Plastic impurities in biowaste treatment: environmental and economic life cycle assessment of a composting plant

<jats:title>Abstract</jats:title><jats:p>The study focuses on an Italian composting plant and aims to investigate the impact of the presence of plastic impurities in the collected biowaste on the environmental and economic performance of the plant. The study is divided into two main steps: firstly, a material flow analysis was conducted to quantify the number of impurities (e.g., conventional plastics and compostable plastics) before and after the composting process. Secondly, a life cycle assessment (LCA) and a complementary life cycle costing (LCC) of the composting process were conducted. The results of the material flow analysis confirmed the initial assumption that conventional plastic remains almost constant before and after the composting treatment, while compostable plastic almost disappears. As far as the life cycle analyses are concerned, the most environmentally damaging phases of the process were the shredding and mixing phases, while the operating costs (OPEX) contributed the most to the total annual costs of the company. Finally, a further scenario analysis was performed, assuming that the plastic contaminants in the treated biowaste consisted exclusively of compostable plastics. The comparison with this ideal scenario can support decision-makers to understand the potential improvements achievable by addressing the presence of plastic impurities in the biowaste. The results show that the treatment of plastic impurities causes relevant environmental and economic impacts, being responsible for 46% of the total waste to treat at the end of the process, almost 7% of the total annual costs covered by the plant owners, and about 30% of all negative externalities.</jats:p>

section_content: Circular economy practices are of utmost importance to strive for a more sustainable and resourcee cient world (Brandao, et al., 2021). In order to close biological material cycles and reduce the linear economy of land lling and incineration, it is crucial to properly treat organic waste. Indeed, it represents a considerable amount of municipal solid waste, a 39. 5% in Italy (ISPRA 2020) and an average of 34% in Europe (EEA 2020). 

Composting is considered a fairly sustainable solution, as it signi cantly reduces the amount of waste that would otherwise go untreated (Pergola, et al. 2018 ). Several studies have proven that composting facilities are extremely bene cial if the composting process is carried out correctly and the leachate is properly treated (Bernstad, et al. 2011 ) (Amlinger et al. 2008 ). Moreover, the compost produced in a composting plant is a high quality, marketable product suitable for small and large applications (Amlinger et al. 2008 ). As a consequence, it is considered preferable compared to other waste management alternatives (Oliveira et al. 2017, Shen et al. 2021, Scoton et al. 2016 ). 

Nevertheless, industrial compost production is still an environmentally damaging process due to the transport and several energy-intensive operations involved. In this context, previous studies (Amlinger et al. 2008, Boldrin et al. 2009 ) have addressed the problem of greenhouse gases produced by fossil fuel transport and aerobic treatment. 

Moreover, when incorrect waste separation occurs, another problematic aspect is the presence of plastic contaminants that get into the organic fraction. In this case, the compost may be compromised, and the quality affected. 

More speci cally, conventional plastics do not degrade in the composting process and therefore, need to be separated, when possible, from the mature mixture (Gómez et al. 2013, Vaverková et Adamcová et al. 2013 ). Additionally, composting processes cause conventional plastic products to fragment and become part of the compost that is eventually used as organic fertilizer in agriculture. This leads to unintentional release of microplastics into the environment (Millican et al. 2021, Braun et al. 2021 ). Research by (Braun et al. 2021 ) also found that the presence in composting facilities of microplastics from conventional plastics, e. g., PE and PVC, contribute signi cantly to greenhouse gas and ammonia emissions. Larger plastic products that can be separated from compost still pose a problem, as they cannot be recycled anymore but have to be sent to incinerators. 

On the other hand, compostable bioplastics could provide a partial solution to this pressing problem by contributing to a real transition towards a circular bioeconomy (Beltran et Issues related to the presence of plastic in composting plants have been addressed by multiple researchers. However, none of the previous studies have attempted to combine a waste ow analysis with the application of an environmental and economic life cycle analysis. The novelty of this project is the enrichment of the research with two speci c assessments that allow the comparison of two possible scenarios. The current condition, in which the waste scenario consists of the treatment of both conventional and biodegradable plastic and an ideal scenario, in which the plastic waste fraction entering the process is ideally composted of only biodegradable plastic, which thus, becomes part of the compost at the end of the treatment. 

Life cycle analysis is indeed also widely applied in the context of organic waste management, and several studies developed comparison analysis based on life cycle analysis (LCA) to nd the best alternative among different end-of-life solutions (Lu et al. 2020, Andersen et al. 2012, Saer et al. 2013 ). Furthermore, several other research combined LCA studies with the complementary Life Cycle Costing (LCC) to assess and compare different waste management system solutions (Lam et al. 2018). Some focused on municipal solid waste (Song et al. 2013 ) (Woon et al. 2014 ), some on food waste (Gómez et al. 2013 ) and some on other kinds of waste (Rocchetti et al. 2013 ) (Simon et al. 2013) This project speci cally addresses the risks and hazards that arise when unintended plastic products are present in the organic waste and thus in the compost mix. Therefore, two follow-up analyses were developed to further investigate the sustainability of a composting plant in the Emilia-Romagna region, Italy. First, an analysis of the waste streams was carried out. Secondly, the main environmental and economic factors were identi ed through a life cycle assessment and life cycle costs analyses.
section_name: Introduction

section_content: In Italy, the organic fraction is mostly collected separately from the rest of the municipal waste and sent for proper treatment. According to (ISPRA 2020), in 2019, 49. 2% of the collected organic waste was disposed of in composting plants, 45. 7% was treated by integrated anaerobic-aerobic methods and the remaining 5. 1% was sent to anaerobic digestion. This case study concerns a composting facility managed by the cooperative "La Città Verde". 

The composting plant of La Città Verde is located in Crevalcore (Emilia-Romagna Region, Italy) and treats around 15,000 tonnes of biowaste every year to produce and commercialize compost, according to the Italian law 75/2010 requirements (La Città Verde, ANPA 2002). It also produces lignocellulosic biomass from green waste (wood chips) to be used as biofuel. More information on the plant and on the cooperative is described in the supplementary information (SI). The organic waste collected at La Città Verde is initially taken into a shed where it is placed in a mixer using an automated mechanical arm and then mixed with the lignocellulosic material. 

The ratio between the two materials depends mainly on seasonality and, generally, it is about 60% of organic waste and 40% of lignocellulosic material. The pile is then treated in two distinct phases, regulated with different temperatures. The rst phase is the mesophilic phase, which works within a temperature range between 18°C and 44°C. Most of the simplest organic substances e. g., carbohydrates, lipids, and proteins, are transformed into water, heat, and CO 2. In particular, heat is an essential element because of its bactericidal action. Subsequently, in the second thermophile phase, the heap is moved into a speci c container having a maximum capacity of 250 tonnes and subject to temperature regulation. During the second phase, microorganisms grow and degrade other organic fractions through hydrolysis. The process lasts around 30 days: it ends when the heap loses the correct amount of water. A considerable amount of leachate is generated during the process and transported to two speci c treatment facilities. Afterward, during the curing phase, which lasts 20 days, the compost maturates inside a particular container reaching a maximum temperature of 55°C. Multiple software can regulate temperature changes and percolate growth from the control cabin. Lastly, the compost is brought outside, where it maturates for a further 40day period in open cells, for a total of 90 days. 

The resulting compost is then sifted and managed according to the size. Composting products having less than 10 mm diameter are the actual compost, i. e. the marketable good ready to be sold. The products ranging between 10 and 50 mm in diameter that mainly consist of ligneous materials are reused in a new composting process with a new fresh organic fraction. The residual waste (e. g., plastics and other non-organic materials) is sent to proper treatment.
section_name: Materials And Methods

section_content: A waste ow analysis can be conducted by using different methods. Compost sampling is the rst step to perform a mass characterization analysis, in which different types of organic materials are involved. 

The quartering method, as described in the UNI EN ISO n° 5667-13 ( 2000 Firstly, the sample is mixed on a surface to form a cone. During this phase, it is recommended to drop the material from the top of the cone to guarantee a proper distribution. Afterwards, the heap is homogenously divided into quarters; then, two quarters are selected and combined. This procedure is repeated until the last two quarters form the mass sample required. 

The quartering method was applied following general guidelines (ARPA Piemonte, 1998) and adapted to our case study which consists of a total amount of initial fresh organic waste was 1,600 kg. 

After the quartering process, a waste ow analysis was performed to identify the amount of plastic and bioplastic entering the plant and the amount remaining after the composting process. 

To this end, the fresh organic waste was rst analyzed before being mixed with the lignocellulosic component, and then the mature mixing was examined before being screened. Subsequently, through a visual inspection, the obtained 115 kilograms of waste was laid on a table and sieved into four categories: organic waste, conventional plastic, biodegradable plastic, and other kinds of wastes e. g., aluminium, textiles, polylaminates, paper, and other non-organic materials. The different groups were then weighted. 

The second analysis was performed on the mature compost before it was sifted. Considering that this mixture was already mixed several times throughout the whole process, 100 kg of material from the pile was selected and analyzed. The resulting sample was signi cantly homogenous and therefore, only part of the sample (40 kg) was investigated. The material under analysis was laid on a table, manually sorted, and divided into plastic, biodegradable plastic, compost, and other waste. Lastly, all the separate groups were weighted for the nal discussion.
section_name: Waste ow analysis

section_content: LCA is a standardized method known and used worldwide (ISO 14040 2006, ISO 14044 2006). Because of its holistic approach, it represents a great support tool for decision-making processes, given the signi cant potential of avoiding burdens shifting problems. 

The methodology allows the users to analyze simultaneously several environmental impact categories related to a product, a service, or even a system throughout the entire life cycle (European Commission 2010a, European Commission 2010b). 

According to the o cial standard of environmental management ISO 14040:2006 (ISO 14040 2006) the LCA methodology consists of four iterative steps: goal and scope de nition, life cycle inventory, life cycle impact assessment, and interpretation of the results. 

The assessment was developed using the SimaPro 8. 5 software. According to the study purposes and the geographical area considered, the modelling was conducted applying the CML-IA baseline methodology. 

The CML-IA baseline method does not take into account biogenic CO 2 emissions in the calculations: this ts the context of the research. Indeed, several authors in the literature suggested to not considering in the calculation the CO 2 emitted during the composting because biogenic and not fossil-based ( The life cycle assessment analysis was performed considering the sole composting process, being the waste management system the main focus of the study. Consequently, potential environmental impacts imputable to the operational activities of the rest of the facility e. g., heating structure system, water consumed as a service, workers equipment, were not considered. 

The functional unit (FU) used is the total amount of food and wood waste delivered to La Città Verde in 2020. Data gathered were mainly primary data provided by the Waste Treatment Manager and adapted upon our scope, guaranteeing a high adherence to reality. When needed, secondary data were gathered from the Ecoinvent v. 3 database. 

A cradle to gate analysis was performed, setting the boundaries from the waste collection to the production of the nal compost. Following the ''zero burden'' assumption, waste has no burdens before it becomes waste (Ekvall et al., 2007). Wood and organic waste are collected from several collection sites and transported to the composting plant. Subsequently, they are mixed and shredded with the residual wood waste from former compost productions. At this point, the product undergoes two maturation stages (indoor and outdoor), and lastly, it is sent to the screening phase, whose output is marketable compost. The diagram represents the maturation steps as part of the shredding and the mixing phase. There is also a supplementary step through which the residual wood material is again screened, and additional wood chip products are gained to be used for a new process. Other kinds of waste are inevitably produced and sent to external treatment facilities. Notably, almost the entire fraction of residual solid waste at the end of the treatment is composed of plastic, while smaller contributions are given by metal, glass, and textile waste. The residual plastic represents 4% of the total waste entering the plant and being treated. Additionally, during the rst stage of the process, 8% of leachate is generated and is currently sent to external treatment facilities. The distribution of the compost to external consumers and its use are not included in the boundaries. Lastly, a total annual amount of 370 UO/m 3 odour emissions were identi ed but not involved in the software calculation since they are below the authorization limits. 

The Life Cycle Inventory (LCI) is reported in Table 1 (see SI for further information).
section_name: Life Cycle Assessment

section_content: The life cycle costing is an economic analysis frequently used to complement LCA in decision -oriented processes (Lu et al. 2020, Elagroudy et al. 2011 ) and calculate the costs along the life cycle of a product or a system (Hunkeler et al. 2008). 

Within this case study, an environmental LCC was performed, referred to the year 2020. Environmental LCC can be de ned as the assessment of all costs associated with the life cycle of a product directly covered by one or more of the actors in the product life cycle (supplier, manufacturer, user or consumer, and/or end of life actor), with the inclusion of externalities that are anticipated to be internalized in the decision-relevant future (Hunkeler et al. 2008 ). The boundaries and the functional unit used for the LCA analysis were maintained, while specifying the different actors involved (i. e., waste collectors who committed the waste in input to the plant, plant owners, waste managers responsible for the treatment of residual waste). 

As far as waste collectors are concerned, the cost for the transport of waste to the plant is the only one considered in the analysis: it includes the costs for fuel, motorway tolls and personnel costs (more details are available in SI). 

The costs covered by the plant owners include operating costs and capital costs, while their incomes come from the waste in input committed to the plant and the sale of compost and wood chips. 

On the one hand, the costs are primary data, since they were provided by the plant manager; on the other hand, some estimations were necessary to calculate the incomes from the waste committed to the plant: to estimate this value, rst, all the contracts which the plant had with customers in 2018 were analyzed, in terms of the maximum amount of waste which each of them could yearly commit to the plant and the price per ton. A weighted average of the price per ton was calculated, both for organic waste and wood waste: this value was used to calculate a proxy value of the incomes. As far as incomes from the selling of compost are concerned, an average price equal to 3 €/ton was considered (primary data), while the unit average price of wood chips is equal to 20 €/ton (moisture content of less than 40% and particle size of up to 40 mm). 

Finally, the cost for the treatment of residual waste in output was also included, as well as the cost for the treatment of the leachate. 

Moreover, following (Magrini et al. 2021 ) who studied the waste management system of the Emilia-Romagna Region, the externalities from the transportation of waste, the consumption of electricity of the plant and the incineration of residual waste were also calculated. The externalities from the emissions of the composting plant were not included because the CO 2 is biogenic, and the use of the bio lter allows the reduction of VOC to a negligible level. Further details on the methodology followed for the assessment of externalities are described in the SI.
section_name: Life Cycle Cost Analysis

section_content: To frame a more complete evaluation and to better tackle plastic-related issues concerning composting activities, the life cycle methodologies were applied to assess an ideal scenario. In this scenario, it was assumed that all the plastic in input consisted of biodegradable plastic, which, therefore, became part of the compost at the end of the treatment without generating additional plastic waste residuals. This assumption is also supported by literature studies that developed speci c works on the degradability of different kinds of bioplastic during the composting process (Ohtaki et al. 2000). In particular, for this case study, biodegradable plastic food packaging and biodegradable plastic bags, whose degradability conditions are suitable for composting, are of interest (Ohtaki et al. 2000). 

Table 4 shows the revised inventory data with the assumption of considering only biodegradable plastic in the treatment and assuming the same prior functional unit equal to the total amount of food and wood waste delivered to La Città Verde in 2020.
section_name: Scenario Analysis

section_content: The results of the waste ow analysis are shown in Table 2 and Table 3, for the fresh organic waste and the mature compost, respectively. The percentage of conventional plastics remains nearly constant before and after the treatment. On the other hand, the biodegradable plastic almost disappears in the composted fraction of waste, consisting of only 0. 63% of the total amount. 

The organic waste analyzed resulted to be relatively homogenous: only a tiny fraction of the total fresh organic waste was extraneous material. This includes plastics and textiles, aluminum, and other materials in smaller shares. Biodegradable plastic products represent 4. 8% out of the total waste fraction. 

It is worth noting that considering the total amount of waste, biodegradable plastic is the only fraction that almost entirely disappeared, whereas conventional plastic and other extraneous waste remain nearly constant. 

From this analysis, it was possible to con rm that biodegradable bioplastic is indeed a good solution to plastic pollution issues if adequately managed. On the other hand, the analysis also con rmed that
section_name: Waste ow analysis results

section_content: The LCIA results are shown in Fig. 2 grouped by the three main phases of the process (i. e., screening, shredding and maturation, transport) and the waste treatment. The shredding and maturation phase is the main contributor in 7 out of the 11 impact categories analyzed. This is a reasonable result, considering the high energy consumption of these stages. This result is also consistent with the study developed by (Lu et al. 2020), in which energy consumption has the most signi cant impact. On the other hand, (Boldrin et al. 2009 ) and (Lundie et al. 2005 ) stated that most environmental impacts were imputable to transportation and waste collection. This may be due to the type of transportation means used and to the distance travelled. In our case, only local and regional collections were considered, and therefore relatively short distances were calculated. 

Acidi cation and eutrophication potential indicators are signi cantly high in the shredding and mixing phase, and this is caused by the generation of leachate, and, to a lesser degree, also by some ammonia emissions, although being the e ciency of the facility in treating and regulating ammonia emissions very high. Indeed, the emissions generated during the decomposition are beneath the legal threshold as a result of the use of a bio lter which constantly captures and biologically degrades pollutants. Similar results have also been demonstrated by (Saer et al. 2013 ), which developed an exhaustive attributional life cycle analysis of a food waste composting system with the novelty of considering the use of the compost as a soil conditioner and as an alternative to peat. (Saer et al. 2013 ) concluded that compost processing was the most environmentally demanding stage in the procedure due to decomposition emissions, which highly impact global warming, acidi cation, and eutrophication. 

Marine aquatic, freshwater aquatic, and human ecotoxicity are primarily associated with the last stage of the process when the compost is nally ready, and the residual waste is separated and then incinerated in speci c treatment facilities. The incineration of waste is, indeed, potentially harmful both for humans and for the environment. 

Considering the scenario analysis, as far as the waste treatment is concerned, the results obtained are signi cantly different. The presence of plastic in the residual solid waste at the end of the treatment has important implications in most of the impact categories analyzed as shown in Fig. 3. 

More speci cally, human, marine and terrestrial ecotoxicity reduce, respectively, of 84. 5%, 92. 6% and 87. 3%. An impressive reduction rate also occurs for Global Warming whose impacts are inevitably related to incineration facilities for solid waste treatment. Further information about the reduction rates from the comparison of the two different scenarios are grouped and shown in SI.
section_name: LCA results and discussion

section_content: The results of the LCC analysis are shown in Fig. 4. For con dentiality reasons, cost and income items of the plant are shown only as a percentage of the total value. As far as internal costs covered by the plant owners are concerned, the operating expenses (OPEX) are the main contributor (69% of total costs), as they include a wide range of expenses e. g., electricity bills, fuel consumption and labour costs. It is worth noticing that the machinery in the plant works at an intensive energy rate all over the year. The cost for treatment of residual waste (12% of the total costs) is mostly related to the incineration of plastic waste. 

The main source of income for the plant is the contribution paid by waste collectors for the committed waste (93%), while the sale of compost is not very pro table, as it represents only 1% of the total income. 

When it comes to externalities, the biggest contribution is given by transport, followed by the electricity consumption of the plant. The positive externalities related to the generation of electricity from incineration of waste are offset by the negative externalities imputable to the emissions from incineration. 

In the scenario analysis (Fig. 5 ), externalities from incineration become positive, but still negligible if compared to the negative externalities related to transport and electricity consumption of the plant.
section_name: LCC results and discussion

section_content: Composting fully embodies sustainability and circular bioeconomy principles by transforming and recycling -under correct and speci c conditions -organic waste that otherwise would be land lled. 

Particularly, this research focuses on an Italian composting facility located in the Emilia-Romagna region. 

The aim was to analyze whether the presence of plastics in biowaste would affect the quality of the nal compost and generate negative environmental and economic consequences. 

To this end, the study was divided into three sections. A waste ow analysis was rstly developed by applying the quartering method described in the UNI EN ISO 5667-13 (2000) to quantify and compare the amount of conventional plastic and biodegradable plastic impurities before and after the composting treatment. The comparison con rmed that conventional plastics and biodegradable plastics show an opposite behavior during the process. On the one hand, the percentage of conventional plastics remains nearly constant, whereas, on the other hand, the amount of biodegradable plastic almost disappeared in the composted waste fraction. 

Additionally, an environmental life cycle assessment (LCA) and a complementary life cycle costing (LCC) were developed. The two analyses showed consistent outcomes in which the mixing and shredding phases represent the most environmentally demanding stages of the process. In line with this, a similar result was also provided by the economic assessment, in which OPEX is the rst contributor to the total annual expenses of the company. 

Moreover, a further analysis was developed to investigate better the impacts imputable to the presence of plastic impurities. To this end, a life cycle comparison was conducted between the current situation with an ideal scenario in which no conventional plastic products were present in the collected biowaste, but, instead, plastic impurities consisted solely of biodegradable plastics. The results show important environmental differences in most of the impact categories analyzed. Conventional plastic impurities mainly in uence human, marine, and terrestrial ecotoxicities. These impacts can respectively reduce of 84. 5%, 92. 6%, and 87. 3%, if no conventional plastic needs to be treated. It was also proved that plastic impurities are responsible for 7% of the total annual costs covered by the plant owners and around 30% of all the negative externalities. These impacts could be potentially avoided if biodegradable plastic would entirely replace the conventional one. 

However, the results partially describe the degree of danger that conventional plastic particles represent when present in the organic fraction. Additional investigations and contributions may be necessary to further extend the analysis outside the boundaries selected, including the use phase of the compost. For To conclude, this study ts into the current Italian context in which biodegradable plastic is experiencing signi cant momentum, representing the primary transitional strategy adopted by the country to reduce plastic pollution (Imbert et al., 2019) (Imbert, 2017). 

On another note, the present results con rm the importance of improving waste separation at source. The intended achievement can wisely be addressed by raising citizens' awareness and intensifying communication strategies regarding the consequences and damages of mismanaged waste to the environment and human health.
section_name: Conclusion

section_content: Giulia Adele Tuci; Writing -review and editing: Sara Bottausci, Chiara Magrini, Giulia Adele Tuci, Alessandra Bonoli; Supervision: Alessandra Bonoli. 

Funding No funding was received to assist with the preparation of this manuscript.
section_name: Declarations

section_content: Acknowledgments The authors thank the Waste Treatment Manager Giorgio Rosso for his fundamental support in the plant knowledge and help in data gathering, and the students Simone Rosso and Annalisa Zoli for their extremely signi cant contribution. 

Authors Conceptualization: Sara Bottausci, Chiara Magrini, Giulia Adele Tuci ; Methodology: Sara Bottausci, Chiara Magrini, Giulia Adele Tuci ; Formal analysis and investigation: Sara Bottausci, Chiara Magrini, Giulia Adele Tuci ; Writing -original draft preparation: Sara Bottausci, Chiara Magrini,

section_name: Disclosures and declarations

section_content: Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

section_name: Ethics approval and consent to participate

section_content: We all declare that manuscript reporting studies do not involve any human participants, human data, or human tissue. So, it is not applicable.
section_name: Ethics approval and consent to participate

section_content: Our manuscript does not contain data from any individual person, so it is "not applicable. "
section_name: Consent for publication

section_content: The authors declare no competing interests.
section_name: Competing interests

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section_name: Supplementary Files

section_name: SupplementaryInformationESPR.docx