Removal and Recovery of Ammonium Nitrogen from Dairy Processing Wastewater Using Air Stripping Technology: A Pilot-Scale Study

Abstract

Ammonium nitrogen (NH₄⁺-N) removal and recovery from wastewater have been critical issues worldwide and key to achieving a sustainable nitrogen cycle and circular economy. In this study, we designed and constructed a pilot-scale air stripping system integrated with a nutrient-capture unit and evaluated the effective pH, temperature, and airflow conditions for maximising NH₄⁺-N removal and recovery from dairy processing wastewater (DPW). Our results demonstrated that increasing pH and temperature substantially enhances NH₄⁺-N removal via air stripping, with higher airflow rates further improving performance. Under these conditions (pH 11, 32 °C, and 300 L min⁻¹), NH₄⁺-N removal from synthetic wastewater reached ≈40% after 6 h air stripping. In comparison, real DPW exhibited slightly lower removal efficiency under the same conditions, achieving ≈34%, likely due to its more complex matrix. Additionally, incorporating a chemical precipitation step followed by filtration prior to air stripping removed NH₄⁺-N from DPW, achieving ≈43%. However, extending the stripping duration under identical conditions significantly improved removal performance, increasing NH₄⁺-N removal in DPW to ≈70%. The downstream capturing system, consisting of acid bath and granulated activated carbon (GAC), consistently recovered 70–95% of the released ammonia (NH₃) when even upstream NH₄⁺-N removal via air stripping was moderate. The GAC effectively adsorbed the volatilised NH₃, achieving adsorption capacities of up to ≈18 mg/kg. Overall, this integrated system demonstrates strong potential for simultaneous NH₄⁺-N removal and recovery from industrial wastewater streams, offering notable environmental benefits.

1. Introduction

An important treatment method employed in the dairy processing industry is anaerobic digestion used for reducing organic load in the wastewater. However, this process does not facilitate the recovery of valuable dissolved nutrients such as phosphorus (as phosphate, PO₄³⁻-P) and nitrogen (as ammonium, NH₄⁺-N) [1]. As a result, dairy processing wastewater (DPW) contains very high concentrations of PO₄³⁻-P and NH₄⁺-N [2,3]. Phosphorus (P) is a vital nutrient for crop production, traditionally sourced from phosphate rock, a non-renewable resource found in limited deposits across only a few countries worldwide [4]. Nitrogen is also an essential plant nutrient; however, while bioavailable forms can be generated through the conversion of gaseous N into ammonium (NH₄⁺), the release of excess N into waterways leads to ecosystem degradation [5]. While chemical precipitation of DPW for P recovery as struvite (NH₄MgPO₄·6H₂O) has proven effective, the process often generates large volumes of sludge and precipitates containing heavy metals and pathogens which require further treatment [6,7]. Furthermore, precipitation has shown to be ineffective at recovering NH₄⁺-N from DPW because it is either in excess concentration compared to the required stoichiometric ratio for struvite precipitation, or non-struvite phosphate containing minerals form at the expense of struvite [8].

Nitrogen removal from wastewater via biological treatment is well established; however, it requires specified management and careful maintenance of microbial communities to maintain optimal conditions. Kinetic limitations can also slow the removal process [9,10,11]. Furthermore, recovering ammonium within biological systems is often difficult. As demand for nutrients such as ammonium and phosphate continues to rise for fertiliser production, nutrient recovery technologies have gained increasing attention [12].

The removal and recovery of ammonia from wastewater is a critical global challenge, yet it is essential for achieving a sustainable nitrogen cycle and supporting a circular economy [13]. Air stripping can be an attractive technology option for ammonia/ammonium removal and recovery [13,14], and successfully applied to various wastewater types, including municipal and industrial effluents, landfill leachates, and anaerobic digestates [15,16,17,18]. This technology is often selected for its simplicity, operational feasibility, and high removal efficiency [19,20]. Over recent decades, substantial progress has been made in the development of air stripping equipment, with improvements primarily focused on enhancing process efficiency. Overall, air stripping remains a relatively low-cost method that relies on simple equipment and can achieve high rates of ammonia removal from wastewater [14,21].

Several advancements in air stripping techniques have been developed, particularly through the use of stripping towers and stripping reactors. Although conventional stripping towers have been widely studied, they often face challenges at larger scales, including calcium carbonate scaling and reduced performance during colder seasons [14,22,23]. In contrast, stripping reactors offer a suitable alternative, helping to overcome space constraints and mitigating scaling or fouling on packing materials commonly observed in stripping towers [14,21]. Examples of investigated ammonia-stripping reactors include the water-sparged aerocyclone reactor [21] and the semi-batch jet loop reactor [24].

The future application of air stripping reactors for ammonia removal appears highly promising, with recent advancements in reactor design contributing to enhanced performance and efficiency. Among the key operational parameters, airflow rate exerts a particularly strong influence on reactor effectiveness [14]. pH and temperature are also recognised as critical factors in ammonia stripping. pH plays a critical role in ammonia air stripping because its adjustment shifts the NH₄⁺/NH₃ equilibrium toward the un-ionised, volatile NH₃ form, thereby increasing the fraction of ammonia available for removal [14,21,24]. Temperature also plays a role in the air stripping of NH₃. Increasing temperature shifts the liquid–gas distribution of ammonia toward the gas phase, thereby enhancing volatilisation. Higher temperatures favour the conversion of ammonium ions (NH₄⁺) to free ammonia (NH₃), which can be more readily transferred into the gaseous phase during air stripping [24,25,26,27]. Increasing temperature enhances ammonia stripping efficiency by increasing molecular diffusion in the gas film and decreasing liquid-phase viscosity, thereby improving gas–liquid mass transfer [24,26]. Although substantial research has examined ammonia air stripping in general wastewater treatment, a significant knowledge gap remains regarding its application to dairy processing wastewater and the corresponding optimal operating conditions. Moreover, most existing studies have been conducted at laboratory scale, with very limited validation under pilot-scale conditions [13]. To the best of our knowledge, no study has yet evaluated NH₄⁺-N removal and recovery from dairy processing wastewater using air stripping technologies.

There is a lack of understanding regarding the optimal operating conditions, specifically pH, temperature, and airflow rate for ammonia air stripping system in dairy processing plant. The novelty of this study lies in the pilot-scale implementation of a water-spray aerocyclone air-stripping reactor specifically applied to dairy processing wastewater, together with a systematic evaluation and optimisation of key operational parameters governing NH₄⁺-N removal under realistic operating conditions. Furthermore, the integration of air stripping with a downstream ammonia recovery process demonstrates the dual potential for efficient nitrogen removal and resource recovery. Therefore, in this study, we designed and constructed a pilot-scale air stripping system and investigated the operating conditions that maximise NH₄⁺-N removal from dairy processing wastewater. In addition, we integrated a chemical precipitation step with the air stripping process. The primary objective of this work was to develop an effective technology for NH₄⁺-N removal and recovery suitable for implementation in dairy processing facilities.

2. Materials and Methods

2.1. Wastewater Collection and Sample Preparation

The DPW was collected in intermediate bulk containers (IBCs) from the outlet of the anaerobic digester at a local dairy processing facility in Casino, NSW, Australia. The elemental composition of the DPW sample was analysed by the Environmental Analysis Laboratory at Southern Cross University, NSW, Australia. Synthetic wastewater was prepared by dissolving ammonium chloride (NH₄Cl) and monopotassium phosphate (KH₂PO₄) in municipal tap water to simulate the NH₄⁺-N and PO₄³⁻-P concentrations present in DPW, respectively. These chemicals, manufactured by Sigma-Aldrich (Darmstadt, Germany), were also purchased from Merck Life Science Pty Ltd., Victoria, Australia.

2.2. Design and Construction of a Pilot-Scale Treatment System

A pilot system for chemical precipitation and air stripping of NH₄⁺-N in dairy processing wastewater was designed and constructed in collaboration with McKeeCo Engineering (East Lismore, NSW, Australia) and subsequently installed at a site of Southern Cross University, NSW, Australia. Figure 1 presents the schematic diagram of pilot system, and Figure S1 (in Supplementary Materials) shows the physical layout of the actual pilot-scale treatment system.

The pilot system is semi-portable plant consisting of individual tank units with pumps to transport wastewater between each stage of the treatment process. The steel mixing tank used for mixing wastewater with chemicals for precipitation is a cone-bottom vessel with a straight side height of 1200 mm, a diameter of 1100 mm, and a cone length of 1000 mm, features a 300 mm diameter outlet. It is the largest vessel in the system, with a capacity of up to 1400 L of wastewater (Figure S1 and Figure 1). It is equipped with a twin-propeller mixing shaft connected to a top-mounted motor and is controlled by a variable speed drive, allowing the mixing speed to be adjusted. The mixing tank has a manhole entry port on top, adjacent to the motor for access into the tank, and an outlet valve connected to the tank’s base. Wastewater is transferred between the intermediate bulk containers (IBCs) and the mixing tank using PVC-reinforced hose connected to a portable centrifugal pump with camlock fittings. A 400 L collection tub is positioned directly below the outlet of the mixing tank to collect the precipitate (solids–liquid slurry) discharged from the bottom of mixing tank. A compressed-air-driven diaphragm pump conveys the solids–liquid slurry from the tub to the filter press for dewatering.

The air stripper is a steel vessel (370 mm diameter × 1250 mm height) with a 140 L capacity (Figure S1 and Figure 1). It features a hinged lid with a silicone gasket and pressure relief valve fitted with 50 mm triclover fitting for infeed. The vessel includes three 100 mm glass inspection ports on the side, a 100 mm outlet with two 50 mm triclover fittings controlled by butterfly valves, and a hinged bottom door for easy access. Multiple inlet and outlet ports allow liquid to pass through the system efficiently. The air stripper is mounted on a bespoke monitoring platform above a 1000 L IBC. The air stripping tank has fine mist-spraying full circle nozzles with an orifice size of approximately two millimetres, which is fed by the incoming wastewater from the tank’s supply line, spraying 6 L/min. The mist sprayers are attached and positioned in the centre of the tank, ensuring the mist is sprayed throughout the whole tank to maximise the contact surface area between the wastewater and air.

The air stripper tank is equipped with airlines that feed through a port hole inside the tank. The air is injected at ambient temperature at the tank’s base using a 7 bar air compressor (Figure S1 and Figure 1). The air is injected counter-currently, creating a vortex action inside the tank to ensure the air contacts the mist, stripping the ammonia efficiently. The air stripping system is equipped with an airflow metre to control the airflow rate, to allow testing on the different airflows to optimise stripping efficiency.

Wastewater is pumped from the IBC into the air stripper above using a centrifugal pump and a filter to remove any unwanted debris that could block the units spray nozzles. The IBC below the air stripper is fitted with a temperature probe, pH metre and heaters, attached using cable glands to ensure the container would remain airtight. The air stripper is connected to the IBC via a solenoid valve that allows wastewater to be gravity-fed back into the IBC for recirculation when opened. Recirculating the wastewater allows for the maximum amount of ammonia recovery after air stripping.

Once the interaction between the air and misted wastewater has commenced, the NH₄⁺-N will be converted from liquid to NH₃ gas. Once the NH₄⁺-N has reached the gaseous state, it is exhausted through an outlet fitted to the top of the tank, which is connected to NH₃ recovery systems (Figure S1 and Figure 1). The recovery system is composed of a filter that captures the NH₃ through the interaction of the NH₃ gas and granular activated carbon (1.5 kg) as the NH₃ contacts the surface area of the activated carbon. Once the NH₃ passes through the filter, the remaining NH₃ will be captured and recovered in an acid bath composed of sulfuric acid (0.10 M), forming ammonium sulphate.

2.3. Experimental Procedure

2.3.1. Air Stripping Trial with Synthetic Wastewater

Figure 1 shows the schematic diagram of pilot setup for the trials. A series of air stripping trials were conducted using synthetic wastewater under varying pH, temperature, and airflow conditions, as summarised in

Table 1. Detailed operating conditions for the pilot-scale air stripping trials using synthetic wastewater.

Trials	Composition of Synthetic Wastewater, mg/L		Synthetic Wastewater, L	Airflow, L/m	Temperature (°C)	pH
	PO43−-P	NH4+-N
1	32.2	71.0	200	150	22	9
2	31.0	67.0	200	150	22	10
3	33.1	70.5	200	150	22	11
4	30.3	75.5	200	150	32	9
5	32.0	73.8	200	150	32	10
6	30.7	70.7	200	150	32	11
7	31.6	76.0	200	300	22	9
8	29.8	77.3	200	300	22	10
9	30.4	72.5	200	300	22	11
10	33.3	75.3	200	300	32	9
11	28.1	70.0	200	300	32	10
12	32.3	73.0	200	300	32	11

.

For each trial, 200 L of synthetic wastewater was prepared in the IBC located beneath the air stripping tower. The target pH and temperature were maintained using NaOH solution and an immersion heater, respectively. During operation, wastewater was pumped from the IBC to the spray nozzles at the top of the air stripper, generating fine droplets inside the stripping column. Compressed air was introduced counter-currently at the base of the tank, creating strong mixing and promoting effective contact between the air and water droplets to maximise ammonia volatilisation. The exhaust gas stream containing NH₃ exited through an outlet at the top of the stripper and was subsequently passed through a GAC column and an acid bath. Sprayed wastewater was allowed to drain back into the IBC for continuous recirculation throughout the six-hour trial period. Samples (50 mL) were collected from both the IBC and the acid bath every two hours. Initial samples taken prior to operation. At the end of each trial, the GAC was removed and analysed for NH₄⁺-N content.

2.3.2. Air Stripping Trial with DPW

A chemical precipitation step was incorporated prior to the air stripping trial with DPW (Figure 1). Magnesium chloride (MgCl₂) was used to precipitate PO₄³⁻-P from the wastewater. Laboratory experiments were conducted to determine the optimum MgCl₂ dosing rate and pH conditions for precipitation, and the corresponding results are provided in the Supplementary Materials (Figure S2). Eight hundred litres (800 L) of DPW were pumped from the IBC into the mixing tank through a flowmeter to ensure accurate volumetric control (Figure 1). MgCl₂ flakes were manually added to the mixing tank at a dose of 2.4 g L⁻¹, and the wastewater was thoroughly mixed. The pH was then adjusted to 9.5 using sodium hydroxide (NaOH) to promote precipitation. The mixture was agitated for one hour to ensure adequate interaction between the added chemicals and the wastewater, followed by one hour of settling. Subsequently, 100 L of the precipitate (solids–liquid slurry) was withdrawn from the bottom of the mixing tank, collected in the collection tub, and pumped to a filter press for dewatering and recovery of the solids. The recovered precipitates were dried over 48 h for elemental and XRD analysis. Clarified wastewater samples were collected from the top of the mixing tank. Initial wastewater samples were taken prior to the precipitation step. All samples were analysed for PO₄³⁻-P and NH₄⁺-N concentrations.

A similar air stripping trial was conducted on treated wastewater (post-precipitation) over an 18 h period. The PO₄³⁻-P depleted clarified wastewater withdrawn from the top of the mixing tank was pumped to the IBC (air stripping). In addition, the PO₄³⁻-P depleted wastewater recovered after filter press was combined with the wastewater in the IBC (air stripping). The operating conditions were selected based on those previously identified as achieving the maximum NH₄⁺-N removal efficiency in the trials using synthetic wastewater. During the trial, 50 mL samples were collected from both the IBC and the acid bath every two hours, including initial samples taken before system start-up. All samples were analysed for NH₄⁺-N. The amount of NH₄⁺-N removed from the wastewater was calculated based on the difference between initial and final aqueous concentrations.

2.3.3. Removal Efficiency and Recovery

The removal efficiency and recovery of NH₄⁺-N and PO₄³⁻-P in wastewater were calculated based on the following equations:

$$Removal efficiency= {\displaystyle \frac{C_{Influent }- C_{Effluent}}{C_{Influent }}}\times 100$$

(1)

where C_Influent and C_Effluent represent the influent and effluent concentrations of NH₄⁺-N or PO₄³⁻-P.

$$Recovery={\displaystyle \frac{Captured {\mathrm{N}\mathrm{H}}_4^{+}-\mathrm{N} \mathrm{o}\mathrm{r} {\mathrm{P}\mathrm{O}}_4^{3-}-\mathrm{P} }{Removed {\mathrm{N}\mathrm{H}}_4^{+}-\mathrm{N} \mathrm{o}\mathrm{r} {\mathrm{P}\mathrm{O}}_4^{3-}-\mathrm{P} }}\times 100$$

(2)

2.4. Analytical Methods

The elemental composition of the raw DPW samples and the solid precipitates was analysed by a commercial laboratory (Environmental Analysis Laboratory, Southern Cross University, NSW). Analysis of NH₄⁺-N and PO₄³-P for the liquid samples from air stripping tank and acid bath were performed according to standard methods (APHA 2017) using Flow Injection Analysis (FIA). To quantify NH₄⁺-N adsorbed onto the solid samples (GAC), the samples were extracted using an HCl solution, and the resulting extracts were analysed by FIA.

The X-ray diffraction (XRD) on solid precipitates was conducted. The solid was firstly powered in a mortar and pestle and bone dried at 50 °C for 24 h. The XRD analysis was performed on a Bruker D4 Endeavour X-ray Diffractometer (Billerica, MA, USA). Each sample ran for a 30 min analysis. Analysis of the diffractograms was performed using Bruker Diffrac (Bruker, MA, USA) evaluation programme: identifying peaks, characterising material and generating a semi quantitative ratio (%) of characterised material.

2.5. Modelling

2.5.1. Kinetic Model

The time series data of NH₄⁺-N removal in wastewater were fitted to mathematical model using pseudo-first-order, pseudo-second-order (PSO), and Elovich kinetic equations. The linear forms of Pseudo-first-order, PSO and Elovich kinetic models can be represented by Equations (1)–(3), respectively:

$$q_t = q_e(1-exp(-K_{1}t\left)\right)$$

(3)

$$q_t =K_2{q_e}^{2}t/1+K_2{q}_{e}t$$

(4)

$$q_t =a \mathit{ln}\left(t\right)+b$$

(5)

where q_t (mg L⁻¹) and q_e (mg L⁻¹) are the amount p-chlorocresol removed from wastewater at time t and at equilibrium, respectively; k₁ (hr⁻¹) and k₂ (L mg⁻¹ hr⁻¹) are the rate constant of Pseudo-first-order and PSO, respectively; a (mg L⁻¹ hr⁻¹) and b are Elovich constants. The Excel Solver is used to calculate the models’ parameters.

2.5.2. Response Surface Model

To identify the optimal factor levels (such as pH, temperature, and airflow) for maximising NH₄⁺-N removal from wastewater, response surface model (RSM) was used to determine the significant factors and their interaction effects. A nonlinear surface response model was developed using 4 adjustable parameters to determine the effects of two variables on NH₄⁺-N removal, namely temperature and pH, as follows:

$$y={\alpha }_0+{\alpha }_1{x}_T+{\alpha }_2{x}_{pH}+{\alpha }_3{x}_T{x}_{pH}$$

(6)

where $y$ is the response NH₄⁺-N removal, ${\alpha }_i$ are the adjustable parameters ($i=0\dots 3$), $x_T$ is temperature, and $x_{pH}$ is potential hydrogen (pH). The model is fitted separately for two flow rates, $150$ and $300$ L/min. The adjustable parameters that minimise error between the experimental data and model predictions are determined using the least squares algorithm (lsqcurvefit), part of MATLAB 2023b.

3. Results

Table 2. Elemental composition of DPW.

Elements	Concentration (mg/L)
pH	7.04
PO43-P	31.8
NH4+-N	72.4
Iron	1.12
Silicon	11.8
Magnesium	11.1
Potassium	28.8
Sodium	390
Chloride	37.4
Sulfur	21.4
Calcium	35

presents the elemental composition of the DPW. On average, the nutrient concentrations were ≈35 mg/L of PO₄³-P and ≈72 mg/L of NH₄⁺-N. The DPW also contains notable levels potassium, sodium, chloride, sulphur and calcium (Table 2).

3.1. NH₄⁺-N Removal

The air stripping trials using synthetic wastewater were conducted to evaluate the effects of pH, temperature, and airflow rate on NH₄⁺-N removal. NH₄⁺-N removal and recovery at three pH levels (9, 10, and 11), two airflow rates (150 and 300 L/min), and two temperatures (22 °C and 32 °C) are presented in Figure 2 and

Table 3. Removal and recovery of NH₄⁺-N from synthetic wastewater.

Airflow (L/m)	Temperature (°C)	pH	NH4+-N Removal		* NH4+-N Captured by Acid Bath and GAC (%)
			mg/L	%
150	22	9	0.8	1.1	95.5
150	22	10	5.5	9.6	86.1
150	22	11	9.0	14.4	80.1
150	32	9	8.5	10.8	85.6
150	32	10	9.0	12.2	81.2
150	32	11	13.6	21.6	79.1
300	22	9	2.0	2.6	91.0
300	22	10	9.8	12.1	85.4
300	22	11	12.0	16.6	83.3
300	32	9	17.0	21.7	78.2
300	32	10	18.3	26.2	76.6
300	32	11	29.1	39.8	72.0

Note: * Percent recovery is calculated based on the removal.

. The results showed a clear trend of enhanced NH₄⁺-N removal with increasing temperature, pH, and airflow rate. Among the tested conditions, pH 11 consistently achieved the highest removal across all experiments. The combination of 300 L/min airflow, 32 °C, and pH 11 resulted in the maximum NH₄⁺-N removal of 39.8% from 200 L of synthetic wastewater after 6 h of air stripping (Figure 2; Table 3).

3.1.1. Effect of pH

A clear trend was observed over the air stripping trials, as the pH of synthetic wastewater increased, the effectiveness of removing NH₄⁺-N improved (Table 3; Figure 3). A significance difference (p > 0.05; Tables S1 and S2 in Supplementary Materials) in the NH₄⁺-N removal rates is observed among the pHs levels across all temperatures and airflow condition. With increased the pH from 9 to 11, the maximum NH₄⁺-N removal was achieved from 1.1% to 39.8%, under conditions of high airflow rate (300 L/min) and elevated temperature (32 °C). However, the lowest NH₄⁺-N removal rate (14.4%) occurred at the higher pH but with a low airflow rate (150 L/min) and a low temperature (22 °C).

Overall, the NH₄⁺-N removal rate at pH 10 was higher than at pH 9, but still notably lower than the removal achieved at pH 11. For instance, at an airflow rate of 300 L/min and 32 °C, the NH₄⁺-N removal rate was 26.2% at pH 10. In comparison, under the same conditions, the NH₄⁺-N removal rate at pH 9 was only 21.7% (Table 3; Figure 3).

Among all tested combinations of airflow and temperature, pH 9 consistently yielded the lowest NH₄⁺-N removal rates, ranging from 1.1% to 21.7% (Table 3; Figure 3). Overall, a one-unit increase in pH of synthetic solution (from 9 to 10) resulted in a 0.13- to 7.73-fold increase in NH₄⁺-N removal rate across all combinations of airflow and temperature. Similarly, a two-unit increase in pH (from 9 to 11) led to a 0.84- to 12.1-fold increase in NH₄⁺-N removal rate under the same conditions.

3.1.2. Effect of Temperature

Raising the synthetic wastewater temperature from 22 °C to 32 °C significantly (p > 0.05; Table S2) improved NH₄⁺-N removal efficiency across all experimental conditions, regardless of pH level or airflow rate (Table 3). At 32 °C, the highest removal efficiency (39.8%) occurred at pH 11 with the maximum airflow rate (300 L/min), demonstrating a strong synergistic effect between elevated temperature, increased aeration, and alkaline pH in promoting NH₄⁺-N volatilisation or transformation. A 10 °C increase from 22 °C to 32 °C raised NH₄⁺-N removal efficiency at pH 11 under low airflow by about 0.5-fold, indicating that even limited aeration is enhanced by thermal input. Under high airflow at the same pH, the same temperature increase improved removal by approximately 1.4-fold, further emphasising the combined benefits of thermal and aeration effects under alkaline conditions. These findings highlight temperature as a critical operational parameter for optimising NH₄⁺-N removal, particularly when combined with high pH and sufficient aeration.

3.1.3. Effect of Airflow Rate

A clear positive correlation was observed between increasing airflow rate and NH₄⁺-N removal efficiency (Figure 2; Table 3). A significance difference (p > 0.05; Table S3) in the NH₄⁺-N removal rates is observed among the airflow levels across all pHs and high temperature condition. For instance, doubling the airflow from 150 to 300 L/min increased NH₄⁺-N removal by approximately 0.15-fold at 22 °C and pH 11. Under the same pH but at the higher temperature of 32 °C, the enhancement was far more pronounced, with NH₄⁺-N removal increasing by approximately 0.84-fold. These findings indicate that both airflow rate and temperature strongly influence NH₄⁺-N removal performance, particularly under highly alkaline conditions.

3.1.4. Response Surface Model

Figure 4 presents 3D RSM plots that illustrate how pH and temperature interact to influence NH₄⁺-N removal under two different airflow rates. The fitted parameters and validation statistics, residual trends, and sensitivity analysis are presented in the Supplementary Materials.

The response surface model indicated that at the lower airflow rate (150 L/min), NH₄⁺-N removal increases gradually with higher pH and temperature, but the overall removal remains relatively modest (up to ≈22%), reflecting the limited stripping capacity at reduced aeration. In contrast, at the higher airflow rate (300 L/min), the response surface shows a steeper gradient, indicating stronger sensitivity of NH₄⁺-N removal to both pH and temperature. Under these conditions, the highest removal occurs (≈40%) at the combination of high pH (11) and elevated temperature (32 °C), demonstrating the synergistic effect of increased alkalinity and enhanced aeration. Overall, the results highlight that greater airflow intensifies the influence of pH and temperature on NH₄⁺-N removal, leading to substantially improved performance compared with the lower-airflow scenario.

3.2. NH₄⁺-N Recovery

A significant variation in the NH₄⁺-N recovery was found across the different pHs levels, regardless of temperature and airflow conditions (p > 0.05; Tables S4 and S5). Despite the modest removal of NH₄⁺-N from the wastewater, the combined acid bath and GAC system consistently together achieved high ammonia-capture efficiencies (72–95%; Table 3). This demonstrates that most of the volatilised NH₃ was successfully recovered downstream, even when stripping rates were not maximised. The particularly high capture efficiencies observed under lower pH and lower stripping intensity such as ≈95.5% at pH 9, 22 °C, and 150 L min⁻¹, suggest that slower and more gradual NH₃ release enhances overall recovery. Under these conditions, the acid bath has sufficient residence time to absorb gaseous NH₃ almost completely, preventing losses to the exhaust stream and resulting in near-quantitative conversion to ammonium salts.

NH₄⁺-N adsorption onto GAC is presented in Figure 5. The results showed that GAC effectively adsorbs NH₃.

Overall, GAC adsorption capacity increased with rising pH and temperature of the wastewater solution (Figure 5). These conditions promote NH₃ volatilisation, thereby enhancing NH₃ availability and interaction with the GAC surface. The highest adsorption occurred at the maximum airflow rate of 300 L min⁻¹, reaching ≈18 mg kg⁻¹, indicating enhanced mass transfer and greater loading of NH₃ onto the GAC. These trends highlight the complementary role of GAC in stabilising and storing recovered nitrogen, thereby enhancing the value of the recovery system and supporting downstream reuse applications.

3.3. NH₄⁺-N Removal and Recovery from DPW

The air stripping trial using DPW was conducted at pH 11, 32 °C, and an airflow rate of 300 L/min. This operational condition was identified as effective for maximising NH₄⁺-N removal in synthetic wastewater (Figure 2; Table 3). The corresponding results for DPW are presented in

Table 4. Removal and recovery of NH₄⁺-N and PO₄³⁻-P from DPW.

Experimental Process/Stage	Airflow, L/m	Temperature (°C)	pH	NH4+-N Removal		PO43−-P Removal		** NH4+-N Recovery %	P Content (%)
				mg/L	%	mg/L	%
Precipitation		22	9.5	20.0	26.0	27.0	83.7
Precipitation plus filtering		22	9.5	33.3	43.4	30.0	93.0
Air stripping	300	32	11	36.3	69.6
* Precipitation, filtering and air stripping				61.0	79.3
Acid bath	300	32	11					68.0
Precipitate solids									7.62

Notes: * Total removal efficiency was calculated based on the initial concentration of NH₄⁺-N in DPW prior to precipitation stage and the final concentration of it after air stripping. The removal efficiency at each stage was calculated based on the influent and effluent concentrations of NH₄⁺-N and PO₄³⁻-P. ** Percent recovery was calculated based on the removal in air stripping.

and Figure 6. The removal efficiencies of PO₄³⁻-P and NH₄⁺-N from DPW in the pilot-scale chemical precipitation trial are presented in Table 4. PO₄³⁻-P removal reached ≈84% prior to filtration and increased to ≈88% following filtration (Table 4). In contrast, NH₄⁺-N removal was lower at ≈26% before filtration and improved to ≈43% after filtration.

XRD analysis confirmed the presence of mineral struvite (MgNH₄PO₄.6H₂O) in the precipitates (Figure S3). Elemental analysis further showed that the precipitate contained high concentrations of magnesium and phosphorus (Table S1).

Air stripping removed the NH₄⁺-N from the DPW by ≈70% over an 18 h period (Table 4; Figure 6). Approximately 50% NH₄⁺-N removal was achieved within the first 10 h, with an additional removal of 20% occurring over the subsequent 8 h. Thus, a total of 18 h of air stripping was required to achieve ≈70% NH₄⁺-N removal. For comparison, synthetic wastewater solution exhibited a faster removal rate of NH₄⁺-N under identical conditions, achieving 40% NH₄⁺-N reduction within 6 h, while DPW achieved only 34% removal over the same duration (Figure 6). Depending on the final concentration of the NH₄⁺-N required in DPW for environmental regulation, the air stripping process could be continued or refined. The 10 h of air stripping appears to be the cost-effective time for air stripping the DPW, as this period resulted in a reduction in NH₄⁺-N concentrations of ≈50%.

The curve fitting indicates that the Elovich model (R² = 0.970) provided the best correlation with the experimental data, followed by the PSO model (R² = 0.925), whereas the pseudo-first-order (R² = 0.845) model showed relatively poorer agreement (Figure 6). This suggests that the removal process is not purely governed by physical stripping but also involves chemisorptive interactions with the gas-liquid interface and possibly with reactor surfaces. The superior fit of the Elovich model further supports the involvement of heterogeneous mass transfer and surface interactions, which may dominate under high pH and long aeration times (Figure 6).

The recovery of NH₄⁺-N in the acid bath is shown in Table 4. The result indicated that the acid bath effectively captured volatilised NH₃. Under the air stripping conditions of pH 11, 32 °C, and an airflow rate of 300 L/min using DPW, approximately 68% of the released NH₃ was recovered in the acid bath (Table 4).

4. Discussion

The results highlighted key operational factors, particularly pH, temperature and airflow rate that strongly influence air stripper performance in maximising NH₄⁺-N removal from wastewater (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6; Table 3 and Table 4).

4.1. NH₄⁺-N Removal

One of the critical factors is the pH of the wastewater, which emerged as a key determinant of air stripper effectiveness for NH₄⁺-N removal. This trend was consistently observed across the results of the trials: as the pH of wastewater increased, there was a corresponding enhancement in NH₄⁺-N removal rate (Table 3; Figure 3). This effect can be explained by the influence of pH on the equilibrium between ammonium ions and free ammonia, which directly affects NH₄⁺-N removal. pH plays a critical role in air stripping of ammonia because adjusting it shifts the balance between NH₄⁺-N and NH₃, thereby increasing the fraction of volatile ammonia available for removal [28,29]. Under alkaline conditions (i.e., pH ≥ 9.0), the equilibrium shifts toward the formation of free ammonia species rather than ammonium ions, thereby, these species can be mass transferred into gaseous phase via air stripping [13,24,25,30].

Once the pH exceeded 9, the results became more consistent, achieving NH₄⁺-N removal rate of over ≈10% in synthetic wastewater (Table 3). At pH 11, the highest NH₄⁺-N removal was observed at ≈40% in the synthetic wastewater over a six-hour period (Table 3), and up to 70% in DPW after 18 h (Table 4; Figure 6). In contrast, solutions at pH 9 showed relatively low NH₄⁺-N removal, with removal dropping below 3% in some cases (Table 3). Increasing the pH from 9 to 11 reduced the time required for the same amount of NH₄⁺-N removal (as at pH 9) from 6 h to ˂3.5 h (Figure 2). Similar results observed in the other studies, where the optimum pH for higher NH₄⁺-N removal via air stripping technique is pH ≈ 11 [14,19,21,26,31]. Maintaining the pH at ≈11 is favourable for ammonia stripping because the increase in hydroxyl ions drives the ammonium ions to transfer to volatile ammonia [13]. This may be attributed to the chemical speciation of ammonia: at higher pH values above 9, the equilibrium between ammonium (NH₄⁺) and ammonia (NH₃) shifts toward the un-ionised, volatile NH₃ form. Our findings indicate that increasing the pH to 11 significantly enhances NH₃ stripping from wastewater, confirming that NH₄⁺-N removal is more efficient at pH 11 than at pH 9 or 10. The results are consistent with the optimisation model predictions (Figure 4). This also aligns with previous research which showed that higher pH levels (over 9) enhance NH₄⁺-N removal from wastewater through air stripping [13,19,26,29].

Along with pH, temperature plays a critical role in the air stripping of NH₃, as higher temperatures consistently enhanced NH₄⁺-N removal from the wastewater (Table 3). For instance, at 22 °C and pH 11 NH₄⁺-N removal rate was below ≈17% after six hours of air stripping, whereas at 32 °C under the same pH, removal increased to ≈40%. These findings indicate that higher temperatures greatly enhance NH₄⁺-N removal efficiency. Notably, raising the temperature by 10 °C (from 22 °C to 32 °C) more than doubled the NH₄⁺-N removal (Table 3), highlighting the strong temperature dependence of air stripping. This effect is likely driven by enhanced NH₃ volatilisation at higher temperatures, consistent with Henry’s law [25]. Similar results have been reported in other studies, including air stripping in a water-sparged aerocyclone reactor [21], a semi-batch jet loop reactor [24], and a rotating packed beds [26], where NH₄⁺-N removal increased with the aqueous phase temperature, and the effect became more pronounced when the temperature exceeds 25 °C. Increasing the temperature enhances the molecular diffusion of ammonia in the gas film and reduces liquid-phase viscosity, which together improve mass transfer. Higher temperatures promote the conversion of ammonium ions (NH₄⁺) to free ammonia (NH₃), which is more readily transferred to the gas phase during air stripping. In addition, elevated temperatures shift the liquid–gas equilibrium toward the gaseous phase, thereby further enhancing ammonia volatilisation [24,26]. Furthermore, higher temperatures reduce droplet size and enhance dissolution, thereby strengthening the driving force for ammonia desorption from wastewater. Together, these effects accelerate the overall removal of ammonia [24,26,32].

The airflow rate enhanced the effectiveness of the air stripping for NH₄⁺-N removal; however, its effect depends on other operating parameters, particularly pH and temperature (Figure 2; Table 3). For instance, at pH 11 and 22 °C temperature, an airflow rate of 150 L/min achieved a ≈14% removal rate, which increased to ≈18% when the airflow was doubled to 300 L/min under the same conditions. At the same pH but at a higher temperature of 32 °C, the removal rate rose from ≈22% to ≈40% with the same increase in airflow to 300 L/min. These results indicate that doubling the airflow rate, particularly under alkaline and higher-temperature conditions, can nearly double the removal efficiency. Similar effects of increasing airflow rate on enhancing NH₄⁺-N removal have been reported in other studies of air stripping systems [14,21,26]. Quan et al. [21] reported that a high airflow rate (>85 L/min) markedly improved both ammonia-removal efficiency and the mass-transfer coefficient during air stripping in a water-sparged aerocyclone reactor. This supports the theory that increasing the airflow rate enhances contact between the air and wastewater, thereby improving removal performance. A higher airflow rate reduces the size of gas bubbles dispersed in the liquid and increases gas holdup, both of which promote more efficient mass transfer [33,34]. Increasing the airflow rate enhances the mass transfer rate by expanding the air–water interfacial area and improving gas–liquid contact [35]. The process begins with the diffusion of dissolved ammonia within the liquid. The molecular ammonia then migrates toward the gas–liquid interface, where it volatilises and escapes into the gas phase. This mass transfer is enhanced by air turbulence, which reduces the boundary layer thickness at the liquid surface. A thinner boundary layer reduces resistance, thereby facilitating more efficient ammonia removal [36]. Moreover, higher airflow rate advances greater sheer stress on the surface of water droplets which correspondingly yields in the driving force for higher ammonia removal and mass transfer coefficient [14,21,37].

Overall, our results suggest that the operating conditions for maximising NH₄⁺-N removal from wastewater via air striping are a pH of 11, a temperature of 32 °C, and an airflow rate of 300 L/min. Under these conditions, up to ≈40% of NH₄⁺-N was removed from the synthetic wastewater within 6 h (Table 3). DPW exhibited a slightly lower removal efficiency, achieving only ≈34% NH₄⁺-N removal after 6 h under the same conditions. This reduced performance may be attributed to the presence of additional metals and biomass in DPW, which are absent in the synthetic wastewater (Table 2). A similar result was reported in a study, where NH₄⁺-N removal rates for dairy processing wastewater [3], anaerobically digested pig slurry [19] and piggery wastewater [38] were lower than those observed for a synthetic wastewater solution. However, after 18 h of air stripping under the same conditions (pH 11, 32 °C, and an airflow rate of 300 L/min), NH₄⁺-N removal reached ≈70%. This result further demonstrates that air stripping becomes more effective under higher pH, elevated temperatures, and increased airflow rates. The results are also consistent with the optimisation model predictions (Figure 4), which showed that higher airflow intensifies the influence of pH and temperature on NH₄⁺-N removal, leading to substantially improved performance compared with the lower-airflow scenario.

The experimental data showed a better fit with the Pseudo-second-order (PSO) model and the Elovich model (R² = 0.970) compared to the Pseudo-first-order model (R² = 0.845). These findings suggest that NH₄⁺-N removal in DPW via air stripping does not follow a simple first-order kinetic process, as commonly assumed in ideal liquid–gas mass transfer systems [39]. The better fit with PSO and Elovich models indicates that the kinetics are governed by more complex mechanisms, potentially involving adsorption–desorption dynamics, surface reactions, and non-ideal diffusion behaviour [40,41]. The Elovich model is often applied to systems where heterogeneous diffusion and surface interactions dominate, which may reflect the complexity of real wastewater matrices, especially in dairy effluents rich in organic and inorganic compounds [42].

The stronger fit to the PSO and Elovich models implies that NH₄⁺-N removal is controlled not only by bulk liquid–gas transfer but also by physicochemical interactions within the wastewater matrix. These may involve buffering effects, competition with coexisting ions, and the influence of high alkalinity at elevated pH [43]. This is consistent with previous reports where NH₄⁺-N removal from complex wastewaters often deviates from classical first-order kinetics [44].

4.2. NH₄⁺-N Recovery

The integrated acid bath and GAC system consistently achieved high overall capture efficiencies (72–95%), demonstrating its effectiveness in recovering volatilised ammonia. Acid solutions, particularly H₂SO₄, are widely recognised for their strong capacity to absorb gaseous NH₃ through rapid protonation and the formation of stable ammonium salts [45]. The higher capture efficiencies observed at lower pH and reduced stripping intensity, for example, ≈95.5% NH₃ capture at pH 9, 22 °C, and an airflow of 150 L min⁻¹, indicate that slower NH₃ volatilisation rates enhance the residence time of the gas in the absorption, enabling near-complete trapping in the acid bath. In contrast, NH₄⁺-N adsorption using GAC reflects a different set of controlling mechanisms. GAC adsorption capacity generally increases with pH and temperature due to enhanced NH₃ volatilisation upstream and improved ion exchange and surface interaction dynamics. The enhancement can be attributed to several complementary mechanisms. First, higher pH and elevated temperature increase the volatilisation of ammonia, resulting in a greater NH₃ flux entering the GAC and acid bath system. Ammonium removal is further supported by electrostatic attraction and cation-exchange interactions, as GAC commonly contains negatively charged functional groups capable of binding NH₄⁺-N [46,47,48]. In addition, the higher NH₃ concentrations generated under these conditions promote greater micropore utilisation within the GAC structure, improving adsorption performance. Overall, the observed adsorption capacities fall below the typical range reported for biochar in previous studies [49,50,51,52]. Nevertheless, the results still demonstrate effective ammonium capture, consistent with findings from similar systems. The lower capacities may be attributed to the placement or configuration of the GAC within the setup, which can influence the contact efficiency and mass transfer between NH₃ the GAC surface.

In comparison with the synthetic wastewater, DPW showed a slightly lower NH₄⁺-N recovery efficiency under the same operating conditions, achieving approximately 68%. This reduction is likely attributable to the more complex matrix of the dairy effluent, which may introduce additional competing compounds or buffering effects that limit ammonia volatilisation and subsequent capture. In the precipitation trial, XRD analysis confirmed the presence of struvite in the recovered solids (Figure S3). In addition to phosphorus, struvite also contains nitrogen, another important plant nutrient. However elevated iron and moderate lead content in the precipitate solids (Table S1) require caution with repeated or extensive land application. The high alkalinity of the precipitate solids may also necessitate pH adjustment prior to agricultural or environmental application.

5. Conclusions

A pilot-scale evaluation of NH₄⁺-N removal via air stripping was thoroughly demonstrated in this study. Air stripping removed approximately 70% of NH₄⁺-N from the DPW over an 18 h period. About 50% removal was achieved within the first 10 h, with a further 20% occurring during the subsequent 8 h. This indicates diminishing removal rates after 10 h; therefore, extended stripping durations are justified primarily when higher removal efficiencies are required to meet discharge limits or downstream treatment objectives. Air stripping effectively removes NH₄⁺-N from wastewater by shifting the ammonium–ammonia equilibrium toward gaseous NH₃, a process strongly influenced by operational conditions such as pH, temperature, and airflow rate. Higher pH and elevated temperature favour ammonia volatilisation, while increased airflow enhances mass transfer, collectively improving stripping efficiency. Higher airflow rates (>300 L min⁻¹), in combination with varying wastewater flow rates, may be explored in future studies, as these operating conditions depend on reactor configuration and gas–liquid contact efficiency. Once released, the volatilised NH₃ is efficiently captured by the biochar (GAC) and acid bath system, where and biochar provides additional adsorption capacity through cation exchange and surface functional groups and acidic solution converts NH₃ into stable ammonium salts. This combined approach not only prevents gaseous NH₃ emissions but also transforms nitrogen pollutants into useful products such as fertiliser-grade ammonium salts and nutrient-enriched biochar. The overall system strongly aligns with circular economy principles by enabling nutrient recovery and reuse, reducing chemical inputs, and minimising waste. This indicates diminishing removal rates after 10 h; therefore, extended stripping durations are justified, primarily when higher removal efficiencies are required to meet discharge limits or downstream treatment objectives. The treated dairy processing wastewater can be beneficially reused for agricultural irrigation and fertigation, industrial non-potable applications, or environmentally compliant discharge, thereby contributing to water conservation and nutrient recycling within a circular economy framework. Our study clearly demonstrated that integrating air stripping with an efficient capture system provides a reliable and sustainable pathway for nitrogen recovery in wastewater treatment. This approach strengthens the feasibility of establishing a more circular nitrogen cycle, which depends on robust and scalable ammonia/ammonium recovery technologies to expand the potential market for reclaimed nitrogen products. Given its strong removal and recovery performance, the proposed NH₄⁺-N recovery system is well suited for practical implementation in dairy processing facilities.