atmosphere Review Odors Emitted from Biological Waste and Wastewater Treatment Plants: A Mini-Review Daniel González , David Gabriel and Antoni Sánchez * Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain; daniel.gonzalez.ale@uab.cat (D.G.); david.gabriel@uab.cat (D.G.) * Correspondence: antoni.sanchez@uab.cat Abstract: In recent decades, a new generation of waste treatment plants based on biological treatments (mainly anaerobic digestion and/or composting) has arisen all over the world. These plants have been progressively substituted for incineration facilities and landfills. Although these plants have evident benefits in terms of their environmental impact and higher recovery of material and energy, the release into atmosphere of malodorous compounds and its mitigation is one of the main challenges that these plants face. In this review, the methodology to determine odors, the main causes of having undesirable gaseous emissions, and the characterization of odors are reviewed. Finally, another important topic of odor abatement technologies is treated, especially those related to biological low- impact processes. In conclusion, odor control is the main challenge for a sustainable implementation of modern waste treatment plants. Keywords: odor; organic waste; composting; anaerobic digestion; biofiltration 1. Introduction Anaerobic digestion and/or composting are currently replacing incineration and Citation: González, D.; Gabriel, D.; Sánchez, A. Odors Emitted from landfills in the framework of waste management in modern societies. These technolo- Biological Waste and Wastewater gies present the main advantage of energy and material recovery in a circular economy Treatment Plants: A Mini-Review. context [1,2]. Atmosphere 2022, 13, 798. https:// Being biological technologies with clear advantages in terms of sustainability, as both doi.org/10.3390/atmos13050798 treatments are focused on the organic fraction of wastes (e.g., municipal solid waste, sewage sludge, farm manures, and agricultural waste), it is evident that they imply the generation Academic Editor: Jo-Chun Kim of volatile compounds that are emitted to atmosphere. Among the main gaseous emissions Received: 25 April 2022 related to the biological treatment of organic wastes, different compounds with different Accepted: 13 May 2022 effects can be found: ammonia [3,4], greenhouse gases (methane and nitrous oxide) [5,6], Published: 13 May 2022 and a heterogeneous group of Volatile Organic Compounds (VOCs) [7,8]. This last case comprises a wide number of families such as alcohols, ketones, esters, Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in organic acids, aldehydes, sulfurs or volatile sulfur compounds (VSCs), aromatics, terpenes, published maps and institutional affil- hydrocarbons, and N-compounds, among others [9]. Many VOCs have an odor. In certain iations. circumstances, emissions of VOCs can give rise to localized odor nuisance problems, even if they are not hazardous to persons. The strength of a given compound’s odor may be expressed by its odor threshold, that is, the concentration at which half the population could not detect an odor [9]. However, it is difficult to predict the odor threshold of a mixture of Copyright: © 2022 by the authors. VOCs and other odorant compounds such as ammonia or hydrogen sulfide, since there are Licensee MDPI, Basel, Switzerland. often complex and nonlinear synergistic effects that can alter both the strength and quality This article is an open access article of the perceived odor. In such cases, the odor threshold of the emitted mixture must be distributed under the terms and determined by practical measurements, usually dynamic olfactometry, that is, with the conditions of the Creative Commons involvement of odor panelists [10]. Attribution (CC BY) license (https:// In this review, the critical aspects regarding the emissions of odors from biological creativecommons.org/licenses/by/ waste treatment are analyzed and discussed. As odors are produced by a mixture of 4.0/). Atmosphere 2022, 13, 798. https://doi.org/10.3390/atmos13050798 https://www.mdpi.com/journal/atmosphere Atmosphere 2022, 13, x FOR PEER REVIEW 2 of 17 Atmosphere 2022, 13, 798 2 of 16 In this review, the critical aspects regarding the emissions of odors from biological waste treatment are analyzed and discussed. As odors are produced by a mixture of chem- ical compounds, the odor composition will be also commented. Finally, technologies for the chemical abatement compounds, thebeodor of odors will alsocomposition will be also discussed, focusing commented. on biological Finally, technologies treatments. for the abatement of odors will be also discussed, focusing on biological treatments. 2. How to Measure Odors 2. How to Measure Odors As mentioned above, odors are the result of the human perception of one or more As mentioned above, odors are the result of the human perception of one or more chemical compounds. Waste treatment plants, when based on biological treatments, have chemical compounds. Waste treatment plants, when based on biological treatments, have several emission sources. In the case of composting, when the process takes place in wind- several emission sources. In the case of composting, when the process takes place in rows (static, aerated, and/or turned), these are the most obvious ones. If a biofilter is used to windrows (static, aerated, and/or turned), these are the most obvious ones. If a biofilter is minimize these emissions, this can be another evident emission source [11]. However, the used to minimize these emissions, this can be another evident emission source [11]. How- emissions of VOCs and other odorants during some pre- (waste conditioning) and post- ever, the emissions of VOCs and other odorants during some pre- (waste conditioning) and treatment operations can be significant [12]. In the case of anaerobic digestion, since only post-treatment operations can be significant [12]. In the case of anaerobic digestion, since closed reactors are possible, odor emissions come from complementary operations rather only closed reactors are possible, odor emissions come from complementary operations than the anaerobic digestion process itself: waste storage, mechanical pretreatment, fermen- rather than the anaerobic digestion process itself: waste storage, mechanical pretreatment, tation preparation, or digestate dewatering [13]. It is evident that odors must be investigated fermentation preparation, or digestate dewatering [13]. It is evident that odors must be according toaccording investigated the emission source. to the emission source. 2.1.Odors 2.1. OdorsSampling Sampling Obtaining representative Obtaining representative gaseous gaseous samples samples from from emitting emitting area area sources sources is is not not simple. simple. Thefirst The first thing thing toto take takeinto intoaccount accountininodor odorsampling sampling is tois avoid to avoid anyanydisturbance disturbance that that sam- pling cancan sampling cause on on cause thethe characteristics characteristics andand conditions conditions of the source. of the In order source. In orderto make to make the results comparable, investigations using different the results comparable, investigations using different sampling methodssampling methods depending on the on the types of sources of odor should be conducted to observe possible types of sources odor should be conducted to observe possible differences [14]. In the differences [14]. In the case of point case sources, of point suchsuch sources, as gas ascollectors, this is gas collectors, notisanot this common a common problem. However, problem. wastewaste However, treat- ment plants treatment often plants havehave often several areaarea several sources. In this sources. In case, it is very this case, it is difficult to cover very difficult the entire to cover the emission entire area during emission sampling. area during In consequence, sampling. In consequence, representative representative sampling sites sites sampling have have to be established, to although be established, no regulations although exist. exist. no regulations Accordingly, several Accordingly, several approaches approachescan canbebefound found in in thetheliterature. For example, literature. For example,in biofil- in biofilters, a relatively ters, a relatively commoncommon strategy strategy is to cover is to cover the entire the entire surface surface to reduce to reduce it to a it to a source point point source [15], although, [15], although, in thisincase, this disturbances case, disturbancesof theofgas theflow gas flow mustmust be carefully be carefully observed observed and and avoided. avoided. OtherOther authors authors havehave usedused a wind a wind tunnel tunnel device device [16,17] [16,17] to obtain to obtain representative representative air air samples samples for for odorodor analysis analysis at different at different pointspoints of plants of plants treatingtreating municipalmunicipal solid (MSW) solid waste waste (MSW) or anaerobic or anaerobic treatment treatment ponds. In ponds. In these these cases, thecases, the airwere air samples samples werebycollected collected sucking by the sucking the air with a depression pump inside inert bags for air with a depression pump inside inert bags for further analysis. Figure 1 shows one further analysis. Figure 1 of the shows one of the most typical devices called most typical devices called a dynamic flux chamber. a dynamic flux chamber. Figure1.1.Dynamic Figure Dynamicflux fluxchamber chamberfor forthe thecollection collectionof ofodors odorsin inaabiofilter biofiltersurface surface(Source: (Source:the theauthors). authors). Atmosphere 2022, 13, 798 3 of 16 Another possibility is to measure the velocity of air emitted using high precision anemometers [16]. Finally, Cadena et al. [18] proposed a methodology to obtain representa- tive gaseous emission samples from surfaces in waste treatment plants. The methodology consisted of determining the air velocity and the concentration of selected compounds in a matrix of points on the emitting surface. Air samples were collected in inert bags for odor analysis. The product of odor concentration and air velocity results in odor mass flow released per area unit. Regardless of the strategy used, it is evident that having a wide number of measurements (at different days, seasons, weather conditions, etc.) will result in more reproducible and realistic odor data. 2.2. Odor Measurement Although several approaches have been used to measure odor, in recent years, the European standard EN 13725:2003 has been extensively used in the European Union and other countries such as Chile, Australia, and Colombia. Today, this standard (that has been recently updated) “BS EN13725:2022—Stationary source emissions. Determination of odour concentration by dynamic olfactometry” is the most used methodology in the world to measure odor concentration [19]. The odor concentration is defined as the number of European odor units in a cubic meter of gas at standard conditions (temperature of 273.15 K and pressure of 105 Pa). Therefore, the odor concentration is measured in “European Odour Units per cubic meter” and its symbol is “ouE /m3 ”. The odor concentration is calculated from the number of times that an odorous gas has to be diluted in order to reach the odor threshold of a group of people or panel specially trained and selected. Although widely accepted as standard, not all the studies related to waste treatment use it. Other alternative measures have been used, such as the number of times that the concentration of an odorant exceeds the threshold values, normally together with the hedonic tone. These approaches are less informative than the odor units, since it is difficult to measure and interpret synergic odor effects, typically found in complex odor mixtures [20]. On the other hand, there have been recent experiences with portable field olfactome- ters for assessing the odor nuisance of odor sources, such as wastewater treatment plants (WWTPs) or municipal solid waste treatment facilities (MSWTFs), and validating the odor dispersion modelling results. From these, the equipment most used are the Nasal Ranger® [21,22] and the Scentroid SM100 portable olfactometer [23,24], which work by gradually diluting an odorous gas with clean odorless air in known ratios until the per- ception of an odor change to obtain the odor concentration. These portable devices have been described as robust and reliable tools to obtain valuable information on source odor concentration, being a cost-effective alternative for odor investigations. 2.3. Electronic Noses An electronic nose offers potential as a portable, rapid, cost-effective, and noninva- sive field diagnostic technique for initial screening to detect odor problems. Essentially, electronic noses consist of an array of nonspecific chemical sensors, which interact with the different VOCs in the sample. The signal is analyzed with pattern recognition and multivariate statistical techniques to differentiate and classify samples according to their volatile composition. Although not completely accepted as a substitute for dynamic olfactometry, several works have highlighted the use of electronic noses for the monitoring of odors in waste treatment plants. In fact, the combination of the two approaches is claimed to have advan- tages for the measurable and objective evaluation of the odor nuisance [25]. Specifically, Jonca et al. (2022) present a recent review on the use of electronic noses in waste treatment plants, focusing on the development of these apparatuses for the constant monitoring of waste treatment processes (composting, anaerobic digestion, and biofiltration, among others) [26]. The conclusion is that electronic noses are promising tools for the control of the waste treatment process and odor impact assessment; however, “current trends clearly Atmosphere 2022, 13, 798 4 of 16 stand that maximum information on the odorous samples investigations is achieved when olfactometry, gas chromatography coupled to mass spectrometry (GC-MS) and e-noses are used as complementary approaches to a given problem” [22]. Other authors have also demonstrated the usefulness of electronic noses in several waste treatment plants, such as composting [27], anaerobic digestion [28], biofiltration [29], landfill [30], complete plants [31], and even biodiesel production facilities [32]. 3. Odor-Producing Chemicals Although odors are often produced by a mixture of different compounds such as VOCs, ammonia, or hydrogen sulfide, there exists a large amount of data about the threshold odor values of several odorants found in waste treatment plants. For instance, the Scottish Environment Protection Agency (2010) published an excellent “Odour Guidance” with detailed information about the characteristic of some odorant compounds, including their threshold value, the hedonic tone, and their possible effects on health depending on their concentration [33]. From this and other sources of information about these compounds, a set of characteristics can be discussed: 1. Some of the threshold values are extremely low; that is, the human nose is a powerful analytical device. 2. Not all the compounds are toxic or, if they are, the concentrations must be very high. 3. A common lack of these tables is the absence of a critical property: biodegradability, which is crucial when a biological treatment for the abatement of these compounds is to be applied. Table 1 presents a summary of the main compounds found in biological treatment plants and their main characteristics. Table 1. Summary of the main odorants emitted from waste treatment plants. Compound Detection Threshold (ppmV ) * Type of Odor Ammonia 0.039 Stringent Hydrogen sulfide 0.00047 Rotten eggs Methyl mercaptan 0.0011 Rotten cabbage Ethylamine 0.026 Fishy, bitter Dimethyl amine 0.047 Fishy, pungent Acetaldehyde 0.004 Fruity Ethyl mercaptan 0.002 Rotten cabbage Dimethyl sulfide 0.001 Rotten vegetables Dimethyl disulfide 0.001 Garlic-like Diethyl sulfide 0.0008 Garlic-like Butyl mercaptan 0.0005 Garlic-like Acetic acid 0.008 Vinegar, acidic Propionic acid 0.0057 Rancid, acidic α-pinene 0.011 Herbal Limonene 0.038 Orange Butyric acid 0.00019 Sweat Skatole (3-methylindole) 0.012 Feces * Values can differ considerably from different sources. Data from [34,35]. Created by the authors. As observed, some of the most unpleasant odorants can be emitted from waste treat- ment plants. This is the reason for typical odor nuisances reported near these facilities. In any case, it is worthwhile to note two important things: (i) most of these products are reported as nontoxic (except hydrogen sulfide and ammonia) even at high concentrations, and (ii) the synergic effect of the mixture of these compounds can significantly alter the threshold value. Atmosphere 2022, 13, 798 5 of 16 4. Odors from Waste Treatment Plants 4.1. Composting Composting and its related operations are probably the most reported regarding gaseous emissions and, particularly, odors. However, there are more onerous processes in terms of odors, such as anaerobic processes, landfills, sewage collection units, or animal waste processing plants, especially fish waste. It is important to know that the composting process has well-defined treatment stages: pretreatment, thermophilic phase, maturation, and post-treatment. In this case, it is obvious that these stages will produce different types of odors. However, it must be taken into account that these stages are not clearly differentiated in full-scale composting plants and sometimes overlap. In such cases, the most efficient monitoring of odors should be their evolution in time [3,7,18]. At the same time, this technology is being applied to a wide variety of organic waste [36] and even for soil remediation purposes [37,38]. Therefore, the odors will be also different. Table 2 exemplifies the characteristics of relevant organic odorants emitted from composting units. The main odorants are those found with the highest concentrations and significantly above their odor detection threshold. Table 2. Relevant organic odorants and families reported in composting processes of MSW or sewage sludge. Main Odorant Main Odorants Waste Type Location Ref. Families (Concentrations, ppmv ) aromatic HC 2-butanone (1.46–4.90) aliphatic HC MSW Composting Toluene (0.19–1.04) [39] ketones Limonene (0.22–1.01) terpenes ketones n-butanol (3.47) Compost MSW terpens Methyl ethyl ketone (0.8) [40] maturation alcohols Limonene (1.93) Decane (0.47–0.60) aromatic HC, aliphatic Toluene (0.38–0.72) Indoor air MSW HC, terpenes Limonene (1.87–3.11) [12] composting hall S compounds H2 S (0.8–0.9) DMS (0.11–0.30) Isovaleraldehyde (0.04) aldehydes 2-butanone (0.46–0.52) Dynamic ketones Sewage sludge DMS (0.55–1.15) [41] windrows S compounds DMDS (0.40–1.39) carboxylic acids Butyric acid (0.09–15) Pinenes (α and β) (0.1–2.8) S-compounds Limonene (0.01–10) Sewage sludge Composting terpens DMS (0.08–1.2) [7] esters DMDS (0.3–18) Ethyl isovalerate (0.1–2.8) DMS (NA) S compounds DMDS (NA) Sewage sludge Composting [42] terpenes Limonene (NA) α-pinene (NA) NA—not available. 4.1.1. Food Waste Food waste or, in general, the organic fraction of municipal solid waste (OFMSW) is mainly transformed into composting. Composting can be performed in windrows or in- vessel systems, which can obviously affect the odor emission. Most of the papers published present data on the concentration of typical odorants (especially ammonia), their effect on health, and some strategies of mitigation. However, the chemicals related to odors during the composting of food waste are a wide variety of chemical compounds: ammonia, amines, acetic acid, and multiple volatile organic compounds (hydrocarbons, ketones, esters, ter- Atmosphere 2022, 13, 798 6 of 16 penes, and S-compounds) [43]. For instance, the co-composting process of kitchen waste and garden waste are reported as a good strategy to decrease the emissions of ammonia and hydrogen sulfide, as well as enhancing compost maturity [44]. In fact, several studies point to a correlation between waste stability and odor [10,45], which is an interesting aspect to consider in the biological stages of an organic waste treatment. In fact, Gutiérrez et al. (2017) presented a successful correlation between Oxygen Uptake Rate (OUR) and odor emissions (in this case, measured as odor units) when using different substrates, including the OFMSW and other substrates [46]. Another topic that is presented in composting is the use of additives to minimize the odor emissions. For instance, Yuan et al. (2015) used a specific bulking agent, dry cornstalks, which were pretreated with ferric chloride (FeCl3 ) during the composting of kitchen waste. The authors conclude that this strategy resulted in a significant decrease in the amounts of ammonia and hydrogen sulfide emitted [47]. Other works have been published in the field of biodrying, a technology based on the same principles as composting [48], with the same conclusion about the use of additives [49]. Regarding odor units, other papers have been published. Colón et al. (2017) presented a complete study on indoor air in modern complex mechanical-biological treatment plants treating mixed municipal solid waste and source-selected OFMSW by composting and anaerobic digestion, observing different patterns during one complete working day, with terpenoids, aromatic and aliphatic hydrocarbons being the VOCs more often detected in odors [7]. This fact has an evident importance on the ventilation required in facilities working with the OFMSW. Other works present extensive studies on the impact of implementing deodorization systems in complex waste treatment plants including composting but a large number of other operations, apart from the biological process itself [43]. In fact, most of the papers related to odor emissions in composting are focused on deodorization, a point that will be treated later in this review. 4.1.2. Sewage Sludge Although less studied, composting is also a technology for the treatment and stabi- lization of sewage sludge. However, in the last decades, and due to the proliferation of anaerobic digestion of wastewater sludge, composting has been increasingly limited to the stabilization of digested sludge. In both cases (sewage sludge and anaerobically digested sludge), benefits from the composting include stabilization, lower phytotoxicity and, what is very important, the reduction in unpleasant odors in its application to soil as organic fertilizer [50]. However, the studies of odor emissions during sewage or digested sludge composting are relatively scarce. In fact, some of the works published are related to the emission of certain compounds, especially hydrogen sulfide, which is produced when anaerobic zones appear in the compost matrix, although ammonia is also often studied. In general, S-compounds are discussed as the most problematic in terms of odors during the composting of sewage sludge [51,52]. Again, in the case of sewage sludge, the correlation between stabilization and the decrease in odor emissions is also observed, especially in the case of ammonia and hydrogen sulfide, the most commonly reported malodors chemicals [53]. Finally, it is important to note that the studies including odor units during the com- posting of sewage sludge are scarce. Particularly, Gonzalez et al. (2019) reported a complete characterization of the VOCs and odor units emitted from a sewage sludge full-scale composting plant. Odor emission factors (OEF) were determined during the progressive stabilization of sewage sludge due to the composting process, resulting in a range of 106 odor units per kg of composted dry mass [7]. The authors also systematically studied the main VOCs detected in odors, which were isovaleraldehyde, indole, skatole, butyric acid, dimethyl sulfide, and dimethyl disulfide. In another study by the same authors, the odor emission factors, when composting sewage sludge at a pilot scale, were in the range of 107 odor units per kg of composted dry mass, whereas the major odor contributors identi- fied were dimethyl disulfide, eucalyptol, and α-pinene [54]. Atmosphere 2022, 13, 798 7 of 16 4.1.3. Manure The term “manure” is somewhat confusing, as it includes different materials in terms of moisture, chemical composition, stability, etc. Therefore, the reader must be careful when consulting studies in this framework. This can be applied to odor units (where the studies are scarce) and, in general, gaseous emissions to the atmosphere. At the same time, this type of waste is often processed some time after its production, which implies a storage period, which again is critical for the emissions during composting. In this regard, Blazy et al. (2015) correlated the chemical composition and the odor concentration of emissions produced during the storage and composting of pig slaughterhouse sludge [55]. The results were positive, as a satisfying correlation between chemical composition and odor concentration was obtained. A relevant aspect from this study is that only three compounds among the 66 identified (trimethylamine, hydrogen sulfide, and methanethiol), accounted for the prediction of odor concentration measured during the composting and storage of this waste. In other studies, manure has been co-composted with other complementary wastes, given the evident lack of porosity and excess of nitrogen and waste found in most of these materials. For example, Toledo et al. (2020) presented a study composting a complex mixture of chicken manure, alperujo (olive pomace paste), olive leaves/pruning, and cereal straw at full-scale, where low odor emissions were reported because of the use of lignocel- lulosic substrates jointly with manure [56]. In a similar work, Zang et al. (2016) reported the effects of the mix ratio, moisture content, and aeration rate on sulfur odor emissions during pig manure composting [57]. Other particular points that are very commonly treated are the study of emissions of anaerobically digested manures [58], where the emissions of some malodourous compounds can be enhanced, and the study of inoculants to decrease the amount of odor units of these materials, although sometimes no quantitative data are provided [59]. 4.2. Anaerobic Digestion Anaerobic digestion is an alternative technology based on biological processes when the main objective is to transform organic waste into biogas, whose high percentage of methane (40–60%) makes it a highly attractive renewable energy source [1]. Nevertheless, it is important to note that anaerobic digestion and composting are complementary tech- nologies that, properly combined, can result in higher material and energy recovery than that of both technologies separately considered [60]. Another important question is that of anaerobic digestion, which, as a net energy producing process, is often considered in global warming studies, and, in consequence, only greenhouse gases are monitored [61]. Regarding odors, as anaerobic digestion is a strict anaerobic process, it is evident that it takes place in enclosed reactors, except in the case of landfills, which are out of the scope of this paper. This is the reason why odors are practically restricted to the complementary operations rather than the process itself: organic waste preconditioning, digested material dewatering, etc. For example, Wisniewska et al. (2020) monitored odors and malodorous compounds in different parts of an anaerobic digestion plant treating food waste. The authors concluded that the zone of waste collection was the main emissions source of the plant [62]. In another similar study [63], waste storage, fermentation preparation, and digestate dewatering were the main emission odor sources in a Polish MSW biogas treatment plant, with ranges of 4 to 78 ou/m3 for fermentation preparation and from 8 to 448 ou/m3 for digestate dewatering. Another important issue is the odor associated with the material during the anaerobic digestion. In fact, the odors emitted when applying digested materials to soil as organic amendments are one of the main concerns related to the use of these materials. In this case, similar to that observed in the composting field, several studies have pointed that odors are strongly correlated to the stability of the material [45,64]. Thus, materials that are partially digested tend to produce more unpleasant odors than those with a low level of biodegradability [65]. Another important issue is the odor associated with the material during the anaero- bic digestion. In fact, the odors emitted when applying digested materials to soil as or- ganic amendments are one of the main concerns related to the use of these materials. In this case, similar to that observed in the composting field, several studies have pointed that odors are strongly correlated to the stability of the material [45,64]. Thus, materials Atmosphere 2022, 13, 798 that are partially digested tend to produce more unpleasant odors than those with8aoflow 16 level of biodegradability [65]. 5.5.Odors Odorsfrom fromWastewater WastewaterTreatment TreatmentPlants Plants Although Althoughlesslessstudied, studied,wastewater wastewatertreatment treatmentplants plants(WWTP) (WWTP)are areobviously obviouslyemitting emitting sources sourcesof ofodors. Although the odors. Although the odorant odorant compounds compoundscan canbe besimilar similartotothose thoseobserved observedin inwaste waste treatment treatment plants, plants, it isitevident is evident that that the emissions the emissions sources sources are different, are different, as the as the config- configuration uration of theofplant the is plant alsoisdifferent. also different. In open-to-air In open-to-air WWTP,WWTP, pretreatment pretreatment units, units, primary primary settlers,settlers, biological biological aeratedaerated reactors, reactors, or the sludge or the sludge treatment treatment (dewatering, (dewatering, anaerobic anaerobic diges- digestion, etc.) can be effective emissions sources [66]. Accordingly, typical odor collection tion, etc.) can be effective emissions sources [66]. Accordingly, typical odor collection de- devices mustbebeadapted vices must adapted(Figure (Figure2).2). Figure2.2. Dynamic Figure Dynamic flux flux chamber chamber for for the thecollection collectionofofodors odorsinina aWWTP WWTP(pretreatment). Source: (pretreatment). the Source: authors. the authors. Regardingodor Regarding odoremissions emissionsgenerated generatedininWWTPs, WWTPs,scientific scientificstudies studiesgenerally generallyfocus focuson on the emission of specific odorant compounds, such as hydrogen sulfide (due to its the emission of specific odorant compounds, such as hydrogen sulfide (due to its odorant odorant asaswell wellasas its its corrosive corrosive nature), nature), ammonia, ammonia, and volatile and volatile organic organic compounds; compounds; however,however, works works targeting targeting actual odoractual odor concentration, concentration, odor emission odor emission rates, rates, and andemission odor odor emission factorsfactors from full-scale WWTPs are scarce. Capelli et al. (2009) integrated odor emission data obtained from 17 different Italian WWTPs differing in constructional and processing features, treating mostly urban wastewaters, and found odor concentrations ranging from 845 to 3840 ou/m3 depending on the emission source considered. From these data, the authors were able to demonstrate that the main odor sources of a WWTP were found among the pretreatment units, specifically at the primary sedimentation step, and that, generally, the odor emission tended to decrease along the depuration cycle [67]. This behavior has also been observed by other authors, who also correlated these higher odor emissions at pretreatment units with an increased presence of H2 S, NH3 , and different VOC families with a low odor detection threshold such as VSCs or ketones [66,68,69]. Recently, De Sanctis et al. (2022) assessed the impact of implementing MULESL technology (MUch LEss SLudge; patent WO2019097463), which aims to reduce sludge generation and the discharge of contaminants of emerging concern (CEC) when treating part of the inlet wastewater from Putignano’s WWTP. It was found that, apart from reducing sludge generation and increasing CEC removal, odor emissions associated with this novel technology were reduced by a percentage of Atmosphere 2022, 13, 798 9 of 16 45% with respect to the those associated with the traditional treatment at Putignano’s WWTP [70]. As explained by the authors, the highest odor concentrations associated to the MULESL technology were found at the initial step of the process, when pumping the sewage (1714 ± 1496 ou/m3 ), and then they were gradually reduced to low values (499 ± 357 ou/m3 ) as the process went on. On the other hand, many works dealing with odor and odorant emissions from WWTPs focused on the design and development of the so-called electronic noses in order to facilitate and automate odor emission quantification and, consequently, odor emission dispersion [71–73]. Typically, the compounds monitored by this kind of system are VOCs, VSCs, and nitrogen compounds, which are the typical odor precursors found in WWTP gaseous emissions. Together with odor concentration measurements, the main aim is to be able to correlate the concentration of different groups of odorant compounds with the odor concentration to finally obtain an electronic system independent of human panels that enables odor emission assessment in WWTPs. In this sense, Burgués et al. (2021) developed an e-nose mounted on a small drone which was capable of generating real- time aerial maps of the odor emissions in WWTPs, using a set of 21 gas sensors together with four environmental parameters sensors to highly correlate the concentrations of H2 S, NH3 , mercaptans, amines, and different VOCs with odor concentrations measured by the dynamic olfactometry of field samples [71,72]. As stated before, the conclusion is that electronic noses are helpful tools for odor impact assessment; however, it is also clear that maximizing the odor information in terms of olfactometric analysis together with powerful analysis by GC-MS and electronic noses is the way to follow for a proper odor emission characterization [26]. Finally, many of the odor emission studies carried out in WWTPs have the ultimate objective of predicting how these odors will impact the surroundings and citizens nearby the facilities, and, subsequently, what actions can be implemented to mitigate these impacts. In this line, air dispersion modeling applied to odor impact assessment is a tool that has been under constant improvement for the last decades, correlating pollutant and/or odor emission data from the emission sources with meteorological data or models to predict the atmospheric dispersion of these pollutants. From the different air dispersion models available, experience has shown that the most appropriate for these purposes are the Gaussian models (e.g., AERMOD) and CALPUFF [74,75]. In this sense, it is important to use as much site-specific data as possible to obtain realistic results. For example, to improve the regulatory framework on odor emissions in Chile, Varela-Bruce and Antileo (2021) gathered experimental olfactometric data from 41 Chilean WWTPs to generate a database on odor emission factors for specific units found in WWTPs. Recently, this database served as the basis to implement a CALPUFF odor dispersion model to assess the odor impact on the surroundings of the Temuco WWTP, concluding that the creation and use of the Chilean specific database showed much more realistic results than the ones obtained using standard databases [76]. In another recent study, Zarra et al. (2021) compared different odor assessment approaches for odor nuisance characterization in an urbanized area near by a full-scale WWTP, concluding that even though air dispersion modeling is a powerful tool for odor nuisance prediction, the fact of coupling and integrating it with other approaches such as field inspections through (i) trained assessors or (ii) questionnaire-based surveys would make the assessment outcomes much more realistic [77]. 6. Odors Abatement Traditionally, odor abatement has been carried out using physical–chemical technolo- gies such as chemical absorption (scrubbing) and adsorption. The former is the most widespread technology at industrial scale, mostly favorable for medium- to highly-soluble compounds (dimensionless Henry coefficient lower than 1). Adsorption is a technology favorable for poorly soluble compounds, such as many hydrophobic VOC (dimensionless Henry coefficient above 1). In both cases, either chemicals’ consumption in scrubbing or adsorbent replacement or regeneration make such technologies much more expensive than Atmosphere 2022, 13, 798 10 of 16 biological processes in terms of operational expenditures (OPEX). Although other common gas treatment technologies such as incineration or condensation could be used, they are also not economical in practice. Incineration needs natural gas to be added due to the low concentrations of odorant flows, whereas condensation costs either by pressure increase or cooling are not justified unless valuable chemicals can be recovered. However, practitioners still need to be convinced that emerging biological processes are a real alternative for odor abatement. Biological gas treatment can be carried out with a range of configurations of bioreac- tors in practice. In all cases, the catalyst for odor stabilization is a microbial culture able to grow under the operating conditions in the bioreactor. However, mainly biofilters and biotrickling filters are used at full-scale for odor abatement. The reasons for that are essen- tially the higher construction costs of bioscrubbers compared to biofilters or biotrickling filters, as bioscrubbers are constituted by an absorption unit for pollutant capture followed by a reaction unit were biodegradation of pollutants takes place. Additionally, the odor is mainly made of very low concentrations of often poorly soluble compounds, which are not suitable to be treated in bioscrubbers where the absorption unit operates under larger L/G ratios (ca. 5–10 times larger) compared to biofilters and biotrickling filters. Thus, out of the large list of factors affecting the biofiltration performance in biofilters and biotrickling filters such as pH, temperature, or nutrient availability; the watering rate, in combination with an adequate gas contact time, is the most critical factor for a proper performance in the removal of odorants [78], as mass transport from the gas to the biofilm (biofilters) or liquid phase (biotrickling filters) is directly dependent on the thickness of the water layer over the biofilm. Selection of the most appropriate configuration is still challenging, but only a few works have systematically addressed that topic. Shammay et al. (2019) proposed a selection flowchart for the treatment of sewer network emissions [79]. Although H2 S removal is properly accomplished by biofilters, biotrickling filters, and activated carbon adsorbers, fluctuating loads and the presence of other odorants make selection complex. Often a combination of different configurations in-series is an appropriate alternative [66,80]. 6.1. Biofilters Biofilters are packed bed type bioreactors in which a complex microbiota grows as a biofilm on the surface of a packing material, which is simply continuously wetted through water humidity condensation or intermittently watered in case of evaporation. Most applications use one single or a combination of organic packing materials such as peat, compost, or wood chips [81], even though some authors have proven that the use of sole plastic materials such as polyethylene films may be also effective for inorganic odorants such as H2 S and NH3 [82]. The packing material exerts an influence on the performance of biofilters [83]. Gas contact times below 10–15 s are enough for medium-to-highly soluble odorants such as H2 S, NH3 , or alcohols, whereas gas contact times above 25–30 s are required to remove odorant VOCs. However, determining an appropriate watering rate is challenging and often depends on the optimum results in adhoc field testing. As a proper starting point for testing, watering rates in the order of 0.1–0.3 m3 ·m−2 d−1 are suggested for odor removal in biofilters. Further research is needed to establish the potential correlations of the impact of the watering frequency and rate over biofilters’ performance. Application of biofilters as an alternative for odor abatement has been extensively reviewed in the past [84]; however, novel applications and opportunities are still arising [85]. Liu et al. (2021) proposed recently a novel three-stage integrated biofilter combining an acidophilic bacterial-based section for S and N removal and fungal-based and heterotrophic bacteria-based sections for the treatment of organic odorants such as VOCs from MSW treatment facilities [86]. Stratification of the bed in different sections was demonstrated as a compact efficient alternative as previously proposed by other authors from a modeling point of view [87]. New opportunities from knowledge findings in the biofiltration field can still be explored. Yao et al. (2019) showed that methanethiol removal in a biofilter was increased when methane was present [88], which may be useful for properly designing and Atmosphere 2022, 13, 798 11 of 16 exploiting the performance in odor mitigation of biocovers of landfills [89], which share many similarities with odor-abatement biofilters from other waste treatment facilities such as MSWTF. 6.2. Biotrickling Filters Biotrickling filters (BTFs) are conceptually similar to biofilters except for the packing materials used, usually inert materials such as plastic or polyurethane foam fillings, and for the fact that a liquid phase is continuously trickled over the bed. Consequently, BTFs are more suitable for medium-to-high solubility compounds, such as H2 S, NH3 , or soluble VOCs such as alcohols and some carboxylic acids or ketones. Their main advantage over biofilters is that they can be built with a taller bed (4–6 m in height), opposite to biofilters often built with 1–1.5 m in height to avoid bed compaction that leads to channeling. Together with the shortest gas contact time needed for relatively soluble compounds (often below 5–10 s), this make BTFs a much more compact technology. In recent decades, BTFs have gained much more interest since Gabriel and Deshusses demonstrated in 2003 the outstanding performance of a polyurethane packed BTF for the removal of H2 S as the main target odorant in WWTPs at gas contact times around 2 s [90]. A range of applications have been developed since then targeting the design of more compact units able to deal with larger pollutant concentrations and gas flow rates including the treatment of odorant concentration levels of VOCs. Strategies such as intermittent trickling of water in the removal of H2 S [91], the effect of natural stratification in multilayer BTFs for simultaneous H2 S and VOCs removal [92], or the optimization of the process parameters such as the packing material configuration and liquid recirculation [93] and the usage of additives, microbial inoculation, and pretreatment techniques to lower odor emission during the process have been recently explored with successful results [94]. Despite BTFs’ benefits in terms of controllability and footprint amongst others, still, biofilters are the preferred configuration at industrial scale, as they are the less expensive configuration among biofiltration technologies [95]. This is particularly important when large gas flowrates, above 40,000–60,000 m3 ·h−1 , have to be treated. Such high flow rates require the use of parallel BTF units, thus leading to much less competitive installations as BTFs present 30–50% larger capital expenditures (CAPEX) than biofilters. However, at the low-end of the range of gas flowrates BTFs offer much more appealing benefits both technically and economically. 6.3. Odor Abatement in In-Series Configurations Because of the complexity of the emissions in many waste treatment facilities and despite recent advances, single-type configurations are usually not able to cope with the more and more restrictive limits in terms of odorant emissions. If bioprocesses are considered, the most widespread combination of technologies for odor treatment at full- scale is the use of in-series chemical scrubbers for VICs’ (volatile inorganic compounds) removal (mainly NH3 and H2 S) followed by biofilter for VOCs’ removal. Alternatives such as the use of plasma systems [96] or the combination of adsorption and biotrickling filtration [97] have been also explored with interesting results. However, several authors have shown that either converting chemical scrubbers to BTFs for VICs’ removal [90,98], combining a BTF followed by a biofilter for complex odorants abatement at WWTP [66], or even using in-series BTFs for S-compounds’ removal may be also effective for biobased-only odor abatement, while, at the same time, a much more sustainable and economical alternative [99]. 7. Conclusions From this review, it is important to highlight the following conclusions: (1) Odors are a very important measure to characterize waste and wastewater treat- ment plants, sometimes being an indicator of some problems in the performance of the plant. Atmosphere 2022, 13, 798 12 of 16 (2) Odors should be measured by dynamic olfactometry to gauge the real effect that they provoke on society. Other types of measures, such as electronic noses, are in a developmental stage, because of the complex mixtures that constitute odors from waste management plants. (3) Odors are not synonymous with toxicity. Although the main contributors of odors in waste and wastewater treatment plants are VOCs (especially nitrogen and sulfur com- pounds), the complete composition must be determined to calculate toxicity values. (4) Among the available technologies for odor abatement, biological treatments such as biofiltration and modifications are the most sustainable in terms of environmental impact and economic cost. Author Contributions: Conceptualization, A.S. and D.G. (David Gabriel); methodology, D.G. (Daniel González); formal analysis, D.G. (David Gabriel); investigation, A.S. and D.G. (David Gabriel); writing—review and editing, A.S., D.G. (David Gabriel) and D.G. (Daniel González). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations BTF Biotrickling filter CAPEX Capital expenditures CEC Contaminants of emerging concern GC-MS Gas chromatography coupled to mass spectrometry MSW Municipal solid waste MSWTF Municipal solid waste treatment facility OEF Odor emission factor OFMSW Organic fraction of municipal solid waste OPEX Operational expenditures OUR Oxygen uptake rate VOC Volatile organic compound VSC Volatile sulfur compound WWTP Wastewater treatment plant References 1. Grando, R.L.; de Souza Antune, A.M.; da Fonseca, F.V.; Sánchez, A.; Barrena, R.; Font, X. Technology overview of biogas production in anaerobic digestion plants: A European evaluation of research and development. Renew. Sustain. Energy Rev. 2017, 80, 44–53. [CrossRef] 2. Cerda, A.; Artola, A.; Font, X.; Barrena, R.; Gea, T.; Sánchez, A. Composting of food wastes: Status and challenges. Bioresour. Technol. 2018, 248, 57–67. [CrossRef] [PubMed] 3. Pagans, E.; Barrena, R.; Font, X.; Sánchez, A. Ammonia emissions from the composting of different organic wastes. Dependency on process temperature. Chemosphere 2006, 62, 1534–1542. [CrossRef] [PubMed] 4. Cáceres, R.; Malińska, K.; Marfà, O. Nitrification within composting: A review. Waste Manag. 2018, 72, 119–137. [CrossRef] 5. Ermolaev, E.; Sundberg, C.; Pell, M.; Smårs, S.; Jönsson, H. Effects of moisture on emissions of methane, nitrous oxide and carbon dioxide from food and garden waste composting. J. Clean. Prod. 2019, 240, 118165. [CrossRef] 6. Tsutsui, H.; Fujiwara, T.; Matsukawa, K.; Funamizu, N. Nitrous oxide emission mechanisms during intermittently aerated composting of cattle manure. Bioresour. Technol. 2013, 141, 205–211. [CrossRef] 7. González, D.; Colón, J.; Sánchez, A.; Gabriel, D. A systematic study on the VOCs characterization and odour emissions in a full-scale sewage sludge composting plant. J. Hazard. Mater. 2019, 373, 733–740. [CrossRef] 8. Font, X.; Artola, A.; Sánchez, A. Detection, Composition and Treatment of Volatile Organic Compounds from Waste Treatment Plants. Sensors 2011, 11, 4043–4059. [CrossRef] Atmosphere 2022, 13, 798 13 of 16 9. Environment Agency. The Categorisation of Volatile Organic Compounds. Available online: http://www.s-t-a.org/Files%20 Public%20Area/Documents/The%20Categorisation%20of%20Volatile%20Organic%20Compounds%20HMIP%20(1996).pdf (ac- cessed on 14 March 2022). 10. Szyłak-Szydłowski, M. Evaluation of Inoculated Waste Biological Stabilization Degree by Olfactometric Methods. Energies 2021, 14, 1835. [CrossRef] 11. Colón, J.; Martínez-Blanco, J.; Gabarell, X.; Rieradevall, J.; Font, X.; Artola, A.; Sánchez, A. Performance of an industrial biofilter from a composting plant in the removal of ammonia and VOCs after material replacement. J. Chem. Technol. Biotechnol. 2009, 84, 1111–1117. [CrossRef] 12. Colón, J.; Alvarez, C.; Vinot, M.; Lafuente, F.J.; Ponsá, S.; Sánchez, A.; Gabriel, D. Characterization of odorous compounds and odor load in indoor air of modern complex MBT facilities. Chem. Eng. J. 2017, 313, 1311–1319. [CrossRef] 13. Wisniewka, M.; Kulig, A.; Lelicinska-Serafin, K. Odour Emissions of Municipal Waste Biogas Plants—Impact of Technological Factors, Air Temperature and Humidity. Appl. Sci. 2020, 10, 1093. [CrossRef] 14. Bockreis, A.; Steinberg, I. Measurement of odour with focus on sampling techniques. Waste Manag. 2005, 25, 859–863. [CrossRef] [PubMed] 15. Defoer, N.; De Bo, I.; Van Langenhove, H.; Dewulf, J.; Van Elst, T. Gas chromatography-mass spectrometry as a tool for estimating odour concentrations of biofilter effluents at aerobic composting and rendering plants. J. Chromatogr. A 2002, 970, 259–273. [CrossRef] 16. Hudson, N.; McGahan, E.; Casey, K.; Lowe, S.; Galvin, G.; Jeston, P.; Dunlop, M. Odor emissions from anaerobic piggery ponds. Results of a three season, 14-month survey. Bioresour. Technol. 2001, 98, 1877–1887. [CrossRef] 17. Sironi, S.; Capelli, L.; Céntola, P.; Del Rosso, R.; Il Grande, M. Odour emission factors for the prediction of odour emissions from plants for the mechanical and biological treatment of MSW. Atmos. Environ. 2006, 40, 7632–7643. [CrossRef] 18. Cadena, E.; Colón, J.; Sánchez, A.; Font, A.; Artola, A. A methodology to develop an inventory of gaseous emissions in a composting plant. Waste Manag. 2009, 29, 2799–2807. [CrossRef] 19. EN 13725:2022; Stationary Source Emissions—Determination of Odour Concentration by Dynamic Olfactometry. UNE Nor- malización Española: Madrid, Spain, 2022. Available online: https://www.en.une.org/encuentra-tu-norma/busca-tu-norma/ proyecto/?c=P0052169 (accessed on 1 May 2022). 20. Hawko, C.; Verriele, M.; Hucher, N.; Crunaire, S.; Leger, C.; Locoge, N.; Savary, G. A review of environmental odor quantification and qualification methods: The question of objectivity in sensory analysis. Sci. Total Environ. 2021, 795, 148862. [CrossRef] 21. Barczak, R.J.; Kulig, A. Comparison of different measurement methods of odour and odorants used in the odour impact assessment of wastewater treatment plants in Poland. Water Sci. Technol. 2017, 75, 944–951. [CrossRef] 22. Wisniewska, M.; Kulig, A.; Lelicinska-Serafin, K. Olfactometric testing as a method for assessing odour nuisance of biogas plants processing municipal waste. Arch. Environ. Prot. 2020, 46, 60–68. [CrossRef] 23. Damuchali, A.M.; Guo, H. Evaluation of a field olfactometer in odour concentration measurement. Biosyst. Eng. 2019, 187, 239–246. [CrossRef] 24. Maurer, D.L.; Bragdon, A.M.; Short, B.C.; Ahn, H.; Koziel, J.A. Improving environmental odor measurements: Comparison of lab-based standard method and portable odor measurement technology. Arch. Environ. Prot. 2018, 44, 100–107. [CrossRef] 25. Littarru, P. Environmental odours assessment from waste treatment plants: Dynamic olfactometry in combination with sensorial analysers “electronic noses”. Waste Manag. 2007, 27, 302–309. [CrossRef] [PubMed] 26. Jonca, J.; Pawnuk, M.; Arsen, A.; Sówka, I. Electronic Noses and Their Applications for Sensory and Analytical Measurements in the Waste Management Plants—A Review. Sensors 2022, 22, 1510. [CrossRef] 27. D’Imporzano, G.; Crivelli, F.; Adani, F. Biological compost stability influences odor molecules production measured by electronic nose during food-waste high-rate composting. Sci. Total Environ. 2008, 402, 278–284. [CrossRef] 28. Savand-Roumi, E.; Mohtasebi, S.S.; Rafiee, S.; Ghanavati, H.; Khoshnevisan, B. Introducing new monitoring indices from the headspace of biogas digester via e-nose: A case study. Measurement 2022, 190, 110769. [CrossRef] 29. López, R.; Cabeza, I.O.; Giráldez, I.; Díaz, M.J. Biofiltration of composting gases using different municipal solid waste-pruning residue composts: Monitoring by using an electronic nose. Bioresour. Technol. 2011, 102, 7984–7993. [CrossRef] 30. Lucernoni, F.; Capelli, L.; Sironi, S. Comparison of different approaches for the estimation of odour emissions from landfill surfaces. Waste Manag. 2017, 63, 345–353. [CrossRef] 31. Giungato, P.; de Gennaro, G.; Barbieri, P.; Briguglio, S.; Amodio, M.; de Gennaro, D.; Lasigna, F. Improving recognition of odors in a waste management plant by using electronic noses with different technologies, gas chromatography–mass spectrome- try/olfactometry and dynamic olfactometry. J. Clean. Prod. 2016, 133, 1395–1402. [CrossRef] 32. Mahmodi, K.; Mostafaei, M.; Mirzaee-Ghaleh, E. Detecting the different blends of diesel and biodiesel fuels using electronic nose machine coupled ANN and RSM methods. Sustain. Energy Technol. Assess. 2022, 51, 101914. [CrossRef] 33. Scottish Environment Protection Agency. Odour Guidance. 2010. Available online: https://www.sepa.org.uk/media/59919/ sepa_odour_guidance.pdf (accessed on 15 March 2022). 34. Haug, R.T. The Practical Handbook of Compost Engineering; Lewis Publishers: Boca Raton, FL, USA, 1993. 35. Nagata, Y. Measurement of Odor Threshold by Triangular Odor Bag Method. Ministry of Environment—Government of Japan. Odor Meas. Rev. 2003, 118, s118–s127. Atmosphere 2022, 13, 798 14 of 16 36. Lin, C.; Cheruiyot, N.K.; Hoang, H.G.; Le, T.H.; Tran, H.T.; Bui, X.T. Benzophenone biodegradation and characterization of malodorous gas emissions during co-composting of food waste with sawdust and mature compost. Environ. Technol. Innov. 2021, 21, 101351. [CrossRef] 37. Tran, H.T.; Vu, C.T.; Lin, C.; Bui, X.T.; Huang, W.Y.; Vo, T.D.H.; Hoang, H.G.; Liu, W.Y. Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission. Bioresour. Technol. Rep. 2018, 4, 145–152. [CrossRef] 38. Tran, H.T.; Lin, C.; Bui, X.T.; Ngo, H.H.; Cheruiyot, N.K.; Hoang, H.G.; Vu, C.T. Aerobic composting remediation of petroleum hydrocarbon-contaminated soil. Current and future perspectives. Sci. Total Environ. 2021, 752, 142250. [CrossRef] 39. Eitzer, B.D. Emissions of volatile organic chemicals from municipal solid waste composting facilities. Environ. Sci. Technol. 1995, 29, 896–902. [CrossRef] 40. Dorado, D.A.; Husni, S.; Pascual, G.; Puigdellivol, C.; Gabriel, D. Inventory and treatment of compost maturation emissions in a municipal solid waste treatment facility. Waste Manag. 2014, 34, 344–351. [CrossRef] 41. Schiavon, M.; Martini, L.M.; Corrà, C.; Scapinello, M.; Coller, G.; Tosi, P.; Ragazzi, M. Characterisation of volatile organic compounds (VOCs) released by the composting of different waste matrices. Environ. Pollut. 2017, 231, 845–853. [CrossRef] 42. Van Durme, G.P.; McNamara, B.F.; McGinley, C.M. Bench-scale removal of odor and volatile organic compounds at a composting facility. Water Environ. Res. 1992, 64, 19–27. [CrossRef] 43. 43Mao, I.F.; Tsai, C.J.; Shen, S.H.; Lin, T.F.; Chen, W.K.; Chen, M.L. Critical components of odors in evaluating the performance of food waste composting plants. Sci. Total Environ. 2006, 370, 323–329. [CrossRef] 44. Zhang, B.; Feifei, F.; Guo, C.; Yu, M.; Zhao, M.; Song, Y.; Li, Y. Evaluation of Maturity and Odor Emissions in the Process of Combined Composting of Kitchen Waste and Garden Waste. Appl. Sci. 2021, 11, 5500. [CrossRef] 45. Scaglia, B.; Orzi, V.; Artola, A.; Font, X.; Davoli, E.; Sanchez, A.; Adani, F. Odours and volatile organic compounds emitted from municipal solid waste at different stage of decomposition and relationship with biological stability. Bioresour. Technol. 2011, 102, 4638–4645. [CrossRef] [PubMed] 46. Gutiérrez, M.C.; Siles, J.A.; Diz, J.; Chica, A.F.; Martín, M.A. Modelling of composting process of different organic waste at pilot scale: Biodegradability and odor emissions. Waste Manag. 2017, 59, 48–58. [CrossRef] [PubMed] 47. Yuan, J.; Yang, Q.; Zhang, Z.; Li, G.; Luo, W.; Zhang, D. Use of additive and pretreatment to control odors in municipal kitchen waste during aerobic composting. J. Environ. Sci. 2015, 37, 83–90. [CrossRef] [PubMed] 48. González, D.; Guerra, N.; Colón, J.; Gabriel, D.; Ponsá, S.; Sánchez, A. Filling in sewage sludge biodrying gaps: Greenhouse gases, volatile organic compounds and odour emissions. Bioresour. Technol. 2019, 291, 121857. [CrossRef] [PubMed] 49. Yuan, J.; Li, Y.; Zhang, H.; Zhang, D.; Chadwick, D.; Li, G.; Wang, G.; Chi, M.; Yang, F. Effects of adding bulking agents on the biodrying of kitchen waste and the odor emissions produced. J. Environ. Sci. 2018, 67, 344–355. [CrossRef] 50. Toledo, M.; Márquez, P.; Siles, J.A.; Chica, A.F.; Martín, M.A. Co-composting of sewage sludge and eggplant waste at full scale: Feasibility study to valorize eggplant waste and minimize the odoriferous impact of sewage sludge. J. Environ. Manag. 2019, 247, 205–213. [CrossRef] 51. Han, Z.; Qi, F.; Wang, H.; Liu, B.; Shen, X.; Song, C.; Bao, Z.; Zhao, X.; Xu, Y.; Sun, D. Emission characteristics of volatile sulfur compounds (VSCs) from a municipal sewage sludge aerobic composting plant. Waste Manag. 2018, 77, 593–602. [CrossRef] 52. Rincón, C.A.; De Guardia, A.; Couvert, A.; Soutrel, I.; Guezel, S.; Le Serrec, C. Odor generation patterns during different operational composting stages of anaerobically digested sewage sludge. Waste Manag. 2019, 95, 661–673. [CrossRef] 53. González, D.; Colón, J.; Gabriel, D.; Sánchez, A. The effect of the composting time on the gaseous emissions and the compost stability in a full-scale sewage sludge composting plant. Sci. Total Environ. 2019, 654, 311–323. [CrossRef] 54. González, D.; Guerra, N.; Colón, J.; Gabriel, D.; Ponsá, S.; Sánchez, A. Characterization of the Gaseous and Odour Emissions from the Composting of Conventional Sewage Sludge. Atmosphere 2020, 11, 211. [CrossRef] 55. Blazy, V.; de Guardia, A.; Benoist, J.C.; Daumoin, M.; Guiziou, F.; Lemasle, M.; Wolbert, D.; Barrington, S. Correlation of chemical composition and odor concentration for emissions from pig slaughterhouse sludge composting and storage. Chem. Eng. J. 2015, 276, 398–409. [CrossRef] 56. Toledo, M.; Gutiérrez, M.C.; Peña, A.; Siles, J.A.; Martína, M.A. Co-composting of chicken manure, alperujo, olive leaves/pruning and cereal straw at full-scale: Compost quality assessment and odour emission. Process Saf. Environ. Prot. 2020, 139, 362–370. [CrossRef] 57. Zang, B.; Li, S.; Michel, F., Jr.; Li, G.; Luo, Y.; Zhang, D.; Li, Y. Effects of mix ratio, moisture content and aeration rate on sulfur odor emissions during pig manure composting. Waste Manag. 2016, 56, 498–505. [CrossRef] [PubMed] 58. Xu, Z.; Li, G.; Huda, N.; Zhang, B.; Wang, M.; Luoa, W. Effects of moisture and carbon/nitrogen ratio on gaseous emissions and maturity during direct composting of cornstalks used for filtration of anaerobically digested manure centrate. Bioresour. Technol. 2020, 298, 122503. [CrossRef] [PubMed] 59. Greff, B.; Szigeti, J.; Nagy, A.; Lakatos, E.; Varga, L. Influence of microbial inoculants on co-composting of lignocellulosic crop residues with farm animal manure: A review. J. Environ. Manag. 2022, 302, 114088. [CrossRef] 60. Colón, J.; Ponsá, S.; Álvarez, C.; Vinot, M.; Lafuente, F.J.; Gabriel, D.; Sánchez, A. Analysis of MSW full-scale facilities based on anaerobic digestion and/or composting using respiration indices as performance indicators. Bioresour. Technol. 2017, 236, 87–96. [CrossRef] Atmosphere 2022, 13, 798 15 of 16 61. Liu, X.; Gao, X.; Wang, W.; Zheng, L.; Zhou, Y.; Sun, Y. Pilot-scale anaerobic co-digestion of municipal biomass waste: Focusing on biogas production and GHG reduction. Renew. Energy 2012, 44, 463–468. [CrossRef] 62. Wisniewska, M.; Kulig, A.; Lelicinska-Serafin, K. The Impact of Technological Processes on Odorant Emissions at MunicipalWaste Biogas Plants. Sustainability 2020, 12, 5457. [CrossRef] 63. Wisniewska, M.; Kulig, A.; Lelicinska-Serafin, K. Odour Nuisance at Municipal Waste Biogas Plants and the Effect of Feedstock Modification on the Circular Economy—A Review. Energies 2021, 14, 6470. [CrossRef] 64. Orzi, V.; Cadena, E.; D’Imporzano, G.; Artola, A.; Davoli, E.; Crivelli, M.; Adani, F. Potential odour emission measurement in organic fraction of municipal solid waste during anaerobic digestion: Relationship with process and biological stability parameters. Bioresour. Technol. 2010, 101, 7330–7337. [CrossRef] 65. Di, Y.; Liu, J.; Liu, J.; Liu, S.; Yan, L. Characteristic analysis for odor gas emitted from food waste anaerobic fermentation in the pretreatment workshop. J. Air Waste Manag. Assoc. 2013, 63, 1173–1181. [CrossRef] [PubMed] 66. González, D.; Colón, J.; Sánchez, A.; Gabriel, D. Multipoint characterization of the emission of odour, volatile organic compounds and greenhouse gases from a full-scale membrane-based municipal WWTP. J. Environ. Manag. 2022, 313, 115002. [CrossRef] [PubMed] 67. Capelli, L.; Sironi, S.; Del Rosso, R.; Céntola, P. Predicting odour emissions from wastewater treatment plants by means of odour emission factors. Water Res. 2009, 43, 1977–1985. [CrossRef] [PubMed] 68. Carrera-Chapela, F.; Donoso-Bravo, A.; Souto, J.A.; Ruiz-Filippi, G. Modelling the odor generation in WWTP: An integrated approach review. Water Air Soil Pollut. 2014, 225, 1932. [CrossRef] 69. Dinçer, F.; Dinçer, F.K.; Sari, D.; Ceylan, Ö.; Ercan, Ö. Dispersion modelling and air quality measurements to evaluate the odor impact of a wastewater treatment plant in Izmir. Atmos. Pollut. Res. 2020, 11, 2119–2125. [CrossRef] 70. De Sanctis, M.; Murgolo, S.; Altieri, V.G.; De Gennaro, L.; Amodio, M.; Mascolo, G.; Di Iaconi, C. An innovative biofilter techonlogy for reducing environmental spreading of emerging pollutants and odour emissions during municipal sewage treatment. Sci. Total Environ. 2022, 803, 149966. [CrossRef] 71. Burgués, J.; Esclapez, M.D.; Doñate, S.; Pastor, L.; Marco, S. Aerial Mapping of odorous gases in a wastewater treatment plant using a small drone. Remote Sens. 2021, 13, 1757. [CrossRef] 72. Burgués, J.; Esclapez, M.D.; Doñate, S.; Marco, S. RHINOS: A lightweight portable electronic nose for real-time odor quantification in wastewater treatment plants. iScience 2021, 24, 103371. [CrossRef] 73. Naddeo, V.; Zarra, T.; Oliva, G.; Kubo, A.; Ukida, N.; Higuchi, T. Odour measurement in wastewater treatment plant by a new prototype of e.Nose: Correlation and comparison study with reference to both European and Japanese approaches. Chem. Eng. Trans. 2016, 54, 85–90. [CrossRef] 74. Capelli, L.; Sironi, S.; Del Rosso, R.; Guillot, J.M. Measuring odours in the environment vs. dispersion modelling: A review. Atmos. Environ. 2013, 79, 731–743. [CrossRef] 75. Onofrio, M.; Spataro, R.; Botta, S. A review on the use of air dispersion models for odour assessment. Int. J. Environ. Pollut. 2020, 67, 108358. [CrossRef] 76. Varela-Bruce, C.; Antileo, C. Assessment of odour emissions by the use of a dispersion model in the context of the proposed new law in Chile. J. Environ. Manag. 2021, 298, 113208. [CrossRef] [PubMed] 77. Zarra, T.; Belgiorno, V.; Naddeo, V. Environmental odour nuisance assessment in urbanized area: Analysis and comparison of different and integrated approaches. Atmosphere 2021, 12, 690. [CrossRef] 78. Lee, S.-H.; Kurade, M.B.; Jeon, B.-H.; Kim, J.; Zheng, Y.; Salama, E.-S. Water condition in biotrickling filtration for the efficient removal of gaseous contaminants. Crit. Rev. Biotechnol. 2021, 41, 1279–1296. [CrossRef] [PubMed] 79. Shammay, A.; Evanson, I.E.J.; Stuetz, R.W. Selection framework for the treatment of sewer network emissions. J. Environ. Manag. 2019, 249, 109305. [CrossRef] 80. Beniwal, D.; Taylor-Edmonds, L.; Armour, J.; Andrews, R.C. Ozone/peroxide advanced oxidation in combination with biofiltration for taste and odour control and organics removal. Chemosphere 2018, 212, 272–281. [CrossRef] 81. Viswanathan, S.; Neerackal, G.; Buyuksonmez, F. Removal of beta-pinene and limonene using compost biofilter. J. Air Waste Manag. Assoc. 2013, 63, 237–245. [CrossRef] 82. Zheng, T.; Li, L.; Chai, F.; Wang, Y. Factors impacting the performance and microbial populations of three biofilters for co-treatment of H2 S and NH3 in a domestic waste landfill site. Process Saf. Environ. Prot. 2021, 149, 410–421. [CrossRef] 83. Márquez, P.; Herruzo-Ruiz, A.M.; Siles, J.A.; Alham, J.; Michán, C.; Martín, M.A. Influence of packing material on the biofiltration of butyric acid: A comparative study from a physico-chemical, olfactometric and microbiological perspective. J. Environ. Manag. 2021, 294, 113044. [CrossRef] 84. Barbusinski, K.; Kalemba, K.; Kasperczyk, D.; Urbaniec, K.; Kozik, V. Biological methods for odor treatment—A review. J. Clean. Prod. 2017, 152, 223–241. [CrossRef] 85. Marycz, M.; Brillowska-Dabrowska,˛ A.; Muñoz, R.; G˛ebicki, J. A state of the art review on the use of fungi in biofiltration to remove volatile hydrophobic pollutants. Rev. Environ. Sci. Biotechnol. 2022, 21, 225–246. [CrossRef] 86. Liu, J.; Yue, P.; Zang, N.; Lu, C.; Chen, X. Removal of odors and VOCs in municipal solid waste comprehensive treatment plants using a novel three-stage integrated biofilter: Performance and bioaerosol emissions. Front. Environ. Sci. Eng. 2021, 15, 48. [CrossRef] [PubMed] Atmosphere 2022, 13, 798 16 of 16 87. López, M.E.; Rene, E.R.; Boger, Z.; Veiga, M.C.; Kennes, C. Modelling the removal of volatile pollutants under transient conditions in a two-stage bioreactor using artificial neural networks. J. Hazard. Mater. 2017, 324, 100–109. [CrossRef] [PubMed] 88. Yao, X.-Z.; Chu, Y.-X.; Wang, C.; Li, H.J.; Kang, Y.-R.; He, R. Enhanced removal of methanethiol and its conversion products in the presence of methane in biofilters. J. Clean. Prod. 2019, 215, 75–83. [CrossRef] 89. Yun, J.; Jung, H.; Ryu, H.W.; Oh, K.-C.; Jeon, J.-M.; Cho, K.-S. Odor mitigation and bacterial community dynamics in on-site biocovers at a sanitary landfill in South Korea. Environ. Res. 2018, 166, 516–528. [CrossRef] 90. Gabriel, D.; Deshusses, M.A. Retrofitting existing chemical scrubbers to biotrickling filters for H2 S emission control. Proc. Natl. Acad. Sci. USA 2003, 100, 6308–6312. [CrossRef] 91. Bua, H.; Carvalho, G.; Huang, C.; Sharma, K.R.; Yuan, Z.; Song, Y.; Bond, P.; Keller, J.; Yu, M.; Jiang, G. Evaluation of continuous and intermittent trickling strategies for the removal of hydrogen sulfide in a biotrickling filter. Chemosphere 2022, 291, 132723. [CrossRef] 92. You, J.; Chen, J.; Sun, Y.; Fang, J.; Cheng, Z.; Ye, J.; Chen, D. Treatment of mixed waste-gas containing H2 S, dichloromethane and tetrahydrofuran by a multi-layer biotrickling filter. J. Clean. Prod. 2021, 319, 128630. [CrossRef] 93. Caicedo, F.; Estrada, J.M.; Silva, J.P.; Muñoz, R.; Lebrero, R. Effect of packing material configuration and liquid recirculation rate on the performance of a biotrickling filter treating VOCs. J. Chem. Technol. Biotechnol. 2018, 93, 2299–2306. [CrossRef] 94. Andraskar, J.; Yadav, S.; Kapley, A. Challenges and Control Strategies of Odor Emission from Composting Operation. Appl. Biochem. Biotechnol. 2021, 193, 2331–2356. [CrossRef] 95. Prado, O.J.; Gabriel, D.; Lafuente, J. Economical assessment of the design, construction and operation of open-bed biofilters for waste gas treatment. J. Environ. Manag. 2009, 90, 2515–2523. [CrossRef] [PubMed] 96. Helbich, S.; Dobslaw, D.; Schulz, A.; Engesser, K.-H. Styrene and Bioaerosol Removal from Waste Air with a Combined Biotrickling Filter and DBD-Plasma System. Sustainability 2020, 12, 9240. [CrossRef] 97. Santos-Clotas, E.; Cabrera-Codony, A.; Martin, M.J. Coupling adsorption with biotechnologies for siloxane abatement from biogas. Renew. Energy 2020, 153, 314–323. [CrossRef] 98. Prado, O.J.; Redondo, R.M.; Lafuente, J.; Gabriel, D. Retrofitting of an Industrial Chemical Scrubber into a Biotrickling Filter: Performance at a Gas Contact Time below 1 s. J. Environ. Eng. 2009, 135, 359–366. [CrossRef] 99. Sun, S.; Jia, T.; Chen, K.; Peng, Y.; Zhang, L. Simultaneous removal of hydrogen sulfide and volatile organic sulfur compounds in off-gas mixture from a wastewater treatment plant using a two-stage bio-trickling filter system. Front. Environ. Sci. Eng. 2019, 13, 60. [CrossRef]