Nitrogen Recovery and CO₂-Assisted Carbonate Formation from High-Ammonium Poultry Digestate via Gas-Driven Ammonia Stripping Coupled with Gypsum-Mediated Absorption

Abstract

High-ammonium poultry digestate from thermophilic dry anaerobic digestion is often recycled, but excessive ammonia accumulation may inhibit anaerobic digestion and reduce process stability. This study developed a gas-driven ammonia stripping process coupled with gypsum-mediated absorption for digestate deammonification, nitrogen recovery, and CO₂-assisted carbonate formation. Laboratory stripping experiments were conducted using simulated biogas to evaluate the effects of pH, temperature, and gas–liquid ratio. Under the selected condition of pH 11, 65 °C, and a gas–liquid ratio of 2, NH₄⁺-N in 10 L digestate decreased from approximately 7980 to 1648 mg L⁻¹ within 12 h, corresponding to about 80% removal. In the absorption step, the slightly soluble CaSO₄ solution showed more stable NH₃ capture than the CaSO₄ suspension, and the corrected NH₃-N recovery reached approximately 90–95%. XRD, SEM-EDS, precipitate mass estimation, and gas-phase CO₂ variation supported the formation of CaCO₃-containing precipitates. Pilot-scale operation using real biogas further reduced NH₄⁺-N from approximately 8000 to 700–800 mg L⁻¹ during 36 h of extended pilot-scale operation. Overall, the coupled process provides a preliminary resource-recovery route integrating ammonia burden reduction, nitrogen recovery, sulfate transfer, and CO₂-assisted carbonate precipitation. However, full-scale sustainability still requires further long-term operation, complete nitrogen–carbon–calcium–sulfur mass balances, complete heat and energy-balance assessment, product-quality evaluation, and techno-economic or life-cycle assessment.

1. Introduction

Poultry manure is an attractive anaerobic digestion feedstock because of its high organic content and biogas potential [1]; however, its high nitrogen content frequently causes ammonia accumulation, process instability, and yield losses, particularly in dry or high-solids systems [2]. In the present study, the poultry digestate contained 8397 ± 347 mg L⁻¹ TN and 7864 ± 286 mg L⁻¹ NH₄⁺-N, further indicating a high ammonia burden that could increase the risk of ammonia accumulation during digestate recirculation. The inhibitory risk increases when pH and temperature shift the ammonium/free-ammonia equilibrium toward NH₃, which is the more membrane-permeable and biologically inhibitory species [3].

A range of ammonia control strategies has therefore been investigated for manure digestion systems, including digestate recirculation [4], in situ or side-stream gas stripping [5], simplified air stripping [6], membrane-based recovery [7], and vacuum thermal stripping coupled with acid absorption [8]. For chicken manure in particular, both digestate-side ammonia control and biogas-mediated stripping [9] have been shown to improve methane production, alleviate volatile fatty acid accumulation, and stabilize digestion under high-ammonia conditions.

Conventional ammonia capture commonly relies on strong mineral acids, especially sulfuric acid, to produce ammonium sulfate solutions or crystals [10]. Although effective, acid scrubbing introduces chemical handling demands and separates nitrogen recovery from carbon management. In parallel, gypsum-based ammono-carbonation [11] and flue-gas desulfurization gypsum carbonation studies have shown that sulfate transfer to ammonium and simultaneous CaCO₃ precipitation are chemically feasible [12], including under near-ambient conditions [13], and that the route has emerging techno-economic relevance for coupled nutrient and carbon valorization [14]. From a sustainability perspective, poultry digestate management should not be limited to ammonia removal alone. An ideal treatment route should simultaneously reduce ammonia inhibition risk, recover nitrogen as a reusable resource, minimize the dependence on strong mineral acids, and couple nutrient recovery with carbon-containing product formation. However, most existing ammonia stripping–absorption studies focus mainly on removal efficiency or ammonium salt production [8], whereas integrated pathways linking biogas-driven NH₃ release, gypsum-mediated NH₃ capture, and CO₂-assisted carbonate precipitation remain insufficiently investigated.

However, these strategies still have practical limitations. Acid scrubbing is efficient for NH₃ capture but relies on strong mineral acids and mainly produces ammonium salts without coupling nitrogen recovery with carbon-containing product formation. Membrane-based recovery can improve selectivity, but membrane fouling and maintenance costs remain major barriers for high-solid digestate [15]. Thermal or vacuum stripping may enhance ammonia release, but the associated energy demand can limit its farm-scale applicability [16]. Therefore, an alternative route that reduces ammonia burden, avoids strong acid dependence, and simultaneously promotes recoverable nitrogen and carbonate-containing products would be valuable for circular digestate management [17].

Therefore, this study proposed a coupled pathway in which NH₃ released from high-ammonium poultry digestate by biogas-driven stripping was captured by a slightly soluble gypsum medium, while the accompanying CO₂ participated in carbonate precipitation. The objectives of this study were to determine suitable operating conditions for NH₃ release from poultry digestate, evaluate NH₃ capture and ammonium enrichment in a gypsum-mediated absorption system, verify nitrogen recovery and CO₂-assisted CaCO₃ formation using liquid-, solid-, and gas-phase evidence, and assess the preliminary transferability of the coupled process under pilot-scale operation.

2. Materials and Methods

2.1. Feedstock, Reagents, and Initial Characterization

Poultry digestate was collected from a high-solids thermophilic dry anaerobic digestion system treating chicken manure at a chicken manure dry anaerobic digestion biogas research base in Shuangliu District, Chengdu, Sichuan Province, China. A representative digestate batch was collected during the stable operation period of the anaerobic digestion system. After collection, the digestate was stored in sealed polyethylene containers at 4 °C and thoroughly homogenized before each experiment.

The digestate had a pH of 8.32 ± 0.03, with COD, TN, NH₄⁺-N, TP, TS, VS, and turbidity values of 32,407 ± 265 mg L⁻¹, 8397 ± 347 mg L⁻¹, 7864 ± 286 mg L⁻¹, 869 ± 15.2 mg L⁻¹, 23.6 ± 0.06%, 11.7 ± 0.05%, and 1254 ± 24.6 NTU, respectively. These characteristics indicate that the digestate was a typical high-ammonium and high-solid poultry digestate and was suitable for evaluating ammonia stripping, nitrogen recovery, and CO₂-assisted carbonate precipitation. The initial physicochemical characteristics of the digestate are summarized in

Table 1. Physicochemical characteristics of poultry digestate.

Parameter	Value
pH	8.32 ± 0.03
TS (%)	23.6 ± 0.06
VS (%)	11.7 ± 0.05
COD (mg L−1)	32,407 ± 265.4
TN (mg L−1)	8397 ± 347
NH4+-N (mg L−1)	7864 ± 286
TP (mg L−1)	869 ± 15.2
Turbidity (NTU)	1254 ± 24.6

.

Industrial by-product gypsum was used as the CaSO₄ source. The gypsum was mainly composed of CaSO₄·2H₂O, with a supplier-reported purity of approximately 98%. Before use, the gypsum was air-dried, ground, and sieved through a 100-mesh sieve, corresponding to a particle size below approximately 150 μm. CaSO₄·2H₂O is only slightly soluble in water; PubChem reports a water solubility of approximately 0.2 g per 100 mL at 20 °C, which supports the design concept of maintaining excess gypsum solids to continuously replenish dissolved Ca²⁺ and SO₄²⁻ during absorption.

Analytical-grade NaOH was used for pH adjustment. A concentrated NaOH stock solution was prepared by dissolving 200 g of NaOH in deionized water and diluting the solution to 500 mL, corresponding to a concentration of 400 g L⁻¹. Deionized water was used for the preparation of the CaSO₄ absorption medium.

$$C_{NaOH}={\displaystyle \frac{200 \mathrm{g}}{500 \mathrm{mL}}}=0.40 \mathrm{g} {\mathrm{mL}}^{-1}=400 \mathrm{g} {\mathrm{L}}^{-1}$$

(1)

2.2. Laboratory-Scale Simulated-Biogas-Driven Ammonia Stripping System

The laboratory-scale ammonia stripping system consisted of a digestate reservoir, a vertical stripping column, a liquid spraying and recirculation unit, a simulated biogas supply unit, and a temperature-control unit. For each batch stripping experiment, 10 L of poultry digestate was added to the digestate reservoir. The digestate was continuously circulated from the reservoir to the top of the stripping column and distributed through a spray nozzle to improve liquid dispersion. Simulated biogas was introduced from the bottom of the column, allowing counter-current gas–liquid contact between the rising gas phase and the sprayed digestate. The presence of both the spray nozzle and packing material was intended to enhance gas–liquid contact and NH₃ transfer from the liquid phase to the gas phase.

The stripping column had an effective height of 1.5 m and an internal diameter of 30 cm, was packed with 15 mm polypropylene Pall rings. The simulated biogas consisted of 75% N₂ and 25% CO₂. No CH₄ or H₂S was added in the laboratory-scale tests to reduce flammability risk and to allow clearer evaluation of CO₂ participation in carbonate precipitation. Therefore, the term “simulated biogas” refers to a controlled CO₂-containing carrier gas, whereas real biogas was used only in the pilot-scale validation. Laboratory-scale results should therefore be interpreted as controlled simulated-gas tests rather than direct real-biogas operation.

The cross-sectional area of the stripping column was calculated as:

$$A={\displaystyle \frac{\pi D^2}{4}}$$

(2)

where A is the cross-sectional area of the stripping column and D is the internal diameter of the column.

For a column diameter of 0.30 m, the cross-sectional area was calculated as:

$$A={\displaystyle \frac{3.1416\times {0.30}^2}{4}}=0.0707 {\mathrm{m}}^2$$

(3)

The gas–liquid ratio was defined as the ratio of gas volumetric flow rate to liquid recirculation flow rate.

$$G/L={\displaystyle \frac{Q_g}{Q_l}}$$

(4)

where G/L is the gas–liquid ratio, Q_g is the gas volumetric flow rate, and Q_l is the liquid recirculation flow rate.

For the laboratory-scale gas–liquid-ratio tests, the gas flow rate was maintained at 2.0 L min⁻¹. The liquid recirculation flow rates were adjusted to 2.0, 1.0, and 0.5 L min⁻¹ to obtain gas–liquid ratios of 1, 2, and 4, respectively. The corresponding empty-column superficial gas velocity under the gas flow rate of 2.0 L min⁻¹ was calculated as 4.72 × 10⁻⁴ m s⁻¹. Although the empty-column superficial velocity was relatively low, gas–liquid contact was enhanced by the combined use of liquid spraying and Pall-ring packing.

The superficial gas velocity was calculated as:

$$u_g={\displaystyle \frac{Q_g}{A}}$$

(5)

For Qg = 2.0 L min⁻¹ and A = 0.0707 m²

$$u_g={\displaystyle \frac{2.0\times {10}^{-3}/60}{0.0707}}=4.72\times {10}^{-4} \mathrm{m} {\mathrm{s}}^{-1}$$

(6)

Three groups of single-factor experiments were conducted to evaluate the effects of initial pH, stripping temperature, and gas–liquid ratio on NH₄⁺-N release. For pH screening, the digestate was adjusted to pH 10, 11, and 12 using the NaOH stock solution, while unadjusted digestate was used as the control. For each 10 L batch, approximately 50, 225, and 500 mL NaOH stock solution were added to reach pH 10, 11, and 12, corresponding to NaOH dosages of approximately 20, 90, and 200 g, respectively. For temperature screening, the stripping temperature was controlled at 55, 65, and 75 °C. For gas–liquid-ratio screening, gas–liquid ratios of 1, 2, and 4 were tested.

The NaOH dosage was calculated as:

$$m_{NaOH}=C_{NaOH}\times V_{NaOH}$$

(7)

For pH 10:

$$m_{NaOH}=0.40 \mathrm{g} {\mathrm{mL}}^{-1} \times 50 \mathrm{mL}=20 \mathrm{g}$$

(8)

For pH 11:

$$m_{NaOH}=0.40 \mathrm{g} {\mathrm{mL}}^{-1} \times 225 \mathrm{mL}=90 \mathrm{g}$$

(9)

For pH 12:

$$m_{NaOH}=0.40 \mathrm{g} {\mathrm{mL}}^{-1} \times 500 \mathrm{mL}=200 \mathrm{g}$$

(10)

Each batch stripping experiment lasted for 12 h. Liquid samples were collected at 0, 2, 4, 6, 8, 10, and 12 h to determine NH₄⁺-N concentration and pH. Liquid volume changes caused by sampling and evaporation were recorded and corrected during mass-based ammonia removal and recovery calculations. Based on NH₄⁺-N removal efficiency, pH stability, NaOH consumption, and practical feasibility, pH 11, 65 °C, and a gas–liquid ratio of 2 were selected as the suitable stripping conditions for the subsequent coupled absorption experiments (see

Table 2. Operating conditions of the laboratory-scale ammonia stripping experiments.

Item	Operating Condition
Digestate volume	10 L
Stripping column	1.5 m height, 30 cm internal diameter
Gas–liquid contact mode	Spray nozzle + 15 mm polypropylene Pall rings
Simulated biogas composition	75% N2 + 25% CO2
Gas flow rate, Qg	2.0 L min−1
Liquid recirculation flow rate, Ql	2.0, 1.0, and 0.5 L min−1
Corresponding gas–liquid ratio	1, 2, and 4
Initial pH	Unadjusted, 10, 11, and 12
NaOH stock solution	400 g L−1
NaOH stock-solution volume for 10 L digestate	50, 225, and 500 mL for pH 10, 11, and 12, respectively
Corresponding NaOH mass	20, 90, and 200 g for pH 10, 11, and 12, respectively
Stripping temperature	55, 65, and 75 °C
Operation time	12 h
Sampling time	0, 2, 4, 6, 8, 10, and 12 h

).

2.3. Laboratory-Scale Gypsum-Mediated Absorption System

The gypsum-mediated absorption system consisted of an absorption reservoir, an absorption column, a liquid recirculation unit, and a multi-stage settling unit. The absorption unit was connected downstream of the stripping unit to receive NH₃- and CO₂-containing gas generated during ammonia stripping. The total absorption liquid volume was 30 L.

To prepare the slightly soluble CaSO₄ absorption medium, 450 g of industrial by-product gypsum was added to 30 L of deionized water. The mixture was continuously stirred for 2 h and then introduced into the multi-stage settling system. The supernatant liquid after dissolution and settling was circulated as the slightly soluble CaSO₄ absorption solution. Excess solid gypsum was retained in the settling and dissolution zone to continuously replenish dissolved Ca²⁺ and SO₄²⁻ during NH₃ absorption and carbonate precipitation. This design ensured that CaSO₄ was supplied in excess relative to the theoretical requirement for NH₃ capture and CaCO₃ formation. Because the recovery liquid may contain an initial NH₄⁺-N background or residual ammonium from previous circulation, NH₃ recovery efficiency was calculated using the net NH₄⁺-N accumulation in the recovery liquid rather than the final NH₄⁺-N concentration alone. The net recovered nitrogen mass was calculated from the difference between the NH₄⁺-N concentration at time t and the initial NH₄⁺-N concentration in the recovery liquid, multiplied by the actual recovery-liquid volume.

The theoretical gypsum demand was calculated according to the simplified stoichiometric reaction between NH₃, CO₂, and CaSO₄·2H₂O [18]. Under the selected stripping condition, NH₄⁺-N in 10 L digestate decreased from approximately 7980 to 1648 mg L⁻¹, corresponding to 63.32 g N removed. Assuming 95% NH₃ recovery, the recovered nitrogen was calculated as 60.15 g N, requiring 369.7 g CaSO₄·2H₂O theoretically. Therefore, 450 g CaSO₄·2H₂O was used to provide an excess solid reserve for the 30 L absorption system.

The absorption experiments were conducted to examine the effects of initial ammonia loading, absorption temperature, absorbent form, and gas–liquid ratio. The initial NH₄⁺-N concentrations in the stripping liquid were adjusted to 4000, 6000, and 8000 mg L⁻¹ by dilution or mixing with the original digestate to provide different NH₃ input loads. The absorption temperatures were controlled at 25, 35, and 45 °C. The absorbent forms included slightly soluble CaSO₄ solution and CaSO₄ suspension. Gas–liquid ratios of 1, 2, and 4 were tested to evaluate the effect of gas–liquid contact intensity on NH₃ capture.

A repeated-cycle operation mode was used to evaluate NH₄⁺-N accumulation in the absorption liquid. The stripping process was operated for a total of 12 h and was divided into six 2 h stripping intervals for sampling and coupling with the absorption unit. After each 2 h stripping interval, the NH₃- and CO₂-containing gas was introduced into the gypsum-mediated absorption unit for 0.5 h absorption. Therefore, the stripping performance was evaluated on the 12 h stripping-time scale, whereas NH₄⁺-N accumulation in the recovery liquid was evaluated according to the six absorption cycles. Absorption liquid samples were collected after each 0.5 h absorption cycle to determine NH₄⁺-N concentration and pH. According to the laboratory absorption results, the suitable absorption condition was determined as an initial NH₄⁺-N concentration of 6000 mg L⁻¹, absorption temperature of 35 °C, slightly soluble CaSO₄ solution, and gas–liquid ratio of 2 (see

Table 3. Operating conditions of the laboratory-scale gypsum-mediated absorption experiments.

Item	Operating Condition
Absorption liquid volume	30 L
Gypsum source	Industrial by-product gypsum
Main gypsum component	CaSO4·2H2O
Supplier-reported gypsum purity	Approximately 98%
Gypsum particle size	<150 μm after 100-mesh sieving
Initial gypsum dosage	450 g CaSO4·2H2O
Absorbent form	Slightly soluble CaSO4 solution and CaSO4 suspension
Initial NH4+-N loading	4000, 6000, and 8000 mg L−1
Absorption temperature	25, 35, and 45 °C
Gas–liquid ratio	1, 2, and 4
Operation mode	Repeated stripping–absorption cycles
Stripping-time scale	12 h
Absorption contact per cycle	0.5 h
Stripping interval	Six 2 h intervals
Number of absorption cycles	6
Absorption-time scale	6 cycles

).

The mass of NH₄⁺-N removed from the digestate was calculated as:

$$M_{N,removed}=(C_{D,0}-C_{D,t})\times V_D$$

(11)

$$M_{N,removed}=(7980-1648)\times 10/1000=63.32 \mathrm{g} \mathrm{N}$$

(12)

The recovered nitrogen mass was estimated as:

$$M_{N,recovered}=M_{N,removed}\times {\eta }_R$$

(13)

For ${\eta }_R$ = 95%:

$$M_{N,recovered}=63.32\times 0.95=60.15 \mathrm{g} \mathrm{N}$$

(14)

2.4. Coupled Stripping–Absorption Operation and Pilot-Scale Validation

After determining the suitable laboratory-scale stripping and absorption conditions, the stripping unit and gypsum-mediated absorption unit were connected through a shared gas circulation loop. In the coupled system, NH₃ released from the digestate in the stripping column was transported by circulating simulated biogas into the absorption column. In the absorption column, NH₃ was captured by the CaSO₄ absorption medium, while CO₂ in the circulating gas participated in carbonate formation. The treated gas was then returned to the stripping unit, forming a closed gas circulation pathway. The configuration of the coupled system is shown in Figure 1.

Pilot-scale validation was performed to evaluate the preliminary transferability and operational stability of the coupled process under enlarged conditions. The pilot-scale system consisted of a stripping unit, an absorption unit, and a gas circulation unit. The stripping unit included a digestate feeding tank, a stripping tower, a heating and temperature-control device, a liquid spray system, packing material, and a gas circulation fan. The absorption unit included an absorption tower, absorption liquid circulation tanks, and a multi-stage precipitation/separation unit.

During pilot-scale operation, the gas and liquid circulation rates were not determined by direct linear scaling of the laboratory-scale flow rates. Instead, they were adjusted according to the target gas–liquid ratio, tower geometry, pressure balance, and operational safety. Although the gas circulation fan and liquid circulation pump had higher maximum capacities, the actual operating flow rates were controlled to avoid negative pressure in the gas-buffering pipeline and to maintain stable gas–liquid contact. Under the target gas–liquid ratio of 2, the actual biogas circulation rate was maintained at approximately 10 m³ h⁻¹, and the corresponding liquid circulation rate was approximately 5 m³ h⁻¹.

The pilot-scale stripping tower was approximately 3.5 m in height. The absorption tower had the same height and was equipped with an atomizing spray head to enhance gas–liquid contact. Instead of the polypropylene Pall-ring packing used in the laboratory-scale column, the pilot-scale stripping tower used a spiral-gradient mass-transfer structure to improve gas–liquid contact while reducing packing-related maintenance. The absorption unit was connected to three 30 L stainless-steel tanks used as absorption-liquid storage and precipitation/separation units; therefore, the total absorption liquid volume in the 30 L pilot test was 90 L. For the 100 L extended pilot-scale operation, the absorption liquid volume was adjusted to 100 L to match the enlarged digestate treatment volume and maintain sufficient NH₃ capture capacity. The NaOH dosage during pilot-scale operation was adjusted according to the actual buffering capacity of the digestate and the target initial pH of 11, rather than by simple linear scaling alone. According to the actual operation records summarized in

Table 4. Preliminary mass-balance, resource-recovery, and direct circulation-electricity indicators linking nitrogen recovery, gypsum utilization, CaCO₃ precipitation, and CO₂ involvement during the coupled stripping–absorption process.

Indicator	Laboratory-Scale Coupled Test	Short-Term Pilot Test	Extended Pilot Test
Digestate volume	10 L	30 L	100 L
Absorption liquid volume	30 L	90 L	100 L
Operation time	12 h	12 h	36 h
Gas source	Simulated biogas	Real biogas	Real biogas
NH4+-N removed	63.32 g N	135 g N	720–730 g N
NH3 recovery efficiency	approx. 95%	approx. 95%	approx. 95%
Net recovered NH3-N	60.15 g N	128.25 g N	684–694 g N
NaOH dosage	90 g	300 g	990 g
Estimated CaSO4·2H2O demand/operational dosage	370 g	800 g	3.73 kg
Measured recovered CaCO3-containing precipitate	213 g	457 g	2.17 kg
Estimated CO2 incorporation based on measured CaCO3-containing precipitate	94.6 g	201.1 g	0.94 kg
Estimated direct electrical consumption for circulation	0.48 kWh	14.4 kWh	43.2 kWh
Specific direct electrical consumption for circulation per unit digestate volume	48.0 kWh m−3	480 kWh m−3	432 kWh m−3
Specific direct electrical consumption for circulation per unit NH4+-N removed	7.58 kWh kg−1 N	106.7 kWh kg−1 N	60.0 kWh kg−1 N
Main resource-efficiency implication	NH3 recovery and CaCO3 formation	Enlarged absorption buffering	NH3 recovery and CO2-reduction-related gas-quality improvement

NH₃ recovery efficiency was calculated from the net NH₄⁺-N accumulation in the recovery liquid after subtracting the initial NH₄⁺-N background. For the short-term pilot test, the actual absorption liquid volume was 90 L; therefore, recovery-liquid concentration data were volume-normalized only for comparison with the 30 L stripping liquid, whereas mass-balance calculations were based on the actual absorption volume. The CaSO₄·2H₂O values represent the estimated stoichiometric demand or operational dosage required to maintain NH₃ capture, sulfate transfer, and CaCO₃ precipitation during the coupled process, rather than directly measured gypsum consumption. The recovered CaCO₃-containing precipitate masses were obtained from the collected solid products after separation and drying. The estimated CO₂ incorporation was calculated from the measured CaCO₃-containing precipitate mass, assuming that the precipitate was mainly CaCO_3, as supported by XRD and SEM characterization. Therefore, the CO₂-related values indicate CO₂-assisted carbonate precipitation rather than complete carbon sequestration. The electricity-related indicators in Table 4 refer only to direct electrical consumption for liquid and gas circulation. Thermal energy required for raising and maintaining the stripping temperature was supplied by purified biogas combustion during pilot-scale operation and was not included in the direct electrical-consumption calculation. Upstream energy inputs associated with NaOH production, gypsum preparation and transport, product separation, heat losses, and other auxiliary operations were also excluded. Therefore, these values should be used only as preliminary circulation-electricity indicators and should not be interpreted as total process energy consumption or final energy-efficiency indicators.

, the NaOH dosages were 90 g for the 10 L laboratory-scale coupled test, 300 g for the 30 L short-term pilot test, and 990 g for the 100 L extended pilot test. The higher dosage in the extended pilot operation reflected the stronger buffering effect of the larger digestate volume and the practical requirement for maintaining the target alkaline condition before stripping.

Excess CaSO₄ solid was maintained in the dissolution/settling zone throughout operation to ensure continuous replenishment of Ca²⁺ and SO₄²⁻. Based on the mass-balance results summarized in Table 4, the CaSO₄·2H₂O dosage or demand was approximately 800 g for the 30 L short-term pilot test and 3.73 kg for the 100 L extended pilot test. During pilot-scale operation, liquid samples were collected from both the stripping liquid and the absorption liquid to determine NH₄⁺-N concentration and pH. Gas-phase CO₂ concentration was monitored before and after the absorption unit to evaluate CO₂ participation in the coupled absorption and carbonate precipitation process. The laboratory-scale tests used simulated biogas with 25% CO₂, whereas the pilot-scale tests used real biogas from chicken manure dry anaerobic digestion.

The initial CO₂ concentration of the real biogas was approximately 28%. After the coupled absorption process, the measured CH₄ proportion increased to approximately 98%, mainly reflecting the decrease in CO₂ proportion in the circulating gas. This result should be interpreted as a preliminary gas-composition change associated with CO₂ removal rather than complete biogas upgrading, because trace gases and full gas-phase mass balance were not quantified in this study.

In the pilot-scale test, real biogas was used to verify the operational transferability of the coupled process under practical gas-circulation conditions. CH₄ was not expected to directly participate in NH₃ absorption or CaCO₃ precipitation under the tested alkaline aqueous conditions, but it could affect the relative gas composition and CO₂ partial pressure in the circulating gas. H₂S was not independently controlled or quantitatively monitored in this study. Therefore, the possible influence of H₂S on corrosion, sulfide dissolution, odor control, and product quality was not included in the present mass-balance calculation and should be further evaluated in long-term pilot-scale operation using real biogas.

Heating energy was not included in the direct electrical-consumption calculation because the heat required to maintain the stripping temperature was supplied by purified biogas combustion rather than by electrical heating. Therefore, only the electricity consumed by the liquid circulation pump and gas circulation fan was considered in the preliminary operational assessment. The calculated values should be interpreted as direct electrical consumption for circulation, not as total process energy demand. Accordingly, the process should be described as reducing external electrical heating demand under the tested pilot-scale configuration rather than as having zero heating energy requirement.

2.5. Reaction Pathway and Product Characterization

The gypsum-mediated absorption process was considered a coupled gas–liquid transfer, dissolution–ionization, and precipitation-driven reaction rather than a simple physical dissolution of NH₃. In the absorption tower, gaseous NH₃ dissolved into the absorption liquid and was converted into ammonium species. Meanwhile, CO₂ dissolved into the liquid phase and participated in the carbonate equilibrium. Slightly soluble CaSO₄·2H₂O supplied Ca²⁺ and SO₄²⁻, where Ca²⁺ reacted with carbonate species to form CaCO₃ precipitates, while SO₄²⁻ contributed to the formation of an ammonium-rich sulfate recovery liquid.

The possible reaction pathways are shown below.

$$N{H}_3+H_{2}O\rightleftharpoons N{H}_4^{+}+O{H}^{-}$$

(15)

$$C{O}_2+H_{2}O\rightleftharpoons HC{O}_3^{-}+H^{+}$$

(16)

$$HC{O}_3^{-}\rightleftharpoons C{O}_3^{2-}+H^{+}$$

(17)

$$CaS{O}_4\cdot 2H_{2}O\rightleftharpoons C{a}^{2+}+S{O}_4^{2-}+2H_{2}O$$

(18)

$$C{a}^{2+}+C{O}_3^{2-}\to CaC{O}_3\downarrow$$

(19)

$$2N{H}_4^{+}+S{O}_4^{2-}\rightleftharpoons {(N{H}_4)}_{2}S{O}_4$$

(20)

The overall reaction can be simplified as:

$$2N{H}_3+C{O}_2+H_{2}O+CaS{O}_4\cdot 2H_{2}O\to {(N{H}_4)}_{2}S{O}_4+CaC{O}_3\downarrow +2H_{2}O$$

(21)

After absorption, solid precipitates were collected from the multi-stage settling unit by solid–liquid separation, washed three times with deionized water, and dried at 60 °C for 12 h before characterization.

The crystalline phase of the recovered precipitate was identified by X-ray diffraction using Cu Kα radiation. The XRD patterns were recorded over a 2θ range of 5–80° at a scanning rate of 5° min⁻¹. The CaCO₃ phase fraction was estimated by semi-quantitative XRD phase analysis. The CaCO₃ purity was expressed as the calculated phase fraction of CaCO₃ in the dried precipitate rather than as directly measured chemical purity.

The morphology and elemental composition of the recovered precipitate were analyzed using field-emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy. SEM-EDS analysis was performed using a ZEISS GeminiSEM 300 field-emission scanning electron microscope equipped with an OXFORD Xplore energy-dispersive spectrometer. Before observation, the dried samples were sputter-coated with a gold–palladium alloy. The SEM system used a Schottky field-emission electron gun. The accelerating voltage range was 0.02–30 kV, the probe current range was 3 pA–20 nA, and the magnification range was 12×–2,000,000×. Secondary electron images were acquired using Inlens and ET detectors. The working distance for EDS analysis was approximately 8.5 mm and was adjusted according to the actual imaging condition.

Gas-phase CO₂ concentration was monitored using a non-dispersive infrared gas analyzer. CO₂ concentration was measured before and after the absorption unit to provide gas-phase evidence for CO₂ participation in carbonate precipitation.

2.6. Calculation Methods

The free ammonia fraction in total ammonia nitrogen was calculated according to the NH₄⁺/NH₃ equilibrium.

$$f_{N{H}_3}={\displaystyle \frac{1}{1+{10}^{p{K}_a-pH}}}$$

(22)

The temperature-dependent $p{K}_a$ was calculated as:

$$p{K}_a=0.09018+{\displaystyle \frac{2729.92}{T}}$$

(23)

where $f_{N{H}_3}$ is the free ammonia fraction and T is the absolute temperature in Kelvin.

The mass of ammonium nitrogen in the digestate at time t was calculated as:

$$M_{D,t}=C_{D,t}\times V_{D,t}$$

(24)

where $M_{D,t}$ is the NH₄⁺-N mass in the digestate at time t, $C_{D,t}$ is the NH₄⁺-N concentration in the digestate, and $V_{D,t}$ is the digestate volume.

$${\eta }_S={\displaystyle \frac{M_{D,0}-M_{D,t}}{M_{D,0}}}\times 100\%$$

(25)

where ${\eta }_S$ is the stripping efficiency, M_D,0 is the initial NH₄⁺-N mass in the digestate, and M_D,t is the NH₄⁺-N mass at time t.

The mass of recovered ammonium nitrogen in the absorption liquid was calculated as:

$$M_{A,t}=C_{A,t}\times V_{A,t}$$

(26)

where M_A,t is the recovered NH₄⁺-N mass in the absorption liquid, C_A,t is the NH₄⁺-N concentration in the absorption liquid, and V_A,t is the absorption liquid volume.

The NH₃ recovery efficiency was calculated as:

$${\eta }_R={\displaystyle \frac{M_{A,t}}{M_{D,0}-M_{D,t}}}\times 100\%$$

(27)

where ${\eta }_R$ is the NH₃ recovery efficiency.

The unrecovered nitrogen fraction was calculated as:

$${\eta }_U={\displaystyle \frac{M_{D,0}-M_{D,t}-M_{A,t}}{M_{D,0}-M_{D,t}}}\times 100\%$$

(28)

To avoid overestimation caused by background ammonium in the recovery liquid, the recovered nitrogen mass was calculated using the net NH₄⁺-N accumulation rather than the final NH₄⁺-N concentration alone:

The apparent stripping rate during a given time interval was calculated as:

$$r_S={\displaystyle \frac{C_{D,t_1}-C_{D,t_2}}{t_2-t_1}}$$

(29)

where $M_{CaS{O}_4\cdot 2H_{2}O}$ is the apparent stripping rate, and C_D,t1 and C_D,t2 are NH₄⁺-N concentrations at times t₁ and t₂, respectively.

The theoretical CaSO₄·2H₂O demand was calculated as:

$$m_{CaS{O}_4\cdot 2H_{2}O}={\displaystyle \frac{M_{N,recovered}}{2M_N}}\times M_{CaS{O}_4\cdot 2H_{2}O}$$

(30)

where $m_{CaS{O}_4\cdot 2H_{2}O}$ is the theoretical CaSO₄·2H₂O demand, $M_{N,recovered}$ is the recovered nitrogen mass, $M_N$ is the molar mass of nitrogen, and $M_{CaS{O}_4\cdot 2H_{2}O}$ is the molar mass of CaSO₄·2H₂O.

The theoretical CaCO₃ production was calculated as:

$$m_{CaC{O}_3}={\displaystyle \frac{M_{N,recovered}}{2M_N}}\times M_{CaC{O}_3}$$

(31)

The theoretical CO₂ participation was calculated as:

$$m_{C{O}_2}={\displaystyle \frac{M_{N,recovered}}{2M_N}}\times M_{C{O}_2}$$

(32)

The specific NaOH consumption was calculated as:

$$C_{NaOH,N}={\displaystyle \frac{m_{NaOH}}{M_{D,0}-M_{D,t}}}$$

(33)

where $C_{NaOH,N}$ is the NaOH consumption per unit nitrogen removed, and $m_{NaOH}$ is the mass of NaOH added.

For the selected laboratory stripping condition:

$$C_{NaOH,N}={\displaystyle \frac{90}{63.32}}=1.42 \mathrm{g} NaOH {\mathrm{g}}^{-1} N_{removed}$$

(34)

The specific gypsum consumption was calculated as:

$$C_{CaS{O}_4,N}={\displaystyle \frac{m_{CaS{O}_4\cdot 2H_{2}O}}{M_{A,t}}}$$

(35)

where $C_{CaS{O}_4,N}$ is the CaSO₄·2H₂O consumption per unit recovered nitrogen.

For the laboratory-scale selected condition, the NH₄⁺-N concentration decreased from 7980 to 1648 mg L⁻¹ in 10 L digestate, corresponding to 63.32 g N removed. Based on approximately 95% NH₃-N recovery, the recovered nitrogen was estimated as 60.15 g N, requiring approximately 370 g CaSO₄·2H₂O, producing approximately 213 g CaCO₃, and involving approximately 94.6 g CO₂.

For the 30 L short-term pilot-scale operation, the NH₄⁺-N concentration decreased from approximately 6000 to 1500 mg L⁻¹, corresponding to 135 g N removed. Based on approximately 95% NH₃-N recovery, the recovered nitrogen was estimated as 128.25 g N, requiring approximately 800 g CaSO₄·2H₂O, producing approximately 457 g CaCO₃, and involving approximately 201.1 g CO₂.

For the 100 L extended pilot-scale operation, the NH₄⁺-N concentration decreased from approximately 8000 to 700–800 mg L⁻¹ after 36 h, corresponding to approximately 720–730 g N removed. Based on the corrected NH₃-N recovery efficiency of approximately 95%, approximately 684–694 g NH₃-N was recovered in the liquid phase. The estimated CaSO₄·2H₂O demand or operational dosage, measured recovered CaCO₃-containing precipitate mass, and estimated CO₂ incorporation during this operation were summarized in Table 4. In the present study, the operational energy-related indicator was defined as direct electrical consumption for circulation. This boundary included only the electricity consumed by the liquid circulation pump and gas circulation fan. Thermal energy required for raising and maintaining the stripping temperature was supplied by purified biogas combustion during pilot-scale operation and was not converted into electrical consumption. Therefore, the reported values do not represent the total process energy demand. A complete energy assessment should include the sensible heat required to increase the digestate temperature, heat losses during temperature maintenance, useful heat supplied by biogas combustion, combustion and heat-transfer efficiencies, and embedded energy associated with NaOH addition, gypsum preparation and transport, product separation, and other auxiliary operations. Because these terms were not fully measured in the present pilot-scale tests, they were not included in the calculated direct electrical-consumption indicators. The direct electrical consumption for circulation was calculated as:

$$E\mathrm{elec}=\mathrm{Epump}+\mathrm{Efan}$$

(36)

where $E\mathrm{elec}$ is the direct electrical consumption for circulation, $\mathrm{Epump}$ is the electrical consumption of the liquid circulation pump, and $\mathrm{Epump}$ is the electrical consumption of the gas circulation fan. Heating electricity was excluded from this calculation because the required heat was supplied by purified biogas combustion rather than electrical heating.

The specific direct electrical consumption for circulation per unit digestate volume was calculated as:

$$SE{C}_V={\displaystyle \frac{E_{\mathrm{e}l\mathrm{ec}}}{V_D}}$$

(37)

The specific direct electrical consumption for circulation per unit nitrogen removed was calculated as:

$$SE{C}_N={\displaystyle \frac{E_{\mathrm{e}lec}}{M_{D,0}-M_{D,t}}}$$

(38)

2.7. Analytical Methods

NH₄⁺-N was determined using Nessler’s reagent spectrophotometry according to HJ 535-2009. This method is applicable to ammonia nitrogen determination in surface water, groundwater, domestic sewage, and industrial wastewater.

COD was determined using the dichromate method according to HJ 828-2017. This standard specifies the dichromate method for COD determination in surface water, domestic sewage, and industrial wastewater.

TN was determined using alkaline potassium persulfate digestion UV spectrophotometry according to HJ 636-2012, which is applicable to total nitrogen determination in surface water, groundwater, industrial wastewater, and domestic sewage.

TP was determined using ammonium molybdate spectrophotometry according to GB 11893-89 [19]. This standard uses potassium peroxydisulfate or nitric acid–perchloric acid digestion to determine total phosphorus in unfiltered water samples. CH₄ concentration in the circulating biogas was measured using a portable biogas analyzer during pilot-scale operation.

TS and VS. were determined by gravimetric methods. The pH was measured using a calibrated pH meter. Turbidity was determined using a turbidity meter and expressed as NTU. Before NH₄⁺-N, COD, TN, and TP analyses, samples were diluted appropriately to ensure that the measured concentrations fell within the linear range of each analytical method.

2.8. Statistical Analysis

All laboratory-scale experiments were performed in triplicate unless otherwise stated, and the results are expressed as mean ± standard deviation. Statistical differences among different laboratory-scale operating conditions were analyzed using one-way analysis of variance followed by Tukey’s post hoc test, and differences were considered statistically significant at p < 0.05. The pilot-scale tests were conducted as operational validation runs rather than independent triplicate scale-up experiments. For pilot-scale liquid samples, NH₄⁺-N was measured in triplicate when available, and the error bars in Section 3.4 represent analytical standard deviations of replicate NH₄⁺-N measurements rather than standard deviations among independent pilot-scale runs. Recovery-efficiency profiles in Section 3.4 were calculated from pilot-scale operational mass-balance data and were used for trend validation rather than statistical comparison among independent scale-up experiments. Regression analysis was used only to describe NH₄⁺-N stripping profiles when necessary. No machine-learning analysis was included in this study.

3. Results

3.1. NH₃ Release from Poultry Digestate and Determination of the Stripping Window

The first requirement for the coupled stripping–absorption process was to generate a stable NH₃-containing gas stream from high-ammonium poultry digestate. Ammonia release from digestate is governed by two linked processes: the conversion of liquid-phase NH₄⁺ into volatile NH₃ through the pH-dependent NH₄⁺/NH₃ equilibrium, and the subsequent transfer of NH₃ from the liquid phase to the gas phase through gas–liquid mass transfer. Therefore, the effects of initial pH, stripping temperature, and gas–liquid ratio were interpreted as coupled chemical-equilibrium and mass-transfer controls rather than as independent operational variables. Accordingly, the observed decrease in NH₄⁺-N concentration should not be attributed solely to the initial pH. Initial pH primarily determined the availability of free NH₃ through the NH₄⁺/NH₃ equilibrium, whereas stripping temperature and gas–liquid ratio further regulated NH₃ volatilization and interfacial mass transfer. Thus, NH₃ release from poultry digestate in this study can be described as a pH-dominated but temperature- and mass-transfer-assisted process.

As shown in Figure 2, NH₄⁺-N concentration decreased gradually with stripping time under all tested conditions, while the corresponding stripping efficiency increased continuously. However, the decrease rate and final stripping efficiency differed markedly among different operating conditions, indicating that NH₃ release from poultry digestate was jointly regulated by alkaline conversion, thermal volatilization, and gas–liquid transfer intensity. The stripping curves generally showed a rapid initial stage followed by a slower stage. This trend suggests that readily transferable NH₃ was removed preferentially during the early stage, whereas the later stage was limited by the decreasing NH₃ availability and reduced concentration gradient between the liquid and gas phases.

Initial pH was the primary factor controlling NH₃ availability in the liquid phase. Increasing the initial pH markedly promoted NH₄⁺-N removal because the NH₄⁺/NH₃ equilibrium shifted toward free NH₃ under alkaline conditions. Compared with the unadjusted digestate and the pH 10 treatment, the pH 11 and pH 12 treatments showed faster NH₄⁺-N reduction and higher stripping efficiency. The calculated free ammonia fraction further supported this mechanism. When the initial pH increased from 10 to 11 and 12, the calculated free NH₃ fraction increased from 0.9856 to 0.9985 and 0.9999, respectively, indicating that pH directly determined the proportion of ammonia available for volatilization.

However, increasing the initial pH beyond 11 did not further improve the stripping performance. The early-stage apparent stripping rates under pH 10, 11, and 12 were 535.23, 1140.23, and 1035.63 mg L⁻¹ h⁻¹, respectively, while the final stripping efficiencies were 38.35%, 81.89%, and 76.02%, respectively. These results indicate that increasing the initial pH from 10 to 11 markedly enhanced NH₃ release, but further increasing the pH to 12 provided no additional benefit under the tested conditions. This may be explained by the combined effects of digestate buffering capacity, carbonate/bicarbonate alkalinity, and the consumption of added OH⁻ by soluble inorganic and organic buffering components. Similar observations have been reported in ammonia stripping studies, where NH₃ release is strongly promoted by alkaline conditions, but the improvement becomes limited once the free NH₃ fraction and mass-transfer driving force are sufficiently high [3,6,20]. In addition, the pH 12 treatment required a much higher NaOH dosage, which increased chemical consumption without improving NH₄⁺-N removal. Therefore, pH 11 was selected as a more practical initial pH by considering stripping efficiency, pH stability, alkali consumption, and operational feasibility.

After sufficient NH₃ was generated through alkaline conversion, temperature further affected NH₃ volatilization and liquid-to-gas transfer. Increasing the stripping temperature from 55 °C to 65 °C enhanced NH₄⁺-N removal, which can be attributed to the reduced solubility of NH₃ at higher temperature and the accelerated molecular diffusion across the gas–liquid interface. The early-stage apparent stripping rates at 55, 65, and 75 °C were 700.04, 1043.23, and 990.23 mg L⁻¹ h⁻¹, respectively, and the final stripping efficiencies were 60.64%, 81.23%, and 79.51%, respectively. Although 75 °C theoretically favored NH₃ volatilization, the final stripping efficiency was not higher than that at 65 °C. This suggests that once sufficient NH₃ availability and mass-transfer driving force were established, additional heating provided limited improvement, especially during the later stage when the remaining transferable ammonia decreased. Considering the trade-off between stripping performance and heat demand, 65 °C was selected as the suitable stripping temperature.

The gas–liquid ratio mainly controlled NH₃ transfer from the liquid phase into the circulating gas phase. When the gas–liquid ratio increased from 1 to 2, NH₄⁺-N removal was enhanced, indicating that an appropriate increase in gas supply intensity renewed the gas-phase boundary layer, reduced the NH₃ partial pressure near the gas–liquid interface, and maintained the driving force for NH₃ transfer. However, further increasing the gas–liquid ratio to 4 did not lead to better stripping performance. Under fixed pH and temperature conditions, the calculated free NH₃ fraction remained nearly unchanged, confirming that the gas–liquid ratio did not alter the NH₄⁺/NH₃ equilibrium but mainly affected gas–liquid contact and mass transfer. Excessive gas flow may shorten effective gas–liquid residence time and reduce the mass-transfer efficiency per unit gas volume. Therefore, a gas–liquid ratio of 2 provided a better balance between gas supply intensity and effective gas–liquid contact.

Overall, NH₃ release from poultry digestate followed a mechanism chain involving NH₄⁺/NH₃ equilibrium regulation, temperature-enhanced volatilization, and gas–liquid mass-transfer control. Among the tested factors, initial pH was the dominant factor because it directly determined the fraction of TAN present as volatile NH₃. However, temperature and gas–liquid ratio were still necessary regulating factors rather than negligible variables. Temperature affected NH₃ solubility, molecular diffusion, and volatilization potential, while the gas–liquid ratio controlled gas-phase renewal and the NH₃ concentration gradient across the gas–liquid interface. The results suggest that once sufficient free NH₃ availability was established at pH 11, further improvement depended more on maintaining an appropriate volatilization and mass-transfer environment than on simply increasing pH, temperature, or gas flow. Under the selected operating window of pH 11, 65 °C, and a gas–liquid ratio of 2, the NH₄⁺-N concentration in 10 L poultry digestate decreased from approximately 7980 to 1648 mg L⁻¹ within 12 h, corresponding to an ammonia stripping efficiency of about 80%. Therefore, this operating window was not selected as the theoretical maximum condition, but as a practical balance among ammonia burden reduction, pH stability, alkali consumption, heat demand, and gas–liquid transfer efficiency.

3.2. Characteristics of Ammonia Capture and Recovery in the Gypsum-Mediated Absorption System

The gypsum-mediated absorption unit was designed to capture the NH₃-containing gas released from the stripping unit and convert gaseous NH₃ into liquid-phase ammonium nitrogen. Unlike conventional acid scrubbing, the CaSO₄-based absorption process was not a simple acid–base neutralization pathway. Instead, it involved gas–liquid NH₃ transfer, NH₃ hydration, ammonium enrichment, CaSO₄ dissolution, and CO₂-assisted carbonate precipitation [21]. Therefore, the absorption performance depended not only on NH₃ solubility but also on the matching relationship among upstream NH₃ input load, absorption temperature, absorbent form, and gas–liquid contact intensity.

As shown in Figure 3, NH₄⁺-N concentration in the recovery liquid increased continuously with the number of absorption cycles under all tested conditions, indicating that NH₃ released from the stripping unit was effectively transferred into the absorption liquid and retained as ammonium nitrogen. However, the accumulation rate and final NH₄⁺-N concentration varied among different operating conditions, suggesting that NH₃ capture in the CaSO₄-mediated system was controlled by the balance between NH₃ supply from the stripping unit and the absorption capacity of the recovery liquid.

The initial NH₄⁺-N concentration in the stripping liquid determined the NH₃ input load entering the absorption unit. At relatively low initial ammonia loading, the NH₃ supply from the stripping unit was insufficient, resulting in slower NH₄⁺-N accumulation in the recovery liquid. Increasing the initial NH₄⁺-N concentration enhanced NH₃ release and promoted ammonium enrichment in the absorption liquid. However, excessively high ammonia loading may exceed the instantaneous acceptance capacity of the absorption system, leading to reduced capture stability and a higher risk of unrecovered NH₃ in the circulating gas. Among the tested conditions, an initial NH₄⁺-N concentration of 6000 mg L⁻¹ provided a more suitable balance between NH₃ generation in the stripping unit and NH₃ capture in the gypsum-mediated absorption unit. Therefore, this condition was selected for subsequent absorption-parameter evaluation.

The stability of the absorption process was therefore determined by the dynamic balance between upstream NH₃ supply and downstream absorption capacity. When the NH₃ input load was low, the absorption liquid still had sufficient capture capacity, but NH₃ delivery from the stripping unit was insufficient, resulting in slow NH₄⁺-N accumulation. In contrast, when the NH₃ input load was excessively high, the instantaneous NH₃ release rate could exceed the effective gas–liquid transfer and liquid-phase retention capacity of the absorption unit, increasing the risk of unstable capture or unrecovered NH₃ in the circulating gas. Therefore, the suitable initial NH₄⁺-N concentration was not simply the highest ammonia loading, but the condition under which NH₃ generation, gas-phase transport, and liquid-phase absorption were better matched.

Absorption temperature affected both gas–liquid NH₃ transfer and ammonium retention in the liquid phase. At 25 °C, NH₄⁺-N accumulation in the recovery liquid was relatively slow, indicating that low temperature limited interfacial transfer and liquid-phase reaction kinetics. When the absorption temperature increased to 35 °C, NH₄⁺-N accumulation was enhanced, suggesting that moderate heating improved NH₃ dissolution and ammonium enrichment. However, further increasing the temperature to 45 °C did not improve NH₃ capture. This may be because higher temperatures reduced the solubility and retention of NH₃ and CO₂ in the aqueous phase, thereby weakening sustained liquid-phase conversion. Therefore, 35 °C represented an appropriate absorption temperature that balanced gas–liquid transfer, NH₃ retention, and reaction stability.

The physical form of CaSO₄ also played an important role in NH₃ capture and ammonium enrichment. Compared with the CaSO₄ suspension, the slightly soluble CaSO₄ solution achieved faster NH₄⁺-N accumulation and higher recovery performance. This result indicates that NH₃ absorption in the CaSO₄ system depended more on a stable and homogeneous ionic environment than on the direct presence of excessive suspended gypsum particles in the absorption liquid. In the slightly soluble CaSO₄ solution, dissolved Ca²⁺ and SO₄²⁻ were more uniformly distributed, which favored NH₃ dissolution, ammonium stabilization, and subsequent carbonate precipitation [22]. In contrast, the CaSO₄ suspension may have caused heterogeneous solid–liquid distribution, disturbed gas–liquid contact, and promoted localized precipitation, thereby reducing the effective absorption performance. Therefore, the slightly soluble CaSO₄ solution was more suitable for continuous NH₃ capture than the CaSO₄ suspension.

The gas–liquid ratio regulated the contact intensity between the NH₃-containing gas and the absorption liquid [20]. When the gas–liquid ratio was too low, the NH₃ delivery rate to the absorption unit was insufficient, limiting the accumulation rate of NH₄⁺-N in the recovery liquid. Increasing the gas–liquid ratio to 2 enhanced gas–liquid contact and promoted NH₃ transfer into the absorption liquid. However, further increasing the gas–liquid ratio to 4 did not lead to better NH₃ recovery. Excessive gas flow may shorten the effective residence time of gas bubbles in the absorption liquid and reduce the absorption efficiency per unit gas volume. Thus, the gas–liquid ratio should be regarded as a matching parameter between NH₃ delivery and liquid-phase capture capacity, rather than a factor that can be increased indefinitely.

Under the selected absorption conditions, namely an initial NH₄⁺-N concentration of 6000 mg L⁻¹, absorption temperature of 35 °C, slightly soluble CaSO₄ solution, and a gas–liquid ratio of 2, the NH₄⁺-N concentration in the 30 L recovery liquid accumulated to approximately 1900 mg L⁻¹ after six absorption cycles. Based on the net NH₄⁺-N accumulation in the recovery liquid, the NH₃ recovery efficiency was approximately 90–95%. This result demonstrates that the gypsum-mediated absorption system could effectively convert stripped NH₃ into an ammonium-rich recovery liquid. From a sustainability perspective, this is important because the process did not simply transfer ammonia from the digestate to another gas phase; instead, it retained the released nitrogen in a recoverable liquid product while providing conditions for CO₂-assisted carbonate precipitation.

Overall, NH₃ capture in the gypsum-mediated absorption system was governed by the coordinated effects of NH₃ input load, absorption temperature, CaSO₄ form, and gas–liquid contact. More importantly, the results indicate that stable nitrogen recovery required a proper match between upstream NH₃ release and downstream absorption capacity. The slightly soluble CaSO₄ solution provided a more homogeneous ionic environment than the CaSO₄ suspension, while 35 °C and a gas–liquid ratio of 2 offered a practical balance between NH₃ transfer, ammonium retention, and carbonate-forming reactions. Under these conditions, the NH₄⁺-N concentration in the 30 L recovery liquid accumulated to approximately 1900 mg L⁻¹ after six absorption cycles, and the corrected NH₃ recovery efficiency reached approximately 90–95%. Therefore, the selected absorption condition was not simply the condition with the highest theoretical absorption potential, but a practical operating window that matched NH₃ supply, liquid-phase capture, and CaSO₄-mediated reaction capacity.

3.3. Product Characterization and Evidence of CO₂-Assisted Carbonate Precipitation

After confirming the effective capture of stripped NH₃ by the gypsum-mediated absorption system, it was necessary to determine whether the absorption process only involved physical dissolution of NH₃ or further induced coupled liquid-phase reactions and solid product formation. Therefore, product verification was conducted from three complementary perspectives: liquid-phase nitrogen recovery, solid-phase precipitate characterization, and gas-phase CO₂ variation. These lines of evidence were used to clarify the role of CaSO₄·2H₂O in linking NH₃ capture with CO₂-assisted carbonate precipitation.

As shown in Figure 4, the absorption process produced both an ammonium-rich recovery liquid and a solid precipitate. The first evidence was the continuous accumulation of NH₄⁺-N in the recovery liquid. Under the selected absorption conditions, namely 35 °C, a slightly soluble CaSO₄ solution, and a gas–liquid ratio of 2, the NH₄⁺-N concentration in the recovery liquid accumulated to approximately 1900 mg L⁻¹ after six absorption cycles, and the NH₃ recovery efficiency was approximately 90–95%. This result indicates that most of the NH₃ released from the stripping unit was captured and retained in the absorption liquid as ammonium nitrogen rather than being lost with the circulating gas. Therefore, the absorption unit functioned as an effective nitrogen recovery step rather than only a gas-phase NH₃ transfer process.

The second piece of evidence was the formation of CaCO₃-containing solid precipitates. As shown in Figure 4a, the XRD pattern of the recovered precipitate showed characteristic diffraction peaks corresponding to CaCO₃, indicating that carbonate precipitation occurred during the gypsum-mediated absorption process. The recovered precipitate was dominated by CaCO₃, and the calculated CaCO₃ phase fraction reached approximately 99.9% based on semi-quantitative XRD analysis. This result suggests that CaSO₄·2H₂O did not simply act as an inert absorption medium. Instead, its slight dissolution supplied Ca²⁺ to the absorption liquid, which subsequently reacted with carbonate species derived from dissolved CO₂ to form CaCO₃ precipitates. The role of CaSO₄ in this process was therefore essential rather than auxiliary. First, slightly soluble CaSO₄·2H₂O acted as a continuous Ca²⁺ source for carbonate precipitation. As dissolved CO₂ was converted into HCO₃⁻ and CO₃²⁻ under the alkaline microenvironment created by NH₃ dissolution, the released Ca²⁺ combined with carbonate species to form CaCO₃ precipitates. Second, the consumption of dissolved Ca²⁺ by CaCO₃ precipitation could further promote the dissolution of residual CaSO₄ solids, thereby maintaining Ca²⁺ replenishment during absorption. Third, SO₄²⁻ released from CaSO₄ remained in the liquid phase and contributed to the formation of an ammonium-rich sulfate-containing recovery liquid. Thus, CaSO₄ provided the chemical bridge linking NH₃ capture, ammonium enrichment, and CO₂-assisted carbonate precipitation.

SEM observation further supported the formation of solid carbonate products. As shown in Figure 4b, the recovered precipitate exhibited distinct crystalline particles and aggregated deposits, rather than amorphous residues or unreacted suspended solids alone. The observed morphology was consistent with the formation of mineral precipitates during the absorption reaction. Combined with the XRD results, the SEM evidence confirmed that the solid product was mainly generated through liquid-phase precipitation reactions involving dissolved Ca²⁺ and carbonate species. In addition to XRD and SEM evidence, the recovered CaCO₃ precipitate mass further supported carbonate formation. According to the mass-balance results, the recovered CaCO₃ precipitate masses were approximately 213 g, 457 g, and 2.17 kg for the laboratory-scale coupled test, short-term pilot test, and extended pilot test, respectively. These values corresponded to estimated CO₂ involvement of 94.6 g, 201.1 g, and 0.94 kg, respectively, indicating that CO₂ in the circulating gas was incorporated into carbonate products through the gypsum-mediated absorption pathway. These values agreed well with the theoretical CaCO₃ masses calculated from the reacted anhydrous CaSO₄ dosages, indicating that Ca²⁺ supplied by CaSO₄ was effectively incorporated into carbonate precipitates.

The third piece of evidence was the decrease in gas-phase CO₂ concentration during the absorption process. As shown in Figure 4c, the CO₂ concentration in the circulating gas decreased after passing through the gypsum-mediated absorption unit. This indicates that CO₂ was not only transported through the system as an inert gas, but partly entered the liquid phase and participated in carbonate formation. In the absorption liquid, dissolved CO₂ could be converted into HCO₃⁻ and CO₃²⁻ under the alkaline microenvironment generated by NH₃ dissolution. The resulting carbonate species then reacted with Ca²⁺ released from CaSO₄·2H₂O to form CaCO₃ precipitates.

Based on these results, the gypsum-mediated absorption process can be interpreted as a coupled gas–liquid reaction involving NH₃ capture, ammonium enrichment, CaSO₄ dissolution, and CO₂-assisted CaCO₃ precipitation [23]. As illustrated in Figure 4d, gaseous NH₃ first dissolved into the absorption liquid and was converted into NH₄⁺. Meanwhile, CaSO₄·2H₂O continuously supplied Ca²⁺ and SO₄²⁻ through slight dissolution. The SO₄²⁻ provided a sulfate-containing ionic environment for ammonium enrichment, while Ca²⁺ reacted with carbonate species derived from dissolved CO₂ to generate CaCO₃ precipitates. Thus, nitrogen recovery and carbonate precipitation occurred simultaneously in the absorption system.

This mechanism differs from conventional strong-acid scrubbing, in which NH₃ is mainly neutralized by mineral acid to form ammonium salts [24]. In the present system, the slightly soluble gypsum medium enabled NH₃ capture without direct strong-acid addition and simultaneously provided Ca²⁺ for carbonate precipitation. From a sustainability perspective, this coupling is meaningful because the process integrates ammonia burden reduction, nitrogen recovery, and CO₂-assisted mineral product formation within one treatment pathway.

However, the current evidence should be interpreted carefully. The combined XRD, SEM, and gas-phase CO₂ results confirm CaCO₃ formation and support CO₂ participation in carbonate precipitation, but they do not provide a complete quantitative carbon balance. Therefore, the process should be described as CO₂-assisted carbonate precipitation rather than complete CO₂ sequestration. A full evaluation of CO₂ conversion efficiency would require quantitative measurement of gas-phase CO₂ input and output, dissolved inorganic carbon in the absorption liquid, and solid-phase carbonate carbon. This limitation should be addressed in future work to further clarify the carbon conversion potential of the coupled process.

Overall, the liquid-, solid-, and gas-phase evidence jointly demonstrated that the gypsum-mediated absorption system was not only an NH₃ capture unit but also a reactive medium for CO₂-assisted CaCO₃ formation. The simultaneous NH₄⁺-N enrichment, CaCO₃ precipitation, and CO₂ decline confirmed the feasibility of coupling nitrogen recovery with carbonate product formation during the treatment of high-ammonium poultry digestate.

3.4. Pilot-Scale Validation of the Coupled Stripping–Absorption Process Using Real Biogas

After confirming the laboratory-scale stripping and gypsum-mediated absorption performance, the coupled system was further evaluated to determine whether stable NH₃ release, gas-phase transport, and liquid-phase capture could be maintained under enlarged operating conditions. The pilot-scale test was performed based on the selected stripping window of pH 11, 65 °C, and a gas–liquid ratio of 2, together with the selected absorption condition using slightly soluble CaSO₄ solution. Therefore, this section focused on the operational transferability of the coupled process rather than further parameter screening. This logic is consistent with the original experimental design, in which the pilot-scale test was used to verify the preliminary transferability of the laboratory-derived operating window.

As shown in Figure 5a, the cumulative total ammonia nitrogen recovery efficiency increased continuously with operation time during the short-term coupled operation. The recovery efficiency increased from a low initial value to approximately 80% at 12 h, indicating that ammonia released from the stripping liquid was progressively transferred and retained in the absorption system. This trend demonstrates that the coupled system did not simply remove ammonia from the digestate, but converted the released NH₃ into recoverable ammonium nitrogen in the absorption liquid.

The real-time ammonia nitrogen recovery efficiency further confirmed the stability of the absorption process. As shown in Figure 5b, the real-time recovery efficiency rapidly increased to above 90% during the early stage and then remained within a relatively narrow range of approximately 93–96% throughout the subsequent operation. This result indicates that the gypsum-mediated absorption unit maintained a stable NH₃ capture capacity after the initial adjustment period. The slight fluctuations in real-time recovery efficiency may be attributed to variations in instantaneous NH₃ release rate, gas–liquid contact intensity, and the dynamic replenishment of dissolved Ca²⁺ and SO₄²⁻ from slightly soluble CaSO₄.

The accumulation profile of ammonia nitrogen in the recycled absorption liquid provided direct evidence for nitrogen transfer from the stripping unit to the recovery liquid. As shown in Figure 5c, ammonia nitrogen concentration in the recycled liquid increased continuously with operation time, reaching approximately 5.8–6.0 g L⁻¹ by the end of operation. This continuous enrichment confirmed that the NH₃-containing gas generated during stripping was effectively absorbed and converted into liquid-phase ammonium nitrogen. Therefore, the absorption unit functioned not only as a gas-cleaning unit, but also as a nitrogen recovery unit.

During the extended operation, the ammonia nitrogen concentration in the stripping liquid decreased markedly with time (Figure 5d). The concentration decreased from approximately 8000 mg L⁻¹ to below 1000 mg L⁻¹ after 36 h, indicating substantial ammonia burden reduction in the high-ammonium poultry digestate. Meanwhile, ammonia nitrogen concentration in the recovery liquid increased from approximately 1.0 g L⁻¹ to nearly 8.0 g L⁻¹ (Figure 5e). The opposite trends between Figure 5d and Figure 5e clearly demonstrate the mass-transfer relationship between ammonia release from the digestate and ammonium enrichment in the absorption liquid.

The recovery efficiency during extended operation remained highly stable. As shown in Figure 5f, ammonia nitrogen recovery efficiency was maintained at approximately 94–97% throughout the 36 h operation. This high and stable recovery efficiency indicates that most of the stripped NH₃-N was captured in the liquid phase rather than being lost with the circulating gas. Because the recovery liquid contained initial ammonium background, the recovery efficiency should be calculated based on net NH₄⁺-N accumulation after subtracting the initial background concentration, which is consistent with the mass-balance calculation principle used in this study.

Overall, Figure 5 confirms that the coupled stripping–absorption process maintained a stable NH₃ release–gas transport–liquid capture relationship during enlarged operation. The simultaneous decrease in ammonia nitrogen concentration in the stripping liquid, increase in ammonia nitrogen concentration in the recovery liquid, and maintenance of high recovery efficiency demonstrate the preliminary transferability of the proposed process. However, this pilot-scale verification should still be regarded as preliminary scale-up validation rather than full-scale engineering demonstration. Because the pilot-scale experiments were conducted as operational validation runs, Figure 5 results should be interpreted as pilot-scale performance trends and mass-balance evidence rather than statistically replicated scale-up comparisons. Long-term continuous operation may still be affected by CaCO₃ scaling, nozzle or pipeline blockage, CaSO₄ consumption, alkali demand, absorption-liquid quality, gas circulation stability, and product separation performance [25].

3.5. Preliminary Sustainability Implications Based on Elemental Flow and Resource-Efficiency Indicators

To highlight the sustainability relevance of the coupled stripping–absorption process, this section integrates the elemental-flow pathway with preliminary resource-efficiency indicators rather than evaluating the process only by NH₄⁺-N removal efficiency. As summarized in Figure 6, the proposed system redistributed nitrogen, carbon, calcium, and sulfur through a coupled gas–liquid–solid transformation pathway. Nitrogen in high-ammonium poultry digestate was converted from liquid-phase NH₄⁺ into gaseous NH₃ during stripping and was subsequently captured in the gypsum-mediated absorption liquid as ammonium-rich nitrogen. Meanwhile, CO₂ in the circulating gas participated in carbonate formation, Ca²⁺ released from CaSO₄·2H₂O contributed to CaCO₃ precipitation, and SO₄²⁻ remained in the recovery liquid as a sulfate source. Therefore, the process functioned not merely as an ammonia removal route, but as an integrated resource-recovery pathway linking digestate deammonification, nitrogen recovery, sulfate transfer, and CO₂-assisted carbonate product formation [26].

Based on this elemental-flow framework, Table 4 further summarizes the preliminary mass-balance and resource-efficiency indicators across laboratory-scale and pilot-scale operations.

The preliminary mass-balance and resource-efficiency indicators summarized in Table 4 provide an operational reference for the coupled process. However, the electricity-related values should be interpreted only as preliminary direct circulation-electricity indicators obtained under non-optimized pilot-scale operating conditions. They do not represent total process energy demand because thermal energy, heat losses, chemical-related embedded energy, and product-separation energy were not included. In the laboratory-scale coupled test, 63.32 g N was removed from 10 L poultry digestate, and approximately 60.15 g NH₃-N was recovered based on the net NH₄⁺-N accumulation in the absorption liquid. When the process was transferred to pilot-scale operation, the removed NH₄⁺-N mass increased to 135 g N in the short-term pilot test and approximately 720–730 g N in the extended 100 L pilot test, while the corrected NH₃ recovery efficiency remained approximately 95%. These results indicate that the coupled process maintained an effective NH₃ release–gas transport–liquid capture relationship during scale enlargement.

From the perspective of material conversion, the CaSO₄·2H₂O dosage or demand increased with the recovered nitrogen load, from approximately 370 g in the laboratory-scale test to 800 g in the short-term pilot test and 3.7 kg in the extended pilot test. Correspondingly, the recovered CaCO₃ precipitate masses were approximately 213 g, 457 g, and 2.1 kg, respectively. Based on the recovered CaCO₃ precipitate, the estimated CO₂ involvement was 94.6 g, 201.1 g, and 0.94 kg for the three operating scales. These results provide quantitative evidence that part of the gas-phase CO₂ was incorporated into carbonate precipitates through the gypsum-mediated absorption pathway. Therefore, Table 4 links the key sustainability outputs of the process, including ammonia burden reduction, nitrogen recovery, gypsum utilization, carbonate product formation, and CO₂-assisted mineralization.

The elemental-flow pathway in Figure 6 and the resource-efficiency indicators in Table 4 also clarify the difference between the proposed process and conventional acid absorption. Conventional acid scrubbing mainly converts stripped NH₃ into ammonium salts through strong mineral acid consumption, whereas the present process used slightly soluble gypsum as a calcium and sulfate source and simultaneously generated ammonium-rich recovery liquid and CaCO₃-containing precipitates. This design may reduce dependence on strong acid absorption and provide a potential route for coupling nutrient recovery with carbon-containing mineral product formation. In addition, the pilot-scale tests using real biogas suggested potential gas-quality improvement associated with CO₂ reduction in the circulating gas. However, this effect should be interpreted as a preliminary indication of gas-quality improvement potential rather than complete biogas upgrading, because trace gas composition and full gas-phase mass balance were not comprehensively quantified [27].

It should also be emphasized that the current sustainability assessment is preliminary and based mainly on laboratory and pilot-scale mass-balance indicators. The CO₂-related values represent CO₂-assisted carbonate precipitation rather than complete carbon sequestration. Full-scale sustainability still depends on several factors, including NaOH consumption, heat supply, gypsum source and purity, long-term CaCO₃ scaling control, absorption-liquid quality, product utilization, operational stability, and the environmental impacts associated with chemical and energy inputs. Therefore, future work should combine long-term continuous operation with complete nitrogen, carbon, calcium, and sulfur balances, product safety evaluation, techno-economic analysis, and life-cycle assessment to further determine the practical sustainability of the coupled process.

4. Discussion

The results of this study demonstrate that gas-driven ammonia stripping coupled with gypsum-mediated absorption can provide an integrated pathway for ammonia burden reduction, nitrogen recovery, and CO₂-assisted carbonate formation from high-ammonium poultry digestate. Unlike conventional treatment routes that mainly focus on ammonia removal or ammonium salt production, the present process links NH₃ release, NH₃ capture, sulfate transfer, and CaCO₃-containing precipitate formation within one coupled system. This integration is particularly relevant for high-solids poultry digestate, where excessive ammonia accumulation may inhibit anaerobic digestion and restrict digestate recycling.

The ammonia stripping results indicate that initial pH was the dominant factor controlling NH₃ release from poultry digestate. Increasing pH promoted the conversion of NH₄⁺ to free NH₃, thereby increasing the concentration of volatile ammonia available for gas-phase transfer. However, the improvement from pH 11 to pH 12 was limited, suggesting that extremely high alkalinity was not necessary for efficient ammonia stripping under the tested conditions. This is important from a practical perspective because excessive NaOH addition would increase chemical consumption and weaken the sustainability advantage of the process. Therefore, the selected condition of pH 11 represented a reasonable balance between ammonia release efficiency and alkali input.

Temperature further regulated NH₃ volatilization and gas–liquid transfer. Increasing the temperature from 55 °C to 65 °C enhanced NH₄⁺-N removal, probably because higher temperature reduced NH₃ solubility and increased molecular diffusion across the gas–liquid interface. However, increasing the temperature to 75 °C did not further improve the final stripping performance. This result suggests that once sufficient NH₃ availability and mass-transfer driving force were established, additional heating provided only limited benefit. Therefore, 65 °C was selected as a more practical temperature considering both stripping performance and heat demand. In farm-scale or biogas-plant applications, this temperature may be supported by waste heat or purified biogas combustion, but the total heat balance should be quantified in future work.

The gas–liquid ratio mainly affected the transfer of NH₃ from the liquid phase to the circulating gas phase rather than the NH₄⁺/NH₃ equilibrium itself. A gas–liquid ratio of 2 improved ammonia removal compared with a lower gas supply intensity, indicating that sufficient gas renewal helped maintain the NH₃ transfer driving force. However, a further increase in the gas–liquid ratio did not lead to better removal, which may be related to shortened effective contact time or reduced mass-transfer efficiency per unit gas volume. Thus, the gas–liquid ratio should not simply be increased to maximize gas flow; instead, it should be optimized according to tower structure, gas residence time, liquid distribution, and direct electrical consumption for circulation.

The gypsum-mediated absorption system showed that slightly soluble CaSO₄ could serve not only as an absorbent medium for NH₃ capture but also as a calcium and sulfate source for coupled product formation. Compared with conventional strong-acid scrubbing, this pathway avoids the direct addition of concentrated mineral acids and provides a route for simultaneous ammonium enrichment and carbonate precipitation. The continuous replenishment of dissolved Ca²⁺ and SO₄²⁻ from CaSO₄·2H₂O supported NH₃ retention in the liquid phase and promoted CaCO₃ formation when CO₂-derived carbonate species were available. The better performance of the slightly soluble CaSO₄ solution compared with the CaSO₄ suspension also indicates that stable dissolution–reaction conditions may be more important than simply increasing the amount of suspended solid.

The formation of CaCO₃-containing precipitates was supported by solid-phase and gas-phase evidence. XRD and SEM-EDS confirmed the presence of carbonate-containing solid products, while the decrease in gas-phase CO₂ concentration after the absorption unit indicated that part of the CO₂ entered the liquid phase and participated in carbonate precipitation. Mechanistically, NH₃ dissolution created a locally alkaline environment, which favored CO₂ conversion to bicarbonate and carbonate species. These carbonate species then reacted with Ca²⁺ supplied by CaSO₄·2H₂O to form CaCO₃ precipitates. Therefore, NH₃ capture and CO₂-assisted carbonate precipitation were not independent processes, but were coupled through gas–liquid transfer, aqueous equilibrium, gypsum dissolution, and precipitation reactions.

Pilot-scale operation further demonstrated the preliminary transferability of the coupled process. When real biogas was used instead of simulated biogas, the system still maintained effective NH₃ release, gas transport, and liquid-phase recovery. In the extended 100 L pilot-scale operation, NH₄⁺-N decreased from approximately 8000 to 700–800 mg L⁻¹, while the corrected NH₃-N recovery efficiency remained approximately 95%. This result suggests that the coupled process is not limited to laboratory simulated-gas conditions and has potential for application in practical biogas systems. However, the pilot-scale results should still be regarded as preliminary because the operation time was limited and the full gas composition, long-term scaling behavior, and dynamic stability of the absorption liquid were not comprehensively evaluated.

The use of real biogas in the pilot-scale test also means that the circulating gas contained components other than CO₂ and NH₃. CH₄ was mainly present as the dominant carrier gas and was relatively non-reactive toward NH₃ capture and CaCO₃ precipitation under the tested conditions. However, the CH₄/CO₂ ratio could influence the CO₂ partial pressure and thus affect the availability of dissolved inorganic carbon for carbonate formation. In contrast, H₂S may have more practical relevance because it can dissolve under alkaline conditions and may contribute to corrosion, odor, sulfide accumulation, or changes in the quality of the recovered liquid and precipitate. Since CH₄ and H₂S were not independently varied and H₂S was not quantitatively balanced in this study, the observed pilot-scale performance should be interpreted as a preliminary validation using real biogas rather than a complete evaluation of all biogas components. Future work should include full gas composition monitoring, especially CO₂, CH₄, H₂S, and residual NH₃, to clarify their effects on mass transfer, carbonate precipitation, product quality, and operational stability.

From a sustainability perspective, the process provides several potential advantages. First, it reduces the ammonia burden in high-ammonium poultry digestate, which may facilitate digestate recirculation or downstream treatment. Second, it recovers nitrogen as an ammonium-rich liquid, which could potentially be used as a fertilizer resource after quality and safety evaluation. Third, it uses gypsum as a low-cost calcium and sulfate source, enabling sulfate transfer and CaCO₃-containing precipitate formation. Fourth, it allows part of the CO₂ in the circulating gas to participate in carbonate precipitation, linking nutrient recovery with carbon-containing mineral product formation. These features make the process more consistent with circular agriculture and resource recovery than ammonia removal alone.

To further clarify the position of the proposed process relative to existing ammonia recovery routes,

Table 5. Comparison of typical ammonia recovery methods for digestate or ammonia-rich waste streams.

Method	Feedstock/Matrix	Operating Feature	NH3/N Recovery or Removal	Main Product	Main Advantages and Limitations
Simplified air stripping [6]	Digested livestock slurry	Mixing, pH adjustment or natural pH, continuous air flow; 30–50 °C	87% NH3 removal at pH 9 and 40 °C; 69% NH3 removal and 47% TN removal without pH adjustment at 40 °C; approximately 85% NH3 removal at 50 °C without pH adjustment	NH3-containing gas requiring downstream capture	Simple and suitable for farm-scale digestate treatment, with reported electrical requirement of approximately 3.8 kWh m−3 digestate; however, the process can be slow and usually requires an additional NH3 capture unit.
Vacuum thermal stripping–acid absorption [8]	Dairy manure digestate	Vacuum-assisted boiling below normal boiling point; sulfuric-acid absorption	93.3–99.9% ammonia stripped within 3 h under tested boiling temperature–vacuum combinations	Ammonium sulfate crystals	High ammonia stripping efficiency and marketable ammonium sulfate production; however, it requires vacuum generation, heat input, demister/liquid-trap control, and sulfuric-acid handling.
Vacuum stripping and absorption pilot process [25]	Sludge digestate and pressate	250 L batch, 3-pool vacuum stripping system, acid absorption	80.7% ammonia stripped from sludge digestate and 91.5% from pressate within 5 h	Ammonium sulfate crystals with 98.4–103.7% calculated purity	Demonstrated pilot-scale scalability and high-quality ammonium sulfate recovery; however, stable vacuum operation, demister configuration, and heat/vapor management remain important engineering challenges.
Side-stream membrane-based NH3 extraction [7]	Poultry manure anaerobic digestion system	3 L CSTR, HRT 15 d, 37 °C; flat-sheet PTFE membrane module; recirculation flow rate of 0.25 L min−1	TAN decreased by 64.5%; TN decreased by 53.4%; methane yield increased from 360 ± 70 to 574 ± 5 N mL CH4 g VSfed−1	Ammonium-rich recovery solution	Selective NH3 extraction can reduce ammonia inhibition and improve anaerobic digestion performance; however, membrane fouling, cost, hydrophobicity loss, and maintenance remain concerns for high-solid digestate.
Alternative scrubbing agents for NH3 recovery [10]	Anaerobic digestate	Organic acids, chilled water, Epsom salt, gypsum, and other scrubbing agents tested as alternatives to sulfuric acid	Citric acid showed scrubbing performance comparable to sulfuric acid but required about double acid dosage; performance varied with scrubbing agent	Ammonium-containing recovery liquid	Reduces dependence on conventional sulfuric-acid scrubbing and broadens absorbent options; however, capture stability, reagent dosage, product concentration, and long-term applicability vary among absorbents.
Biogas stripping coupled with gypsum absorption [28]	Anaerobically digested dairy manure effluent	Biogas stripping integrated with CaSO4·2H2O absorption; effects of gas loading rate, CO2 content, absorption temperature, and operating time evaluated	Gypsum solution recovered approximately 95% of stripped ammonia at 10% CO2 and approximately 100% when CO2 exceeded 30%; observed total ammonia recovery ranged from 46% to 71%, with modeled optimum of 81.2%	Ammonium sulfate fertilizer	Avoids direct sulfuric-acid absorption and links NH3 recovery with CO2-containing stripping gas; however, total recovery depends strongly on CO2 content, gas loading, absorption temperature, and operating time.
Present study	High-ammonium poultry digestate	pH 11, 65 °C, gas–liquid ratio of 2; slightly soluble CaSO4 absorption coupled with CO2-assisted carbonate precipitation	Laboratory stripping removed about 80% NH4+-N; corrected NH3 recovery was approximately 90–95%; pilot-scale recovery remained approximately 95%; extended pilot operation removed 720–730 g N and recovered 684–694 g NH3-N	Ammonium-rich sulfate-containing liquid and CaCO3-containing precipitate	Avoids direct strong-acid scrubbing and couples ammonia burden reduction, nitrogen recovery, sulfate transfer, and CO2-assisted CaCO3 precipitation; however, NaOH demand, heat requirement, CaCO3 scaling, product quality, complete energy balance, TEA, and LCA still require further evaluation.

compares typical ammonia stripping, acid absorption, membrane-based extraction, and gypsum-mediated absorption strategies. Because different studies used different feedstocks, operating scales, ammonia loads, pH values, temperatures, and performance indicators, the comparison should be interpreted as a systematic qualitative–quantitative overview rather than a direct one-to-one ranking of process performance.

As shown in Table 5, conventional acid absorption and vacuum thermal stripping can achieve high ammonia removal or recovery efficiencies, but they generally rely on sulfuric-acid absorption and require heat, vacuum generation, or acid handling. Membrane-based NH₃ extraction provides selectivity and can reduce ammonia inhibition in poultry manure digestion, but fouling and maintenance remain important concerns for high-solid digestate. Compared with these routes, the present process does not aim to simply maximize NH₃ recovery efficiency. Its main contribution is the integration of ammonia burden reduction, nitrogen recovery, sulfate transfer, and CO₂-assisted CaCO₃ precipitation in one coupled pathway. Nevertheless, this comparison also shows that the proposed process still requires further optimization of NaOH consumption, heat supply, CaCO₃ scaling control, product quality, and complete techno-economic and life-cycle performance before full-scale application.

Nevertheless, the current study should not be interpreted as a complete full-scale sustainability demonstration. Although direct electrical consumption for circulation was estimated in this study, the total process energy demand remains to be quantified. The exclusion of thermal energy does not mean that heating demand was negligible; rather, it reflected the tested pilot-scale configuration in which heat was supplied by purified biogas combustion instead of electrical heating. Therefore, the reported electricity-related indicators should be interpreted as partial operational indicators. Future work should establish a complete heat and energy balance including digestate heating, heat retention, heat losses, biogas combustion efficiency, heat-transfer efficiency, heat recovery, NaOH- and gypsum-related embedded energy, product separation, and transport before making full-scale energy-efficiency claims. Although CaCO₃-containing precipitates were obtained and CO₂ participation was supported, the CO₂-related results represent CO₂-assisted carbonate precipitation rather than complete carbon sequestration. A complete carbon balance would require quantitative measurement of gas-phase CO₂ input and output, dissolved inorganic carbon in the absorption liquid, carbonate carbon in the precipitate, and possible CO₂ losses during operation. Similarly, although the direct electrical consumption of liquid and gas circulation was estimated, the total energy and environmental costs of NaOH production, gypsum preparation, heating, product separation, and transport were not fully included. Therefore, future techno-economic analysis and life-cycle assessment are necessary before the process can be claimed as fully sustainable at engineering scale.

In addition to ammonia, poultry digestate contains soluble organic compounds and microbial metabolites, such as volatile fatty acids, soluble carbohydrates, and other low-molecular-weight organics. These components may influence the coupled stripping–absorption process in several ways. First, organic acids and their salts may contribute to the buffering capacity of the digestate, thereby increasing alkali demand and affecting pH stability during ammonia stripping. Second, soluble organics and suspended colloidal substances may affect liquid viscosity, foaming behavior, spray distribution, and gas–liquid mass transfer in the stripping column. Third, these components may be partially transferred into the recovery liquid or interact with the precipitated solids, thereby affecting the quality and potential utilization of the ammonium-rich liquid and CaCO₃-containing precipitate. In the present study, VFAs and soluble sugars were not individually quantified; therefore, their specific effects could not be separated from the overall matrix effect of poultry digestate. Future studies should evaluate digestates with different organic compositions and include VFA, soluble carbohydrate, alkalinity, and product-quality analyses to clarify their influence on process stability and product utilization.

Several practical issues also require further investigation. Long-term operation may lead to CaCO₃ scaling in the absorption tower, pipelines, or spray system, which could reduce mass-transfer efficiency and increase maintenance requirements. The quality of the ammonium-rich recovery liquid should be evaluated in terms of nitrogen concentration, sulfate content, salinity, heavy metals, pathogens, and agronomic suitability. The recovered CaCO₃-containing precipitate should also be assessed for phase fraction, stability, and potential use as a soil amendment or industrial by-product. In addition, the process should be tested using digestates from different poultry farms or digestion conditions to evaluate the influence of feedstock variability, buffering capacity, suspended solids, and organic matter on ammonia release and recovery.

Overall, the coupled gas-driven stripping and gypsum-mediated absorption process provides a promising resource-recovery strategy for high-ammonium poultry digestate. Its main contribution lies in converting a single-purpose ammonia removal process into an integrated pathway for digestate deammonification, nitrogen recovery, gypsum utilization, and CO₂-assisted carbonate product formation. Future studies should focus on long-term continuous operation, complete nitrogen–carbon–calcium–sulfur mass balances, scaling control, product safety, energy optimization, and techno-economic evaluation to further verify the engineering feasibility and sustainability of this process.

5. Conclusions

This study developed a gas-driven ammonia stripping process coupled with gypsum-mediated absorption for nitrogen recovery and CO₂-assisted carbonate precipitation from high-ammonium poultry digestate. Under the selected stripping condition of pH 11, 65 °C, and a gas–liquid ratio of 2, NH₄⁺-N in 10 L digestate decreased from approximately 7980 to 1648 mg L⁻¹ within 12 h, corresponding to an ammonia removal efficiency of about 80%. The slightly soluble CaSO₄ solution provided more stable NH₃ capture than the CaSO₄ suspension, and the corrected NH₃-N recovery reached approximately 90–95% under the selected absorption condition.

The coupled process also achieved measurable gypsum-mediated carbonate product formation. With the increase in treatment scale, the estimated CaSO₄·2H₂O demand or operational dosage increased from approximately 370 g in the laboratory-scale coupled test to 800 g in the short-term pilot test and 3.73 kg in the extended pilot test. Correspondingly, the measured recovered CaCO₃-containing precipitate masses reached approximately 213 g, 457 g, and 2.17 kg, respectively. Based on these measured precipitate masses, the estimated CO₂ incorporation was 94.6 g, 201.1 g, and 0.94 kg at the three operating scales. Together with XRD, SEM, and gas-phase CO₂ variation, these results collectively support the occurrence of NH₃ capture, sulfate transfer, and CO₂-assisted carbonate precipitation in the gypsum-mediated absorption system.

Pilot-scale operation using real biogas further verified the operational transferability of the process. During the extended 100 L pilot test, NH₄⁺-N in the digestate decreased from approximately 8000 to 700–800 mg L⁻¹ after 36 h, while the corrected NH₃-N recovery efficiency remained approximately 95%. These results demonstrate that the coupled system can simultaneously reduce ammonia burden in poultry digestate, recover nitrogen as an ammonium-rich liquid, consume gypsum as a sulfate and calcium source, and generate measurable CaCO₃-containing precipitates. Therefore, the process provides a resource-recovery route linking digestate deammonification, nitrogen recovery, gypsum utilization, and CO₂-assisted mineral product formation.

Nevertheless, this work should still be regarded as a laboratory-to-pilot validation rather than a complete full-scale sustainability demonstration. Although measurable CaCO₃-containing precipitates supported CO₂-assisted carbonate formation, the CO₂-related results should not be interpreted as complete carbon sequestration. Long-term continuous operation, complete nitrogen–carbon–calcium–sulfur balances, CaCO₃ scaling control, absorption-liquid quality assessment, product safety evaluation, complete heat and energy-balance accounting, chemical input assessment, and techno-economic or life-cycle assessment are still required before full-scale application. The agronomic suitability and safety of the ammonium-rich liquid and CaCO₃-containing precipitate should be further evaluated before practical product utilization.