A Wet Scrubber and Electrooxidation System for the Efficient Removal of Odor: A Bench-Scale Study

Abstract

Odor emissions are a crucial component of atmospheric pollution. As odor is a sensory pollutant, its management and treatment are recalcitrant. A wet scrubber (WS) is an efficient technique for odor removal, but disposal of waste liquid discharge leads to secondary pollution and CO₂ emissions during transportation. In this study, a system consisting of WS and electrooxidation (EO) was developed and installed in a swine manure fermentation facility. The absorption and EO characteristics were estimated through the practical implementation of a bench-scale WS (BSW). For EO, a dimensionally stable anode and Cl⁻ were applied. When the BSW was operated without EO, an L/G ratio of 8.88 was essential for securing the simultaneous removal rate of the four odorants (hydrogen sulfide, methyl mercaptan, ammonia, and total volatile compound). With the operation of the EO, the period to change the liquid based on equilibrium was postponed due to the continuous oxidation of the odorants absorbed in the liquid. As the applied current increased, the change period was further prolonged. However, the oxidation and absorption rates differed depending on the odor substances, due to differences in their physicochemical characteristics. Hydrogen sulfide and methyl mercaptan exhibited similar absorption and oxidation rates. Ammonia had a high absorption rate and a low oxidation rate. The acetaldehyde oxidation rate was the most sluggish among the substances. These findings demonstrate that simultaneous consideration of Henry’s constant and the reactivity of the target pollutant with HOCl renders the design of BSW appropriate for treating odor gases containing various odorants. This study contributes to efforts to address environmental problems concerning odors and also to global climate threats.

1. Introduction

Odor, a sensory pollutant, causes considerable problems, including olfactory nuisance, annoyance, stress, and emotional health threats [1,2]. In addition to negative influences on residents living adjacent to odor-emitting facilities, odor compounds are associated with air pollution, such as photochemical smog, acid rain, and secondary particulate emissions [3]. Due to the increase in civil complaints, regulations that strain odor emissions from industrial or environmental installations are being strengthened to alleviate the risk of odors [4].

Over the past decades, various conventional technologies have been developed and applied to efficiently remove odors [5]. Among these technologies, wet scrubber (WS) is one of the most effective and widely applied systems due to its low cost, ease of handling, and capacity for simultaneous removal of various substances [6,7]. WS, in particular, has a superior ability to remove odorous substances with high water solubility and polarity, such as ammonia, hydrogen sulfide, and organic acids. Due to its robust performance, it is often employed to mitigate odor emissions [8]. However, the scrubbing liquid used for a given period must be changed to meet odor discharge standards [9]. Chemical absorption processes have been used to improve the removal rate and capacity of WS. Nonetheless, chemical absorption periodically replaces water and requires the addition of chemicals. Additionally, chemical absorption must be further improved because of the large scale of reactors and their high operating costs [7].

Waste liquid discharged from WS contains various contaminants that require physicochemical treatment before safe discharge into nature [10]. Due to economic and efficiency issues, the waste liquid is transferred to a consignment company rather than having its own treatment facility. In this regard, the transportation of the liquid from a production area to a treatment facility is crucial, and unnecessary costs and CO₂ emissions are incurred in this process. CO₂ emissions depend on transport distance. Ulrich et al. have reported that the CO₂ emission rate for tank lorry is 1.33 kgCO₂e/km [11]. According to the European Chemical Transport Association, trucks emit approximately 62 gCO₂e/ton/km in road transport mode [12]. Therefore, the reduction in waste liquid discharged from WS would allow CO₂ emission reduction. In addition to CO₂ emissions, fresh water issues are also associated with WS. The world is suffering from water shortages and water quality degradation due to pollution, climate change, and population growth. The desalination process producing fresh water consumes a large amount of energy and accelerates climate change. Although many efforts have been made to overcome water shortages, the risk of water shortages continues to worsen over time [13,14].

Traditional chemical absorption techniques were used to enhance the absorption capacity and efficacy of WS. Liu et al. injected hypochlorous acid to remove NO_x, which is insoluble in water [15]. Yan et al. showed that ozone injection enhanced the simultaneous removal efficiency of SO₂ and NO_x from diesel exhaust gas [16]. Lee et al. used ozone and potassium iodide to treat NO₂ and SO₂ [17]. Although chemical injection offers a higher removal efficiency, it has some drawbacks, such as the cost of chemicals, secondary wastewater generation, and the complicated determination of the appropriate dosage. To evade the external injection of chemicals, electro-oxidation (EO) is a possible alternative. EO has attracted considerable attention for wastewater treatment due to its outstanding performance, fast oxidation, and ease of operation. Pollution substrates in water are oxidized through both direct and indirect oxidation, depending on the characteristics of the anode, supporting electrolyte, and pH. In direct electro-oxidation, the reactant is directly oxidized at the anode, whereas in indirect electro-oxidation, an intermediate oxidizes the reactants [18]. If Cl⁻ exists in water as a supporting electrolyte, reactive chlorine species (RCSs), which are strong oxidants, are electrochemically generated. The dominant type of RCSs depends on pH, as described below [19]:

2Cl⁻ → Cl₂ + 2e⁻

(1)

Cl₂ + H₂O → HOCl + Cl⁻ + H⁺ (pKa = 2.0)

(2)

HOCl → H⁺ + OCl⁻ (pKa = 7.54)

(3)

Considering that the absorption capacity is governed by the solubility of the solvent and the present concentration, changing the period of liquid in WS to maintain removal efficiency can be prolonged by means of the EO of dissolved odorants.

In this study, a coupled WS and EO system was developed and applied to a swine manure fermentation facility in Gongju-si, Korea, to remove odor gases. The odor gas emitted from the fermentation room was collected and introduced into the WS. The odor removal rate from the gas stream was estimated at various electrolyte concentrations and compared with and without EO. The proper period for changing the scrubbing water was verified based on odor removal efficacy. This approach is expected to be an economical and efficient method for reducing CO₂ emissions in odor treatment technology.

2. Materials and Methods

2.1. Bench-Scale BSW

BSW was used to evaluate the deodorization efficacy and estimate the change period for the scrubbing liquid in a fermentation facility at Gongju-si, Korea. The facility treats 200 tons of swine manure daily via aerobic fermentation through both aeration and trickling in the atmosphere. The facility is far from residential areas, and a nasty and intensive odor is emitted into the surroundings. The BWS was installed next to the aeration tank.

Figure 1a shows an image of the BSW installed in the fermentation facility, and Figure 1b describes the schematic diagram. The BSW consists of three major parts: a scrubber, an EO reactor, and a control panel. The scrubber was manufactured using a PVC (poly vinyl chloride) pipe with an internal diameter of 0.5 m and a length of 2.3 m. A two-stage medium layer (height = 0.3 m) was installed inside the pipe. The media used was 1-inch polling made of polypropylene material (P-series 25, The Pall Rings Company, King’s Lynn, UK). The liquid for scrubbing was sprayed using a magnetic gear pump (EMG-5000, EMS Tech, Yongin, Republic of Korea) and a hydro jet nozzle. The liquid flow rate was regulated using a ball-type flow meter (VFC-151, Dwyer, Michigan, IN, USA). Ambient air in the fermentation facility was collected and introduced without further pretreatment. The airflow rate was quantified by substituting the cross-sectional area and gas velocity measured using a digital anemometer (testo 440 100 mm Vane, Testo Ltd., Alton, UK). The gas velocity was adjusted using a ring blower and current source inverter (SV008iG5A-1, LS Electric, Anyang, Republic of Korea). The liquid-to-gas ratio (L/G), which is a crucial parameter in WS, was calculated using Equation (4):

$$\mathrm{L}/\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o} (\mathrm{L}\mathrm{i}\mathrm{t}\mathrm{e}\mathrm{r}/{\mathrm{m}}^3)=\frac{\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d} \mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} (\mathrm{L}/\mathrm{m}\mathrm{i}\mathrm{n})}{\mathrm{g}\mathrm{a}\mathrm{s} \mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} ({\mathrm{m}}^3/\mathrm{m}\mathrm{i}\mathrm{n})}$$

(4)

The gas retention time was calculated using Equation (5):

$$\mathrm{G}\mathrm{a}\mathrm{s} \mathrm{r}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}(\mathrm{s})=\frac{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m} \mathrm{l}\mathrm{a}\mathrm{y}\mathrm{e}\mathrm{r} \left(\mathrm{L}\right)}{\mathrm{A}\mathrm{i}\mathrm{r} \mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} (\mathrm{L}/\mathrm{s})}$$

(5)

Figure 1. BSW at the fermentation facility: (a) a picture after installation, and (b) schematic diagram.

Figure 1. BSW at the fermentation facility: (a) a picture after installation, and (b) schematic diagram.

The EO reactor was fabricated using rectangular stainless steel (SS304) with a length of 300 mm, width of 150 mm, and depth of 160 mm, corresponding to a volume of 7.2 L. The liquid that dropped from the medium was collected in a reservoir located at the bottom of the scrubber and overflowed into the EO reactor. A commercial dimensionally stable anode (DSA) was purchased from a manufacturer (Sungwon Foaming Inc., Ansan, Republic of Korea) and used as the anode, while a stainless-steel plate (SS304) was utilized as the cathode. The anodes and cathodes had a rectangular shape with a width of 150 mm, a depth of 160 mm, and a thickness of 1 mm. Seven anode and eight cathode plates were arrayed 10 mm apart. The EO was conducted in the galvanostatic mode at applied currents of 15 A and 30 A using a direct current power supply (TDP-3020B, TOYOTECH, Incheon, Republic of Korea). NaCl was added to tap water at a given concentration as a supporting electrolyte.

2.2. Lab-Scale EO Reactor

To identify the EO characteristics of various odorant mixtures over DSA and Cl⁻, the electrochemical reactor was operated. A rectangular electrochemical oxidation reactor with dimensions of 150 mm (width) × 90 mm (length) × 155 mm (height) was used for the EO experiments. The two odorants were mixed in distilled water with 1 mM of each odor substance, and 10 mM Cl⁻ was added simultaneously. Ammonia, hydrogen sulfide, methyl mercaptan, and acetaldehyde were selected as representative malodorous substances. To make a surrogate odorants mixture, Na₂S (98%, Daejung, Siheung, Republic of Korea), CH₃SNa (95%>, Sigma-Aldrich, Burlington, MA, USA), NH₄Cl (99.5%, Daejung), and acetaldehyde (90%, Daejung) were dissolved in distilled water. Two anodes and one cathode were installed, and the distance between them was set at 5 mm. The electrode plates had the following dimensions: height of 100 mm, width of 60 mm, and thickness of 1 mm. A direct current (DC) was applied to the electrodes using a DC power supply (TDP-3020B, TOYOTECH, Incheon, Republic of Korea). The current density was 50 mA/cm².

2.3. Analytical Methods

The concentrations of the odorants in the air were quantified using a portable instrument (MultiRAE Pro PGM-6248; RAE Systems, San Jose, CA, USA). The electrochemical detectors were installed for ammonia (0.1–100 ppmv), hydrogen sulfide (0.1–100 ppmv), and methyl mercaptan (0.1–10 ppmv), while the photoionization detector (0.01–2000 ppmv) was positioned for total volatile organic compound (TVOC) in the instrument.

The aqueous concentration of ammonia was measured using an analytical kit (Hach, Loveland, CO, USA) that applies the salicylate colorimetric method. The concentration of hydrogen sulfide in the liquid phase was quantified using an H₂S analyzer (ECH, Halle, Germany). Methyl mercaptan and acetaldehyde were analyzed by gas chromatography-mass spectrometry (GC-MS, QP2050, Shimadzu, Kyoto, Japan).

3. Results

3.1. Preliminary Test of BSW

Before conducting an experiment on the deodorization performance of electrochemical oxidation, a preliminary test was conducted to identify an appropriate amount of odor gas for securing odor removal efficacy through scrubbing only. The water flow rate was fixed at 16 L/min due to the capacity limitation of the water pump. The gas flow rates were 1.8, 3.6, and 7.2 m³/min, respectively. The influent concentrations of the four odorants (NH₃, CH₃SH, H₂S, and TVOC) were monitored. The influent concentration was measured prior to the start of the experiment. When the effluent concentration approached the influent concentration, it was presumed that scrubbing water had reached a breakthrough point. The liquid/gas flow rate ratios (L/G) and gas retention times (GRT) corresponding to the gas flow rates have been listed in

Table 1. L/G ratio and GRT depending on set of the gas flow rates.

Gas Flow Rate (m3/min)	L/G (L/m3)	GRT (s)
1.8	8.88	1.24
3.6	4.44	0.62
7.2	2.22	0.31

.

Figure 2 depicts the change in the effluent concentrations of the four odorants as operation time elapsed. At an L/G ratio of 2.22, the ammonia removal rate was greater than 90%, and no increase in the effluent concentration was observed at an influent concentration of 18 ppm. Contrastingly, methyl mercaptan reached an influent concentration of 10 ppm within 8 min. Even though the concentration after 8 min decreased to 7 ppm, this was due to a decrease in the influent concentration rather than an increase in the removal efficiency. The ambient concentration varied minute by minute due to variations in wind direction and velocity. Accordingly, the liquid reached the breakthrough point of methyl mercaptan. Hydrogen sulfide showed a removal tendency similar to that of methyl mercaptan. A removal rate of 70% for TVOC, compared to the influent concentration of 200 ppb, continued after 10 min, indicating that the absorption capacity was retained even after 10 min. In summary, hydrogen sulfide and methyl mercaptan reached equilibrium, whereas ammonia and TVOC did not.

The amount of substances dissolved in a gas has a constant value, which is defined by Henry’s law constant as follows:

C = H × P

(6)

where C is the concentration of the dissolved gas, H is the Henry’s law constant, and P is the partial pressure of the gas. According to Equation (6), a smaller Henry’s law constant leads to a lower equilibrium concentration in the liquid.

Table 2. Henry’s law constants for representative odorants.

Substrate	Henry’s Law Constant (mol/m3∙Pa)	Reference
Ammonia	5.9 × 10−1	[20]
Hydrogen sulfide	1.0 × 10−3	[20]
Methyl mercaptan	3.8 × 10−3	[21]
Acetaldehyde	1.3 × 10−1	[20]
Propionaldehyde	9.9 × 10−2	[22]
Acetic acid	4.0 × 101	[22]
Propionic acid	1.5 × 101	[22]

summarizes Henry’s law constants for the odorous substances monitored in this study. The Henry’s law constants of hydrogen sulfide and methyl mercaptan were nearly 100 times smaller than those of ammonia or organic substances. The inlet concentration of TVOC was 200 ppb. Considering that TVOC is the total concentration of each VOC substance, the inlet concentration of each organic odor substance was expected to be lower. Therefore, the scrubber liquid must be replaced when the concentrations of hydrogen sulfide and methyl mercaptan decrease.

As the L/G ratio increased, the removal efficiencies of the four odorants also increased. In a scrubber, the L/G ratio refers to the spraying rate of liquid droplets compared to the incoming gas flow rate. Therefore, an increase in the L/G ratio led to an increased probability of gas and liquid contact, thereby enhancing the removal efficiency. The applied L/G ratio varied depending on the inflow concentration of the target gas, air volume, size of the packed bed, type and density of the filler, and empty-bed retention time. Generally, the WS is operated within a range of 2–10 L/m³. Considering the removal efficiency, the appropriate L/G ratio for the WS used in this study was 8.88. Therefore, the entire experiment was conducted at an L/G ratio of 8.88.

In this study, the liquid spraying rate was fixed at 16 L/min, and the gas flow rate varied to change the L/G ratio from 2.22 to 8.88. The GRT increased with an increasing L/G ratio, indicating that the removal efficiency was influenced by the L/G ratio and GRT (Table 1). Therefore, this study aimed to examine changes in the liquid replacement cycle depending on the presence or absence of an EO and its additional effects. Changes in the mass transfer efficiency or absorption performance depending on the L/G ratio were not covered in the study.

3.2. Water Replacement Periods Test

The absorption capacity can be maintained by continuously removing absorbed odorous substances through the EO. In the EO process, the applied current governs the number of electrons participating in the chemical reaction and affects the direct and indirect oxidation efficiencies. Therefore, the odor removal efficiency was analyzed under applied currents of 15 and 30 A, respectively. Figure 3 shows the change in the odor concentration of the effluent gas according to the applied currents. The supporting electrolyte concentration was 10 mM NaCl.

Unlike the methyl mercaptan and hydrogen sulfide concentrations, which reached the breakthrough point after approximately 10 min when the EO was turned off, it took approximately 20 min to reach equilibrium regardless of the substance under the condition of 15 A. Furthermore, the breakthrough point was achieved after 40 min when the applied current was doubled to 30 A. Briefly, the time required to reach equilibrium was proportional to the applied current. The electrolysis device constructed in this study used a DSA electrode as the anode. DSA is suitable for chlorine generation, and hypochlorous acid is mainly generated by the electrochemical oxidation of chlorine ions. It is believed that hydrogen sulfide and methyl mercaptan are mainly oxidized by the generated hypochlorous acid. As mentioned earlier, the applied current is an important factor that affects the amount of hypochlorous acid produced and is proportional to the amount of chlorine produced. Therefore, when the applied current was doubled from 15 A to 30 A, the breakthrough time also doubled.

However, the breakthrough time and applied current were not directly proportional. Ammonia tended to increase in effluent concentration when current was applied. Hydrogen sulfide increased threefold when the applied current was 30 A compared to 15 A. Acetaldehyde tended to be proportional to the amount of the applied current. It is believed that this was not due to the efficiency of the EO but because the inlet concentration was varied, as described in chapter 3.2. At the site, the inlet concentration was not constant, and concentration variations occurred depending on the type of odorous substance. Particularly, the concentration of ammonia varied greatly, from a few ppm to several tens of ppm. Therefore, it is believed that the ammonia removal efficiency is affected by changes in the inlet concentration, regardless of EO operation. TVOC, which fluctuated less due to its low concentration compared to that of other odorants, was more influenced by the EO. Accordingly, the applied current had a relatively large effect on the TVOC removal efficiency. Hydrogen sulfide and methyl mercaptan exhibited similar inlet concentration ranges and removal trends.

In addition to the applied current, the concentration of electrolyte (Cl⁻) is a crucial design factor that affects the amount of oxidant produced. Increasing the concentration of the electrolyte increased the amount of HOCl produced, thereby improving the reaction rate in the reactor. However, more than a given concentration produces an excessive amount of oxidant that can be reduced at the cathode, which may cause a decrease in current efficiency. Accordingly, the changes in the breakthrough time of the liquid were verified through tests at various concentrations (10, 50, 100, and 200 mM). Figure 4 shows the relationship between electrolyte concentration and breakthrough time. The breakthrough time increased proportionally with the electrolyte concentration.

Generally, an increase in Cl⁻ concentration leads to an increase in HOCl generation. Therefore, the oxidation rate is enhanced. However, excessive HOCl, which does not react with odorants, diffuses to bulk. The HOCl in bulk can be reduced at the cathode, indicating a parasitic reaction that causes a decrease in current efficiency. This study showed a direct proportional tendency in the Cl⁻ concentration range of 10–200 mM. It must be attributed to the fact that HOCl generated by EO was consumed to remove odorants. This finding demonstrates that the absorption rate was faster than the oxidation rate.

3.3. Electro-Oxidation Characteristics of the Odorants Mixture

To evaluate the reactivity of the odorous substances absorbed by the RCS and determine how to design a BWS, the two odorous substances were mixed and electrochemically treated. The reaction characteristics were confirmed by observing the concentration changes over time. The pH ranged from 6 to 8 during the experiment. Thus, it was assumed that HOCl was the dominant oxidant that reacted with odorous substances. The following Equations (7)–(10) describe the oxidation paths [23,24,25].

RCHO + HOCl → RCOOH + HCl

(7)

HS⁻ + HOCl → S_{(precipitate)} + H₂O + Cl⁻

(8)

NH₄⁺ + HOCl → NH₂Cl + H₂O + H⁺

(9)

2RSH + Cl₂ → RSSR + 2HCl

(10)

As shown in Figure 5a, hydrogen sulfide exhibited a faster removal rate than acetaldehyde. When methyl mercaptan and acetaldehyde were mixed, methyl mercaptan showed a faster removal rate than acetaldehyde (Figure 5b). Mixtures of hydrogen sulfide or methyl mercaptan with ammonia showed that sulfur-based odorous substances were removed faster than ammonia (Figure 5c,d). However, the removal rates were similar in the presence of hydrogen sulfide and methyl mercaptan. When ammonia and acetaldehyde were mixed, ammonia showed a faster removal rate. Therefore, the reactivity of the four odorous substances with HOCl followed the order H₂S = CH₃SH > NH₃ > CH₃CHO.

As seen in Figure 5c,d, the concentration of ammonia remained unchanged until the concentrations of hydrogen sulfide or methyl mercaptan were completely removed. The concentration of ammonia began to decrease only after the sulfur-based odorous substances were completely removed. In an earlier study, it was confirmed that the reaction characteristics were due to the different Gibbs free energies for the oxidation of hydrogen sulfide and ammonia [21]. Additionally, considering that methyl mercaptan shows removal characteristics similar to those of hydrogen sulfide, it must be considered that the generated HOCl reacts first with methyl mercaptan.

Another factor that influences the reaction rate is bond dissociation energy. The energies required for bond dissociation for the four odorants are listed in

Table 3. Dissociation energy list for the various bonds related to the odorants monitored in this study [26].

Substrate	Bond	Dissociation Energy (kJ/mol)
Acetaldehyde	C-C	350
	C-H	410
	C=O	732
Hydrogen sulfide	S-H	340
Methyl mercaptan	C-S	260
	S-H	340
Ammonia	N-H	390

[26]. The bond with the highest dissociation energy was the C=O bond, which required approximately 732 kJ/mol. The C-H bond has the next highest value at 410 kJ/mol. However, the C-C bond of acetaldehyde and the S-H bonds of hydrogen sulfide and methyl mercaptan showed similar dissociation energies of 350 and 340 kJ/mol, respectively. Contrastingly, the N-H bond of ammonia has a high dissociation energy of 390 kJ/mol. Specifically, the N-H bond must be broken to decompose ammonia, which consumes more energy than other bonds. These results demonstrate that HOCl attacks and removes bonds that are relatively easily broken. The chemical characteristics of HOCl also affect its odor oxidation rate. The sulfur and carbonyl carbons are electrophiles [27,28]. Contrastingly, HOCl and ammonia are nucleophiles [29,30]. The thiol group (S-H) of hydrogen sulfide, or methyl mercaptan, is easily broken by the nucleophilic attack of HOCl. However, ammonia does not readily react with HOCl because both compete to donate electrons to each other. Therefore, ammonia is hardly oxidized by HOCl. This may occur because ammonia oxidation proceeds after the termination of the reaction with hydrogen sulfide or methyl mercaptan.

Although acetaldehyde coexisted with hydrogen sulfide, methyl mercaptan, or ammonia, its concentration decreased. Because the carbonyl carbon in acetaldehyde is an electrophile, acetaldehyde was removed simultaneously with other odorants. The acetaldehyde removal rate was not significantly different for any of the mixtures. Acetaldehyde contains various bonds that can be attacked by HOCl. This facilitates acetaldehyde oxidation because the destruction of bonds in acetaldehyde leads to its decomposition. Therefore, broken bonds are expected to differ depending on the coexisting substance.

It is important to note that the organic concentration (COD) was hardly reduced despite the removal of acetaldehyde. Even if some bonds were broken, acetaldehyde was converted into another form of organic matter, as described in Equation (7). However, if acetaldehyde, which affects equilibrium, can be removed, the absorption performance can be maintained continuously. It is not necessary to oxidize acetaldehyde to CO₂. This allows the BSW to be more feasible for removing odors in practical applications.

The oxidation rate of hydrogen sulfide and methyl mercaptan was higher than that of ammonia and acetaldehyde. Their breakthrough time was simultaneously affected by Henry’s law constant as well as the oxidation rate. The ammonia removal rate is governed by absorption rather than oxidation due to its high water solubility and the low reactivity with HOCl. Acetaldehyde was directly proportional to the applied current and had a high Henry’s law constant. This implies that the simultaneous consideration of Henry’s constant and reactivity of the target pollutant with HOCl renders the design of BSW appropriate for treating odor gases containing various odorants.

The reactivity information derived from the EO results for the two odorant mixtures provides a crucial operating factor for the design and operation of BSW. Generally, an odorous gas is a mixture of various odorants. To apply the BSW, the liquid change period must be determined by simultaneously considering the reactivity of odorants with HOCl and their water solubility. Ammonia and aldehydes are the main odor substances in livestock odors. The decision regarding the liquid period must be focused on the removal efficiency of aldehydes, due to their low solubility and slow removal rate compared to those of ammonia. If odors from wastewater treatment plants are the target, hydrogen sulfide and ammonia are the main odor substances [31]. As discussed earlier, hydrogen sulfide is oxidized first compared to ammonia, but its solubility in water is significantly lower than that of ammonia. Therefore, it is essential for the design to consider the inlet load of hydrogen sulfide.

4. Conclusions

BSW was helpful in prolonging the liquid change period through the continuous oxidation of the dissolved odorants. Postponement enables savings in water usage and wastewater treatment costs, convenient operation of the WS, and even a reduction of CO₂ emissions due to the transport of wastewater. This study revealed the efficacy of BSW on absorption and EO characteristics. Furthermore, informative results for appropriately designing BSW are covered in this study through practical application tests. A bench-scale test in this study presented feasibility in practical application, water reuse efficacy, and possible CO₂ emission reduction. The next step would be a pilot-scale study to quantify economic benefit assessment and CO₂ reduction rate. Furthermore, the long-term stability of BSW should be identified. Finally, BSW could be commercialized through a further study, which will contribute to efforts to address environmental problems concerning odor problems, global climate threats, and water risk.