Solid waste management is becoming a global problem in developed countries. According to international recommendations and legislation, different technologies are being used to reduce landfill disposal of organic wastes, improving recycling of organic matter and nutrients [1]. Among the technologies used to treat the source-separated organic fraction of municipal solid wastes (OFMSW), anaerobic digestion and composting are widely recommended as environmentally friendly technologies. Regarding this, although the objective of composting and anaerobic digestion, as well as other waste treatment technologies, is to safely transform wastes into less polluting and/or hazardous substances or, when possible, useful products reducing their possible impact on the environment, there are some inherent environmental impacts associated with waste recycling in large-scale facilities. Odorous compound emissions and atmospheric pollution are the most common of these impacts. In fact, composting plants present numerous odor and pollution sources, including reception and materials handling, forced aerated composting, stockpiling, etc. Gaseous emissions in composting facilities are typically constituted by nitrogen-based compounds, sulphur-based compounds and a wide group of compounds denominated Volatile Organic Compounds (VOCs) [2]. VOCs are also produced during the anaerobic digestion process [3], although their composition can be very different from that of aerobic processes.

Volatile Organic Compounds is a denomination used to refer to a wide group of organic compounds whose vapor pressure is at least 0.01 kPa at 20 °C [4]. VOCs are also characterized by their low water solubility. Once in the atmosphere, VOCs participate in photochemical reactions producing photochemical oxidants. According to Eitzer [2], who undertook a pioneering study on the exhaustive characterization of the different VOCs emitted at the different stages of the composting process, most VOCs in composting plants are emitted during the early stages of process: i.e., at the tipping floors, at the shredder and during the initial forced aeration composting period. Also, incomplete or insufficient aeration during composting can produce sulphur compounds of intense odor, whereas incomplete aerobic degradation processes result in the emission of alcohols, ketones, esters and organic acids [5]. Van Durme et al. [6] identified dimethyl sulphide, dimethyl disulphide, limonene and α-pinene as the most significant odorous VOC in a wastewater sludge composting facility. According to this study, the latter two compounds were released from the wood chips used as bulking agent. In fact, Büyüksönmez and Evans [7] also found six different terpenes (α-pinene, β-pinene, 3-carene, camphene, β-myrcene and D-limonene) as the major compounds responsible for VOC emissions from green waste composting processes (within 33 and 95% of total emissions). These compounds were also predominant in feedstock natural breakdown.

Delgado-Rodríguez et al. [8] reported that volatile compounds emissions are closely related to the composting process phases: aldehydes, alcohols, carboxylic acids, esters, ketones, sulphides and terpenes are mainly emitted during the initial acid phase, while in the thermophilic phase ketones, organosulphur-compounds, terpenes and ammonia become predominant. During the cooling phase sulphides, terpenes and ammonia are the main volatile compounds emitted. These authors also investigated the effect of the process control parameters (moisture, aeration and C/N ratio) on volatile compound emissions in MSW (Municipal Solid Waste) composting. The C/N ratio had the most significant effect on VOC emissions, followed by the aeration level and the moisture content. The effect of aeration on emissions from composting mixtures was also highlighted by Staley et al. [9], when comparing emissions from aerobic and anaerobic treatments. However, in both cases it must be considered that the C/N ratio should be related to the biologically available carbon and nitrogen forms, a point that is not often considered [10]. At the laboratory scale, total VOC concentration in exhaust gases from composting processes of different wastes has been also studied and it was concluded that the highest concentrations of VOCs were emitted during the first 48 h of the process [11]. These authors also stated that total VOC emissions could not be correlated with the biological activity of the process as measured by respirometry. Pierucci et al. [12] found the same emission pattern (main VOC release during the first days of process), although the possible correlation with the biological activity was not studied.

Tolvanen et al. [13] and Komilis et al. [14] studied VOC emissions from the composting process considering the possible negative effects on plant workers and the nearby population. Tolvanen et al. [13] stated that VOC concentrations detected at a composting plant treating source-separated catering waste were much lower than the national threshold limit values, thus avoiding primary health effects, although many of the compounds exceeded their threshold odor concentrations; thus secondary symptoms as nausea and hypersensibility might be expected from exposure to unpleasant odors. In fact, the effect of workers’ simultaneous exposure to several VOCs is not very well known since synergic effects should be also considered. On the other hand, Komilis et al. [14] found various xenobiotic VOCs in gaseous emissions during laboratory scale composting of different MSW components.

VOC emissions in biological treatment processes have also been studied as environmental loads related to these processes. Regarding this, VOC emissions have been reported as emission factors (i.e., amount of VOC emitted per weight of waste treated) or from a Life Cycle Assessment (LCA) perspective, since their contribution is mainly related to the Photochemical Oxidation Potential (POP). Emission factor values of 1.70 and 0.59 kg of VOC Mg⁻¹ of OFMSW treated were reported by Baky and Eriksson [15] and Smet [3] from studies at laboratory scale, while Cadena et al. [16] found 0.2 and 7.3 kg VOC Mg⁻¹ of OFMSW treated in two different full scale composting plants using different technologies. In LCA studies, emissions of VOC to the atmosphere are expressed in kg ethylene equivalent/kg of emission and included in the POP category [17].

The presence of VOCs in gaseous emissions from waste treatment plants has been reported as total VOCs emitted [16–19], as concentration of different families of VOCs [14,19] or as individual VOC concentrations [3,20]. Table 1 summarizes some values reported for VOC emissions in biological treatment processes of organic wastes by different authors. Some values in Table 1 are reported as a range of VOC concentrations obtained in a same plant during different sampling days or as different values obtained by the same authors in different plants. As seen in Table 1, values of VOC concentration are very variable, even for the same facility and the same type of waste. In the case of total terpenes, for instance, values reported in Table 1 for an effluent of a biofilter in a composting plant range between 20 and 12,350 μg/m³, while the same type of compounds for an anaerobic digestion plant show concentrations between 683 and 4,750 ppbv for non-treated OFMSW and 414 and 1,151 ppbv for the anaerobically digested residue. As a general trend observed from the wastes and treatments reported, the compounds with the highest presence in the emissions are terpenes and alcohols followed by ketones.

Sulphur compounds, with a relatively low concentration, are significant for their contribution to the odor level. In fact, dimethyl sulfide and dimethyl disulfide together with carbon disulfide and methyl and ethyl mercaptan have been found in gaseous emissions from aerobic and anaerobic waste treatments. Actually, these compounds are very common in organic waste decomposition processes, originating mainly in the microbial degradation of sulphur-containing amino acids found in proteins [3,23].

Regarding nitrogen compounds, there is an important difference within values reported resulting from the analysis of samples using GC-MS (Gas Chromatography-Mass Spectrometry) and gas detection tubes, with those obtained with tubes being higher (plants studied in both cases correspond to different treatments). It is clear that both analytical methodologies need a deeper and more complete comparison to be fully reliable being, at present, GC-MS the most powerful tool to quantify VOC emissions.

There are also other compounds that have not been listed in Table 1 but have been detected by some authors in low concentrations. This is the case, for instance, of halogenated and aromatic compounds such as trichlorofluoromethane, 1,1,1-trichloroethane, dicloromethane, 1,3-dichlorobenzene, naphthalene or p-isopropyltoluene [2,14,20] or other terpenes such as camphene or thujone [3,20].

The presence of VOCs has also been investigated in landfill gas and ambient air surrounding landfill facilities [24,25]. Aromatic VOCs (benzene, toluene, ethylbenzene and xylene) and reduced sulphur compounds (RSC) were found in these studies, with their concentration and relative percentages in landfill gas being strongly dependant on landfill aging [26]. Among the RSCs found in these emissions, hydrogen sulfide was predominant both in emissions from active and inactive landfill stages [27]. In addition to this, the conditions of the waste in the landfill, which are mainly anaerobic, must be related to the composition of the emitted gas [9].

The main sources of VOCs and other gaseous contaminants in composting plants are area emission sources. Composting or, in general, waste treatment plants, can be completely confined with process emissions treated through scrubbers and/or biofilters or completely open, where the composting process takes place commonly in windrows (static, aerated and/or turned), or a mixture of both situations. Then, biofilter and composting windrows surfaces are the sources of VOCs. In addition, emissions from reception and storage areas should be considered if not confined. Also the emissions of VOCs during some pre (waste conditioning) and post treatment (compost sieving) operations can be significant. In fact, Albretch et al. [30] stated that odors emission during turning, sieving and shredding may exceed those from biofilters. Also, Pagans et al. [11] showed a continuous emissions of VOCs from the biofilter even when this was not working. In anaerobic digestion treatment plants, area emission sources also exist as a focus of VOCs (biofilters external surfaces), together with some point emission sources, such as safety flares for biogas.

Although gaseous emissions can be directly released to the atmosphere in non-confined waste biological treatment facilities [16], effective gas treatment equipment is usually required in most cases, especially when the distance to inhabited areas is low [18]. As can be deduced from the numerous VOC detected in gaseous emissions from waste treatment plants reported above, the selection of the adequate equipment for gaseous pollutants abatement in these installations is not an easy task. There is no specific technology developed for this application and normally a combination of the available options must be studied. Adsorption, absorption (scrubbing), thermal/catalytic oxidation and biological treatments have been used for VOC removal in gaseous emissions [39]. Adsorption and scrubbing involve the generation of a liquid or solid effluent containing the contaminants removed from gaseous flows and thus further treatment will be needed for these waste flows. Sulphur oxides could be generated in a thermal treatment depending on VOCs present in the gaseous effluent from a waste treatment plant. Although presenting acceptable VOC removal rates, physico-chemical treatments involve higher economical costs than biological treatments.

Biological treatments of waste gases are based on the use of naturally selected microbial strains capable of contaminant removal which act as carbon source, energy source or both [39]. Different types of bioreactors are used in VOC contaminated gas treatment including biofilters, biotrickling filters and bioscrubbers. Contaminant removal mechanisms are similar in these bioreactors differences relying in the use of microorganisms, either suspended or immobilized, packing media, pollutant concentration and others [40]. Among the available options, biofiltration is the most commonly used technology to reduce emissions from the biological waste treatment processes [41]. At large facilities biofiltration is usually preceded by an absorption step under acidic conditions, mainly to remove ammonia that is produced in large amounts during the composting process and that can inhibit the biological activity of the biofilter [42]. Biofiltration is considered economically and environmentally viable and a suitable technology in terms of waste recycling and filtering effect [43].

Biofilters are a type of bioreactors in which a humidified polluted air stream is passed through a porous packed bed on which a mixed culture of pollutant-degrading organisms is immobilized. Generally the pollutants in the air flow are transported from the gaseous phase to the microbial biofilm (through liquid phase or moisture) where the biological oxidation of VOCs occurs [40]. The by-products of biological oxidation are water, carbon dioxide, mineral salts, some volatile organic compounds and microbial biomass [44]. Organic packing materials are commonly used such as wood chips mixed with compost or other bulking materials used in the composting process. Figure 1 shows a scheme of a biofilter. The performance of a biofilter is not uniform and it is influenced by several important variables such as the composition of the gas, the packing material, nutrient supply, temperature, pH, pressure drop and gas residence time [45]. Maximum elimination capacities of 100–120 g·m⁻³·h⁻¹ have been reported for typical biofilters colonized with bacteria [46]. These authors developed a method to assess biofilters performance for a wide range of VOCs with different air/water partition coefficients. Some authors indicate that the addition of activated carbon improves the biofilter degradation capacity [40]. However, the prediction of the biofilters efficiency in the removal of a mixture of VOCs such as the one present in the exhaust gases of a waste treatment plant is difficult, being limited by the solubility and biodegradability of pollutants.

Pagans et al. [11] reported biofilter removal efficiencies up to 97% for VOCs generated in the composting process of different organic wastes at pilot plant scale with loading rates ranging from 0.55 to 40 g·C·m⁻³·biofilter·h⁻¹. Removal efficiencies were highly dependent on the composted waste. The same authors also pointed a biofilter basal emission of VOCs of approximately 50 mg of C·m⁻³. However, in the literature, VOC biofiltration is frequently studied in laboratory scale biofilters using synthetic gases with two or three mixed compounds or even a single compound [47,48], which can be very different from those conditions found in full-scale facilities, as previously commented.

Another aspect to comment in biofilter operation and maintenance at an industrial level is the need for attention to packing material needs for a correct performance of the gas treatment equipment. In fact, watering of the material is needed, as well as a periodic replacement of the biofilter material. Colón et al. [18] reported real data on two full scale biofilters treating gases from the OFMSW composting process during filtering material replacement. Old biofilter material showed lower removal efficiencies than the new material. The average VOC removal efficiencies with the old material for the two biofilters studied were 42 and 65%, whereas the average values of all data with new material were 74 and 71%. These efficiencies correspond to average elimination capacities of 11 and 8.6 g·C·m⁻³ biofilter h⁻¹ for the old material and 17.1 and 27 g·C·m⁻³·biofilter·h⁻¹ for the new material. These results indicate that the biofilter performance was improved as a result of material replacement. In one of the two biofilters the authors observed the pattern reported by Devinny et al. [45] in relation to VOC removal in biofilters: a first stage of dominance of the adsorption process followed by a decrease of the removal efficiency attributable to the saturation of the adsorption/absorption capacity and to the microorganisms acclimation period and, finally, an increase in the removal efficiency because of the biodegradation dominance [49]. For the second biofilter, the removal efficiency followed a similar pattern to the applied VOC loading rate during the entire period of the study (before and after biofilter material replacement). Even though biofilters are widely implemented in full scale treatment plants for organic waste treatment facilities, the complete removal of VOC is difficult to achieve [20]. Proper maintenance operations are thus needed to maximize biofilter performance.

VOCs present in gaseous emissions of biological treatment facilities of organic wastes have been investigated and characterized by different researchers. The main VOCs related to organic waste handling and treatment are terpenes, although ketones, alcohols and organic acids have also an important contribution to the total VOC content measured. The presence of sulphur and nitrogen compounds has to be also highlighted. Different relationships have been established between VOC concentration and odor level on specific situations and for a specific range of VOCs and odor concentrations. There is no universal correlation covering the whole concentration range and the entire group of VOC families.

The analysis of VOCs is mainly performed by GC-MS although other methods such as gas detection tubes or electronic noses have been used in some cases. GC-MS appears as the most reliable method covering a wide range of VOC concentrations and being capable to identify and quantify a large number of compounds from different families. Gas detection tubes permit the measurement of many compounds present at relatively high concentrations. As an economical and quick method, it can be useful for the self management of gaseous emissions in waste treatment plants. Electronic noses permit the characterization of the families of VOC present in a gaseous sample and the comparison between VOC emissions from different points of a treatment plant of different installations, although they must be considered a first approach.

Different methods for air emissions sampling in waste treatment plants have been proposed. The lack of homogeneity among these methods is one of the drawbacks in VOC emission measurement from this type of facilities.

Finally, it must be highlighted that biofiltration is the gaseous pollutant abatement technology that is most widely used in organic waste treatment plants for VOC removal.