Proceeding Paper Home Composting: A Review of Scientific Advances † Raquel Barrena and Antoni Sánchez * Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona (UAB), 08193 Bellaterra, Barcelona, Spain; raquel.barrena@uab.cat * Correspondence: antoni.sanchez@uab.cat † Presented at the 1st International Electronic Conference on Processes: Processes System Innovation, 17–31 May 2022; Available online: https://sciforum.net/event/ECP2022. Abstract: Composting has been demonstrated to be a sustainable technology for treating organic wastes. The process is based on the microbial decomposition of organic matter under aerobic conditions to obtain compost: An organic amendment that can be safely used in agriculture and other applications. Among the composted wastes, the organic fraction of municipal waste is commonly used. In this sense, the interest in composting at home or community scale is exponentially growing in recent years, as it permits the self-management of waste and obtaining a product that can be used by the own producer. However, some questions about the quality of the obtained compost or the environmental impact of home composting are in an early stage of this development. In this review, the main points related to home composting are analyzed in detail according to the current scientific knowledge. Among them: (i) The performance of the process, especially in the temperature reached and the fact that if home compost is sanitized, (ii) the quality of home composting, especially in terms of maturity and stability, (iii) the main environmental impacts of home composting, that is, gaseous emissions and (iv) the main trends related to community composting, a step forward from home composting, to make this alternative attractive for municipal organic waste management. The main advantages and possible drawbacks of home composting are also highlighted. Keywords: organic fraction of municipal waste; home composting; process performance; compost quality; environmental impact Citation: Barrena, R.; Sánchez, A. Home Composting: A Review of Scientific Advances. Eng. Proc. 2022, 1. Introduction 19, 35. https://doi.org/10.3390/ Among municipal waste, food waste (FW) comprises the main fraction (45%) of total ECP2022-12625 municipal solid waste in Europe [1]. This percentage can increase up to 55% in developing Academic Editor: Dariusz Dziki countries [2]. Some years ago, the final destination of FW was either the disposal in controlled/uncontrolled landfills or incineration with/without energy recovery. Published: 20 May 2022 FW treatment is usually performed by biological processes, such as composting and Publisher’s Note: MDPI stays neutral anaerobic digestion, although new strategies are being developed to obtain added value with regard to jurisdictional claims in bioproducts from organic waste [3]. Composting is based on the biological degradation of published maps and institutional affil- organic matter under aerobic conditions, and compost is the product of composting. The iations. process is considered a sustainable and environmentally friendly alternative for treating FW and is used worldwide at a commercial scale in industrial composting plants [4]. In contrast to these big facilities, an increasing number of initiatives using home or community composting have emerged in different parts of the world. Initially, these Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. experiences were explained as hobbies and low scientific information could be obtained This article is an open access article from them [5]. Today, we have scientifically based information about home and community distributed under the terms and composting in different aspects, such as the performance of the process, the quality of home conditions of the Creative Commons compost, or the environmental impact and Life Cycle Assessment. Attribution (CC BY) license (https:// The objective of this communication is to present a general perspective of home creativecommons.org/licenses/by/ composting, especially in those studies with scientific data, to provide updated information 4.0/). to the readers about the state-of-the-art of this technology. Eng. Proc. 2022, 19, 35. https://doi.org/10.3390/ECP2022-12625 https://www.mdpi.com/journal/engproc Eng. Proc. 2022, 19, 35 2 of 6 Eng. Proc. 2022, 19, 35 2 o 2. Home Composting 2. Home Composting 2.1. Process Performance 2.1. Process Performance Home composting Home performance compostingisperformance different from is that of industrial different from that full-scale compost- of industrial full-scale com ing. On the oneposting. hand, itOn is difficult to maintain the thermophilic conditions the one hand, it is difficult to maintain the thermophilic conditionsthat allow the that allo compost to be sanitized [6] or the presence of considerable stratification of temperature the compost to be sanitized [6] or the presence of considerable stratification of temper found in home ture composters found in[7]. homeThecomposters results obtained [7]. Thein results the composters obtained in indicate that small- the composters indicate th scale composting was viable since thermophilic sanitization temperatures ◦ C) were (55 temperatures small-scale composting was viable since thermophilic sanitization (55 ° maintained for were threemaintained consecutive fordays. threeHowever, consecutive stability indicators days. However, showed stability a different indicators showed a d organic matter ferent biodegradation organic matter process along the compost biodegradation process bin’s alongheight, with the the compost bottom bin’s height, with t layer requiringbottom a longer period layer to be stable requiring a longerthan the upper period layers.than to be stable Thetheauthors upperconclude layers. The autho that these phenomena conclude can have that importance these phenomena when candesigning commercial have importance whenhome composting designing commercial hom reactors. On thecomposting other hand,reactors. other works studied On the other other hand,important other works aspects studiedof home compost- aspects other important ing performance,homesuch as mass balances composting [8], which performance, such are verybalances as mass important [8],for further which are studies very important f related to Life Cycle Inventories and Life Cycle Assessment (LCA) [9]. further studies related to Life Cycle Inventories and Life Cycle Assessment (LCA) [9]. Regarding the Regarding performance of the process, the performance of theother aspects process, otherare considered aspects in recent are considered in recent lit ature. literature. Among Among them, thethem, needthe forneed for a bulking a bulking agent agent has special has special relevance relevance to provide poros to provide to the mixture porosity to the mixture to haveto have a strict a strict aerobic aerobic process process andand to avoid to avoid unpleasantodors unpleasant odorsand and unwant unwanted gaseousgaseous emissions emissions in in thethe formofofgreenhouse form greenhouse gases, gases, especially especiallymethane methane and andnitrous oxi (Figure1). nitrous oxide (Figure 1). Figure Figure 1. Detail of 1. Detail a home of a home composter withcomposter with a proper a proper porosity porosity adjustment. adjustment. In this sense,Guidoni In this sense, Castiglioni Castiglioni Guidoni et al. (2018)et investigated al. (2018) investigated how different how different ratios ratios of of bu bulking agent andingfood agentwaste and food wastethe can affect can affect the progress progress of the of the composting composting process [10].process Results[10]. Resu showed that the showed ratio ofthat the ratio bulking of bulking agents has an agents has an effect important important effect on the on the biodegradation biodegradation organic of organic matter, matter, nitrogen nitrogen dynamics, dynamics, and theoftoxicity and the toxicity of the product. the product. It is evident It is evident that that th ratio must be carefully studied in home composting to reach this ratio must be carefully studied in home composting to reach an equilibrium between an equilibrium between b biodegradation and environmental impact [11]. Regarding including FW in a home com- compo degradation and environmental impact [11]. Regarding including FW in a home posting process,ing process, there there is some is some discussion discussion on including on including meat and, meat and, in general, in general, foodof food waste waste of a animal origin. Storino et al. (2016) studied the comparison of home composting with and and wi mal origin. Storino et al. (2016) studied the comparison of home composting with out the presence of meat in the initial mixture [11]. The authors concluded that meat h without the presence of meat in the initial mixture [11]. The authors concluded that meat several positive effects on the process activity and an acceleration of the biodegradation has several positive effects on the process activity and an acceleration of the biodegradation organic matter without altering the main physicochemical characteristics (pH, salinity, of organic matter without altering the main physicochemical characteristics (pH, salinity, phytotoxicity) and a low pathogen level with proper handling of the home composte or phytotoxicity) and a low pathogen level with proper handling of the home composters. However, no information on gaseous emissions expected from meat waste, such as amm However, no information on gaseous emissions expected from meat waste, such as ammo- nia, is provided. Other authors showed low gaseous emissions when composting leftove nia, is provided. Other authors showed low gaseous emissions when composting leftovers of raw fruits and vegetables without animal wastes [12]. In the case of inoculation, hom of raw fruits and vegetables without animal wastes [12]. In the case of inoculation, home composting is a perfect system to use the obtained compost as a source of active microo composting is aganisms perfect to system to use maintain the obtained compost a semi-continuous process,as a source although of active some microor- authors point out that t ganisms to maintain a semi-continuous process, although some authors point out that the Eng. Proc. 2022, 19, 35 3 of 4 Eng. Proc. 2022, 19, 35 3 of 6 presence of the so-called “Effective Microorganisms” is positive in only particular aspects, such as odor control and humification [13]. presence of the so-called “Effective Microorganisms” is positive in only particular aspects, suchFinally, as odor some controlauthors comment [13]. and humification that collection is a critical step to having a good home composting process. Thus, Puyuelo Finally, some authors comment that et al. (2013) performed collection a comparison is a critical of several step to having methods a good to collect home food waste composting before process. composting, Thus, Puyuelo etshowing al. (2013)that the useaof performed perforatedofpassively comparison several aer- ated bins to methods jointly with collect foodcompostable waste beforebags was superior composting, showingto other conventional that the systems [14]. use of perforated For instance, passively this system aerated did with bins jointly not imply more gaseous compostable bags was emissions, superior and it was to other suitable for pre- conventional systemsthe paring [14]. For instance, organic thisfurther waste for systemcomposting. did not implyBesides, more gaseous in termsemissions, of weightand it was loss, tempera- suitable ture, for preparing respiration theand index, organic wastematter organic for further composting. reduction, Besides, the best in terms results wereofalso weight achieved loss, temperature, respiration with the aerated system (Figure 2). index, and organic matter reduction, the best results were also achieved with the aerated system (Figure 2). Figure Figure 2. Collection Collectionofoffood foodwaste waste using using compostable compostable bagsbags and and aerated aerated bins. bins. 2.2. Home Compost Quality 2.2. Home Compost Quality In reference to the quality of compost obtained from home systems, there is a consensus in theInsense reference to the that this quality quality of compost is similar, obtained if not better, than from that ofhome systems, industrial thereThe facilities. is a con- sensus in the sense that this quality is similar, if not better, than that absence of impurities in the initial mixture is the main reason for this high quality of home of industrial facilities. The absence of composting. impurities Thus, Vázquezinand theSoto initial mixture (2017) presentis the mainincluding a study reason for 880this high quality of experiences home of home composting. compostingThus, Vázquez in rural and Soto areas, using (2017) biowaste, household present aincluding study including meat and880 fishexperi- leftovers ences [15]. Ninety of home home compost composting in ruralsamples were analyzed, areas, using household showing excellent biowaste, properties: including meat and A low fish C/N ratio leftovers (10–15), [15]. Ninety nohome physical contaminant compost samples materials (less than showing were analyzed, 0.3% in dry matter), prop- excellent low heavy erties: A lowmetal C/N content, and highno ratio (10–15), nutrient physical content (2.1% N, 0.6% contaminant P, 2.5%(less materials K, 0.7% thanMg, andin dry 0.3% 3.7% Ca, respectively). Other studies show similar results of home compost matter), low heavy metal content, and high nutrient content (2.1% N, 0.6% P, 2.5% K, 0.7% quality in terms of physicochemical characterization (pH, moisture, carbon, nitrogen, and C/N ratio) [16,17]. Mg, and 3.7% Ca, respectively). Other studies show similar results of home compost qual- Regarding the presence of pathogenic microorganisms, they were not detected, even when ity in terms of physicochemical characterization (pH, moisture, carbon, nitrogen, and C/N thermophilic temperatures were not fully reached [11]. ratio)Apart [16,17]. fromRegarding the presence these studies, of pathogenic the literature is scarce on microorganisms, the comparison of they homewereand not de- tected, even when thermophilic temperatures were not fully reached industrial composting. An especially interesting study is that of Barrena et al. (2014), where [11]. Apart a large numberfrom of these home andstudies, the literature industrial composts is scarce were on theincomparison compared of home terms of stability, using and in- dustrial respirationcomposting. techniques An [18].especially The main interesting conclusion isstudy is that that home of Barrenawhen composting, et al.properly (2014), where amanaged, large numbercan reach high levels of home of stability,composts and industrial similar to werethose compared of full-scaleincomposting. If terms of stability, only physicochemical properties are compared, industrial and using respiration techniques [18]. The main conclusion is that home composting, when home composts are not significantly properly different. managed, can reach high levels of stability, similar to those of full-scale compost- ing. If only physicochemical 2.3. Environmental Impact properties are compared, industrial and home composts are not significantly different. Regarding environmental impacts of home composting, they have been quite exten- sively studied in the literature. On the one hand, the main advantage of home composting 2.3. Environmental is the Impact absence of waste transport and lower energy requirements, whereas a possible disad- Regarding vantage environmental is the lack impacts of gaseous emissions of home control composting, as in full-scale theywhich facilities, have is been quite exten- practically unavoidable [19]. On the other hand, there is a need to compare home composting sively studied in the literature. On the one hand, the main advantage of home composting with isindustrial composting the absence of wastefrom the environmental transport point ofrequirements, and lower energy view to have an experimentally- whereas a possible dis- advantage is the lack of gaseous emissions control as in full-scale facilities, which is prac- tically unavoidable [19]. On the other hand, there is a need to compare home composting Eng. Proc. 2022, 19, 35 4 of 6 based comparison. The preliminary studies on this topic were presented by Colón et al. (2010) and Andersen et al. (2012) [9,20]. Both papers performed a very complete study on the environmental impacts, which were compiled in an LCA. The conclusions of both works are similar: Home composting performed better than other waste management technologies in most of the impact categories. Both studies also agree with the fact that gaseous emissions are the main contributors to negative environmental impacts in different environmental categories, especially in the global warming potential. Accordingly, further studies on the environmental impact focused on gaseous emis- sions during the home composting process [21]. For instance, Ermolaev et al. (2014) studied the greenhouse gases (GHG) emissions of several home composters treating FW and compared them with literature data from full-scale composting [22]. In this case, home composting emitted less methane than large-scale composts but similar amounts of nitrous oxide. Other works have focused on this comparison in rigorous studies with the same initial mixture, season, and location, with more environmental categories studied. This is the case of Colón et al. (2012) [23]. In this study, four different full-scale facilities treating a source-selected organic fraction of Municipal Solid Waste (OFMSW) were environmentally evaluated with an LCA, including composting technologies (in-vessel, turned piles, and home composting) and anaerobic digestion plus composting. In this case, home composting was better in terms of environmental impact than the other composting technologies. In a previous work, Martínez-Blanco et al. (2010) directly compared a complete LCA home and industrial composting [24]. In summary, the results were as expected: Ammonia and GHG emissions (methane and nitrous oxide) released from home composting were considerably higher than those of industrial composting. However, this latter option involved 2 and 53 times more needs for transport, energy, water, and infrastructure. 3. Community Composting Although used extensively in many countries, especially in central Europe [25], and having an extraordinary development in recent years, community composting has received low attention in the world of research. Thus, there are starting experiences in universities, hospitals, municipal markets, or just groups of households. However, the information found in the scientific literature is very scarce. Most of the studies on community composting are related to economic feasibility. From these studies, it is evident that many entities can use community composting. In this case, universities can play an important role as the first stakeholders to impulse this strategy that can be easily extrapolated to other organic waste producers, such as hotels, hospitals, or schools. Community composting can have an important influence on two items: On the one hand, it can treat a significant amount of organic waste and, on the other hand, it can be a stimulus to promote home composting among citizens [26]. In Figure 3, a recently developed program of community composting in Pontevedra (Spain) is presented. Eng. Proc. Eng. 2022, Proc. 19,19, 2022, 3535 5 5ofof46 Figure 3. Community composting in Pontevedra (Spain). Figure 3. Community composting in Pontevedra (Spain). 4. Conclusions 4. Conclusions It can be concluded that home and community composting has a great potential to be a It canimplemented massively be concludedstrategy that home and community in organic compostingwhen waste management has adeveloping great potential to national and be aregional massivelyprograms. Onestrategy implemented important reason waste in organic for this expansion is management the developing when fact that home composting national andhas passedprograms. regional from being a hobby One to a scientifically-based important technology. reason for this expansion From is the fact the that environmental home composting has passed from being a hobby to a scientifically-based technology. to and economic points of view, home and community composting appears be superior From to industrial composting the environmental and economic in most pointsofof the environmental view, categories, which home and community again compost- makes this strategy ing appears attractive to be superior to to be included industrial in wasteinmanagement composting programs. most of the environmental catego- ries, which again makes this strategy attractive to be included in waste management Author Contributions: Conceptualization, A.S.; methodology, R.B.; formal analysis, A.S.; investi- programs. gation, R.B. and A.S.; writing—original draft preparation, A.S.; writing—review and editing, A.S.; funding acquisition, A.S. All authors have read and agreed to the published version of the manuscript. Author Contributions: Conceptualization, A.S.; methodology, R.B.; formal analysis, A.S.; investiga- tion, R.B.This Funding: andresearch A.S.; writing—original draft preparation, was funded by Agència A.S.;Universitaris de Gestió d’Ajuts writing—review and editing, i de Recerca A.S.; of Generalitat defunding acquisition, Catalunya, A.S. All grant number authors have Pandèmies read Project and Ref. agreed to the published version of the manu- 2020PANDE00021. script. Institutional Review Board Statement: Not applicable. Funding: This research was funded by Agència de Gestió d’Ajuts Universitaris i de Recerca of Generalitat Informed Consent de Catalunya, grantStatement: Not applicable. number Pandèmies Project Ref. 2020PANDE00021. Data Availability Institutional Statement: Review Not applicable. Board Statement: Not applicable. Acknowledgments: Not applicable. Informed Consent Statement: Not applicable. 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