effectiveness of MBT when MW collection system is ineffective. The present paper considered a facility in an area with a high-performance MW collection system. A long-term (1999–2019) on-site sampling allowed the comparison between two sites of the facility: the old site (before the MBT activation) and the new area, where the stabilized waste is disposed of. Monitoring of biogas, leachate (analyzed parameters: pH, BOD5, COD, ammonia-nitrogen) and odorous emissions was performed to verify the effect of the stabilization process. The considered long period and the on-site sampling support the relevance of the results, compared to the available literature, often referred to as laboratory scale. The results proved the relatively low benefit of stabilization at the considered facility, which cannot justify the energy consumption of MBT.

Hydrothermal carbonization of the wet fraction from mixed municipal solid waste: Hydrochar characteristics and energy balance

Mixed municipal solid waste (MSW) may be pre-treated in a mechanical–biological treatment (MBT) plant to produce an exiting stream with improved combustible characteristics. The process also produces a second waste stream, which is generally separated on a size basis by industrial sieving equipment. It contains fractions with a high moisture content such as residual food waste, soiled paper and cardboard, and small fragments of other materials. Samples of this stream, collected at an existing plant, were characterized and processed by hydrothermal carbonization (HTC) at laboratory scale, at various temperatures (180, 200 and 220 °C), reaction times (1, 4 and 8 h) and solid to water ratios (0.15 and 0.07). The primary energy balance, on a hypothetical industrial scale, was performed. In brief, the results confirmed that the produced hydrochar was a brittle, hydrophobic, solid carbonaceous product which gave a better combustion performance as the residence time of the HTC process was increased. Moreover, the dewaterability of the carbonized waste was greatly improved when compared to raw, wet samples. The results of the primary energy balance confirmed that the energy contained in the produced hydrochar was higher than the energy consumption for the process itself, under all the HTC working conditions. The energy consumed in the process was in the range of 40–70 % of the energy content of the produced hydrochar.

Microplastics generation and concentration during mechanical-biological treatment of mixed municipal solid waste

Mechanical-biological treatment (MBT) is a popular solution for the processing of mixed municipal solid waste (MSW). However, it is assumed that the treatment processes can lead to the generation of microplastics in large quantities and their concentration in the organic output. Organic outputs from MBT as a source of microplastics are still poorly understood. The current article aims to fill this gap and investigate microplastics formation during MBT and their abundance in ready stabilized organic output. Seasonal samples were taken from the four stages of the possible microplastics pathway in MBT to study changes in microplastics numerical and mass concentration, shape and size. Large microplastics were identified by Fourier transform infrared spectroscopy, and small microplastics by Nile Red dye staining method. The results showed that both mechanical pre-treatment and aerobic treatment had a significant impact on microplastics formation, while mechanical post-treatment only resulted in the enrichment of the output with microplastics. Moreover, microplastics became finer during treatment. Microplastics abundance in ready organic output ranged from 8925 ± 1344 particles/kg in winter 2021 to 17407 ± 4319 particles/kg in summer 2020, and up to 160.5 t of microplastics were emitted from the Kaunas MBT treatment facility during the study year. In addition, a relationship between the microplastics abundance and plastic content of the incoming waste was found by a regression analysis. Therefore, to reduce the formation and emission of microplastics by MBT, the organic fraction of MSW should be collected and treated separately.

Can the biological stage of a mechanical–biological treatment plant that is designed for mixed municipal solid waste be successfully utilized for effective composting of selectively collected biowaste?

Although the requirements for overall recycling rates can only be met when organic recycling is not overlooked, information is scarce regarding adaption to biowaste composting of existing mechanical–biological treatment (MBT) plants originally designed for stabilization of organic municipal solid waste (OFMSW). Thus, this study aimed to assess the suitability of the operational conditions in the biological part of a full-scale MBT plant now used for stabilization of OFMSW (working line: closed-module–covered-pile–open-pile) with a view to producing compost from biowaste. Temperatures above 75 °C were maintained in the closed module and reached again in the covered pile, indicating that intensive organic-matter mineralization occurred in both stages. In the covered pile, the temperature sharply decreased, indicating depletion of easily biodegradable organic matter. An aerobic 4-day respiration test (AT4) value below 10 mg O2/g dry matter, the cut-off for assessing compost stability, was obtained after 8 weeks. However, a high content of humic substances (HS), reflecting compost maturity, was obtained only after 120 days. The increase in HS content proceeded in two phases. In the first phase (45–84 day), the rate constant and the rate of HS formation were lower than in the second phase (84–120 day) (0.072 vs. 0.087 day−1, 1.97 vs. 3.06 mg C/(g organic matter·d)). All the above-mentioned indicators and the nutrient content (N, P, K, Mg, Ca) in the compost indicates that the biological stage of an MBT plant can successfully treat biowaste. This is in accordance with a circular economy and will contribute to increasing recycling rates.

Optimization of the Biostabilization Process of an Italian Mechanical–Biological Treatment Plant to Account for Changes in Waste Composition

In this paper, we present a case study on the optimization of the biostabilization process of an Italian mechanical–biological treatment (MBT) plant to account for changes in feed waste composition related to a progressive increase in separate collection of MSW fractions, biowaste in particular. After ten years of operation (2009–2019), a decrease of the stabilization degree of the output material of the plant was detected, with Dynamic Respiration Index (DRI) values above the established limit of 1,000 mgO2/kgVS/h (average values of 4,000 mgO2/kgVS/h determined weekly for eight weeks). The investigations carried out in 2019 on the waste samples feeding the MBT plant showed that paper and plastic materials constitute around 75% of the input waste to the two aerobic bioreactors of the MBT plant, against 55% at the start-up (2009). Furthermore, the airflow rates and the moisture content analyzed weekly for eight weeks in the bioreactors resulted below the optimal values suggested in the literature. To improve the performances of the biostabilization process, a series of modifications were implemented in the plant. The main modification involved the primary mechanical treatment by varying the mesh size of the screens to 50 mm circular holes mesh. Furthermore, the configuration of the aerobic bioreactors was changed by placing the two bioreactors in series (instead of the previous configuration in parallel) and using a screening unit (25 mm) between the two bioreactors instead of at the end of the process. In this way, the residence time of the materials in the aerobic treatment was enhanced from 16 days with the previous configuration to 27 days. Together with an increase of the airflow rates of around 40% and a water supply of approximately 10% in the bioreactors, these modifications allowed to achieve the desired stability of the output waste, with DRI values below 1000 mgO2/kgVS/h.Graphical

Environmental Comparison of Different Mechanical–Biological Treatment Plants by Combining Life Cycle Assessment and Material Flow Analysis

The role of Mechanical–Biological Treatment (MBT) is still of the utmost importance in the management of residual Municipal Solid Waste (MSW). These plants can cover a wide range of objectives, combining several types of processes and elements. The aim of this work is to assess and compare, from an environmental point of view, the performance of seven selected MBT plants currently operating in different countries, which represent the main MBT layout and processes. For the scope, a combined Life Cycle Assessment (LCA) and Material Flow Analysis (MFA) approach has been adopted to assess plant-specific efficiencies in materials and energy recovery. Metals recovery was a common and high-efficiency practice in MBT; further recovery of other types of waste was often performed. Each assessed MBT plant achieved environmental benefits: among them, the highest environmental benefit was achieved when the highest amount of waste was recovered (not only with material recycling). Environmental results were strongly affected by the recycling processes and the energy production, with a little contribution from the energy requirement. The impacts achieved by the MBT process were, on average, 14% of the total one. The main condition for a suitable MBT process is a combination of materials recovery for the production of new raw materials, avoiding disposal in landfill, and refuse-derived fuel production for energy recovery. This work can be of help to operators and planners when they are asked to define MBT schemes.