Ammonia nitrogen, which represents a primary nitrogen constituent in wastewater, can exist as either ammonia (NH₃) or ammonium ions (NH₄⁺), with their equilibrium described by Eq. (1). At standard temperature (25 °C), the pK_a for this equilibrium is 9.24 [51]. Since the pH in most wastewater streams tends to be below this threshold, ammonia nitrogen is most commonly present as ammonium ions (NH₄⁺). This factor is critical in the selection of adsorbent materials and the design of effective adsorption processes.

Ammonium ion (NH₄⁺) is a highly soluble, non-volatile polyatomic cation that forms stable hydrated clusters due to strong hydration interactions [52–54]. In contrast, non-ionic ammonia (NH₃) is a neutral molecule exhibiting significant volatility, which is attributed to its relatively high vapor pressure (25 °C, 9.9 atm) [55]. The equilibrium between ammonium ions and volatile ammonia in wastewater is dynamically affected by environmental conditions such as pH and temperature [51]. At neutral pH levels, typical of municipal wastewater, ammonium ions predominantly represent the main nitrogen species. Due to the prevalence of ammonium ions in most wastewater matrices, the design and optimization of adsorbent materials and adsorption techniques focus specifically on targeting ammonium removal [56]. Conversely, in some specific alkaline industrial wastewaters with elevated ammonia content (e.g., caprolactam industry, semiconductor industry), volatile ammonia (NH₃) becomes the dominant species, in which case gas-liquid mass transfer-based approaches, such as ammonia stripping and membrane contactors, are more appropriate for treatment [13, 31, 57–59].

The mechanisms governing ammonium ion adsorption in wastewater depend on both the properties of the adsorbent and the chemical conditions of the aqueous solution. Fundamentally, the driving force of adsorption is the electrostatic attraction between ammonium ions and negatively charged functional adsorption sites [7, 56, 60]. In the present study, we classify the key adsorption mechanisms of ammonium ions into reversible and irreversible categories, and examine them in detail. Reversible adsorption facilitates ammonia recovery via desorption, whereas irreversible adsorption prevents effective recovery [56, 61].

Zeolites are crystalline microporous materials consisting of interconnected SiO₄ tetrahedra and AlO₄ tetrahedra arranged in a three-dimensional framework via oxygen bridges [73]. Because of aluminum (Al) substituting for silicon (Si) in the structure, the entire lattice exhibits a net negative charge, which is counterbalanced by exchangeable cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ [73]. This ion-exchange property is among the primary attributes that make zeolites valuable for environmental engineering uses.

Typically, zeolites exhibit a cation exchange capacity (CEC) in the range of 1.5 to 5.5 meq/g [74, 75]. The Si/Al ratio in zeolites plays a critical role in determining CEC, with lower Si/Al ratios associated with increased CEC values [76, 77]. The general chemical formula for zeolites can be represented as M_xD_y[Al_x+2ySi_n-(x+2y)O_2n] mH₂O, where M and D indicate exchangeable monovalent and divalent cations, respectively [78]. Zeolites possess pore sizes that generally range from 0.2 to 10 nm [79]. The combination of these pore structures and cation exchange properties facilitates the selective adsorption and desorption of ions by zeolites, depending on both charge and size [74]. The ion-exchange selectivity of natural clinoptilolite, a widely used zeolite, generally follows the sequence: K⁺ > NH₄⁺ > Pb²⁺ > Ba²⁺ > Na⁺ > Ca²⁺ > Li⁺ > Cd²⁺ > Cu²⁺ > Zn²⁺ [80, 81]. The estimated global production of zeolites is approximately 1.1 million tons per year, and market prices depend on factors such as purity and application, with typical values ranging from $50–300 per ton [82, 83].

Numerous studies have examined ammonium ion removal techniques employing both pristine and modified zeolites (Table 2). Muscarella et al. (2021) investigated the removal of ammonium ions utilizing natural clinoptilolite with a particle size of 2–2.5 mm to treat synthetic wastewater containing an initial ammonium concentration of 778 mg NH₄-N/L, and determined a maximum adsorption capacity of 9.1 mg NH₄-N/g [43]. Within the initial 8 hours, 90% of the equilibrium adsorption capacity was reached, after which the adsorption rate markedly decreased [43]. The researchers also demonstrated the possibility of ammonium recovery as well as clinoptilolite reuse through the application of 100 mL of 1 M NaCl regenerant solution per gram of zeolite, leading to an ammonium ion recovery rate of 80% [43]. Nevertheless, the long-term performance and stability of the adsorption and regeneration processes across multiple cycles were not evaluated, indicating a requirement for further extended and repetitive regeneration studies.

Taddeo et al. (2017) utilized natural clinoptilolite to treat sludge return wastewater artificially enriched with NH₄Cl to replicate ammonium-rich conditions (2500 mg NH₄-N/L), and reported a maximum adsorption capacity of 20 mg NH₄-N/g [84]. Their results demonstrated that adsorption capacity was minimally affected by pH values ranging from 6 to 8.5 [84]. Ammonium ion adsorption primarily operates via an ion-exchange mechanism; therefore, pH only significantly influences the process under highly alkaline conditions, which favor the conversion of ammonium ions to gaseous ammonia, or extremely acidic conditions, which elevate proton (H⁺) concentrations. Thus, in designing ammonium ion removal systems using ion-exchange-based zeolites for wastewater with neutral pH, pH adjustment typically does not play a significant role. This feature distinguishes ion-exchange adsorption technology from ammonia stripping and membrane contactor processes, which require stringent pH control for optimal performance.

Kučić et al. (2012) examined the ammonium ion removal efficiency of natural clinoptilolite with particle size ranges of 1–2 mm, 2–4 mm, and 4–10 mm [85]. For synthetic wastewater with an initial ammonium concentration of 7000 mg NH₄-N/L, maximum adsorption capacities of 56, 43, and 42 mg NH₄-N/g were observed, respectively [85]. The study found that 1–2 mm zeolite exhibited an adsorption capacity 23% greater than those with larger particle sizes, which was attributed to its increased specific surface area [85]. Nevertheless, the use of excessively fine particles can lead to operational challenges, such as clogging in full-scale adsorption towers and difficulties with settling and recovery in batch systems. Therefore, process design should balance adsorption capacity with practicality to ensure efficient and manageable operation.

Gagliano et al. (2022) synthesized zeolite from Mt. Etna volcanic ash using hydrothermal methods for the purpose of ammonium adsorption [86]. When tested with wastewater containing 100 mg NH₄-N/L, the synthesized zeolite achieved a maximum adsorption capacity of 13.7 mg NH₄-N/g [86]. X-ray diffraction (XRD) analysis confirmed the presence of X-type zeolite and sodalite in the product, both recognized for their strong ammonium adsorption abilities [87, 88]. As volcanic ash is frequently regarded as a waste product that can have detrimental economic effects on local communities, converting it into high-performance zeolite adsorbents offers a promising and sustainable solution [86].

Lu et al. (2022) modified synthetic NaY zeolite with MgCl₂ and FeCl₂ to introduce surface sites for struvite precipitation and magnetic characteristics [89]. In experiments treating synthetic wastewater containing 156 mg NH₄-N/L of ammonium, the zeolite exhibited a maximum adsorption capacity of 110 mg NH₄-N/g [89]. Ion exchange and surface struvite precipitation were identified as the dominant adsorption mechanisms [89]. Furthermore, the presence of iron oxide allowed the used zeolite to be separated efficiently by applying a magnetic field [89]. In the context of sustainable adsorption processes, effective recovery of spent adsorbents is essential. Therefore, further investigations into strategies for separating used adsorbents, as explored in this study, are highly significant.

Muscarella et al. (2023) investigated ammonium ion adsorption using four types of zeolites: two natural zeolites (ZNS, ZNC) and two NaCl-modified zeolites (ZSS, ZSC) [90]. When treating synthetic wastewater with an ammonium concentration of 15556 mg NH₄-N/L, maximum adsorption capacities were observed: 23 mg NH₄-N/g for ZNS, 24 mg NH₄-N/g for ZNC, 26 mg NH₄-N/g for ZSS, and 27 mg NH₄-N/g for ZSC [90]. Treatment with high concentrations of NaCl was found to enhance the specific surface area and pore diameter of zeolites, thereby improving the structure for monovalent ion adsorption [90, 91].

Research into composite materials based on zeolite remains relatively limited. Salam et al. (2021) fabricated a composite adsorbent by integrating zeolite with geopolymer, achieving a maximum ammonium adsorption capacity of 103 mg NH₄-N/g, nearly double that of pure geopolymer (58 mg NH₄-N/g) [92]. After five consecutive adsorption-regeneration cycles, the composite retained more than 85% of its original adsorption performance, illustrating significant recyclability [92].

Nguyen et al. (2022) also developed a composite by synthesizing zeolite/layered double hydroxide (LDH) using Mg/Al LDH [93]. This composite displayed a maximum ammonium adsorption capacity of 21 mg NH₄-N/g, which was credited to the combined effects of LDH surface complexation with ion-exchange processes [93]. These findings indicate that integrating different materials may overcome certain adsorption limitations and offers a basis for designing advanced adsorbents that utilize multiple adsorption mechanisms.

A review of prior research demonstrates that zeolite’s high efficiency in ammonium removal is predominantly attributed to its ion-exchange mechanism. Under near-neutral pH conditions, the majority of ammonia nitrogen exists as ammonium ions (NH₄⁺), which can readily occupy ion-exchange sites [43, 84]. Additional studies have reported the enhancement of adsorption capacity through mechanisms such as surface precipitation and surface complexation [89, 93]. While reducing particle size tends to increase adsorption efficiency, it may also result in clogging of adsorption columns and complicate the separation of exhausted adsorbent, highlighting the need to balance operational practicality with performance in the design of full-scale processes [85]. Due to its cost-effectiveness and strong ammonium adsorption capability, future studies that focus on advancing the long-term regeneration and field applicability of zeolites are anticipated to be key for establishing nitrogen recovery-based wastewater treatment systems.

Ion-exchange resins consist of polymeric beads synthesized primarily from styrene and divinylbenzene monomers [94]. Although these resins are insoluble in water, they exhibit swelling upon water uptake, which opens their internal porous structure and allows ions to diffuse into the matrix for adsorption [95]. Post-synthesis modification is typically performed to introduce functional groups that impart selectivity for specific ions [96]. For example, cation exchange resins are functionalized with fixed -SO₃⁻ or -COO⁻ groups to mediate ion-exchange reactions with cations present in water, including ammonium ions, hardness ions (Ca²⁺, Mg²⁺), and certain heavy metals. Strong acid cation (SAC) exchange resins characteristically display a CEC in the range of 1.8–2.5 eq/L, and their simplified molecular formula is represented as R-SO₃⁻ [97, 98]. The ion selectivity hierarchy of conventional SAC resins is Ba²⁺ > Pb²⁺ > Ca²⁺ > Cu²⁺ > Cs⁺ > K⁺ > NH₄⁺ > Na⁺ > H⁺ > Li⁺ [99]. Weak acid cation (WAC) exchange resins typically possess a higher CEC (3.5–5.0 eq/L), with R-COO– as their representative molecular structure [100, 101]. WAC resins exhibit strong affinity for divalent cations but have lower selectivity for monovalent cations [102, 103]. The selectivity order for WAC resins can be summarized as: Cu²⁺ > Co²⁺ > Zn²⁺ > Ni²⁺ > Ca²⁺ > Mg²⁺ > NH₄⁺ [99, 102, 103].

Numerous research groups have investigated the removal of ammonium ions from wastewater using ion-exchange resins (Table 3). Chen et al. (2002) examined ammonium removal with SAC exchange resin (Amberjet 1200 Na), which contains -SO₃⁻ functional groups [104]. In experiments using synthetic wastewater with an initial ammonium concentration of 31 mg NH₄-N/L, they reported a maximum adsorption capacity of 21 mg NH₄-N/g [104]. The introduction of competing cations such as Na⁺, K⁺, Mg²⁺, and Ca²⁺ led to a substantial decrease in ammonium removal efficiency, from 73% to 12.4%, underlining the critical influence of competition in ion-exchange processes [104]. These findings emphasize that process design for actual applications must take into account both the presence and temporal variation of competing cations in influent wastewater.

Wang et al. (2022) evaluated ammonium removal using two SAC exchange resins, D61 and D708 [105]. Using synthetic wastewater with an initial ammonium concentration of 2333 mg NH₄-N/L, they achieved maximum adsorption capacities of 11 mg NH₄-N/g and 7 mg NH₄-N/g, respectively [105]. Variations in temperature from 25–40 °C caused less than a 10% change in adsorption efficiency, indicating that the performance of these ion-exchange resins remains relatively consistent despite temperature fluctuations [105]. Their feasibility study estimated the cost of an ion-exchange-resin-based ammonium removal process at approximately $1.78/m³ for wastewater treatment [105]. It is important to note that this cost assessment was based on laboratory-scale research; thus, further pilot-scale testing on site is required to more accurately estimate operational expenses.

Ding and Sartaj (2016) investigated ammonium removal using SAC exchange resin (Amberlite IR120H) and found that treating synthetic wastewater with an initial ammonium concentration of 3000 mg NH₄-N/L yielded a maximum adsorption capacity of 27 mg NH₄-N/g [106]. The adsorption behavior conformed to the pseudo-second-order kinetic model, attaining equilibrium within 30 minutes, which indicates a fast adsorption process [106]. The spent resin was effectively regenerated using 2 N H₂SO₄, completely restoring its original adsorption capacity, and enabling the recovery of ammonium ions [106]. These results demonstrate the viability of repeated use of ion-exchange resins within ammonium recovery systems. Nevertheless, extended validation is necessary to evaluate regeneration effectiveness across multiple adsorption-regeneration cycles.

Vignoli et al. (2015) evaluated the ammonium adsorption capacities of four distinct ion-exchange resins in synthetic wastewater [107]. SAC exchange resins (Amberlyst 15wet and Lewatit VPOC1800), which feature sulfonate groups (-SO₃⁻) with a strong affinity for ammonium, demonstrated maximum adsorption capacities of 58 mg NH₄-N/L and 55 mg NH₄-N/L, respectively, when applied to wastewater with an initial ammonium concentration of 1556 mg NH₄-N/L [107]. By contrast, the WAC ion-exchange resin (Dowex MAC-3), characterized by low ammonium affinity, and the non-ionic resin (Purolite MN 250) showed negligible ammonium adsorption [107]. These findings indicate that in designing ion-exchange-resin-based ammonium removal processes, careful selection of resins is essential, prioritizing those capable of maintaining deprotonated functional groups across a broad pH spectrum and exhibiting high ammonium affinity (such as SO₃⁻-based resins) [108].

Chartrand (2018) investigated ammonium removal using SAC exchange resins (Amberlite IR120Na and Purolite SSTC60) [109]. With an initial ammonium concentration of 70 mg NH₄-N/L in synthetic wastewater, the maximum adsorption capacities achieved were 24.9 mg NH₄-N/g for Amberlite IR120Na and 25.3 mg NH₄-N/g for Purolite SSTC60 [109]. Nevertheless, when these resins were utilized for treating real mining wastewater, the existence of competing cations (e.g., K⁺, Ca²⁺) resulted in an 80% reduction in adsorption capacity [109]. These results highlight the necessity of pre-treatment steps to eliminate competing cations or the development of more selective resin materials to optimize performance in real wastewater scenarios.

A review of prior research demonstrates that SAC exchange resins consistently provide high ammonium adsorption performance across a range of concentrations, primarily attributed to the presence of sulfonate functional groups with strong ammonium affinity [104, 105, 107]. However, several studies have also revealed a marked decline in ammonium adsorption efficiency in the presence of competing cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺, with observed efficiency reductions of up to 80% in real wastewater contexts [104, 109]. As these competing cations significantly influence the achievable adsorption capacity, they consequently exert a pronounced effect on overall treatment costs [109].

To enhance the practical applicability of ion-exchange resins, future studies should emphasize the optimization of resin functional groups and structural modification strategies to increase ammonium selectivity while reducing interference from competing cations. The development of pre-treatment processes aimed at eliminating or lowering the concentration of competing cations prior to ammonium adsorption is also critical. Moreover, comprehensive assessments concerning the structural stability of ion-exchange resins over extended adsorption-regeneration cycles remain limited. Therefore, prior to full-scale deployment, long-term experimental evaluation is essential, including the investigation of diverse competing ions and influent parameters to verify the overall feasibility of the process.

Activated carbon and biochar are typical carbon-based adsorbents extensively employed in wastewater treatment due to their significant specific surface area and the presence of various functional groups. Activated carbon is obtained through the physical or chemical activation of carbon-rich raw materials such as wood, coal, and coconut shells at elevated temperatures [110]. In general, steam activation (Eq. (8)) and carbon dioxide activation (Eq. (9)) are conducted at temperatures between 800 and 1100°C, removing volatile substances and creating well-developed pore structures [110].

Activated carbon typically possesses a substantial specific surface area of 100–1500 m²/g and a pore volume of 0.1–0.8 cm³/g, with its carbon content varying from 30% to 95% [111–114]. The presence of both micropores and macropores allows for the efficient adsorption of a range of aquatic contaminants. Furthermore, oxygen-containing functional groups (e.g., -COOH, -OH, -SO₃H) are formed on the activated carbon surface during activation. These groups are polar, promoting stronger interactions with polar substances in aqueous environments [115]. Numerous studies have explored modifications to introduce new functional groups onto the surface of activated carbon, substantially increasing its adsorption efficiency for specific contaminants [116]. The combination of large surface area and modifiable surface chemistry thus renders activated carbon a highly efficient and versatile adsorbent.

Another carbon-based adsorbent, biochar, is a carbon-rich material derived from organic feedstocks (e.g., agricultural waste, sewage sludge) via pyrolysis under oxygen-limited conditions [117]. Biochar is generally produced by pyrolyzing organic feedstocks at temperatures between 300–800 °C in an oxygen-restricted environment, resulting in a carbon content that varies from 25% to 90% [118]. In contrast to activated carbon, biochar generated solely through pyrolysis (without activation) typically exhibits a specific surface area of 8–132 m²/g and a pore volume of 0.02–0.08 cm³/g [119]. However, following activation with various gases (e.g., NH₃, CO₂, CO, CH₄, H₂), biochar can reach specific surface areas of 300–800 m²/g and pore volumes of 0.2–0.4 cm³/g, becoming comparable to activated carbon and incorporating a variety of functional groups [120]. Owing to its increased surface area and variety of functional groups, biochar shows considerable potential for the adsorption of diverse water pollutants [120].

Many studies have investigated the performance of carbonaceous adsorbents, including activated carbon and biochar, for the removal of ammonium ions (Table 4). In addition to investigations with unmodified adsorbents, several reports have focused on modification techniques designed to improve adsorption performance.

Zhu et al. (2016) examined ammonium ion adsorption utilizing activated carbon derived from avocado seeds and reported a maximum adsorption capacity of 3 mg NH₄-N/g when treating synthetic wastewater with an initial ammonium concentration of 350 mg NH₄-N/L [121]. Their analysis identified electrostatic attraction involving carboxyl functional groups as the primary mechanism for adsorption [121]. The authors further explained that at pH values below 4, protonation of carboxyl groups on the activated carbon surface diminished charge interactions, leading to lower adsorption efficiency. At pH above 8, ammonium ions are converted to volatile ammonia due to deprotonation, causing a reduction in adsorption efficiency [121]. These findings indicate that pH control and the characteristics of surface functional groups are critical factors in ammonium ion adsorption processes.

Vu et al. (2018) investigated the use of corncob-based activated carbon for ammonium ion removal from synthetic wastewater and observed a maximum adsorption capacity of 10 mg NH₄-N/g at an initial ammonium concentration of 82 mg NH₄-N/L [122]. During regeneration, attempts were made to desorb ammonium ions using high concentrations of HCl, NaCl, and NaOH; however, desorption efficiency remained below 70%, suggesting constraints in the adsorbent’s regeneration capability [122]. Because the CEC of the adsorbent was more than 30% lower than the ammonium adsorption amount, the authors inferred that irreversible adsorption processes were involved [122]. The observed limited regeneration efficiency, resulting from irreversible adsorption, implies that corncob-based activated carbon may not be suitable for applications requiring sustainable ammonium ion recovery.

Ghising and Jha (2022) investigated the use of activated carbon derived from waste tires for the adsorption of ammonium ions from synthetic wastewater, achieving a maximum adsorption capacity of 105 mg NH₄-N/g at an initial ammonium concentration of 467 mg NH₄-N/L [123]. The study also included adsorption tests on four groundwater samples from the Kathmandu region, revealing that the application of 1 g/L activated carbon yielded ammonium ion removal efficiencies between 17% and 28% [123]. The study did not provide an in-depth analysis of the factors contributing to the observed variations in removal efficiency among the groundwater samples. The authors proposed that these efficiency differences were likely attributable to variations in competing cation concentrations present in the groundwater [123].

Ren et al. (2021) investigated the use of Fe₂O₃-modified activated carbon for removing ammonium ions from synthetic wastewater, reporting a maximum adsorption capacity of 0.2 mg NH₄-N/g at an initial ammonium concentration of 12 mg NH₄-N/L [124]. The Fe₂O₃ modification process, which utilized high-temperature calcination, resulted not only in the deposition of Fe₂O₃ on the activated carbon surface but also generated a range of surface functional groups, including carboxyl groups [124]. Consequently, ammonium ions were retained through several mechanisms, such as electrostatic attraction, ion-exchange, and physical adsorption [124]. Previous research has shown that iron-based surface modifications facilitate the introduction of polar functional groups and oxidized metal structures, thereby improving the adsorption capacity for cationic pollutants [125].

Lee et al. (2018) employed a surfactant treatment to modify coconut shell-based activated carbon, introducing sulfonate functional groups with heightened affinity for ammonium ions [11]. In experiments using synthetic wastewater at an initial ammonium concentration of 1000 mg NH₄-N/L, the modified activated carbon reached a maximum adsorption capacity of 6.7 mg NH₄-N/g [11]. This represented an 85% increase compared to unmodified coconut shell-based activated carbon. The improvement was ascribed to the presence of sulfonate groups, which strengthened both electrostatic and ion-exchange interactions with ammonium ions [11]. Collectively, these results indicate that targeted incorporation of ion-exchange functional groups onto carbon-based adsorbents can significantly enhance their performance in ammonium ion adsorption.

Hou et al. (2016) prepared biochar by pyrolyzing giant reed at 500 °C and found a maximum adsorption capacity of 0.9 mg NH₄-N/g during synthetic wastewater treatment [126]. Their results showed that increasing the reaction temperature from 283 K to 313 K led to a 43% improvement in adsorption capacity, indicating an endothermic adsorption process [126]. While there is limited research specifically addressing the influence of temperature on ammonium ion adsorption, some evidence suggests that temperature variations may affect adsorption efficiency. As a result, assessing temperature sensitivity is advisable when designing adsorption processes.

Wang et al. (2020) synthesized biochar from peanut shells at 500 °C, followed by post-treatment with 2 M H₂O₂ [127]. The biochar modified with H₂O₂ exhibited a quadrupled adsorption capacity compared to unmodified biochar, a result ascribed to the formation of oxygen-containing functional groups such as carboxyl and carbonyl groups during oxidation [127, 128]. In addition, Wang et al. (2015) documented that extended oxidation using greater H₂O₂ concentrations could further improve ammonium ion adsorption capacity [128]. Collectively, these results highlight that enriching the biochar surface with negatively charged oxygen-containing functional groups supports the creation of efficient biochar-based adsorbents.

A comparison of previous findings indicates that the effectiveness of ammonium ion removal by carbonaceous adsorbents depends significantly on the types and abundance of surface functional groups. Consequently, to achieve a high-performance ammonium ion adsorption system with carbonaceous adsorbents, future work should investigate targeted modifications of surface functional groups to improve ammonium ion selectivity.

To assess the ammonium ion adsorption performance across various adsorbents (i.e., zeolite, ion-exchange resin, activated carbon, and biochar), both the average maximum adsorption capacity (mg NH₄-N/g) and the cost normalized adsorption capacity (g NH₄-N/$) were evaluated (Fig. 2). The cost normalized adsorption capacity was determined by dividing the average maximum adsorption capacity by the market price of each adsorbent ($/g) (Table S1). The analysis revealed that zeolite achieved the highest average maximum adsorption capacity (39.8 mg NH₄-N/g), followed by ion-exchange resin (31.2 mg NH₄-N/g), biochar (30.2 mg NH₄-N/g), and activated carbon (21.4 mg NH₄-N/g). This trend aligns with established research, which attributes zeolite’s superior ammonium adsorption to its negatively charged lattice structure, elevated cation exchange capacity, and extensive surface area [80, 81, 86, 89]. When normalization was applied based on unit cost, zeolite (199 g NH₄-N/$) displayed a markedly greater cost-effectiveness in comparison to biochar (30 g NH₄-N/$), activated carbon (11 g NH₄-N/$), and ion-exchange resin (6 g NH₄-N/$). Although adsorption capacities of these adsorbents differed by less than a factor of two, the unit price varied by as much as 25 times—most notably between zeolite and ion-exchange resin—leading to pronounced disparities in cost normalized performance. Thus, selecting suitable adsorbents for real-world applications requires consideration of both adsorption capacity and the economic implications of material costs.

It is important to acknowledge that this cost normalized adsorption capacity analysis relies on laboratory-scale data and limited market price sources (Table 2–4 and S1). In operational settings, adsorbent prices may fluctuate with purchase volume, logistics, available storage, and supplier arrangements. Additionally, long-term operational expenses and environmental sustainability are affected by the chosen regeneration technique and the number of possible reuse cycles for each adsorbent. Therefore, adsorbent choice should be guided by a holistic assessment encompassing adsorption efficiency, unit cost, durability, and potential for regeneration. To support effective decision-making, pilot-scale demonstrations under actual operating scenarios are advised before proceeding to full-scale deployment.

Zeolites, ion-exchange resins, and carbon-based adsorbents, which are commonly employed for ammonium ion removal, function primarily through ion-exchange processes involving acidic anionic functional groups and negatively charged inorganic frameworks [7]. Ammonium ions adsorbed by ion-exchange can be desorbed using regenerant solutions with high concentrations of acids, bases, or salts (e.g., HCl, NaOH, NaCl). This enables both the recovery of ammonium ions and the regeneration of the adsorbents for subsequent reuse [129].

The chemical regeneration process displaces ammonium ions sorbed to adsorption sites by introducing alternative cations, thereby enabling efficient desorption. In practice, concentrated aqueous solutions of HCl (1–10 wt%), H₂SO₄ (0.5–6 wt%), NaCl (8–26 wt%), and NaOH (2–5 wt%) are used to extract pollutant ions, such as ammonium, from ion-exchange matrices [130, 131]. The chemical regeneration mechanisms for ammonium ions adsorbed on zeolite, SAC resin, WAC resin, and activated carbon are depicted in Eqs. (10)–(13), respectively. Upon regeneration, the process produces waste solutions enriched in ammonium ions. Generally, these ammonium-laden regenerant wastewaters require subsequent purification and concentration prior to their use as ammonia feedstock. Section 5 will present a detailed discussion of strategies for utilizing recovered ammonium.

For the application of chemical regeneration processes, it is critical to assess not only the variations in adsorption performance throughout successive cycles but also the consequent consumption and costs associated with chemical reagents. For example, previous studies have shown that ammonium-saturated zeolites regenerated with NaCl (20–30 g/L) and NaOH (1–2 g/L) solutions can achieve desorption and regeneration efficiencies exceeding 95% [132, 133]. Turan and Celik (2003) demonstrated that natural zeolite could retain similar ammonia removal efficiency after three regeneration cycles when using a mixed solution containing 30 g/L NaCl and 1.5 g/L NaOH [133]. Sancho et al. (2017) utilized 2 g/L NaOH to regenerate natural zeolite, finding that the adsorption capacity remained above 95% beyond two regeneration cycles [132]. Likewise, Rahmani et al. (2004) and Hlavay et al. (1982) observed that zeolite preserved stable ammonium adsorption capacity following regeneration with NaCl solutions [134, 135]. For ion-exchange resins, Tarpeh et al. (2018) evaluated the regeneration of ammonium-saturated Dowex Mac 3 resin and reported that H₂SO₄ delivered the highest regeneration efficiency among several equinormal regenerants, such as HCl, HNO₃, and NaCl [136]. Regarding chemical consumption and associated costs, Sancho et al. (2017) noted that regenerating 1 BV of zeolite typically required around 20 bed volumes (BV) of 2 g/L NaOH solution [132]. Using this scenario and projected average reagent costs in 2025, the estimated chemical expense is about $100–$120 for regenerating 1 m³ of zeolite [137]. This demonstrates that, although regeneration effectively recovers adsorption performance, the demand for regenerant chemicals can significantly impact overall operating expenses. In the case of ion-exchange resins, regeneration usually requires 2–3 BV of regenerant solution per 1 BV of resin, employing 5–10% HCl, H₂SO₄, or NaCl [130]. The resulting chemical expenses for regenerating 1 m³ of resin are projected to be within $300–$900 for HCl, $100–$300 for H₂SO₄, and $150–$450 for NaCl [137]. However, these estimates may fluctuate depending on variations in experimental design, reagent concentrations, and local market conditions. Consequently, they should be considered comparative references for evaluating the relative economic viability of distinct regeneration methods rather than absolute values.

Among ammonium ion adsorbents, zeolites are particularly notable for their ability to be regenerated through a thermal desorption process. The underlying mechanism of thermal regeneration involves the desorption of ammonium ions adsorbed onto zeolites as ammonia gas at elevated temperatures, with only protons remaining, thus restoring the ion-exchange capacity of the zeolite [138]. Specifically, when zeolites loaded with ammonium ions are heated within the temperature range of 150–500 °C, the ammonium ions decompose to produce ammonia gas, which is subsequently released. The effectiveness of this process is strongly dependent on the zeolite’s pore structure, acidity, and the activation energy required for desorption [139, 140]. Gordina et al. (2022) identified an activation energy of 70–80 kJ/mol for ammonia desorption from zeolites and demonstrated that both physical and chemical desorption occur in succession as temperature increases [139]. The thermal regeneration process for ammonium-saturated zeolites is illustrated by Eq. (14).

Thermal regeneration offers a promising approach for desorbing ammonium ions from adsorbents and facilitates the recovery of pure ammonia gas without the use of high-concentration regenerant solutions. In comparison to chemical regeneration, thermal regeneration eliminates the need for chemical regenerants and avoids the generation of secondary wastewater, enhancing its environmental sustainability. Additionally, the technique enables the recovery of ammonium ions in the form of high-purity ammonia gas, which can be used as a valuable precursor for various nitrogen-based chemicals [141].

Despite its potential, research into the thermal regeneration of ammonium ion adsorbents for wastewater treatment applications remains limited. Although laboratory-scale studies have demonstrated the fundamental processes of ammonia thermal desorption [139, 140], limited focus has been given to evaluating the practical implementation of thermal regeneration in actual wastewater treatment systems. Notably, the impact of repeated thermal cycling on adsorbent performance under operational conditions is still insufficiently understood. Furthermore, assessments of the economic feasibility and energy demands associated with thermal regeneration are scarce, despite their importance for real-world application. For thermal regeneration to advance sustainable nitrogen resource recovery, additional research is required to optimize operational parameters and to thoroughly evaluate the prospects for large-scale deployment.

Ensuring the structural stability and consistent adsorption performance of zeolite following multiple thermal regeneration cycles remains a significant challenge in real-world applications. Exposure to high temperatures may cause zeolites to experience structural collapse and modifications in surface chemistry, thereby decreasing their ion-exchange capacity and overall adsorption efficiency [142]. Additionally, zeolites can undergo partial dealumination at elevated temperatures, which diminishes the number of active sites essential for ammonium ion adsorption [143]. Consequently, there is a critical need to develop thermally stable zeolites that can sustain high adsorption efficiency after repeated thermal regeneration. Upcoming research efforts should prioritize the implementation of structural modification strategies to improve thermal resistance, as well as the investigation of innovative heat-resistant synthetic zeolites that preserve adsorption performance under elevated temperatures [144]. Effectively addressing these issues will facilitate the development of a sustainable thermal regeneration-based ammonium ion recovery system and support the advancement of more efficient nitrogen resource recovery technologies.

Worldwide, the annual production of ammonia exceeds 180 million tons, with over 80% allocated to the synthesis of ammonia-based fertilizers [145, 147]. The Haber-Bosch process requires operation at elevated temperature (430 °C) and pressure (100 atm), which leads to very high energy use and significant emissions of carbon dioxide [148]. Conversely, reclaiming ammonium ions from wastewater and utilizing them as a fertilizer precursor provide several notable benefits.

In the ammonium recovery process, ion-exchange-based adsorbents undergo multiple adsorption-regeneration cycles to enable repeated use. The selection of the regenerant solution plays a crucial role in determining the purity and suitability of the recovered ammonium brine for fertilizer applications. Regenerant solutions comprising concentrated acids (e.g., HCl, H₂SO₄, H₃PO₄, HNO₃), salts (e.g., NaCl, KCl), or bases (e.g., NaOH, KOH) are commonly utilized for adsorbent regeneration, with each type resulting in regenerant brine of varying chemical composition. For example, when regenerants such as NaCl are applied, the adsorbent can be effectively regenerated and reused, but the resulting spent brine contains both desorbed NH₄⁺ and residual Na⁺. The coexistence of additional cations in the recovered ammonium brine diminishes its suitability for ammonium fertilizer production and may restrict broader industrial applications. Furthermore, employing base-based regenerants such as NaOH leads to two primary issues: it introduces Na⁺ into the recovered solution and promotes the release of ammonium ions as ammonia gas (NH₃), thereby complicating subsequent use as a fertilizer.

Because nitrogen fertilizers are principally produced as ammonium salts, acid regenerants are required to achieve high-purity ammonium salts free from extraneous cation contamination. Typical acid regenerants, including HCl, H₂SO₄, H₃PO₄, and HNO₃, promote the efficient desorption of ammonium ions by substituting them with proton (H⁺) on the adsorbent surface [149, 150]. The end result of this regeneration procedure is a spent regenerant solution containing elevated concentrations of ammonium salts such as NH₄Cl, (NH₄)₂SO₄, (NH₄)₃PO₄, and NH₄NO₃, accompanied by residual proton, which imparts acidity to the solution [130, 151, 152].

Achieving a high-purity ammonium salt solution with low residual proton content requires the introduction of a controlled amount of ammonia gas to the spent regenerant solution, facilitating neutralization of remaining protons [153]. This approach enables the production of highly pure ammonium salt solutions—such as ammonium sulfate devoid of contaminants like Na⁺—to meet industrial demand. In particular, the regeneration of ion-exchange resins fully loaded with ammonium ions using sulfuric acid is depicted by Eq. (15). Post-regeneration, the sulfuric acid-based spent regenerant solution consists predominantly of ammonium ion (NH₄⁺), residual proton (H⁺), and sulfate ion (SO₄²⁻), rendering the solution acidic. To neutralize residual protons and enhance ammonium ion concentration, a precise amount of ammonia gas is introduced, triggering the neutralization reaction outlined in Eq. (16) and ultimately yielding a high-purity ammonium sulfate solution.

In conventional ammonium fertilizer production, significant quantities of ammonia gas are directly introduced into concentrated acids (e.g., H₂SO₄) to synthesize ammonium salts [153]. In contrast, neutralizing the spent regenerant solution with a smaller amount of ammonia gas to obtain high-purity ammonium salt fertilizers represents a more sustainable and environmentally responsible process [153]. The high-purity ammonium salt solution generated using this approach can be subsequently processed into solid ammonium salt fertilizers (e.g., ammonium sulfate, ammonium phosphate) through the use of a mechanical vapor recompression evaporator, supporting the commercialization of the final product [154].

A noteworthy application of recovered ammonia is the synthesis of urea, which is an extensively utilized nitrogen-based chemical [155]. Urea is the most prominent nitrogen-based chemical, exhibiting an annual global production volume of 76 million tons [147]. Principal uses of urea consist of nitrogen fertilizer, melamine, urea-formaldehyde resin, and diesel exhaust fluid (DEF) manufacture [156].

During urea synthesis from recovered ammonia, the release of ammonia gas from adsorbed ammonium ions is a vital process. The effectiveness of extracting adsorbed ammonium ions as ammonia gas depends substantially on the choice of regenerant solution. Different regenerant solution types, such as acid, salt, and base, are capable of desorbing ammonium ions from adsorbents. Nevertheless, only base regenerant solutions can transform the desorbed ammonium ions into ammonia gas. The principal advantage of using base regenerant solutions lies in their capability to drive the ammonium-ammonia equilibrium toward ammonia gas production. In the presence of a strong base such as NaOH, hydroxide ions interact with desorbed ammonium ions, resulting in effective deprotonation to generate non-ionic ammonia. Because non-ionic ammonia is volatile, it can transition efficiently from the liquid to the gas phase, which facilitates its extraction. This mechanism is optimized under alkaline conditions with elevated pH values, thereby ensuring a high ammonia gas recovery rate. In contrast, acid and salt regenerant solutions do not support this conversion. Spent regenerant solutions derived from acid and salt regeneration maintain acidic and neutral pH, respectively. These pH conditions inhibit the transformation of desorbed ammonium ions into volatile ammonia gas. Under acidic conditions, ammonium ions persist in their ionic state, preventing volatilization. Similarly, neutral pH fails to sufficiently promote the conversion of ammonium ions to ammonia gas (pK_a = 9.24 at 25 °C) [51].

Therefore, for the production of nitrogen-based chemicals such as urea, in which ammonia gas acts as a raw material, it is necessary to employ a base regenerant solution, such as NaOH. In particular, the process of regenerating exhausted ion-exchange resins using an NaOH regenerant solution and subsequently extracting ammonia gas is illustrated by Eq. (17) below [131].

Following the volatilization and separation of ammonia, the recovered ammonia gas may be introduced into the conventional ammonium carbamate synthesis reaction (Eq. (18)), which, after undergoing the ammonium carbamate dehydration reaction (Eq. (19)), yields urea [155].

Nitric acid is a significant nitrogen-based chemical, with an annual global output of 9 million tons [147]. Evaluating the feasibility of synthesizing nitric acid from recovered ammonia is therefore warranted. When producing nitric acid from recovered ammonia, as is the case for urea, extracting ammonia gas from adsorbed ammonium ions is an essential step. According to Eq. (17), the application of base regenerant solutions (such as NaOH or KOH) achieves both the release of ammonium ions and subsequent ammonia gas extraction [131]. The recovered and purified ammonia gas is suitable for the Ostwald process, the primary commercial route for nitric acid manufacture [157]. The Ostwald process encompasses three principal stages: ammonia oxidation (Eq. (20)), nitric oxide oxidation (Eq. (21)), and nitrogen dioxide absorption (Eq. (22)) [157].

Nitric acid produced by these processes can be used as a precursor for nitrate-based fertilizers, nitrobenzene, and other nitrogen-based chemicals [147, 158]. Integrating ammonia recovered from wastewater into nitric acid synthesis directly links treatment with nitrogen chemical production, advancing a sustainable nitrogen cycle.

Hydrogen gas (H₂) is recognized as a clean energy carrier, since only H₂O forms when H₂ reacts with O₂ in combustion or fuel cell processes to release energy [159]. In contrast to carbon-based fuels like petroleum or natural gas, which discharge carbon dioxide (CO₂), nitrogen oxides (NO_x), and sulfur oxides (SO_x) during combustion, hydrogen generates no such pollutants, positioning it as an environmentally advantageous alternative [160]. Consequently, the hydrogen economy is considered a pivotal approach for achieving carbon neutrality. Despite hydrogen’s superior gravimetric energy density (120 MJ/kg), its volumetric energy density is very low at only 9.8 kJ/L under standard temperature and pressure (STP), which presents a considerable barrier [161]. For comparison, gasoline and methanol under STP provide volumetric energy densities of 32 MJ/L and 16 MJ/L, respectively [162]. Therefore, the inherently low volumetric energy density of hydrogen gas remains a prominent limitation that must be resolved.

To address the limitations of hydrogen gas, ammonia (NH₃) has attracted attention as a next-generation hydrogen energy carrier [163]. Ammonia possesses three hydrogen atoms, resulting in a high hydrogen density, and it can be conveniently transported in liquid form [164]. The strategy of transporting hydrogen as ammonia and subsequently cracking ammonia on-site to release hydrogen gas for energy utilization has emerged as a promising solution [161]. Nonetheless, transitioning to an ammonia-to-hydrogen conversion-based hydrogen economy presents several challenges. The Haber-Bosch process for ammonia synthesis has a substantial carbon footprint, raising important environmental concerns [165]. In response, research is increasingly directed toward synthesizing ammonia using renewable energy sources and establishing more environmentally benign ammonia production methods [166]. In light of the growing demand for green ammonia, producing hydrogen from ammonia recovered from wastewater offers a compelling alternative [167, 168].

In this regard, the recovery of ammonia from wastewater not only supports the hydrogen economy as a sustainable hydrogen carrier but also provides a valuable method for extracting energy from wastewater. Advancing toward a hydrogen-based energy infrastructure requires solving two principal challenges [169]. First, hydrogen production must minimize greenhouse gas emissions throughout the process. Second, storage and transportation systems for hydrogen should ensure safety and efficiency. Extracting ammonia from wastewater and employing it as a transportable hydrogen carrier for subsequent on-site hydrogen generation can address both of these obstacles. This strategy unites wastewater treatment processes with sustainable energy generation, presenting a viable alternative to conventional hydrogen supply pathways.

Successful implementation of this approach requires that ammonia recovered from wastewater be efficiently transformed into hydrogen. This process is enabled through catalytic ammonia cracking, which achieves highly efficient hydrogen generation [164]. The procedure begins with the recovery of ammonium ions from wastewater via adsorption, followed by conversion to ammonia gas (Eq. (17)), which is then directed into a catalytic ammonia cracking system. Within this system, conventional metal catalysts (such as Ru, Ni, and Fe) promote the thermal decomposition of ammonia at operational temperatures of 450–700 °C, resulting in a conversion efficiency exceeding 98% (Eq. (23)) [162]. The hydrogen produced using ammonia sourced from wastewater can subsequently be used as an energy resource in diverse sectors through the application of fuel cells. In sum, the generation of hydrogen from wastewater-derived ammonia advances the realization of waste-to-energy technologies.

Recently, ammonia has attracted considerable attention not only as a hydrogen carrier but also as a direct fuel, presenting a sustainable approach for energy recovery from ammonia derived from wastewater [170]. In contrast to fossil fuels, ammonia combustion does not release carbon dioxide, and its existing storage and transportation infrastructure supports its viability as an alternative energy source [171]. Furthermore, ammonia can be directly used in fuel cells, creating a promising route toward carbon-free energy generation [172].

Although ammonia offers several benefits, direct combustion is associated with a number of technical challenges. The long ignition delay time of ammonia contributes to decreased combustion efficiency [171]. Additionally, nitrogen oxides (NO_x) may form as byproducts during ammonia combustion [173]. To overcome these challenges, research has targeted the blending of fossil fuels with ammonia and the development of catalytic NO_x reduction techniques [172]. Concurrently, ammonia-based fuel cells have gained attention as a distinct method for energy conversion, providing improved efficiency and lower emissions [170]. Specifically, solid oxide fuel cells (SOFCs) enable the direct electrochemical oxidation of ammonia, producing only nitrogen (N₂) and water (H₂O) as byproducts [174]. Employing ammonia fuel cells achieves highly efficient energy conversion systems and substantially mitigates NO_x emissions, thereby improving environmental outcomes. This technology supports a circular economy by converting waste into energy and serves as a pivotal solution for attaining carbon neutrality in upcoming energy infrastructures.

The adsorption-based ammonium ion recovery process is characterized by a straightforward mechanism and reactor configuration; however, additional investigation into advanced adsorbents is necessary for successful industrial implementation. It is essential to develop adsorbents with significantly improved adsorption capacity, regeneration ability, and selectivity. Enhanced adsorption capacity increases the amount of ammonium ions recovered per unit of adsorbent, thus improving process efficiency and lowering both operational costs and waste generation [108, 175]. Further, elevated adsorption capacity can reduce both storage and reactor space requirements, making wastewater treatment systems more economically viable. Improving the efficiency of adsorbent regeneration and extending adsorbent lifespan are also critical. Robust, regenerable adsorbents decrease replacement rates, which supports greater economic viability, sustainability, and operational effectiveness [176].

For example, the effective adsorption capacity (Q_n) after n regeneration cycles can be described by Eq. (24), where Q₀ represents the initial adsorption capacity and η (0 < η < 1) indicates the retention rate of regeneration performance. A higher value of η signifies reduced performance loss following regeneration, which supports the sustained recovery of resources over extended periods. In addition, adsorbents with high regeneration capability allow for longer replacement intervals during prolonged operations, simplifying both system design and operational procedures. Enhancing the regeneration characteristics of adsorbents is therefore vital for ensuring the reliable performance of wastewater treatment plants.

The improvement of adsorbent selectivity is also an important area of research. Enhanced selectivity allows the adsorbent to preferentially capture ammonium ions even when various competing ions are present [177]. Such advancements offer multiple advantages for resource recovery processes. Moreover, highly selective adsorbents can produce ammonium ions with increased purity. In real wastewater systems, ions such as Na⁺, Ca²⁺, and Mg²⁺ coexist and compete with NH₄⁺. Adsorbents with high selectivity exhibit a stronger binding affinity toward ammonium, enabling more efficient adsorption of NH₄⁺ in the presence of these competing ions. This results in improved purity of the recovered ammonium, which in turn alleviates the demands on downstream purification stages. Ultimately, enhanced selectivity boosts the efficiency of the resource recovery system. However, if selectivity becomes excessively high and irreversible, it can impede the regeneration process, making resource recovery unfeasible. It is therefore essential to achieve an optimal balance between adsorbent selectivity and regenerability [108].

Investigating advanced materials that are relatively unexplored for ammonium adsorption may further improve the recovery of ammonium ions. The synthesis of metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) introduces a novel methodology in this context [178]. Characterized by their high specific surface area, tunable pore architectures, and designed functional sites, these materials offer the potential for superior selectivity, increased adsorption capacity, and enhanced regeneration capability [179]. Although research on the use of MOFs and COFs for ammonium ion adsorption remains scarce, optimizing these materials for ammonium recovery holds significant promise for future advancements.

Adsorption-based technology for ammonium ion removal has shown excellent efficacy in various laboratory-scale experiments. In contrast, pilot- and full-scale investigations of its practical application in wastewater treatment plants are extremely limited [19]. This scarcity is attributed to multiple obstacles associated with the implementation of large-scale operations.

The primary reasons for the scarcity of pilot- and full-scale studies are operational challenges, such as clogging and fouling within the adsorption tower, frequent replacement and regeneration of saturated adsorbents, and the logistical challenges associated with handling high-concentration regenerant solutions (e.g., HCl, H₂SO₄) [19]. Although these difficulties are manageable at the laboratory scale, they present far greater complications in scaled-up applications. Larger-scale deployment must overcome several industrial constraints, including the challenge of maintaining continuous wastewater treatment during adsorbent regeneration, requirements for infrastructure to manage strong acid regenerants, and heightened safety risks to workers during adsorption tower maintenance. These substantial barriers have hindered industries from actively implementing and assessing adsorption processes on a larger-scale.

Another complicating factor in pilot testing is that adsorption processes under evaluation cannot substitute for existing biological wastewater treatment systems, as they function differently in terms of target contaminant removal. Conventional biological treatment systems are capable of simultaneously eliminating organic matter, total nitrogen (including organic nitrogen, ammonia, and nitrate), and phosphorus, whereas adsorption processes predominantly focus on ammonium ions. Accordingly, adsorption trials must be conducted while retaining full-scale biological treatment operations, thereby increasing the operational complexity within wastewater treatment plants [108]. This heightened complexity creates greater staffing demands and introduces operational uncertainties, further discouraging the initiation of large-scale pilot studies.

Given the absence of sufficient pilot- and full-scale investigations, implementing pilot-scale tests to validate the long-term functionality of adsorption processes in actual wastewater is especially critical. Unlike laboratory-scale research, which frequently relies on synthetic wastewater or pretreated specimens, real wastewater includes colloidal matter, suspended solids, and diverse microorganisms, all of which can repeatedly obstruct adsorption columns. Additionally, both flow rate and water quality are subject to substantial fluctuations due to seasonal and daily changes [108]. Pilot-scale testing facilitates the assessment of operational efficiency over extended periods (weeks to months) under authentic wastewater conditions, enabling continuous monitoring of pressure drop variations in fixed-bed adsorption systems. This assessment is fundamental for defining vital process optimization parameters, such as adsorbent replacement schedules, pretreatment approaches, and flow management techniques [180]. Furthermore, for stirred-tank adsorption systems, pilot-scale evaluation is necessary to determine practical cycles for adsorbent dosing, reaction, settling, separation, and subsequent regeneration [181]. The processes of regenerating saturated adsorbents and recovering ammonium also mandate rigorous pilot-scale investigation. While laboratory-scale tests manage high-concentration regenerant solutions (e.g., HCl, H₂SO₄) with relative ease, scaling up requires well-developed rinsing protocols post-regeneration and robust management strategies for spent regenerant solution storage [182].

Additionally, pilot-scale experimentation is essential for assessing the integration of adsorption processes within existing treatment frameworks. Conventional wastewater facilities typically incorporate biological nitrogen removal, sedimentation, filtration, and disinfection units. As a result, pilot testing must be conducted to identify the most appropriate stage for incorporating adsorption, or to determine whether parallel integration with current treatment modules is preferable [183]. Adsorption column operations ordinarily necessitate partial stoppages for adsorbent regeneration or replacement. Therefore, developing protocols to limit downtime during these procedures is a primary objective [108]. Comprehensive demonstration studies are the only means by which the true benefits (e.g., cost reduction, reduced facility footprint, improved nitrogen removal, enhanced nitrogen recovery) and possible operational hurdles can be accurately evaluated.

Finally, operational data obtained from pilot tests provide a robust basis for conducting techno-economic feasibility analyses and life cycle assessments (LCA) [184]. Laboratory-scale studies alone are insufficient to accurately predict chemical consumption rates, energy requirements, or waste generation for full-scale systems. In contrast, extended pilot studies yield direct data on chemical usage, workforce needs, and maintenance intervals, which support precise cost estimations ($/m³) and detailed quantitative assessments of the environmental impacts, such as reductions in carbon footprint and pollutant discharge. These outcomes collectively establish a dependable framework for decision-making by industry stakeholders and policymakers.

In summary, pilot testing is indispensable for the practical deployment and widespread adoption of ammonium ion adsorption technologies. As further operational data and technological advancements emerge through pilot testing, the prospect of constructing a demonstration-scale facility becomes increasingly realistic. Over time, merging adsorption with biological treatment processes is expected to be achievable, which could significantly enhance nitrogen removal performance in wastewater treatment plants.

Based on fundamental adsorption principles, higher concentrations of the target contaminant result in increased adsorption per unit mass of adsorbent [185]. Therefore, effective management requires both technical and policy approaches to selectively divert high-concentration ammonium ion streams from municipal and industrial wastewater systems.

Within municipal wastewater, ammonium ions primarily derive from human excreta, especially urine, which is responsible for approximately 75% of the total nitrogen load in influent to wastewater treatment plants [186]. However, traditional treatment systems typically mix these concentrated nitrogen sources with large volumes of domestic wastewater, resulting in significant dilution of ammonium and limiting the efficiency of recovery by adsorption [187]. Industrial wastewater from sectors such as leather processing, semiconductor manufacturing, and food production can also exhibit elevated ammonium concentrations [188, 189]. When these industrial flows are combined with other wastewater streams, dilution further impairs the effectiveness of ammonium recovery.

In light of these constraints, it is critical to implement targeted waste stream separation (i.e., source separation) strategies in both municipal and industrial wastewater management. Preventing the dilution of high-ammonium waste streams prior to mixing can significantly enhance ammonia recovery efficiency. The development of dedicated pre-separation piping systems for distinct wastewater sources will be fundamental to achieving optimal recovery. Accordingly, advancing both policy measures for source separation and innovations in separation technology should be prioritized.

Moreover, source separation streamlines the overall wastewater treatment process [190]. Conventional wastewater treatment plants utilize biological nutrient removal (BNR) processes, employing anaerobic, anoxic, and aerobic reactors in a complex arrangement [108]. However, processing pre-separated ammonia sources through an adsorption process can substantially reduce the biological nitrogen treatment load in wastewater treatment plants [190]. This improvement can lead to enhanced operational stability, alongside reductions in energy usage and operational expenditures.

Despite the various prospects for ammonia source separation, only a limited number of pilot projects and case studies have been documented, necessitating focused policy research and support for practical implementation. For instance, certain European research organizations have recently initiated assessments of ammonium recovery efficiency by implementing urine-separating toilets. In Finland, pilot programs are ongoing to collect urine separately and extract ammonium ions for fertilizer production, while Switzerland has conducted empirical trials utilizing biological and distillation methods for nitrogen recovery from source-separated urine [191, 192]. Similarly, Taiwan Semiconductor Manufacturing Company (TSMC) has adopted a process to separate high-concentration ammonia wastewater and transform it into solid ammonium sulfate for resource reutilization [193]. Nevertheless, large-scale adoption remains extremely limited, and comprehensive policy frameworks to support broad application are still lacking. Accordingly, expanded policy research is vital to develop regulatory standards, economic incentives, and infrastructure planning measures to enable ammonia source separation and recovery at a wider scale.

To promote the broad acceptance of source separation methods, robust policy frameworks are indispensable. An important approach involves requiring the installation of urine-separating toilets in urban and industrial developments to ensure that high-concentration ammonium streams are not diluted. While the separation of high-concentration ammonia streams at the source presents operational obstacles—such as the incorporation of new decentralized piping networks, prevention of pipe scaling, and mitigation of precipitation-induced valve blockages—emerging toilet and piping technologies are being advanced to resolve these technical challenges, indicating that these barriers may be gradually mitigated [194]. In addition, financial support should be introduced to foster implementation, for example through subsidies provided to businesses and households that adopt source separation techniques [194]. Finally, reinforcing legislative requirements to mandate the segregation of high-concentration ammonia streams from other wastewater at the initial stages of urban planning could support the construction of more robust ammonia source separation and recovery systems in new buildings and public infrastructure [195].

Beyond the separation process, developing a viable market for recovered ammonia is crucial to encourage industry engagement. Governments must establish regulatory frameworks governing the sale and distribution of recovered ammonia, with provisions to ensure quality standards and safety compliance. Awarding carbon credits to companies producing recovered ammonia is a promising approach, as this reduces dependence on the energy-intensive Haber–Bosch process. This incentive may be incorporated into existing emissions trading schemes by recognizing ammonia recovery from wastewater as a qualified offset activity. Likewise, companies purchasing recovered ammonia for applications in fertilizers, chemical production, or energy generation could be eligible for tax incentives, thereby further supporting the adoption of sustainable nitrogen sources. Eco-friendly certifications for products made with recovered ammonia can also be integrated into established environmental labeling and public procurement programs, such as the Korea Eco Label program and the European Union’s Green Public Procurement (GPP) framework. Such alignment allows governments and public institutions to prioritize these products in public procurement processes, supporting broader market uptake. Once the economic and environmental benefits of source separation and ammonia recovery are substantiated through quantitative analyses and incorporated into regulatory frameworks, the advancement towards a circular nitrogen economy in municipal and industrial wastewater management systems can be achieved.