Waste Management 171 (2023) 350–364 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Research Paper Life cycle assessment of biowaste and green waste composting systems: A review of applications and implementation challenges E.R. Oviedo-Ocaña a, C. Abendroth b, c, I.C. Domínguez a, A. Sánchez d, *, C. Dornack b a Universidad Industrial de Santander, Facultad de Ingenierías Fisicomecánicas, Grupo de Investigación en Recurso Hídrico y Saneamiento Ambiental - GPH, Carrera 27 Calle 9 Ciudad Universitaria Bucaramanga, Colombia b Technische Universität Dresden, Institute of Waste Management and Circular Economy, Pratzschwitzer Str. 15. 01796 Pirna, Germany c Brandenburg Technical University Cottbus-Senftenberg, Faculty of Environment and Natural Sciences, Lehrgebäude 4 A R2.25, Siemens-Halske-Ring 8 03046 Cottbus, Germany d Universitat Autònoma de Barcelona, Department of Chemical Engineering, Composting Research Group, 08193, Barcelona, Bellaterra, Spain A R T I C L E I N F O A B S T R A C T Keywords: Composting is one of the most widely applied methods for recycling organic waste. This process has been pro­ Life Cycle Assessment posed as one option that facilitates the reincorporation of materials into the production cycle. However, com­ Composting posting also generates environmental impacts. Life Cycle Assessment (LCA) is the most common approach to Environmental impact evaluate the environmental impacts of a process at different system stages. Nevertheless, applying LCA in Biowaste Green waste composting facilities is challenging due to the extensive information required, the lack of standardization on the initial assumptions, the definition of system boundaries, and the high diversity of existing composting tech­ nologies. This paper systematically reviews LCA studies in biowaste and/or green waste composting. The study highlights the challenges that should be met in order to improving the application of LCA to evaluate the environmental impacts of this type or waste treatment strategy. The review protocol used identified 456 papers published between 2010 and 2022. After the screening, 56 papers were selected, read, and thoroughly analyzed. The results show that: i) about 68% of the studies aimed to compare composting with other solid waste man­ agement options; ii) there was a wide diversity among the impact categories considered, which predominantly included climate change and ozone depletion; iii) there was no consensus on the functional unit or the system boundaries; iv) the main gaseous emissions studied were ammonia, methane, and nitrogen oxide, which were generally determined by emission factors; v) the avoided environmental impacts associated with the end-product quality and its application as an organic amendment or soil improver were ignored. This work demonstrates the complexity of conducting credible and valid composting LCA studies and proposes seven recommendations for improving the application of this assessment methodology to analyze this waste management alternative. 1. Introduction (Jensen et al., 2016). Composting is commonly suggested as an ideal treatment method for Green waste (GW) and biowaste (BW) represent an important frac­ organic waste, as it complies with circular economy strategies thanks to tion of municipal solid waste (MSW) (Thi et al., 2015; Zhang et al., the possibility of material recovery (Oldfield et al., 2018; Zhang et al., 2021). According to Lu et al. (2020), the organic fraction makes up 46% 2021). Composting is the aerobic biological decomposition and stabili­ of the total MSW globally and up to 64% of the MSW in low-income zation of organic waste, which was accurately defined by Haug (1993), countries. Disposing BW and GW in dumps or sanitary landfills results thirty years ago. During composting, the main process parameters (i.e., in the generation of high-strength leachate and contributes to the porosity, moisture, interstitial oxygen, temperature, C/N ratio, avail­ emission of greenhouse gases that can cause adverse environmental and ability of nutrients, pH and biological activity) play an important role in public health effects (Oviedo-Ocaña et al., 2016). Proper waste man­ the process performance, as it has been extensively review by Sayara et agement can contribute to a more sustainable use of Earth’s resources, al (2020) and by Cerda et al. (2018) and Reyes-Torres et al. (2018), better environmental protection, and reduced climate change emissions which are specific compilations of all the issues related to biowaste and * Corresponding author. E-mail address: antoni.sanchez@uab.cat (A. Sánchez). https://doi.org/10.1016/j.wasman.2023.09.004 Received 23 August 2022; Received in revised form 21 August 2023; Accepted 4 September 2023 Available online 12 September 2023 0956-053X/© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 green waste composting, respectively., If all these parameters are summary, applying LCA in composting is challenging due to the exten­ controlled and evolve properly, composting results in a suitable product sive information required, the lack of standardization on the initial as­ for agricultural and horticultural use and erosion control (Saer et al., sumptions, the definition of system boundaries, and the high diversity of 2013). Saer et al. (2013) highlighted the environmental benefits of existing composting technologies. composting for improving soil quality, including i) enhanced carbon On the other hand, several review studies addressed the application storage capacity in the soil, thus, reducing global warming, ii) reducing of LCA in MSW management (e.g., Laurent et al., 2014a, 2014b; Schott the need for fertilizers, pesticides, and peat use, iii) improvements in soil et al., 2016; Yadav and Samadder, 2017; Bernstad and la Cour Jansen, structure, density, and porosity, which increases water retention ca­ 2012; Morris et al., 2013; Yadav and Samadder, 2018a). However, these pacity and reduces erosion and nutrient leaching, and iv) incorporation studies focused on a particular waste (e.g., solid or food) or waste of organic matter, nutrients and electrolytes into the soil. The incorpo­ treatment system (Khandelwal et al., 2019a). None of these studies ration of organic matter includes the formation of humic acid, which is systematically review the LCA application to BW and/or GW known to improve soil quality due to a complex interplay with plants composting. and microbes (Ampong et al., 2022). It is important to mention, ac­ To bridge this gap, this research systematically reviews scientific cording to these benefits, that a complete LCA should cover not only the literature regarding LCA on GW and/or BW composting. This review biological issues related to composting as a biological process or identifies differences, uncertainties, and the lack of descriptive data in compost as organic fertilizer, but the potential impacts of the entire existing LCA studies. The study highlights the challenges to the robust organic matter collection, conditioning of organic matter prior to com­ and reliable application of this environmental assessment approach to posting or compost refining and application, as well as transportation of analyze BW and/or GW composting processes. To the best of the authors organic waste and compost and composting plant building and main­ knowledge and despite a tremendous amount of existing literature on tenance. In fact, even the type of composting plant can have an impor­ composting, the present work is the first to develop an integrated tant effect on the environmental performance of composting, in relevant analysis of multiple LCA studies on biowaste and green waste com­ aspects such as gaseous emissions (Colón et al., 2010). For instance, posting and propose recommendations to improve its implementation. Cadena et al. (2009a) revealed the results obtained when investigating At this point, it is also important to highlight what LCA studies on two typical composting systems such as in-vessel reactor and turned biowaste and green waste composting do not consider in the assessment. piles, with different environmental impacts on critical points such as The most important issue, in quantitative terms, is the benefits of gaseous emissions (directly measured), energy, or water consumption. compost use. It is true that some LCA studies consider compost as a Colón et al. (2012) highlighted these differences again in a highly- substitute of chemical fertilizers as it has a certain content of NPK detailed paper where four full-scale biowaste composting plants were (Quirós et al., 2014a). However, this underestimates other benefits environmentally assessed, including home composting. Unfortunately, related to the presence of stabilized and fertile organic matter, such as as commented later, it is difficult to find papers covering the entire reducing pesticide use, improving soil tilth and workability, or higher process or comparing different composting plants or scenarios (Martí­ carbon sequestration (Favoino and Hogg, 2008). Other types of compost nez-Blanco et al., 2010). uses, such as soil bioremediation, in landfill covers or as biofilter media Conversely, composting can have negative environmental impacts, are typically not considered in LCA studies. It is evident that these are such as carbon dioxide (CO2) emissions from fossil fuel use in trans­ particular situations, but they are resulting in a large number of expe­ portation and processing equipment. Due to oxygen depletion, further riences and publications (Sayara et al., 2020). emissions can be generated, such as methane, nitrous oxide, and ammonia, which are also greenhouse gases and potential odor sources 2. Methodology (Saer et al., 2013; Oliveira et al., 2016). Therefore, it is critical to evaluate the impacts of composting using sustainability assessment 2.1. Literature search methods and tools (Weligama Thuppahige et al., 2022). Life cycle assessment (LCA) is considered a holistic approach that A systematic review method was considered the most suitable to covers the life cycle of a product or process from the cradle to the grave. evaluate LCA studies of BW and/or GW composting since this is a sound This technique has been extensively used at different stages of waste approach to searching, selecting, analyzing, and synthesizing research treatment technologies (Yadav and Samadder, 2018a) to identify and evidence (Dastjerdi et al., 2021). This review included the definition of a evaluate environmental impacts such as global warming, acidification, search protocol, identifying keywords, and available information sour­ ecotoxicity, and human toxicity. LCA could help to select the processes ces. For this, we chose the Scopus® academic database. with lower environmental impacts and avoid the activities with higher The search focused on scientific research articles using the following environmental impacts (Dastjerdi et al., 2021). According to Zhang et al. protocol: (i) publication years between 2010 and 2022 (February); (ii) (2021), LCA studies applied to solid waste management systems typi­ the title and abstract should include the keywords “green waste” or cally have three objectives: i) analyze the environmental performance of “biowaste” AND “composting” AND “LCA” OR “life cycle analysis”; (iii) specific technologies, ii) compare different waste treatment alternatives, only scientific indexed articles; (iv) keywords should contain at least one and iii) provide practical modifications of existing treatment processes of the words: “composting”, “green waste”, “biowaste”, “LCA”, “life to minimize related environmental impacts. Regarding organic waste cycle analysis”, “environmental assessment”, “environmental impact”. composting, the LCA approach identifies the processes and stages with This specific search resulted in 127 papers. the most significant environmental impacts, including the impacts associated with the collection of organic waste, production, and distri­ 2.2. Screening and selection criteria bution of compost, and its use as soil conditioner (Saer et al., 2013). Different studies have applied LCA to analyze BW and/or GW com­ A screening process followed the literature search to narrow the posting (Bong et al.,2017; Abeliotis et al., 2012; Jensen et al., 2016; relevant articles according to their research focus. The following pa­ Oliveira et al., 2016; Martínez-Blanco et al., 2010). These studies show rameters were considered: that the environmental impacts of aerobic composting are very sensitive a) Title and Abstract: papers with a title and/or abstract that lacked to compost facility management practices for maintaining aerobic con­ relation to GW or BW composting and LCA were excluded despite the ditions (e.g., the technology used and operation conditions). Therefore, inclusion of the selected keywords. local circumstances, data sources, subjective assumptions by different b) Abstract: Each abstract was read to verify that the work addressed researchers, and other influencing factors lead to different LCA results LCA studies of GW and/or BW composting. even across studies with common objectives (Zhang et al., 2021). In c) Content: the entire article was read to identify the studies on LCA 351 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 applications in BW or GW composting, including information such as 3. Results and discussion methodology, substrate characteristics, impact categories, goal and functional unit, system boundaries, technology, and environmental 3.1. Research trends and current status impacts. The studies that included information on one or more of the previous topics, were selected for analysis. The composting systems Fig. 1 shows the final number of articles considered in this study. considered included both industrial and domestic (i.e., home compost­ From the initial Scopus® report (including 456 articles), only 56 articles ing) applications. Similarly, studies comparing composting systems with (12.2%) were included for further analysis. Of the 456 documents that other organic waste treatment options were included. Studies in which met the search criteria, 43 were discarded because they were not articles composting was a complement to anaerobic digestion (e.g., digestate (e.g., conference papers, book chapters, or other reports). Subsequently, composting) were discarded. 37 documents were discarded for being outside the analysis period defined for the study (2010–––2022). Furthermore, articles that did not 2.3. Organization and data structure include the keywords “composting”, “green waste”, “biowaste”, “LCA”, “life cycle analysis”, “environmental assessment”, and “environmental An Excel® database was used to organize the bibliography of impact” were discarded. The remaining 127 articles were checked to selected articles, facilitate the search of the different research units ac­ determine if they met the review purpose. As shown in Fig. 2, from 2016 cording to categories (e.g., authors, journals, publication year, and to 2021, between 12 and 18 papers on composting processes were keywords), and classify the articles into folders according to topics. The published. All 127 articles were also sorted according to national origin. supplementary material includes the database (Tables S1 and S2). The countries with the largest number of articles in this review were Italy (18 articles), Spain (14 articles), the United States (14 articles), 2.4. Review and analysis of documents China (13 articles), Thailand (9 articles), Brazil (7 articles), Canada, France and India (6 articles each) and Australia (5 articles). Most of The Excel® database included information from the articles that these studies were conducted in developed countries, showing the need addressed LCA on BW and/or GW composting. The following data were to collect information that facilitates the application of LCAs in devel­ extracted from each article: (i) Objective; (ii) LCA Methodological oping countries. The necessity of reliable and standardized LCAs is even approach; (iii) LCA Software; (iv) Impact categories; (v) Goal and more apparent when considering that many of the composting facilities Functional Unit; (vi) Description of systems boundaries; (vii) Data in developing countries are characterized by poor monitoring and a lack sources (i.e., primary or secondary data); (viii) Information on sub­ of information on their design, operation, and maintenance (Oviedo- strates (e.g., BW, GW, municipal solid waste), technology (process type), Ocaña et al., 2016). emissions and treatment of emissions (e.g., leachate and gases), and A final selection was performed on the 127 pre-selected papers, since energy consumption; (ix) Information on offsets and substitutions rates only a fraction of these addressed BW or GW composting. 71 articles for electricity, soil improvers, and carbon sequestration/storage; (x) were discarded being incomplete LCA studies or addressing substrates Sensitive variables and assumptions that might drive the particular other than BW or GW. Thus, 56 articles were included for a detailed LCA’s results; (xi) Data on the end-product quality. review; of these, 25 articles performed a detailed LCA of BW and/or GW The information was tabulated and graphed. Each topic of items (i) composting systems and were used to extract the critical information for to (xi) was analyzed, and trends were identified. In addition, each the review (tabulated and graphed). The remaining (31) articles did not analyzed topic was compared to information reported by the scientific provide detailed information on applying LCA to evaluate green waste literature. Based on the analysis, challenges and recommendations for and/or biowaste composting systems. However, they were examined to applying LCA to evaluate BW and/or GW composting systems were broaden the discussion on the challenges of applying LCA in this field. proposed. In addition to the 56 articles included, other scientific documents Fig. 1. Outline of the screening of articles for the literature review of Life Cycle Assessment of Biowaste and/or Green waste composting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 352 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 Fig. 2. Distribution of the number of relevant articles addressing the LCA of biowaste and/or green waste composting according to year (2010–2022). Note: Until February 2022, 3 articles were published. Source: Scopus® (2022). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) were consulted that describe information related to mass balances in the 25 articles regarding the application of LCA to BW and/or GW composting processes, gaseous emissions, and relevant technical data composting is included in the supplemental material, such as: title, for the environmental analysis of composting.Specific information on objective, methodology, software, impact assessment, impact Fig. 3. Phases of the LCA of biowaste and/or green waste composting. Source: adapted from Kobayashi et al. (2020); Note: BW: biowaste; GW: green waste. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 353 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 categories, goal, and functional unit, description of the system bound­ 3.2.1.2. Tools used for life cycle assessment. Numerous tools for con­ aries, quality, and origin of data in the inventory, substrate, technology, ducting LCA or supporting the different applications and phases of LCA quality of compost, gaseous emissions, leachate, environmental analysis, exist. Several LCA computer programs have been developed to model and conclusions. and assess the products and processes involved. Some of these programs have focused on solid waste management. These computer-based tools help collect and analyze data, analyze waste management systems, and 3.2. Life cycle assessment of biowaste or green waste composting assess emissions and their environmental impacts (Khandelwal et al., 2019b). Although using modeling software to perform an LCA is not 3.2.1. Methodology and tools mandatory, this could help acquire, organize, and analyze the inventory. These tools are designed explicitly considering the LCA framework, thus 3.2.1.1. Description of the life cycle assessment methodology. LCA is the facilitating systematically performing complex and lengthy calculations most common approach to evaluate the environmental impacts of (Iqbal et al., 2020). different waste management strategies (Komilis and Sánchez, 2017; The LCA models developed to analyze waste management solutions Dastjerdi et al., 2021). According to Christensen et al. (2020), there are include integrated waste management (IWM)-1 and 2, WARM (Waste six waste management-associated areas where LCA is expected to play a Reduction Model), WASTED (Waste Analysis Software Tool for Envi­ role in the future: (i) understanding an existing waste management ronmental Decisions), EASWASTE (Environmental Assessment of Solid system; (ii) improving existing waste management systems; (iii) Waste Systems and Technologies), WRATE (Waste and Resources comparing alternative technologies (technology performance); (iv) Assessment Tool for the Environment), MSW-DST (Municipal Solid technology development (prospective technologies); (v) policy devel­ Waste - Decision Support Tool). SimaPro and Gabi (Ganzheitliche opment/strategic development, and 6) reporting. Bilanzierung – holistic balancing) are other tools used, even though that Generally, in the reviewed studies analyzing composting processes, are not designed specifically for analyzing solid waste management the LCA methodology agrees with the International Organization for systems. The most widely used software for performing LCA studies on Standardization (ISO) standards (ISO 14001, 14040, 14041, 14044). MSW is SimaPro, and the most employed database is Ecoinvent. SimaPro However, the wording in the given LCA standards is rather general and and other software usually treat the waste as separate fractions, not as a does not give detailed guidance about specific areas, such as organic whole mass, an advantage over other commonly used software. Using an waste management (Bernstad and Cour Jansen, 2012). The ISO standard LCA modeling tool is a user choice that can vary based on the study’s defines four phases (See Fig. 3): i) goal and scope definition, ii) Life objectives, purchasing cost of the tool, the use of the database from the Cycle Inventory (LCI), iii) Life Cycle Impact Assessment (LCIA), and iv) software, and the software’s user-friendliness (Iqbal et al., 2020). In this interpretation. Variations observed in the studies were associated with review, the SimaPro software predominated (15 of 25 studies); other differences in system boundaries, technological assumptions, and in­ computer programs used were GaBi, EASWASTE, and IWM-2. ventory analysis (Bong et al., 2017). The goal and scope definition are critical steps since the appropri­ 3.2.2. Goal and scope definition ateness of LCA methodology depends on the purpose of the specific study (e.g., selection of functional unit or impact categories). It is also 3.2.2.1. Description of the analyzed composting systems. Table 1 includes where the system boundaries (geography, time, technology, etc.) are the 25 articles addressing detailed LCAs of BW and/or GW composting. described, and the functional unit is defined (Christensen et al., 2020). Type of waste, composting technology, functional unit, and system The functional unit refers to a product, a service, or a system whose boundaries (i.e., pre-composting, composting, post-composting) are re­ impacts are addressed. ported. In the following sections, these aspects are individually One of the most critical stages of the LCA is the life cycle inventory discussed. analysis, which involves collecting and quantifying inputs and outputs for a specific product or process throughout its life cycle (Liu et al., 3.2.2.2. Goal. Fig. 4 presents the general goal set in the reviewed LCA 2022). Common inventories for composting include transportation, studies. It is important to note that composting is normally compared operational machinery, emissions during maturation, and end-product with other technologies (17 studies), which are quite different in terms utilization. of objectives and results (e.g. landfill and incineration). In this case, it The methods of the Life Cycle Impact Assessment (LCIA) for char­ performs much better when composting is compared with these other acterizing, aggregating, and normalizing the inventory information into options for organic waste treatment or disposal from an environmental common potential impacts (e.g., global warming, acidification, and perspective.. In more recent studies (10), different composting ap­ toxicity) are not specifically developed for waste management but are proaches (e.g., industrial composting and/or home composting) are generally applicable (Christensen et al., 2020). In this review, all studies compared, which results in much more similar environmental impacts. included used midpoint indicators to assess the environmental impact, Even though there are no many studies where composting is compared and only 6 used end-point indicators (Abduli et al., 2011; Blengini, with anaerobic digestion, the preferable option is the combination of 2008; Buratti et al., 2015; Martinez-Blanco et al., 2010; Quirós et al., both technologies in the case of biowaste (Font and Sánchez, 2021). 2014b; Song et al., 2013). In the interpretation phase, the results from the LCIA are evaluated in 3.2.2.3. System boundaries. System boundaries define the inclusion and relation to the goal and scope of the study to reach conclusions and exclusion of a subprocess or a variable from the inventory analysis and recommendations. Often, the results of the first modeling can be greatly influence the evaluation results (Iqbal et al., 2020). Typically, incomplete. Therefore, the input data must be adjusted to obtain a good the system boundaries of a particular composting system comprise three balance and system coverage between goal and scope definition and stages: pre-, during-, and post-composting (Bong et al., 2017) (Fig. 5). interpretation (Christensen et al., 2020). On the other hand, LCA re­ However, these three stages might be addressed differently. Morris et al. quires several assumptions. These assumptions significantly impact the (2013) performed a meta-analysis of 82 studies that compared multiple final LCA results. Therefore, a careful examination should be conducted end-of-life management options, including composting. Comparing of crucial technical parameters affecting waste recovery and recycling different strategies to address an LCA, they concluded that environ­ processes in this step (Zhang et al., 2021). Such uncertainties may cause mental impacts might be considered only based on emissions at the undesirable mistakes in decision-making. Therefore, the identification organic waste management facility. Other authors might include the and quantification of uncertainty in the interpretation phase is essential impacts of organic waste collection and transport. Finally, one might to test the robustness and reliability or the assessment. 354 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 Table 1 Information about studies on the Life Cycle Assessment of Biowaste and/or Green waste composting. Reference Waste Type of Technology Functional unit System boundaries input1 Pre- composting Post- composting composting Abduli et al. (2011) OFMSW Windrow composting 1 ton of OFMSW X X Andersen et al. (2012) OHW Home composting 1 ton of OHW X X X Banias et al. (2020) OFMSW Not specified 1 ton of OFMSW/year X X X Blengini (2008). BW In-vessel composting 0.001 ton of input BW X X X Buratti et al. (2015) OW Enclosed agitated bed system/ outdoor aerated 1 ton of OW X X X static piles Colon et al. (2012) OFMSW composting in-vessel Reduction of 1 DRI unit in 1 X windrow composting ton of OFMSW Home composting Yay (2015) KW/YW Aerobic static pile composting 1 ton of MSW X X Jensen et al. (2016) OHW In-vessel and Windrow composting 1 ton of OHW X X GKhandelwal et al. OFMSW windrow composting 1 ton of MSW X X (2019b) Keng et al. (2020) FW Open-air static pile 0.2 ton/day of OW X X X Liu et al. (2022) OSW Static heap, windrow composting, membrane- 1 ton of OSW X covered composting, and reactor composting Lu et al. (2020) GW / FW windrow composting, In vessel, home composting, 1 ton of OW X X X home composting community Martínez-Blanco et al. OFMSW Home composting, in vessel and windrow 1 ton of OW X X 2010 Manfredi et al. (2011) OW Tunnel composting 1 ton of OW X X Mu et al. (2017) FW In-vessel 1 ton FW X X X Oldfield et al. (2018) FW/GW Windrow and in-vessel composting 0.001 tons of FW X X X Oliveira et al. (2016) MOW Home composting waste input X X Padeyanda et al. (2016) FW Not specified 1 ton of FW X X X Pergola et al. (2020) GW windrow composting with turning and forced 1 ton of compost X X X aeration Quirós et al. (2014b) FW Home composting 6.8 tons ha− 1of horticultural X X crops of cauliflower Rana et al. (2019) OFMSW Not specified 1 ton of MSW X X X Saer et al. (2013) FW Windrow composting 1 ton of compost X X X Song et al. (2013) FW Not specified 321,752 tons of MSW X X X Yadav & Samadder FW/YW Windrow composting 1 ton of MSW X X X (2018b) Weligama Thuppahige OFMSW windrow composting 1 ton of OFMSW X X et al. (2022) Note: Pre-composting: collection and pre-treatment; composting: emission composting, energy usage, water usage, emission water, leachate; Post-composting: fer­ tilizer replacement, peat replacement, product distribution, post-application. 1 DRI: Dynamic Respiration Index; FW: food waste; GW: Green waste; KW; kitchen waste; YW: Yard waste; MOW: Municipal organic waste; MSW: Municipal solid waste; OFMSW: Source-separated organic fraction of the municipal solid waste; OHW: Organic household waste; 20 18 16 16 14 Number of studies 12 10 8 6 4 4 3 2 2 0 Comparison with other Comparison between Comparison between Comparison between technologies of MSWM industrial composting home composting and home composting technologies industrial composting technologies Fig. 4. General goal of LCA of biowaste or green waste composting (n = 25 studies). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 355 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 Fig. 5. System boundaries typically considered in LCA of biowaste and green waste composting systems. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) consider implications such as those from compost applied to soil (i.e., 3.2.2.4. Functional unit (FU). When comparing alternative technolo­ agriculture, horticulture, landscaping, or improvement soil). The degree gies, these must be well-understood, and the FU must be well-defined of quantification of resource inputs, product outputs, and emissions (Christensen et al., 2020). The FU is defined as the reference flow. It from these processes must agree with the system boundary chosen for may be selected considering the waste input, product output, services the study (Iqbal et al., 2020). provided, or the technologies being compared. The FU must be consis­ In comparative analyses, the system boundaries for different options tent with the system goal and scope, according to ISO 14,040 and must be, if not the same, at least justly comparable. When comparing 14,044. different facilities, they must have similar objectives regarding the end- LCA studies of waste management systems have adopted several product (compost) quality and stability. types of FUs with different purposes (Zhang et al. (2021)). The Composting systems have different operational phases (construction, commonly used FU in the reviewed studies is based on mass. Oldfield operation, and decommissioning). Zhang et al. (2021) state that emis­ et al. (2018) highlight that the FU based on mass is inappropriate when a sions in the construction and decommissioning phase are insignificant system focuses on waste valorization and circular economy. Instead, compared to the operation phase; thus, they are usually ignored in the they suggest selecting a FU that reflects the downstream and secondary LCA of the municipal solid waste management (MSWM) system. In processing function. For example, the FU should allow the comparison addition, Buratti et al. (2015) indicate that construction, implementa­ of value-added products such as nitrogen (kg N), energy, or other tion, maintenance, and/or demolition (decommissioning) of the me­ second-generation products. Likewise, Colon et al. (2012) assessed four chanical and biological plants and equipment are frequently not full-scale facilities treating source-separated OFMSW using the Respi­ included due to a lack of data. In this review, all studies considered the ration Index Efficiency as the novel FU instead of the classical LCA operation phase, and only three included the construction phase (Mar­ approach based on the total mass treated (i.e., reduction of 1 Dynamic tínez-Blanco et al., 2010; Pergola et al., 2020; Liu et al., 2022). Respiration Index unit in 1 Mg of OFMSW). The authors conclude that to In agreement with the above-described findings by Morris et al. assess the environmental impacts of biological waste treatment plants, (2013), Bong et al. (2017), and Buratti et al. (2015), our review shows the FU must consider the performance of the biological treatment (waste variations between system boundaries in studies regarding LCA on BW stabilization). and/or GW composting. Table 1 shows a summary of the system Table 1 summarizes the FU selected for the LCA studies on BW and/ boundaries of each examined study. 20 studies considered the pre- or GW composting. Most studies (23/27) use a FU based on the input composting stage (Upstream process), 25 studies the composting mass. Three works are specific (Saer et al., 2013; Quirós et al., 2014b; stage, and 17 studies the post-composting stage (Downstream process). Pergola et al., 2020), considering as FU the amount of composting In general, the active operational phase was always included. At the product or productivity of a specific crop type. same time, some studies discarded pre and post-composting. According to Bong et al. (2017), researchers often exclude pre-composting activ­ 3.2.3. Life cycle inventory (LCI) ities such as waste transportation and collection. These authors assumed the travel distance for all waste management scenarios to be the same. 3.2.3.1. Data used. The comprehensive nature of LCA studies demands On the other hand, some studies also exclude the benefits of post- data representing foreground and background processes (Henriksen composting, such as using compost to improve soil amendments, et al., 2019). The foreground data refer to data required to describe a enhance crop health, increase water holding capacity, and reduce waste management system to model the environmental impacts. These pesticide consumption. These types of choices are made typically on the data include waste composition and the total amount of waste managed basis of the goal of the study. in a process. The background data (or the life cycle inventory data) are 356 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 the basis for assessing the environmental emissions and considering all managing household organic waste (Martínez-Blanco et al., 2010). Like process-relevant inputs and outputs, used resources, and energy per FU centralized composting, home composting can produce a nutrient-rich (Ikhlayel, 2018). humus-like material to use as a soil amendment (Lu et al., 2020). Information on the characteristics of composting facilities is essential The reviewed articles implemented LCA in BW/GW composting to for applying the LCA. However, achieving adequate life cycle inventory evaluate the environmental impacts of this technology and other organic data has been reported as difficult in various studies. Buratti et al. waste management options (See Table 1). Some studies compare the (2015) mention that the LCA methodology’s applicability is firmly environmental impacts of industrial composting and home composting subordinated to data available on the processes involved. Some of the (Oliveira et al., 2016; Colon et al., 2010; Colon et al., 2012), and others required information include plant capacity, land occupation, input data compare composting with different organic waste management options (i.e., quantities and characteristics of treated waste, energy, fuel, and (i.e., anaerobic digestion, incineration, composting, landfill) (Blengini, water consumption), and output data (i.e., quantities and characteristics 2008; Oliveira et al., 2016; Jensen et al., 2016; Keng et al., 2020; Beh­ of the end-product and destination, gaseous, solids and liquid emissions) rooznia et al., 2018, Yay, 2015). Other research compares composting (Cadena et al., 2009b; Weligama Thuppahige et al., 2022). technologies (i.e., static heap, windrow composting, membrane-covered Some data from composting facilities can be obtained from their composting, reactor composting) (Colon et al., 2012; Liu et al., 2022). managers through surveys or reports, but others must be directly Table 1 shows the type of composting technology considered in the measured at the facility (Martínez-Blanco et al., 2010). Komilis and different studies. Windrows, in-vessel, and home composting predomi­ Sánchez (2017) indicated that original inventory data are essential for a nate among the composting technologies evaluated. thorough analysis. A researcher must seek these data by performing Equipment, aeration mechanisms, and other technological compo­ original laboratory research or field measurements. In addition, meet­ nents can modify the energy requirements and the emissions generated. ings and interviews with people in full-scale facilities are required to According to Liu et al. (2022), static heap composting transforms obtain data, especially about water consumption, effluent discharges, organic matter into compost via a natural fermentation process that can electrical energy and diesel consumption, among other data. take long periods (e.g., 3 to 6 months) to be completed. However, the The studies analyzed in this review relied on mixed methods for process can be accelerated when forced aeration and turning equipment collecting the data needed to apply the LCA (i.e., primary and secondary are applied, as example in windrow composting. Although much faster, sources). Data on performance was obtained directly at the composting these options are associated with a strong odor, significant ammonia facility. In some cases, experimental tests were carried out to estimate losses, and large nitrous oxide emissions. The membrane-covered aer­ information associated with gas emissions (Martínez-Blanco et al., 2010; obic composting technology has been verified to effectively reduce odor Quirós et al., 2014b). In other studies (Olivera et al., 2017; Jensen et al., and greenhouse gas (GHG) emissions. Finally, in-vessel composting is a 2016; Behrooznia et al., 2018), emission factors were assumed from closed process integrating feeding and discharging, aeration, stirring, literature reports. and exhaust gas control for aerobic digestion. Despite data collection efforts, it is common that some data remain In another study, Lu et al. (2020) compare windrows and in-vessel missing. Data gaps could be defined as unavailable values. However, composting. They found that in-vessel composting allowed greater filling the gaps would improve the accuracy and reliability of LCA re­ control of gaseous emissions and undesirable odors than windrow sults. Data gaps that are ignored, i.e., not filled by some alternative data, composting. In addition, in-vessel composting can process the same may bias the LCA results (Henriksen et al., 2019). Researchers amount of waste with less space than the windrow system. However, in- frequently fill data gaps using bibliographic data. However, biblio­ vessel composting requires higher technical skills and energy con­ graphic data may not accurately represent the actual operating condi­ sumption. Although better controlled in vessel composting technologies, tions of a facility (or case) under study (Komilis and Sánchez, 2017). the emission of odors and other contaminants (e.g., volatile organic Therefore, it is essential to consider criteria that facilitate the incorpo­ compounds) cannot be neglected. ration of relevant data when these are not available. Apart from the previously mentioned impacts, the different com­ posting technologies have different energy requirements for waste 3.2.3.2. Substrates. The BW or GW characteristics influence the com­ transport and processing. Moreover, advanced systems show a higher posting process, end-product quality, and emissions of gases and demand for water, energy, and infrastructure (Oliveira et al., 2016; leachate. For example, an imbalance in the C/N ratio, depending on the Haug, 1993). From this perspective, home composting is more process conditions, could affect the emission of gases such as ammonia environmentally-friendly and has the advantage that collection and (NH3) or nitrous oxide (N2O). However, the studies reviewed rarely transport of organic waste are not needed (i.e., a significant amount of report the quality of the processed waste. Only in a few cases (Saer et al., GHG emissions emitted during fuel combustion, mainly fossil CO2, can 2013; Keng et al., 2020; Oldfield et al., 2018), the physicochemical be avoided) (Oliveira et al., 2016; Lu et al., 2020). Since home com­ characteristics of substrates were analyzed. Most of the studies only posting avoids the use of transport, it is an attractive and complementary specified the amount of waste to be processed and its main composition alternative to industrial composting in low-population-density areas (i.e., yard waste, fruit waste, pruning waste, kitchen waste, among (Oliveira et al., 2016). However, there are also some environmental others). concerns; for example, gas treatment systems are absent (Colon et al., On the other hand, in this review, only three studies (Pergola et al., 2010). Lu et al. (2020) argue that savings from avoiding waste collection 2020; Lu et al., 2020; Oldfield et al., 2018) explicitly mentioned the and transport might not significantly improve environmental perfor­ incorporation of GW. The remaining studies refer to organic solid waste, mance since the distance between the residential area and the central­ food waste, biowaste, organic fraction of municipal solid waste, source- ized facility is another factor to consider. Regarding decentralized separated organic fraction of municipal solid waste, organic household composting, in recent years, there has been increasing interest in com­ waste, and municipal organic waste. munity composting, including techno-economic and environmental analysis (Marcello et al., 2021). However, to our knowledge, this al­ 3.2.3.3. Composting technologies. Composting can be implemented at ternative’s gas emissions or environmental impacts have not been different scales. Centralized composting (or industrial) requires waste studied yet through approaches such as LCA considering the direct collection and transport to a single facility. Different technologies, such measurement of GHG emissions (Sánchez, 2022). as in-vessel systems, biodynamics, and windrow composting, can be Although various investigations have shown the reduction of gaseous applied. On the other hand, home-scale composting is an alternative to emissions and odors using biofilters in composting systems (Pagans centralized composting, promoted as a simple and low-cost solution for et al., 2006; Colon et al., 2009; Saraya and Sánchez, 2021), few papers evaluated the reduction of environmental impacts by the use of biofilters 357 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 in LCA studies. For example, Bernstad and Cour Jansen (2012) indicated benefits of compost (Lu et al., 2020). However, some studies use stan­ that the reduction of emissions of NH3, methane (CH4), and N2O using dardized compost application factors; for example, Banias et al. (2020) biofilters in composting facilities has a significant impact on the mentioned that 1 ton of compost is equivalen to 23 kg of N-fertilizer, 9.5 “Climate Change” category (i.e., it could reach up to 25% in Global kg of P-fertilizer, and 9 kg of K-fertilizer. Weligama Thuppahige et al. Warming Potential - GWP). (2022) indicated that the average amounts of N, P2O5, and K2O in 1 ton On the other hand, LCA has been used to choose solid waste man­ of compost are 6.9, 1.61, and 7.3 kg, respectively. Studies are needed to agement alternatives from an environmental perspective by comparing delve into the relationship between end-product quality, compost use, different technologies – 17 studies in this review. Bernstad and Cour and reduction of environmental impacts. Jansen (2012) compared twenty-five LCA studies, which addressed food One of the difficulties in estimating the environmental benefits of waste treatment processes, including landfills, thermal treatment, using compost is its diverse nutrient composition and quality, notably in composting (small and large scale), and anaerobic digestion. The climate developing countries where the quality labeling for compost is poor. change impact related to these treatment alternatives varied among the There is also limited data on the environmental performance of the studies. Significant differences in the setting of system boundaries, compost after its application. This limitation leads to the exclusion of the methodological choices, and variations in user input data were high­ post-application phase in most of the life cycle inventory analyses on lighted. Therefore, it is critical to distinguish between the direct emis­ composting (Bong et al., 2017). sions generated by the composting process that are related to the Finally, some studies indicate that compost use generates environ­ biodegradation of organic matter and the volatilization of some com­ mental impacts. For instance, Silvenius et al. (2016) suggested that the pounds of the organic waste and the indirect ones associated with pre- compost application affects the emissions of gases and leachate during and post-composting processes and the equipment necessary for these the life cycle of lawn areas. The impact of eutrophication from N and P operations (mainly diesel and electricity consumption and related leachate can also be important since compost-based substrates can be emissions, among others). Bernstad and Cour Jansen (2012) precisely nutrient-rich. describe these emissions sources as follows: 3.2.3.5. Gaseous emissions. The composting of organic matter can a) Direct emissions (directly linked to the waste management), origi­ generate gaseous emissions, as previously mentioned. The amount and nate from collection/transportation, treatment, and post-treatment characteristics of the gases emitted from composting processes vary and of the waste; are related to the initial feedstock materials and the composting meth­ b) Indirect emissions or avoided emissions (or indirect burdens or odology adopted (Dhamodharan et al., 2019). Saer et al. (2013), Sayara avoided burdens occurring in the background system). - Upstream and Sánchez (2021), and Pergola et al. (2020) indicated that the four indirect activities (e.g., production of materials and energy carriers main gases resulting from feedstock decomposition are CO2, CH4, N2O, used in the treatment chain or construction of machinery and and NH3 (together with volatile organic compounds - VOC, 99% of the treatment facilities used in the treatment) and - Downstream indirect total emission). activities (e.g., avoided emissions when substituting materials and CO2 emissions from the degradation of organic material are consid­ energy carriers by activities in the waste management chain). ered biogenic (and not fossil-derived). Therefore studies ignore these emissions in assessing climate change impacts (Cadena et al., 2009b; In summary, environmental impacts from different composting Bernstad and Cour Jansen, 2012; Saer et al., 2013; Bong et al., 2017; technologies are related to energy consumption, which varies across Manfredi et al., 2011). In addition, CO2 emissions from produced bio- different composting technologies, operation modes, and management fertilizers later used on land are also considered carbon neutral (Bern­ practices. Environmental issues such as energy consumption, gaseous stad and Cour Jansen, 2012). emissions, leachate generation, and compost production should be Otoma and Diaz (2017) state that the assumption made generally in analyzed to evaluate environmental performance (Bong et al., 2017). current models that the global warming potential of CO2 of biogenic origin is zero, and that only CH4 emissions are accounted for, is erro­ 3.2.3.4. End-product quality. Compost utilization allows diverting the neous. They argue this is an erroneous assumption. Instead, they organic waste fraction from landfills, recycling nutrients, and improving developed a model that quantifies all CO2 and CH4 emissions to the soil characteristics. In addition, compost utilization promotes the atmosphere from the degradation of organic Carbon (DOC) and then restoration of soil carbon by adding organic matter and the formation of discounts the amount of CO2 originating from that DOC during photo­ humic substances, thus counteracting the continuous organic matter loss synthesis. This approach is considered more realistic to balance emis­ from soils due to agricultural activities (Bong et al., 2017; De Feo et al., sions from biogenic sources. In this regard, Christensen et al. (2009) 2016). suggest that biogenic carbon emissions can be either positive or neutral, Many factors determine compost quality, such as waste stream and long-term carbon storage neutral or credit, as long as the same composition, production management, and weather conditions. The evaluation methodology is adopted (Iqbal et al. 2020). Therefore, the physicochemical and biological characteristics and the heavy metals parameter choice is an arbitrary practice based on the boundary settings content (e.g., Zn, Cu. Ni, Cr, Pb, and Cd) in the end-product influence the and assumptions of the studies. However, a transparent analysis requires agronomical and environmental performance of systems where compost a carbon mass-flow (mass balance) for all scenarios (Bernstad and is used (Quirós et al., 2014b). LaCour Jansen, 2012). Few studies (Rashid and Shahzad, 2021) analyzed the end-product On the other hand, there is no consensus in the literature on ac­ characteristics and their agronomic value to estimate its use’s environ­ counting for CH4 emissions in LCA on composting since only some mental and economic benefits. Bong et al. (2017) highlight the impor­ studies report methane production depending on the operating condi­ tance of studying the relationship between end-product quality and the tions of the composting process. Given the importance of CH4 as a benefits of using compost. However, other authors (Burrati et al., 2015) greenhouse gas, the selection of the CH4 emission factor could signifi­ assume that the environmental impacts or benefits of compost applica­ cantly change the analysis (Saer et al., 2013; Bernstad and Cour Jansen, tion are similar to those of other conventionally employed soil amend­ 2012). In this review, 15 out of 25 studies included methane emissions to ments and therefore ignore them. estimate environmental impacts. Buratti et al. (2015) indicate that In summary, the studies typically ignore the end-product quality or several studies ignore methane emissions, arguing that they were its potential use (e.g., agriculture – a different type of crop, garden, negligible or undetectable. improvement soils, erosion reduction) to estimate the environmental N2O is also formed in anaerobic pockets and is mainly produced at 358 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 the end of the composting process when the available carbon has been environmental impact through LCA (i.e., 17 studies). However, it is utilized without stabilization of all nitrogen (i.e., during denitrification necessary to be careful when adopting these factors. These emission that reduces NO–3 to N2O) (Boldrin et al., 2009). A poorly managed factors may be helpful in specific situations but do not provide infor­ composting process can generate N2O emissions, which have 298 times mation about biomass degradation processes. It is necessary to know if the global warming impact per unit weight of CO2 (Saer et al., 2013). the degradation occurs under anaerobic, semi-aerobic, or aerobic con­ The inclusion of N2O emissions can have a significant effect on the ditions (Otoma and Diaz, 2017). Some examples of emission factors are: overall climate change impact of these systems (Boldrin et al., 2009). In a) Cadena et al. (2009b) reported emission factors from the biolog­ this review, 15 studies included N2O in gaseous emissions. ical treatment process for NH3 and VOCs were 3.9 kg Mg OFMSW− 1 and Similarly, NH3 could be produced when temperatures rise above 40 0.206 kg Mg OFMSW− 1 respectively (1 Mg of OFMSW was considered as to 50 ◦ C, and therefore some studies included ammonia in assessing the the functional unit). Although in this study, the methodology developed impacts (i.e., 13 of 25 studies) (Saer et al., 2013). Aeration rate, pH, and was applied to a specific configuration of a composting plant, it can be initial total ammonium nitrogen content influence ammonia emissions used with different technologies and wastes. directly in a composting process (Beck-Friis et al., 2001; Cadena et al., b) Saer et al. (2013) reported emission factors of organic waste 2009b). According to Bernstad and Cour Jansen (2012), studies composting using windrow technology (i.e., CH4: 0.021 – 11.9 kg/1000 including NH3 emissions show an increased environmental impact on kg-feedstock; N2O: 0.003 – 0.252 kg/1000 kg-feedstock; NH3: 0.025 – acidification and eutrophication. 1.3 kg/1000 kg-feedstock). Another prominent group of gaseous pollutants from composting c) Weligama Thuppahige et al. (2022) adopted values proposed by facilities is VOCs. According to Dhamodharan et al. (2019) and Cadena the Intergovernmental Panel on Climate Change (IPCC) for aerobic et al. (2009b), most VOCs in composting plants are emitted at the composting: 4 kg CH4 and 0.3 kg N2O per ton of OFMSW. beginning of the process (tipping floors, shredder, and during the initial Bong et al. (2017) indicate that gaseous emissions are also due to forced aeration composting period). Incomplete or insufficient aeration other system components (i.e., not only the composting operation: but during composting can result in the emission of sulfur compounds with also shredding, turning, transferring, and screening). This aspect in­ intense odor, whereas incomplete aerobic degradation processes release cludes fuel consumption in waste collection and transport, energy con­ emissions in the form of alcohols, ketones, esters, and organic acids sumption (diesel or electricity) during pre-treatment, and energy (Cadena et al., 2009b; Sayara and Sánchez, 2021). In the present work, consumption in tool manufacturing (e.g., composters and gardening seven studies reported VOCs emissions in their LCAs. tools). In addition, emissions savings refer to the avoided emission from Finally, Buratti et al. (2015) consider H2S and particulate emissions the waste collection and transport to the landfill, emissions from the in their study. H2S was measured over static piles, while particulates landfill, replacement of mineral fertilizer (MF), and peat extraction were measured in the exhaust air from the compost refining area. They (Bong et al., 2017). Enhanced soil carbon sequestration through found that only a few data on H2S and particulate emissions in full-scale compost utilization was reported as the key contributor for reducing the composting facilities are reported in the literature. The emissions esti­ emissions of Greenhouse Gases (GHG) (Saer et al., 2013). mated were 1.56 kg/ton of organic fraction for H2S and 0.074 kg/ton of In summary, activities related to GHG emissions in composting sys­ organic fraction for particulate. Fig. 6 shows gaseous emissions tems are: a) Upstream: waste collection, waste transportation, and ma­ considered in the LCA of composting systems. terials consumption; b) Operational: screening (pre/post), shredding, Despite the importance of estimating gaseous emissions in the turning, composter, compost packing; c) Emissions during composting composting process, actual data on gaseous emissions released from full- of: CO2, N2O, CH4, VOCs. Activities related to savings of GHG are: scale composting plants are challenging. An exhaustive sampling avoided landfill emissions, avoided transportation to landfills, avoided campaign is necessary to get representative and reliable data on a single post-extraction, avoided use of conventional soil amendments, and plant (Cadena et al., 2009b). Therefore, some studies (Saer et al., 2013; carbon sequestration (Bong et al., 2017). Oliveira et al., 2016; Jensen et al., 2016; Behrooznia et al., 2018; Yay, Compost post-application could also contribute to the GHG emis­ 2015) applied emission factors for different gases to estimate the sions. During the cultivation phase, the compost is transported to the Fig. 6. Gaseous emissions considered in LCA of biowaste and green waste composting systems (n = 25 studies). Note: * include H2S, SO2, HCl, CO, particulate material. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 359 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 crop area; the crop is harvested and transported to the storage buildings. production). However, home composting uses little energy input among These activities consume materials and energy for infrastructure and the available composting technology options. Home composting relies machinery production and use, and cause emissions from the field and almost entirely on human labor, which is not considered in LCAs that nursery plot (Quirós et al., 2014b). It has been reported that inventories compare energy consumption (Morris et al., 2013). Some values re­ from other studies often exclude the post-application of compost due to ported are: the complexity of quantifying the emissions and the lack of data (Bong a) Bernstad and Cour Jansen (2012): showed that the energy input in et al., 2017). composting varies largely in different studies; the values found by the On the other hand, for industrial composting, the emissions depend authors are 15.1–55.0 kW h electricity/ton and/or 0.01–15.3 L diesel/ on the operation mode of the composting system. Windrow systems ton (i.e., to windrow, closed reactor, and home composting seldom include emissions for the manufacturing of composting equip­ technologies). ment. The system consumes energy from the continuous air or oxygen b) Buratti et al. (2015) reported a total electricity demand of 26.03 supply for the aerated static pile. For the in-vessel system, the GHG kWh/ton of OW and a diesel demand of 1.06 kg/ton of OW (i.e., emissions for the manufacturing of the composter is often considered. enclosed agitated bed system technology). The GHG emissions due to different operational modes and energy c) Cadena et al. (2009b) quantified emissions associated with energy consumption are accounted for; this includes turning, aeration, shred­ use and production (60.5 kg CO2 Mg OFMSW1 and 0.66 kg VOC Mg ding, screening, and compost spreading. Saer et al. (2013) conclude that OFMSW− 1) in closed composting reactors with controlled aeration, gas the major contributor to GHG emissions in composting systems is the collection, and treatment through a wet scrubber and a biofilter. The direct emission during organic matter decomposition. Other processes, maturation phase took place in forced-aerated windrows open to the such as waste collection and transport, infrastructure, ancillary mate­ atmosphere). rials production, and energy consumption (e.g., for ventilation equip­ d) Banias et al. (2020) reported that the energy used for the opera­ ment), had a low contribution to GHG emissions. Despite this, these tion of the composting facility was 19.67 kWh of electricity, and 0.36 L emissions cannot be neglected. of diesel, per ton of treated waste (i.e., technology not specified). The lower GHG emissions due to the avoidance of waste collection e) Di Maria and Sisani (2017) reported the electrical energy con­ and the lower investment costs are advantages of home composting. For sumption was assumed to be 40 kWh/Mg OFMSW at the plant inlet. In home composting, inventories of other studies often included the GHG contrast, diesel fuel consumption was assumed to be 0.134 kg/Mg emissions for manufacturing the related tools such as the bin and garden OFMSW (i.e., technology not specified). chipper, direct GHG emissions, energy consumption during composting, f) Colon et al. (2012) reported the electrical energy consumption was and the compost post-application. Home composting shows a significant 740 MJ/Mg OFMSW, 235.8 MJ/Mg OFMSW, 33 MJ/Mg OFMSW, and variation in GHGs inventories among various studies that have limited 33 MJ/Mg OFMSW for composting in-vessel, composting in confined its application from a scientific perspective (Colon et al., 2010). Quirós windrows, turned windrow composting, and home composting, et al. (2014b) compared two home composting systems with high and respectively (7.435, 91, 3.000 and 0.43 t/year of BW processed, low GHG emissions. They identified a significant difference of 4-, 5-, and respectively). The authors indicate that the energy consumption was 52-times higher emissions of CH4, N2O, and NH3 in the high-emission highly dependent on the machinery and the composting technology system with no mixing compared to the low-emission one with employed. The greatest energy consumption was for in-vessel and frequent turning. Colon et al. (2012) compared home composting and windrows aeration. These two technologies included additional pro­ industrial composting (composting in-vessel and turned windrow com­ cesses to treat gas emissions (scrubbers and biofilters) that accounted for posting). They measured the gaseous emissions finding the following 45% of the total energy consumption in these facilities. values (per ton of processed material), respectively: NH3: 0.84 kg, 0.11 kg, and 8.63 kg; VOCs: 0.56 kg, 0.36 kg, and 5.70 kg; N2O: 0.676 kg, 3.2.4. Life cycle impact assessment (LCIA) 0.075 kg, and 0.251 kg; CH4: 0.16 kg, 0.34 kg, and 4.37 kg. Home composting did not always present have the lowest gaseous emissions 3.2.4.1. Impact categories. Fig. 7 shows the impact categories consid­ for the three systems compared. ered in LCA studies on BW or GW composting. The studies used different impact categories depending on the goal and scope definition, system 3.2.3.6. Energy and water consumption and other emissions. Energy and boundaries, and impact assessment method. According to Iqbal et al. water consumption and leachate generation are rarely addressed in (2020), the coverage of a greater number of impact categories represents composting studies. Some studies assume that the production of leachate a more detailed analysis. Despite a high diversity of impact categories and emissions related to water do either not occur or are insignificant (i. considered in the studies, some categories are generally evaluated e., due to the low water content and the high temperature in the piles) popular in the LCAs of composting systems. Among the most frequently (Bernstad and Cour Jansen, 2012). However, industrial-scale compost­ selected categories are climate change (24/24 studies), terrestrial ing often requires the addition of water to maintain optimal biological acidification (23/24 studies), freshwater eutrophication (21/24 conditions for converting organics into marketable compost. Also, in- studies), ozone depletion (19/24 studies), marine eutrophication (19/24 sink food waste disposers need running tap water to facilitate grinding studies), human toxicity (18/24 studies), photochemical oxidant for­ and flushing (Morris et al., 2013). Therefore, water-related emissions mation (17/24 studies), fossil fuel depletion (14/24 studies), and should not be neglected. In this review, only 3 of the analyzed studies terrestrial ecotoxicity (11/24 studies). As observed, the number of cat­ considered leachate in their assessment (Andersen et al., 2012; Oldfield egories analyzed in these studies is different. In some cases, this is due to et al., 2018; Bong et al., 2017). Some studies motivated the fact that the specific objectives of the work (some of them are only focused on water-related impacts were not considered, due to low quantities of climate change or carbon footprint), but in the case of a complete LCA leachate generated (Keng et al., 2020; Martínez-Blanco et al., 2010; there are two main groups: articles published before the implementation Colon et al., 2012). A third option was suggested by Yadav and Sam­ of ReCiPe2016 from the National Institute for Public Health and the mader (2018b) and Colon et al. (2012), who assumed that leachate Environment (The Netherlands) and other more detailed (Serafini et al., could be recirculated into the windrow to avoid the necessity of treating 2023) and recent articles using these modern methodologies (ReCiPe the generated leachate. 2016, CML latest versions, IPCC values, Eco-indicator, etc.). Modern and Industrial composting typically does not provide an energy benefit to previous methodologies follow upon the same strategy as ReCiPe2008, counterbalance the energy needed for collection, transport, and com­ where different sources of uncertainty and different choices are grouped posting facility operations (other than the offset energy for amendment into a limited number of perspectives or scenarios, although the number 360 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 Fig. 7. Impact categories used in LCA of biowaste or green waste composting (n = 24 studies)*. Note: CC: Climate change; OD: Ozone depletion; HT: Human toxicity; POF: Photochemical oxidant formation; PMF: particulate matter formation; IR: ionizing radiation; TA: terrestrial acidification; FE: Freshwater eutrophication; ME: Marine eutrophication; TET: Terrestrial ecotoxicity; FET: Freshwater ecotoxicity; MET: Marine ecotoxicity; ALO: agricultural land occupation; ULO: urban land occupation; NLT: Natural land transformation; WD: water depletion; MRD: metal depletion; FD: fossil fuel depletion. * One article only evaluated endpoint indicators. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) of categories is different, being more detailed in the case of current current limitations to this standardization include data collection and its methodologies, which are widely applied in most recent papers (Catalán quality, system definition, system and time boundaries, and process et al, 2019). Regarding end-point indicators (three damage impacts: modeling. Three approaches for the determination of system boundaries human health, natural resources, and biodiversity), it is the opinion of when conducting LCAs regarding the composting of biowaste or green this review that they are a critical future trend, but no concluding results waste were identified: can be obtained nowadays in the composting field. 1) Pre-composting, composting, and post-composting can be For example, Zhang et al. (2021) compared 45 LCA studies on included when evaluating different solid waste management options (e. organic solid waste management. They found GWP, also called ‘climate g., incineration, anaerobic digestion, landfill, composting). Comparisons change impact’, was commonly considered in all the analyzed studies. between industrial composting and home composting are also included The second most common impact was acidification potential and human in this approach. toxicity potential. Similarly, Morris et al. (2013) reviewed 82 studies on 2) Composting and post-composting can be included when evalu­ end-of-life management methods for source-separated organic waste; ating different organic solid waste management options (e.g., anaerobic they found that climate change was the environmental impact most digestion and composting). frequently investigated. On the other hand, only a few studies (Oldfield 3) Composting can be included when evaluating different compost­ et al., 2018; Abduli et al., 2011) justify their selection of the impact ing technologies or when the effect of some process improvement stra­ categories. tegies is being investigated (e.g., adding inoculant, bulking or amendment materials, and aeration systems). Depending on the study scope, post-composting could also be included to assess the environ­ 3.3. Perspectives and challenges for LCA applications to biowaste and mental impacts of end-product use. green waste composting processes Defining system boundaries is essential since this influences aspects such as the functional unit, including or excluding waste and end- The credibility and validity of the results of an LCA of biowaste and product transport, allocation of environmental burdens, and avoiding green waste composting face several challenges, for instance, data environmental loads. Particularly the specification of system boundaries inconsistency, frequent neglect of the goal definition, variation in sys­ is crucial to reduce inconsistencies between different studies (Komilis tem boundaries, multi-input allocation, difficulties in capturing influ­ and Sanchez, 2017). ential local specificities (e.g., the inclusion of representative waste On the other hand, a relative consensus exists around ignoring the compositions into the inventory), and unclear user assumptions. These infrastructure construction and dismantling stages. In this review, the challenges create complexity and limit the ability to compare several studies that included these two stages found their contribution to studies as the basis of system definition may differ. Similarly, the results environmental impacts to be lower compared to those generated in the of an LCA study are closely related to the specific conditions of each operation stage. context, such as lifestyles, climatic conditions, composition, and waste characteristics. Based on this review, the challenges to address for 3.3.2. Data collection improving the application of LCA in biowaste and/or green waste This review shows that a large amount of data is necessary to composting are described as follows. determine the environmental impacts of a composting system. Original data still need to be carefully collected (i.e., primary sources such as 3.3.1. Determination of system boundaries measured or estimated data after field investigation, official data from The methodological framework for LCA application to biowaste or statistical reports, interviews, surveys, and site visits) and should not be green waste composting needs standardization. This review found that 361 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 solely based on the data contained in the “black box” of commercial forced aeration systems. However, there is a lack of studies assessing the software (Komilis and Sánchez, 2017). However, this does not exclude effect of these strategies on process emissions (e.g., gaseous emissions consolidating a database with information from rigorous scientific such as CH4 and N2O) and, therefore, on the impacts in terms of envi­ studies. This database would make it possible to compare the data ob­ ronmental impacts. Research is needed to compare facilities of different tained in different studies and improve the inventory phase of the LCA. sizes, operating systems, and changes in substrate quality to draw more The database could be organized according to country-specificities, convincing conclusions on potential mitigation strategies. technology-specificities, waste characteristics, end-product characteris­ Other technological aspects of BW or GW composting that can be tics, and other input and output data (e.g., emissions), considering that studied through LCA were also identified in this review, for example, (i) the assumptions or data from other studies may be specific to the loca­ the effect of feedstock porosity in the composting process using different tions where the studies were conducted. technological systems (i.e., porosity affects process conditions and gaseous emissions, and therefore, environmental impacts); (ii) the effect 3.3.3. Gaseous emissions of biofilters in reducing greenhouse gas emissions and, therefore, envi­ This review highlights the importance of considering all the gaseous ronmental impacts; (iii) the environmental impacts of decentralized compounds emitted from the process (CO2, NH3, N2O, COV, and CH4) management approaches, such as community composting (hotels, hos­ and analyzing their contribution. Estimating actual gaseous emissions pitals, universities, and schools); (iv) the environmental impacts of the data is essential, considering that these depend on the process condi­ residues generated during the operation of the BW composting plants (e. tions, the technology used, the type of substrate (or a mixture of sub­ g. greater area requirement and extra energy use). Works have advanced strates), and the treatment processes applied. Since this is not always on the environmental impacts of residues generated in the organic possible, especially in developing countries, information from databases fraction of municipal solid waste anaerobic digestion plants (Colazo can be used. Still, validating the data source (i.e., conditions, origin, et al., 2015). Still, research is scarce with regard to residues from BW or process characteristics) is necessary. Additionally, the incorporation of GW composting systems. Finally, although this study did not include indicators of odors generation in biowaste composting needs further vermicomposting, future studies could employ LCA to consider aspects study to allow its inclusion in LCAs. Recent studies on odor estimation associated with the emissions of this technology. Some works have (Cadena et al., 2018) or mitigation (Nguyen et al., 2023; Gao et al., advanced in estimating gaseous and other emissions from vermi­ 2022) offer important information for environmental impact composting (Lleó et al., 2013). assessment. 3.3.7. Use of software 3.3.4. Environmental impacts from compost utilization There is an increasing need for predictive models to support envi­ There is a lack of information related to the environmental impacts of ronmental policy and decision-making. The validity of these models compost utilization as soil amendment to determine the replacement depends on the availability of reliable, rigorously generated data rele­ capability of compost. Further studies are required that relate the quality vant to the study context. This work found several LCA software tools of the end-product with the resulting environmental impacts for that evaluate the environmental impacts of BW and/or GW composting. different uses (e.g., agriculture, horticulture, soil amendment, land­ Even though all of these tools contain databases and information, very scaping). This relation depends on the quality and physicochemical few users are aware of certain software features, specific terms, and characteristics of the compost and on the environmental conditions of underlying computational processes. In addition, the data contained in the system where the product is applied. Recent studies show that using the databases are restricted to limited scenarios (Komilis and Sanchez, compost generates more benefits than simply adding nutrients (for 2017). Therefore, it is necessary to train users to improve the application instance, organic compost has significantly higher antioxidant and anti- of these tools. The training may include aspects that increase the un­ carcinogenic properties than chemical fertilizers) (Komilis and Sánchez, derstanding of: (i) composting processes, (ii) emissions generated in the 2017). Future studies should determine: i) the avoidance of conven­ process (e.g., mass and energy balances), (iii) basics of LCA, (iv) input tional soil improvers or amendments, ii) long-term carbon sequestra­ data to the software, (v) use of specific software, (vi) interpretation of tion, iii) improvement of soil properties, iv) contribution to climate results. change mitigation and v) other uses of compost such as biofilter media, soil bioremediation or landfill cover. 4. Conclusions 3.3.5. Impact categories In this study, a review of LCA articles assessing the environmental Only a few studies (Oldfield et al., 2018; Abduli et al., 2011) justify impacts of biowaste and/or green waste composting was systematically their selection of impact categories. These studies allow for standard­ performed. This work demonstrates the complexity of conducting izing the categories used in a composting system according to the system credible and valid LCA studies on biowaste composting. Future research goal and scope and depend on the impact assessment methodology should be carried out with the aims of: i) improving input data, priori­ selected. These arbitrary choices seriously influence the results and can tizing the collection of primary information, and the creation of regional restrict the completeness and reliability of an LCA. Although coverage of databases in the context of data scarcity mainly in developing countries a greater number of impact categories allows a more detailed analysis, in (e.g., waste characteristics, end-product characteristics, emissions, and this review, few studies analyzed the following impact categories: other input and output data); ii) improving the definition of the system ionizing radiation, agricultural land occupation, urban land occupation, boundaries in agreement with the objectives of the LCA study (pre- natural land transformation, water depletion, and marine ecotoxicity. composting, composting, post-composting); iii) achieving a standardi­ All LCA in BW and/or GW composting should justify the choice of the zation of the environmental impact categories (i.e., midpoint and impact categories evaluated in agreement with the studý s goal and endpoint) used in accordance with the goal and scope of the studies; iv) scope. conducting studies on the environmental loads avoided by applying compost for different purposes; v) assessment of the environmental 3.3.6. Environmental impacts of strategies for improving composting impacts of specific strategies for improving composting technologies technologies (compare facilities of different operating systems and technologies); and Generally, the strategies for improving the composting of BW and vi) training practitioners in the application of LCA models, considering GW are addressed from a technological perspective (e.g., reduction of criteria for data selection, understanding process material and energy composting time and improvement of product quality). These strategies balances, and interpreting results. may include amendment or bulking materials, inoculum addition, and 362 E.R. Oviedo-Ocaña et al. Waste Management 171 (2023) 350–364 Declaration of Competing Interest Cerda, A., Font, X., Barrena, R., Gea, T., Sánchez, A., 2018. Composting of food wastes: Status and challenges. Bioresource Technology 248A, 57–67. 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