atmosphere Article Characterization of the Gaseous and Odour Emissions from the Composting of Conventional Sewage Sludge Daniel González 1,2 , Nagore Guerra 3 , Joan Colón 3 , David Gabriel 2 , Sergio Ponsá 3 and Antoni Sánchez 1, * 1 Composting Research Group (GICOM), Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; daniel.gonzalez.ale@uab.cat 2 Group of Biological Treatment of Liquid and Gaseous Effluents (GENOCOV), Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; David.Gabriel@uab.cat 3 BETA Technology Centre: “U Science Tech”, Univeristy of Vic-Central University of Catalonia, 08500 Vic, Spain; nagore.guerra@uvic.cat (N.G.); joan.colon@uvic.cat (J.C.); sergio.ponsa@uvic.cat (S.P.) * Correspondence: antoni.sanchez@uab.cat  Received: 15 December 2019; Accepted: 15 February 2020; Published: 19 February 2020  Abstract: Many different alternatives exist to manage and treat sewage sludge, all with the common drawback of causing environmental and odour impacts. The main objective of this work is to present a full inventory of the gaseous and odorous emissions generated during the bench-scale composting of conventional sewage sludge, aiming at assessing the process performance and providing global valuable information of the different gaseous emission patterns and emission factors found for greenhouse gases (GHG) and odorant pollutants during the conventional sewage sludge composting process. The main process parameters evaluated were the temperature of the material, specific airflow, average oxygen uptake rate (OUR), and final dynamic respiration index (DRI), resulting in a proper performance of the sewage sludge composting process and obtaining the expected final product. The obtained material was properly stabilized, presenting a final DRI of 1.2 ± 0.2 g O2 ·h−1 ·kg−1 Volatile Solids (VS). GHGs emission factor, in terms of kg CO2eq ·Mg−1 dry matter of sewage sludge (DM–SS), was found to be 2.30 × 102 . On the other hand, the sewage sludge composting odour emission factor (OEF) was 2.68 × 107 ou·Mg−1 DM–SS. Finally, the most abundant volatile organic compounds (VOC) species found in the composting gaseous emissions were terpenes, sulphur compounds, ketones, and aromatic hydrocarbons, whereas the major odour contributors identified were dimethyldisulphide, eucalyptol, and α-pinene. Keywords: sewage sludge; composting; gaseous emission; greenhouse gases (GHG); odour; VOCs 1. Introduction The large amount of sewage sludge (SS) production is becoming a worldwide environmental problem. In Europe, the increasing number of wastewater treatment plants due to the application of the Council Directive 91/271/EEC concerning urban wastewater treatment [1] has led to the generation of more than 7.5 million Mg of SS per year (in dry basis) by the different European Member States [2], with the existing necessity of handling and managing it. Nowadays, different treatment processes are used to handle and manage all SS produced, such as land application, incineration, composting, or anaerobic digestion, aiming at reducing its volume and quantity as well as stabilizing it [2]. In fact, one of the most typical and applied stabilization approach, composting, is considered an environmentally friendly technology that reduces sludge volume and transport costs, decomposing organic matter into a stable end product and removing malodorous compounds as well as pathogens and weeds [3]. Atmosphere 2020, 11, 211; doi:10.3390/atmos11020211 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 211 2 of 16 The aim of SS composting is obtaining a biological stable product that can be used as fertilizer or soil amendment by decomposing the organic matter present in the initial feedstocks [4]. Nevertheless, different unavoidable environmental and social concerns arise from this SS biological treatment, such as the emission of different gaseous compounds responsible for odour nuisance and atmospheric pollution. In this sense, ammonia, hydrogen sulphide, and a wide range of volatile organic compounds (VOCs) have been described as the main odorant compounds present in SS composting gaseous emissions [5,6], which can appear at different levels due to the biological degradation of organic matter and nitrogen-and sulphur-based compounds depending on the conditions (aerobic/anaerobic) of the SS composting process. Apart from the previously cited odorant compounds, different greenhouse gases (GHG) such as carbon dioxide, methane, and nitrous oxide are emitted during the SS composting process. From those, even though CO2 is the principal product of the biological respiration emitted during SS biodegradation, it is not accounted into the total amount of GHG in composting processes due to its biogenic origin [7]. Moreover, CH4 and N2 O present a higher global warming potential (GWP), 28 and 265 times that of CO2 , respectively [8]. Due to its importance in environmental impact assessment and the valuable information that it represents when it comes to proposing mitigation strategies and avoiding population nuisance, gaseous emission of GHG and odorant compounds has been widely reported for SS composting in pilot-scale [6,9], and some studies have been conducted in full-scale facilities [10,11]. Emission factors referring to specific pollutants are a useful tool for environmental impact assessment and for estimating the overall emission rate of a treatment process based on a specific activity index, which should represent the process evaluated [12,13]. The US Environmental Protection Agency (USEPA) defined the emission factor of a chemical compound as the relation between its concentration and the emitted air flow and then normalized with respect to one or more reference parameters such as the mass of waste to be treated, the emitting surface, or time units amongst others [14]. Analogously, in the last years this tool has been referenced for the assessment of odour impact from different treatment processes and facilities, namely the odour emission factor (OEF) [15–17]. Odour emissions can be characterized using high-performance analytical techniques for the identification and quantification of odorous compounds such as gas chromatography coupled to mass spectrometry (GC/MS). However, it is difficult to relate the information obtained from those techniques with the real sensation perceived by humans. In this sense, sensorial analytical techniques such as dynamic olfactometry are frequently used for odour impact assessment purposes. Their use helps into quantifying odours in terms of intensity and/or concentration [18]. Moreover, complementary information about the odorous emissions generated during the SS composting process can be obtain by means of the individual odour activity values (OAVi ), which can help in identifying the main odour contributors present in a complex gaseous mixture by relating the concentration of a specific odorant compound measured in a complex gas mixture with its odour detection threshold (ODT). The main objective of this work is to provide a full and global inventory of the gaseous and odorous emissions generated during the conventional sewage sludge composting process, which could be later extrapolated to higher process scale. The authors intend to enlarge the scientific knowledge about gaseous and odorous emissions generated during the composting of SS due to its importance for further development of the composting technology and the development of mitigation and control strategies. Emission factors for specific pollutants are provided, as well as a screening on the diversity of VOC families found in the gaseous emissions from both treatment technologies, which can be valuable information for further environmental assessment studies. 2. Experiments 2.1. Characteristics of the Feedstocks SS was obtained from the wastewater treatment plant (WWTP) of Navàs, Barcelona, which treats 1500 m3 ·d−1 of domestic wastewater. Diatomaceous earth (DE) was obtained from the organic fraction Atmosphere 2020, 11, 211 3 of 16 of municipal solid wastes (OFMSW) biomethanization plant of Can Barba in Terrassa, Barcelona, and pruning waste (PW), used as bulking agent, was obtained from the composting plant of Manresa, Barcelona. The mixing ratio of the three feedstocks was 1:0.25:3 (SS:DE:PW) on a volume basis (1:0.15:0.48 w/w basis), which is a typical mixing ratio used in SS composting [10]. Table 1 shows the physical–chemical characteristics of each material and mixture. Table 1. Physical–chemical properties of the feedstocks and mixture used during the sewage sludge (SS) composting process. Physical–Chemical Properties SS PW DE Composting Mixture Moisture content (%) 83.5 ± 0.1 18.0 ± 0.0 3.9 ± 0.0 59.2 ± 0.0 Organic matter content (%) 68.9 ± 0.2 88.7 ± 0.0 35.2 ± 0.1 85.9 ± 0.0 C/N ratio 9.1 31.6 1321.6 13.6 pH 6.7 8.2 6.2 – Electrical conductivity (mS·cm−1 ) 2.5 1.7 1.2 – DRI24 h (g O2 ·h−1 ·kg−1 VS) 7.3 ± 0.9 – – 6.3 ± 1.7 PW—pruning waste; DE—diatomaceous earth; DRI—dynamic respiration index. 2.2. Sewage Sludge Composting Biorreactor Operation Figure 1 shows the bioreactor with cylindrical structure and operative volume of 100 L that was used to carry out the SS composting process. The reactor was filled with 43.15 kg of the mixture with a density of 395.9 kg·m−3 . The reactor was 0.85 m high and had a diameter of 0.50 m. The internal and external walls of the reactor were made of stainless steel, and they had a thermal isolation of polyurethane foam (2 cm) to avoid the loss of temperature and to maintain adiabatic conditions. A perforated plate was fixed at the bottom of the reactor to support the material and to facilitate aeration, working as a diffusor. The composting reactor was hermetically closed. Temperature of the material was measured with a thermometer probe (PT-100, Iserntech S.A., Vilanova i la Geltrú, Spain) introduced into the composting reactor, with sensors located at the top, middle, and bottom of each one. An air compressor (Dixair DNX 2050, Worthington Creyssensac, Pinto, Spain) and gas mass-flowmeters/controllers (D-6311-DR, Bronkhorst High-Tech B. V., Ruurlo, the Netherlands) were used for continuous aeration. Aeration flow rate was controlled by the oxygen uptake rate (OUR) [19], a composting key parameter, which determines the oxygen consumption and the level of biological activity present in the solid matrix. To follow the evolution of the composting process in terms of biodegradation, the oxygen content of the outlet gases of the reactor was monitored using an O2 sensor (O2 A1, Alphasense, Great Notley, Essex, UK), after passing them through a condensation train to prevent sensors being damaged. The composting process lasted 20 days, and the material was mixed once on the 10th day. Arduino ONE was used for data acquisition, and LabView 2017® was used for data analysis and airflow control. Atmosphere 2020, 11, 211 4 of 16 Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 17 Figure 1. Schematic of the bench-scale composting reactor. Figure 1. Schematic of the bench-scale composting reactor. 2.3. Gas Sampling Methodology 2.3. Gas Sampling Methodology For the composting reactor, gas sampling was done directly from the outlet gas tube to obtain the gaseousFor samples the composting reactor,® gas into Nalophan sampling bags. was done All Nalophan ® bagsdirectly used from to storethegasoutlet gas tube samples weretoflushed obtain the gaseous two times before samples into the obtaining Nalophan ® bags.sample final gaseous All Nalophan ® bags used to store gas samples were for analysis. flushed two times To perform VOCbefore obtaining the final characterization, gaseous sample two different gaseousfor analysis. samples of 1 L each were prepared in sorbent tubes packed with two different hydrophobic sorbents (Tenax ofTA To perform VOC characterization, two different gaseous samples ® 1 Land each were prepared Carbograph™ 1TD,in sorbent International, Markes tubes packed with Inc., two Golddifferent River, CA,hydrophobic USA) by meanssorbentsof (Tenax a manual ® TAsampling and Carbograph™ pump (Markes 1TD, Markes International, Inc., Gold River, CA, USA) by means of a manual sampling International, Inc.). The gaseous samples were obtained on the 2nd and 11th day of running, coinciding pump (Markes International, with Inc.). The the thermophilic andgaseous samples the mesophilic wereofobtained phases on the 2nd the SS composting and 11th day of running, process. coinciding with the thermophilic and the mesophilic phases of the SS composting process. 2.4. Analytical Methodology 2.4. Analytical Methodology Moisture content, dry matter, organic matter content, N-Kjeldhal, C/N ratio, pH, and electrical Moisture conductivity content, were dry matter, determined organic according matter to the content, standard N-Kjeldhal, procedures [20,21]. C/N Theratio, pH, and electrical experimental measure conductivity of were determined OUR was determined from onlineaccording to the on data acquired standard procedures airflow and [20,21].[19,22]. oxygen content The experimental To evaluate measure the of OUR was final biological determined stability from online of the composted data acquired material, on airflow the dynamic and oxygen respiration content index (DRI) [19,22]. was used. To evaluate The the final determination biological of the DRI was stability done usingof the composted a dynamic material, the respirometer [23]dynamic respiration as explained in [10]. index (DRI)Methane was used. The determination analysis was performedofusing the DRI was done an Agilent 6890using N gas a chromatograph dynamic respirometer [23] as (GC) (Agilent explained in [10]. Technologies, Inc., Santa Clara, CA, USA), equipped with a photoionization flame detector (PID). Methane The column usedanalysis for thewas performed analysis using an QAgilent was a HP-PLOT 6890 N column semi-capillary gas chromatograph (30 m × 0.53 mm (GC)× (Agilent 40.0 µm, Technologies, Agilent Inc., Santa Technologies, Clara, Inc., CA, Santa USA),CA, Clara, equipped USA), with with aNphotoionization 2 as carrier gas flame detector at 2 psi (PID). pressure, and Thea column used particle post-column for the analysis trap (2 m, was a HP-PLOT no. 5181–3352,QAgilent semi-capillary columnInc., Technologies, (30 m × 0.53 Santa mm CA, Clara, × 40.0 µm, USA). Agilent The Technologies, injector temperature Inc., was 240 ◦Clara, Santa C, the CA, USA), detector with N2 as was temperature carrier 250gas◦ C at and2 psi thepressure, oven, whichandworked a post- column particle isothermally, wastrap ◦ at 60(2C.m,Theno.injection 5181–3352, Agilent volume usedTechnologies, for each sample Inc., wasSanta 500 µL Clara, CA,total and the USA). timeThe of injector temperature analysis was 4 min. was 240 °C, the detector temperature was 250 °C and the oven, which worked isothermally, Nitrous oxidewas atanalysis 60 °C. Thewasinjection performed volume usingused for each6890N an Agilent sample GCwas 500 µLTechnologies, (Agilent and the total Inc.), time of analysiswith equipped was 4anmin.electron capture detector (ECD). The column used for the N2 O analysis was a HP-PLOTNitrous oxide analysiscolumn Q semi-capillary was performed (30 m × 0.53using mman×Agilent 40.0 µm,6890N AgilentGCTechnologies, (Agilent Technologies, Inc.), with N Inc.), 2 as equipped carrier gaswith at 2 psian electron pressure,capture detector (ECD). and a post-column The column particle used trap (2 m, for the N2Agilent 5181–3352, O analysis was a HP- Technologies, PLOTThe Inc.). Q semi-capillary injector temperaturecolumnwas (30120m ×◦ C, 0.53 themm × 40.0temperature detector µm, Agilentwas 345 ◦ C, and Inc.), Technologies, withwhich the oven, N2 as carrier gas worked at 2 psi pressure, isothermally, 60 ◦ C. was at and a post-column The injectionparticle volumetrap used(2 for m, 5181–3352, each sample Agilent was 500Technologies, µL and the Inc.).time total The injector of analysis temperature was 6 min. was 120 °C, the detector temperature was 345 °C, and the oven, which worked isothermally, was at 60 °C. The injection volume used for each sample was 500 µL and the total time of analysis was 6 min. Atmosphere 2020, 11, 211 5 of 16 Total volatile organic compounds (tVOCs), ammonia, and hydrogen sulphide concentration of the outlet gases of the composting reactor were measured daily in situ with a MultiRAE Lite portable analyser (RAE Systems, San José, CA, USA), equipped with a 10.6 eV PID lamp for tVOCs measurement and two electrochemical sensors for NH3 and H2 S measurement, respectively. tVOCs detection ranged from 0 to 1000 ppmveq isobutylene with 1 ppmv increments, NH3 detection ranged from 0 to 100 ppmv with 1 ppmv increments, and H2 S detection ranged from 0 to 100 ppmv with 1 ppmv increments. During each analysis, the portable analyser was placed inside a hermetic recipient with inlet and outlet ports and the outlet gases of the reactor were passed through it until a steady value was read on the analyser. For the composting reactor, the outlet gas flowrate was the same as the inlet airflow as the reactor was hermetically closed. The gases were measured just before the water trap installed to protect the rest of the measurement devices from the gaseous flow moisture. An SM-100 portable field olfactometer provided by Scentroid (IDES Canada, Inc., Whitchurch- Stouffville, ON, Canada) was used to perform the odour concentration analysis of the different gaseous samples obtained during the experiment, as explained by [24]. Briefly, the portable field olfactometer, which is factory calibrated, works by diluting a fixed flow of clean breathable air with the odorous sample until the panellist perceives an odour change. Both the clean air and the odorous sample flows are mixed by means of a Venturi valve, and the dilution ratio is controlled by means of a manual screw and different interchangeable measurement plates, depending on the expected odour concentration. All samples were analysed in triplicate by the same panellist in a closed, well-ventilated room the same day of sampling. To perform the VOCs’ characterization, first a liquid VOC custom solution was prepared with 35 standard compounds (considered as representative VOCs in gaseous emissions from composting processes) in methanol, for calibration purposes. To analyse the obtained samples, first a UNITY-2 thermal desorber (TD) (Markes International, Inc., Gold River, CA, USA) was used to desorb the gaseous samples retained in the sorbent tubes. These sorbent tubes were heated at 290 ◦ C for 8 min while flowing high-purity He at a flow rate of 50 mL·min−1 to desorb the VOCs onto a cold trap at −10 ◦ C. Then the cold trap was heated up to 305 ◦ C at a rate of 40 ◦ C·s−1 for 5 min to desorb the VOCs and to inject them into the chromatographic column, using a 1:10 split ratio to prevent column overload. The gas was directed to the chromatographic column through a transfer line heated at 250 ◦ C to avoid condensation. Then, an Agilent 7820 GC coupled to an Agilent 5975 mass spectrometer (MS) (Agilent Technologies, Inc.) was used to analyse and characterize the different VOCs present in the gaseous samples obtained at different moments of the process. The chromatographic column used was an Agilent DB-624 capillary column (60 m × 0.25 mm × 1.4 µm, Agilent Technologies, Inc.), with an He gas flow rate of 1 mL·min−1 as the carrier gas. The temperature program for the GC was an initial isothermal stage at 50 ◦ C during 2 min, then a first temperature ramp to 170 ◦ C at a rate of 3 ◦ C·min−1 , followed by a second ramp up to 280 ◦ C at a rate of 8 ◦ C·min−1 . Finally, the MS acquired data in scan mode with m/z interval ranging from 35 to 355 amu. The VOCs were identified by mass spectra matching with the Wiley275 mass spectrum library available in the TD-GC/MS system. The gaseous samples were analysed the same day that they were obtained in order to preserve the stability of VOCs in sorbent tubes [25]. 2.5. Estimation of the Emission Factors The emission factor calculation was based on the measured concentration of a specific pollutant, the aeration flow through the reactor and, in this case, it was normalized by the total mass of dry matter of sewage sludge (DM–SS) introduced in the reactor. The first step was to obtain the daily emission rate of each pollutant (Equation (1)). ERi = Ci ·F, (1) where ERi is the emission rate of the target pollutant (mg pollutant·d−1 ); Ci is the measured concentration of the target pollutant (mg pollutant·m−3 ); F is the aeration flow through the reactor (m3 ·d−1 ). Atmosphere 2020, 11, 211 6 of 16 Atmosphere 2020, 11, x FOR PEER REVIEW 6 of 17 Then, Then, the the emission emission rates rates of of aa target target pollutant pollutant obtained obtained for for each each sampling sampling day day were were represented represented versus versus process process time, time, so so the the area area below below the the curve curve obtained obtained corresponds corresponds to to the the emitted emitted total total mass mass of of the pollutant throughout the composting process studied. Finally, the emitted total mass the pollutant throughout the composting process studied. Finally, the emitted total mass of a target of a target pollutant pollutant was was divided divided byby the the total total DM–SS DM–SS introduced introduced inin the the reactor reactor to to obtain obtain the the emission emission factor factor (Equation (Equation (2)). (2)). EFii = EF /I, ER,ii/I, = ER (2) (2) where EF EFii is is the the emission emission factor factor of of the the target −1 SS); ER is the emission rate where target pollutant pollutant (mg (mg pollutant·kg pollutant·kg−1 SS); ERii is the emission rate of the target pollutant (mg pollutant·d −1 ); I is the total DM–SS treated as specific activity index index (kg (kg of the target pollutant (mg pollutant·d ); I is the total DM–SS treated as specific activity −1 DM–SS). DM–SS). 2.6. Potential 2.6. Potential Odour Odour Contributors Contributors The individual The individual odour odouractivity activityvalue value(OAV (OAV ) can be calculated as the relation between i) ican be calculated as the relation between the the concentration concentration of a compound of a target target compound and its and its odour odour threshold, detection detection as threshold, describedasbydescribed by Equation (3) Equation (3) [26,27]. OAV is an indicator of the contribution to odour of a specific [26,27]. OAVi is an indicatori of the contribution to odour of a specific quantified compound present quantified compound in a gaseouspresent samplein a gaseousa sample containing mixturecontaining of odorantacompounds mixture of odorant compounds [28,29]. In this sense, [28,29]. In this the higher the sense,i, the OAV the more higherlikely the OAV i , the more an odorant likely ancontributes compound odorant compound contributes to the perceived to the perceived odour. odour. OAVi i == Cii/ODTii,, OAV (3) (3) where OAVi is the individual odour activity value (dimensionless); Ci is the measured concentration where OAVi is the individual odour activity value (dimensionless); Ci is the measured concentration of of the target pollutant (ppbv); ODTi is the odour detection threshold of the target compound (ppbv). the target pollutant (ppbv ); ODTi is the odour detection threshold of the target compound (ppbv ). 3. 3. Results Results and and Discussion Discussion 3.1. 3.1. Sewage Sewage Sludge Sludge Composting Composting Performance Performance The aimofofthe The aim the SSSS composting composting process process is to obtain is to obtain a biologically a biologically stable with stable product product high with high agronomic agronomic properties properties that that is mature is mature enough, to beenough, used as to be used fertilizer oras fertilizer soil or soilby amendment, amendment, by biological biological decomposition decomposition of the organic of the organic matter matter2 [30]. [30]. Figure showsFigure 2 shows the performance the performance of the composting of the composting process process during the during the 20 day run. 20 day run. 70 25 T avg (ºC) Specific airflow (L·min ·kg VS of SS) 60 Avg OUR 24 h (g O2·h-1·kg-1 VS) -1 -1 20 Avg OUR 24h (g O2·h-1·kg-1 VS) Specific airflow (L·min ·kg VS of SS) 50 Temperature (ºC) -1 15 40 -1 30 10 20 5 10 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (d) Figure 2. Figure Performance of 2. Performance of the the composting composting process. process. Black Black dots dots represent represent the the evolution evolution of of the the average average temperature of the reactor ( ◦ C); green crosses represent de average oxygen uptake rate (OUR) value temperature of the reactor (°C); green crosses represent de average oxygen uptake rate (OUR) value for 24 24 h −1 ·kg−1 VS); solid line represents the specific airflow passed through the reactor ·h−1 for h (g(g OO22·h ·kg−1 VS); solid line represents the specific airflow passed through the reactor −1 −1 ·kg−1 VSVS (L·min−1·kg (L·min ofof SS). SS). Atmosphere 2020, 11, 211 7 of 16 Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 17 In In Figure Figure 2, 2, thetheevolution evolutionofofthe theaverage average temperature temperature of of the thebottom, bottom,middle, middle,and andtoptopofofthethe solid solid matrix; the specific airflow; and the average OUR of the last 24 h are represented matrix; the specific airflow; and the average OUR of the last 24 h are represented for the whole process for the whole process timetimeof the composting of the composting treatment. treatment. As expected, As expected, during duringthe composting the composting process, two two process, temperature temperature stages canstages be differentiated, a thermophilic stage (>40 ◦ C) and a mesophilic stage (<40 ◦ C). Temperature can be differentiated, a thermophilic stage (>40 °C) and a mesophilic stage (<40 °C). increased rapidly during rapidly the firstduring 48 h to ◦ C). Thermophilic Temperature increased thethe maximum first 48 h to the value registered maximum (62.9 value registered (62.9 °C). conditions Thermophilic wereconditions reached atwere the reached 24 h of at process the 24 andh of maintained process and until the 9thuntil maintained daytheof 9th composting. day of The composting. composting The composting reactor reactorjust was turned wasonceturned on just once day the 11th on theof 11th day of theHowever, the process. process. However, despite the despitethe turning, thetemperature turning, thetended temperature tended to decrease to never and decrease and never reached reached the peak the peak again. temperature temperature again. During the composting process, the OUR was monitored to control the aeration rate of the reactor Duringthe and to verify theproper composting process, performance of the the OUR was process. biological monitored to control Specific the supplied airflow aeration to rate theofreactor the reactor and to verify the proper −1 performance −1 of the biological process. varied from 2.9 to 0.3 L·min ·kg VS of SS during the composting process, with an average value Specific airflow supplied to the reactor −1varied −1 from 2.9 to 0.3 L·min −1·kg−1 VS of SS during the composting process, with an average of 1.2 L·min ·kg VS of SS, which is typical of values used during SS composting [6,31]. As it can be value of 1.2inL·min −1·kg−1 VS of SS, which is typical of values used during SS composting [6,31]. As it observed Figure 2, the curve described by the OUR corresponds to a typical SS composting can be observed in Figure 2, the curve described by the OUR corresponds to a typical SS composting process [6,32], where the peak of maximum biological activity in terms of organic matter biodegradation process [6,32], where the peak of maximum biological activity in terms of organic matter is found during the 2nd and 3rd day of the process, coinciding with when the maximum temperature biodegradation is found during the 2nd and 3rd day of the process, coinciding with when the is reached inside the reactor. Then, the tendency of the OUR was to decrease as easily degradable maximum temperature is reached inside the reactor. Then, the tendency of the OUR was to decrease organic matter is being consumed. According to the temperature and OUR profiles and the DRI as easily degradable organic matter is being consumed. According to the temperature and OUR −1 ·kg−1 VS), which was reduced an 81% with respect to the value of the final material (1.2 ± 0.2 g O 2 ·h profiles and the DRI value of the final material (1.2 ± 0.2 g O2·h−1·kg−1 VS), which was reduced an 81% mixture’s initialtoDRI, with respect the the composting mixture’s initial was DRI,performed the composting properlywas and the material performed wasand properly stabilized. The SS the material composting process time was fixed at 20 days on the basis of following similar was stabilized. The SS composting process time was fixed at 20 days on the basis of following similar procedures that have been generallythat procedures described have been by the GICOMdescribed generally research groupby theinGICOM differentresearch SS composting group in studies different[6,10,22]. SS Moreover, composting thestudies fact that non-digested [6,10,22]. Moreover, SS the wasfactused, thatwhich is a waste non-digested SS wasthatused, presents whichhigher biological is a waste that activity, presents was a reason higher to establish biological activity, this wasduration a reasonintoorder to ensure establish a proper this duration biological in order stabilization to ensure a proper of thebiological treated mixture. stabilization of the treated mixture. 3.2.3.2. Greenhouse Gases Greenhouse GasesEmission Emission Greenhouse Greenhouse gases gases(GHGs) (GHGs)emissions emissions from from different SS SS composting compostingprocesses processeshave have been been widely widely reported in in reported thethe past years past yearsdueduetototheir theirpotential potential impact globalwarming impact to global warming[31,33,34]. [31,33,34].Figure Figure 3 shows 3 shows thethe daily daily emissionrates emission ratesofof CH CH44 and andNN2O2 obtained O obtained during duringthe composting the process, composting altogether process, with altogether the with thetemperature temperatureprofile ofof profile thethe material materialand thethe and specific airflow specific supplied airflow to the supplied to reactor at theatmoment the reactor of the moment sampling. of sampling. Figure 3. CH4 and Figure 3. CH N2 O emission rate profiles during the composting process. Black dots represent the 4 and N2O emission rate profiles during the composting process. Black dots represent the average temperature ofofthe ◦ C); white diamonds represent the specific airflow passed through average temperature thereactor reactor( (°C); white diamonds represent the specific airflow passed through the reactor (L·min ·kg VS of SS); blue squares represent the emission rate of CH4 (mg CH4 ·d−1 ); −1 −1 green triangles represents the emission rates of N2 O (mg N2 O·d−1 ). Atmosphere 2020, 11, 211 8 of 16 Two different emission dynamics can be observed for CH4 and N2 O in Figure 3. First, CH4 emission followed a similar profile as the reactor’s temperature profile, presenting its emission peak when maximum temperature and maximum aeration occurred. Differently, N2 O showed an emission peak on the first sampling day due to the stripping of some remaining N2 O present in the SS. Then, the N2 O emission rapidly decreased to low values, possibly due to the inhibition of some mechanisms of N2 O generation by the high temperature achieved by the system [35]. Later, a great increase of the N2 O emission was observed when the temperature of the material was below 40 ◦ C. Generally, CH4 is formed by methanogens under anaerobic conditions. Hence, because of the rapid degradation of organic matter during thermophilic phase, an anaerobic environment can be created by the oxygen demand at this temperature range [31]. As observed in Figure 3, the peak of methane emission coincided with that of temperature, and then CH4 emission decreased as temperature decreased into the mesophilic range. The maximum CH4 concentration measured during SS composting was 27.01 ppmv , whereas the minimum concentration obtained was 1.34 ppmv . On the other hand, the formation and emission of N2 O is complex, which can be produced by denitrification, when nitrates are converted to N2 O and nitrogen gas, and incomplete nitrification, when during the conversion of ammonia to nitrite there is some formation of N2 O [36]. As it can be observed in Figure 3, during the first sampling day, a peak of N2 O emission appeared, mainly because of the stripping effect of the remaining N2 O present in the SS used for the composting experiment. Once this first N2 O was stripped of the SS, low levels of N2 O formation and emission were observed until the temperature of the material decreased to a mesophilic range. At that point, an increase of the N2 O emission appeared until the end of each process. The results suggest that the observed N2 O formation has a direct relation with the material’s temperature at the moment of the emission [35]—below 40 ◦ C—and with the low carbon availability at the latter stages of the process. At these conditions, some denitrifying bacteria can be reactivated and promote the N2 O formation by nitrate denitrification at the end of the processes [35,37,38], which could be confirmed by further bacterial community characterization in future works [39]. Moreover, the increase in the N2 O emission coincide with low aeration of the material. This fact possibly promoted the formation of anaerobic spots around the composting material and hence, the higher formation and emission of N2 O during the last phase of the process. Peaks of N2 O emission have been reported in different composting studies [35,40,41] mainly in non-aerated composting piles, when carbon availability was low, which supports that a combination of the previously mentioned conditions (low temperature, low C availability, and low oxygenation) can be the reason for the observed N2 O increase at the end of the present SS composting process. Maximum N2 O concentration measured during SS composting was 317.59 ppmv , whereas the minimum concentration obtained was 0.28 ppmv . Table 2 shows the emission factor of each GHG for the SS composting process, expressed in kg of CH4 or N2 O per Mg DM–SS and in kg of CO2eq per Mg DM–SS. Table 2. CH4 and N2 O emission factors, expressed in kg of pollutant per Mg of dry matter of sewage sludge (DM–SS) and in kg of CO2 equivalent per Mg of dry matter of SS. CH4 emission factor (kg CH4 ·Mg−1 DM–SS) 1.37 × 10−1 N2 O emission factor (kg N2 O·Mg−1 DM–SS) 8.53 × 10−1 CH4 emission factor (kg CO2eq ·Mg−1 DM–SS) a 3.83 N2 O emission factor (kg CO2eq ·Mg−1 DM–SS) a 2.26 × 102 GHG emission factor (kg CO2eq ·Mg−1 DM–SS) a 2.30 × 102 aGreenhouse gas (GHG) emission for CH4 and N2 O, on a 100-year frame, is 28 and 265 times higher than that of CO2 , respectively [8]. To contextualize the obtained results, the GHG emission factors have been contrasted with other similar studies. For example, [31] reported a maximum GHG emission factor of 284.13 kg CO2eq ·Mg−1 Atmosphere 2020, 11, 211 9 of 16 DM–SS during the composting of dewatered non-digested sewage sludge in a 60 L aerated pilot-scale reactor. On the other hand, [6] found lower GHG emission factors referred to the SS composting in a 50 L aerated pilot-scale reactor, ranging from 4.98 to 9.90 kg CO2eq ·Mg−1 DM–SS. Another bench-scale composting study conducted with dewatered sludge with wheat straw in a 150 L reactor [34] reported GHG emission factors ranging from 36.05 to 134.56 kg CO2eq ·Mg−1 DM of mixture. Generally, a high variability exists in terms of GHG emission depending on different factors such as the characteristics of the feedstocks, the operational parameters (aeration strategy) or even the scale of the process. In the present study, the fact that N2 O emission increased sharply at the end of the SS composting process contributed to a higher GHG emission factor. A shorter SS composting process would not be the best solution to minimize N2 O emission at the end of the process because the biological stabilization of the material could be compromised. In this sense, a better aeration strategy or an increase of the mixing frequency at the latter stages of the process to avoid or at least minimize the formation of anaerobic spots around the composting material could have been beneficial to mitigate N2 O emissions and, hence, to decrease GHG emissions. 3.3. Odorant Compounds Emission 3.3.1. NH3 , H2 S, tVOCs and Odour Emission The emission of NH3 , H2 S, tVOCs, and odour from the SS composting treatment was monitored during the whole process. Table 3 presents the emission factors obtained for all these pollutants, expressed in kg of pollutant per Mg DM–SS and ou per Mg DM–SS, respectively, when monitoring the composting process. Table 3. NH3 , H2 S, tVOCs and odour emission factors, expressed in kg of pollutant per Mg of dry matter of SS and in ou per Mg of dry matter of SS, respectively. NH3 emission factor (kg NH3 ·Mg−1 DM–SS) 5.13 H2 S emission factor (kg H2 S·Mg−1 DM–SS) 1.66 × 10−1 tVOCs emission factor (kg C-VOC·Mg−1 DM–SS) 6.20 Odour emission factor (ou·Mg−1 DM–SS) 2.68 × 107 To put the obtained results into context, the emission factors obtained have been compared with other reported emission factors from different composting processes. Researchers in [6] reported NH3 and tVOCs emission factors in the range of 1.33–4.12 kg NH3 ·Mg−1 DM–SS and 0.66–1.00 kg C-VOC·Mg−1 DM–SS, respectively, during composting of raw sludge in a 50 L aerated pilot-scale reactor. González et al. [10] studied the emission of NH3 and tVOCs in a full-scale sewage sludge composting plant, reporting emission factors of 25.32 kg NH3 ·Mg−1 DM-SS and 6.17 kg C-VOC·Mg−1 DM-SS. Maximum concentrations measured during SS composting were 2600 ppmv of NH3 , 66 ppmv of H2 S, and 1650 ppmv of tVOCs, which were observed during the peak of maximum temperature of the reactor. On the other hand, few works dealing with the OEFs of SS biological treatment have been found. However, [42] evaluated the OEFs of different substrates in a 300 L aerated pilot-scale reactor, reporting an OEF of 9.35 × 108 ou·Mg−1 DM-SS for SS composting and [24] obtained an OEF of 9.45 × 107 ou·Mg−1 DM-SS in a full-scale sewage sludge composting plant. For SS composting, different ranges of target pollutants and odour concentrations can be found in literature depending on the characteristics of the feedstocks or the composting process itself [43–45]. Figure 4 shows the daily odour emission rates (OER) obtained during the SS composting process, together with the average temperature of the reactor and the specific airflow. Atmosphere 2020, Atmosphere 2020, 11, 11, 211 x FOR PEER REVIEW 10 10 of 17 of 16 Figure 4. Odour emission rate (OER) profile during the composting process. Black dots represent the Figure 4. Odour emission rate (OER) profile during the composting process. Black dots represent the average temperature of the reactor (◦ C); white diamonds represent the specific airflow passed through average temperature the reactor of−1 (L·min−1 ·kg theVS reactor of SS);(°C); cyanwhite diamonds squares represent represent the emission the odour specific airflow passed rate (ou·d−1 ). through the reactor (L·min ·kg VS of SS); cyan squares represent the odour emission rate (ou·d ). −1 −1 −1 Similarly to other works where odour emission from SS composting were studied [24,42], Similarly the profiles to other of odour works reached emission where odour emissionduring its maximum from SS thecomposting thermophilic were studied phase, when[24,42], the biological profiles of odour emission reached its maximum during the thermophilic phase, activity is higher and then tended to decrease as temperature and biological activity decreased. In fact,when biological activity the is higherofand distribution thethen odour tended to decrease emissions duringasthetemperature and biological whole SS composting activity process decreased. shows that more In fact, the distribution of the odour emissions during the whole SS composting than 64% of these emissions happened during the thermophilic phase of the process, whereas the process shows that more thanwas rest 64%divided of these between emissionsthe happened during theand pre-thermophilic thermophilic phasestages. the mesophilic of the process, whereas Temperature andthe rest hence was divided biological between activity the pre-thermophilic have been reported to be keyand the affecting factors mesophilic stages. odour Temperature emission, andduring being higher hence biological activity have been reported to be key factors affecting odour emission, being the thermophilic phase of the composting process [24,46,47], due to the fact that easily biodegradable higher during the thermophilic organic matter, as phase wellofasthe compostingsulphur-based nitrogen-and process [24,46,47], due to theare compounds, factconsumed, that easily biodegradable leading to the organic matter, as well as nitrogen-and sulphur-based compounds, formation and emission of higher quantities of odorant compounds such as ammonia, are consumed, leading to the hydrogen formation and emission of higher quantities of odorant compounds such sulphide, and a wide range of VOCs. Maximum odour concentration measured during composting as ammonia, hydrogen sulphide, was and a−3wide 3200 ou·m range . As the of VOCs. processes wentMaximum odour on, odour concentration emission decreasedmeasured during progressively composting until reaching was 3200 ou·m −3. As the processes went on, odour emission low odour concentration values between 280 and 290 ou·m . −3 decreased progressively until reaching low odour concentration values between 280 and 290 ou·m−3. 3.3.2. VOCs’ Characterization 3.3.2. VOCs’ Characterization A chemical characterization was made by comparing the VOC families found in the different A chemical samples as well ascharacterization by quantifying was made by comparing the concentration of 17 outthe VOC of the 35 families detectedfound in the different compounds. Figure 5 samples shows theasdistribution well as by of quantifying the differenttheVOCconcentration of 17inout families found theof the 35 of samples detected compounds. two different Figure days obtained 5 shows the distribution during the SS composting process.of the different VOC families found in the samples of two different days obtained during the SS composting process. Atmosphere 2020, 11, Atmosphere 2020, 11, 211 x FOR PEER REVIEW 11 of 11 of 16 17 100 Acetals Alcohols Aldehydes 80 Alkanes Alkenes % VOC category Aromatic HC 60 Carboxylic acids Esters Eters Furans 40 Halogenated HC Ketones N compounds Phenols 20 Siloxanes S compounds Terpenes 0 Day 2 Day 11 Figure 5. Distribution of the different volatile organic compound (VOC) categories found in the off Figure 5. Distribution of the different volatile organic compound (VOC) categories found in the off gases of the composting reactor, expressed in relative abundance with respect to the whole sample. gases of the composting reactor, expressed in relative abundance with respect to the whole sample. As it can be observed in Figure 5, terpenes—predominantly α-pinene—are the main VOC family As it can be observed in Figure 5, terpenes—predominantly α-pinene—are the main VOC family found in the gaseous samples analysed, with relative abundances between 50–80%. Furthermore, other found in the gaseous samples analysed, with relative abundances between 50–80%. Furthermore, odorant species appeared in different percentages, such as sulphur compounds—dimethyl sulphide other odorant species appeared in different percentages, such as sulphur compounds—dimethyl (DMS) and dimethyl disulphide (DMDS), ketones—mainly 2-butanone and 2-pentanone, aromatic sulphide (DMS) and dimethyl disulphide (DMDS), ketones—mainly 2-butanone and 2-pentanone, hydrocarbons—with toluene and m-xylene, and some alkanes. Generally, terpenes such as α-pinene, aromatic hydrocarbons—with toluene and m-xylene, and some alkanes. Generally, terpenes such as β-pinene, or limonene may originate as intermediate products of aerobic biodegradation of organic α-pinene, β-pinene, or limonene may originate as intermediate products of aerobic biodegradation matter. Furthermore, terpenes are also related to the PW used as bulking agent for the composting of organic matter. Furthermore, terpenes are also related to the PW used as bulking agent for the process [24,48,49]. Particularly in this case, its abundance was that high due to the high SS:PW v/v composting process [24,48,49]. Particularly in this case, its abundance was that high due to the high ratio used. On the other hand, the higher abundance of sulphur compounds over other VOC families SS:PW v/v ratio used. On the other hand, the higher abundance of sulphur compounds over other in both samples is normal as those compounds are typical chemicals present in the gaseous emissions VOC families in both samples is normal as those compounds are typical chemicals present in the generated during the SS handling and treatment [29,49]. The relative abundances of the different gaseous emissions generated during the SS handling and treatment [29,49]. The relative abundances species found in the samples of the 2nd day of each process were similar, with nearly 80% of terpenes, of the different species found in the samples of the 2nd day of each process were similar, with nearly 10% of sulphur compounds, and 7% of ketones among others. However, there was a variation in the 80% of terpenes, 10% of sulphur compounds, and 7% of ketones among others. However, there was relative abundances of the different VOC families found in the samples of the 11th day; terpenes were a variation in the relative abundances of the different VOC families found in the samples of the 11th still the predominant species in the gaseous samples, but decreased to 50%, and sulphur compounds day; terpenes were still the predominant species in the gaseous samples, but decreased to 50%, and were maintained about 9%, while the abundance of other species such as aromatic hydrocarbons and sulphur compounds were maintained about 9%, while the abundance of other species such as alkenes increased. aromatic hydrocarbons and alkenes increased. There exists a clear relation on how the concentration of different individual VOCs can contribute There exists a clear relation on how the concentration of different individual VOCs can to the odour concentration of a complex gas mixture. In this sense, the individual odour activity value contribute to the odour concentration of a complex gas mixture. In this sense, the individual odour (OAVi ) of the VOCs quantified in the gaseous samples obtained from the sewage sludge composting activity value (OAVi) of the VOCs quantified in the gaseous samples obtained from the sewage process was determined, as it can be a useful tool to indicate how an individual compound contributes sludge composting process was determined, as it can be a useful tool to indicate how an individual to odour in a gaseous mixture of odorant compounds [28]. The calculated OAVi are presented in compound contributes to odour in a gaseous mixture of odorant compounds [28]. The calculated Table 4 altogether with the concentration and the odour detection threshold of each specific VOC. OAVi are presented in Table 4 altogether with the concentration and the odour detection threshold of each specific VOC. Atmosphere 2020, 11, 211 12 of 16 Table 4. VOCs quantified in the gaseous emissions generated during the sewage sludge composting process (concentration expressed in ppbv ), with its odour detection threshold (ODT, ppbv ) and the Atmosphere 2020, 11, calculated OAVx FOR PEER REVIEW 12 of 17 i (“1 determined for the gaseous6.samples Figure Odour obtained duringand concentration the individual thermophilic phase odour (day 2)value activity and the mesophilic (OAV phase (dayfor i) >1 determined 11)the of the conventional gaseous samples sewage obtainedsludge composting during process.phase (day 2) and the mesophilic phase (day 11) the thermophilic of the conventional sewage sludge composting process. As it can be observed in Figure 6, it is clear that DMDS is the major contributor to odour found in the Asgaseous it can beemissions observed generated in Figure 6,during thethat it is clear thermophilic phase DMDS is the (day major 2) of the to contributor sewage odoursludge found in the gaseous emissions generated during the thermophilic phase (day 2) of the sewage sludge composting process, which coincides with the maximum odour concentration measured during the whole process. Moreover, other compounds such as eucalyptol, α-pinene, or m-xylene also contributed to a large extent to odour emission generated during the thermophilic phase. On the other hand, during the mesophilic phase (day 11), odour emission decreased to very low levels Atmosphere 2020, 11, 211 13 of 16 composting process, which coincides with the maximum odour concentration measured during the whole process. Moreover, other compounds such as eucalyptol, α-pinene, or m-xylene also contributed to a large extent to odour emission generated during the thermophilic phase. On the other hand, during the mesophilic phase (day 11), odour emission decreased to very low levels around 600 ou·m−3 , and the VOCs’ concentration and, hence, the OAVi decreased as well. Nevertheless, sulphur compounds such as DMS and DMDS were quantified and determined as the major odour contributor found during the mesophilic phase. Even though in the present work there was not enough information to deal with that, some authors have attempted to correlate odour concentration with the numerical addition of the OAV (OAVSUM ) of all the individual compounds quantified in the emissions, seeking a relation between the concentration of odorants and odour concentration, obtaining uneven results. For example, [26] found a good linear correlation between the OAVSUM and odour concentration measured during composting of pig slaughterhouse sludge at lab-scale (R2 = 0.87). However, [24,52,53] were unable to strongly correlate the odour concentration with OAVSUM in a full-scale sewage sludge (R2 = 0.60), in a swine farm (0.30 < R2 < 0.52) and in a sanitary landfill (R2 = 0.39). These results point out that OAVSUM may not be a robust approach or a good odour concentration predictor when dealing with complex odorous mixtures because many different aspects can interfere, such as the type or characteristics of the waste to be treated, the existence of synergistic and masking effects between odorants found in the gaseous mixtures, the limitations that can be encountered when quantifying different odorants (lack of quantification of different odorant species), or the over- or under-estimation of odour concentration [54]. 4. Conclusions Results indicated a proper performance of the SS composting process, obtaining a biologically stable material. The GHGs emission (in terms of kg of CO2eq ) during the SS composting process were mainly dependant on the emission of N2 O due to its higher global warming potential. N2 O emission was found to greatly increase at the end of the process, coinciding with the decay of the material to a mesophilic temperature stage, together with the low aeration provided. On the other hand, odour emissions followed the temperature dynamics as expected, presenting their maximum during the thermophilic phase of the composting process. Regarding this, DMDS, eucalyptol, and α-pinene were found to be the major odour contributors present in the gaseous emissions. In conclusion, this study encloses a full inventory of the gaseous and odorous emission related to the SS composting process, which can help in the development and further implementation of mitigation and control strategies of the mentioned emissions. Author Contributions: Conceptualization, D.G. (David Gabriel), A.S., J.C., N.G., and D.G. (Daniel González); methodology, D.G. (David Gabriel), A.S., J.C., N.G., and D.G. (Daniel González); sampling and analysis, N.G. and D.G. (Daniel González); writing—original draft preparation, D.G. (Daniel González); writing—review and editing, D.G. (Daniel González), N.G., D.G. (David Gabriel), A.S., J.C., and S.P.; supervision, D.G. (David Gabriel), A.S., J.C., and S.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. 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