Film and Diffusion Kinetics of Ammonium Removal from Leachate in an Up-Flow Anaerobic Sludge Blanket Reactor

Abstract

Understanding NH₄ ⁺ removal from leachate mediated by anaerobic sludge (AS) development could lead to the creation of effective, designed biodegradation technologies in the future. This study utilized an up-flow anaerobic sludge blanket (UASB) reactor equipped with a gas–liquid–solid separator to investigate NH₄ ⁺ removal from landfill leachate, enhancing understanding of the biosorption kinetics involved. AS development was initially unstable but progressively improved, exhibiting greater stability with increasing NH₄ ⁺ concentration over the experimental duration. The UASB process performance increased from 15.32 to 63.38% efficiency with an increase in NH₄ ⁺ concentration from 141.71 to 360.59 mg/L. The trend in increased NH₄ ⁺ removal closely mirrored CH₄ production but exceeded COD removal over the experimental duration. The internal mass transfer (IMT) rate variation closely tracked the global mass transfer rate but remained significantly higher than the external mass transfer (EMT) rate, indicating EMT-controlled mass transfer resistance throughout the 197-day experiment. Analysis of NH₄ ⁺ biosorption kinetics in leachate provides new insights into UASB process performance, highlighting the importance of IMT rates, which could inform advanced landfill leachate treatment plant design in the future. Understanding the COD/N/P ratio is crucial for optimizing UASB process efficiency, warranting further research on comparative biosorption kinetics of COD, NH₄ ⁺, and PO₄ ^3– removal from leachate to gain comprehensive insights into landfill leachate treatment.

1. Introduction

Ammonium (NH₄ ⁺), a chemical component in leachate, contributes to increased toxicity and reduced aquatic environment quality. The presence of NH₄ ⁺ in leachate from various anthropogenic sources affects leachate quality and impacts the surrounding environment. Leachate treatment can release biogases (e.g., CH₄ and CO₂) into the atmosphere while removing chemical oxygen demand (COD) and NH₄ ⁺, thereby improving environmental quality. , Leachate treatment involves the controlled biodegradation of organic matter (OM) by anaerobic microorganisms, converting it into products like CH₄, H₂, and CO₂, while recovering valuable resources such as biogas (primarily CH₄). , The use of an up-flow anaerobic sludge blanket (UASB) reactor to treat leachate can effectively remove COD and NH₄ ⁺ and convert OM into gaseous products, including CH₄, CO₂, and H₂, through biological processes. , CO₂ is a dominant component of greenhouse gases (GHGs), and anaerobic bioconversion in UASB can convert CO₂ into CH₄, potentially increasing its content into the atmosphere. However, the presence of O₂ in an UASB reactor can intrinsically inhibit key enzymes involved in leachate degradation, potentially causing process instability. Hydrogenotrophic methanogens utilize H₂ as an electron source, requiring reduced ferredoxin as a crucial electron donor for methanogenesis, producing CH₄. H₂ is rapidly consumed in methanogenic environments, even at low concentrations. The self-immobilization of methanogens forms a sludge blanket, allowing leachate to flow upward through the UASB reactor with a low-velocity up-flow pattern, facilitated by a peristaltic pump. A high concentration of anaerobic microorganisms in a UASB reactor can achieve a higher conversion rate of NH₄ ⁺ removal than conventional anaerobic processes, while tolerating fluctuations in organic loading rate (OLR), pH, and temperature. , This study builds on previous research showing that UASB reactors can effectively remove COD and NH₄ ⁺ from leachate, with a focus on exploring the correlation between COD and NH₄ ⁺ removal efficiencies over time. Understanding the mechanisms and mass transfer kinetics of NH₄ ⁺ removal from leachate is crucial for determining mass transfer resistance (MTR), which can be predicted using the Generalized Fulazzaky (GF) equations by simulating film and diffusion mass transfers.

Understanding the mechanisms and mass transfer kinetics of NH₄ ⁺ removal, involving transport from bulk water to surface precipitation and subsequent diffusion, is crucial for determining MTR. The interdependence of physical, chemical, and biological processes influences NH₄ ⁺ release from organic compounds in leachate and anaerobic sludge (AS) development (Torretta et al.). NH₄ ⁺ transport from bulk water to extracellular precipitation on AS surfaces, followed by diffusion catalyzed by extracellular enzymes from anaerobic bacteria, can be predicted using kinetics-based simulation approaches. The global mass transfer (GMT) of NH₄ ⁺, encompassing film mass transfer and diffusion catalyzed by AS development, is influenced by various factors, including temperature, pH, dissolved oxygen (DO), COD, hydraulic retention time (HRT), initial sludge content, and UASB reactor volume. The increased activity of catalysts in a UASB reactor’s colloidal state, characterized by a large surface area, can be attributed to the enhanced reaction rate resulting from the increased contact area between AS and reactants. Increased O₂ availability in sludge’s interior channels can inhibit OM catalysis, impact NH₄ ⁺ release and CH₄ production, and destabilize AS structure, thereby altering film mass transfer and diffusion rates. , The complete biodegradation of OM via the UASB process depends on the biological characteristics of sludge formed under anaerobic conditions. , Adsorption and biosorption kinetics of NH₄ ⁺ removal have been studied in various contexts, including biosorption from palm oil mill effluent (POME) using a sequencing batch reactor, and adsorption from surface water using a downflow glassy plug-flow column reactor, as well as from wastewater using an up-flow hybrid hydrodynamic plug-flow column reactor, contributing to the understanding of NH₄ ⁺ removal kinetics. However, the biosorption kinetics of NH₄ ⁺ removal from leachate, catalyzed by diverse anaerobic bacterial communities and subsequently accumulated into biomass to form AS, remains poorly understood. The breakdown of complex organic molecules catalyzed by AS development, which absorbs NH₄ ⁺ from leachate, necessitates evaluating the mass transfer kinetics of biosorption, controlled by either film mass transfer or cellular diffusion. Therefore, evaluating the biotransformation of OM in leachate mediated by AS growth, which facilitates NH₄ ⁺ transport from bulk water to surface precipitation and subsequent diffusion into biomass, is crucial for understanding NH₄ ⁺-reducing bacteria engineering. Anaerobic biodegradation of organic molecules in landfill leachate can mitigate pollution by breaking down organic pollutants and releasing NH₄ ⁺. However, further research is needed to develop effective engineering designs for NH₄ ⁺ removal, ultimately enhancing environmental quality.

This study’s objectives were: (1) to simulate NH₄ ⁺ mass transfer kinetics data from UASB-treated leachate experiments using GF equations, (2) to evaluate NH₄ ⁺ biosorption kinetics and mass transfer mechanisms, comparing external and internal mass transfer rates to determine NH₄ ⁺ removal resistance, and (3) to predict UASB performance for leachate treatment catalyzed by AS development, based on external and internal mass transfer factors.

2. Materials and Methods

2.1. Leachate Sampling and Sludge Cultivation

Leachate samples were collected from the first pond of a sanitary landfill in Tanjung Langsat, Pasir Gudang, Johor Bahru, Malaysia (see Figure a), every 2 weeks for 8 months. Samples were scooped from the liquid surface using a pail and dipper, capturing anaerobic conditions conducive to CH₄ and CO₂ production from complex OM breakdown. The temperature range at the leachate collection pond was 24–31 °C. Samples were stored in 20 L containers, tightly sealed and refrigerated at 4 °C. AS was cultivated by mixing POME sludge from Felda Palm Industries Sdn Bhd (Kulai, Johor) with domestic sewage sludge (DSS) from Indah Water Konsortium (Pantai Dalam, Kuala Lumpur). Figure b presents a schematic diagram of UASB operation, illustrating how experimental data collected over 197 days were utilized in simulations with GF equations to predict mass transfer mechanisms and determine MTR.

1.

(a) Location for landfill leachate sampling and (b) schematic diagram of UASB reactor.

2.2. UASB Reactor Operation and Experimental Procedures

The UASB reactor, covered with double-jacketed plexiglass, was used for landfill leachate treatment, facilitating continuous AS development. The reactor measured 230 mm in height and 84 mm in internal diameter, with a working volume of 1.25 L (see Figure b). The double-jacketed plexiglass was maintained at 38 °C to create mesophilic conditions, promoting optimal growth of AS. The UASB process was initiated by seeding anaerobic POME sludge mixed with DSS and feeding it at an OLR of 1.0 g COD/L·d and an HRT of 24 h to facilitate acclimation of the microbial community. After 4 weeks of acclimation, with COD removal efficiency reaching around 47% and NH₄ ⁺ removal efficiency at 23.32%, the UASB process was stabilized by feeding OM with a COD concentration of approximately 1200 mg/L (OLR: 1.2 g COD/m³·d). Daily biogas production was 0.001 L, captured by the gas–liquid–solid (GLS) separator (see Figure b). After reaching steady state within 6 days, the UASB reactor was operated at an OLR of 1.0 kg COD/m³·d and an HRT of 24 h. The OLR was then gradually increased to 3.2 kg COD/m³·d, which was maintained for an extended period. pH was monitored throughout the 197-day experiment at the inlet, inside, and outlet of the UASB reactor using an Orion Star A211 Benchtop pH Meter (Cole-Parmer, Illinois, USA). The GLS separator, featuring an inverted V-shaped baffle, was designed to capture CH₄ while preventing solids washout. This design promoted separation of CH₄ from solids, allowing AS to return to the sludge blanket zone and enhancing suspended solids removal from the effluent. The UASB reactor system was automated using a programmable logic controller (PLC) connected to all electrical sensors, ensuring accurate measurements throughout the experiment.

2.3. Analytical Methods

NH₄ ⁺ concentrations were measured using the Nessler method, and COD was determined spectrophotometrically after digestion in a COD reactor (HACH DRB200 COD Digester, Loveland, USA), following standard methods. The concentrations of NH₄ ⁺ and COD were determined using a HACH DR 6000 UV–vis spectrophotometer. Suspended solids (SS) and volatile suspended solids (VSS) were quantified gravimetrically: SS by drying residue at 103–105 °C and VSS by measuring weight loss after ignition at 550 °C. pH was monitored throughout the 197-day experiment using a portable pH meter (Jenway Model 350, Cole-Parmer, Staffordshire, UK). Daily CH₄ production was measured using a peristaltic pump and magnetic proximity sensor, enabling real-time monitoring of CH₄ accumulation in the GLS separator, with data directly recorded in the PLC system.

2.4. Simulation and Statistical Analysis

The NH₄ ⁺ removal efficiency in the UASB process can be calculated using the formula

$$E=\frac{C_0-C_{\mathrm{s}}}{C_0}\times 100\%$$ 1

where E is the NH₄ ⁺ removal efficiency of the UASB process (%), C ₀ is the inlet NH₄ ⁺ concentration of the UASB reactor (mg/L), and C _s is the outlet NH₄ ⁺ concentration of the UASB reactor (mg/L).

The accumulated concentration of NH₄ ⁺ removed from the landfill leachate was calculated using the method described by Fulazzaky

$$q_{\mathrm{a}}={\int }_0^V\frac{(C_0-C_{\mathrm{s}})\mathrm{d}V}{m}$$ 2

where, q _a is the accumulated concentration of NH₄ ⁺ removed from the leachate (mg/g), V is the volume of leachate flowed into the UASB reactor (L), and m is the initial COD content in 1 L of AS in the UASB reactor (g).

Mathematical modeling enables the representation of green chemistry principles through symbolic statements. The modified Gompertz equation can estimate CH₄ production from experimental data. Furthermore, the GF equations, formerly known as the modified mass transfer factor, describe the mechanisms and mass transfer kinetics of CH₄ desorption. The GF equations, developed by Fulazzaky et al. based on experimental data from Khamidun’s PhD project, aimed to modify the original Fulazzaky equations (formerly known as mass transfer factor models); for broader applicability in analyzing mass transfer kinetics. The GF equations have been validated for various applications, including biosorption kinetics of oil and grease mediated by Serratia marcescens SA30, OM removal from POME using aerobic granules, phosphorus removal kinetics via aerobic granule development, and decolorization kinetics of the mixed azo dyes using magnetic-activated carbon aerobic granular sludge. Understanding the mass transfer kinetics of NH₄ ⁺ removal from leachate, catalyzed by AS development, enables prediction of UASB process effectiveness. The mechanisms and kinetics of NH₄ ⁺ removal were thus investigated using the GF equations

$$\ln \left(\frac{C_0}{C_{\mathrm{s}}}\right)={[k_{\mathrm{L}}a]}_{\mathrm{g}}\times {\mathrm{e}}^{-\beta \times \ln (q_{\mathrm{a}})}\times t$$ 3

The mathematical expression of eq can be linearized as follows

$$\ln (q_a)=\frac{1}{\beta }\times \ln (t)+B$$ 4

with

$$B=\frac{\ln ({[k_{\mathrm{L}}a]}_{\mathrm{g}})-\ln \left\{\ln \left(\frac{C_0}{C_{\mathrm{s}}}\right)\right\}}{\beta }$$ 5

The relationship between external mass transfer (EMT) and GMT factors can be described as follows

$${[k_{\mathrm{L}}a]}_{\mathrm{f}}={[k_{\mathrm{L}}a]}_{\mathrm{g}}\times {\mathrm{e}}^{-\beta \times \ln (q_{\mathrm{a}})}$$ 6

where [k _L a]_g is the global mass transfer factor (1/d), β is the molecular affinity parameter (d/g·mg), t is the time of running the UASB experiment (d), B is the potential mass transfer index related to the driving force of NH₄ ⁺ transferred during AS development in the UASB reactor (mg/g), and [k _L a]_f is the external mass transfer factor (1/d), as used in the calculation of NH₄ ⁺ removal kinetics.

The variation of [k _L a]_g with respect to the C ₀/C _s ratio was calculated using eq , while the variation of [k _L a]_f was determined based on the q _a value calculated using eq , with parameters β and B verified through linear regression analysis of ln­(q _a) versus ln­(t) according to eq . The internal mass transfer (IMT) factor, defined as the difference between GMT and EMT factors, is given by

$${[k_{\mathrm{L}}a]}_{\mathrm{d}}={[k_{\mathrm{L}}a]}_{\mathrm{g}}-{[k_{\mathrm{L}}a]}_{\mathrm{f}}$$ 7

where [k _L a]_d is the internal mass transfer factor (in 1/d).

Analyzing the trends of [k _L a]_f, [k _L a]_d, and [k _L a]_g variations, representing EMT, IMT, and GMT rates over time t, helps predict the mechanisms and kinetics of NH₄ ⁺ biosorption, providing insight into how EMT and IMT influence NH₄ ⁺ removal. Comparing [k _L a]_f with [k _L a]_d during the experiment can determine the MTR of NH₄ ⁺ from bulk water to AS’s acceptor sites. This insight can inform future UASB reactor designs, enhancing accuracy and efficiency in industrial applications for NH₄ ⁺ removal from leachate and CH₄ production as a sustainable energy source.

3. Results and Discussion

3.1. Leachate Quality and Composition

The landfill leachate samples had average concentrations of COD (2300 ± 105 mg/L), NH₄ ⁺ (400 ± 21.4 mg/L), suspended solids (180 ± 14.8 mg/L), and volatile suspended solids (87 ± 8.7 mg/L), with a pH range of 6.9–8.0. The BOD/COD ratios of the raw leachate at 8, 47, and 103 days were 0.4, 0.25, and 0.11, respectively, indicating a decrease over time. These observations indicate a decrease in the biodegradation rate of OM in the leachate over time. The initial alkalinity concentrations of the landfill leachate ranged from 400 to 3450 mg/L as CaCO₃. The pH values varied as follows: inlet (7.47–8.61, average 8.07), inside the UASB reactor (6.29–7.96, average 7.30), and outlet (5.01–6.69, average 5.86). The observations support the breakdown of complex molecules into simpler ones, increasing H⁺ concentration, consistent with the higher H⁺ ion levels observed in the effluent compared to the UASB reactor and influent.

3.2. Anaerobic Sludge Development

The average granule counts and granule characteristics were analyzed using Plaxis image analysis software (see Figure ). Analysis of scanned images revealed a steady increase in granule size from week 1 to week 31, with notable growth over this period. The granulation development process was monitored according to the aerobic granulation potential protocol, , following standard analytical procedures to control nutrient levels within the UASB reactor. The progression of sludge development is shown in Figure a–f for weeks 1, 9, 12, 15, 24, and 31, respectively. At week 1 (see Figure a), the sludge was unstabilized, as evidenced by methane production in blank batches (no substrate). However, by week 24 (see Figure e), all sludge samples showed increased stability, with progressive improvement from week 1. The sludge bed growth rate increased after COD was increased as a carbon source to the UASB reactor, suggesting that the feeding scheme and substrate composition can impact sludge bed development. Due to technical issues with the feeder, including short-circuits and electrical breakdowns, continuous leachate flow was disrupted. Consequently, it took 5 weeks to achieve significant granule enlargement and stabilize the water bath temperature within the reactor. To prevent fermentation during extended electrical breakdowns, the sludge bed should be flushed out of the UASB reactor and stored in a cool room at a temperature around 4 °C. The components of the feedstock are selectively utilized by diverse bacteria in the sludge bed, particularly in response to varying COD-to-N ratios. For high-strength organic wastewater, the nutrient requirements for biomass growth often deviate from the conventional COD:N:P ratio of 100:5:1. The development of AS via the UASB process requires understanding the optimal COD/N/P ratio, which warrants future assessment of PO₄ ^3– removal over the experimental duration.

2.

Granulation process visualized with PLAXIS: (a) week 1, (b) week 9, (c) week 12, (d) week 15, (e) week 24, and (c) week 31.

3.3. Performance of Up-Flow Anaerobic Sludge Blanket Reactor

The NH₄ ⁺ concentration at the UASB reactor inlet fluctuated between 139.22 and 360.59 mg/L, with an average of 236.22 mg/L. Over the 197-day experiment, the concentration showed an increasing trend, reaching 360.59 mg/L by the end (see Figure ; black round dots and line). The NH₄ ⁺ concentration at the UASB reactor outlet remained relatively stable, fluctuating slightly between 108.84 and 136.40 mg/L, with an average concentration of 123.61 mg/L over the 197-day experiment (see Figure ; red triangle dots and line). The UASB reactor’s NH₄ ⁺ removal performance was evaluated by plotting removal efficiency (E) versus time (t), as shown in Figure with blue square dots and line. The UASB reactor’s NH₄ ⁺ removal efficiency from leachate, calculated using eq and plotted in Figure with blue square dots and line, ranged from 15.32 to 63.38%. Notably, the removal efficiency showed an increasing trend over time, reaching 63.38% by the end of the experiment. The AS development increased substantially from 15.32 to 63.38%. This significant growth likely enhanced UASB performance, potentially due to electrostatic effects and molecular interactions within the biomass. , Although microbial characterization was not conducted in this study, previous research has shown that self-immobilized microbial aggregates can attach to the inner walls of UASB reactors, forming AS comprising diverse microorganisms with specific methanogenic activity on OM in leachate. The diverse microbial assemblage forming a bacterial community has the potential to degrade various complex molecules present in leachate. The self-assembled microbial community in AS attached to the UASB reactor’s inner wall likely supported bacterial communities, enabling the degradation of complex molecules and removal of soluble matter, including NH₄ ⁺, from leachate. This mechanism contributed to the UASB reactor’s overall efficiency of approximately 63% after 197 days. The UASB reactor achieved 63.38% NH₄ ⁺ removal efficiency and 10 L CH₄/L d methane production when treating sugar cane vinasse at an OLR of 33 kg COD/m³ d. The removal of NH₄ ⁺ ions in the UASB reactor, facilitated by anaerobic bacteria developing biomass, likely occurred through biodegradation and biotransformation of complex molecules into simpler ones, with concurrent CH₄ production, a potent greenhouse gas. The breakdown of complex molecules into simpler ones likely drives methanogenesis, mediated by AS development utilizing NH₄ ⁺ as a nutrient source. To optimize NH₄ ⁺ removal efficiency in the UASB process, the kinetics of EMT and IMT under anaerobic conditions can be evaluated using the GF equations.

3.

Variations of NH₄ ⁺ influent, NH₄ ⁺ effluent, and removal efficiency over time.

3.4. Correlations between COD and NH₄ ⁺ Removal with Time and CH₄ Production

Figure a (black round dots) illustrates that COD removal via the UASB process increased linearly with experimental duration from 0 to 197 days. A strong linear correlation (COD = 1.2519 × t – 2.5255, R ² = 0.9996) effectively demonstrates the role of AS in enhancing COD removal over time. Strictly anaerobic conditions promote the transformation of various organic and inorganic contaminants, such as NH₄ ⁺, although the range of observed reactions is limited, with anoxic ammonia removal and chemical bond cleavage being the most prevalent. NH₄ ⁺ removal increased with experimental duration (see Figure a; red triangle dots), exhibiting a strong power correlation with CH₄ production as shown in Figure a with blue square dots (CH₄ = 0.108 × t ^1.916, R ² = 1), indicating a higher removal rate compared to COD. NH₄ ⁺ removal may be most effective in reducing in-digester NH₄ ⁺ concentrations across various organic loading rates. Meanwhile, the UASB process shows promise due to its flexibility in controlling EMT and IMT processes separately. CH₄ production is linked to OM degradation in leachate, increasing powerfully with COD removal (see Figure b). This relationship is described by the power correlation CH₄ = 0.0771 × COD^1.9072 (R ² = 0.9994). The power correlation between CH₄ production and COD removal from leachate describes methanogenesis of OM in the UASB process, informing EMT and IMT kinetics of CH₄ release. CH₄ production under anaerobic conditions closely parallels NH₄ ⁺ removal over time, suggesting a strong linear correlation (CH₄ = 0.4352 × NH₄ ⁺ - 23.216, R ² = 0.9988; see Figure c). This relationship indicates that NH₄ ⁺ utilization supports balanced AS development and CH₄ production. Adjusting the NH₃/NH₄ ⁺ ratio can stabilize processes and prevent CH₄ production inhibition when treating nitrogen-rich substrates during AS development. Table reveals linear and power correlations between COD removal and CH₄ production with time (see Figure a; black round and blue square dots), and power and linear correlations between COD and NH₄ ⁺ removal with CH₄ production (see Figure b,c). A previous study validated a power correlation between COD removal and CH₄ production, as confirmed in Figure b. Figure c shows a linear correlation between NH₄ ⁺ removal and CH₄ production. Future analysis of the correlation between PO₄ ^3– removal and CH₄ production is necessary to provide a comprehensive understanding of CH₄ production while simultaneously reducing COD, NH₄ ⁺, and PO₄ ^3–. This study suggests that future research should evaluate the correlation between COD and PO₄ ^3– removal to gain insight into the optimal COD/N/P ratio for optimizing biomass growth under anaerobic conditions.

4.

Correlations: (a) COD and NH₄ ⁺ removal and CH₄ production over time, (b) CH₄ production versus COD removal, and (c) CH₄ production versus NH₄ ⁺ removal (197-day experiment).

1. Correlations between COD Removal and CH₄ Production over Time and CH₄ Production versus COD and NH₄ ⁺ Removal (197-Day Experiment).

correlation	curve	equation	R 2
COD versus t	linear	$$\mathrm{COD}=1.2519\times t-2.5255$$	0.9996
CH4 versus t	power	$${\mathrm{CH}}_4=0.108\times t^{1.916}$$	1.0000
CH4 versus COD	power	$${\mathrm{CH}}_4=0.0771\times {\mathrm{COD}}^{1.9072}$$	0.9994
CH4 versus NH4 +	linear	$${\mathrm{CH}}_4=0.4352\times {{\mathrm{NH}}_4}^{+}-23.216$$	0.9988

3.5. Linear Regression Analysis of ln­(q) versus ln­(t)

The linear plot of ln­(q) versus ln­(t) yielded a slope of 1/β (β = 0.63255 g/d mg) and y-intercept B (0.2424 mg/g) (see Figure ), with a strong fit to experimental data (R ² = 0.9840). These parameters (β and B) were used to simulate NH₄ ⁺ removal mechanisms and mass transfer kinetics in the UASB process. The biosorption rate of NH₄ ⁺ removal in the UASB reactor, catalyzed by AS development, depends on three successive steps: (1) transport of NH₄ ⁺ from bulk water to the surface, (2) cell surface sorption passing through the film zone into the sludge, and (3) NH₄ ⁺ diffusion within the AS. , The structure and formation of AS influence NH₄ ⁺ removal mechanisms and mass transfer kinetics, with NH₄ ⁺ biosorption kinetics controlling UASB process performance. The β value (0.63255 g/d mg) reflects the interaction strength between NH₄ ⁺ ions and the AS surface during NH₄ ⁺ biosorption, influencing the transport and precipitation of NH₄ ⁺ ions. The B value (0.2424 mg/g) relates to the enzymatic role of methylthio-alkane reductase in AS development, influencing the driving force for NH₄ ⁺ transport from the sludge surface into the biomass. , Increased NH₄ ⁺ concentration outside the AS can decrease the driving force for NH₄ ⁺ transport, thereby reducing the EMT rate, which requires verification. AS development is a promising strategy for NH₄ ⁺ removal from leachate, but determining whether film mass transfer or diffusion within the biomass controls the MTR is crucial.

5.

Linear plot of ln­(q) versus ln­(t) for determining β and B.

Others have observed that NH₄ ⁺ biosorption mechanisms catalyzed by AS development involve surface functional groups and intramolecular/intermolecular forces related to ion exchange and chemical bonding. , The presence of other anions and cations may affect NH₄ ⁺ biosorption from leachate, potentially prolonging the lag phase of NH₄ ⁺ removal. The structural features and components of AS, characterized by distinct geometries and chemical functions, facilitate various mechanisms for NH₄ ⁺ biosorption. , Self-immobilization of AS on the UASB reactor wall can form a cell-structured shape, characterized by dense biomass. The AS’s cell walls, composed of lipids, polysaccharides, and proteins, provide abundant functional groups (amino acids, carboxyl, hydroxyl, and phosphate) and hydrophobic adsorption sites (aliphatic carbon chains and aromatic rings). Cell surface hydrophobicity and polysaccharides influence AS development during leachate NH₄ ⁺ biosorption. , Simulating NH₄ ⁺ transport using GF equations can help determine MTR by distinguishing between external (bulk water to sludge surface) and internal (surface to biomass cavity) mass transfer rates.

3.6. External, Internal, and Global Mass Transfer Kinetics of NH₄ ⁺ Biosorption

Analyzing EMT, IMT, and GMT mechanisms and kinetics simulated using GF equations helps determine whether MTR is controlled by EMT or IMT during NH₄ ⁺ transport from bulk water to acceptor sites in developed AS. NH₄ ⁺ biosorption kinetics for EMT, IMT, and GMT in landfill leachate treatment using a UASB reactor can be monitored through variations in [k _L a]_f, [k _L a]_d, and [k _L a]_g values, respectively. Variations in [k _L a]_f, [k _L a]_d, and [k _L a]_g values over the 197-day UASB experiment are shown in Figure a,b. NH₄ ⁺ biosorption from leachate, involving transport from bulk leachate to surface precipitation and accumulation in AS (see Figure c), can be kinetically analyzed to understand liquid–solid mass transfer behaviors and mechanisms. , The structural features of diverse anaerobic microbial communities in the UASB reactor, without sufficient DO content, create anaerobic conditions in both inner and outer sludge layers. , At the start of UASB operation, extracellularly precipitated NH₄ ⁺ ions on AS surface can easily diffuse through sludge cavities due to a high driving force, resulting from the significant concentration difference between the outer layer and biomass cavity. , Anaerobic biodegradation of leachate via methanogenesis, catalyzed by a complex microbial community, generates gaseous products (CH₄, H₂, and CO₂) and facilitates NH₄ ⁺ removal. The [k _L a]_g value fluctuated from 0.19 to 1.17 1/d (see Figure a; blue square dots), and [k _L a]_d ranged from 0.17 to 1.17 1/d (see Figure a; red triangle dots), with both being close and higher than [k _L a]_f (see Figure a; black round dots), which decreased from 0.028 to 0.005 1/d over 197 days (see Figure b).

6.

Numerical analysis of NH₄ ⁺ biosorption kinetics and mechanisms for EMT, IMT, and GMT based on variations in [k _L a]_f, [k _L a]_d, and [k _L a]_g over time: (a) plot with uniform y-axis scaling, (b) plot with detailed y-axis scaling, and (c) graphic display of external and internal mass transfers across a cross-section from bulk leachate to accumulate into acceptor sites within bacterial cells.

Leachate OM biodegradation occurs in three fermentation steps: (1) hydrolysis and fermentation, (2) acetogenesis, and (3) methanogenesis, , potentially causing [k _L a]_d fluctuations during NH₄ ⁺ removal. The similar variation in [k _L a]_d and [k _L a]_g values (see Figure a; red triangle and blue square dots) indicates that NH₄ ⁺ biosorption kinetics depends on NH₄ ⁺ availability in the leachate and microbial breakdown of organic molecules into H₂ and CO₂. The driving force for NH₄ ⁺ transport from bulk water to sludge surface decreases with experiment duration, consistent with the observed decrease in [k _L a]_f value over 197 days. NH₄ ⁺ diffusion from the sludge surface to acceptor sites within the biomass cavity is linked to AS development. The [k _L a]_f value decreased sharply over the first 44 days (see Figure a; black round dots), while the [k _L a]_d value increased more significantly (see Figure a; red triangle dots), likely due to anabolic reactions during sludge formation requiring energy to synthesize complex molecules, which were probably slower initially. Comparing the EMT rate (based on [k _L a]_f) with the IMT rate (based on [k _L a]_d) allowed determination of the MTR for the removal of NH₄ ⁺ from leachate using the UASB process. The [k _L a]_f value variation was significantly lower than [k _L a]_d, indicating that EMT controlled NH₄ ⁺ biosorption resistance over 197 days. The [k _L a]_d value increased consistently for 92 days, followed by a zigzag pattern with periodic peaks, indicating variability in NH₄ ⁺ biosorption mass transfer kinetics driven by intramolecular diffusion and biochemical reactions. The zigzag pattern in [k _L a]_d value with an increasing trend (see Figure a; red triangle dots) corresponds to the gradual increase in OLR from 1.0 to 3.2 kg COD/L·d at an HRT of 24 h. This resulted in enhanced NH₄ ⁺ biosorption, mediated by a mixed methanogenic community, increasing NH₄ ⁺ removal from leachate in an UASB reactor. The rate-limiting step of NH₄ ⁺ biosorption over 197 days was controlled by EMT, attributed to the readily available carbon source and nutrients promoting biomass growth. Meanwhile, the driving force for NH₄ ⁺ transport from bulk water to the sludge surface decreased continuously, as evidenced by the decrease in [k _L a]_f (see Figure b).

The primary objective of this study was to expand the application of GF equations, scientifically describing the biosorption kinetics of NH₄ ⁺ from leachate mediated by AS development. To our knowledge, comparison with other models was not possible because the GF equations are specifically designed to predict the kinetics and EMT and IMT mechanisms, including biosorption kinetics of NH₄ ⁺ removal from leachate, thereby allowing determination of MTR. The applicability of the GF equations provided improved insight into the implications of NH₄ ⁺ biosorption from leachate on the dynamic behaviors of EMT and IMT, enhancing understanding of response mechanisms and mass transfer kinetics of NH₄ ⁺ from bulk water to surface precipitation and subsequent AS accumulation (see Figure c). The development of AS in the UASB reactor accelerated NH₄ ⁺ removal by stimulating biomass growth, which in turn increased the IMT rate of NH₄ ⁺ biosorption. Empirical evidence (Figure a,b) shows that the [k _L a]_d value rapidly increased over the first 110 days (see Figure a; red triangle dots), counterbalanced by a gradual decrease in the [k _L a]_f value (see Figure b) over 197 days. Subsequently, the [k _L a]_d value exhibited a zigzag pattern (0.91–1.17 1/d), while the [k _L a]_f value decreased slightly from 0.007 1/d to almost constant value of 0.005 1/d after 160 days. The increased removal of NH₄ ⁺ may be attributed to the growth of AS biomass blanketing the UASB reactor’s interior walls, potentially resulting from an enhanced IMT rate. , AS development can enhance NH₄ ⁺ removal by driving the diffusion of NH₄ ⁺ ions from surface precipitation to acceptor sites, facilitating their accumulation within bacterial cells. The development of anaerobic bacteria has been shown to promote stable AS formation with good biomass retention throughout the UASB reactor blanket. The growth potential of biomass increased over time with the increased availability of smaller molecules (CO₂ and H₂), likely due to CO₂ reduction using H₂ as an electron donor to form acetate via homoacetogenesis, thereby facilitating acetoclastic methanogenesis as the dominant pathway for CH₄ production, which relies on the biosorption kinetics of NH₄ ⁺ as a nutrient to develop AS. − The kinetic rate of IMT closely approximated the GMT rate for NH₄ ⁺ removal from leachate (see Figure a; red triangle and blue square dots), indicating that NH₄ ⁺ biosorption was primarily dependent on the IMT rate, warranting further correlation analysis with UASB reactor efficiency.

3.7. Correlation between E and [k _L a]_d

Numerical simulation of E versus [k _L a]_d (Figure ) enabled prediction of UASB process performance based on the IMT rate. The plot of E versus [k _L a]_d exhibited a near-linear power relationship, indicating increased NH₄ ⁺ removal efficiency over 197 days, which can be described by the equation: E = 58.155 × ([k _L a]_d)^0.7519 (R ² = 0.9983). The UASB process’s increasing efficiency in removing NH₄ ⁺ from leachate over time can be attributed to the dominance of IMT kinetics in NH₄ ⁺ removal, which is influenced by the driving force transporting NH₄ ⁺ ions from bulk water to AS surface precipitation. , Future research could reasonably explore the anammox process for NH₄ ⁺ removal from leachate, focusing on microbial community dynamics under stressful operating conditions. , NH₄ ⁺ removal efficiency in landfill leachate treatment via AS development in the UASB reactor increased with rising [k _L a]_d, suggesting that chemical reactions converting NH₄ ⁺ ions to biomass play a crucial role in accelerating the IMT rate. NH₄ ⁺ removal, dependent on the IMT rate, can be predicted over the 197-day experiment, with optimal UASB process performance achievable through accelerated IMT rates. Therefore, optimizing PO₄ ^3– concentration to support AS development in the UASB process could enhance NH₄ ⁺ removal efficiency. The application of suitable microbial communities for NH₄ ⁺ removal from leachate highlights anaerobic digestion’s potential as an attractive bioprocess for leachate treatment in low-temperature regions, , offering a pathway to economic competitiveness through CH₄ production as a valuable energy source. The significance of anaerobic bacteria in NH₄ ⁺ biosorption from leachate likely stems from their ability to enhance the IMT rate, facilitating NH₄ ⁺ ions transport into bacterial cells, which in turn promotes new biomass formation. This study provides a valuable reference for technological advancements in NH₄ ⁺ removal from landfill leachate, informing decision-makers without preconceived preferences.

7.

Analysis of the performance of UASB reactor based on IMT rate.

4. Conclusions

This study validated the applicability of the GF equations in describing NH₄ ⁺ biosorption kinetics in leachate, mediated by AS development in a UASB reactor. As the experiment progressed, AS development stabilized, marked by linearly increasing COD removal and a sharp increase in NH₄ ⁺ removal that closely mirrored CH₄ production. The best-fit kinetic parameters β and B, obtained from plotting ln­(q) versus ln­(t) with R ² = 0.9840, were used to simulate experimental data using the GF equations, revealing a trend where the IMT rate closely follows the GMT rate. The comparison of high EMT rates with low IMT rates indicates that the rate-limiting step in NH₄ ⁺ biosorption kinetics from leachate is controlled by EMT. Investigating COD’s impact on EMT, IMT, and GMT rates alongside NH₄ ⁺ and PO₄ ^3– removal provides a comprehensive understanding of UASB treatment performance. The UASB process’s performance in removing NH₄ ⁺ and COD while producing CH₄ biogas, mediated by AS development, improved with increasing NH₄ ⁺ concentration, highlighting the need for future research on optimizing the COD/N/P ratio to advance UASB treatment systems. Future studies should examine PO₄ ^3– removal to inform optimal COD/N/P ratios for scaling up UASB-based leachate treatment processes.

The authors gratefully acknowledge the financial support from the Ministry of Higher Education, Science, and Technology (MoHEST), Indonesia, under the Fundamental ResearchRegular Scheme (Grant No. 125/C3/DT.05.00/PL/2025). All the funding bodies have no role in the design of the study and collection, analysis, and interpretation of data and in writing this manuscript.

The authors declare no competing financial interest.