A Review on Landfill Leachate Treatment Technologies: Comparative Analysis of Methods and Process Innovation

Abstract

Landfill leachate, characterized by its high concentration of organic matter (high COD), elevated ammonia and nitrogen levels, high salinity, and toxicity, poses a significant challenge for environmental pollution control. In recent years, extensive research efforts have been dedicated to treating landfill leachate, resulting in the implementation of various engineering technologies. However, with the advancement of analytical techniques, an increasing number of emerging contaminants (ECs) have been detected in landfill leachate. These pollutants pose potential environmental and health risks, yet traditional wastewater treatment technologies struggle to effectively remove them, necessitating innovative upgrades to existing methods. This paper reviews the current research status of landfill leachate treatment technologies, compares the advantages and disadvantages of various techniques, and emphasizes the importance of technological innovation in treatment processes.

1. Introduction

The rapid development of the social economy and urbanization has significantly enhanced the living standards of both urban and rural residents. Nevertheless, this progress has concurrently led to a substantial and accelerated increase in domestic and municipal solid waste generation [1,2,3]. It is anticipated that the global amount of domestic waste will reach 2.2 billion tons by the year 2025 [4,5]. The principal waste disposal methodologies encompass landfilling, incineration, and composting, with landfilling remaining the predominant global choice due to its operational simplicity and cost-effectiveness. This technology’s logistical advantages and economic viability have established it as the cornerstone of contemporary municipal solid waste management systems [6,7]. While sanitary landfills offer significant waste management advantages, they inevitably produce landfill leachate as a byproduct [3,8,9]. Landfill leachate is the main secondary pollution product of landfills [10,11], and originates from complex biogeochemical transformations of waste materials. The formation mechanism involves synergistic hydrological interactions (precipitation infiltration and groundwater intrusion) coupled with biochemical degradation processes (physical compaction, chemical dissolution, and microbial decomposition), ultimately generating this recalcitrant wastewater containing elevated concentrations of organic and inorganic contaminants [12,13]. This contains toxic heavy metals, organic pollutants, pathogens, and inorganic salts with pollutant levels often several orders of magnitude higher than those found in domestic sewage [4,14,15]. These pollutants contaminate surface water, groundwater, soil, etc., posing a serious threat to ecosystems, plants, animals, and human health [16,17,18]. Furthermore, toxicity analyses using various test organisms (Vibrio fischeri, Daphnia magna, Artemisia salina, Phragmites, etc.) have confirmed the potential hazards of landfill leachate [19,20,21,22,23]. Consequently, there has been an increasing emphasis on the disposal of landfill leachate in recent years, particularly in urban areas [24,25,26,27]. Although research on landfill leachate has increased since 2016, it is crucial to develop economical and environmentally friendly disposal technologies [28,29].

In recent years, various methods for purifying landfill leachate have become a hot research topic, including physical, chemical, and biological technologies [30]. However, with the advancement of analytical technologies (such as high-resolution mass spectrometry (HRMS)) [31], an increasing number of previously undetected pollutants in landfill leachate are being identified [32], and these pollutants are defined as emerging contaminants (ECs). ECs encompass a wide range of substances, including pharmaceuticals, endocrine-disrupting compounds, nanomaterials, microplastics, and industrial chemicals, which significantly increase the complexity and difficulty of landfill leachate treatment [33]. Single methods have certain limitations and cannot completely purify the landfill leachate. Physical methods, such as adsorption, are beneficial for removing small molecular weight organics, but they are not suitable for removing large molecular weight contaminants (such as humus) in landfill leachate [34]. Biological methods are widely used to remove nitrogen and organics from young landfill leachate. However, with the extension of landfill time, refractory compounds such as humic acids and fulvic acids make biological processes inefficient [35]. Advanced oxidation processes (AOPs) can effectively compensate for the limitations of physical and biological methods, and are often used to treat refractory organics. However, their high economic cost remain a challenge [36].

The treatment of leachate depends on its characteristics and age, as well as factors such as the required capital investment, operational costs, and the desired level of pollutant removal. The ultimate objective is to adhere to legal requirements [37] that ensure the discharge of leachate has minimal environmental and health impacts on humans. A diverse range of treatment methods exists, often necessitating multiple stages that integrate physical, chemical, and biological processes. These include sedimentation, filtration, coagulation, aerobic and anaerobic biological treatments, constructed wetlands, membrane technologies, and advanced oxidation processes [38,39,40,41]. In landfill leachate treatment, it is essential to emphasize sustainable development and circular economy principles. For instance, humic substances can be recovered from leachate and utilized as liquid fertilizers [42], while anaerobic fermentation can be employed to produce methane [43], achieving energy self-sufficiency. These practices not only transform waste into valuable resources but also reduce environmental impacts and promote economic benefits.

This paper comprehensively reviews the main techniques employed for landfill leachate treatment. Specifically, the composition and characteristics of landfill leachate are discussed and subsequently, various landfill leachate treatment methods and their treatment efficiencies are provided. In addition, this paper further analyzes the advantages and limitations of each treatment technology, emphasizing the necessity of a synergistic approach that integrates different methods to ensure effective purification of leachate, as well as highlighting the importance of innovating existing technologies. Unlike previous reviews, this study systematically evaluates landfill leachate treatment technologies, emphasizing the challenges of emerging contaminants and the integration of novel membrane-based and oxidation techniques. Additionally, it highlights recent advancements in analytical methods for detecting persistent pollutants, providing a broader perspective on future research needs.

2. The Formation, Characteristics, and Composition of Landfill Leachate

2.1. Formation and Characteristics of Landfill Leachate

Landfill leachate is generated through physical, chemical, and biological interactions between moisture and decomposing waste within landfills. The formation process begins when external water sources—such as precipitation, inherent moisture in waste, or groundwater intrusion—penetrate the landfill through its cover layer or cracks [44]. As water percolates through the waste strata, it initiates the aerobic degradation phase, where organic matter is broken down by microbial activity into carbon dioxide and water, accompanied by heat release. Once oxygen is depleted, the system transitions to an anaerobic phase, during which complex organic compounds are converted into small-molecule organic acids, ammonia nitrogen, and methane. Concurrently, DOM and heavy metals leach from components such as electronic waste and batteries. Environmental factors—including elevated temperatures and pH shifts from neutral to alkaline—further regulate pollutant transformation and migration. Ultimately, these processes yield leachate characterized by high COD, elevated ammonia nitrogen levels, and potential toxicity [35].

Landfill leachate exhibits significant temporal and spatial heterogeneity in composition, governed by a complex interplay of waste characteristics (type and composition), landfill engineering parameters (compaction density and cover system design), hydrogeological conditions (infiltration patterns and groundwater interactions), and temporal variables (landfill age and seasonal precipitation variations) [45]. From an analytical chemistry perspective, conventional leachate characterization typically employs these critical parameters: chemical oxygen demand (COD), total organic carbon (TOC), 5-day biochemical oxygen demand (BOD5), pH, ammonia nitrogen (NH₃-N), and heavy metal profiles. The biodegradation potential of organic constituents is frequently assessed through two key indices: the BOD5/COD ratio, reflecting microbial metabolic accessibility, and the COD/TOC ratio, indicating organic carbon oxidation states [46].

A multitude of parameters influence the composition of leachate, including the type and composition of waste, operational mode, climatic conditions, hydrogeological factors, and the age of the landfill site [47]. Table 1 provides a summary of the typical characteristics of landfill leachate. It has been observed that the characteristics of landfill leachate vary, primarily due to factors such as the composition and moisture content of the waste, the age of the landfill facility, seasonal variation (including temperature and precipitation), and other contributing elements [48]. Zhao Jun [49] discovered that the levels of pH, COD, TOC, total nitrogen (TN), and conductivity were remarkably higher in winter than in summer. However, there were no significant differences in redox potential and the concentrations of metals, total suspended solids, and volatile suspended solids between the two seasons. This seasonal fluctuation is primarily attributed to the inhibitory effects of low temperatures on microbial metabolic activity: during winter, reduced temperatures significantly decelerate the biodegradation rates of organic matter, resulting in the accumulation of refractory organic compounds and nitrogenous compounds (e.g., ammonia nitrogen) in leachate. Concurrently, the cold environment potentially enhances volatile fatty acid production through anaerobic fermentation processes, further elevating pH levels.

Landfill age is also a key factor influencing leachate characteristics, and leachate age is typically classified into three stages, including young (<5 years), middle (5–10 years) and mature (>10 years). Young landfill leachate usually contains high concentrations of organic matter and high BOD/COD ratios (0.5–1.0), which is applicable for biological treatment. Disposal of mature leachate with a low BOD/COD ratio (<0.1) becomes more inefficient with biological treatment than with the physical–chemical method [13,50,51]. Ammonia is recognized as a primary contaminant in landfill leachate systems. The coexistence of elevated ammonia concentrations with persistent organic compounds poses significant environmental risks, including eutrophication of aquatic systems and toxicity to aquatic organisms [46,50,52,53].

Table 1. Typical characteristics of landfill leachate [49,54,55,56].

Number	Parameters	Values (mg/L)
1	pH	2.69–8.11
2	BOD5	98–50,000
3	COD	140–157,200
4	TOC	3963–78,640
5	NH3-N	400–6000
6	TN	395–5332
7	Calcium (Ca)	69–2330
8	Potassium (K)	28–1700
9	Magnesium (Mg)	4–780
10	Sodium (Na)	85–3800
11	Lead (Pb)	0.001–5
12	Arsenic (As)	0.0001–0.578
13	Chlorine (Cl−)	47–6000
14	Sulphate (SO42−)	0.01–3240
15	Volatile suspended solids (VSS)	800–11,320
16	Total Suspended Solids (TSS)	930–13,333
17	Volatile phenol	1.3–800
18	Total hardness	15–2241

Table 1. Typical characteristics of landfill leachate [49,54,55,56].

Number	Parameters	Values (mg/L)
1	pH	2.69–8.11
2	BOD5	98–50,000
3	COD	140–157,200
4	TOC	3963–78,640
5	NH3-N	400–6000
6	TN	395–5332
7	Calcium (Ca)	69–2330
8	Potassium (K)	28–1700
9	Magnesium (Mg)	4–780
10	Sodium (Na)	85–3800
11	Lead (Pb)	0.001–5
12	Arsenic (As)	0.0001–0.578
13	Chlorine (Cl−)	47–6000
14	Sulphate (SO42−)	0.01–3240
15	Volatile suspended solids (VSS)	800–11,320
16	Total Suspended Solids (TSS)	930–13,333
17	Volatile phenol	1.3–800
18	Total hardness	15–2241

2.2. Composition of Landfill Leachate

Landfill leachate varies widely in composition according to the age of the landfill as well as the type and the mixture of waste and the contents of these wastes. Usually these wastes contain dissolved, non-dissolved, and suspended materials [35]. Landfill leachate typically manifests as a deeply pigmented liquid exhibiting pronounced malodorous properties, comprising a complex mixture of elevated concentrations of both organic compounds and inorganic constituents [57]. Landfill leachate generally comprises four principal constituent groups [58]: (1) dissolved organic matter (DOM) (100–10,000 mg/L), encompassing volatile fatty acids (VFAs) and aquatic humic substances (AHSs) such as humic and fulvic acids; (2) heavy metal ions including Cu²⁺, Mn²⁺, Cd²⁺, and Ni²⁺ (<2 mg/L); (3) xenobiotic organic compounds comprising phenolic derivatives, aliphatic alcohols, aldehydes, pesticides, and related compounds (0.1–500 ug/L); and (4) macro-inorganic species such as NH₄⁺, Na⁺, Ca²⁺, Fe²⁺, and HCO₃⁻ (10–6000 mg/L). Notably, emerging contaminants of global concern (ECGCs) in leachate matrices have been increasingly identified through advanced analytical techniques, including endocrine-disrupting compounds (EDCs), polybrominated diphenyl ethers (PBDEs), and per- and polyfluoroalkyl substances (PFASs) (0.1–292 μg/L). These ECGCs exhibit distinct environmental persistence characteristics, demonstrating resistance to natural degradation through their inherent chemical stability, bioaccumulation potential, and high environmental mobility [59].

Table 2. Composition of landfill leachate [34].

Species	Examples	Ref.
Dissolved organic matter	Organic acid	[56,60,61]
	Humic acid-like compounds
	Fulvic acid-like compounds
Metal ions	Fe2+, Fe3+, Mn2+, Zn2+, Pb2+, Cr3+, Cr6+, Ni2+	[62,63,64]
Metalloid ions	Cl−, PO43−, SO42−, As3+, As5+	[65,66]
Antibiotics	Macrolides	[67,68]
	Quinolones
	Sulfonamides
	Tetracyclines
Endocrine disrupters	Polycyclic aromatic hydrocarbons	[69,70,71]
	Polychlorinated biphenyls
	Bisphenol A
Pesticides	Dichlorodiphenyltrichloroethane	[72,73]
	Hexachlorocyclohexane
Pharmaceutical and personal care products	Bezafibrate	[74,75,76]
	Carbamazepine
	Sulfamethoxazole
	Metoprolol
Commercial organic matter	Perfluoroalkyl acids	[77,78,79]
	Perfluorooctane sulphonate
Microplastics	Polystyrene	[80,81,82]
	Polypropylene
	Polyethylene
	Polyvinyl chloride

shows the composition of landfill leachate.

2.3. Emerging Contaminants

In recent years, advancements in analytical technologies have led to the detection of a growing number of emerging contaminants (ECs) in landfill leachate [32]. These pollutants encompass both inorganic (e.g., heavy metals) and organic (e.g., detergents and pesticides) compounds, including personal care products (PCPs), pharmaceuticals, insecticides, flame retardants, surfactants, and related chemical substances [83]. These compounds are typically either recently detected in environmental matrices or have only recently had their impacts recognized. Such contaminants lack comprehensive regulatory frameworks due to insufficient data regarding their environmental and health implications.

Notably, many persist in environmental matrices and bioaccumulate through food chains, posing latent risks to ecosystems and human health over extended temporal scales [33]. These substances exhibit bioaccumulative potential, environmental persistence, and intermittent endocrine-disrupting activity, demonstrating significant ecotoxicological impacts on biota and public health even at environmentally relevant concentrations [84]. For example, certain pharmaceuticals possess endocrine-disrupting capabilities, including interfering with the reproduction and development of animals, and these substances may enter the environment and ultimately come into contact with humans through various pathways, thereby increasing the likelihood of cancers and birth defects in humans [44,85,86]. However, traditional water treatment methods are often ineffective in removing them, and as a result, these substances are frequently unintentionally released into the environment [84,87]. Therefore, technological retrofitting and upgrades of existing systems are imperative. For example, Tan et al. [88] innovatively engineered a zirconium-based metal–organic framework (Zr-MOF) to enhance both selective adsorption and photocatalytic degradation efficiency of EC, and the synthesized catalyst VNU-1 exhibited 100% photodegradation efficiency for ampicillin, lincomycin, and sulfamethoxazole. Wang et al. [89] fabricated a heterojunction catalyst by supporting Co₃O₄ nanoparticles on β-MnO₂ nanorods, and about 99.4% of tetracycline underwent rapid oxidative degradation within 60 min in this system.

3. Technologies for Treating Landfill Leachate

Landfill leachate poses severe pollution risks to the ecological environment and indirectly threatens human life safety. To prevent ecotoxicity and environmental damage caused by landfill leachate, proper treatment is imperative. The treatment of landfill leachate has evolved significantly with technological proliferation, as evidenced by diverse reported methodologies (Figure 1). Contemporary strategies predominantly employ an integrated approach combining physical, chemical, and biological technologies. Notably, membrane separation processes—especially pressure-driven membrane technologies—have been effectively implemented in full-scale applications due to recent innovations in material science and modular design [90,91]. Physical–chemical methods, such as coagulation–flocculation and advanced oxidation processes (AOPs), demonstrate high efficacy in eliminating refractory organic compounds [92,93]. Concurrently, biological methods, particularly sequential anaerobic–aerobic bioreactors and biological nitrogen removal (BNR) systems, are optimized for ammonium and nitrate reduction [94,95,96]. Furthermore, adsorption processes leveraging porous materials (e.g., activated carbon and biochar) [97,98,99] and membrane-based separation technologies (e.g., nanofiltration and reverse osmosis) [100,101] are increasingly integrated into hybrid systems to address the complex pollutant matrix.

3.1. Physical–Chemical Method

3.1.1. Adsorption

The adsorption (ADS) method has been extensively applied in landfill leachate treatment due to its high efficiency in removing organic compounds and ammonia [102]. Among the diverse range of adsorbents, activated carbon remains the most efficient and promising material for this purpose [103,104]. Activated carbon is primarily categorized into powdered activated carbon (PAC) and granular activated carbon (GAC) based on its physical morphology. In particular, stabilized leachate from the Goslar landfill, Germany, was firstly evaluated using a GAC column in 1995, illustrating a COD removal of 91% with an initial concentration of 940 mg/L [11]. Liu [105] demonstrated the efficacy of PAC in removing low-molecular-weight organic pollutants from river water. However, it is important to note that the ADS method primarily functions through phase separation and pollutant transfer mechanisms rather than chemical degradation, which limits its ability to fully mineralize contaminants.

To investigate the interaction of different PACs with DOM in membrane bioreactor (MBR) effluent at the molecular level, Liu [104] respectively compared DOM adsorption efficiency over wood-based PAC (WoAC) and coal-based PAC (CoAC), and found the 89.2% removal of DOC by CoAC was significantly higher than the 80.9% of WoAC. CoAC’s superiority is likely due to its Fe component, as iron oxides on the surface of CoAC may interact with certain functional groups (e.g., -OH groups) of unsaturated compounds, thereby enhancing DOM adsorption efficiency.

Humic acid poses a significant challenge in landfill leachate treatment. To address this issue, Wang [106] conducted a comprehensive study on the selective adsorption of humic acid using graphitic carbon nitride (g-CN) across leachate samples from 10 different landfills. The research demonstrated that g-CN achieved a UV₂₅₄ removal rate of 30–70% across all leachate samples, significantly surpassing TOC removal efficiency. This innovative selective adsorption technique not only effectively addresses UV quenching substances (UVQSs) in leachate treatment but also enables the recovery of humic acid for potential reuse as liquid fertilizer in agroforestry applications. Furthermore, the adsorption method has proven effective in reducing key leachate parameters, including COD, suspended solids, and color intensity, thereby offering a promising solution for leachate treatment challenges. However, the adsorption process can only be used as a pretreatment of landfill leachate, and the effective purification of landfill leachate requires an integrated approach combining biological degradation and chemical treatment processes.

3.1.2. Coagulation–Flocculation Method

Coagulation–flocculation (CF) represents a relatively straightforward yet efficient process, particularly effective in the treatment of mature landfill leachate. For young leachate, it serves as an advantageous pretreatment step, enhancing the efficacy of subsequent physicochemical or biological treatment processes in municipal domestic landfills [107]. CF employs chemical coagulants to neutralize and aggregate suspended particles in leachate through charge neutralization and aggregation mechanisms, facilitating the formation of denser flocs that are subsequently efficiently separated via sedimentation. Table 3 lists the efficiency of some physicochemical methods for landfill leachate. M. Arbabi [108] conducted a comparative study on leachate treatment efficiency using ultrasonic-assisted CF with sodium ferrate versus standalone ultrasonication. Experimental results demonstrated that the integrated process achieved significantly higher removal rates of 87.5% for COD and 88.6% for color under optimized parameters (15 min ultrasonication duration, pH 5.5). This performance markedly surpassed that of ultrasonic treatment alone, which exhibited limited efficacy, with both COD and color removal rates below 50%.

Z. Chaouki [109] investigated the efficacy of ferric chloride-based coagulation pretreatment and its synergistic combination with powdered activated carbon (PAC) adsorption for leachate treatment. Experimental data revealed that the optimized ferric chloride dosage (12 g/L as Fe³⁺) achieved maximum removal efficiencies of 50% COD, 89.9% turbidity, and 80% color in single-stage coagulation. Notably, the integration with PAC adsorption under optimized operational parameters significantly enhanced treatment performance, attaining 59% COD removal along with 99% reductions in both turbidity and color. This comparative analysis demonstrates that the coagulation–adsorption integrated system provides substantially superior contaminant removal capabilities compared to standalone coagulation processes, confirming its technical viability for advanced leachate treatment.

Bouyakhsass [110] systematically investigated the application of a cationic polymer flocculant (Himoloc DR3000) in CF treatment for landfill leachate. Through central composite design optimization, the study established optimal operational parameters: pH 7.66, coagulant dosage 9.48 g/L, flocculant dosage 9.05 mL/L, and mixing duration 10.22 min. Under these optimized conditions, the process achieved significant contaminant removal efficiencies of 68.81% for color, 77.46% for polyphenols, and 80.99% for nitrate. This parametric optimization demonstrates the technical feasibility of cationic polymer-enhanced CF processes in advanced leachate treatment.

Landfill leachate treatment through physical methods demonstrates distinct advantages, including straightforward operability, reduced infrastructure demands, enhanced process efficiency, and favorable energy economics. However, the efficacy of these methods diminishes when confronting leachate containing recalcitrant macromolecular organic compounds or endocrine-disrupting chemical mixtures (EGCMs), presenting significant technical limitations in contaminant speciation management.

Table 3. Efficiency of physicochemical methods at laboratory scale.

Method	Adsorbent/ Coagulant	Dosage (g/L)	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Ref.
ADS	PAC	0.1	7.8	TOCi	2.5	37.0	[105]
				TOCf	1.6
ADS	WoAC	2.0	8.0	DOCi	197.5	70.8	[104]
				DOCf	57.7
ADS	CoAC	2.0	8.0	DOCi	197.5	78.4	[104]
				DOCf	42.7
ADS	Palygorskite	133.0	8.0	CODi	1998.0	54.7	[111]
				CODf	905.0
ADS	Palygorskite	133.0	8.0	NH3-Ni	675.0	41.9	[111]
				NH3-Nf	392.0
CF	Na2FeO4	120.0	5.5	CODi	32,000.0	87.1	[108]
				CODf	4144.0
CF	FeCl3	34.8	7.6	CODi	22,000.0	50.0	[109]
				CODf	11,000.0
CF	Fe2 (SO4)3	0.2	7.6	CODi	3855.0	74.5	[112]
				CODf	983.0

Note: i refers to initial; f refers to final; ADS refers to adsorption; CF refers to coagulation–flocculation; PAC refers to powdered activated carbon; WoAC refers to wood-based PAC; CoAC refers to coal-based PAC.

Table 3. Efficiency of physicochemical methods at laboratory scale.

Method	Adsorbent/ Coagulant	Dosage (g/L)	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Ref.
ADS	PAC	0.1	7.8	TOCi	2.5	37.0	[105]
				TOCf	1.6
ADS	WoAC	2.0	8.0	DOCi	197.5	70.8	[104]
				DOCf	57.7
ADS	CoAC	2.0	8.0	DOCi	197.5	78.4	[104]
				DOCf	42.7
ADS	Palygorskite	133.0	8.0	CODi	1998.0	54.7	[111]
				CODf	905.0
ADS	Palygorskite	133.0	8.0	NH3-Ni	675.0	41.9	[111]
				NH3-Nf	392.0
CF	Na2FeO4	120.0	5.5	CODi	32,000.0	87.1	[108]
				CODf	4144.0
CF	FeCl3	34.8	7.6	CODi	22,000.0	50.0	[109]
				CODf	11,000.0
CF	Fe2 (SO4)3	0.2	7.6	CODi	3855.0	74.5	[112]
				CODf	983.0

Note: i refers to initial; f refers to final; ADS refers to adsorption; CF refers to coagulation–flocculation; PAC refers to powdered activated carbon; WoAC refers to wood-based PAC; CoAC refers to coal-based PAC.

3.2. Advanced Oxidation Progresses

3.2.1. Electrooxidation

Electrooxidation (EO) represents an advanced oxidation technology for organic pollutant degradation, operating through two distinct pathways: direct electron transfer at the electrode surface and indirect oxidation via electrogenerated reactive species. The process involves in situ generation of oxygen and chlorine-derived oxidants (e.g., •OH and HClO) under controlled electrochemical conditions, typically achieved through anode polarization at optimized potentials [113]. The principal merit of this technology lies in its mineralization capability, converting organic contaminants into benign end-products (CO₂ and H₂O) through complete oxidation, thereby preventing secondary contamination through phase transfer phenomena.

Electrochemical processes such as electrocoagulation (EC) have proved to be a promising option in the treatment of various types of industrial effluents, for the removal of dyes, pesticides, heavy metals, and pharmaceutical compounds [114]. Oliveira et al. [114] innovatively utilized electrodes fabricated from steel shavings (SFEs) for EC to remove heavy metals and thermotolerant coliforms from landfill leachate, and the novel electrodes demonstrated removal efficiencies of 51–95% for heavy metals and achieved 100% elimination of thermotolerant coliforms present in the leachate. Ghanbari [113] studied the efficiency of different processes such as EC, EO, and sulphate-based advanced oxidation processes (SR-AOPs) in landfill leachate treatment, and found that the removal of COD was 60.0%, 50.0%, and 77.9%, respectively. The total removals of COD, TOC, BOD, and ammonia (NH₃-N) by the sequential method were 95.6%, 90.5%, 91.6%, and 99.8%, respectively. Converting dissolved organic pollutants into separable insoluble matter is an energy-efficient strategy. Liang [115] engineered a novel dual-anode electrochemical system (Fe²⁺/HClO) for advanced treatment of landfill leachate concentrates, employing an innovative oxidative coupling-mediated insolubilization mechanism. This configuration demonstrated exceptional performance, with the DOC removal efficiency reaching 89.8%, while simultaneously achieving superior environmental compatibility by successfully suppressing toxic byproduct generation, maintaining total organic chlorine (TOCl) concentrations below the 0.1 mg/L threshold.

3.2.2. Ozone Oxidation

Ozonation (O₃), as a robust advanced oxidation process (AOP), has gained prominence in leachate remediation for its exceptional capability in mineralizing refractory organic contaminants (ROCs) through radical chain reactions (•OH-mediated oxidation). The decomposition of O₃ generates •O₂⁻ and •OH⁻ radicals in water (as shown in Equations (1)–(6)), as the highly reactive oxidative radicals non-selectively attack refractory organic compounds in landfill leachate (e.g., humic acids and polycyclic aromatic hydrocarbons, denoted as CHOSs) through electron transfer, hydrogen abstraction, and addition reactions. These radical-mediated processes progressively cleave large organic molecules into low-molecular-weight, biodegradable intermediates such as carboxylic acids and ketones, while simultaneously oxidizing them through bond-breaking mechanisms. Ultimately, the degraded organic constituents are mineralized into environmentally benign end-products, predominantly carbon dioxide (CO₂) and water (H₂O). Figure 2 shows macromolecular substances can produce highly resoluble small molecular substances, and even degrade them into CO₂ and H₂O [116].

The total reaction is as follows:

$$O_3+H_{2}O\to 2·\mathit{OH}+O_2$$

(1)

The chain reaction is as follows:

$$O_3+{HO}^{-}\to ·O_2^{-}+{HO}_2$$

(2)

$$O_3+·OH\to O_2+{HO}_2·$$

(3)

$$O_3+H{O}_2·\to 2O_2+HO·$$

(4)

$$2H{O}_2·\to O_2+H_2{O}_2$$

(5)

The degradation reaction is as follows:

$$·OH+RH\to ·R+H_2{O}_2\to Further oxidation\to {CO}_2+H_{2}O$$

(6)

Recent advancements in landfill leachate treatment have demonstrated the progressive development of ozone-based oxidation technologies. Yuan [118] implemented an ozonation process for pretreatment of high-concentration leachate, achieving 80.38% COD removal efficiency through direct oxidative degradation. However, recognizing the inherent limitations of single-stage ozonation in handling complex leachate matrices, Scandelai [119] engineered a hybrid O₃/supercritical water oxidation (O₃/ScWO) system, establishing an innovative synergistic oxidation platform. Under optimized operational parameters (reaction temperature 450 °C, pressure 25 MPa, O₃ dosage 15 mg/L), this integrated process attained remarkable removal efficiencies of 95% BOD₅, 92% COD, and 79% TOC, representing 1.6–2.3-fold enhancement compared to standalone ozonation processes.

Zhu [120] engineered a manganese–cerium composite oxide supported on a gamma-alumina (MnCeOx/γ-Al₂O₃) catalyst to enhance ozone activation in landfill leachate treatment. Comparative analysis revealed significant performance enhancement in catalytic ozonation systems: COD removal efficiency increased by 48% (from 55.8% to 82.4%) and UV₂₅₄ absorbance reduction improved by 15% (from 83.6% to 96.3%) compared to conventional ozonation. This performance enhancement stems from the catalyst’s ability to generate reactive oxygen species (ROS), including hydroxyl radicals (•OH) and superoxide radicals (O₂•−), which exhibit superior oxidative potential for refractory organic compound mineralization. Recent mechanistic studies [121] have further validated that heterogeneous catalytic ozonation provides threefold advantages: (1) enhanced organic mineralization rates (>85% TOC reduction), (2) optimized ozone utilization efficiency (up to 92% mass transfer efficiency), and (3) facile catalyst separation via γ-Al₂O₃’s macroscopic structure, making it technically viable for continuous-flow operations.

3.2.3. Fenton Oxidation

Fenton oxidation employs hydrogen peroxide (H₂O₂) as an oxidizing agent to produce hydroxyl free radicals (•OH) in the presence of ferrous ions, which then attack organic compounds as active component of the Fenton reagent, resulting in their partial or complete oxidation and eventually producing CO₂ and H₂O [57,122] (as shown in Equations (7)–(15)).

$${Fe}^{2+}+H_2{O}_2\to {Fe}^{3+}+·OH+O{H}^{-}$$

(7)

$$RH+·OH\to H_{2}O+R·$$

(8)

$${Fe}^{3+}+H_2{O}_2\to {Fe}^{2+}+H{O}_2·+H^{+}$$

(9)

$${Fe}^{3+}+H{O}_2·\to {Fe}^{2+}+O_2+H^{+}$$

(10)

$$·OH+H_2{O}_2\to H_{2}O+H{O}_2·$$

(11)

$${Fe}^{2+}+H{O}_2·\to {Fe}^{3+}+H{O}_2^{-}$$

(12)

$$·OH+{Fe}^{2+}\to {Fe}^{3+}+O{H}^{-}$$

(13)

$$COD/TOC+H_2{O}_2({Fe}^{2+},{Fe}^{3+})\to Intermediate product$$

(14)

$$H_2{O}_2({Fe}^{2+},{Fe}^{3+})+Intermediate product\to {CO}_2+H_{2}O+Inorganic salt$$

(15)

The Fenton oxidation process, operating through the in situ generation of hydroxyl radicals (•OH) via Fe²⁺/H₂O₂ catalytic reactions under acidic conditions (pH 2.5–3.5), has emerged as a benchmark chemical oxidation technology for leachate pretreatment [58]. Fenton oxidation degrades landfill leachate by generating highly reactive hydroxyl radicals (•OH) through the reaction of Fe²⁺ with H₂O₂ under acidic conditions (pH 2–4). These radicals non-selectively attack C-H bonds, aromatic rings, and functional groups (such as carboxyl and amino groups) in organic molecules, gradually mineralizing refractory organic compounds (e.g., humic acids and polycyclic aromatic hydrocarbons) and metal complexes into CO₂, H₂O, and inorganic salts.

Recent advancements in Fenton-based technologies for landfill leachate treatment have demonstrated significant progress in process optimization and catalyst development. Jegadeesan et al. [123] pioneered the aeration-enhanced electrochemical Fenton process for domestic leachate treatment, achieving exceptional removal efficiencies of 99% COD and complete color elimination (100%) in stabilized leachate under optimized aeration parameters. Parallel developments by Yang et al. [124] in bio-electro-Fenton systems demonstrated 70% COD removal under baseline conditions (pH = 2, applied potential 0.6 V), which significantly improved to 91.3% through cathode modification using platinum-group metal coatings, highlighting electrode engineering’s critical role in process enhancement.

Despite these advancements, conventional homogeneous Fenton processes face three fundamental limitations, as critically analyzed by Smith et al. [125]: (1) iron sludge production (0.8–1.2 kg/m³ treatment capacity), (2) stringent pH requirements (2.5–3.5), and (3) non-recoverable catalysts. Addressing these challenges, Guo et al. [126] developed a novel heterogeneous Fenton system employing iron oxide-impregnated granular activated carbon (Fe-GAC) catalysts for mature leachate treatment. Through orthogonal experimental design, optimal conditions (H₂O₂/COD = 1.5, Fe²⁺/H₂O₂ = 0.3, reaction time = 90 min) achieved 93.7% COD removal, 65.5% NH₃-N reduction, and 84% decolorization in batch mode. Continuous operation in a fluidized-bed reactor demonstrated process stability with >90% COD removal sustained over 12 h of operation. The electro-Fenton variant exhibited superior performance, attaining 97.4% COD removal, complete color elimination (99%), TN reduction of 94.2%, and full nitrification of NH₃-N (100%) through synergistic radical oxidation (·OH yield = 2.3 mM) and electrochemical reduction mechanisms.

3.2.4. Photocatalytic Oxidation

Photocatalytic oxidation has emerged as a cutting-edge remediation technology for landfill leachate treatment, leveraging semiconductor-mediated photoredox mechanisms. The process initiates through photon absorption (UV spectrum, λ < 380 nm) by wide-bandgap semiconductors (e.g., TiO₂ [3.2 eV], ZnO [3.37 eV], and CdS [2.4 eV]), inducing electron excitation from the valence band (VB) to the conduction band (CB) with subsequent generation of electron–hole (e⁻-h⁺) pairs possessing redox potentials of +2.4–3.0 V vs. NHE [127]. Photocatalysis can degrade the macromolecular pollutants from leachate into small molecules or directly mineralize the pollutants into CO₂ and H₂O [29,128,129].

Recent studies on semiconductor material modification have significantly advanced the application efficacy of photocatalytic technology in landfill leachate treatment. Azadi et al. [130] synthesized p-type TiO₂ nanoparticles through silicon doping (2.5 wt.%), achieving 84.7% COD removal efficiency under visible-light irradiation (λ > 420 nm) with optimized parameters (calcination temperature 500 °C, pH 6). Their subsequent research [28] comparatively evaluated n-type, p-type, and n–p heterojunction TiO₂ photocatalysts for treating high-strength leachate (initial COD 2050 mg/L), demonstrating COD removal rates of 46.0%, 60.0%, and 64.0%, respectively, after 46 h irradiation, which substantiated the enhanced photocatalytic activity through interfacial charge separation effects in heterojunction systems. Mishra et al. [131] further developed perovskite-type CaTiO₃ nano-photocatalysts, achieving simultaneous removal of multiple pollutants under UV-A irradiation (33 W, 8 h) at optimal conditions (catalyst dosage 3.33 g/L, pH 6): 72.3% COD removal, 43.6% chroma reduction, 41.8% turbidity improvement, 59.0% total dissolved solids elimination, 35.7% NH₃-N degradation, and heavy metal removal efficiencies of 52.3% for chromium (Cr), 63.5% for nickel (Ni), and 87.5% for zinc (Zn).

Table 4. Efficiency of advanced oxidation processes at laboratory scale.

Method	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Ref.
EO	7.3	CODi	3400.0	50.0	[113]
		CODf	1700.0
EO	3.0	CODi	3015.7	90.1	[115]
		CODf	297.7
O3	7.2	CODi	12,320.0	80.4	[118]
		CODf	2417.0
O3	7.6	CODi	1062.0	55.8	[120]
		CODf	469.0
Fenton	2.0	CODi	970.0	70.0	[124]
		CODf	291.0
Fenton	5.0	CODi	1851.0	97.4	[126]
		CODf	48.0
Photocatalytic	6.0	CODi	600.0	85.0	[130]
		CODf	90.0
Photocatalytic	6.0	CODi	47,089.0	72.0	[131]
		CODf	13,185.0

Note: i refers to initial; f refers to final; EO refers to electrooxidation.

lists the efficiency of AOPs for landfill leachate treatment.

These technologies can completely degrade pollutants into CO₂ and H₂O, but the degradation efficiency depends on several factors, such as the oxidation potential, pollutant concentration, and reaction kinetics. AOPs degrade pollutants by generating highly oxidative free radicals (e.g., ·OH and SO₄⁻·); the oxidation potentials of hydroxyl radicals and sulfate radicals are 2.8 V and 2.6 V, respectively, theoretically enabling the mineralization of most organic compounds [132]. However, in practical applications, the selection of reactive species must align with the specific properties of the target pollutants. Pollutant concentration also significantly impacts degradation efficiency: excessive concentrations can lead to rapid oxidant consumption, insufficient radical generation, and accumulation of intermediate products [130]. Additionally, factors like pH, temperature, and catalyst dosage critically influence the process. These variables can be optimized by establishing reaction kinetics models to enhance overall performance [29,133].

AOPs demonstrate exceptional technical merits in landfill leachate remediation, particularly in three operational domains: (1) effective mineralization of refractory organic compounds [46], (2) complete decolorization and odor elimination [123,125], and (3) broad-spectrum applicability to leachates of varying stabilization stages [132]. However, their industrial implementation encounters three principal constraints: (i) elevated operational costs [46], (ii) high energy consumption [35], and (iii) catalyst deactivation issues [133].

3.3. Biological Treatment Processes

Biological treatment processes demonstrate superior efficacy in sustainable leachate remediation, particularly through cost-effective removal of organic and nitrogenous contaminants [134]. The biological treatment processes for landfill leachate can be classified into aerobic processes and anaerobic processes [135].

3.3.1. Aerobic Treatment Processes

The activated sludge process (ASP) and sequencing batch reactor (SBR) are widely used methods for aerobic treatment [135,136,137]. Elmaadawy et al. [138] assessed the efficiency of activated sludge in treating landfill leachate, achieving a COD degradation rate of 69.9% at an initial COD concentration of 950 mg/L. Ren [139] conducted a comparative study using two 3 L sequencing batch reactors (SBRs)—a granular sludge SBR (GSBR) and an activated sludge SBR (ASBR)—for young leachate treatment (TAN: 1200–1500 mg/L). The GSBR system exhibited remarkable efficiency, with 99% TAN removal and a COD reduction of 67–87% under organic loading rates of 3.2–4.8 kg COD/m³·d. In contrast, the ASBR showed inferior performance, with TAN removal fluctuating between 60–80% and COD removal of 52–83% at identical hydraulic retention times (HRT = 24 h). Saxena et al. [140] systematically validated the multi-pollutant removal capability of a granular sludge sequencing batch reactor (GSBR) in treating high-strength landfill leachate. Under controlled operational parameters (HRT = 24 h, DO = 2.5–3.5 mg/L, MLSS = 8500 mg/L), the system demonstrated exceptional removal efficiencies of 94% COD (initial 668 ± 110 mg/L), 85% NH₃-N (30 ± 3.3 mg/L), and 83% PO₄-P (147 ± 18 mg/L). Figure 3 shows the process of nitrification and denitrification.

3.3.2. Anaerobic Treatment Processes

Anaerobic biotechnologies have emerged as cornerstone processes in advanced landfill leachate treatment, with two predominant pathways demonstrating exceptional nitrogen removal capabilities: anaerobic ammonium oxidation (Anammox) and heterotrophic denitrification. Ren [141] engineered a continuous plug-flow multistage A/O system combining first-stage partial nitrification (DO 0.5–1.0 mg/L, pH 7.8–8.2) with anaerobic ammonium oxidation (PNA) and fixed-film activated sludge. During long-term operation (HRT = 32 h, 35 ± 1 °C), the system achieved 98.1% inorganic nitrogen removal and 52.9% COD reduction over 400 days (days 301–405), with PNA contributing 94.3–95.0% nitrogen elimination through selective enrichment of anammox bacteria (Candidatus Brocadia abundance: 15–18%, VSS/TSS ratio > 0.85). Wang [142] investigated the partial nitrification (PN), carbon oxidation, denitrification, and anaerobic ammonia oxidation performance of a continuous tower biofilter (TBFR) with a large height-to-diameter ratio of 13 for treatment of real landfill leachate. The average removal of TN and COD by the TBFR reached 87.47% and 88.45% at the maximum nitrogen loading rate and organic loading rate (OLR) of 1.31 kg-N/(m³·d) and 4.76 kg/(m³·d), respectively.

3.3.3. Constructed Wetlands

On-site treatment using constructed wetlands (CWs) is one of the low cost methods of landfill leachate treatment which has been widely practiced in several countries for many years [41]. A wetland system comprises a permeable substrata, such as gravel, which is typically planted with emergent wetland plants, such as Schoenoplectus, Typha, Phragmites, and Cyperus [64]. Białowiec et al. [41] employed three discontinuous batch shallow CW systems with reed, willow, and plant-free configurations featuring recirculating subsurface horizontal flow to evaluate nitrogen removal from landfill leachate in relation to HRT and climatic conditions. This study revealed vegetation significantly influenced both conditions and processes within the shallow CW systems. Zhuang et al. [143] expressed that more than 50% of nitrogen can be eliminated by microbial activities, such as the nitrification/denitrification process, while around 25% of nitrogen may be absorbed by plant roots. Dan et al. [144] achieved good removal efficiencies of phenol (88–100%), bisphenol A (9–99%), and 4-tert-butylphenol (18–100%) from synthetic landfill leachate using a vertical flow constructed wetland system.

Biological treatment demonstrates distinct advantages in landfill leachate management, particularly through efficient organic matter mineralization and cost-effective nitrogen elimination under optimized conditions [10]. However, its application encounters three principal limitations: (1) prolonged treatment cycles due to refractory organic fractions and microbial inhibition from complex contaminants [10]; (2) inadequate removal of non-biodegradable organics [35]; (3) little effect on mature leachate [36].

Table 5. Efficiency of biological treatment at laboratory scale.

Method	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Ref.
ASP	7.85	CODi	950.0	69.9	[138]
		CODf	286.0
SBR	8.4	CODi	4975.0	50.0	[145]
		CODf	2487.0
GSBR	7.5	TANi	498.0	99.0	[139]
		TANf	5.0
GSBR	7.5	CODi	810.0	67.0	[139]
		CODf	267.0
AGR	7.5	CODi	1149.0	75.0	[140]
		CODf	287.0
A/O	8.2	CODi	3387.7	52.9	[141]
		CODf	1605.6
USAB	7.5	CODi	6000.0	85.0	[146]
		CODf	900.0
USAB	7.1	CODi	73,000.0	82.4	[147]
		CODf	12,848.0
TBFR	7.6	CODi	2360.8	88.5	[142]
		CODf	272.7

Note: i refers to initial; f refers to final; ASP refers to activated sludge process; SBR refers to sequencing batch reactor; GSBR refers to granular sludge SBR; AGR refers to aerobic granular reactor; A/O refers to continuous plug-flow multistage anoxic/oxic; USAB refers to up-flow anaerobic sludge blanket; TBFR refers to tower biofilter.

lists the efficiency of biological treatment for landfill leachate.

3.4. Membrane Technology

Currently, the most widely used membrane technologies for landfill leachate are membrane bioreactors (MBRs) and membrane distillation (MD). Table 6 presents the efficiencies of membrane treatment for landfill leachate.

In the last decade, nanofiltration (NF) and reverse osmosis (RO) were extensively used for treatment of landfill leachate due to stringent legal regulations. Dolar [148] investigated the treatment of landfill leachate using RO membranes (XLE) and NF membranes (NF90 and NF270), achieving significant treatment efficiency. Both the XLE and NF90 membranes demonstrated COD and TOC removal rates exceeding 80%. However, the application of RO and NF membranes posed challenges of membrane fouling. Ultrafiltration (UF) is a low-pressure (40–1000 kPa) membrane process that yields higher permeate flux and saves significant operating costs compared to NF/RO [149]. Esfahani [149] investigated the removal of selected heavy metals from landfill leachate by electrospun polyacrylic acid (PAA)/polyallylamine hydrochloride (PAH)-laminated UF membranes (PAA/PAH-UF). The effect of the leachate matrix on metal removal by the PAA/PAH-UF membrane was also examined. In synthetic metal solutions, the PAA/PAH-UF membrane exhibited 38–85% higher removal efficiency when compared to the unmodified membrane. However, due to the relatively large pore size of UF, it is not commonly employed.

MBR technology has emerged as a state-of-the-art biological treatment solution for mature landfill leachate management, particularly through its unique combination of microbial degradation and precise membrane separation [150]. Li et al. [151] combined a MBR and NF technology to treat leachate, and this system provided good removal of COD (87%). A two-stage MBR system was applied to the treatment of partially stabilized leachate from solid waste landfill in Thailand, and during a steady operation of 200 days, BOD, COD, and NH₃-N removals were found to be 99.6, 68, and 89%, respectively [152]. Lindamulla et al. [153] treated landfill leachate using MBR with different configurations. An aerobic MBR (AMBR) system was operated in three phases. In the first phase, an AMBR alone, in the second phase, an anaerobic reactor followed by an anoxic reactor and an AMBR, and in the third phase an anoxic reactor followed by an AMBR were operated. The three MBR configurations removed more than 93%, 64.8%, and 59% of BOD₅, COD, and total nitrogen, respectively. Zhou Chu et al. [154] engineered an innovative hybrid membrane bioreactor–nanofiltration (MBR-NF) system through strategic membrane replacement, substituting conventional microfiltration (MF, 0.1–0.2 μm pore size) with polyamide thin-film composite nanofiltration membranes. By comparing the treatment effect of the two different reactors on landfill leachate, as a result, the performance of the MBR-NF was 20–30% higher than that of the MBR-MF in removing refractory organics.

MD, an emerging thermally-driven desalination technology, operates through vapor pressure differentials across hydrophobic microporous membranes (pore size 0.1–0.5 μm). This process demonstrates exceptional hypersaline concentration capabilities, achieving brine salinity levels of 25–30% w/w—3–5 times higher than conventional pressure-driven membrane technologies like NF (5–8% salinity limit) and RO (7–10% salinity limit) [155,156]. The technical merits of MD in landfill leachate treatment are threefold: (1) effective operation at moderate temperatures (60–80 °C) with low-grade thermal energy (specific energy consumption 120–180 kWh/m³), (2) simultaneous removal of multi-contaminants through combined vapor–liquid equilibrium mechanisms, and (3) compatibility with high osmotic pressure solutions (>5 MPa) [157,158,159]. However, due to the high operational energy consumption required to maintain the necessary temperature driving force in MD processes, their cost-effectiveness may be lower compared to other membrane technologies [160].

Zoungrana [161] pioneered the application of surface-modified direct contact membrane distillation (DCMD) for leachate treatment. Under optimized conditions (feed temperature 65 °C, permeate temperature 20 °C, cross-flow velocity 0.3 m/s), the system achieved remarkable contaminant removal efficiencies: 94.2% COD, 91.5% sulfate, and 93.8% hardness (as CaCO₃ equivalent), with concurrent 98.4% ammonia rejection and >90% heavy metal removal (Cr⁶⁺, Zn²⁺, and Pb²⁺). Yang [162] engineered an integrated two-stage membrane distillation (ITMD) system with optimized thermal gradients (feed stage: 75 ± 2 °C, permeate stage: 15 ± 1 °C), achieving exceptional contaminant removal efficiencies of 99.2–99.8% COD, >99.5% inorganic salts (TDS < 50 mg/L), and 99.1–99.7% metal ions (Cr⁶⁺, Pb²⁺, and Cd²⁺) through synergistic vapor–liquid separation mechanisms. However, practical MD applications face two critical challenges: (1) irreversible membrane fouling caused by organic foulants (humic acids) and scaling (CaSO₄/CaCO₃ deposition), and (2) concentrated brine management (salinity >25% w/w) requiring specialized disposal protocols. To address these limitations, Aftab [163] developed a novel pretreatment strategy using lignin-derived biochar coupled with MD. Compared to the MD system without adsorption assistance, this MD showed better performance, such as 40% total membrane flux recovery, a 42% increase in filtration flux, and a 53% reduction in concentrate yield.

Membrane treatment for landfill leachate offers several advantages, including stable effluent quality, effective removal of impurities, small footprint, and ease of operation and maintenance. However, its disadvantages are relatively high treatment costs, susceptibility to damage and fouling, which results in frequent replacement, and specific requirements for raw wastewater quality.

Table 6. Efficiency of membrane treatment.

Method	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Scale	Ref.
MBR-NF	/	CODi	4670–6700	87%	Pilot	[151]
		CODf	800
AMBR	7.1–8.3	CODi	2482.6	80.7	Lab	[153]
		CODf	479.6
DCMD	8.0	CODi	198.7	99.0	Lab	[164]
		CODf	2.0
ITMD	12.0	CODi	1291.8	99.9	Lab	[162]
		CODf	0.5
MD	7.0	TOCi	602.0	98.9	Lab	[165]
		TOCf	6.5
PVDF	8.3	CODi	1188.0	57.1	Lab	[166]
		CODf	510.0
MBR	8.0	CODi	5000.0	91.7	Lab	[167]
		CODf	417.0

Note: i refers to initial; f refers to final; AMBR refers to aerobic membrane bioreactor; DCMD refers to direct contact membrane distillation; ITMD refers to integrated two-stage membrane distillation; MD refers to membrane distillation; PVDF refers to polyvinylidene fluoride; MBR refers to membrane bioreactor.

Table 6. Efficiency of membrane treatment.

Method	pH	Parameter	Concentration (mg/L)	Removal Rate (%)	Scale	Ref.
MBR-NF	/	CODi	4670–6700	87%	Pilot	[151]
		CODf	800
AMBR	7.1–8.3	CODi	2482.6	80.7	Lab	[153]
		CODf	479.6
DCMD	8.0	CODi	198.7	99.0	Lab	[164]
		CODf	2.0
ITMD	12.0	CODi	1291.8	99.9	Lab	[162]
		CODf	0.5
MD	7.0	TOCi	602.0	98.9	Lab	[165]
		TOCf	6.5
PVDF	8.3	CODi	1188.0	57.1	Lab	[166]
		CODf	510.0
MBR	8.0	CODi	5000.0	91.7	Lab	[167]
		CODf	417.0

Note: i refers to initial; f refers to final; AMBR refers to aerobic membrane bioreactor; DCMD refers to direct contact membrane distillation; ITMD refers to integrated two-stage membrane distillation; MD refers to membrane distillation; PVDF refers to polyvinylidene fluoride; MBR refers to membrane bioreactor.

3.5. Leachate Recirculation

Leachate recirculation is a technique that involves reintroducing the leachate generated in landfills back into the landfill to accelerate the biological degradation process of the waste. This process helps reduce the volume of leachate produced and enhances the stability of the landfill [168]. However, successful implementation of leachate recirculation requires specific conditions to be met. Otherwise, it may pose environmental risks. Therefore, the conditions for leachate recirculation must be strictly controlled: (1) a comprehensive anti-seepage system, drainage system, and gas collection system must be in place; (2) the leachate must exhibit high biodegradability, with pollutant concentrations (e.g., COD, NH₃-N, and heavy metals) maintained within acceptable limits to prevent cumulative contamination; (3) environmental conditions must be considered, prioritizing geologically stable regions with moderate precipitation levels.

3.6. Cost of Technologies

The cost of technologies for treating landfill leachate varies depending on factors such as the treatment type, scale of operation, equipment costs, and operation and maintenance expenses. Below is an overview of several common technologies and their associated cost profiles (

Table 7. The cost of technologies for treating landfill leachate.

Treatment Process	Technologies	Equipment Investment (CNY/ton·Day)	Operational Cost (CNY/ton)	Characteristics
Physicochemical treatment	Coagulation sedimentation, adsorption, membrane separation (ultrafiltration, reverse osmosis)	500–2000	10–30	Suitable for small-scale treatment; low initial investment but high maintenance costs for membrane separation in later stages.
Biological treatment	Aerobic, anaerobic, MBR (Membrane Bio-Reactor)	2000–5000	20–50	Effective for refractory organic compounds; high operational costs.
Advanced oxidation	Fenton oxidation, ozone oxidation, photocatalytic oxidation	5000–10,000	50–100	Suitable for high-concentration leachate; high energy consumption and capital/operational costs.

).

3.7. Advantages and Disadvantages of Treatment Technologies

The treatment of landfill leachate encompasses a diverse array of technological approaches, each exhibiting unique strengths and limitations, with no singular universally optimal solution currently established [169]. This technological diversity reflects the complex and variable nature of leachate characteristics across different landfill sites and operational conditions. Each technical approach presents distinct advantages and disadvantages (as shown in

Table 8. Advantages and disadvantages of treatment technologies [170].

Technologies		Advantages	Disadvantages
Physical method	Adsorption	• Simple operation • Adsorbent reusability • Effective for low-concentration organics and heavy metals	• High adsorbent costs • Complex regeneration protocols • Limited efficacy for high-concentration wastewater
	Coagulation–flocculation method	• Cost-effective • Minimal infrastructure requirements • Efficient removal of suspended solids and select organics	• Limited effectiveness against soluble organic compounds • Generates sludge requiring additional treatment
Advanced oxidation progress		• Rapid degradation of refractory organics • High reaction efficiency	• Elevated operational costs • Secondary pollution risks
Biological method	Aerobic	• Low operational costs • Effective organic matter and ammonia nitrogen removal	• Inefficient for refractory organics • Performance instability under fluctuating water quality
	Anaerobic	• Suitable for high-organic wastewater • Energy-efficient with biogas recovery potential	• Prolonged startup period • Sensitive to temperature/pH variations • Post-treatment effluent requirements
Membrane technology		• High separation efficiency • Superior effluent quality • Adaptable to high-concentration wastewater	• Membrane fouling susceptibility • High membrane replacement costs • Significant energy demand

).

4. Conclusions and Prospects

The treatment of landfill leachate currently faces both challenges and opportunities. This article provides an overview of the characteristics and composition of landfill leachate, summarizes existing treatment methods, and compares their respective advantages and disadvantages. While multiple treatment technologies have been implemented in this field, each exhibits unique advantages and limitations. From an economic perspective, physical methods stand out for their low cost and operational simplicity. In terms of treatment efficacy, chemical oxidation demonstrates strong performance, yet its high expenses and risk of secondary pollution remain concerns. Under green development principles, biological methods show distinct advantages—particularly in removing nitrogen- and carbon-containing organic compounds—though their effectiveness in degrading refractory substances remains limited. Membrane separation technology has achieved notable success in leachate treatment, but challenges such as membrane fouling, contamination risks, and high operational costs hinder its widespread adoption. Moreover, as the variety of emerging contaminants detected in landfill leachate continues to grow, traditional treatment technologies struggle to effectively remove them. Compounded by the limitations of single-process approaches, which fail to address the diversity and complexity of pollutants, hybrid treatment systems integrating physical, chemical, and biological processes have emerged as a critical strategy to resolve these challenges.

Despite ongoing advancements, landfill leachate treatment still faces unresolved challenges, including emerging contaminants and efficiency-cost trade-offs in existing technologies. Given its high environmental risks, the field urgently requires innovative, integrated solutions to achieve efficient, eco-friendly, and cost-effective treatment, driving sustainable development.