An Extensive Analysis of Combined Processes for Landfill Leachate Treatment

Abstract

Sanitary landfilling is the predominant process for solid urban waste disposal, but it generates leachate that poses environmental, economic, and social concerns. Landfill leachate (LL) contains complex and refractory pollutants and toxic compounds that can vary depending on landfill maturity, age, and biochemical reactions, making its treatment challenging. Due to its unique characteristics and occurrence in remote locations, LL requires separate treatment from wastewater. Various conventional treatment processes involving biological, chemical, and physical processes have been used for LL treatment, but a single treatment process is insufficient to meet environmental standards. This review demonstrates that combined treatment processes are more effective and efficient for LL treatment compared to single processes. Among the various combinations, chemical–chemical and chemical–biological treatments are the most commonly used. Specifically, the integration of Fenton with adsorption and a membrane bioreactor (MBR) with nanofiltration (NF) processes shows promising results. The combined processes of MBR with NF, Fenton with adsorption, and PF with biological treatment show maximum removal efficiencies for COD, reaching 99 ± 1%, 99%, 98%, and 97%, respectively. Additionally, the combined Fenton with adsorption process and EC with SPF process enhance biodegradability as indicated by increased BOD₅/COD ratios, from 0.084 to 0.82 and 0.35 to 0.75, respectively. The findings emphasize the importance of developing and implementing enhanced combined treatment processes for LL, with the aim of achieving efficient and comprehensive pollutant mineralization. Such processes have the potential to address the environmental concerns associated with LL and contribute to sustainable waste management practices.

1. Introduction

The increasing industrialization, urbanization, and global population have led to a greater emphasis on achieving a high quality of life and well-being for the population. As a consequence, there has been a significant increase in the generation of municipal solid waste (MSW). The sanitary landfill process has been used and will continue to be the most popular choice for disposing of MSW due to its economic benefits [1]. Usually, at the landfill site, the waste is placed in thin layers and compressed to reduce its size as much as possible. Additionally, it is covered periodically with suitable material. During that, many chemical, physical, and biological reactions take place, causing organic compounds to become immersed and undergo decomposition. As rainwater infiltrates the waste, it combines with these decomposed materials, resulting in the formation of a highly contaminated liquid known as “leachate” [2,3,4]. This leachate may eventually seep into the soil and then into the groundwater as it drains to bodies of surface water [5,6]. Leachates may include high concentrations of organic matter (OM), ammonium nitrogen [7], heavy metals [8], and chlorinated inorganic and organic salts [9]. Organic pollutants in leachate are characterized using global parameters, namely total organic carbon (TOC) [10], 5-day biochemical oxygen demand (BOD₅) [8], and chemical oxygen demand (COD) [11].

The characteristics of leachate can vary depending on parameters such as climate changes, temperature, precipitation, waste composition, and moisture content of buried solid waste [12,13]. These variations may occur from site to site and from time to time [13]. Landfill leachate (LL) is characterized by its composition and generation rate, both of which are affected by the age of the landfill site. Based on the age of the landfill, leachate can be classified into three categories: young (acid-phase, <5 years), intermediate (5–10 years), and mature or “stabilized” (methanogenic-phase, >10 years) [14]. In young landfills, leachate exhibits high levels of COD (>10,000 mg/L) and a high ratio of BOD₅/COD (ranging from 0.5 to 1). On the other hand, leachate from stabilized landfills is characterized by relatively lower COD levels (less than 4000) and a low BOD₅/COD ratio (less than 0.1) [14]. Improper disposal of leachate from landfills represents a significant contamination source that can result in severe risks to soil and water ecosystems. Additionally, it poses a substantial risk to the health of residents, disrupts ecological balance, and disrupts regional element cycles [14,15,16]. Reports by Foo and Hameed [14], SQ et al. [17], and Bashir et al. [18] have highlighted the potential risk of highly polluted leachate seeping into the earth and polluting groundwater, surface water, and soils. Therefore, it is of utmost importance to ensure proper treatment of generated leachate before its release into the environment.

Different treatment processes have been used for the treatment of LL, with varying degrees of success [6]. Conventional processes, including physical, biological, and chemical treatment processes, have been used for LL treatment [19]. Chemical treatments, including reverse osmosis (RO) [20] and coagulation–flocculation (C/F) [21], and electrochemical treatments such as electro-Fenton (EF), electro-oxidation (EO), and electro-coagulation (EC) [22,23,24], have also been effective. Physical treatment processes such as adsorption [25] and filtration [25] are commonly used to treat LL. Additionally, various biological treatment processes have been investigated successfully under both aerobic and anaerobic conditions. For instance, Contrera et al. [26] implemented the sequencing batch reactor (SBR) process to effectively treat LL. However, each treatment process has its own set of advantages and disadvantages. After increasing the age of LL, the effluent becomes more complex, the biodegradability (BOD₅/COD) decreases, and bio-refractory pollutants emerge, presenting challenges for the aforementioned treatment processes when used individually [27,28]. Frequently, a single treatment process fails to treat leachate from landfills to a level that meets the necessary standards for environmental release [19,29,30]. Therefore, Mojiri et al. [19] and Jamrah et al. [29] strongly recommend the implementation of a combination of chemical, biological, and physical treatment processes to achieve effective treatment outcomes.

Recently, there has been a growing recognition of the significance of utilizing combined treatment processes that harness the synergistic efficiency of biological, physical, and chemical approaches. Numerous integrated processes have also been put into full practice and tested in laboratories. Up to 85–90% of the pollutants can be reduced by employing two or more of the aforementioned treatment processes [31,32]. In this sense, Jegadeesan et al. [33] showed successful results in eliminating color and COD from stabilized LL (for more than 15 years) by using the combined EC and solar photo Fenton (SPF) processes. Under optimal conditions, the sole employment of the EC process resulted in a removal efficiency of 76% for color and 75% for COD. However, when the leachate was subjected to the SPF process after EC treatment, notable enhancements were observed. The combined treatment approach led to a significant improvement, with color and COD removal efficiencies reaching 91% and 90%, respectively. Additionally, the biodegradability of the leachate increased from 0.35 to 0.73. Tałałaj et al. [34] utilized a combined SBR and RO to eliminate ammonia nitrogen, total organic carbon (TOC), BOD, Cl⁻, and Fe from two types of LL (stabilized and young leachates). The implementation of SBR as a pre-treatment process proved to be highly effective, achieving ammonia nitrogen removal rates exceeding 98% for both leachates. The study has demonstrated that the combined SBR and RO process has a high removal efficiency (more than 80%) for all parameters examined. By combining various treatment processes, the limitations associated with relying solely on a single treatment approach can be overcome, leading to enhanced treatment efficiency and improved outcomes [29,35].

Numerous research and review studies have been conducted, examining both single treatment processes and combined treatment processes for different types of LL treatment. Notably, these studies have primarily focused on the characterization, composition, and generation of LL [6,36]. However, none of these reviews have specifically addressed the need for a comprehensive analysis of the existing literature solely pertaining to integrated treatment processes for LL treatment. The aim of this review is to evaluate the efficacy of different combined treatment processes and to discuss limitations and future perspectives for each process, in addition to discussing its mechanism of action. This manuscript aims to enhance researchers’ understanding of the characteristics of various combined treatment processes, their combinations, and the rationale behind arranging each process as pre- or post-treatment based on leachate characteristics. Additionally, it addresses the identified gaps encountered by many researchers in previous studies. The authors conclude the manuscript by offering recommendations for the effective implementation of combined treatment for LL. The major findings and conditions of the available research will be summarized for each combined process in a table to make comparisons easier. The suggested treatment process for LL is shown schematically in Figure 1.

As depicted in Figure 1, the suggested approach for treating LL is illustrated, which is based on previous research studies conducted by scholars and involves a combination of treatment processes.

2. Methodology

In this review study, a systematic technique was employed to assess the efficacy and efficiency of integrated treatment processes for LL. The initial stage involved identifying relevant, high-quality research articles discussing integrated treatment processes in LL. Subsequently, studies that utilized appropriate analysis processes to ensure the applicability and reliability of their findings were selected. This review encompasses a comprehensive compilation of papers published between 2011 and November 2023, which was then screened to include any research that could be pertinent. A filtering process was applied to exclusively include articles from scholarly journals with a high impact factor that are indexed in the Web of Science and Scopus databases. According to the type of applied process, the relevant, high-quality research articles were categorized, and their key findings were compared and discussed (see Figure 2). Through this analysis, several conclusions were drawn regarding the main objectives of integrated treatment processes. These objectives include achieving high efficiency in treating LL and attaining a level of wastewater discharge that complies with specific standards and requirements. This review also highlights the importance of implementing integrated treatment processes to achieve these objectives effectively.

3. Leachate from Landfill

The quality of leachates is influenced by various factors, including their waste type, seasonal weather variations, chemical composition, age, and precipitation. Particularly, the composition of LL is significantly influenced by landfill age [37]. The fundamental metrics of heavy metals, total Kjeldahl nitrogen (TKN), suspended solids (SSs), COD, BOD₅, pH, the BOD₅/COD ratio, and ammonia nitrogen (NH₃-N) are often utilized to characterize the properties of leachate. According to the age of the landfill, leachate can be categorized into three different categories, as illustrated in

Table 1. Summary of the relationship between landfill age and LL characteristics (NA: not available, VFAs: volatile fatty acids, HA: humic acid, and FA: fulvic acid) adapted from [14]. (CCC License Number 5751491181329.)

Type of Leachate	Young	Intermediate	Stabilized
Age (years)	<5	5–10	>10
pH	<6.5	6.5–7.5	>7.5
COD (mg/L)	>10,000	4000–10,000	<4000
BOD5/COD	0.5–1.0	0.1–0.5	<0.1
Ammonia nitrogen (mg/L)	<400	NA	>400
TOC/COD	<0.3	0.3–0.5	>0.5
Heavy metals	Low to medium	Low	Low
Biodegradability	High	Medium	Low
Kjeldahl nitrogen (g/L)	0.1–0.2	NA	NA
Organic compound	80% VFA	(5–30%) HA + FA + VFA	HA + FA

[14].

As shown in Table 1, it can be seen that the water quality of stabilized LL is characterized by alkaline with pH > 7.5, low COD below 4000 mg/L, high contamination with NH₃-N content over 400 mg/L, and poor biodegradability (BOD₅/COD). The water quality of fresh LL is characterized by being acidogenic with a pH < 6.5, high COD over 10,000 mg/L, low ammonia nitrogen concentration below 400 mg/L, and strong biodegradability. The water quality of intermediate LL is between the fresh phase and the stabilized phase.

Precipitation plays a significant role in leachate production. Waste is typically layered, compressed, and periodically covered. Within waste cells, various chemical, physical, and biological processes occur, transforming waste materials into pollutants. Rainfall, along with the moisture present in the waste and cover material, leads to the generation of leachate (Figure 3) [38,39]. Leachate generation is also influenced by factors such as surface runoff, groundwater infiltration, evapotranspiration, oxidation, and microbial degradation [40,41,42].

As shown in Figure 3, rainfall contributes to the distribution of water within the waste and thus distributes the polluted water to the soil and in a lateral direction. Throughout the landfill’s operational lifespan, the quantity and composition of leachate vary based on rainwater percolation, waste moisture content, landfill compaction, and biological/chemical processes [43,44]. Less compacted waste generally results in higher leachate production due to reduced filtration rates [44].

The biological and chemical reactions that occur within the landfill were divided into four stages, as illustrated in Figure 4 [45]:

• Aerobic stage;
• Anaerobic and acidogenic stage;
• Unstable methanogenic stage;
• Stable methanogenic stage.

Figure 3. Generation of leachate in landfill adapted from [6,19].

Figure 3. Generation of leachate in landfill adapted from [6,19].

Figure 4. Scheme of biological treatment of LL. 1 = aerobic stage; 2 = anaerobic acidogenic stage; 3 = unstable methanogenic stage; 4 = stable methanogenic stage [45]. (CCC License Number 5751390347312).

Figure 4. Scheme of biological treatment of LL. 1 = aerobic stage; 2 = anaerobic acidogenic stage; 3 = unstable methanogenic stage; 4 = stable methanogenic stage [45]. (CCC License Number 5751390347312).

According to Figure 4, decomposition takes place under the influence of aerobic bacteria during the aerobic stage of the biochemical activities that take place inside the landfill. When there is a solid–liquid contact between the waste and the surrounding liquids, this process occurs quickly. This process, which ultimately decomposes organic matter into sulfate, nitrate, H₂O, CO₂, and other biodegradable chemicals, is catalyzed by aerobic bacteria. This occurs quite quickly because the O₂ is consumed within a few days [46]. In the anaerobic stage, a series of reactions occur that take place in the absence of O₂. Acid fermentation takes place, bringing the pH down to 5.5–5.6, mostly because CO₂ and VFA are produced. This fermentation is the consequence of anaerobic bacteria interacting with organic waste during its degradation [45,46]. The initial disruption of the unstable methanogenic stage occurs due to the accumulation of a significant quantity of VFA produced during the anaerobic stage. In this stage, both CO₂ and VFA are converted into methane. The duration of this stage can range from a few months to approximately two years, and it leads to a gradual increase in pH levels [45]. Finally, in the stable methanogenic stage, methane production continues until all of the biodegradable organic matter is completely consumed. This process, which takes 15 to 20 years to complete, marks the end of the biodegradation of organic matter in landfills [46]. The methane created during this stage can be utilized as fuel or to produce electricity.

The assessment of the landfill’s projected production of leachate is another important factor. Traditionally, the amount of LL has been estimated based on the concepts of hydrological balance. Equation (1) demonstrates the utilization of the hydrologic evaluation of landfill performance (HELP) model, as recommended by the EPA, to determine the quantities of LL [47,48,49]. This model takes into account various factors, such as the total water volume entering the landfill, including infiltration and percolation, and subtracts it from the water volumes consumed during biological and chemical decomposition, as well as those lost through water evapotranspiration [50,51].

$$\mathrm{L}=\mathrm{P}-\mathrm{R}+{\mathrm{R}}^{*}-\mathrm{E}\mathrm{T}+\left(∆\mathrm{U}\mathrm{S}-∆\mathrm{U}\mathrm{W}\right)+\mathrm{I}\mathrm{G}+\mathrm{I}\mathrm{S}+\mathrm{J}+\mathrm{b}$$

(1)

The hydrologic evaluation of landfill performance involves several components. These components include the amount of leachate (L), surface runoff from outside areas (R*), runoff (R), precipitation (P), evapotranspiration (ET), variations in water content of the capping material (∆US), variation in the water content of the disposed waste volume (∆UW), irrigation and leachate recirculation (J), infiltration water from groundwater (IG), infiltration water from surface water bodies (IS), and the consumption or production of water related to the various anaerobic and aerobic biochemical degradation reactions of organic substances (b). These factors are considered in the evaluation process, and their interactions play a crucial role in determining the overall performance of the landfill [48,49]. The amount of waste generated in many countries ends up in landfills, especially in developing countries, which contributes significantly to the generation of a large amount of leachate. Several studies have detailed the amount of leachate generation and waste disposed of at various landfill locations globally as shown in

Table 2. Information of waste generation, landfill deposits, and climate conditions in different countries.

Country	Climate Conditions	Waste Generation (m3/d)	Amount of Waste Deposited in Landfill (tones/day)	Year of Waste Assessment	Ref.
Koprivnica, Croatia	Alluvial depression with a reductive aquifer, and high iron content	320	-	2011	[52]
Istanbul, Turkey	Humid subtropical	-	2000	2010	[53]
Kulim, Kedah, Malaysia	Humid tropical	-	240	2009–2010	[54]
Jiangmen, Guangdong Province, PR China	Humid subtropical	150–200	750	2011	[55]
Samsun, Turkey	Humid subtropical	-	400	2012	[56]
Wuhan, China	Humid subtropical	-	800	2012	[57]
Penang, Malaysia	Hot and humid	-	2200	2011	[58]
Belo Horizonte/MG, Brazil	Tropical Savanna	-	4200	2009	[59]
Beirut, Lebanon	Mediterranean	-	200	2013	[60]
Odayeri, Istanbul, Turkey	Moderate	2000	-	2013	[61]
Chang Shankou, China	Humid subtropical	400–500	2800	2014	[62]
Çorlu, Tekirdağ, Turkey	Between Mediterranean and humid subtropical	-	110	2014	[63]
Tehran, Iran	Semi-arid	-	3500	2014	[64]
Vila Real, Portugal	Mediterranean	-	205.48	2015	[65]
Zagreb, Croatia	Continental	1000–21,000	800	2015	[66]
Rebat, Morocco	Mediterranean with influences from the Atlantic Ocean	-	700	2017	[67]
Guilan Province, Iran	Humid subtropical	-	500	2016	[68]
Quebéc, Canada	Humid continental	100	180	2016	[69]
Herten, Germany	Temperate Maritime	-	746.73	2017	[70]
Vale dos Sinos, Brazil	Humid subtropical	-	220	2017	[71]
Tallinn, Estonia	Humid continental	16.67–83.33	-	2014–2015	[72]
Shanghai, China	Humid subtropical	3000	10,000	2017	[73]
Dongguan, China	Humid subtropical	315–400	1000	2017	[74]
Konya province, Turkey	Semi-arid	150	-	2018	[75]
Bucaramanga, Colombia	Tropical monsoon with relatively consistent temperatures throughout the year	-	734.3	2017	[76]
Wuhan, China	Humid subtropical	400	2000	2019	[77]
Rio de Janeiro, Brazil	Tropical Savanna	1000	10,000	2019	[78]
Ariyamangalam, India	Tropical Savanna	-	400–600	2020	[79]
Southwest China	Humid subtropical	2000	3527.4	2020	[80]
Silesia, Poland	Humid continental	60	355	2021	[81]
Nantong Jiangsu Province, China	Humid subtropical	-	800–1000	2021	[82]
Setúbal, Portugal	Mediterranean	81.6	273.97	2021	[83]
Mediouna, Casablanca (Morocco)	Mediterranean with influences from the Atlantic Ocean	-	3500	2021	[84]
Penang, Malaysia	Tropical rainforest with consistently high temperatures and abundant rainfall throughout the year	200	-	2021	[85]
Hanoi, Vietnam	Humid subtropical	-	6499.46	2021	[86]
Sabar’a/Minas Gerais, Brazil	Humid subtropical	600	3400	2021	[87]
North-west Poland	Humid continental	50–90	-	2021	[34]
Pongsu, Perak, Malaysia	Tropical rainforest	-	198.416–220.46	2022	[88]
Rabat-Salé-Kénitra, Morocco	Mediterranean with influences from the Atlantic Ocean	480	-	2022	[89]
Vellakal, Avaniyapuram, India	Tropical Savanna	-	450	2022	[33]
Dhapa, Kolkata, India	Tropical Savanna	-	3000	2022	[90]
Ranchi, Jharkhand, India	Tropical Savanna	-	700	2023	[91]

.

Data on the waste generation and the amount of waste deposited in landfills, as indicated in Table 2, are an important and crucial element in leachate management and should not be disregarded easily. Climate conditions greatly affect the amount of waste generated from landfills. During the rainy season, there is a lot of rain, which increases the moisture content in landfills and thus increases the amount of waste generated from landfills. Based on Table 2, it was found that countries with climate conditions tropical Savanna, continental, and humid subtropical, which are characterized by heavy rain, generate more waste compared to climate conditions humid continental, Mediterranean, and semi-arid. Croatia had the highest amount of waste generation, ranging from 1000 to 21,000 mg/L, due to continental climate conditions.

The composition of LL varies in different regions around the world based on local factors related to waste management, the conversion process used, and the age of the landfills. The classification of LL in various countries around the world is shown in

Table 3. Composition of young leachate from some significant landfills.

Country	COD (mg/L)	TOC (mg/L)	BOD5 (mg/L)	BOD5/COD	pH	TSS (mg/L)	TN (mg/L)	NH4+-N (g/m3)	Ref.
Young Landfills
China	14,768				7.9				[92]
Iran	11,280 ± 300	3750 ± 100	1300 ± 100	0.1152	6.21 ± 0.05	3940 ± 350			[64]
	68,250		8500	0.1245	6	8300			[93]
Turkey	11,000		6400	0.582	7.95	2853		1247	[56]
Morocco	22,000		8500	0.39	7.61	33,940			[84]
Mexico	14,680 ± 3225		1500 ± 500	0.102	8.04 ± 0.17				[94]
	10,193	4950	861	0.0845	8.13	360	2113		[95]
Saudi Arabia	16,874				6.21	5604			[96]
India	19,800–24,000		4250–5100	0.215	6–8.1	110–498		1530–4000	[97]
	14,420				5.1				[98]
Spain	14,200	5600			8.56				[99]
Chile	11,950				8.9				[100]

,

Table 4. Composition of intermediate leachates from some significant landfills.

Country	COD (mg/L)	BOD5 (mg/L)	BOD5/COD	pH	TSS (mg/L)	TN (mg/L)	NH4+-N (g/m3)	Ref.
India	5120	200–150	0.04–0.06	8–9.5				[33]
China	6880 ± 180	572 ± 56	0.083	8.5 ± 0.5			2402 ± 48	[101]
	4500 ± 113			8.6 ± 0.06				[102]
Morocco	6730	600	0.089	8	19,128			[103]
Turkey	6400	1418	0.22	9		1836.6		[104]

and

Table 5. Composition of stabilized leachates from some significant landfills.

Country	COD (mg/L)	TOC (mg/L)	BOD5 (mg/L)	BOD5/COD	pH	TSS (mg/L)	TN (mg/L)	NH4+-N (g/m3)	Ref.
China	1763.8 ± 33.6	294.3 ± 11.7			7.9 ± 0.1		1496.3 ± 11.5	884.3 ± 6.8	[105]
	3200.35	1200	75	0.0234	7.6			55	[106]
	3424–3680				8.5–8.6		2713–2796		[57]
	2290 ± 57.83	431 ± 10.21			7–9		2760 ± 25		[82]
Malaysia	2025	860	93	0.043	8.5				[58]
Portugal	4505		300	0.07	7.6	337	1780		[107]
Brazil	3332		141	0.042	8.3	53			[108]
Vietnam	2100–4500				7.5–8.3				[109]
Algeria	3847.7		388	0.11	8.1	10.4			[110]
Poland	3135 ± 14.5	614 ± 3.15	165 ± 2.5	0.053	8.4 ± 0.2		824.5 ± 11.8		[81]

based on the age of the landfill: young, medium, and mature, respectively.

As summarized in Table 3, Table 4 and Table 5, the leachate compositions reported in the literature from different sites show a wide range of variation. In contrast to stabilized LL, young LL processes extremely high concentrations of all chemical parameters, with the exception of pH, as seen in Table 3 and Table 4. The range of COD concentration is from 10,193 mg/L [95] to 68,250 mg/L [93] for young LL and 1150 [111] to 4505 [107] for stabilized LL samples. The biodegradability (BOD₅/COD ratio) of young LL varied from 0.0845 [95] to 0.582 [56]. The biodegradability (BOD₅/COD ratio) of stabilized LL varied from 0.0234 [106] to 0.11 [110]. The pH value of the young LL was observed in the range of 5.1 [98] to 8.9 [100], but the stabilized LLs were alkaline with a pH range of 7 to 9 [82]. This might be explained by the fact that the quantity of free volatile acids in the anaerobic component has decreased, as fatty acids can partially ionize and raise pH levels. Based on Table 3, Table 4 and Table 5, most of the organic LLs referred to as COD were located in Morocco (COD = 22,000 mg/L), China (COD = 6880 ± 180 mg/L), and Algeria (COD = 3847.7 mg/L) for young, intermediate, and stabilized LL.

4. Leachate Transfer

4.1. CO-Treating Landfill Leachate and Urban Wastewater

Due to the high ammonium concentration, high COD/BOD₅ ratio, low biodegradability, and presence of heavy metal ions in LL, the biological treatment of leachate is more challenging than that of treating industrial and municipal wastes [54,112]. LLs are currently commonly treated in urban wastewater treatment plants with urban wastewater. However, the need for separate treatment and disposal of LL has grown as a result of tighter regulations on nitrogen discharge and issues with the possible impact of resistant leachate elements on the biological treatment phase [113]. Researchers have also suggested treating wastewater and LL together [112].

Co-treatment of LL with urban wastewater sources at wastewater treatment plants has emerged as a cost-effective, easy maintenance, and viable solution [114]. Co-treatment of LL with wastewater has the advantage that neither of the nutrients has to be added at the wastewater treatment plant because the former often includes surplus nitrogen and the latter excess phosphorous (whereas leachates typically have low phosphorous levels).

Table 6. Merits and demerits of co-treatment of LL with wastewater.

Merits	Cost-effective and convenient solution with easy maintenance and operational simplicity, requiring only LL transportation. It enhances biodegradability, increases the COD/BOD5 ratio, and promotes microbial degradation, making it suitable for biological treatment. No additional nutrients are required for the wastewater treatment plant, as LL provides the nitrogenous fraction and the wastewater supplies the necessary phosphorus. The higher organic matter content fosters microbial degradation and the stabilization of microorganisms. The alkalinity present in LL during the methanogenic stage supports anaerobic treatments by maintaining pH levels, eliminating the need for external pH correction processes. The dilution of LL with wastewater and the adaptability of activated sludge facilitate the biodegradation of organic pollutants. This approach is particularly well suited for developing countries that have ample available land and limited financial resources.	[41,91,115,116,117,118,119,120,121,122]
Demerits	Metals require particular consideration due to their inhibitory effects on heterotrophic and nitrifying bacteria. Notably, zinc levels in LL during the acidogenic stage are significantly elevated. Refractory compounds and heavy metals can suppress the activity of microorganisms responsible for breaking down organic matter in the activated sludge process and render it unsuitable for utilization as a fertilizer. Increased sludge production in wastewater treatment plants includes the inability to reuse it, which is caused by the additional organic LL load and alterations in its composition. The presence of heavy metals and organic inhibitory compounds with low BOD5/COD in the LL can lead to increased effluent concentrations and reduce treatment efficiency, raising concerns about the viability of this option. The high concentrations of inorganic and organic components are attributed to the stabilized and fresh LL, respectively. The removal efficiency decreased as the leachate ratio in the wastewater mixture and LL increased. The precipitation of iron oxides can lead to corrosion, hindered sludge settling, and operational problems in the plant. The removal of bioaccumulative substances and refractory organic compounds may not be efficiently achieved. The treated effluent may exhibit an elevation in recalcitrant compounds. Insufficient alkalinity can hinder the nitrification process.	[41,116,118,120,122,123,124,125]

summarizes the merits and demerits of co-treating LL with wastewater.

However, due to the demerits in Table 6, efforts were required to optimize the volume distribution of LL and wastewater in order to mitigate the coexistence of nitrogen in LL and phosphorous in wastewater [118,121]. Few studies have been recently conducted on the feasibility of co-treatment and the ideal ratio of leachate to wastewater that ensures optimal treatment performance and preserves the quality of the final effluent [118]. A summary of some co-treatments of LL with wastewater and their efficiency are listed in

Table 7. A summary of co-treatment of LL with wastewater processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Adsorption + SBR	Stabilized	Aeration rate = 2.87 L/min, time = 11.7 h.	The removal efficiencies significantly increased to 76.96%, 79.29%, 73.38%, and 79.57% for Cd, Ni, Mn, and Fe, respectively.	______	[116]
Constructed wetland + adsorption	Stabilized	Reaction time was 50.2 h for color, Cd, NH4+-N, and COD removal, while the reaction time was 49 h for Ni removal.	The optimum removal efficiencies are 99.2% for NH4+-N, 87.1% for Cd, 86% for Ni, 86.7% for COD, and 90.3% for color. Use a plant for heavy metal sequestration.	______	[124]
SBR + EC	Intermediate	The SBR process has a 16 h oxic phase and a 7 h anoxic phase. For the EC process, reaction time = 30 min and current density = 257 A/m2, with Al as the anode and stainless steel as the cathode with electrode spacing of 5 cm.	The overall reduction efficiencies reached 98% for COD, 98% for TSS, and 99% for ammonia, nitrate, and phosphate. Potential for wastewater reuse.	The study reported that the efficiency of the EC process declined over time due to an increase in pH. This decline in efficiency may affect the removal of COD, particulate matter, and nutrients, potentially impacting the overall treatment performance.	[119]
Adsorption + SBR	Stabilized	Contact time = 22 h, HRT = 10 d, SRT = 30 d.	The GAC-SBR system demonstrated efficient removal of nutrients (80–90%) and COD (83%) from stabilized LL.	______	[122]
SBR + C/F	Stabilized	For the SBR process, the HRT is 6 days, and the SRT is 30 days. For the C/F process, the ferric chloride dosage is 470 mg/L, and the alum dosage is 2800 mg/L.	In the SBR and C/F processes, the total removal rates were 93% for TSS, 89% for COD, 98% for turbidity, 82% for nitrate, and 83% for ammonia using alum. With ferric chloride, the combined treatment resulted in removal rates of 94% for TSS, 90% for COD, 98% for turbidity, 83% for nitrate, and 86% for ammonia. The combined treatment met international standards for discharges to inland surface water.	The study mentions the use of chemicals such as aluminum and ferric chloride, along with the innovative flocculant, in the C/F process. The use of these chemicals may raise environmental and economic impacts, so it is necessary.	[91]

.

As indicated in Table 7, two studies conducted by Mojiri [116,124] examined the co-treatment of LL with different wastewater sources and evaluated the efficiency of various treatment processes. In the first study conducted by Mojiri [116], the researchers compared the effectiveness of an SBR system with and without the use of ZELIAC powder (PZ) in removing heavy metals from LL and household wastewater. The results showed that the use of PZ-SBR was more effective in removing heavy metals compared to SBR alone. The study highlights the effectiveness of integrating different treatment processes for treating LL and household wastewater and improving pollutant removal and treatment efficiency.

The second study by Mojiri [124] focused on co-treating LL and municipal wastewater (MWW) using a constructed wetland (CW) and an adsorption process, as illustrated in Figure 5.

The study showed that using a CW consisting of two layers of adsorbent materials (zeolite and ZELIAC) resulted in effective pollutant removal, as shown in Figure 5. The accumulation of nickel and cadmium in the roots and shoots of Typha domingensis plants cultivated in the CW was also monitored. These studies shed light on the effectiveness of integrating different treatment processes for treating wastewater and dairy wastewater and improving pollutant removal and treatment efficiency.

Chakraborty [119] conducted an assessment of a combined process involving SBR and EC for the co-treatment of LL and MWW. In the experiment, SBR was initially used to co-treat a mixture of 20% (v/v) LL and MWW. The effluent from the SBR was then treated further using EC for post-treatment. When subjected to post-treatment by EC, the overall reduction efficiencies reached 98% for COD, 98% for TSS, and 99% for ammonia, nitrate, and phosphate. During the EC process, the pH increased over time, resulting in a decline in EC efficiency, leading to the removal of COD, particulate matter, and nutrients. Based on these findings, the study suggests that using SBR followed by EC as a post-treatment option can be an effective approach for treating a mixture of LL and MWW. However, the study indicates that the efficiency of the EC process decreases over time due to an increase in pH, which may affect the removal of COD and other organic matter.

In a study by Kumar [91], innovative flocculants combined with an SBR were found to be effective in treating LL and MWW to reduce pollution. The SBR treatment efficiency decreased as leachate concentration increased, with COD removal rates ranging from 58% to 70%, phosphate removal rates from 69% to 95%, ammonia removal rates from 86% to 93%, and nitrate removal rates from 76% to 83%. C/F treatment using ferric chloride and alum combined with a novel flocculant (GGI-g-PAM) achieved COD removal rates of 77% and 74%, respectively, at a leachate concentration of 20%. The combined SBR and C/F treatment at a 20% LL-to-MWW ratio resulted in total removal rates of 93% for TSS, 89% for COD, 98% for turbidity, 82% for nitrate, and 83% for ammonia using alum. The study concludes that using novel flocculants with SBR offers an efficient approach for treating LL and MWW, meeting international standards for water discharge.

Verma [122] conducted a study using granular activated carbon–SBR (GAC-SBR) to co-treat LL with MWW. The researchers examined the GAC-SBR system at different mixing ratios of LL and MWW and various HRTs. The treatment efficiency of the GAC-SBR was evaluated based on parameters such as COD, phosphate, nitrate, ammonia, turbidity, mixed liquor suspended solids (MLSSs), and mixed liquor volatile suspended solids (MLVSSs) removal. A univariate analysis of variance (ANOVA) was employed to determine the statistical significance of the treatment at different LL-to-MWW ratios. The findings indicated that increasing the GAC concentration resulted in enhanced removal of ammonia and COD from stabilized LL. However, the adsorption efficiency either decreased or remained constant after reaching a GAC concentration of 15 g/L.

The combination of SBR with other techniques has been demonstrated to be a highly efficient approach compared to alternative methods. Scientific studies have provided evidence that incorporating SBR with techniques such as adsorption or chemical processes like C/F can significantly enhance the removal efficiency of various pollutants. Specifically, the combined method of SBR with adsorption has shown promising results in eliminating contaminants such as ammonia, nickel, and phosphate. To achieve high removal efficiency of these pollutants, adsorbent materials like zeolites and ZELIAC materials can be employed. Furthermore, experimental findings have indicated that integrating adsorption followed by SBR can improve the removal efficiency of food and organic substances. The use of GAC in this process can effectively remove COD. However, it is important to consider additional factors in order to further enhance the method. One potential factor worth exploring is the impact of temperature on removal efficiency. Research has shown that higher temperatures can enhance adsorption processes and improve overall efficiency. Therefore, investigating the relationship between temperature and removal efficiency could be beneficial in optimizing the combined SBR with the adsorption process.

4.2. Recycling

Leachate recycling is a process used in landfills to control and promote physical, biological, and chemical processes. The aim is to use the substances and resources found in decomposed solid waste that collect in landfills, add them again to the landfill, and create a procedure known as a bioreactor landfill [115,126,127]. This process has been shown to have several benefits. The recycling landfill eliminates the need for treatment plants, and thus the costs of the process become low [115,128]. Studies have demonstrated that leachate recirculation increases moisture content and facilitates the distribution of nutrients and enzymes, leading to improved leachate quality [129]. It also accelerates landfill stabilization, which leads to reducing land use, reducing the required time from several decades to 2–3 years, and reducing environmental pollution [130]. Leachate recycling can decrease the COD and BOD₅ concentrations of leachate over time, but it may also result in a low BOD₅/COD ratio and an increase in nitrogen concentration [131,132]. Leachate recirculation enhances waste decomposition, landfill gas production, settlement, and leachate treatment. It also provides conditions for advanced leachate treatment. Long-term recirculation, however, may result in a buildup of ammonia nitrogen, which prevents waste from decomposing [133].

Micro-aeration can mitigate this issue by promoting in situ nitrification and denitrification [134]. Overall, leachate recycling is an effective technique for landfill management, but careful management is required to avoid potential drawbacks and optimize results [127].

5. Combined Treatment Processes

Previous research has proven the effectiveness of applying integrated treatment processes to treat LL and has shown that a variety of combined processes are quite successful at eliminating specific contaminants from LL. The treatment processes of LL are classified into four categories, including (1) chemical processes; (2) physical processes; (3) biological processes (anaerobic or aerobic); and (4) leachate transfer [135]. Chemical treatment processes, such as C/F [65], precipitation [136], electrocoagulation (EC) [7], and Fenton [137,138], are applied to enhance biodegradability and remove metals, colloids, and organic compounds from LL [38]. Physical treatment processes, such as reverse osmosis (RO) [34], adsorption [82], filtration (nanofiltration (NF) [139], ultrafiltration (UF) [140], and microfiltration (MF)) [141], agitation [62], air stripping [142], and sedimentation [143], are used to remove metals, organic compounds, and ammonia [144]. Biological treatment processes, such as sequencing batch reactors (SBRs) [37], air lagoons [71], trickling filters [145], and membrane bioreactors (MBRs) [146], are utilized to remove biodegradable total nitrogen and organics due to their reliability, operational cost, and suitability. The evaluation of the biological technique operating circumstances is made possible by control relationship conditions such as the sludge retention time (SRT), food–microorganism ratio (F/M), and hydraulic retention time (HRT) and cell residence time (sludge age) [1]. Leachate transfer processes, such as co-treatment of leachate with wastewater [91], evaporation [11], and recycling of nutrients from wastewater, thus save energy consumption or produce biofuels [135]. The final waste of treatment (such as sludge and biomass) can be used as agricultural fertilizers [144]. Due to the large quantity of OM, chlorinated inorganic and organic salts, and heavy metals present in LL [7,8], none of these treatment processes can successfully decrease contaminants to an acceptable level on their own [27,147,148].

Therefore, combining treatment processes can leverage the advantages of each process to overcome challenges and limitations, resulting in treated wastewater that meets environmental regulations and can be safely discharged into water bodies. The following sections provide a breakdown of the different treatment processes categorized by the type of integrated treatment processes.

5.1. Chemical + Chemical Integrated Processes

The treatment of LL is based on the chemical composition of the leachate. LL typically contains various challenging components such as recalcitrant and heat-resistant organic matter, toxic substances, suspended and colloidal solids, heavy metals, and humic substances [64,105]. Humic substances are large anionic molecules with different molecular weights, and LL often has a low ratio of BOD₅/COD < 0.1 [149]. The presence of humic substances and non-degradable organic matter makes biological treatment processes less effective [149]. Therefore, a combination of chemical processes is employed for the treatment of LL. Chemical processes, including Fenton, C/F, and EC, have been successfully used for LL treatment. These processes are often used in combination with each other. Other AOPs such as EF, PF, SPF, oxidation, ozonation, and photocatalysis are also used. While individual chemical processes have limited effectiveness in treating LL, their efficiency can be enhanced by employing pre- and post-treatment steps [58]. Pre-treatment steps are employed to remove suspended and colloidal solids from LL, while post-treatment steps focus on decomposing soluble organic compounds and removing refractory organic matter, including heavy metals, pesticides, humic and fulvic acids, and non-biodegradable substances [64]. These steps are necessary due to the presence of these substances in LL. The sequential execution of combined chemical treatment processes enables the efficient removal of COD from LL.

The combination of chemical processes is particularly advantageous for treating organic compounds that are difficult to biodegrade and have high toxicity.

Table 8. A summary of the reviewed chemical + chemical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Fenton + ozone-based advanced oxidation processes (AOPs)	Stabilized	Reaction time = 40 min, Fe2+ dosage = 4 mmol/L, H2O2/Fe2+ molar ratio = 3, and pH = 3.	COD removal efficiency was 46% and 72% for single Fe2+/H2O2 and (Fe2+/H2O2 + ozone), respectively. Enhanced BOD5/COD from 0.01 to 0.15 and 0.15 to 0.24 for single Fe2+/H2O2 and (Fe2+/H2O2 + ozone), respectively. Resulted in a significant improvement in leachate quality.	Moderate COD removal rate. The achieved BOD5/COD ratio of 0.4 fell short of the minimum requirement for effective biological treatment. This indicates that the LL is refractory in nature.	[147]
EC + EF	Young	For EC: Fe sacrificial anode, initial pH = 6.54, conductivity = 16.4 mS/cm, EC time = 180 min, and current density = 30 mA/cm2. For EF: operating time = 60 min, and H2O2 = 5000 ppm.	COD removal efficiency of 65.85% and 74.21% for single EC and EC+EF, respectively. No production of secondary pollutants.	Moderate COD removal efficiency. High energy consumption of 11.092 kWh/kg of COD for the EC and EF processes. This means that a significant amount of energy is consumed to achieve a certain level of COD removal. Cathode consumption of 0.418 kg of COD/kg of Al during the EC process.	[56]
Ozone + per sulfate oxidation	Stabilized	pH = 10, COD/S2O82− ratio = (1/7), and ozonation time = 210 min.	The removal efficiency of 55% for NH3-N, 93% for color, and 72% for COD. Enhanced the biodegradability from 0.05 to 0.29. Ozonation increases the oxidation potential of organic compounds.	Moderate removal rates. High ozone consumption rate of 0.76 kg of O3/kg of COD for COD removal. Potential formation of harmful by-products.	[58]
Coagulation + Fenton	Young	pH = 7, FeCl3 dosage = 1500 mg/L.	The removal efficiency of TSS, color, and COD was 97%, 73%, and 65%, respectively. Enhanced the biodegradability from −0.51 to +1.7. Improved oxidative quality of the treated LL. Reduced photo-toxicity.	Potential limitations related to the experimental setup.	[64]
C/F + (Fenton or PF)	Stabilized	For C/F: FeCl3 dosage = 1400 mg/L, and pH = 4. For (C/F + Fenton): H2O2 dosage = 1330 mg/L, Fe2+ dosage = 266 mg/L. For single PF: H2O2 dosage = 2720 mg/L, Fe2+ dosage = 544 mg/L.	The removal efficiency of COD was 83.3%, demonstrating its effectiveness. Easy operation. Cost-effective. High oxidation performance. Process simplicity. Uses inexpensive chemicals.	-	[108]
C/F + (Fenton or PF)	Stabilized	pH = 5, FeCl3 as coagulant with dosage = 2 g/L, settlement time = 1 h, slow mixing of 20 rpm for 20 min, and rapidly mixing of 120 rpm for 3 min.	The removal efficiency of COD was 89%, demonstrating its effectiveness. High reduction in DOC of 75%. Increased biodegradability. Non-toxicity. Improvement in the quality of the treated LL to a level suitable for release into public wastewater. Meeting the legal limit for COD of 1000 mg/L.	Required several days to achieve effective results. Inability to discharge the treated water into natural water bodies.	[65]
C/F + microelectrolysis Fenton	Intermediate	Initial pH of 3.2, 400 mL of LL, Fe-C dosage of 104.52 g/L, H2O2 dosage of 3.57 g/L, and polyaluminum chloride (PAC) as a flocculant.	The removal efficiency of COD and humic acid (HA) was 90.27% and 93.79%, respectively. Cost-effective. Environmentally safe.	-	[101]
C/F + UV oxidation	Stabilized	For C/F, FeCl3 was used as the coagulant. Reaction time = 1 h, oxidant dosage = 12 Mm, settlement time = 1 h, slow mixing of 50 rpm for 30 min, fast mixing of 150 rpm for 2 min, Potassium peroxy-mono-sulfate (PMS) and potassium persulfate (PS) dosages are 12 mM.	The combined C/F + UV/PMS and C/F + UV/PS demonstrates its efficiency by COD removing 91.5% and 90.9%, respectively. Reduction in ecotoxicity of the treated LL, as demonstrated by a decrease in toxicity units (TUs) from 10.1 to 1.74 following the C/F process.	The presence of sulfate radical ions in the treated effluent resulted in a slight increase in ecotoxicity after the UV/PMS process, rising to 1.8.	[148]
Coagulation/sedimentation + SPF	Intermediate	pH = 8, slow mixing of 40 rpm for 30 min, fast mixing of 300 rpm for 2 min, PAC as coagulant with a dosage of 1.5 g/L, and pH = 8.	The COD removal efficiency was 73% and 80% for C/S using PAC followed by SPF and C/S using FeCl3:6H2O followed by SPF, respectively. Cost-effective: The combined (C/D using PAC + SPF) reduction in the usage of PAC coagulant can lead to cost savings in the long run.	The combined C/S using FeCl3:6H2O and SPF showed a 13.3% increase in total cost compared to the combined C/D using PAC and SPF.	[149]
C/F + ozonation	Stabilized	500 mL of leachate volume, SnCl4 as coagulant with a dosage of 15.408 g/L, pH = 8, (COD/SnCl4) ratio = 1:4, O3 dosage = 31 g/m3, and mixing of 5, 15, and 30 for rapid, slow, and settlement mixing, respectively.	The color and COD removal efficiencies of SnCl4-O3 were 97.1% and 88%, respectively. Increased BOD5/COD from 0.03 to 0.28, transitioning the LL from a stabilized condition to an intermediate state.	Additional concentration of Zr and Sn ions: The ICP test revealed that there was an increased concentration of Sn and Zr ions in the sample after the treatment process. This finding raises concerns about the potential environmental impact of these elements. Potential additional cost.	[88]
EC + SPF	Intermediate	For EC: NaCl dosage = 2 g/L, voltage = 4 V, pH = 7, distance between electrodes = 3 cm, Fe and Al electrodes, monopolar arranged, and EC time = 60 min. For SPF: H2O2 dosage = 10 g/L, Fe2+ dosage = 1 g/L, pH = 3, and SPF time = 30 min.	The COD and color removal efficiencies of the EC-SPF were 90% and 91%, respectively. Increased BOD5/COD from 0.35 to 0.73. The addition of oxygen to the reaction medium significantly improved the effectiveness of color removal.	Chemical costs. The process may have negative implications for waste management. Operation and maintenance requirements.	[33]
EC + electro-oxidation (EO)	Stabilized	As bipolar electrodes, stainless steel as a cathode, aluminum as anode, and BDD as an anode. pH = 6.5, current intensity = 0.7 A, and EC-EO treatment time = 24 h.	The removal efficiency of 98.5% for COD, 62.2% for TN, 85.5% for Cr (VI), and 85.8% for NH4+-N, demonstrating its effectiveness. Simple and easy operation eliminates the need for specialized labor or lengthy startup processes. Small reactor spaces. Low capital: The process required 847 kWh/kg and 50 kWh/kg to remove 85% and 98% of NH+4-N and COD, respectively.	The process requires the use of chemicals such as electricity, which may have an impact on the overall costs. The process may require periodic maintenance and cleaning to ensure its effective performance.	[111]
EO + EC	Stabilized	Current density = 25 mA/cm2, magnetic stirrer conditions = 150 rpm, distance between electrodes = 1 cm, and treatment times of EO and EC were 180 and 60, respectively.	The removal efficiency of 80.32% for COD and 79% for TOC.	Sludge generation: This sludge generation can increase the complexity and cost of sludge management and disposal.	[106]
Sulfate radical electrochemical AOPs + EC (SR-EAOP + EC)	Stabilized	Applied voltage = 3 V, pH = 3, Fe2+ dosage = 100 mg/L, and persulfate (PS) dosage = 500 mg/L.	Effective COD removal with a removal efficiency of 88.67% and 74.51% for SR-EAOP + EC and EC + SR-EAOP, respectively. Simulates the production of active chlorine and hydroxyl radicals through electrochemical oxidation processes, enhancing the reduction in COD in the LL.	Reliance on chemicals such as persulfate and Fe2+ ions. Proper handling and disposal of the resulting by-products from the process may be needed to ensure environmental sustainability.	[150]
Electrochemical peroxidation (ECP) + EF	Stabilized	Current density = 30 mA/cm2, initial pH = 9, and 3.5 for ECP and EF, respectively.	The removal efficiency of 99.2 ± 0.1% for TN, 99.4 ± 0.1% for NH+4-N, 85.1 ± 0.8% for TOC, and 95.9 ± 0.4% for COD, demonstrating its effectiveness. Reduction in acute cytotoxicity of the LL. Enhanced degradation of organic compounds. Retention of Fe2+: Fe2+ produced by the sacrificial anode in the combined process can be retained in the effluent, driving the subsequent EF process. Reduction in acute cytotoxicity (ACT).	Proper handling and disposal of resulting by-products may be required for environmental sustainability.	[105]

provides an overview of a few chemical + chemical combination treatment processes along with an analysis of their effectiveness.

As indicated in Table 8, Cortez [147] examined several pre-treatment processes for stabilized LL to enhance the organic matter biodegradability in preparation for subsequent biological treatment. They tested the Fenton process (Fe²⁺/H₂O₂) and various ozone-based AOPs such as O₃/H₂O₂, O₃/OH⁻, and O₃. Ozone demonstrated the highest levels of biodegradability and removal effectiveness, particularly when used in combination with H₂O₂ and at higher pH values. Overall, the study found that, at natural and neutral pH levels, Fenton treatment, O₃/OH⁻, and O₃/H₂O₂ processes were more effective in removing TOC and COD compared to O₃ treatment. The results presented in Table 8 clearly demonstrate that the Fe²⁺/H₂O₂ + O₃ process significantly enhanced the removal of TOC and COD compared to the individual process (Fe²⁺/H₂O₂). Despite the leachate quality improvements, a biodegradability of 0.4 was reached, which is thought to be required for efficient biological treatment. Further research should focus on improving pre-treatment techniques such as ozonation and Fenton treatment at an alkaline pH or in conjunction with H₂O₂ to obtain a leachate that is more biodegradable. A cost analysis revealed that the Fe²⁺/H₂O₂ system was the most economical option for treating the LL.

In a study conducted by Orkun and Kuleyin [56], the researchers examined the effectiveness of using an EC process to remove COD from young LL. They found that the EC process alone achieved a certain level of COD removal efficiency. Furthermore, the researchers explored the combined application of EC and EF processes for COD removal from LL. They observed that the EF process effectively removed COD from LL, and the combined process resulted in an improvement in COD removal efficiency compared to the EC process alone.

In studies conducted by Moradi and Ghanbari [64], Cheibub [108], and Amor [58], they focused on the removal of COD from LL using the combined C/F and Fenton or photo-Fenton treatment processes. Moradi and Ghanbari [64] employed the combined C/F and Fenton processes for young LL treatment. In the study, the C/F process using FeCl₃ was employed as a pre-treatment process for young LL. After that, the leachate was subjected to the Fenton process to degrade it. The research employed RSM for the experiment design, modeling, and data analysis. Based on the germination index, which significantly increased following the C/F and Fenton processes, a phytotoxicity test was also carried out.

Similarly, Cheibub [108] studied the removal of COD from stabilized LL using the combined C/F followed by Fenton and photo-Fenton (PF) treatment processes. Treatment with FeCl₃ in the C/F removed approximately 53% of COD. After C/F, the remaining Fe³⁺ ions were unable to catalyze H₂O₂ breakdown into hydroxyl radicals, which enhanced COD elimination. The LL was first treated by the C/F process, and the addition of Fe²⁺ ions and subsequent interaction with H₂O₂ in the Fenton step enhanced the COD removal to 83.3%. A 75% decrease in COD was observed, demonstrating the effectiveness of the PF process when used directly on the raw effluent. The COD of the LL was significantly decreased by the AOPs. However, the COD of the raw LL was dramatically lowered by combining the C/F process with the Fenton or PF process.

Amor [58] investigated the treatment of stabilized LL with a focus on meeting discharge limits. They first investigated the effect of initial pH and different coagulants on the C/F process. They then evaluated the efficiency of two AOPs, namely Fenton and solar photo-Fenton (SPF), for leachate remediation. The C/F pre-treatment step removed a substantial amount of polyphenols, turbidity, and COD. Combining C/F with the Fenton reagent achieved a COD removal rate of 89% in 96 h. However, this combined treatment required several days for effective results. Furthermore, combining C/F with SPF yielded higher reductions in dissolved organic carbon (DOC) (75%), compared to single SPF (54%). The treated LL exhibited increased biodegradability and showed no toxicity in biodegradability and toxicity tests.

In summary, these three studies [58,64,108] focused on the treatment of LL using the integration of chemical-based processes such as C/F and AOPs (Fenton and SPF). The studies demonstrated the effectiveness of these processes in removing pollutants from the leachate. The combined treatments resulted in improved biodegradability and reduced toxicity of the treated leachate.

Luo [101] used flocculation with PAC as a flocculant followed by microelectrolysis-Fenton (MEF) to effectively remove COD and humic acids (HAs) from LL. Superior decontamination performance was achieved by combining the PAC coagulation with MEF processes; the expected clearance rates of HA and COD were 93.79% and 90.27%, respectively. Whereas MEF was more successful in destructing organic contaminants that were resistant to removal, particularly those that have humic-like and fulvic-like substances, PAC coagulation was more successful in eliminating compounds that resembled protein-like substances.

Ishak [148] investigated the combination of C/F and UV-based sulfate radical advanced oxidation (UV/SRAOP) for COD removal from stabilized LL. C/F treatment achieved 76.9% COD removal, and subsequent UV/SRAOP treatment using UV/PMS or UV/PS resulted in approximately 91.5% and 90.9% COD removal, respectively. The presence of sulfate radical ions in the treated effluent maintained a stable toxicity level after UV/PS treatment, but it slightly increased after UV/PMS treatment. These findings suggest that UV/SRAOP could be a viable alternative for LL treatment.

Rebolledo [149] conducted experiments to treat stabilized LL using two different combination treatments. The first treatment involved coagulation/sedimentation (C/S) with polyaluminum chloride (PAC), followed by the SPF process. The second treatment involved C/S with FeCl₃:6H₂O, followed by SPF induced by ferrioxalate. The objective was efficient COD removal. The researchers suggested exploring solar radiation as a sustainable and renewable energy source in LL treatment. These alternative treatments offer potential for efficient COD reduction in LL while considering cost and sustainability factors.

Zakaria [88] investigated the use of tin tetrachloride (SnCl₄) and zirconium tetrachloride (ZrCl₄) as coagulants for treating stabilized LL, as illustrated in Figure 6. SnCl₄ showed favorable performance, removing 77% of COD and 97% of color. The researchers also explored combined treatments using ozonation (O₃) and C/F processes.

As depicted in Figure 6, four treatment sequences were implemented: SnCl₄-O₃, ZrCl₄-O₃, O₃-SnCl₄, and O₃-ZrCl₄. The SnCl₄-O₃ treatment achieved the highest removal efficiencies, with 88% for COD and 97.1% for color. The ZrCl₄-O₃ treatment had lower removal percentages (70.6% for COD and 90% for color). The SnCl₄-O₃ treatment significantly improved the leachate’s biodegradability ratio (BOD₅/COD), transitioning it from a stabilized to an intermediate state (from 0.03 to 0.28). Overall, the combined treatment process was more effective than C/F alone in treating stabilized LL.

In research by Sun [106], the performance of a combination treatment process that included electrooxidation (EO) and EC (EO + EC) to treat stabilized LL was confirmed. For COD and TOC, the treatment’s efficacy was 80.32% and 79%, respectively. EO could remove HA and FA concurrently and encourage the conversion of FA to hydrophilic (HyI) humus. EC was extremely successful in the removal of a significant amount of humic-like humus from the concentrate but weak in the removal of FA and long-wave humus. When it came to eliminating aromatic organic debris, EC + EO performed better than EO+EC. While EC only effectively removes the HA component with significant sludge generation, EC and EO both have the ability to totally degrade the HA component on their own.

Nidheesh [150] conducted a study to investigate the treatment of stabilized LL utilizing a combination of sulfate-radical-associated electro-chemical AOPs (SR-EAOPs) followed by EC (SR-EAOP + EC). The electrochemical processes were carried out in a single step, as shown in Figure 7.

As shown in Figure 7, the polarity is switched in the process of EC. In SR-EAOP, Fe was used as the cathode and Pt/Ti was used as the anode. The treatment processes involved the generation of sulfate radicals through the cathodic reduction of persulfate (PS), the anodic oxidation of Pt/Ti, and the activation of these radicals through the addition of externally provided Fe²⁺ ions in the water medium. The system stimulated the production of active chlorine and hydroxyl radicals through the indirect electrochemical oxidation and anodic oxidation processes, which significantly enhanced the reduction in COD in the leachate.

The study conducted by Lu [105] aimed to enhance the treatment of stabilized LL in China by combining two chemical processes: the electrochemical peroxidation (ECP) process and the EF process. This combined process resulted in a reduction in acute cytotoxicity (ACT) as well as an improvement in the removal of nitrogen and organic contaminants. The ECP process primarily removes the organic compounds in stabilized LL, which are then further mineralized into smaller hydrophobic molecules in the EF process. The combined ECP-EF process effectively reduces the complexity of the organic compounds and aromaticity, leading to decreased ACT. The study demonstrates that the combined ECP-EF process allows for advanced treatment of old LL. Under the optimal parameters outlined in Table 8, this combined process achieved removal efficiencies of 99.2 ± 0.1%, 99.4 ± 0.1%, 85.1 ± 0.8%, and 95.9 ± 0.4% for total nitrogen (TN), NH⁺₄-N, TOC, and COD, respectively. Additionally, the retention of Fe²⁺ produced by the sacrificial anode in the combined process facilitated the subsequent EF process. However, the study identified a limitation in the proper handling and disposal of resulting by-products, which may be necessary for environmental sustainability. Addressing this issue is crucial to ensure the overall sustainability and environmental impact of the ECP and EF process.

5.2. Chemical + Physical Integrated Processes

The integration of chemical and physical treatment processes to treat LL has gained wide attention due to its ability to remove biodegradable and non-biodegradable organic compounds and toxic metals and nitrogen compounds in a more efficient and effective manner [82].

Table 9. A summary of the reviewed chemical + physical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Coagulation + Adsorption	Young	0.6 g/L and 0.8 g/L for alum and FeCl3, respectively.	In the combined alum and fly ash adsorbent process, the overall removal efficiency of COD was 82%.	The use of ferric chloride resulted in higher sludge production compared to aluminum sulfate.	[151]
	Stabilized	0.6 g/L and 0.7 g/L for alum and FeCl3, respectively; fly ash adsorbent dosage = 6 g/L; and equilibrium time = 210 min.	Cost-effective materials: Fly ash was used as a cost-effective adsorbent, while alum and ferric chloride were used for coagulation. By combining coagulation with alum and adsorption with fly ash, an overall COD removal efficiency of 82% was achieved for stabilized LL.
Fenton + Adsorption	Young	GAC used as adsorbent, pH = 4, H2O2 30% (w/w), COD/H2O2 = 9, and Fe2+/H2O2 = 0.6.	Demonstrates its efficiency by removing COD, TOC, TN, and color by up to 99%. Enhanced BOD5/COD from 0.084 to up to 0.82. Demonstrated effectiveness in treating LL.	The combined Fenton and adsorption process was more expensive due to the use of adsorption materials. This could limit its feasibility and practical application. Further studies are needed to optimize the use of GAC as the adsorption material, which may help improve cost-effectiveness and overall performance.	[95]
EC + Fiber Filtration	Intermediate	Fe and Al are used as electrodes with a dosage of 100 mg/L, a pH of 7, and a current density of 0.05 A/m2, with an operating time of 30 min.	The removal efficiency of 86% for phosphorus, 96% for iron, 87% for arsenic, and 94% for COD. The first-step fiber filter retained the flocs formed during the EC process, enhancing the removal of SS. The second-step biofilter facilitated the removal of organic pollutants through biodegradation.	The process effluents still require an extra treatment step in order to comply with requirements because of the LL high pollutant content. Implementing the combined processes may involve higher costs and technical complexity compared to conventional treatment processes.	[152]
C/F + Adsorption	Intermediate	For the C/F process, the optimum conditions were contact time = 8 h, a temperature of 25 °C, a pH of 8, and FeCl3 used as a coagulant with a dosage of 20 g/L. For the adsorption process, the optimum conditions were contact time = 8 h, pH = 7, and Cupressus sempervirens cones (CupSem) used as a natural bioadsorbent with a dosage of 1 g/L.	The removal efficiency of 93% for color, 96% for BOD5, and 86% for COD. Cost-effective.	______	[103]
C/F + Adsorption	Young	For the C/F process, the optimum condition was contact time = 3 h, and FeCl3 was used as a coagulant with a dosage of 12 g Fe3+/L. For the adsorption process, the optimum condition was contact time = 3 hr, and palm bark powder (PBP) was used as a bioadsorbent with a dosage of 9 g/L.	The palm bark powder (PBP) demonstrates its efficiency by removing color, turbidity, and COD at 90%, 99%, and 59%, respectively. Simplicity and cost-effectiveness.	While the combined process shows promise in treating LL, the effluents produced still require further treatment to meet the standards set by local regulations. Due to the high pollutant load, an additional treatment step is necessary to ensure that the treated effluents fully comply with local regulations.	[84]
C/F + Adsorption	Stabilized	For the C/F process, the optimum condition was pH = 5, polyacrylamide (PAM) dosage = 2 mg/L, and ferric trichloride (FTC) = 750 mg/L. For the adsorption process, the optimum condition was activated coke used as an adsorbent with a dosage of 5 g/100 L.	The activated coke demonstrates its efficiency by removing color, COD, and TOC at 100%, 57%, and 91%, respectively. The removal efficiencies of protein-like and humic matters both exceeded 95%. The study showed that over 80% of nitrogen compounds were retained in the effluent, indicating the potential for their recovery. Provides a feasible alternative for addressing the global concern over the secondary pollution caused by LL and its effective disposal.	______	[82]
Fenton + Adsorption	Stabilized	Adsorbent time = 24 h, sodium percarbonate (SPC) H2O2 dosage = 2.5:1, and biochar = 2 g/L.	For the H2O2 + biochar adsorption (BC) process, the COD decreased from 3155 mg/L to 549 mg/L. For the sodium percarbonate (SPC) + BC process, the COD value decreased from 256 mg/L to 944 mg/L. The TOC values were 119 mg/L and 186 mg/L for (H2O2+BC) and (SPC+BC), respectively. Increased the biodegradability from 0.053 to 0.27 (H2O2) and 0.14 (SPC). Cost-effectiveness of biochar as an adsorbent. SPC is an eco-friendly and safer reagent.	_____	[81]
Electro-chemical + Membrane Distillation	Intermediate	Operating time = 8 h, pH < 2, and Na2SO4 = 15 mmol/L.	The MED-MD process efficiently removes pollutants from LL, with high rejection rates for metal ions, ammonia, total phosphorus, and CODCr of around 96%, 99%, 99.5%, and 97.3%, respectively. The MED-MD process recovered 48% of humic acid (HA) and 31% of ammonia nitrogen from the raw LL. The MED-MD process demonstrates excellent resistance to membrane fouling and wetting, producing high-quality water. The MED-MD process reduces overall energy consumption and carbon emissions by approximately 20% compared to MD alone and by 8% compared to the EO-MD process.	The thermal energy consumption of MD still results in carbon emissions, indicating a potential environmental impact.	[102]

provides an overview of a few chemical + physical combination treatment processes along with an analysis of their effectiveness. Chemical and physical treatment processes are often used as pre-treatment or combined treatments to treat the LL [151]. In the study, chemical treatment processes such as EC, C/F, and Fenton were used as pre-treatments to remove suspended and colloidal solids from LL to make it a viable biological treatment in the young LL, and to decompose some of the heat-resistant organic compounds present in the stabilized LL [84,151]. Although chemical processes have shown effectiveness in reducing COD and toxic metals [95], they alone could not achieve a reduction of more than 69%. Therefore, combining chemical and physical processes can improve treatment efficiency.

By combining different processes such as adsorption, filtration, C/F, EC, and Fenton, the limitations of each individual process can be overcome, leading to an improvement in overall treatment performance.

As indicated in Table 9, research on the treatment of LL utilizing a combination of pre-treatment processes, including coagulation and adsorption, was undertaken by Gandhimathi [151]. The researchers utilized fly ash as a cost-effective adsorbent for adsorption studies, while alum and ferric chloride were employed to study coagulation. Coagulation experiments were performed on both stabilized and young leachate, whereas adsorption experiments were conducted using alum-pre-treated stabilized leachate. The researchers found that the maximum COD removal for stabilized leachate, which had been pre-treated with alum, was 28% when fly ash was utilized as the adsorbent. By combining coagulation with alum and adsorption with fly ash, an overall COD removal efficiency of 82% was achieved for leachate. During the coagulation experiments, it was observed that the COD removal efficiency using ferric chloride was 59% for stabilized leachate, whereas for alum, it was 75% without prior pH adjustment. Comparatively, for young leachate, the COD removal efficiency using alum was 55%, while it was 35% for ferric chloride without pH correction. The researchers also reported that the removal efficiency of COD using ferric chloride decreased from 53% at pH 2 to alkaline pH levels. Conversely, alum exhibited a COD removal efficiency of 55% at pH 8.

For the purpose of removing contaminants from LL, Pedro-Cedillo [95] compared the Fenton–adsorption and adsorption treatments. In comparison to the adsorption process on raw leachate, the Fenton–adsorption process demonstrated greater removal rates for TOC, TN, COD, and color (over 99%), as well as removal rates for TDS and BOD₅ up to 95%. The primary organic substance in the leachate was bisphenol-A, which was only eliminated during the Fenton–adsorption process’s adsorption step. The most abundant organic compound in the leachate was bisphenol-A, which was retained by the carbon in the adsorption process. The combined Fenton–adsorption process was successful for treating LL, but it was expensive because of the adsorption material. Further studies are needed to optimize the use of GAC.

Li [152] conducted a study that examined the use of EC and fiber filtration for treating intermediate LL. The research focused on assessing the effectiveness of EC in removing COD, arsenic, iron, and phosphorus from the LL. By employing a combination of EC and fiber filtration, the study achieved significant reductions in the concentrations of contaminants responsible for suspended solids, color, and odor. Following the EC process using Al or Fe electrodes, the treated LL was passed through two steps of fiber filters. The first-step fiber filter retained the flocs formed during EC, while the second-step biofilter facilitated the removal of organic contaminants through biodegradation.

El Mrabet [103] conducted a study on LL treatment using a combined C/F with FeCl₃ as a coagulant, followed by adsorption with a Cupressus sempervirens (CupSem) bioadsorbent. Combining C/F with CupSem adsorption further improved the removal percentages, with 93% color, 96% BOD₅, and 86% COD removal. The study found that the pseudo-second-order kinetics and Langmuir isotherm models described the adsorption process well. These findings suggest that the combined C/F and adsorption treatment of LL offers an affordable alternative to existing processes and can be successfully implemented in large-scale sanitary landfills. Similarly, Chaouki [84] used C/F with FeCl₃, followed by adsorption onto palm bark powder (PBP), to treat young LL. The Dubinin–Radushkevich isotherm models describe the adsorption process.

In order to assess the effectiveness of treating LL, Kwarciak-Kozłowska and Fijałkowski [81] combined the AOPs with biochar adsorption (BC), as indicated in Figure 8.

Two systems, H₂O₂+BC and sodium percarbonate (SPC) with BC, were tested, as shown in Figure 8. COD values decreased significantly in both systems, with the H₂O₂+BC system showing higher elimination efficiencies. The SPC+BC system exhibited better germination and longer plant roots. In comparison to the conventional Fenton process, the suggested system has the benefit of utilizing a safer and more ecologically friendly reagent (SPC), and it is also more affordable to use biochar as an adsorbent than granular activated carbon (GAC).

Yan [102] investigated the combination of a membrane electrochemical reactor (MER) with membrane distillation (MD) for LL treatment. The aim was to control membrane fouling, recover resources, and reduce energy consumption and carbon emissions. The MER-MD system efficiently removed pollutants, with high rejection rates for metal ions, ammonia, total phosphorus, and COD_Cr. It also recovered humic acid and ammonia nitrogen from LL. The MER-MD system showed greater resistance to membrane fouling and wetting compared to the MD and EO-MD systems. Energy consumption and carbon emissions were reduced by around 20% compared to MD alone and by 8% compared to EO-MD. The MER-MD system’s recovery of ammonia nitrogen can compensate for 8.25 kg of a carbon dioxide equivalent.

5.3. Chemical + Biological Integrated Processes

Biological treatment processes are commonly used to treat LL containing high concentrations of COD and BOD₅ due to their simplicity and high cost-effectiveness [29]. Biological treatment processes have proven to be satisfactory. However, the presence of toxic pollutants, recalcitrant organic materials, and non-degradable organic compounds with little carbon and oxygen in LL limits the application of biological treatment alone [97]. In such cases, chemical processes are used to pre-treat LL to make it amenable to biological treatment and to convert non-degradable organic compounds into more stable, non-hazardous materials. It has been found that the use of an integration of chemical and biological treatment processes is the most common and widely used among the combined treatments to treat LL.

Table 10. A summary of the reviewed chemical + biological integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Fenton + SBR	Stabilized	For the Fenton process, pH = 4, Fe2+ dosage = 0.08 mol/L, H2O2/Fe2+ ratio = 10, and mixing time = 20 min. For the SBR process, water drainage and supply, settlement, and aeration times were 0.5 h, 0.5 h, and 11 h, respectively; HRT = 4 d; and SRT = 15.2 d.	The removal efficiency of 88.2% for FA and 87.3% for HA. The removal rates for DOC and COD in both FA and HA exceeded 80%. Non-destructive to sample structures. High sensitivity and selectivity.	_____	[153]
SPF + Biological Oxidation	Stabilized	Fe2+ dosage = 60 mg/L and 3 h of sedimentation.	Achieved complete removal of nitrites, nitrates, and ammonium from the LL. The process resulted in significant reductions in aromatic content (86%), polyphenol reduction (57%), and leachate mineralization (57%), indicating effective removal and degradation of organic contaminants.	The process requires the addition of chemicals such as NaOH for nitrification and H2SO4 for denitrification. The estimated optimal energy requirement for treating biodegradable effluent using 90 mM H2O2 when in excess was 29.2 KJUV/L.	[107]
C/F + SBR	Young	For the C/F process, FeCl3 is used as a coagulant with a dosage = 1000 mg/L and a pH of 6.2 and 12. For the SBR process, HRT = 6 days, SRT = 21.8 days, and cycle = 8 h.	Achieved removal efficiencies of up to 72% for TN, 60% for COD, 73.3% for NH4+-N, and 89% for BOD5. The study found that the biodegradable fractions were between 1.27 and 1.34 times the corresponding BOD5. This indicates the potential for biomass growth using biodegradable fractions as a food source. Reduced cycle time, sequence, and footprint are required.	Phosphorus dependency: The study highlights the importance of phosphorus nutritional balance in the process. There may be difficulties in controlling and managing appropriate phosphorus levels in the feed, requiring careful monitoring and control of this nutrient.	[60]
EC + Bioaugmentation	Stabilized	For the EC process, Al electrodes with a spacing of 2 cm, an EC time of 150 min, and a current density of 95 A/m2. For the biological treatment process, the biological time is 166 h.	The removal efficiency is 56% for COD and 80% for NH4+. The addition of activated sludge (AS) also improved the efficiency of nitrification by 10%. Nitrification is a biological process that converts ammonium into nitrate, which is less harmful to the environment.	Incomplete ammonium degradation. Process scalability and applicability.	[154]
PEF + MBR	Young	For the PEF process, the pH is 2.9, the reaction time is 45 min, the current density is 140.5 A/m2, and the UV light capacity = 8 W. For the MBR process, the HRT is 6 days and the reactor aeration rate is 5 l/min.	For the PEF + MBR process, the removal rates increased to 88.2% for chloride, 93.3% for sulfide, 82.7% for sulfate, 100% for phosphate, 88.3% for ammonia nitrogen, 96.2% for COD, 90.2% for BOD, and 95.5% for TSS.	Energy dependence. The disposal of the waste generated by the combined process may require special measures.	[97]
Electro-Ozonation + SBR	Stabilized	The reaction time is 97 min, and the optimum COD removal is 120 mg/L of O3. The reaction time is 117.4 min, and optimum color removal is 120 mg/L of O3. The reaction time is 96.7 min, and the optimum Ni removal is 120 mg/L of O3. The optimum of the combined process is 96.9 min of time, 7.3 pH, 4A of current, and 9 V of voltage.	The removal efficiency of 73.4% for Ni, 96.1% for color, and 88.2% for COD, demonstrating its effectiveness.	The ozone utilization in this process ranged from 0.3 kg to 1.4 kg of COD removed per kg of ozone consumed.	[117]
EC + Biological	Young	The pH is 7, the applied voltage is 7 V, the Fe–Fe electrode has a spacing of 1 cm, and the electrode surface area is 40 cm2.	The removal efficiency of 95% for Ni, 99.6% for Zn, 98% for Pb, 97.2% for Cu, 73% for Fe, and 60% for COD. The study also included the production of biogas as a by-product of the treatment process, along with 13.4% bio-hydrogen production. Reduces the processing time required for treatment. The combined process reduces the need for large amounts of water for dilution purposes. The biological fermentation of organics with mixed consortia results in the production of renewable energy carrier gas. This offers an additional benefit by providing a low-cost and sustainable energy resource.	The study revealed that the metal removal rate decreased with increasing distance between electrodes. These limitations may pose difficulties in implementing the process in certain factories or plants that lack sufficient space.	[98]
EC + Anaerobic Batch Reactor	Intermediate	Initial pH of 5, retention time of 54 min, spiral Fe electrodes, and current density of 50 mA/cm2.	The removal efficiency of 92% for COD, demonstrating its effectiveness. Ease of construction.	Additional costs may be required.	[104]
EF + Biological	Stabilized	The pH is 2, the electrode area is 25 cm2, the distance between electrodes is 3 cm, the applied voltage is 5 V, and the catalyst dosage is 50 mg/L.	The removal efficiency of 97% for COD. Improved the biodegradability from 0.03 to 0.4 using EF. This suggests that the EF process enhanced the potential for further biological treatment. The investigation revealed that the catalyst could be effectively reused for up to three cycles. This indicates that the catalyst has good stability and can be economically viable for multiple treatment cycles.	_____	[155]
PF + Biological	Young	In the PF process, the optimum parameters were pH (2.4), reaction time (120 min), 3400 mg of H2O2/L, and 80 mg of Fe2+/L. For the biological process, the optimum parameters were reaction time = 40 h, 0.571 ± 0.04 vvm (L/L.min), and 2.36 ± 0.1 mg/mg (BOD5/MLSS). For PF and biological processes, the optimum parameters were 150 h of reaction time, 1.571 ± 0.06 vvm (L/L.min), and 4.41 ± 0.3 mg/mg (BOD5/MLSS).	The removal efficiency of 98% for COD and BOD5, demonstrating its effectiveness. This satisfies the requirements for releasing treated wastewater into water bodies.	The overall process requires a significant amount of time, as the biological treatment period lasts for 40 h. This may result in increased operational costs and time.	[156]

provides an overview of a few chemical + biological combination treatment processes along with an analysis of their effectiveness.

As indicated in Table 10, in a study conducted by Liu [153], the researchers examined the effectiveness of the combined Fenton–SBR process in removing FA and HA from stabilized LL. The results indicated that the combined Fenton–SBR process was capable of efficiently removing humus from the leachate, with removal rates of 88.2% for FA and 87.3% for HA. Furthermore, the removal rates for DOC and COD of both FA and HA exceeded 80%.

Vilar [107] investigated combining an SPF process with biological oxidation (denitrification and nitrification) for residual DOC and nitrogen removal from LL. They achieved complete removal of nitrites, nitrates, and ammonium through biological nitrification and denitrification after neutralizing Fe sludge. The highest rate of nitrification, 68 mg of NeNH₄⁺/day, was achieved with 33 mmol NaOH/L and 27.5 mmol H₂SO₄/L of denitrification, respectively.

El-Fadel [60] studied the use of an SBR preceded by a C/F process with phosphorus nutritional balance for LL treatment. The study also identified several factors that influenced the rate of biomass growth resulting from substrate consumption. These factors included a YH (yield of heterotrophic biomass) of 0.18 mg of VSS/mg of COD (volatile suspended solids per milligram of COD), Yobs (observed yield) ranging from 0.12 to 0.17 mg of VSS/mg of COD, qmax (maximum specific substrate utilization rate) ranging from 2.81 to 2.99 day⁻¹, mmax (maximum specific growth rate) ranging from 0.51 to 0.54 day⁻¹, and Kd (endogenous decay coefficient) of 0.10 day⁻¹. COD removal followed a pseudo-first-order model with a constant k of 0.36–1.54 day⁻¹ depending on phosphorus levels.

Djelal [154] investigated the combined use of EC and biological treatment for stabilizing LL in a non-hazardous waste storage facility. The EC treatment reduced COD, with removal rates of 33% and 56% at current densities of 23 and 95 A/m², respectively. However, ammonium removal was not effective. Subsequent biological treatment using activated sludge (AS) did not significantly enhance the degradation of OM, which mainly consisted of recalcitrant compounds. Both biological treatments, with and without AS, reduced NH₄⁺ with a removal efficiency of 62%. The addition of AS improved nitrification efficiency by 10%. However, the combined treatment achieved only around 80% degradation of NH₄⁺ at a current density of 95 A/m².

Nivya and Pieus [97] investigated the treatment of highly contaminated LL using the photo-electro-Fenton process (PEF), followed by the MBR. The combined approach showed higher pollutant removal efficiencies compared to PEF alone. Furthermore, the study examined the biodegradability of the synthetic wastewater by measuring the BOD/COD ratio, which increased from 0.19 to 0.34. This indicated improved biodegradability, leading to the adsorption of MBR as a post-treatment. However, when the combination of PEF followed by MBR was applied, the removal percentages increased to 88.2% for chloride, 93.3% for sulfide, 82.7% for sulfate, 100% for phosphate, 88.3% for ammonia nitrogen, 96.2% for COD, 90.2% for BOD, and 95.5% for TSS.

Mojiri [117] conducted a study in which they treated concentrated LL using a combination of electro-ozonation and a composite adsorbent-augmented SBR process, as depicted in Figure 9. The treatment process resulted in removal efficiencies of 52.9% for nickel, 90.4% for color, and 64.8% for COD. The ozone utilization in this process ranged from 0.3 kg to 1.4 kg of COD removed per kg of ozone consumed.

As shown in Figure 9, following the optimal electro-ozonation treatment, the concentrated leachate was then transferred to a powdered composite adsorbent (P-BAZLSC)-augmented SBR reactor (PB-SBR). By employing the PB-SBR, the removal efficiencies for nickel, color, and COD improved to 73.4%, 96.1%, and 88.2%, respectively, compared to the initial values of 52.9%, 90.4%, and 64.8%.

Bhagawan [98] investigated the use of the EC process and biogas generation for treating LL. The study examined various parameters affecting the EC process, including electrode material, electrode reactive surface, in situ peroxi-EC, inter-electrode distance, applied voltage, and initial pH. The metal removal rate increased with higher electrode surface area and applied voltage but decreased with increasing inter-electrode distance. The results showed high removal efficiencies for COD, Fe, Cu, Pb, Zn, and Ni, as shown in Table 10.

Tezcan Un [104] studied the combined treatment of LL using an EC process as pre-treatment followed by anaerobic treatment. The EC process was optimized to achieve a maximum COD removal efficiency of 69%. The effluent was then subjected to anaerobic treatment, and the process was further optimized using a factorial design process. The maximum COD removal through anaerobic treatment was 74%. When the combined EC and anaerobic treatment were applied, it resulted in an overall COD removal efficiency of 92%. The study emphasizes the effectiveness of the combined treatment approach compared to using either process alone for leachate wastewater.

Baiju [155] synthesized iron molybdophosphate (FeMoPO) and used it as a catalyst for the EF treatment of LL. EF achieved 82% COD removal, improving the biodegradability as indicated by an increased BOD₅/COD ratio from 0.03 to 0.4. The catalyst could be reused for up to three cycles. When EF was combined with biological treatment, an overall COD removal of 97% was achieved, resulting in a final COD concentration of 192 mg/L. The combination of EF and biological processes proved to be highly efficient for LL treatment.

Colombo [156] investigated the combination of physical–chemical and biological processes for reducing pollutants in LL. They employed a conventional biological treatment followed by a photo-Fenton (PF) process and then combined PF with biological treatment. The PF process was optimized using a central composite rotatable design (CCRD). After a decantation process, the biological treatment was performed over 40 h. When the improved PF process was combined with the biological process, a removal effectiveness of 98% for COD and BOD₅ was achieved, meeting the requirements for releasing treated wastewater into water bodies.

5.4. Physical + Physical Integrated Processes

The utilization of physical treatment processes to treat LL has proven effective in terms of high flexibility, high process performance, greater mobility, and low production costs, especially membrane filtration [157]. Membrane filtration processes have been commonly used in recent years, including RO, NF, MF, and UF [135]. Among these, NF and RO have been found to be efficient in the removal of suspended and colloidal solids and organic pollutants from LL [131,135]. Forward osmosis membranes’ excellent treatment efficiency has made them appropriate for a variety of uses [157].

Table 11. A summary of the reviewed physical + physical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Forward osmosis + membrane distillation (FO-MD)	Intermediate	For the FO process, the optimal flow rate was 0.87 L/min and 0.31 L/min for FS and DS, respectively, and the DS concentration was 4.82 M. For the MD process, the optimal concentration was 60,000 mg/L and 25,000 mg/L for NaCl and FS, respectively. Also, the optimal temperature was 62.5 ± 0.5 °C and 72.5 ± 0.5 °C for NaCl and FS, respectively.	Demonstrates high salt rejection rates (>96%) and excellent removal rates for TN, TOC, and NH4+-N, as well as the toxic elements Hg, As, and Sb (>98% removal). FO requires low pressure or even no pressure. MD is a thermally driven separation process, which means it relies on temperature differences to drive the separation of molecules. Low membrane fouling tendency. Obtaining high-quality production water. Solving the problem of drawing solution dilution.	-	[157]
Adsorption + UF	Stabilized	PAC dosage = 1 g/L.	The removal efficiency of 54.7% for DOC, and 34.5% for COD, demonstrating its effectiveness.	Membrane blockage and reduced permeate flux. Lower permeate flux in the two-stage processes. Limited applicability due to membrane pore blockage. Significantly dropped membrane capacity.	[140]
Adsorption + membrane (MF, UF, fine UF)	Stabilized	For the adsorption process, the adsorption time is 30 min, and the PAC dosage is 3 g/L. For PAC adsorption and the MF process, the permeate flux is 167.6 L/(m2.h).	The removal efficiency of 97.2% for color, and 87.8% for COD.	Reduction in membrane flux with PAC and membrane combinations: The study revealed that combining PAC with membrane processes (UF and MF) led to a reduction in membrane flux. These combinations can be ranked as PAC + MF > PAC + UF > PAC + fine UF in terms of membrane flux.	[158]
Nanoparticle adsorption + electro-flotation	Stabilized	For the Fe2O3 process, contact time is 40 min, pH is 4, and Fe2O3 dosage is 20 g/L. For the electro-flotation process, contact time is 40 min, pH is 4, and current density is 40 A/m2. For the Fe2O3 NPs/electro-flotation process, contact time is 2 h, and pH is 4.	The removal efficiency of 99% for turbidity, 100% for color, 99% for TDS, 96% for COD, and 99% for NH3-N, demonstrating its effectiveness.	NH3-N had the lowest removal rate at 48% using the Fe2O3 nanoadsorbents, and 68.3% using the electro-flotation process. Although the combined treatment process enhanced the removal efficiency of NH3-N, it may still have limitations in effectively removing this particular pollutant.	[85]

provides an overview of a few physical + physical combination treatment processes along with an analysis of their effectiveness.

According to Table 11, forward osmosis (FO) combined with MD processes for treating high-salinity hazardous waste LL gave excellent performance in TN, TOC, and NH₄⁺-N, as well as the toxic element Hg, in a study conducted by Zhou [157]. They used RSM to optimize the operating parameters of the FO process using the Box–Behnken design (BBD), aiming to maximize pollutant removal efficiency, water flux, and salt reverse flux. The independent variables considered were the flow rate of the draw solution (DS) and the feed solution (FS), as well as the concentration of the DS. The FO-MD combined system demonstrated high salt rejection rates (>96%) and excellent removal rates for TN, TOC, and NH₄⁺-N, as well as the toxic elements Hg, As, and Sb (>98% removal). The results demonstrated that the FO-MD hybrid system was effective in treating high-salinity hazardous LL and outperformed individual FO or MD processes in contaminant rejection.

Kulikowska [140] conducted a study to investigate the removal of organics from stabilized LL using a combination of adsorption with Norit SX2 powdered activated carbon (PAC) and fine UF. Fine UF alone showed low efficiency in organics removal. To improve the removal efficiency, two doses of PAC were used before fine UF: 0.2 g/L and 1 g/L. However, Norit SX2 particles caused membrane blockage and reduced the permeate flux, even at the lower dose. The combination of adsorption and fine UF was more effective in removing organics but led to a lower permeate flux compared to fine UF alone. Using a lower dose of Norit SX2 with fine UF provided similar treatment efficiency with less reduction in permeate flux.

Shadi [85] conducted a study to investigate the treatment of stabilized LL utilizing a combination of iron oxide nanoparticles (Fe₂O₃ NPs) as adsorbents and an electro-flotation process, as shown in Figure 10. The aim was to remove organic and inorganic pollutants, increase DO levels, and improve water quality.

As shown in Figure 10, combining both processes (Fe₂O₃/electro-flotation) significantly improved removal efficiencies. After 120 min, almost all pollutants were removed (>99% removal) for COD, color, TDS, NH₃-N, and turbidity. The combined treatment process was highly effective, achieving over 99% pollutant removal with an estimated operational cost of 3.18 USD/m³. The study demonstrates an efficient treatment process for removing inorganic and organic contaminants from stabilized LL.

5.5. Physical + Chemical Integrated Processes

The efficacy of treating LL by combining chemical and physical treatment modalities has also been reported in the literature. Adsorption and UF by themselves are not sufficient to decrease contaminants to levels that are tolerable. Based on the previous literature, the UF treatment process can remove only 50% of COD from LL [118]. Therefore, the most effective way to satisfy the allowable limits of pollutant discharge is to combine it with chemical treatments such as Fenton and C/F [29,159].

Table 12. A summary of the reviewed physical + chemical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Ref.
Activated Persulfate + Fenton	Young	UV-LED radiation, pH = 3, PS dosage = 234 mM, FeTiO3 dosage = 1 g/L, current density = 200 mA/cm2, time = 480 min.	The combined use of electrolysis/FeTiO3/UV-LED, followed by Fenton oxidation, led to an overall mineralization exceeding 90%. PS has high stability, high solubility in water, a wide operative pH range, effective heat activation, and versatile applications.	[99]
Adsorption + PF	Young	pH of 3; FeSO4 and H2O2 dosage = 0.3 g/L and 0.67 g/L, respectively; and O3 generation = 3 g/h with power =100 W.	The removal efficiency of 97.9% for HA, 94.5% for ammonium, 95% for color, and 95.1% for COD, demonstrating its effectiveness. Reduction in toxicity.	[100]
UF + C/F	Stabilized	Sedimentation time = 30 min; FeCl3–0 dosage = 0.4 g/L.	The optimized combination of FeCl3 and UF membrane effectively removed TDS at a level from 3200 to 1500 mg/L, pH from 8.23 to 4.86, color from 2930 to 10 Pt-Co units, and turbidity from 226 to 1 FAU, producing clear permeate water for reuse. The polyethersulfone (PES) polymer-based membrane exhibited good stability and was suitable for experimental work. This suggests that the membrane can maintain its performance over the long term and effectively remove impurities. The study demonstrates that up to 70% of the water from the liquid waste can be reclaimed for industrial or domestic applications.	[159]

provides an overview of a few physical + chemical combination treatment processes along with an analysis of their effectiveness.

According to Table 12, in order to mineralize and decolorize LL, Silveira [99] undertook research to assess the viability of using electrolysis and FeTiO₃, increased by UV-LED, for the activation of persulfate (PS), followed by Fenton oxidation. Increasing current density converted PS to SO₄⁻ and generated hypochlorite, enhancing light penetration and promoting Fe(III) photoreduction on the FeTiO₃ surface for improved decolorization. The combined use of electrolysis, FeTiO₃, and UV-LED demonstrated synergistic effects, achieving a mineralization rate of 53%. Subsequent Fenton oxidation at pH 3 resulted in over 90% overall mineralization. This combined treatment process proved effective for refractory and highly colored effluents, with the remaining TOC consisting mainly of short-chain organic acids and a small amount of chlorinated compounds. However, more improvement is needed. While PS has several advantageous properties, such as high stability, solubility, and versatile applications, there is room to enhance its efficiency. Future research could focus on optimizing the PS dosage and reaction conditions to achieve even higher mineralization rates. Additionally, exploring alternative catalysts or modifications to the Fenton process may help improve the overall performance of the combined approach.

Poblete and Pérez [100] studied the depuration of LL using sawdust (SD) as an activated carbon material for adsorption processes as a pre-treatment for SPF and SPF+O₃. Treatment with SD resulted in significant reductions in pollutant concentrations, including copper, iron, ammonium, color, COD, and HA, with reductions of 61.1%, 70.2%, 87%, 19.5%, 33.7%, and 18.3%, respectively. Additionally, the combined process of SPF+O₃ resulted in the removal of organic matter, with HA, ammonium, nitrate, color, and COD showing reductions of 73.3%, 12.8%, 50%, 74.9%, and 76.4%, respectively. The overall treatment achieved removal efficiencies of 97.9%, 94.5%, 95%, and 95.1% for pollutants such as HA, ammonium, color, and COD, respectively. The improved LL quality led to reduced toxicity, as indicated by a germination index (GI) of 20% for Lactuca sativa using a 50% diluted LL. The incorporation of SD as a pre-treatment in LL treatment proves to be beneficial. However, more improvement is needed. Future studies could investigate the selection of suitable absorbents and explore different operational parameters to enhance the absorption process. Additionally, optimizing the PF reaction conditions, such as catalyst concentration, pH, and irradiation intensity, could lead to improved degradation efficiency and reduced treatment time.

In the study conducted by Nazia [159], the researchers explored the effectiveness of different coagulants in combination with alum Al(SO₄)₃ and lime Ca(OH)₂ for the pre-treatment of LL. They also developed a polyethersulfone (PES) polymer-based UF membrane for the treatment process. The aim was to reduce impurities such as COD, TDS, turbidity, and color on the membrane. They found that FeCl₃ alone was the most effective and cost-efficient coagulant for impurity reduction in LL. The synthesized PES-based UF membrane exhibited good stability. The optimized combination of FeCl₃ and a UF membrane effectively removed turbidity, TDS, electrical conductivity, pH, and color, producing clear permeate water for reuse. The integrated process demonstrated the reclamation of at least 70% of LL water for various applications, offering advantages such as a small equipment footprint, cost-effectiveness, environmental safety, low energy consumption, and scalability. While the PES membrane used in the UF process exhibited good stability, further research could focus on enhancing the membrane’s fouling resistance and long-term performance. Additionally, investigating alternative coagulants and flocculants, as well as optimizing the dosage and reaction conditions, may further improve the removal efficiency of the C/F process. This combined approach has the potential to reclaim a significant amount of water from liquid waste for industrial or domestic applications, and further optimization efforts can enhance its overall effectiveness.

5.6. Physical + Biological Integrated Processes

Biological treatment processes are commonly utilized to treat LL containing high concentrations of COD and BOD₅ due to their simplicity [29]. Biological treatment processes have proven to be satisfactory. However, the presence of recalcitrant pollutants in LL limits the application of biological treatment alone as they are often difficult to degrade [116,124]. Combined physical and biological treatment processes have been proven to be effective for the treatment of LL.

Table 13. A summary of the reviewed physical + biological integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Ref.
Air Stripping + Anaerobic Digestion/Aerobic AS	Stabilized	For air stripping, temperature = 25 ± 2 °C, reaction time = 18 h, pH = 11, and aeration rate = 7 L/min. For aerobic treatment, temperature = 30 °C, and aeration flow = 7 l/min.	The removal efficiency of 77% for COD and 97% for organic compounds initially present in the LL. The application of an AS reactor as a post-treatment process effectively removed resistant organic matter, further improving the quality of the treated effluent.	[142]

provides an overview of a few physical + biological combination treatment processes along with an analysis of their effectiveness.

According to Table 13, Smaoui [142] investigated the efficiency of a combined process involving air stripping, anaerobic digestion (AD), and aerobic AS treatment for LL with a high organic matter content. To address the nitrogen load that hampers biological development, air stripping pre-treatment was conducted. The research aimed to evaluate a combined process of aerobic activated sludge, AD, and air stripping treatment. Air stripping achieved an impressive ammonia removal rate of over 80%, improving biodegradability and the carbon-to-nitrogen (C/N) ratio. The anaerobic treatment effectively removed COD and produced biogas. This indicates the potential for energy recovery from the leachate through biogas production. The raw leachate exhibited a 78% COD removal rate and generated 4 L/d of biogas with a 70% methane content. This approach removed 77% of COD and over 97% of the initial organic compounds in the leachate. Finally, the application of an activated sludge reactor as a post-treatment process efficiently eliminated resistant organic matter. However, additional information on the specific experimental setup, system performance, and any limitations encountered would provide a more comprehensive scientific evaluation of the study.

5.7. Biological + Biological Integrated Processes

Biological treatments are used for biodegradability of organic compounds, particularly in LL, that contain high concentrations of COD and BOD, due to their ease and affordability [29]. By using microorganisms’ degrading activity, these strategies facilitate the transition of the compounds found in LL to the following: under aerobic conditions, biomass in sludge form and carbonic gas; and biogas, when exposed to anaerobic conditions, preventing the contaminants’ phase transition [118,160,161,162,163,164,165,166,167,168,169].

Table 14. A summary of the reviewed biological + biological integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Trickling filter + SBR	Stabilized	The dissolved oxygen in SBR and TF was 6.1 and 4.7 mg/L, respectively.	For the TF, the reduction in SS, turbidity, BOD5, and NH3 was 73.17%, 71.96%, 76.69%, and 59.5%, respectively. For the SBR process, the reductions in SS, BOD5, and NH3 were 62.28%, 84.06%, and 64.83%. Significant nitrification occurred within permissible limits during the SBR process.	The effluents produced by both treatment processes did not meet the national regulatory standards for discharge into surface water bodies. Residual concentrations of color, SS, DS, BOD5, and Fe exceeded the limits set by regulatory standards. Additional treatment processes may be required to achieve higher-quality effluents that comply with regulatory standards.	[145]
Anoxic/oxic (A/O) + MBR	Young	Operation period = 113 d.	The removal efficiency of 74.87% for TN, 99.04% for NH4+-N, and 80.60% for COD, demonstrating effectiveness.	-	[146]

provides an overview of a few biological + biological combination treatment processes along with an analysis of their effectiveness.

According to Table 14, Aluko and Sridhar [145] conducted a study to examine the effectiveness of bench-scale treatment processes, specifically a trickling filter (TF) and SBR, in treating LL. The results showed that the TF treatment process resulted in significant reductions in NH₃ (59.50%), BOD₅ (76.69%), turbidity (71.96%), and SS (73.17%) in the effluents. On the other hand, the SBR process achieved reductions in NH₃ (64.83%), BOD₅ (84.06%), and SS (62.28%). Although there was a marginal reduction in NH₃ levels, significant nitrification occurred within permissible limits. However, the effluents produced by both treatment processes did not meet the national regulatory standards for discharge into surface water bodies, as the residual concentrations of BOD₅, iron, dissolved solids, SS, and color exceeded the limits. Therefore, it is recommended to combine the SBR process with other treatment processes in a sequential manner to achieve higher-quality effluents.

Liu [146] conducted an experiment using a laboratory-scale two-step anoxic/oxic (A/O) combined MBR for 113 days to treat LL with a high NH₄⁺-N concentration and low C/N ratio, as depicted in Figure 11. The average removal rates achieved were 74.87% for TN, 99.04% for NH₄⁺-N, and 80.60% for COD. A mass balance assessment indicated that the second-step A/O process played a crucial role in contaminant removal, with total removal capacities of 22.40 g/d for TN, 24.35 g/d for NH₄⁺-N, and 125.60 g/d for COD. The high-throughput sequencing analysis revealed that Chloroflexi (3.13–4.80%), Firmicutes (3.31–4.53%), Planctomycetes (6.94–8.47%), Bacteroidetes (22.09–27.25%), and Proteobacteria (44.57–50.36%) were the dominant phyla in the bacterial community throughout the operation period. Two-step A/O and MBR showed a pretty good performance for LL treatment.

The TF process showed substantial reductions in SS, turbidity, BOD₅, and NH₃. However, the effluents produced by both treatment processes did not meet the national regulatory standards for discharge into surface water bodies. The concentrations of color, SS, dissolved solids, BOD₅, and Fe exceeded the limits set by regulatory standards. Additional treatment processes may be necessary to achieve higher-quality effluents that comply with regulatory standards. The performance of this combined process can be enhanced by modifying the filter design or enhancing the filter media. For the combined A/O process followed by the MBR process, the results demonstrated effectiveness in removing TN, NH₄⁺-N, and COD.

According to the previous literature, two studies have investigated combined biological and biological treatment processes for treating LL.

Both studies achieved significant removal of pollutants. The combined TF and SBR processes in Aluko and Sridhar’s work [145] showed reductions in various parameters, but additional treatment processes were recommended for meeting discharge standards. On the other hand, the two-stage A/O-MBR in Liu’s work [146] demonstrated high removal rates for TN, NH₄⁺-N, and COD, indicating its effectiveness for LL treatment.

5.8. Biological + Chemical Integrated Processes

Biological treatment processes are widely used for LL treatment due to their many advantages [29]. Because single biological treatment processes for LL treatment are often unable to break down refractory organic matter, discharge standards are not met. Biological treatments are frequently combined with chemical and physical processes in order to get over this restriction [170,171,172,173,174].

Table 15. A summary of the reviewed biological + chemical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Sequencing batch biofilm reactor (SBBR) + EF	Stabilized	Reaction time = 60 min, H2O2 dosage = 5.05 mol/L, and FeSO4 dosage = 0.42 mol/L with a rate of 0.5 mL/min.	For the SBBR process, the reduction in COD, BOD5, and NH4+-N was 21.6%, 54.7%, and 56.1%, respectively. For the EF process, the reduction rate in BOD5, COD, and TOC was 61%, 71.6%, and 40.5%, respectively. The study identified a strong correlation between the absorbance of LL at 254 nm (UV254) and the levels of COD and TOC during the EF process, providing a potential process for estimating these parameters.	-	[160]
Biofiltration + EC	Stabilized	Hydraulic load = 0.17 m3/m2/j, flow rate = 5 L/min, initial pH = 8.4, NaHCO3 dosage = 1 g/L, stainless steel as cathode, magnesium as anode, treatment time = 30 min, and current density = 10 mA/cm2.	For the biofiltration process, the removal efficiency of phosphorus, turbidity, BOD, and NH4+-N was over 98%, 95%, 94%, and 94%, respectively. For the EC process, the removal efficiency of color and COD was 85% and 53%, respectively.	Significant pH increase was observed after the EC process, indicating a potential impact on the environmental balance of the treated water. The efficiency of the EC process was affected by water alkalinity, suggesting a need for optimization of treatment conditions to improve performance.	[161]
SBR + coagulation		For the SBR process, the pH is 7, and the aeration rate is 3 L/min. For the coagulation process, we used alum as a coagulant with a dosage of 5 g/L.	Improved the removal efficiencies to 85.81% for color, 91.82% for TSS, 94.25% for NH3-N, and 84.89% for COD.	-	[162]
SBR + electro-Fenton oxidation	Stabilized	Time = 5 days, <10 kDa MW	For the SBR+EF oxidation process, the COD removal efficiency was 96.3% and 77.3% for >10 KDa MW and <1 KDa MW, respectively. The biological chemistry in the SBR process is capable of breaking down organic compounds, enhancing the processes’ effectiveness in pollutant removal.	-	[163]
SBR + EC	Intermediate	For the EC process, pH = 8.2, electrical content = 2.1 A, Al used as electrode with spacing = 3 cm, settling time = 35 min, reaction time = 60 min, and clinoptilolite dosage = 110 g/750 mL as the regeneration solvent. For the SBR process, fill time is 5 min, mixing homogeneously lasts for 1 min, draw time is 5 min, and idle time is 1 min.	The removal efficiency of 95% ± 1% for color, and 83% ± 2% for NH3-N. Utilization of clinoptilolite, a readily available natural adsorbent, as a key component in the treatment process. This highlights the use of sustainable and easily accessible materials for LL treatment. No chemical was added.	-	[164]

provides an overview of a few biological + chemical combination treatment processes along with an analysis of their effectiveness.

As shown in Figure 12, the proposed system operated continuously. The adsorption kinetics of color and NH₃-N were well described by the pseudo-second-order kinetic model (R² ≈ 1). This research successfully established an advanced technological process for treating leachate using a readily available natural adsorbent, clinoptilolite. The system demonstrated efficient treatment of concentrated leachate at a relatively low overall operating cost of 8.71 USD/m³ per cycle. Therefore, combining biological, physical, and chemical processes into a single process could be a viable option for effectively removing concentrated color and NH₃-N from LL.

5.9. Biological + Physical Integrated Processes

The study by Lim [165] proposed the use of an aerobic SBR (ASBR) followed by zeolite adsorption for treating locally obtained real LL. The ASBR demonstrated the ability to remove 30% of COD and 65% of ammoniacal nitrogen over a seven-day treatment period. Subsequently, zeolite, an effective adsorbent, was employed as a post-treatment step to further polish the LL by adsorbing COD and ammoniacal nitrogen. The combined treatment process exhibited significant removal of COD and heavy metals such as magnesium, aluminum, and lead. In conclusion, this combined treatment approach offers a feasible option for effectively removing contaminants from LL.

Table 16. A summary of the reviewed biological + physical integrated treatment processes and their results.

Study	Leachate Age	Parameters	Advantages	Limitations	Ref.
Aerobic SBR (ASBR) + Adsorption	Stabilized	Treatment time = 7 days, and zeolite as an adsorbent with 24 h of adsorption time.	Improved the removal efficiencies to 43% for COD and 96% for ammoniacal nitrogen, respectively. More effective, environmentally friendly, and cost-effective.	-	[165]
MBR + RO	Stabilized	-	The MBR-alone process reduced the COD from 12,000 to 718 mg/L. The RO process reduced COD from 718 to 10 mg/L. Resulted in treated effluent that met applicable discharge standards. Lower environmental risk.	The effluent from the combined process mainly consisted of phenolic compounds and aliphatic compounds. It should be noted that some of the phenolic compounds may originate from anti-sludge bulking agents used in the treatment process.	[80]

provides an overview of a few biological + physical contamination treatment processes along with an analysis of their effectiveness.

According to Table 16, Chen [80] studied the treatment of LL using a combination of MBR and RO processes. The MBR process alone had limited efficiency in removing organic contaminants from the LL, but when combined with RO, it completely removed macromolecular compounds and produced a treated effluent meeting discharge standards. The LL contained various types of dissolved organic matter (DOM) with a high molecular weight. MBR effectively removed DOM with low saturation and high bioavailability, while RO significantly reduced residual DOM, particularly heteroatom DOM. The combined MBR+RO process produced an effluent mainly consisting of phenolic and aliphatic compounds. The effluent was highly biodegradable, reducing environmental risk compared to untreated LL.

In summary, both studies offer innovative approaches for treating LL, but they have limitations that need to be addressed. The first study [165] lacks information on the long-term performance and stability of the treatment system, while the second study [80] raises concerns about the composition of the treated effluent. Further research and evaluation are needed to fully assess the effectiveness and potential impacts of these treatment approaches on the environment and human health.

6. Conclusions

LL contains complex and refractory pollutants and toxic compounds that can vary depending on landfill maturity, age, and biochemical reactions, making its treatment challenging. Due to its unique characteristics and occurrence in remote locations, LL requires separate treatment from wastewater. Conventional treatment processes, such as biological, physical, and chemical treatment processes, when used alone, are unable to totally remove these contaminants. In order to treat LL, many combinations of treatment techniques have been proposed between 2011 and 2023. Based on the findings of the reviewed studies, the following key conclusions can be drawn:

• It was found that combining successive chemical processes was the most popular and widely used treatment.
• The most successful treatment processes were found to be a sequential combination of chemical treatments, followed by biological–physical treatment processes.
• In comparison to single treatment processes, the use of various combined treatments demonstrated an improvement in efficiency in terms of not only removing COD, TN, TDS, NH₃-N, SS, color, turbidity, and heavy metals such as Ni, Cd, Mn, and Fe, but also in terms of process simplicity and sludge reduction.
• Compatible processes that showed higher efficiency in COD removal included MBR with NF (with removal rates reaching 99 ± 1%), Fenton with adsorption (with removal rates up to 99%), PF with biological treatment (with removal rates up to 98%), EF with biological treatment (with removal rates up to 97%), PEF with MBR (with removal rates up to 96.2%), and electrochemical with EF (with removal rates reaching 95.9 ± 0.4%).
• The compatible combination of the Fenton process with adsorption and the EC process with SPF showed improvements in biodegradability, with the BOD₅/COD ratio increasing from 0.084 to 0.82 and from 0.35 to 0.75, respectively.
• The co-treatment of LL with wastewater using combined treatment processes has proven to be highly beneficial in terms of pollutant removal and treatment efficiency. Among the commonly employed co-treatment processes, the combination of the SBR with EC demonstrated exceptional performance. This integrated approach achieved remarkable removal efficiencies, with COD, TSS, ammonia, nitrate, and phosphate reaching levels as high as 98% and 99%, respectively. These findings underscore the effectiveness of co-treatment processes in addressing the challenges associated with landfill leachate and wastewater, leading to enhanced removal of pollutants and improved treatment outcomes.
• Countries with heavy rainfall, such as those with tropical Savanna, continental, and humid subtropical climates, tend to generate more waste compared to regions with humid continental, Mediterranean, and semi-arid climates.
• Croatia exhibits the highest waste generation levels, ranging from 1000 to 21,000 mg/L, attributed to its continental climate conditions.
• Significant concentrations of organic LL, measured as COD, were found in Morocco (COD = 22,000 mg/L), China (COD = 6880 ± 180 mg/L), and Algeria (COD = 3847.7 mg/L) for young, intermediate, and stabilized LL.

7. Recommendation

For many years, conventional chemical, physical, and biological treatment processes have been considered the most suitable technologies for managing and manipulating high-strength effluents such as LL. When treating stabilized LL that contains less biodegradable substances, physical and chemical treatment processes have been found to be effective as a refining step after a biological treatment process to remove organic refractory substances. However, when dealing with young LL, biological treatment processes can achieve reasonable treatment performance in terms of NH₃-N, COD, and heavy metals. In recent years, due to increasingly stringent discharge standards in most countries and the aging of landfill sites resulting in more stabilized leachates, conventional treatments are no longer sufficient to achieve the required level of purification necessary to fully mitigate the negative environmental impact of LL. This indicates the need for the proposal of new alternative treatment processes.

The combination of chemical, physical, and biological treatment processes, regardless of the order in which they are implemented, addresses the limitations of individual processes and enhances the overall treatment efficacy [29]. The concept of combining treatments has proven to be a more efficient and successful approach. After conducting a thorough literature review on the assessment of combined treatment processes for treating LL, it is recommended that

• The utilization of environmentally friendly technologies, such as solar energy, on an industrial scale, offers a cost-effective solution. By incorporating solar energy into industrial processes, businesses can reduce their environmental footprint while also benefiting from the economic advantages associated with renewable energy sources. This approach aligns with the principles of sustainability and promotes a cleaner and more sustainable industrial sector.
• The recovery and utilization of valuable substances from LL presents a significant opportunity for resource optimization. LL is rich in metal elements, making it a potential source for secondary resources. To fully harness this potential, it is crucial to focus on the advancement of scientifically grounded processes that enable efficient separation and enrichment of valuable substances present in LL. By doing so, we can capitalize on the extensive resources found within LL, consequently reducing the reliance on costly synthetic substances.
• The utilization of kinetic and statistical models to comprehensively describe the various combined processes is currently constrained. However, if these models accurately depict the experimental outcomes, they hold the potential for application in the upscaling of such systems [29,175,176,177,178].