Full-Scale Odor Abatement Technologies in Wastewater Treatment Plants (WWTPs): A Review

: The release of air pollutants from the operation of wastewater treatment plants (WWTPs) is often a cause of odor annoyance for the people living in the surrounding area. Odors have been indeed recently classiﬁed as atmospheric pollutants and are the main cause of complaints to local authorities. In this context, the implementation of effective treatment solutions is of key importance for urban water cycle management. This work presents a critical review of the state of the art of odor treatment technologies (OTTs) applied in full-scale WWTPs to address this issue. An overview of these technologies is given by discussing their strengths and weaknesses. A sensitivity analysis is presented, by considering land requirements, operational parameters and efﬁciencies, based on data of full-scale applications. The investment and operating costs have been reviewed with reference to the different OTTs. Bioﬁlters and biotrickling ﬁlters represent the two most applied technologies for odor abatement at full-scale plants, due to lower costs and high removal efﬁciencies. An analysis of the odors emitted by the different wastewater treatment units is reported, with the aim of identifying the principal odor sources. Innovative and sustainable technologies are also presented and discussed, evaluating their potential for full-scale applicability.


Introduction
Odorants arise from many anthropogenic sources, such as refineries, petrochemical industries, livestock production, food processing, chemical factories, and sanitary environmental facilities [1]. This phenomenon is mostly encountered in developing countries due to the proliferation of industries and lack of environmental protection policies [2,3]. Meanwhile, the advent of EU environment and climate change policies demonstrates a growing concern for the quality of the environment in terms of focusing on an improvement of air quality and reduction of greenhouse gas (GHG) emission [4]. In the case of air quality management, odor emission is considered an air pollutant that required immediate attention [5]. Unwanted odors are mostly generated from environmental treatment facilities such as wastewater treatment plants (WWTPs), sanitary landfills, composting, etc. In the past, WWTPs were engineered and designed primarily for the removal of inorganic and organic pollutants in the influent. However, there is no comprehensive program in odor management in WWTPs implemented by the operators [6]. The identification of effective solutions to reduce odor emissions and related complainants are thus of fundamental importance to increase the acceptability and sustainability of the facilities needed in the urban water cycle and to limit the negative impacts on the surrounding area to ensure correct process management [7,8].
There is no comprehensive approach used in odor regulatory systems, and methods and tools for management and control can derive from the characterization of the odor concentration or of individual chemicals. Jurisdictions have not yet promulgated regulations with standardized odor methodologies and objective criteria commonly use the principles of nuisance law to fundament the management of odor annoyance [17]. Some European countries generally determine odor exposure limits set as emission limit values (ELV) in ouE·s −1 or ouE·h −1 . On the other hand, in several U.S. states, the dilution-to-threshold (D/T) field olfactometry approach is used to set the limits [18].
Nowadays, wastewater treatment industries have to adapt to a stricter law on odor management by providing more efficient, cost-effective and environmental-friendly odorcontrol technologies to make WWTPs management more sustainable. There are two main strategies for controlling malodorous emissions released from WWTPs, which consist in (1) the prevention of odor production as a result of good management of the plant [19,20], and (2) the applications of abatement and control technologies for the identified odorous compounds [21][22][23]. Barriers to contain odor emissions within certain areas using trees as buffer zones is a passive strategy used for reducing annoyance among the residents. However, the efficiency of this solution relies on weather conditions (i.e., wind speed, direction, etc.) [24]. On the other hand, chemical agents are used to control malodorous molecules released from WWTPs by masking and/or destroying them [25] but this method is only ideal at low concentration levels [11]. Although some chemicals can stabilize odors, they may potentially determine even more odor if they are not properly dosed [26,27]. An odor emission characterized by high odor concentration may indeed still be present, with a different degree of pleasantness or unpleasantness (i.e., hedonic tone) due to the numerous byproducts which may be produced during the reaction.
In areas where there is a high density of people, the primary strategy is to convey the odor source and treat the emissions. This strategy can be implemented by isolating the source with confinement structures and then collecting the conveyed flue gases to an Odor treatment technology (OTT) system. OTTs can treat odorous compounds chemically and/or biologically, removing or turning odorants into odorless compounds [28]. The utilization of structures for covering the different treatment units in WWTPs minimizes odor emissions, disperses them in the atmosphere and reduces evaporation. In this way, less water and chemicals are required in the wastewater treatment process.
The odor treatment technologies are classified mainly into hybrid (e.g., physical and chemical) and biological techniques. Physical and chemical techniques have a high abatement efficiency and robustness when operated and maintained properly, low empty bed retention time (EBRT), rapid start-up. However, some drawbacks are still present [6], mainly consisting in the fact that regular use of consumables (i.e., adsorption material and chemical agents) can be a contributing factor to a high operating cost and the disposal of waste materials is a challenge in a circular economy perspective [27]. On the other hand, biological techniques constitute a more cost-effective and environmental-friendly alternative [29,30], but can present significant investment costs (e.g., bioscrubbers) or land requirements (e.g., biofilter) [31]. Among the biological techniques, the biotrickling filter has been identified as one of the promising solutions due to its efficiency, cost-effectiveness and sustainability [32].
The present review aims to categorize and critically analyze different abatement and control technologies applied to WWTPs for odor management. A proof and updated analysis of the state-of-the-art about full-scale OTTs installation in WWTPs is needed due to the scarcity of comparative analyses in terms of cost-benefits balance. It is essential to compare technologies at the industrial scale to show the robustness of the process under working flows fluctuation and wide range of pollutant concentrations.

Odor Emissions in WWTPs
During the wastewater collection and treatment operations, a mixture of several chemical compounds that can produce an unpleasant odor are generated through anaerobic decomposition of organic matter [1, 23,33]. The odor is generated from the mixture of different volatile chemical species which can trigger the sensation of odor [33]. It is thus due to the interaction of different volatile chemical species, in particular sulfur compounds (e.g., sulfides, mercaptans), nitrogen compounds (e.g., ammonia, amines) and volatile organic compounds (e.g., esters, acids, aldehydes, ketones, alcohols) [34]. Volatile organic compounds (VOCs) are a large group of compounds, with different functional groups such as volatile fatty acids, alcohols, aldehydes, amines, carbonates, esters, sulfides, disulfides, mercaptans, and heterocyclic nitrogen compounds, characterized by a certain volatility. Conversely, inorganic compounds (H 2 S, NH 3 , Cl 2 ) due to their low molecular weights can bind olfactory receptors and affect odor level [35]. Table 1 summarized the threshold levels of principal malodorous compounds detected in WWTPs. Over the years, scientists analyzed many WWTPs and odor emission capacity (OEC) measurements were realized in primary sedimentation units (10,000 OU m −2 h −1 ), in sludge-digestion tanks (8200 OU m −2 h −1 ), and sludge thickening and dewatering facilities (2500 OU m −2 h −1 ). Lower values have been identified in the denitrification (anoxic) (730 OU m −2 h −1 ) and nitrification (aerobic) tanks (510 OU m −2 h −1 ). Primary sedimentation units, sludge thickeners and dewatered sludge are considered the main responsible for odor nuisance. To further justify this finding, Giuliani et al., (2015) [36] and Zarra et al., (2014) [37] demonstrated that raw wastewater and sludge thickening account for roughly 52% of the total emissions and for 40% of disposal activities. The major odor sources are indeed associated with pretreatment units (pumping station, grid), primary sedimentation and sludge thickening ( Figure 1) [13,15,38,39]. Over the years, scientists analyzed many WWTPs and odor emission capacity (OEC) measurements were realized in primary sedimentation units (10,000 OU m −2 h −1 ), in sludge-digestion tanks (8200 OU m −2 h −1 ), and sludge thickening and dewatering facilities (2500 OU m −2 h −1 ) . Lower values have been identified in the denitrification (anoxic) (730 OU m −2 h −1 ) and nitrification (aerobic) tanks (510 OU m −2 h −1 ). Primary sedimentation units, sludge thickeners and dewatered sludge are considered the main responsible for odor nuisance. To further justify this finding, Giuliani et al., (2015) [36] and Zarra et al.,(2014) [37] demonstrated that raw wastewater and sludge thickening account for roughly 52% of the total emissions and for 40% of disposal activities. The major odor sources are indeed associated with pretreatment units (pumping station, grid), primary sedimentation and sludge thickening (Figure 1) [13,15,38,39].

Volatile Organic Compounds (VOCs)
Volatile organic compounds (VOCs) are toxic organic chemicals that evaporate under normal atmospheric conditions due to their high vapor pressures, low boiling points and low water solubility [31,40]. The main alarming VOCs emissions are related to the presence of "BTEX" (benzene, toluene, ethylbenzene and xylenes) that are considered harmful gases and detrimental to the environment. In particular, petrochemical WWTPs have been characterized among the main VOCs emissions sources, with consistent emissions of BTEX [38,39,41].
The World Health Organization (WHO) identified many VOCs as the most dangerous for human health [42]. In fact, benzene is known as one of the strongest carcinogenic agents [43]. Others are suspected to be carcinogens but also can have toxic effects on human health and destroy the stratospheric ozone, produce tropospheric ozone and form photochemical smog [10].

Hydrogen Sulfide (H2S)
Hydrogen sulfide (H2S) is an extremely toxic gas, and it is responsible for the typical odor of rotten eggs in WWTPs. When the concentration of H2S is around 1000-2000 ppm

Volatile Organic Compounds (VOCs)
Volatile organic compounds (VOCs) are toxic organic chemicals that evaporate under normal atmospheric conditions due to their high vapor pressures, low boiling points and low water solubility [31,40]. The main alarming VOCs emissions are related to the presence of "BTEX" (benzene, toluene, ethylbenzene and xylenes) that are considered harmful gases and detrimental to the environment. In particular, petrochemical WWTPs have been characterized among the main VOCs emissions sources, with consistent emissions of BTEX [38,39,41].
The World Health Organization (WHO) identified many VOCs as the most dangerous for human health [42]. In fact, benzene is known as one of the strongest carcinogenic agents [43]. Others are suspected to be carcinogens but also can have toxic effects on human health and destroy the stratospheric ozone, produce tropospheric ozone and form photochemical smog [10].

Hydrogen Sulfide (H 2 S)
Hydrogen sulfide (H 2 S) is an extremely toxic gas, and it is responsible for the typical odor of rotten eggs in WWTPs. When the concentration of H 2 S is around 1000-2000 ppm with an exposure time of minutes, it can be rapidly absorbed through the lungs causing instant death [44]. Table 2 reported the hazardous concertation at different exposure times. Crude petroleum and natural gas contain H 2 S. However, in WWTPs H 2 S is also a byproduct of bacteria digestion of organic materials. In WWTPs, sulfur exist as either organic sulfur from feces or inorganic sulfur from the sulfate ion (SO 4 2− ). Typically, microbial reduction of SO 4 2− is the dominant mechanism of H 2 S formation. Besides, H 2 S can be formed whenever sulfur-containing compounds are exposed to organic materials at high temperatures [45]. Hydrogen sulfide has low solubility in wastewater. The TLV-STEL (threshold limit values at short term exposure limit) is the maximum concentration that workers can be exposed to for 15 min during a workday. TLV-STEL for H 2 S in the air is 24 mg m −3 35 ppmv), and the concentrations of H 2 S founds nearby WWTPS without odorous compounds control systems generally exceeds the following limit. If the workers' exposure is below the preset daily limit, they may avoid the adverse health effects.

Ammonia (NH 3 )
Ammonia (NH 3 ) is another malodorous compound in WWTPs caused by bacterial decomposition of urea generated from human activities. Its pungent character makes it easy to identify, among other gases. Furthermore, it can cause nose and throat irritation, bronchiolar and alveolar edema, and airway destruction. Due to the low evaporation temperature, ammonia can easily evaporate and release odors in the atmosphere. The TLV-STEL for NH 3 in the air is 69 mg m −3 (50 ppm). Usually, the concentrations of NH 3 that arise in wastewater is lower than the threshold value. However, an air pollution control system is still mandatory to avoid NH 3 reacting with other compounds, thus, reducing the overall emissions. Table 3 reported the hazardous concertation at different exposure times.

Odor Emissions Management in WWTPs
During the last decade, national and international authorities have increased their interest in resolving odor problems. In Europe, according to the Directive 2008/98/CE, "Member States shall take the necessary measures to ensure that waste management is carried out without endangering human health, without harming the environment and, in particular: (b) without causing a nuisance through noise or odors".
There are two main approaches to odor emission control, the first one is to apply a different strategy without any treatment unit, and the second one is to apply an OTT for the specific treatment of emissions.
The main strategies for reducing or masking the odorous emissions from WWTPs are good process design, good operational practices [46], implementation of buffer zones [51] and spraying masking agents [25].
Different technologies are applied for the odor emission treatment, they can be divided into three main groups: physical, chemical, and biological technologies. For the treatment of the emissions from the different units of the WWTP, it is necessary to cover the odorous sources. OTTs are based on the collection and treatment of the odorous emissions generated in WWTPs, reducing or removing the concentration of odorants before being released to the atmosphere [31].
Chemical scrubbers and activated carbon filters are the most widespread pilot plant scale physical/chemical technologies used in WWTPs for odor treatment [47]. These odor abatement techniques are based on chemical oxidation [48] and solid-phase adsorption [52]. Biological OTTs such as biofilters, biotrickling filters, bioscrubbers and activated sludge diffusion, are based on the biological oxidation of chemical agents by microorganisms once they have been transferred from the gaseous emission to an aqueous phase [23,30]. Different methods can be used for odor measurement such as sensorial, analytical and senso-analytical techniques [53,54]. Sensorial approaches such as dynamic olfactometry, field inspection and recording from residents are based on how humans respond to emissions, while analytical methods, such as gas chromatography-mass spectrometry (GC/MS), identification of specific compounds, infrared and electrochemical sensors, etc., are based on a laboratory Senso-analytical methods are the most promising. They overcome the main drawbacks of using analytical instruments (e.g., expense and inability to quantify the odor of a gas mixture), in the field for the prediction of the odor released on-site [53]. Among senso-instrumental methods, instrumental odor monitoring systems (IOMSs), also known as "electronic noses" (e.Noses), represent the tool with the greatest potential for future development for the continuous monitoring of environmental odors, with a view to obtaining real-time information [5].
One of the main sensorial approaches used to measure odor concentration (OUE m −3 ) is dynamic olfactometry regulated by EN13725:2003 [14,45]. According to European standardization, 1 OUE m −3 is defined as the amount of odorant that, when evaporated into 1 m 3 of gas air at standard conditions, causes a physiological response from a panel (detection threshold) equivalent to that of n-butanol (reference gas) evaporated into 1 m 3 of neutral gas [1,55]. Meanwhile, under analytical methods, GC-MS has been widely used for the measurement of chemical concentration. This tool can only measure the mass concentration (ppm or mg m −3 ) of a single or multiple gaseous compounds that is/are responsible for odor, but not the odor concentration of the emission [37]. Nonetheless, the quantity of gas determined by GC-MS can correlate to acquire insights on the odor concentration [56].
During the last decades, IOMSs have been improved by hardware components and the selection of the array of the sensors [38,39]. A set of nonspecific sensors are used to characterize an odor by IOMS, where each sensor is responsive to a variety of odorous compounds, but reacts differently to each other [5,57]. It can provide a total response output from a simple or complex odor immediately [37]. In contrast, the measurements in conventional GC-MS required further interpretation of a statistical program to obtain the analysis [56,58]. Moreover, IOMS can be applied on-site while sensorial and analytical analysis of odor can mostly be carried out in the laboratory.
OTTs are installed, principally, where the odor emissions are higher in terms of flow rate and odorant loads. As reported in Figure 2, 52% of the OTTs analyzed were installed at the headworks of the plant (e.g., pumping station, screening systems, grit systems). Twentynine percent of the OTT installations investigated were located at the sludge treatment units, while only 19% of OTTs were implemented to treat odorous emissions at primary analysis [56,58]. Moreover, IOMS can be applied on-site while sensorial and analytical analysis of odor can mostly be carried out in the laboratory.
OTTs are installed, principally, where the odor emissions are higher in terms of flow rate and odorant loads. As reported in Figure 2, 52% of the OTTs analyzed were installed at the headworks of the plant (e.g., pumping station, screening systems, grit systems). Twenty-nine percent of the OTT installations investigated were located at the sludge treatment units, while only 19% of OTTs were implemented to treat odorous emissions at primary treatment. The results obtained from this analysis agree with the data shown in Figure 3, where the principal malodorous units in WWTPs were reported.

Full-Scale OTTs in WWTPs
Only a few reviews [6,12] explored, collected and summarized chemical/physical and biological technologies for the treatment of odorous emission. Figure 3 reports the configuration of the main chemical/physical and biological technologies applied in WWTPs for the treatment of odorous compounds emissions.
Biologically based odor treatment technologies, such as biofilters, biotrickling filters, and bioscrubbers have gained more and more popularity due to their lower O&M cost, reduced energy and chemical consumption and the absence of expensive adsorbent materials. Biotechnologies also have a more environmentally friendly profile because pollutants are finally converted into innocuous compounds such as CO2, H2O and biomass at ambient pressure and temperature. Best Available Techniques (BAT) Reference Document for Common Waste water and Waste gas Treatment/Management Systems in the Chemical Sector (2016) reported an overview of end-of-pipe odor treatment techniques. This document reported the advantages of biofiltration, including (i) low shift of pollution to any other media, (ii) few chemical agents added, and (iii) low energy consumption. Moreover, it also suggested the combination of biofiltration and bioscrubbing since the bioscrubber may act as a humidifier and degrade a high portion of the odorous load. In a biofilter system (Figure 3a), the odorants are forced through a packed bed (compost, peat, bark or a mixture of these) on which the microorganisms are attached as a biofilm. The pollutants are absorbed by the filter material and degraded by the biofilm.
In BTF (Figure 3b), the odorous gas is forced through a packed bed filled with a chemically inert carrier material that is colonized by microorganisms, similar to trickling filters in wastewater treatment. The liquid medium is recirculated over the packed bed and the pollutants are first taken up by the biofilm on the carrier material and then degraded by the microorganisms. The liquid medium can be recirculated continuously or discontinuously and in a co-or countercurrent to the gas stream. Flow directions will not affect the efficiency of the process.

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In BS (Figure 3c), the pollutant is adsorbed in an aqueous phase in an absorption tower then converted by the active microorganisms into CO2, H2O and biomass in a separate activated sludge unit. The effluent is circulated over the absorption tower in a co-or countercurrent direction to the gas stream.
Physical/chemical technologies consist of two types of reactors, namely adsorption systems and chemical scrubbing. Adsorption systems (Figure 3d) generally consist of static beds of granular materials in vertical cylindrical columns. Among purification methods, adsorption is simple and easy to apply to real-scale wastewater treatment plants [59]. Several sorbents have been studied, including fly ash, carbon, activated carbon, polymers, carbon-coated polymers, ceramics, micro-and mesoporous materials, metal-organic frameworks, natural zeolites, and synthetic zeolites. Chemical scrubbers (Figure 3e) are among the most mature abatement techniques employed in WWTPs due to the extensive experience and high robustness as well as the short gas retention time (as low as 1-2.5 s). The most common configuration ( Figure 3) is a vertical shell with gas flow going up through packing and the liquid solution (depending on the target compounds) going down. The liquid solution is usually circulated over the packing by pumping from a collection sump in the bottom of the tower, while chemicals are added either in the sump or in the recirculation piping. To the best of our knowledge, the current work is the first review paper to analyze and compare more than 50 full-scale odor treatment technologies (chemical/physical and biological) applied in WWTPs. The main characteristics of full-scale OTTs found in the scientific literature are reported and critically analyzed for each treatment method.

Full-Scale OTTs in WWTPs
Only a few reviews [6,12] explored, collected and summarized chemical/physical and biological technologies for the treatment of odorous emission. Figure 3 reports the configuration of the main chemical/physical and biological technologies applied in WWTPs for the treatment of odorous compounds emissions.
Biologically based odor treatment technologies, such as biofilters, biotrickling filters, and bioscrubbers have gained more and more popularity due to their lower O&M cost, reduced energy and chemical consumption and the absence of expensive adsorbent materials. Biotechnologies also have a more environmentally friendly profile because pollutants are finally converted into innocuous compounds such as CO 2 , H 2 O and biomass at ambient pressure and temperature. Best Available Techniques (BAT) Reference Document for Common Waste water and Waste gas Treatment/Management Systems in the Chemical Sector (2016) reported an overview of end-of-pipe odor treatment techniques. This document reported the advantages of biofiltration, including (i) low shift of pollution to any other media, (ii) few chemical agents added, and (iii) low energy consumption. Moreover, it also suggested the combination of biofiltration and bioscrubbing since the bioscrubber may act as a humidifier and degrade a high portion of the odorous load. In a biofilter system (Figure 3a), the odorants are forced through a packed bed (compost, peat, bark or a mixture of these) on which the microorganisms are attached as a biofilm. The pollutants are absorbed by the filter material and degraded by the biofilm.
In BTF (Figure 3b), the odorous gas is forced through a packed bed filled with a chemically inert carrier material that is colonized by microorganisms, similar to trickling filters in wastewater treatment. The liquid medium is recirculated over the packed bed and the pollutants are first taken up by the biofilm on the carrier material and then degraded by the microorganisms. The liquid medium can be recirculated continuously or discontinuously and in a co-or countercurrent to the gas stream. Flow directions will not affect the efficiency of the process.
In BS (Figure 3c), the pollutant is adsorbed in an aqueous phase in an absorption tower then converted by the active microorganisms into CO 2 , H 2 O and biomass in a separate activated sludge unit. The effluent is circulated over the absorption tower in a co-or countercurrent direction to the gas stream.
Physical/chemical technologies consist of two types of reactors, namely adsorption systems and chemical scrubbing. Adsorption systems (Figure 3d) generally consist of static beds of granular materials in vertical cylindrical columns. Among purification methods, adsorption is simple and easy to apply to real-scale wastewater treatment plants [59]. Several sorbents have been studied, including fly ash, carbon, activated carbon, polymers, carboncoated polymers, ceramics, micro-and mesoporous materials, metal-organic frameworks, natural zeolites, and synthetic zeolites. Chemical scrubbers (Figure 3e) are among the most mature abatement techniques employed in WWTPs due to the extensive experience and high robustness as well as the short gas retention time (as low as 1-2.5 s). The most common configuration ( Figure 3) is a vertical shell with gas flow going up through packing and the liquid solution (depending on the target compounds) going down. The liquid solution is usually circulated over the packing by pumping from a collection sump in the bottom of the tower, while chemicals are added either in the sump or in the recirculation piping.
To the best of our knowledge, the current work is the first review paper to analyze and compare more than 50 full-scale odor treatment technologies (chemical/physical and biological) applied in WWTPs. The main characteristics of full-scale OTTs found in the scientific literature are reported and critically analyzed for each treatment method.

Biofilter
Different studies [60][61][62][63] reported H 2 S and NH 3 as the main pollutants removed by the biofilters (BFs). In the Subiaco Wastewater Treatment Plant (Western Australia, the waste gas flowrate of 65,000 m 3 h −1 , 75 ppm H 2 S and 5 ppm NH 3 ), a biofilter installed after the acid scrubber to promote the formation of a biofilm for H 2 S removal, was then moved to the inlet of the scrubber to treat H 2 S and NH 3 mixtures [63]. BFs were also used to treat odors from the sludge thickeners, effluent channel and influent splitter box at the Mill Creek WWTP of the Metropolitan Sewer District of Greater Cincinnati [64] and in Shandong, China with PU packing materials [65]. The REs, in terms of H 2 S and NH 3 concentrations, to be higher than 90%. The removal yields thus reduced odor emissions to under detection limits. Compared to scrubber operations which entail using of acid/alkali as scrubbing media, BFs can provide less negative environmental impacts because water is added instead of chemicals and small amounts of leachate are produced. However, the capital and operating costs must require further investigation to consider this target. High concentrations of H 2 S were detected at pumping stations in the WWTP of the City of Birmingham (Alabama), at a WWTP of South Walton (Florida) and at Etaples-Le Touquet's WWTP (Artois-Picardie Region, France) [66]. The H 2 S levels fluctuations ranged between 4-26 ppm. A total of six BF units were installed at the Birmingham WWTP (waste gas flowrate of 51,000 m −3 h −1 ), while BF with inorganic bed media was utilized in Le Touquet's WWTP. REs higher than 99% were obtained by utilizing the biofilters. Owing to the significant waste gas volume to treat and considering that these sites were mostly located in sensitive areas, even a slight exceeding of the threshold limits due to accidental leaks may be annoying and, consequently, strict monitoring is required also using a dispersion model [67], and/or, multiple BF units in series can be installed to increase the treatment efficacy [68].
Some papers dealt with the use of different packing materials to enhance biofiltration in municipal WWTP including peat [69] (Charguia, Tunisia with inlet H 2 S concentrations ranging between 200-1300 mg m −3 ), seashells [70] (Lake Wildwood WWTP, California with 55,200 L h −1 of wastewater flowrate and air flowrate of 28,300 L min −1 ), polyurethane foam [65] (Shandong, China, H 2 S, NH 3 and VOC inlet concentrations were 0.5-28.4, 0.9-34.3 and 0-0.9 mg m −3 , respectively), advanced biofiltration with organic and inorganic phase in the medium [71] (Mallorca, Spain with air flow rate of 15,000 m 3 h −1 ), packed waste straw and cortex [72] (refinery WWTP in Shanghai, China). Using the modified packing materials, biofiltration was demonstrated to be an optimum OTT by having 90-99% RE. The goal of the authors was to provide a nutrient-rich environment for the bacteria in the packing material, which may increase the efficiency of the process. However, the efficiencies were dependent on the different operating conditions since the packing materials are sensitive to shock loadings. In a real case scenario, the H 2 S inlet loads fluctuate, and sometimes, the loading rates overcome the microbial activity capacities. This scenario is challenging because the microbial population in the medium must be enough and must not be as a limiting factor [32]. Moreover, some articles assess removal yields in terms of odor concentrations measured with dynamic olfactometry in OU E m −3 [71,73]. In Harnaschpolder WWTP, a full-scale biofilter (headworks and the sludge handling units air flowrate: 60,000 m 3 h −1 and activated sludge including aerobic and anaerobic tanks air flowrate: 70-100,000 m 3 h −1 ) is applied [73], while in Middelfart's municipal waste water treatment plant (Norway), a BF is implemented in order to treat 1500 m 3 h −1 of odorous emissions from headworks and primary treatment areas [71]. Both BFs have performance higher than 96% RE. BFs were able to withstand an acidic environment without adding NaOH or NaOCl. Even though the filter must be periodically replaced and there are savings in chemical consumption, this phenomenon can bring to the production of high amount of acid leachate that might be difficult to dispose of.
Evaluating the studies, the type of packing material influenced the efficiency of biofilter, as well as other parameters such as pH and moisture content. Heterotopic bacteria are the dominant microorganisms. Moisture levels in the packing materials must be maintained only at the ideal point because, at low levels, the microbial activity might decrease, while at high levels, anaerobic zones can be present and decrease the amount of oxygen for biological activity, affecting OTT's performances. The bed must be continuously aerated to avoid anaerobic conditions. Table 4 summarizes the mean removal efficiencies of malodorous compounds using plant-scale biofilters.

Biotrickling Filter
Kasperczyk et al. [38] tested a semi-industrial scale biotrickling filter in a WWTP in Poznań (Poland) for the treatment of odor in the exhaust air with 440 ppm v H 2 S and 240 ppm v VOCs at maximum. The authors used biocatalysts such as Pseudomonas fluorescens bacteria and bacterial strains Thiobacillus sp. to promote the formation of the BTF's biofilm to metabolize the odorants. Yang et al. [16] studied biotrickling filters in a chemical fiber WWTP at both lab-and pilot-scale to degrade TVOCs. At the laboratory scale, the degradation seemed to be due to the combination of adsorption and biological reactions (i.e., 90% RE on the fourth day and a declined during the fifth to eighth day). However, in the pilot-scale WWTP, RE was affected by the EBRT, since REs decreased by more than 40% when the EBRT was reduced to 32 s. This condition might be due to the scale-up of the BTF. Furthermore, Wu et al. [77] achieved 95% of RE in a pilot-scale BTF in a Singapore WWTP, while Cox et al. [78] obtained 98% of RE for H 2 S and VOCs at the Hyperion WWTP in Los Angeles, (California). Chen et al. [79] achieved RE of 96% in BTF using activated carbon-loaded polyurethane packing materials to remove H 2 S in the upper layer and modified organism-suspended fillers in the lower layer, with EBRTs lower than 1 min.
The BTFs in the investigated studies [9,44,75,76] demonstrated high efficiencies (higher than 85% of RE). Guerrero and Bevilaqua [80] evaluated the performance of a BTF to treat H 2 S emissions from a UASB reactor. Only 50.9% of RE for H 2 S was obtained in the experiment, carried out on a real case scenario (brewery WWTP) with EBRT of 1.6 min and, thus, values were higher than in other studies. This condition in the scenario might be due to the type of microorganisms in the packing materials utilized. These were an autotrophic H 2 S-degrading culture obtained from the anaerobic sludge of the UASB reactor of the WWTP, sensitive to a temperature lower than 29 • C.
Biotrickling filters are capable of treating high inlet loads compared to other OTTs, but their efficacy is still strongly dependent on the type of packing material. In fact, the study of Lakey [81], reported that a BF in the WWTP of Perth (Australia) with an inlet air flowrate of 79,000 m −3 h −1 achieved a H 2 S RE of 99.5% and a VOCs RE of 95%. The replacement of chemical scrubbers with BTFs can be thus considered economically viable since the theoretical consumption of need chemicals for the absorption and oxidation of both H 2 S and VOCs [82].
Plastic fibers (i.e., polyurethane foams) are preferred in some studies to enhance the BTFs' removal performances [65,83]. In terms of operation, the BTF requires relatively low power, since only the pumping phase requires energy and an aeration blower is not needed. Moreover, less sludge is produced than by suspended-growth systems. Despite the high manufacturing costs of this technology, their life span is longer than ordinary packing material. On the other hand, the clogging incidence is expected and the packing material's porosity has to be periodically maintained by back-washing. The generated sludge needs further treatment and disposal and the final effluent must be treated in the WWTP [83]. Table 5 summarizes the mean removal efficiencies of malodorous compounds using pilot plant-scale application of bio-trickling filters. Note: Unit for EC: g m −3 h −1 ; unit for odor concentration: OU E m −3 ; unit for volumetric air flowrate: m 3 h −1 . WWTP types: (') municipal WWTP, ('') chemical fiber,) ('*') tannery WWTP, (''*) brewery WWTP.

Scrubber System
Baawain et al. [45] reported the application of a chemical wet scrubber (with two identical parallel-train cross-flow systems) as OTT in Al-Ansab WWTP (Oman), with wastewater flowrate of 2300 m −3 h −1 , waste gas flowrate of 160,000 m 3 h −1 and H 2 S inlet concentrations of 65-170 ppm. The removal efficiency ranged between 80 and 96%, but declined to 67% during the maintenance period. Meanwhile, some papers investigated the usage of oxidants in the scrubbing medium to enhance wet scrubbing efficiency. For example, Kerc and Olmez [92] analyzed different scrubbing compounds (i.e., water, ozonated water, caustic and ozone injected caustic) to remove H 2 S in Tuzla WWTP (Istanbul, Turkey) in which 99% RE was achieved using caustic scrubbing and ozonation, while Yang et al. [93] utilized peroxymonofulfate (PMS) as an oxidant for odor reduction (e.g., methyl mercaptan, CH 3 SH(G)) in a wet scrubbing process. Furthermore, in Orange County Sanitation District, California, Zhou et al. [56] used both chemicals and bioscrubbers in one plant (headworks and primary treatment) and another (headworks), respectively, while Biard et al. [94] investigated a conventional chemical scrubber to treat H 2 S using NaOH and NaOCl solution.
Zhou et al. [56] revealed that chemical scrubbers and biofilters performed best among other odor control technologies (OCTs), while Kerc and Olmez [92] offered ozonation as an effective scrubbing enhancement. However, the cost of installation and complexity of the operation must be taken into account. To accelerate the mass transfer of gas pollutant to a liquid solution, Yang et al. [93] showed that synthetic oxidants can be applied. The approach of Kerc and Olmez [92] and Yang et al. [93] offered a promising technique to enhance the efficiency of wet scrubbing, but the production of byproducts in the liquid solution has to be further investigated.
Wet treatment techniques such as scrubbers in odor control are mostly applied because the gaseous pollutant can be dissolved in liquid phase and temporarily stabilized for further treatment [5]. Chemical scrubbers have the ability to deal with a wide range of gas pollutants from sulfur to acidic gases and can tolerate fluctuating temperatures, which is ideal for operation in almost any environment. However, they require periodic maintenance and suffer from corrosion due to chemical attacks. Table 6 summarizes the mean removal efficiency of malodorous compounds using pilot plant-scale application by scrubber.

Combined OTT
Integrated OTTs designs are implemented to address situations in which different typologies of odor compounds or high inlet loads are present. These cases are usually detected in refineries where high odorant concentrations, mainly BTEX, are present and, thus, a combination of different OTTs is [29,95,96]. Rada et al. [97] utilized a bioscrubber, two biotrickling filters and a biofilter with an overall RE higher than 70% to remove benzene (C 6 H 6 ), while Torretta et al. [98] implemented water scrubbing followed by biofilter (Italy) with an overall RE of almost 95% (benzene inlet concentration of 12.4 mg m −3 , benzene outlet concentration of 1.02 mg m −3 , toluene inlet concentration of 11.1 mg m −3 , toluene outlet concentration of 0.25 mg m −3 , ethylbenzene inlet concentration of in: 2.7 mg m −3 , ethylbenzene outlet concentration of 0.32 mg m −3 , xylene inlet concentration of 9.5 mg m −3 , xylene outlet concentration of 0.26 mg m −3 ). Another study of Raboni et al. [29] implemented water scrubbing as pretreatment, followed by a biotrickling filter and a biofilter (inlet air flowrate of 600 m 3 h −1 , Refinery WWTP in Milan, Italy) with an overall RE of 96%, while Zhou et al. [56] used bioscrubbers and biotrickling filters at the headworks and primary treatment units respectively, with a RE of 50-70% in terms of odour removal.
Torretta et al. [98] and Raboni et al. [29] utilized water scrubbing without adding chemicals (i.e., NaOH or NaOCl) with low REs (lower than 50% of total BTEX removal) since BTEX have moderate solubility in water. However, this condition might lead to the fact that the leachate is less dangerous than using chemicals and the lifespan of the wet scrubber is higher due to fewer corrosion problems. Biological methods (i.e., biofilters) can be regarded as polishing techniques or can be installed in points where the odor threshold is low (i.e., headworks) [73]. Lafita et al. [73] converted chemical scrubbers with biofilters to biotrickling filters (air flowrate of 2000-3500 m 3 h −1 ) with 95% RE in terms of H 2 S removal at Hoogheemraadschap van Delfland WWTP (Netherlands). Municipal WWTPs have lower loads of sulfide and VOCs compared to refineries. Consequently, Martinez et al. [99] and Jones et al. [76] utilized the combination of a biotrickling filter and a biofilter to treat H 2 S and VOCs (>91.00% RE of H 2 S and >74.00% for VOCs) in real urban WWTPs. The biological systems successfully removed low concentrations of VOCs in the presence of highly fluctuating H 2 S concentrations, but chemical scrubbing still needed pretreatment in heavy industries (i.e., refineries) that are characterized by high levels of odorous gases. Although chemical scrubbing is complex in terms of NaOH handling and material corrosion, biofilters' efficiency may be affected by the pressure drops due to compaction, water retention and excessive microbial growth that may cause clogging.
Other conventional methods are still used by some research such as air stripping [100] and carbon adsorption (at bioscrubber outlet) [84,101]. Finke et al. [101] managed odor emissions by a bioscrubber followed by four activate carbon filters (air flowrate of 52,000 m 3 h −1 ) in Merrimac WWTP (Gold Coast, Australia) with a 99.5% RE for VOCs, while a combination of absorption and a bioscrubber with 99% RE for H 2 S was achieved by Hansen and Rindel [102] in a WWTP in Copenhagen (Denmark) (inlet flowrate of 6000 m 3 h −1 ). Behnami et al. [100] implemented a steam stripping technique which has been demonstrated as an effective solution for the pretreatment of the waste gas prior to biofiltration in a WWTP in East Azerbaijan (Iran) with a flowrate of 4800 m 3 d −1 . The method was able to achieve a higher removal of VOCs. Further research must be carried out for H 2 S loads fluctuation. Table 7 summarizes the mean removal efficiency of malodorous compounds using pilot plant-scale applications with a combination of different OTTs.

Photo-Bioreactor Based on Algae-Bacteria Synergism
Environmentally friendly technology for the abatement of all types of emissions coming from plants are necessary to achieve the 17 sustainable development goals (SDGs) of the United Nations [104]. Biotechnologies have gained popularity thanks to the improvements driven by scientists. They contribute to the development of more robust and cost-effective biotechnologies. Algae-based technologies use low-cost materials and are proven to be effective at laboratory scale as odor control processes in WWTPs [104]. The synergism between algae and bacteria biodegrades H 2 S and VOCs while CO 2 biofixation occurs in open and closed photobioreactors has been studied and proved [30,31,105]. Biotechnologies used to treat odorous compounds released in the atmosphere [105]. Algal-bacteria photo-bioreactors could be an optimum choice due to the simultaneous treatment of odor compounds (e.g., VOC and H 2 S) contained in waste gas and the capture of CO 2 [106]. Table 8 reports data from the scientific literature that prove the applicability of algalbacteria photobioreactors for the biodegradation of contaminants emitted in the atmosphere from the wastewater treatments process. Moreover, due to the photosynthetic activities of microalgae, CO 2 biofixation is possible, while odorants (e.g., H 2 S and VOCs) are oxidized. The algal biomass generated could be used to produce valuable byproducts (e.g., biofuel, fertilizers, pharmaceuticals, biopolymers, etc.) [110,111]. Even though it has been studied at a laboratory scale and demonstrated good efficiency in terms of oxidation of odorant compounds (e.g., VOCs and H 2 S), a scaled-up analysis is needed for the evaluation of the robustness at full-scale application on a WWTPs with a real mixture of odorants.

Sensitive Analysis
The empty-bed residence time (EBRT), removal efficiency (RE), elimination capacity (EC) and odor reduction of different OTTs were compared.
The empty bed residence time (EBRT) is defined as the contact time between the gaseous emissions and the biofilter media. EBRT is considered one of the principal operational parameters that influence the performance of gaseous compounds treatment technologies, particularly when hydrophobic odor compounds such as VOCs are involved [112]. Figure 4 presents the average OTTs' EBRT organized for the different full-scale technologies examined. Biofilters are operated at an average EBRT of 48 ± 30 s, which is considered the highest among odor treatment technologies, depending on the type of packing material and the contaminant inlet load. On the other hand, the bioscrubbers showed the lowest EBRT (7.5 ± 2.5 s). Biotrickling filters and the chemical scrubbers showed an EBRT of 22 [112]. Figure 4 presents the average OTTs' EBRT organized for the different full-scale technologies examined. Biofilters are operated at an average EBRT of 48 ± 30 s, which is considered the highest among odor treatment technologies, depending on the type of packing material and the contaminant inlet load. On the other hand, the bioscrubbers showed the lowest EBRT (7.5 ± 2.5 s). Biotrickling filters and the chemical scrubbers showed an EBRT of 22.2 ± 26.2 s and 20 ± 8.1 s, respectively.     Limited results are reported in terms of OU E m −3 for the evaluation of OTT performance and some of them reported results only in terms of RE of odors. As reported in Table 11, biofilter and biotrickling exhibit an average RE in terms of odor equal to 97.7 ± 1.9% and 87.7 ± 15.6%, respectively. The bioscrubber and the chemical scrubber demonstrated an RE of 89 ± 9% and 70 ± 0%.

Cost Analysis
The operational and investment costs of the full-scale OTT assessed are reported in Table 12. High chemical and water requirements are necessary for the absorption of odorants in chemical scrubbers; thus, it is less sustainable than others even with competitive investment costs. Adsorption systems have a lower investment cost per unit flow rate (5-12 EUR m −3 h −1 ), but a very high operating cost (10-200 EUR m −3 h −1 ) compared to the other technologies because of the periodic replacement of adsorptive material. Biofilters generally need more land than other options; however, its low investment (6-15 EUR m −3 h −1 ) and operating costs (2-4 EUR m −3 h −1 ) ensures a cost-effective technology. The investment cost (8-28 EUR m −3 h −1 ) of BTFs is mainly related to the packing material used in the design (e.g., inorganic salts, polyurethane foam, activated carbon fibers, multisurface hollow balls etc.). However, the competitive operating cost (3-6 EUR m −3 h −1 ) and the capability to treat high-load odorants ensure BTF as one of the most diffused technologies. Bioscrubbers, due to the high investment cost (10-32 EUR m −3 h −1 ) and lower robustness at high loading rates, are not widely implemented for the treatment of malodorous emissions coming from WWTPs. The wide variation of the investment and operating costs reported in Table 9 depends on several factors such as flow rate (investment cost per m −3 h −1 decrease with increasing the working flow rate), EBRT (increasing EBRT significantly increases the investment and operating costs, especially in biofilters and BTF), packing material (in adsorption systems depending on the type of packing material) and odorant load.
Several obsolete chemical scrubbers applied in WWTPs have been upgraded to biological systems. Gabriel and Deshusses [116] developed a general procedure for the conversion of chemical scrubber to BTF, successfully showing a reduction of operating cost.

Future Perspective
Several pilot-scale applications of OTTs in WWTPs were critically examined. The greater importance of treating odors, key atmospheric pollutants in the urban water cycle, has boosted advancements in full-scale technologies for odor removal. Chemical/physical systems were developed and gradually replaced by low-cost and environmental friendly biologically based processes. Biofilters dominate among conventional odor treatment applications, but more sophisticated types of biotechnologies such as biotrickling filters and bioscrubbers have gained attention in real case applications. The data reported in Tables 4-7 confirmed that the biotrickling filter is one of the most reliable technologies due to the efficiency of treating VOCs, H 2 S and NH 3 to reduce odor emissions from WWTPs. These results also confirmed BTFs' moderate investment and operational costs and lower land requirements than biofilters.
Algal-bacteria-based processes are emerging as a promising solution to convert the traditional biotechnologies implemented to control odorous emissions in WWTPs, with a high-potential hybrid configuration of photo-bioreactors and a membrane. Algal-based biotechnologies have indeed been confirmed as effective solutions to increase the sustainability of the management of odor treatment facilities in the urban water cycle. These aforementioned innovative solutions promise to be a turning point for environmentally friendly development and the circular economy when applied at real scale wastewater treatment plants. The use of algae biomass for the production of valuable bioproducts, such as biofuels opens a new prospect of converting WWTPs into green biorefineries.