Proceeding Paper Implementing Community Composting in Primary Schools: First Experiences at Universitat Autònoma de Barcelona, Spain † Marta García-Prats 1 , Daniel González 1, * , Javier Moral-Vico 1 , Cristina Madrid-López 2 and Antoni Sánchez 1 1 GICOM Research Group, Department of Chemical, Biological and Environmental Engineering, Escola d’Enginyeria, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; marta.garcia.prats@uab.cat (M.G.-P.); antoniojavier.moral@uab.cat (J.M.-V.); antoni.sanchez@uab.cat (A.S.) 2 Sostenipra Research Group, Institute of Environmental Science and Technology (ICTA-UAB), Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; cristina.madrid@uab.cat * Correspondence: daniel.gonzalez.ale@uab.cat † Presented at the 2nd International Electronic Conference on Processes: Process Engineering—Current State and Future Trends (ECP 2023), 17–31 May 2023; Available online: https://ecp2023.sciforum.net/. Abstract: Composting is one of the most viable alternatives to landfill disposal to reduce the environ- mental impacts of organic waste management, such as the emission of greenhouse gases (GHGs). A community composting system consisting of four 1 m3 modules was installed in a selected primary school in Bellaterra (Spain) and monitored through daily analysis of the main process parameters (tem- perature, moisture content and interstitial oxygen) and weekly analysis of gaseous emissions (CH4 , N2 O and VOCs). The composting process was successful and gaseous emissions were maintained under desirable values, which can be used to support and promote this kind of initiatives. Keywords: community composting; biowaste; compost; resource; valorisation 1. Introduction Waste management is one of the main challenges of modern society, and its importance Citation: García-Prats, M.; González, is expected to increase as the world’s population keeps growing. The organic fraction of D.; Moral-Vico, J.; Madrid-López, C.; municipal solid waste (OFMSW) is especially sensitive, as its mismanagement results in Sánchez, A. Implementing serious environmental impacts, such as global warming due to greenhouse gases emissions Community Composting in Primary (GHG) [1]. Some decades ago, the final destination of organic waste was the disposal in Schools: First Experiences at controlled/uncontrolled landfills and the incineration with/without energy recovery; it is Universitat Autònoma de Barcelona, still the case in many places [2]. In recent years, biological processes, such as composting Spain. Eng. Proc. 2023, 37, 37. and anaerobic digestion, have appeared as a much more sustainable alternative for organic https://doi.org/10.3390/ waste management. These strategies offer a possibility to obtain value-added products from ECP2023-14730 residues, including energy and other valuable bioproducts like compost [3], which can help Academic Editor: Kuo-Lin Huang to close the organic matter (OM) cycle and thus the move towards a more circular economy. EU member states are obligated by the Landfill Directive (1999/31/EC) and the Waste Published: 10 July 2023 Framework Directive (2008/98/EC) to reduce the amount of biodegradable municipal solid waste (MSW) sent to landfills and to recycle organic fractions using more environmentally friendly technologies [4,5]. The European Commission (EC) adopted the “Circular Econ- omy Package” to lower the limits for municipal waste going to landfills and set a target Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. for recycling 65% of municipal waste by 2035 [6]. Recently, European regulations have This article is an open access article stated that biowaste must be source-separated and collected for its proper treatment and distributed under the terms and valorisation for resource recovery [7]. In Spain, a new legislation requires municipalities to conditions of the Creative Commons totally separate domestic biowaste in origin by the end of 2023 [8]. These policies, together Attribution (CC BY) license (https:// with the rising prices and obstacles set to landfill disposal, will increase the demand for creativecommons.org/licenses/by/ biological treatment processes. 4.0/). Eng. Proc. 2023, 37, 37. https://doi.org/10.3390/ECP2023-14730 https://www.mdpi.com/journal/engproc Eng. Proc. 2023, 37, 37 2 of 8 Composting is a widely known biological process in which OM is decomposed mainly by microorganisms under aerobic conditions producing compost, which can be used as fertilizer [9]. Currently, composting is mostly applied through centralized systems that treat the organic wastes of several municipalities at an industrial scale. The reasons behind using this approach are that (1) industrial composting is more efficient, (2) different odorous compounds, such as ammonia, hydrogen sulphide or a wide range of volatile organic compounds (VOCs) are produced during the composting process, which can be partially or totally treated in those industrial plants and (3) ordinary citizens might be unable to select the suitable waste and properly operate the process [10]. However, in recent years, there has been a rise in small-scale composting initiatives in diverse communities (villages, neighbourhoods, apartment buildings, schools, hospitals, hotels, prisons, etc.) [11–13]. In these decentralized composting systems, the location where waste is generated and treated is close to where the compost is used, which minimizes material transportation and, therefore, reduces process costs, GHG emissions, road wear, traffic and noise generation [14]. Thus, the utilization of the compost produced not only improves the soil quality, but also avoids the environmental impacts associated with the production of mineral fertilizers [15]. Decentralized waste management systems have a high potential to involve users and promote environmental education. Specifically, community composting systems have an even higher potential, as the process is relatively simple and can be understood by all social groups [9]. In decentralized composting systems, waste generators become both the people in charge of the process and the recipients of the final product, which increases their awareness on the impacts involved in MSW management and their own waste generation and thus, this tends to reduce the amount of waste generated [16]. The waste separation at source also improves as it is critical to the success of the initiative, which, in turn, improves the quality of the compost obtained [17]. The main drawbacks of house and community composting are the problems in ob- taining stable mature compost and the unpleasant odours produced, which can be usually dampened by adding a suitable fraction of bulking agent to the raw material [3,18]. There- fore, the quality of the compost and the gaseous emissions related to environmental impact and unpleasant odours are key for the successful application of community-scale compost- ing systems. In the context of promoting the advantages of decentralized composting and ensuring the quality and efficiency of the process and the final product, the “CARE: Citizen Arenas for Resource Use and Waste Management” project aims to bring the composting science to children at primary schools, raising their awareness on the environmental impacts that their own biowastes can generate if they are not properly managed, and to give them the opportunity to learn the benefits that compost represents to the environment. With that purpose, a community composting system has been installed in a selected school in Bellaterra (Cerdanyola del Vallès, Spain), where organic kitchen waste is transformed into compost, which can later be used in the school’s green spaces. During the process, typical parameters, such as material temperature, interstitial O2 or humidity, as well as gaseous emissions, are monitored continuously to ensure the proper functioning of the community composting system. 2. Materials and Methods 2.1. Characteristics of the Feedstock The biowaste fed to the composting system is the organic fraction of waste generated at the school, which comes mainly from the meals prepared at the restaurant. Every day, approximately 400 meals are served, accounting for 50–60 kg OFMSW/day on average. Shredded pruning waste supplied by the gardening services at Universitat Autònoma de Barcelona is used as the bulking agent. The residue and the bulking agent are mixed in a 1:1–1:1.25 volumetric ratio to adjust the porosity of the raw mixture and promote the air flow through it, which is one of the key parameters to ensure the efficiency of the process Eng. Proc. 2023, 37, x 3 of 8 Eng. Proc. 2023, 37, 37 Barcelona is used as the bulking agent. The residue and the bulking agent are mixed3 ofin8a 1:1–1:1.25 volumetric ratio to adjust the porosity of the raw mixture and promote the air flow through it, which is one of the key parameters to ensure the efficiency of the process andthe and thequality qualityofofthe theresulting resultingcompost. compost.This Thistranslates translatesinto intoaadaily dailyvolumetric volumetricload loadofof around 100 L of mixture to the composting system. around 100 L of mixture to the composting system. 2.2.Community 2.2. CommunityComposting CompostingSystemSystem Thecommunity The communitycomposting compostingsystem system installed installed in in thethe school school consists consists of four of four compost- composting ing modules modules withwith a volume a volume of 1of m13 ,m 3, and a storage compartment to keep the bulking agent and a storage compartment to keep the bulking agent (Figure1). (Figure 1).The Thesystem systemconfiguration configurationallows allowscontinuous continuousoperation operationof ofthe theprocess, process,as aseach each moduleisisdevoted module devoted toto aa specific specific phase phase of the composting process. process. Specifically, Specifically,module module1 1(feeding) (feeding)receives receives the the daily daily loads loads and is where the the process process begins, begins,and andthethetemperature temperature starts startsrising, rising,reaching reachingthe thethermophilic thermophilicrange. range. When When module module 11 isis full, full, the thematerial materialisis moved movedtotomodule module2,2,where wherethe thethermophilic thermophilcphase phasecontinues. continues.TheThematerial materialtransfer transferhelps helps totoproperly properly mix and oxygenate the material and to liberate module 1 to continue withthe mix and oxygenate the material and to liberate module 1 to continue with the feeding. feeding.When Whenmodule module1 1isisfull again, full again,thethematerial material from module from module 2 is2moved is moved to module to module3 or3 4or(maturation), where 4 (maturation), it is it where kept untiluntil is kept it is it fully stable is fully and and stable ready to betoused ready as fertilizer; be used the as fertilizer; material fromfrom the material module 1 is moved module to module 1 is moved to module2, and2, so andon.so on. Figure1.1.Community Figure Communitycomposting compostingsystem systeminstalled installedatatthe theselected selectedprimary primaryschool. school. 2.3.Composting 2.3. Compostingand andCompost CompostAnalytical AnalyticalProcedures Procedures Therewere There werefour fourclasses classesofof3rd 3rdand and4th 4thgrade gradeinvolved involvedin inthe theproject, project,with withaatotal totalofof 80 80students studentsaged aged8–10 8–10years yearsand and33teachers. teachers.Through Throughaaseries seriesof offormation formationandandinformation information sessions, sessions,thethe children and the the teachers teacherswere wereintroduced introducedtoto composting composting terms, terms, process process pa- parameters rameters and andoperation. operation. Thereafter, Thereafter, there there were were joint joint practical practical sessions sessions forfor preparing preparing the the biowaste biowaste (weighting (weighting thethe biowaste biowaste andand the the bulking bulking agent, agent, mixing, mixing, sorting sorting impurities impurities and and loading loading modulemodule 1), measuring 1), measuring andand understanding understanding thethe keykey parameters parameters (temperature, (temperature, in- interstitial terstitial OO2,2 ,and andmoisture moisturecontent) content) and and observing observing the process process development development(checking (checkingthethe module modulefillfilllevel, level,mixing mixingthe thematerial, adding material, addingwater or bulking water agent or bulking to adjust agent the moisture to adjust the mois- content, noticing odours, etc.). ture content, noticing odours, etc.). 2.3.1. 2.3.1.Routine RoutineAnalytical AnalyticalMethods Methods Solid Solid samples obtainedalong samples obtained alongthe theoperation operationwere werecharacterized characterizedininterms termsof ofmoisture moisture content, dry matter, OM, pH and electrical conductivity according to standard procedures [19]. content, dry matter, OM, pH and electrical conductivity according to standard procedures For a qualitative control of moisture content, the “fist test” or “squeeze test” was [19]. performed, which is based on taking a handful of mixture and squeezing it. If the material For a qualitative control of moisture content, the “fist test” or “squeeze test” was per- drips liquid is too wet, bulking material must be added, whereas if it totally disintegrates formed, which is based on taking a handful of mixture and squeezing it. If the material is too dry, it must be watered. If the material remains aggregated without leaching, the drips liquid is too wet, bulking material must be added, whereas if it totally disintegrates moisture content is appropriate. is too dry, it must be watered. If the material remains aggregated without leaching, the moisture 2.3.2. content isand Temperature appropriate. Interstitial Oxygen Temperature was measured daily using three different temperature probes simultane- ously, placing one in the centre of the composter (more active and warmer zone), another one in an intermediate zone and the last one next to the composter wall (theoretically less active and colder zone). The action was repeated for all modules with material inside, Eng. Proc. 2023, 37, 37 4 of 8 even though in this case there was only one active module. The ambient temperature was also recorded. Interstitial O2 was measured in the same locations as the temperature using an O2 probe equipped with a manual air pump connected to an O2 sensor (Sensotran, Barcelona, Spain). 2.4. Gaseous Emissions Sampling and Analytical Procedures A semi-spherical stainless steel flux chamber (0.443 m of base diameter, 0.154 m2 of base area and 0.045 m3 of volume) provided by Scentroid (IDES Canada Inc., Whitchurch- Stouffville, ON, Canada) was used to perform emissions sampling [20]. Nalophan® bags were used to store gas samples, which were obtained before and after mixing the material inside the corresponding composting module. The gaseous samples were obtained once per week, corresponding to days 15th, 23rd, 32nd, 37th, 44th and 49th of running. CH4 and N2 O analysis were performed using an Agilent 6890 N Gas Chromatograph (GC) and an Agilent 8860 GC, respectively (Agilent Technologies, Inc., Santa Clara, CA, USA). For CH4 analysis, the Agilent 6890 GC was equipped with a flame ionization detector (FID), whereas for N2 O analysis, the Agilent 8860 GC was equipped with an electron capture detector (ECD). Both GCs were equipped with a HP-PLOT Q semi-capillary column (30 m × 0.53 mm × 40.0 µm, Agilent Technologies, Inc.) with N2 as carrier gas at 13.79 kPa pressure coupled to a post-column particle trap (2 m, n◦ 5181-3352, Agilent Technologies, Inc.). The injection volume used for each sample was 250 and 500 µL and the total time of analysis was 4 min and 6 min for CH4 and N2 O, respectively. To perform VOC analysis, a MiniRAE 3000 portable analyser was used (RAE Systems, San José, CA, USA), which is equipped with a 10.6 eV PID lamp with a detection range from 0 to 15,000 ppmveq isobutylene [20]. 3. Results and Discussion 3.1. Composting Performance The daily material loads accounted on average for 65 kg mass, accounting both for the organic waste (OFMSW) and the bulking agent (VF). Considering the feeding ratio in the school and that the material was loaded 3 to 4 days per week, a total of 3 weeks were needed to fill module 1, reaching an accumulated mass of 450 kg. Due to the school’s summer holidays, only one round of compost will be produced, but the material was transferred to module 2 once the first module was full to promote the mixing. The accumulated mass shown in Figure 2 represents the fresh material loaded into the composting system. The subsequent volume reduction due to degradation and water loss has not been considered, but by the end of the period depicted in Figure 2, the remaining material had 60% of the initial volume. The moisture content, although not shown, was maintained within a range of 40–60%, which is recommended to facilitate the activity of the microbial degrading populations and to avoid the generation of leachates throughout the process. Temperature is the key parameter ruling the composting performance. The curve observed from the composting system established in the school shows the typical phases of the composing process: first, a mesophilic phase from ambient temperature up to 45 ◦ C, followed by a thermophilic phase up to 70 ◦ C, where maximum OM biodegradation occurs, and end up with a cooling and maturation period (below 45 ◦ C), where the material is finally stabilised. It is considered that the thermophilic phase should last for at least 14 days to ensure the material sanitisation and that, after the peak, the temperature must descend back to ambient values to consider the material stable and optimally mature [9]. Both conditions were accomplished in the system, which guarantees the quality of the composting process and the compost produced. Eng. Proc. 2023, 37, 37 5 of 8 Eng. Proc. 2023, 37, x 5 of 8 Figure 2. Accumulated material mass, considering the organic and vegetal fractions (OFMSW + VF) Figure 2. Accumulated material mass, considering the organic and vegetal fractions (OFMSW + VF) and main process parameters (average temperature and interstitial O2) measured during the com- and main process parameters (average temperature and interstitial O ) measured during the compost- posting process. The vertical lines represent the different phases of 2the composting process (meso- ing process. The vertical lines represent the different phases of the composting process (mesophilic, philic, thermophilic and maturation). thermophilic and maturation). The moisture content, although not shown, was maintained within a range of 40– Interstitial O2 gives valuable information about the biological activity. There is an 60%, which is recommended to facilitate the activity of the microbial degrading popula- inverse relation between temperature and interstitial O2 , as high temperatures entail high tions and to avoid the generation of leachates throughout the process. biological activity, which leads to an increase in O2 consumption and a decrease in its Temperature is the key parameter ruling the composting performance. The curve ob- concentration. A concentration of 10% O2 is considered the limit value to make sure that served from the composting system established in the school shows the typical phases of microorganisms have enough O2 to degrade the OM aerobically; below that value, anaero- the composing process: bic degradation processesfirst, maya take mesophilic phase from place, resulting ambient related in problems temperature up torotting, to material 45 °C, followed unpleasant odours, etc. [9]. Interstitial O2 was maintained over 18.5% throughout oc- by a thermophilic phase up to 70 °C, where maximum OM biodegradation the curs, andthus process, endavoiding up with aanaerobic cooling and maturation degradation andperiod (below ensuring 45 °C), aerobic where the conditions material throughout is thefinally stabilised. It is considered that the thermophilic phase should last for at least 14 process. days to ensure the material sanitisation and that, after the peak, the temperature must descend backoftoGHGs 3.2. Emission ambient and values VOCs to consider the material stable and optimally mature [9]. Both Composting conditions were accomplished of organic wastes in incentralized/decentralized the system, which guarantees the quality facilities of the are a green al- composting processthe ternative to reduce andenvironmental the compost produced. impacts of landfill deposition, but still poses some Interstitial problems O2 gives regarding valuableofinformation the emissions GHGs includingaboutcarbon the biological activity. dioxide (CO There is an 2 ), methane (CH4 ) inverse relation between temperature and interstitial O 2 , as high temperatures and nitrous oxide (N2 O) [21], and compounds responsible for odour pollution like entail high volatile biological activity, which leads to an increase in O consumption and organic compounds (VOCs) [22]. During the composting process operation, gaseous emis- 2 a decrease in its concentration. A concentration of 10% O is considered the limit value sion samples were obtained weekly to monitor the associated emission of CH4 , N2 O and 2 to make sure that microorganisms total VOCs, as shown have enough in FigureO3.2 to degrade the OM aerobically; below that value, anaer- obic degradation processes may take place, resulting in problems related to material rot- ting, unpleasant odours, etc. [9]. Interstitial O2 was maintained over 18.5% throughout the process, thus avoiding anaerobic degradation and ensuring aerobic conditions through- out the process. Composting of organic wastes in centralized/decentralized facilities are a green alter- native to reduce the environmental impacts of landfill deposition, but still poses some problems regarding the emissions of GHGs including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) [21], and compounds responsible for odour pollution like Eng. Proc. 2023, 37, 37 volatile organic compounds (VOCs) [22]. During the composting process operation, 6gas- of 8 eous emission samples were obtained weekly to monitor the associated emission of CH4, N2O and total VOCs, as shown in Figure 3. Figure 3. Figure 3. Temperature Temperature curve curve and and gaseous gaseous emission emission rates rates (CH (CH44,, N N22O O and tVOCs) measured and tVOCs) measured during during the composting process. The vertical lines represent the different phases of the composting process the composting process. The vertical lines represent the different phases of the composting process (mesophilic, thermophilic and maturation). (mesophilic, thermophilic and maturation). The average The averageCH CH 4 concentration measured was around 2.4 ppmv, except for a 39.7 4 concentration measured was around 2.4 ppmv, except for a 39.7 ppmv peak observed at the endthe ppm v peak observed at endthermophilic of the of the thermophilic phase, coinciding phase, coinciding withofa 10 with a period period days of 10 with days no with no mixing, mixing, which maywhich lead tomaythelead to the formation formation of some anaerobic of some anaerobic spots withinspots the within the material material and and the subsequent the subsequent punctualpunctual emission. emission. In terms Inofterms CH4ofemission CH4 emission rate, the rate, the ob- observed served average average was 183.7wasmg/d. 183.7 mg/d. N2O emissions N2 O emissions were werelow from low from the beginning the beginning of the ofoperation the oper- ation through the thermophilic phase—around 2.0 ppm through the thermophilic phase—around 2.0 ppmv —until the material’s temperature went v—until the material’s tempera- ture went down below 45 ◦below down C, when 45 a°C, when peak of N a 2peak O was of observed—14.5 N2O was observed—14.5 ppmv —and ppmitsv—and its emission emission followed followed a regular aincrease. regular increase. At this mesophilic At this mesophilic temperature temperature conditions conditions together together with a with a possible possible limitation limitation in carbon in carbon sources, sources, denitrifying denitrifying bacteriabacteria tend totendreduceto reduce available NO3 − available NO3 forming − forming N2 O N as2Oanasintermediate, an intermediate, provoking provoking its subsequent its subsequent emission emission [23–26]. [23–26]. Regard- Regarding ing N 2 ON O emission emission 2 rates, rates, the the observedobserved average average was 299 was mg/d. 299 mg/d. Considering Considering the Global the Global Warming Warmingon Potential Potential a 100-yearon aframe 100-year frame of CH 4 and of NCH2 O4 and (27 N and 2 O (27 273 and times 273 times higher higher than that than for that CO 2, respectively) [27], the process average GHG emission for CO2, respectively) [27], the process average GHG emission rate was 86.8 rate was 86.8 g CO 2 -eq/d. Finally, g CO2-eq/d. VOCs Finally,were VOCs found weretofound be emitted mainlymainly to be emitted duringduring the thermophilic the thermophilicphase,phase, wherewhere the most the easily biodegradable most easily biodegradable OM isOM consumed is consumed and volatiles and volatiles are much moremore are much easilyeasily formed and formed emitted and emitted[20]. [20]. The highest The highest total VOC concentration total VOC concentrationobserved was 255 observed wasppm255v , ppm whereas VOCs v, whereas average concentration VOCs average was 53.9 concentration wasppm v . ppm 53.9 Therefore, it is important v. Therefore, to notice it is important that community to notice that com- composting munity compostingsystems must systemsbe managed must be properly managedtoproperlyavoid undesirable gaseous emissions to avoid undesirable to gaseous ensure not only the comfort of the people nearby the system, but also the environmental sustainability of the process. 4. Conclusions The composting process presented here has been successfully operated, both in terms of loading and transferring the material and analysing the main process parameters. The temperature curve shows that the treated material has gone through all the expected phases of a composting process and therefore is properly sanitized and matured. Interstitial O2 Eng. Proc. 2023, 37, 37 7 of 8 decreased during the temperature peak as a result of the intense microbial activity but remained way over 10% throughout the process, thus avoiding anaerobic degradation processes. Emissions of GHGs (CH4 and N2 O) and odour pollutants (VOCs) were generally maintained under desirable limits, except for sporadic peaks. The correct management of the process is key to ensure the successful implementation of community composting systems like the one installed in the school, while avoiding any significant detrimental effects on the environment from the process. Author Contributions: Conceptualization, D.G., J.M.-V., C.M.-L. and A.S.; methodology, M.G.-P., D.G. and J.M.-V.; validation, M.G.-P., D.G., J.M.-V. and A.S.; investigation, M.G.-P., D.G. and J.M.-V.; resources, D.G., J.M.-V. and A.S.; data curation, M.G.-P. and D.G.; writing—original draft preparation, M.G.-P.; writing—review and editing, D.G. and J.M.-V.; visualization, M.G.-P. and D.G.; supervision, D.G., J.M.-V., C.M.-L. and A.S.; project administration, D.G. and J.M.-V.; funding acquisition, D.G., J.M.-V. and C.M.-L. All authors have read and agreed to the published version of the manuscript. Funding: This research and the CARE project was funded by the European Union’s Horizon 2020 research and innovation programme under the grant agreement nº 101016888 (SMART-ER project). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request due to restrictions. 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