Optimizing Source-Control Systems for Ammonia Mitigation in Swine Manure Pits: Performance Assessment and Modeling

Abstract

Ammonia (NH₃) emissions from swine manure pits contribute significantly to odor nuisance, health risks, and secondary PM_2.5 formation. This study assessed the pilot-scale performance of three source-control technologies: surface sealing with surfactant-based foam system (FOAM SYSTEM), swine manure wiping and removing system (WIPING SYSTEM), and belt-conveyor-based solid–liquid separator system (BELT SYSTEM). Each technology targets a different pathway in the ammonia generation process. The FOAM SYSTEM suppresses volatilization by forming a foam barrier at the air–liquid interface. The WIPING SYSTEM reduces precursor contact time by periodically removing feces. The BELT SYSTEM separates feces and urine upon excretion, inhibiting enzymatic ammonia formation. Among the individual systems, the BELT SYSTEM achieved the highest ammonia reduction efficiency of 91.7%, followed by the FOAM SYSTEM (73.6%) and WIPING SYSTEM (64.4%). The combined FOAM SYSTEM + BELT SYSTEM yielded the best performance with an ammonia reduction efficiency of 94.4%, showing modest synergy without operational interference. In contrast, the FOAM SYSTEM + WIPING SYSTEM configuration achieved 71.1%, slightly lower than the FOAM SYSTEM alone, likely due to foam disruption. Environmental sensitivity tests revealed that higher temperatures and alkaline pH elevated NH₃ emissions, whereas systems that maintained near-neutral pH, like the FOAM SYSTEM, demonstrated greater stability. These findings highlight the importance of integrating physical and source-control mechanisms while considering environmental variability for effective on-farm ammonia mitigation.

1. Introduction

With the rapid growth of the swine industry worldwide, the treatment of livestock manure and the control of associated odor emissions have emerged as critical environmental and societal issues [1]. Swine manure contains various organic nitrogen compounds, including urea and proteins, which are biologically converted into ammonia (NH₃) through enzymatic hydrolysis by microbial urease and microbial deamination of protein-derived amino acids [2]. Volatilized ammonia contributes to the formation of secondary particulate matter (PM2.5), which not only poses health risks to nearby residents and farm workers but also exacerbates air pollution [3,4]. These environmental burdens, coupled with increasingly stringent regulations, are becoming major obstacles to the sustainable development of the swine industry [5].

Ammonia emissions in buildings primarily originate from manure pits located beneath pig housing floors. These pits temporarily store swine feces and urine, where active biochemical reactions among urea; urease, which is an enzyme causing conversion of urea to ammonia; and other nitrogenous compounds continuously generate ammonia [6]. Due to the enclosed structure of the pit, ammonia tends to accumulate to high concentrations, and the rate of volatilization increases markedly under elevated temperature and pH conditions [6].

To reduce ammonia emissions within manure pits, various studies have investigated floor-structure designs, and pilot- and field-scale experiments on equipment such as scraper- and belt-type systems have been conducted [7,8]. However, high concentrations of ammonia are still volatilized even when these technologies are applied. In addition, existing ammonia mitigation strategies have typically relied on single-process systems, which often lack adaptability to diverse environmental conditions or operational scenarios. This limits their long-term performance stability and practical applicability on commercial farms [9]. Consequently, there is an urgent need for reliable, field-deployable mitigation approaches that can effectively control ammonia emissions at the source within the manure pit while ensuring operational robustness.

To address this challenge, the present study proposes and systematically evaluates three practical mitigation technologies designed to suppress ammonia generation and volatilization inside swine manure pits. These technologies operate through distinct mechanisms—physical barrier formation, mechanical removal, and source-level inhibition via solid–liquid separation—thereby providing complementary functions and enhanced adaptability under diverse conditions [10,11].

(1)

Surface sealing with surfactant-based foam (FOAM SYSTEM): This technique forms a stable foam layer on the manure surface using a surfactant, which physically blocks the transfer of ammonia from the liquid to the gas phase [11].

(2)

Swine manure wiping and removing system (WIPING SYSTEM): This system employs a mechanical wiper that periodically removes deposited manure from the pit surface and transfers it to an isolated chamber, thereby minimizing the contact time between swine manure and the atmosphere [7,12]

(3)

Belt-conveyor-based solid–liquid separator system (BELT SYSTEM): This system rapidly separates feces and urine immediately after excretion, preventing interactions among urea, urease, and proteins that lead to ammonia formation [13].

The objective of this study was to quantitatively assess the ammonia reduction performance of each system under pilot-scale conditions and identify optimal operating parameters. Key experimental factors included surfactant type and concentration (FOAM SYSTEM), wiping frequency and wiping plate angle (WIPING SYSTEM), and belt velocity and inclined plate angles (BELT SYSTEM). Furthermore, the systems were tested under varying temperature and pH conditions to evaluate their applicability to real-world swine housing environments.

To overcome the limitations of single-system approaches and to validate the potential for synergistic effects, combined configurations in which the FOAM SYSTEM was integrated with either the WIPING SYSTEM or BELT SYSTEM were also evaluated. These experiments aimed to determine whether combined strategies could achieve enhanced ammonia mitigation through complementary physical and source-level mechanisms.

2. Materials and Methods

2.1. Preparation of Synthetic Swine Manure, Urine, and Urease

To ensure experimental significance and reproducibility while minimizing external variables, synthetic manure samples were prepared to reflect the physical and chemical characteristics of actual swine manure. These synthetic samples served as a standardized basis for quantitatively comparing the ammonia mitigation performance of each treatment system. The major characteristics of synthetic and/or actual swine manure, urine, and feces are shown in

Table 1. The major characteristics of synthetic and/or actual swine manure, urine, and feces.

	Swine Manure *	Urine *	Feces *
	Concentration of NH4+-N (mg/L)	Concentration of Urea-N (mg/L)	Urease Activity (unit/L)	Viscosity (mPa·s)
Synthetic	3000 ± 80	3000 ± 60	≥250 ± 10	1300 ± 100
Actual	1907–3086	1400–2800	156–1104	1200–1500

* Synthetic swine manure: prepared to reflect the typical ammonia concentration found in swine manure [14]. * Synthetic swine urine: formulated with a urea concentration comparable to that in actual swine urine [15]. * Synthetic swine feces: formulated with a urease activity level exceeding that typically observed in actual swine feces [16,17].

.

For the evaluation of the FOAM SYSTEM and WIPING SYSTEM, a synthetic ammonium solution was used to isolate the physical suppression effect on ammonia volatilization, excluding any biological processes such as urea hydrolysis or protein deamination. The solution was prepared by dissolving ammonium chloride (NH₄Cl, 99.5%, Daejung Chemicals & Metals Co., Siheung, Republic of Korea) in ammonia-free tap water to achieve an NH₄⁺ concentration of 3000 mg N/L, which reflects the general concentration of ammonia in swine manure [14].

In contrast, the BELT SYSTEM experiments were designed to simulate both the urea hydrolysis and the effect of solid–liquid separation. To this end, synthetic feces and urine were prepared separately and then combined into a representative mixture. The synthetic urine was prepared by adding urea (Urea, 99.5%, Daejung Chemicals & Metals Co., Siheung, Republic of Korea) to ammonia-free tap water to reach a nitrogen concentration of 3000 mg N/L, representing the precursor form of ammonia. The synthetic feces were formulated to mimic the viscosity of real swine manure by adding clay (Jijemto, specific viscosity of 10,000–100,000 mPa·s, SMILESCIENCE, Seoul, Republic of Korea) to achieve a target viscosity. Urease (specific activity of ≥10,000 u/g, Urease, 98% pass 80 mesh, XI’AN SENTIAN BIOTECHNOLOGY CO., Ltd., Xi’an, China) was then added to the mixture.

2.2. Experimental Setup and Procedures

All experiments for ammonia mitigation were conducted using a pilot-scale swine pit simulation reactor (PSSR, RED Inc., Paju, Republic of Korea), which was equipped with nine swine manure inlets, two foam inlets, an air inlet, an air outlet, and four internal fans to simulate realistic airflow and mixing conditions. The PSSR had dimensions of 2000 mm × 1500 mm × 1200 mm [L × W × H] and a total volume of 3600 L (Figure 1). Fresh air was continuously supplied at 32 L/min, while four internal fans ensured homogeneous mixing of air and ammonia gas inside the reactor. The mixed air was passively discharged through the air outlet. For the FOAM SYSTEM, the built-in foam inlets located at the top of the PSSR were used for surfactant solution injection, without requiring any additional equipment installation. For the WIPING SYSTEM and BELT SYSTEM experiments, the respective systems were mechanically installed at the bottom of the reactor.

The internal ammonia concentration obtained without the application of any system was used as the control. For this, synthetic swine manure was continuously introduced into the PSSR at a flow rate of 60 L/h, and the gas released from the air outlet was collected for 30 min. The same procedure was then applied to evaluate the ammonia mitigation efficiency of the three systems. For the FOAM SYSTEM, the optimal surfactant identified from the foam stability tests was injected into the PSSR through the foam inlets. For the WIPING SYSTEM, a wiping device was installed inside the PSSR and operated during the test. For the BELT SYSTEM, a belt-based device was installed inside the PSSR and operated under the same manure loading condition.

In addition to optimizing system-specific operating parameters, the influence of environmental factors such as temperature and pH was evaluated for all systems. These parameters were selected to reflect conditions commonly found in swine facilities under seasonal and storage variations. For consistency, temperature was tested at 20, 25, and 30 °C, and pH was adjusted to 6, 7, 8, 9, and 10. Although it is well known that ammonia volatilization generally increases with temperature and pH, this study aimed to further evaluate whether each system could mitigate ammonia emissions and prevent chemical volatilization. These tests were conducted across all three systems to assess their robustness under realistic farm conditions. pH was measured using a pH meter equipped with a Vernier pH sensor (Vernier Science Education, model LQ2-LE; manufactured in China, company headquartered in Beaverton, OR, USA). To control and maintain the ambient temperature during the experiments, all tests were conducted in a temperature-controlled incubator, and the air temperature was monitored using a digital thermometer (SDT-25, −50 °C to 1000 °C, SUMMIT, Seoul, Republic of Korea).

Additionally, after each experiment, the interior was rinsed with ammonia-free tap water, and residual moisture was removed using a dry towel. Ventilation was applied to ensure that no gaseous ammonia remained inside the PSSR before the next experiment.

2.2.1. Surface Sealing with Surfactant-Based Foam System

The FOAM SYSTEM was a physical barrier method that aimed to reduce ammonia volatilization from swine manure by forming a stable foam layer on the manure surface. This foam layer inhibits the mass transfer of ammonia from the liquid phase to the gas phase [7]. To evaluate the FOAM SYSTEM technology, three representative surfactants were tested for foam stability: anionic sodium dodecyl sulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB), and non-ionic Triton X-100 (TX-100). SDS is a typical anionic surfactant; it binds to polypeptides, denaturing them and imparting a negative charge that masks their intrinsic charge [18]. CTAB is a representative cationic surfactant that is soluble in water and other polar solvents and exhibits excellent surface-active properties, reducing the surface tension of water and promoting micelle formation [19]. Finally, TX-100 is a non-ionic surfactant with a hydrophilic polyethylene oxide headgroup and a hydrophobic tail, widely used for protein and organelle extraction, cell lysis, and membrane permeabilization [20].

Foam stability was evaluated by comparing the initial foam volume and the foam volume retained after 2 h, and the surfactant exhibiting the highest foam retention was selected as the optimal coating agent. Following the selection of the optimal surfactant, foam retention tests were conducted at concentrations ranging from 1 to 5 g L⁻¹ to determine the optimal injection concentration. Additionally, foam retention was further assessed under varying temperatures (20, 25, 30 °C) and pH conditions (6–10) using the optimal surfactant at its determined optimal concentration.

The ammonia mitigation efficiency of the FOAM SYSTEM was evaluated within the PSSR. The surfactant identified as optimal from the foam stability tests was injected at its determined optimal concentration through the foam inlets of the PSSR.

2.2.2. Swine Manure Wiping and Removing System

The WIPING SYSTEM, which had dimensions of 1800 mm × 800 mm × 600 mm [L × W × H], was a mechanical control strategy designed to reduce ammonia emissions by minimizing the residence time of manure in the pit. By removing fresh feces at regular intervals, the system interrupts the contact between air and swine manure, thereby reducing the generation of free ammonia. The WIPING SYSTEM consisted of a wiping plate, wiper, wiping plate angle adjustment, and an isolated manure storage container (Figure 2). Manure was periodically transferred by the wiper into a sealed chamber to minimize ammonia release. Operational parameters were optimized by varying wiping frequency (6–48 times/h) and wiping plate angle (0–6°). Subsequently, the wiping plate angle was varied while maintaining the optimal frequency to further refine system performance.

2.2.3. Belt-Conveyor-Based Solid–Liquid Separator System

The BELT SYSTEM, which had dimensions of 1400 mm × 1200 mm × 800 mm [L × W × H], was designed to reduce ammonia emissions by rapidly separating feces and urine at the pit level, thereby interrupting the enzymatic hydrolysis of urea by urease. The BELT SYSTEM included a belt conveyor made of rubber, inclined plate angle adjustment, and separated containers for solid and liquid fractions (Figure 3). In the device, the solid fraction of swine manure, containing urease, is transported upward by the movement of the belt and collected in a separated solid manure container, while the liquid fraction, containing urea, flows downward along the inclined surface of the belt, driven by gravity, and is collected in a separated liquid manure container. This mechanism enables efficient solid–liquid separation and prevents odor volatilization.

When the angle of the inclined plate is small, the swine manure tends to accumulate near the center of the belt, whereas a larger angle allows for more uniform distribution across the entire belt surface.

According to Vaddella et al. [21], ammonia volatilization can be reduced by up to 99% when solid and liquid fractions of swine manure are separated. In this study, the majority of the solid fraction was assumed to be in the form of feces, while most of the liquid fraction was assumed to be urine. The solid–liquid separation efficiency (SLSE) was evaluated through the urine–feces separation rate, and accordingly, SLSE was used as one of the performance indicators of the equipment.

Key operational parameters—including belt velocity (2.7–41.3 m/h), inclined plate angle (20–30°), and belt slope angle (7–14°)—were optimized to achieve maximum solid–liquid separation efficiency (SLSE). Environmental performance under different temperatures and pH values was also evaluated following the common protocol.

2.2.4. Combined Systems

The FOAM SYSTEM physically suppresses ammonia volatilization through foam coverage, while the WIPING SYSTEM and BELT SYSTEM target the source of ammonia formation by removing or separating precursors such as urea and urease. Two combined systems, FOAM SYSTEM + WIPING SYSTEM and FOAM SYSTEM + BELT SYSTEM, were tested to evaluate potential synergistic effects and compared with individual systems. Each system was operated under its optimal conditions. The ammonia reduction performance of the combined systems was evaluated and compared with the single systems.

2.3. Manure Feeding and Gas Collection Procedures

To evaluate ammonia reduction efficiency, synthetic swine slurry was continuously introduced into the reactor at a flow rate of 60 L/h for 30 min in the FOAM SYSTEM, WIPING SYSTEM, and FOAM SYSTEM + WIPING SYSTEM. In contrast, for the BELT SYSTEM and FOAM SYSTEM + BELT SYSTEM, synthetic feces and urine were introduced separately without mixing to better simulate actual swine pit conditions. Each substrate was injected through the manure inlets of the PSSR in a distributed manner, with synthetic feces and urine supplied at flow rates of 15 L/h and 45 L/h, respectively, for 30 min.

All feeding operations were performed via the upper manure inlets of the PSSR to ensure even distribution, and the flow rates were precisely controlled using a peristaltic pump (Masterflex L/S, Cole-Parmer, Vernon Hills, IL, USA). This setup ensured consistency in feeding conditions and maintained operational stability throughout the experiments. To quantitatively analyze ammonia volatilization, a low-pressure vacuum pump (Model DA120 OEM, YLKTECH, Shenzhen, China) continuously supplied air into the PSSR to maintain a consistent flow. To prevent short-circuiting of the internal airflow, the internal fans were operated at 1100 rpm. The ammonia-laden air generated inside the reactor was discharged through the gas outlet and completely captured using impingers (Ace Glass Inc., Vineland, NJ, USA) filled with 4% boric acid solution (Daejung Chemicals & Metals Co., Siheung, Republic of Korea). The internal pressure of the reactor was maintained at 1 atm throughout the experiments.

Collected gas samples were analyzed for ammonia concentration using the indophenol blue method [22]. Each condition was tested in triplicate, and only data with a coefficient of variation (CV) of 5% or less were considered valid and used for further analysis. Subsequently, the validated datasets were subjected to statistical analysis using one-way analysis of variance (ANOVA) to evaluate the significance of differences among experimental groups. Where appropriate, post hoc multiple comparison tests (Tukey’s HSD) were employed. Statistical significance was defined at p < 0.05.

2.4. Performance Indicators and Regression Models

To evaluate the performance of the FOAM SYSTEM, foam stability was determined using the foam volume retention ratio, as suggested in previous studies. Specifically, the ratio of retained foam volume to the initial volume was calculated and used as an index of foam stability [23].

Each test was conducted using a 100 mL transparent glass bottle, into which the surfactant solution was added at the specified concentration. The solution was manually shaken up and down 10 times to generate foam, and the initial foam volume (V_i, mL) was immediately recorded. After allowing the sample to stand undisturbed for 2 h, the remaining foam volume (V_e, mL) was measured. Foam stability was calculated using Equation (1).

$$\mathrm{F}\mathrm{o}\mathrm{a}\mathrm{m} \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}\left(\%\right)={\displaystyle \frac{V_e}{V_i}}\times 100$$

(1)

where V_i is foam volume immediately after generation (mL) and V_e is foam volume after 2 h at rest (mL). Foam stability tests were repeated under different environmental conditions to assess the influence of temperature (20, 25, and 30 °C) and pH (6, 7, 8, 9, and 10) on foam persistence.

Ammonia reduction efficiency was used to quantify the performance of each mitigation system based on the change in ammonia concentration within the PSSR. The ammonia reduction efficiency was calculated by comparing the ammonia concentration of each system to that of the control (no mitigation applied) using Equation (2) [24].

$$\mathrm{Ammonia} \mathrm{reduction} \mathrm{efficiency} \left(\%\right)={\displaystyle \frac{(C_i-C_s)}{C_i}}\times 100$$

(2)

where C_i is ammonia concentration inside the reactor under control conditions (mg/L); C_s is ammonia concentration under system operation (mg/L). Ammonia reduction efficiency was measured in triplicate, with samples collected independently at three separate time points to account for temporal variability, and the reported values represent the mean of these replicates.

The relationship between wiping frequency and ammonia concentration in the WIPING SYSTEM was modeled using a nonlinear regression equation. An exponential decay function was applied and is expressed as Equation (3):

$${\mathrm{Y}}_{\mathrm{M}\mathrm{W}\mathrm{S}}=a+b {· e}^{-c·f}$$

(3)

where Y_MSW is ammonia concentration under WIPING SYSTEM operation (mg/L), f is wiping frequency (times/h), and a–c are empirically fitted constants. The curve fitting was performed using the least squares method in SigmaPlot (Version 11, Systat Software Inc., San Jose, CA, USA), and model fit was evaluated using the coefficient of determination (R²). This model was used to determine the minimum wiping frequency required to achieve 90% suppression of ammonia volatilization compared to the untreated control.

The physical separation performance of the BELT SYSTEM was evaluated using the SLSE. The total dry weight of solids introduced into the system (Mt) and the dry weight of solids collected in the feces storage tank (Ms) were measured. The SLSE was calculated using Equation (4).

$$\mathrm{S}\mathrm{L}\mathrm{S}\mathrm{E}\left(\%\right)=\left(1-{\displaystyle \frac{M_t-M_s}{M_t}}\right)\times 100$$

(4)

where SLSE is solid liquid separation efficiency (%), M_t is the total dry weight of solids in the influent (g), and M_s is the dry weight of solids recovered in the feces storage container (g). All solid samples were dried at 105 °C for 24 h in a laboratory oven and subsequently cooled in a desiccator before final weighing.

The relationship between belt velocity and SLSE in the BELT SYSTEM was modeled using a nonlinear regression equation. A saturation-type function was applied and is expressed as Equation (5):

$${\mathrm{Y}}_{\mathrm{B}\mathrm{e}\mathrm{l}\mathrm{t} \mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}=a+bln\left(v\right)$$

(5)

where Y_{Belt system} is SLSE under BELT SYSTEM operation (%), v is belt velocity (m/h), and a and b are empirically fitted constants. The curve fitting was performed using the least squares method in SigmaPlot, and the coefficient of determination (R²) was calculated to evaluate the goodness of fit. This model was used to identify the optimal belt velocity required to achieve 90% of the maximum SLSE.

In the BELT SYSTEM, if the belt velocity exceeds a critical threshold, the flow direction of the urine may reverse upward, causing unintended transport of liquid into the solid storage zone. This backflow can compromise solid–liquid separation and reduce ammonia reduction efficiency.

To avoid this issue, the theoretical critical belt velocity (v) was calculated using Equation (6) [25], and all experiments were conducted within sub-critical velocity ranges:

$$\mathrm{v}={\displaystyle \frac{\rho gsin\left(\theta \right)h^2}{2\eta }}$$

(6)

where v is the critical belt velocity (m/s), ρ is the density of swine urine (1.01 g/cm³), g is the gravitational acceleration (9.80 m/s²), θ is the inclined plate angle (°), h is the fluid thickness (assumed as 0.1 m), and η is the dynamic viscosity of swine urine (assumed as 0.5 Pa·s). The calculated critical velocity was used as an upper limit for belt velocity during SLSE and ammonia reduction efficiency testing.

3. Results and Discussion

3.1. Ammonia Mitigation Through Surface Sealing with Surfactant-Based Foam System

Foam stability tests were conducted using three representative surfactants: nonionic Triton X-100, cationic CTAB, and anionic SDS. The results are shown in

Table 2. Comparative foam stability of surfactants for surface sealing and detailed performance of sodium dodecyl sulfate (SDS) under varying concentrations, temperatures, and pH conditions.

Experimental Step	Sample No.	Surfactant	Concentration (g/L)	pH	Temperature (°C)	Foam Stability (%)
I (surfactant screening)	1	Triton X-100	1 ± 0.05	8.0 ± 0.1	25	23.4 ± 0.4
	2	CTAB 1	1 ± 0.05	8.0 ± 0.1	25	33.8 ± 0.6
	3	SDS 2	1 ± 0.05	8.0 ± 0.1	25	35.1 ± 0.5
II (dose variation)	4	SDS 2	2 ± 0.05	8.0 ± 0.1	25	65.4 ± 0.3
	5	SDS 2	3 ± 0.05	8.0 ± 0.1	25	66.2 ± 0.2
	6	SDS 2	4 ± 0.05	8.0 ± 0.1	25	66.4 ± 0.4
	7	SDS 2	5 ± 0.05	8.0 ± 0.1	25	64.6 ± 0.4
III (temperature variation)	8	SDS 2	2 ± 0.05	8.0 ± 0.1	20	67.7 ± 0.4
	9	SDS 2	2 ± 0.05	8.0 ± 0.1	25	65.4 ± 0.3
	10	SDS 2	2 ± 0.05	8.0 ± 0.1	30	61.2 ± 0.5
IV (pH variation)	11	SDS 2	2 ± 0.05	6.0 ± 0.1	25	73.9 ± 0.4
	12	SDS 2	2 ± 0.05	7.0 ± 0.1	25	67.9 ± 0.5
	13	SDS 2	2 ± 0.05	8.0 ± 0.1	25	65.4 ± 0.3
	14	SDS 2	2 ± 0.05	9.0 ± 0.1	25	75.7 ± 0.3
	15	SDS 2	2 ± 0.05	10.0 ± 0.1	25	72.2 ± 0.5

¹ CTAB = cetrimonium bromide, ² SDS = sodium dodecyl sulfate.

. Triton X-100 showed the lowest foam stability (23.4 ± 0.4%), likely due to the absence of ionic charge, which limits the formation of a stable electric double layer and reduces repulsive forces between bubbles [26]. In contrast, CTAB and SDS showed higher foam stabilities of 33.8 ± 0.6% and 35.1 ± 0.5%, respectively (p = 1.43 × 10⁻⁷ < 0.05). SDS was considered superior, as its sulfate group forms strong ionic interactions with water molecules, enhancing interfacial stability and reducing surface tension [18]. CTAB, despite its relatively high initial performance, was judged less favorable for field application due to its higher ecological toxicity and environmental sensitivity [27]. Its strong electrostatic interactions with negatively charged cell membranes induce membrane disruption, metabolic inhibition, and ultimately cell death in both microorganisms and mammalian cells, which underscores its potential risks in practical use [28]. Consequently, SDS was selected as the optimal surfactant for further application.

To determine the optimal SDS concentration, foam stability was evaluated at concentrations ranging from 1 to 5 g/L. At 1 g/L, the foam stability was 35.1 ± 0.5%, whereas concentrations of 2 g/L or higher achieved stability rates exceeding 64%. This enhancement is attributed to increased molecular density at the air–liquid interface, resulting in thicker and more stable foam films. However, exceeding the critical micelle concentration (CMC) of SDS—reported as approximately 2.3 g/L—may cause excessive micelle formation and environmental risks upon discharge into water systems [29]. Therefore, 2 g/L was selected as the optimal concentration, balancing performance and ecological safety.

To assess environmental sensitivity, foam stability was further tested under varying temperatures (20, 25, and 30 °C) and pH values (6, 7, 8, 9, and 10). The results indicated a declining trend in foam stability with increasing temperature: 67.7 ± 0.4%, 65.4 ± 0.3%, and 61.2 ± 0.5%, respectively (p = 3.45 × 10⁻⁶ < 0.05), which can be explained by accelerated gas expansion and bubble collapse at elevated temperatures (Table 2, Step III) [30]. At higher ambient temperatures, such as during summer, a decrease in FOAM SYSTEM effectiveness may be expected.

Foam stability was also affected by pH. At pH 6 and 9, stability was higher (73.9 ± 0.4% and 75.7 ± 0.3%, respectively) than that at pH 8 (65.4% ± 0.3) (p = 5.19 × 10⁻⁸ < 0.05) (Table 2, Step IV). This is likely due to enhanced bubble cohesion in acidic conditions and increased electrostatic interactions in alkaline environments, particularly for anionic surfactants such as SDS [31,32]. However, higher pH also increases the conversion of ammonium (NH₄⁺) to free ammonia (NH₃), potentially leading to increased volatilization [33]. Therefore, maintaining manure pH below 8 is desirable to optimize the effectiveness of the FOAM SYSTEM.

The ammonia reduction performance of the FOAM SYSTEM technology was evaluated by comparing ammonia concentrations and total emissions between the control and SDS-treated samples. In the control (no SDS), the ammonia concentration reached 82.1 ± 1.0 mg-N/L, with a total emission of 164.2 ± 2.0 g-N over 30 min in a 2 m³ reactor. In contrast, the SDS-treated condition (2 g/L) resulted in a concentration of 21.7 ± 0.5 mg-N/L and an emission of 43.4 ± 1.0 g-N, corresponding to a 73.6 ± 1.6% reduction in ammonia volatilization (

Table 3. Ammonia concentrations under different ammonia mitigation strategies, including single and combined system configurations.

	Control	Single			Combined
		FOAM SYSTEM 1	WIPING SYSTEM 2	BELT SYSTEM 3	FOAM SYSTEM + WIPING SYSTEM	FOAM SYSTEM + BELT SYSTEM
Ammonia Concentration (mg/L)	82.1 ± 1.0	21.7 ± 0.5	29.2 ± 0.6	6.8 ± 0.1	24.5 ± 0.2	4.6 ± 0.1

¹ FOAM SYSTEM = surface sealing with surfactant-based foam system, ² WIPING SYSTEM = swine manure wiping and removing system, ³ BELT SYSTEM: belt-conveyor-based solid–liquid separator system.

and Figure 4). These findings confirm that SDS-generated foam forms an effective physical barrier to suppress ammonia release.

3.2. Ammonia Mitigation Through the Swine Manure Wiping and Removing System

To determine the optimal wiping frequency, ammonia concentrations in the reactor were measured under five conditions: 0 (control), 6, 12, 24, and 48 times per hour. After each operating condition was assessed, internal cleaning was performed to prevent the accumulation of synthetic swine manure. As shown in Figure 5, the ammonia concentration in the control was 82.1 ± 1.0 mg-N/L, while the concentrations under the wiping frequencies of 6, 12, 24, and 48 times/h were 45.2 ± 0.8, 42.1 ± 1.2, 39.8 ± 0.9, and 38.0 ± 1.0 mg-N/L, respectively. The relationship was modeled using nonlinear regression based on Equation (3), and the resulting function is expressed in Equation (7) with a coefficient of determination (R²) of 0.9960:

$${\mathrm{Y}}_{MWS}=39.6+42.{4e}^{-0.33f}$$

(7)

The regression model predicted a maximum ammonia reduction efficiency of approximately 53%. The optimal frequency was defined as the minimum frequency required to achieve 90% of the maximum ammonia reduction efficiency, which was determined to be 11.25 times/h.

Subsequently, the optimal wiping plate angle was evaluated by fixing the frequency at 11.25 times/h and varying the wiping plate angle (0°, 2°, 4°, and 6°). As presented in Step I of

Table 4. Ammonia concentrations in the swine manure wiping and removing system under varying wiping plate angles, temperatures, and pH levels at a fixed wiping frequency of 11.25 times/h.

Experimental Step	Sample No.	Wiping Plate Angle (°)	Temperature (°C)	pH	NH3 Concentration (mg/L)
I (slope variation)	1	0	25	8 ± 0.1	40.3 ± 1.2
	2	2	25	8 ± 0.1	32.7 ± 2.5
	3	4	25	8 ± 0.1	29.2 ± 0.6
	4	6	25	8 ± 0.1	29.5 ± 2.5
II (temperature variation)	5	4	20	8 ± 0.1	18.1 ± 2.0
	6	4	25	8 ± 0.1	29.2 ± 0.6
	7	4	30	8 ± 0.1	33.4 ± 1.3
III (pH variation)	8	4	25	6 ± 0.1	2.9 ± 0.5
	9	4	25	7 ± 0.1	5.8 ± 0.7
	10	4	25	8 ± 0.1	29.2 ± 0.6
	11	4	25	9 ± 0.1	145.2 ± 5.8
	12	4	25	10 ± 0.1	689.5 ± 13.5

, the ammonia concentrations at each angle were 40.3 ± 1.2, 32.7 ± 2.5, 29.5 ± 0.6, and 29.2 ± 2.5 mg-N/L, respectively. These results indicate that steeper angles enable faster feces removal and further reduce ammonia emissions, up to 26.8% additional reduction at 6° compared to 0°. However, no significant difference was observed between 4° and 6° (p = 0.708 > 0.05). Assuming a typical pit length of 4000 mm, the required pit depth at 6° would be 421 mm compared to 280 mm at 4°, and the corresponding plate lengths would be 4022 mm and 4010 mm, respectively. Such geometric increases may compromise the economic feasibility of WIPING SYSTEM installation. Therefore, 4° was selected as the optimal wiping plate angle, balancing performance and constructability.

The implementation of the identified optimal operational parameters resulted in a final ammonia reduction efficiency of 64.4 ± 1.6%. This efficiency markedly exceeds the 10–40% reduction reported in previous studies, suggesting that the application of optimal wiping frequency and belt angle provides superior mitigation performance compared to the conventional wiping system [7].

To assess the environmental sensitivity of the system, the effect of temperature and pH on ammonia reduction efficiency was further investigated. Under constant pH 8, the reactor was operated at 20, 25, and 30 °C. The analysis confirmed that ammonia volatilization was highest at 30 °C (p = 0.0069 < 0.05) (Table 4, Step II). These findings suggest that higher temperatures, such as those encountered in summer, can significantly elevate ammonia emissions, highlighting the need for complementary measures under such conditions.

In pH-related experiments conducted at a constant temperature of 25 °C, the lowest ammonia concentration (2.9 ± 0.5 mg-N/L) was observed at pH 6. The concentration doubled at pH 7 (5.8 ± 0.7 mg-N/L) and increased sharply increased to 145.2 ± 5.8 and 689.5 ± 13.5 mg-N/L at pH 9 and 10, respectively (Table 4, Step III). This trend aligns with the chemical equilibrium shift from ammonium to free ammonia at higher pH levels [32]. Since the pH of manure tends to rise during extended storage due to microbial activity and increased alkalinity, prolonged retention can significantly increase ammonia emissions even with the WIPING SYSTEM [34]. These results indicate that minimizing manure storage time and controlling pH are essential for maintaining the effectiveness of the WIPING SYSTEM in practical applications. According to Overmeyer et al. [35], the initial pH of freshly excreted swine manure is approximately 7, but it increases to above 8 after around 10 days. To enhance the efficiency of the present technology, maintaining the residence time of scraped and isolated manure within 10 days is expected to allow effective ammonia management.

3.3. Ammonia Mitigation Through the Belt-Conveyor-Based Solid–Liquid Separator System

To prevent reverse flow and separation failure due to excessive belt velocity, theoretical critical velocities were calculated using Equation (4) at three belt slope angles (7°, 10.5°, and 14°), yielding limits of 289, 433, and 574 m/h, respectively. All experimental belt velocities were far below these thresholds.

For velocity optimization, the belt slope angle and inclined plate angle were fixed at 10.5° and 30°, respectively, while the belt velocity was varied from 50 to 600 rpm, corresponding to linear velocities of 2.7, 6.2, 13.2, 20.3, and 41.3 m/h. The resulting SLSE values are shown in Figure 6 and

Table 5. Solid–liquid separation efficiency of the belt-conveyor-based separator system under varying belt velocities, inclined plate slopes, and belt slope angles.

Experimental Step	Sample No.	Belt Velocity (m/h)	Inclined Plate Angle (°)	Belt Slope Angle (°)	Solid–Liquid Separation Ratio (%)
I (belt velocity)	1	2.7	30	10.5	22.2 ± 1.2
	2	6.2	30	10.5	43.0 ± 2.3
	3	13.2	30	10.5	62.4 ± 3.8
	4	20.3	30	10.5	80.1 ± 2.1
	5	41.3	30	10.5	99.5 ± 1.5
II (inclined plate variation)	6	31.4	20	10.5	32.8 ± 1.5
	7	31.4	25	10.5	43.1 ± 2.3
	8	31.4	30	10.5	90.0 ± 7.3
III (belt slope variation)	9	31.4	30	7	77.1 ± 5.2
	10	31.4	30	10.5	90.0 ± 7.3
	11	31.4	30	14	61.9 ± 3.7

.

The relationship was modeled using a logarithmic regression function as shown in Equation (5), yielding Equation (8) with an R² value of 0.9924:

$${\mathrm{Y}}_{BCSLS}=-16.7+31 \mathrm{l}\mathrm{n}\left(v\right)$$

(8)

Based on this model, the optimal belt velocity was determined to be 31.4 m/h, which achieves 90% SLSE. Using this optimal belt velocity, the inclined plate angle was varied (20°, 25°, and 30°) while the belt slope angle was kept at 10.5°.

When the internal plate angle is small, the injected solid and liquid fractions of swine manure tend to concentrate in the central part of the belt. Conversely, when the internal plate angle is large, the injected manure spreads over a wider area on the belt. This higher concentration in a narrow area increases the downward flow velocity, which may cause the solid and liquid fractions to be swept into the lower liquid collection compartment, potentially causing critical problems for solid–liquid separation.

From experiments conducted at different internal angles, solid–liquid separation efficiencies of 32.8 ± 1.5%, 43.1 ± 2.3%, and 90.0 ± 7.3% were achieved at angles of 20°, 25°, and 30°, respectively. This indicates that setting the internal plate angle to 30° can achieve approximately 57% higher solid–liquid separation efficiency compared to an angle of 20°.

Next, belt incline was evaluated at fixed conditions (belt velocity: 31.4 m/h, inclined plate angle: 30°), at belt slope angles of 7°, 10.5°, and 14°. The SLSEs were 77.1 ± 5.2%, 90.0 ± 7.3%, and 61.9 ± 3.7%, respectively (Table 5, Step III). These results indicate that at low angles, prolonged contact reduces separation, while overly steep angles cause gravity-driven downward flow to override belt transport, reducing collection efficiency [20]. Hence, 10.5° was selected as the optimal belt incline. Under the final optimized conditions (belt velocity: 31.4 m/h, inclined plate angle: 30°, belt slope angle: 10.5°), ammonia concentration in the reactor was reduced to 6.8 ± 0.1 mg-N/L, corresponding to a 91.7 ± 1.7% reduction in ammonia volatilization compared to the control (Table 3). Previous studies have reported that the equipment could reduce ammonia emissions by up to 75%. However, in the present study, the application of specific operational parameters enabled a reduction of more than 90% in ammonia volatilization. This enhanced performance is likely attributable not only to the belt operation itself, but also to the incorporation of new operational factors such as belt angle adjustment, installation of internal inclined plates, and optimization of their angles [8].

To assess environmental sensitivity, BELT SYSTEM performance was tested under varying temperature and pH conditions. At pH 8, increasing the temperature from 20 °C to 30 °C led to ammonia concentrations of 5.7 ± 0.3, 6.8 ± 0.1, and 9.0 ± 0.5 mg-N/L, respectively (

Table 6. Ammonia concentrations from the belt-conveyor-based solid–liquid separator system under varying pH and temperature conditions.

Experimental Step	Sample No.	Belt Velocity (m/h)	Inclined Slope Angle (°)	Belt Slope Angle (°)	Temperature (°C)	pH	NH3 Concentration (mg/L)
I (temperature variation)	5	31.4	30	10.5	20	8 ± 0.1	5.7 ± 0.3
	6	31.4	30	10.5	25	8 ± 0.1	6.8 ± 0.1
	7	31.4	30	10.5	30	8 ± 0.1	9.0 ± 0.5
II (pH variation)	8	31.4	30	10.5	25	6 ± 0.1	2.1 ± 0.2
	9	31.4	30	10.5	25	7 ± 0.1	3.4 ± 0.8
	10	31.4	30	10.5	25	8 ± 0.1	6.8 ± 0.1
	11	31.4	30	10.5	25	9 ± 0.1	9.0 ± 0.4
	12	31.4	30	10.5	25	10 ± 0.1	12.3 ± 1.2

). At a constant temperature of 25 °C, pH values of 6 to 10 resulted in ammonia concentrations of 2.1 ± 0.2, 3.4 ± 0.8, 6.8 ± 0.1, 9.0 ± 0.4, and 12.3 ± 1.2 mg-N/L, respectively. These results demonstrate that the BELT SYSTEM, like the WIPING SYSTEM, is affected by environmental conditions, with higher temperature and pH leading to increased emissions. However, comparative analysis showed that the BELT SYSTEM is more vulnerable to temperature rise but less sensitive to pH. At 30 °C, the BELT SYSTEM showed a 32% increase in ammonia emission, compared to a 14% increase for the WIPING SYSTEM. Conversely, at pH 10, the BELT SYSTEM showed only an 80% increase in ammonia emissions, while the WIPING SYSTEM showed a dramatic 2200% increase. This difference is attributed to the complete physical separation of feces (urease source) and urine (urea substrate) in the BELT SYSTEM, which suppresses the enzymatic formation of ammonia [9]. As a result, even under elevated pH, the ammonia concentration remains relatively low, enhancing the robustness of the BELT SYSTEM in alkaline conditions.

3.4. Evaluation of Ammonia Mitigation Performance in Single and Integrated System Configurations

To enhance ammonia mitigation, combined applications of the FOAM SYSTEM and source-control systems such as the WIPING SYSTEM and the BELT SYSTEM were tested. These configurations aim to leverage complementary mechanisms—the FOAM SYSTEM inhibits ammonia volatilization at the air–liquid interface, while the WIPING SYSTEM and BELT SYSTEM suppress ammonia generation by removing or isolating precursors such as urea and urease.

The results of combined system experiments are presented in Table 3. The FOAM SYSTEM + BELT SYSTEM configuration demonstrated the highest performance, reducing ammonia concentration to 4.6 ± 0.1 mg-N/L, corresponding to an ammonia reduction efficiency of 94.4 ± 1.7%. This represents an improvement over the BELT SYSTEM alone (91.7 ± 1.7%) (p = 1.69 × 10⁻⁶ < 0.05), indicating a modest but meaningful synergistic effect. The foam layer provided by the FOAM SYSTEM likely remained stable due to the non-disruptive operation of the BELT SYSTEM, allowing both systems to function concurrently without interference.

In contrast, the FOAM SYSTEM + WIPING SYSTEM configuration yielded an ammonia reduction efficiency of 70.2% ± 1.5, which improved upon the WIPING SYSTEM alone (64.4 ± 1.6%) (p = 0.00016 < 0.05) but was lower than that of the FOAM SYSTEM alone (73.6 ± 1.6%) (p = 0.0018 < 0.05). This result suggests possible functional interference between the two systems.

Specifically, the repetitive wiping motion in the WIPING SYSTEM may have physically disturbed the foam layer generated by the FOAM SYSTEM—by either shearing or redistributing the foam—thereby reducing its sealing capacity. Such interaction highlights the importance of ensuring operational compatibility when integrating dynamic mechanical elements with surface-based control technologies. Taken together, these results demonstrate that the effectiveness of integrated ammonia mitigation systems is determined not only by the individual performance of their components but also by their mechanistic interaction. Configurations with minimal physical conflict and complementary functions—such as the FOAM SYSTEM + BELT SYSTEM—showed improved stability and cumulative performance.

When the BELT SYSTEM is operated alone, a small amount of ammonia may volatilize during the short residence time on the belt before solid–liquid separation, due to reactions between urea and urease (i.e., with an ammonia mitigation efficiency of 91.7 ± 1.7%, approximately 10% of the ammonia may still volatilize on the belt conveyor). However, when it is combined with the FOAM SYSTEM, ammonia that could volatilize during the belt residence time is physically blocked, providing an additional reduction in ammonia emissions. Considering that the annual ammonia emission per pig has been reported to be approximately 3.7 kg/pig/year, the application of the FOAM SYSTEM combined with the BELT SYSTEM is expected to reduce ammonia emissions by approximately 3.5 kg/pig/year [8].

Conversely, combinations involving mechanical agitation, such as the FOAM SYSTEM + WIPING SYSTEM, may require further optimization of timing, frequency, or structural design to minimize interference and fully realize synergistic potential.

4. Conclusions

This study systematically evaluated three ammonia mitigation technologies (FOAM SYSTEM, WIPING SYSTEM, and BELT SYSTEM) to control ammonia volatilization directly within swine manure pits. Pilot-scale experiments were conducted to determine the optimal operational parameters and to assess performance under both individual and integrated configurations. Among the single systems, the BELT SYSTEM showed the highest ammonia reduction efficiency, achieving a 91.7 ± 1.7% reduction through optimized belt velocity (31.4 m/h), inclined plate angle (30°), and belt slope angle (10.5°). The FOAM SYSTEM reduced ammonia emissions by 73.6 ± 1.6% via the formation of a stable foam layer using 2 g/L of SDS, while the WIPING SYSTEM achieved 64.4 ± 1.6% reduction through intermittent removal of feces at an optimized frequency of 11.25 times/h and wiping plate angle of 4°. When systems were operated in combination, the FOAM SYSTEM + BELT SYSTEM configuration achieved the highest overall performance, reducing ammonia concentration to 4.6 mg ± 0.1-N/L (94.4% ± 1.7 of ammonia reduction efficiency). This enhancement was attributed to the synergistic interaction between gas-phase blocking and source-level inhibition. In contrast, the FOAM SYSTEM + WIPING SYSTEM configuration, while improving upon the WIPING SYSTEM alone, showed slightly lower performance than the FOAM SYSTEM alone, suggesting the potential for mechanical interference. Environmental sensitivity tests revealed that all systems were affected by elevated temperature and pH, with the BELT SYSTEM maintaining greater stability under alkaline conditions due to its physical separation mechanism. These results highlight the importance of understanding not only the individual performance of mitigation systems but also their functional interactions under variable field conditions. This research provides foundational data for the practical implementation of multi-functional ammonia reduction systems, and future studies should focus on full-scale field applications and cost-effectiveness assessments for commercial adoption.