Abstract

Landfill leachate is characterised by high chemical and biological oxygen demand and generally consists of undesirable substances such as organic and inorganic contaminants. Landfill leachate may differ depending on the content and age of landfill contents, the degradation procedure, climate and hydrological conditions. We aimed to explain the characteristics of landfill leachate and define the practicality of using different techniques for treating landfill leachate. Different treatments comprising biological methods (e.g. bioreactors, bioremediation and phytoremediation) and physicochemical approaches (e.g. advanced oxidation processes, adsorption, coagulation/flocculation and membrane filtration) were investigated in this study. Membrane bioreactors and integrated biological techniques, including integrated anaerobic ammonium oxidation and nitrification/denitrification processes, have demonstrated high performance in ammonia and nitrogen elimination, with a removal effectiveness of more than 90%. Moreover, improved elimination efficiency for suspended solids and turbidity has been achieved by coagulation/flocculation techniques. In addition, improved elimination of metals can be attained by combining different treatment techniques, with a removal effectiveness of 40–100%. Furthermore, combined treatment techniques for treating landfill leachate, owing to its high chemical oxygen demand and concentrations of ammonia and low biodegradability, have been reported with good performance. However, further study is necessary to enhance treatment methods to achieve maximum removal efficiency.

HIGHLIGHTS

  • Membrane bioreactors and integrated biological techniques could remove up to 100% of ammonia.

  • Enhanced elimination of metals can be gained by combining different treatment methods.

  • Better elimination efficiency for suspended solids has been achieved by coagulation/flocculation.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

Urban solid waste landfills are commonly used for household, industrial nonhazardous and commercial solid wastes as well as nonhazardous sludge (Mojiri et al. 2016a). Sanitary landfilling continues to be employed in waste management plans despite its potentially hazardous effect on the environment (Mojiri et al. 2017). Compared with other methods, such as incineration, sanitary landfilling generally entails lower operation costs (Gotvajn & Pavko 2015). Waste may undergo a series of biological and physicochemical transformations after being landfilled, thereby producing extremely polluted wastewater called leachate. Such wastewater may pollute nearby ground and surface water as well as soil (Zamri et al. 2017).

Landfill leachate is characterised by high chemical and biological oxygen demand (COD, BOD) and often consists of high concentrations of organic contaminants, heavy metals, toxic materials, ammonia and inorganic materials as well as refractory compounds, such as humic substances (Chávez et al. 2019) as well as contaminants of emerging concern (Eggen et al. 2010). The characteristics of landfill leachate may differ depending on the degradation procedure, climate, hydrology conditions and age of a landfill. Ecological pollution and health issues are commonly connected to the insufficient treatment of landfill leachate (Mojiri et al. 2016a).

Minimising risks to the environment and human health is a serious concern in open dumping and sanitary landfills (Xaypanya et al. 2018). Appropriate key techniques for landfill leachate treatment consist of biological methods and chemical and physical processes. However, a comprehensive assessment of landfill leachate, including its characteristics, influences and treatment techniques, is lacking. Thus, this article serves to provide such a critical review.

LANDFILL LEACHATE AND ITS CHARACTERISTICS

Leachate forms when water penetrates waste in a landfill and transfers certain forms of contaminants (Mojiri et al. 2017). Municipal landfill leachate contains pollutants that can be categorised into four key groups, namely, organic contaminants and substrates, inorganic compounds, heavy metals, total dissolved solids (TDS) and colour (Mojiri et al. 2016a). Based on its age, landfill leachate may be divided into three key groups (Table 1), namely, young, intermediate and old (Aziz 2012; Tejera et al. 2019). Aziz (2012) and Vaccari et al. (2019) stated that in ‘young’ landfills (i.e. the acid phase), leachate is characterised by low pH levels, high concentrations of volatile acids and simply degraded organic matter. In mature landfills (i.e. the methanogenic phase), leachate methane production and pH are high, and the organic materials present are mainly humic and fulvic fractions. However, there is a slightly difference in some other studies (Wang et al. 2018a, 2018b) due to the waste characteristics based on the countries. Table 2 shows the characteristics of landfill leachate around the world. Based on Table 2, most concentrated landfill leachates were located in China with COD (mg/L, 28,000) and in Riyadh (Saudi Arabia) with Fe (167.6 mg/L) for concentrated landfill leachate.

Table 1

Leachate characteristics and treatability based on the landfill age

Age (years) Young Intermediate Old 
0–5 5–10 >10 
pH <6.5 6.5–7.5 >7.5 
COD (mg/L) >10,000 5,000–10,000 <5,000 
BOD5/COD 0.5–1.0 0.1–0.5 >0.1 
NH3–N (mg/L) <400 – >400 
H.M Medium to low Low Low 
VFA/HFA VFA (80%) VFA (5–30%) + HFA HFA (80%) 
Biodegradability High Medium Low 
Age (years) Young Intermediate Old 
0–5 5–10 >10 
pH <6.5 6.5–7.5 >7.5 
COD (mg/L) >10,000 5,000–10,000 <5,000 
BOD5/COD 0.5–1.0 0.1–0.5 >0.1 
NH3–N (mg/L) <400 – >400 
H.M Medium to low Low Low 
VFA/HFA VFA (80%) VFA (5–30%) + HFA HFA (80%) 
Biodegradability High Medium Low 

H.M, heavy metals; VFH, volatile fatty acids; HFA, humic and fluvic acids.

Table 2

Characteristics of landfill leachate around the world

RemarksCOD (mg/L)BOD5BOD5/CODAmmonia (mg/L)Heavy metals (mg/L)
LocationReferences
FeMnZnCdNi
Concentrated leachate 28,000 950 0.04 3.50 30.00 4.03 17.80 NR 3.70 MSW incineration plants, China Ren et al. (2018)  
Semi-aerobic 935 83 0.09 483 7.9 NR 0.6 NR NR Pulau Burung, Malaysia Kamaruddin et al. (2015)  
– 6,140 558 0.09 1,856 NR NR NR 0.01 NR Heimifeng, Changsha, China Hu et al. (2016)  
Covered landfill 24,040 15,021 0.59 2,281 10.37 NR 0.96 NR 0.95 Istanbul Kömürcüoda Landfill, Turkey Akgul et al. (2013)  
– 2,350 NR NR 310 NR NR 0.05 0.02 0.54 Sivas, Turkey Atmaca (2009)  
Sanitation landfill 2,305 105 0.04 1,240 NR NR NR NR NR Beijing, China Wang et al. (2016)  
Semi-aerobic 1,343 96 0.07 NR 3.41 0.17 2.3 NR 0.17 Matuail landfill, Bangladesh Jahan et al. (2016)  
– 10,400 1,500 0.14 NR 11.16 NR 3.00 0.03 1.33 Mavallipura landfill, India Naveen et al. (2014)  
– 17,003 NR NR NR 167.61 10.83 0.18 NR 0.50 Riyadh City, Saudi Arabia Al-Wabel et al. (2011)  
Semi-sanitary 3,380 760 0.22 1,150 NR NR 1.35–1.60 0.13–0.3 NR Nonthaburi Landfill, Thailand Xaypanya et al. (2018)  
Concentrated landfill leachate 1,281 NR – 14.2 NR 0.692 – – 0.233 Jiangsu Province, China Cui et al. (2018)  
 7,700 1,300 0.16 1,780 10.03 NR 1.06 NR NR Xiangtan, China Hu et al. (2011)  
– 3,308–3,540 823–1,274 0.24–0.35 1,006–1,197 NR NR NR NR NR Nam Binh Duong, Vietnam Luu (2020)  
– 781 1,16 0.14 212 21 NR NR NR NR Jones County Municipal Landfill, Iowa, USA Nivala et al. (2007)  
Sanitation landfill 4,737 NR NR 1,897 NR NR NR NR NR Virginia, USA Iskandar et al. (2017)  
NR 765 70 0.09 342 2.6 NR 0.07 NR NR Saint-Rosaire's City, Québec, Canada Oumar et al. (2016)  
Old and active landfill 1,380 NR NR 665.2 NR NR NR 0.004 NR Jakuševec landfill, Zagreb, Croatia Dolar et al. (2016)  
Operated for 2 years (very young). Non-hazardous wastes, no fermentable wastes 260 47 0.18 187 NR NR NR NR NR France Ricordel & Djelal (2014)  
– 3,847 388 0.11 3,158.98 21.50 NR NR 1.70 NR Ouled Fayet landfill site, Algeria Boumechhour et al. (2012)  
Sanitation landfill 4,425–4,860 433–588 0.09–0.12 NR NR NR NR NR NR Sao Carlos, Brasil Ferraz et al. (2014)  
– 1,013 NR NR 398.02 6.84 0.42 NR 6.26 NR Guaratinguetá, Brasil Peixoto et al. (2018)  
RemarksCOD (mg/L)BOD5BOD5/CODAmmonia (mg/L)Heavy metals (mg/L)
LocationReferences
FeMnZnCdNi
Concentrated leachate 28,000 950 0.04 3.50 30.00 4.03 17.80 NR 3.70 MSW incineration plants, China Ren et al. (2018)  
Semi-aerobic 935 83 0.09 483 7.9 NR 0.6 NR NR Pulau Burung, Malaysia Kamaruddin et al. (2015)  
– 6,140 558 0.09 1,856 NR NR NR 0.01 NR Heimifeng, Changsha, China Hu et al. (2016)  
Covered landfill 24,040 15,021 0.59 2,281 10.37 NR 0.96 NR 0.95 Istanbul Kömürcüoda Landfill, Turkey Akgul et al. (2013)  
– 2,350 NR NR 310 NR NR 0.05 0.02 0.54 Sivas, Turkey Atmaca (2009)  
Sanitation landfill 2,305 105 0.04 1,240 NR NR NR NR NR Beijing, China Wang et al. (2016)  
Semi-aerobic 1,343 96 0.07 NR 3.41 0.17 2.3 NR 0.17 Matuail landfill, Bangladesh Jahan et al. (2016)  
– 10,400 1,500 0.14 NR 11.16 NR 3.00 0.03 1.33 Mavallipura landfill, India Naveen et al. (2014)  
– 17,003 NR NR NR 167.61 10.83 0.18 NR 0.50 Riyadh City, Saudi Arabia Al-Wabel et al. (2011)  
Semi-sanitary 3,380 760 0.22 1,150 NR NR 1.35–1.60 0.13–0.3 NR Nonthaburi Landfill, Thailand Xaypanya et al. (2018)  
Concentrated landfill leachate 1,281 NR – 14.2 NR 0.692 – – 0.233 Jiangsu Province, China Cui et al. (2018)  
 7,700 1,300 0.16 1,780 10.03 NR 1.06 NR NR Xiangtan, China Hu et al. (2011)  
– 3,308–3,540 823–1,274 0.24–0.35 1,006–1,197 NR NR NR NR NR Nam Binh Duong, Vietnam Luu (2020)  
– 781 1,16 0.14 212 21 NR NR NR NR Jones County Municipal Landfill, Iowa, USA Nivala et al. (2007)  
Sanitation landfill 4,737 NR NR 1,897 NR NR NR NR NR Virginia, USA Iskandar et al. (2017)  
NR 765 70 0.09 342 2.6 NR 0.07 NR NR Saint-Rosaire's City, Québec, Canada Oumar et al. (2016)  
Old and active landfill 1,380 NR NR 665.2 NR NR NR 0.004 NR Jakuševec landfill, Zagreb, Croatia Dolar et al. (2016)  
Operated for 2 years (very young). Non-hazardous wastes, no fermentable wastes 260 47 0.18 187 NR NR NR NR NR France Ricordel & Djelal (2014)  
– 3,847 388 0.11 3,158.98 21.50 NR NR 1.70 NR Ouled Fayet landfill site, Algeria Boumechhour et al. (2012)  
Sanitation landfill 4,425–4,860 433–588 0.09–0.12 NR NR NR NR NR NR Sao Carlos, Brasil Ferraz et al. (2014)  
– 1,013 NR NR 398.02 6.84 0.42 NR 6.26 NR Guaratinguetá, Brasil Peixoto et al. (2018)  

NR, not reported.

Colour and TDS

Colour is a common pollutant in landfill leachate. The decomposition of certain organic compounds, such as humic acid (HA), may cause water to turn yellow to dark brown (Naveen et al. 2016). Gotvajn & Pavko (2015) emphasised that substances and particles produce colour and turbidity. TDS display the integrative influence of certain cations and anions, such as calcium, chlorides, magnesium, sodium, potassium and bicarbonates, on water/wastewater. Furthermore, TDS can be produced from small amounts of dissolved organic matter (Sakizadeh 2019) and may inhibit or diminish the biological degradation of dissolved organic carbon (Hanson et al. 2019). Hussein et al. (2019) expressed that high electrical conductivity and TDS may specify dissolved organic and inorganic substances in samples.

Organic and inorganic pollutants, and heavy metals

The organic composition of leachate varies depending on waste characteristics, the age of a landfill and climatic conditions (Mojiri et al. 2016a). Urban solid waste and landfill leachate contain a wide variety of organic compounds (Scandelai et al. 2019). In landfill leachate, dissolved organic matter makes up 80% of total organic compounds and is generally composed of refractory humic substances and volatile fatty acids (Jiang et al. 2019). Such refractory organics may not be efficiently degraded by conventional biological treatments. Dissolved organics may be signified by BOD5 and COD (Samadder et al. 2017). Moreover, persistent organic pollutants may be found in landfill leachate. Scandelai et al. (2019) indicated that various organic compounds with medium and low polarity, such as amines, alcohols, carboxylic acids, aldehydes, benzothiazolone, ketones, phenols, chlorinated benzenes, phosphates, nitrogen compounds, pesticides and aromatic and polyaromatic hydrocarbons, have been frequently noticed in leachate. Contaminants of emerging concern – pharmaceuticals, personal care products, surfactants, plasticisers, fire retardants, pesticides and nanomaterials – are also found in many municipal landfills, requiring attention on their management (Ramakrishnan et al. 2015; Qi et al. 2018).

Inorganic macro components, such as sulphates, chloride, iron, ammonia, aluminium and zinc, comprise anions and cations (Agbozu et al. 2015). Tałałaj (2015) argued that landfill leachate generally consists of large amounts of compounds, 80–95% of which are inorganic and approximately 52% are organic. Inorganic ions contain chloride (Cl), nitrites and nitrates, cyanide (CN), sulphides (S) and sulphates (). Moreover, inorganic cations contain ammonia and ferrous (Tałałaj 2015).

One of the most toxic contaminants in landfill leachate is heavy metals. In most developing countries, the segregation of nonhazardous wastes from hazardous wastes before disposal into a landfill is uncommon (Edokpayi et al. 2018); therefore, several heavy metals in high concentrations have been reported in the landfill leachates (Chuangcham et al. 2008). Removal of heavy metals is a difficult task; consequently, we pay more attention to the removal of metals from landfill leachate in this study. Dan et al. (2017a) reported that the most common heavy metals in landfill leachate are chromium (Cr), manganese (Mn), cadmium (Cd), lead (Pb), iron (Fe), nickel (Ni) and zinc (Z). Metal concentrations in young (acetogenic) leachate are generally higher than those in old leachate (Dan et al. 2017a).

LANDFILL LEACHATE TREATMENT METHODS

The different landfill leachate treatment methods are shown in Figure 1 and Table 3.

Table 3

Reported landfill leachate treatment methods

CompoundsRemoval (mg/L) or Removal efficiency (%)Treatment methodRemarksCategoryReferences
Ammonia 94.5% Adsorption/Photo-Fenton-Ozone Pre-treatment was done via activated carbon (Sawdust) activated by H3PO4. After the adsorption process, the leachate was moved to a solar photo-Fenton/O3 process. Advanced oxidation process/Adsorption Poblete & Pérez (2020)  
COD 95.1% 
Colour 95.0% 
HA (ABS25497.9% 
COD 94% Electrocoagulation/Fiber filtration Anodic electrodes were arranged in parallel. After electrocoagulation with aluminium or iron electrodes, the treated landfill leachate was applied to two stages of fiber filters. Advanced oxidation process/Coagulation/Adsorption Li et al. (2017)  
As 87% 
Fe 96% 
86% 
COD 3,381.9 mg/L Electro-catalytic ozonation The current density was 42.1 mA/cm2, and ozone concentrations varied 100–400 mg/h. This method increased biodegradability index from 0.27 to 0.45. Advanced oxidation process Ghahrchi & Rezaee (2020)  
BOD 1,521 mg/L 
Ammonia 90% Supercritical water oxidation (ScWO)/Zeolite ScWO was operated under a pressure of 23 MPa at 600 and 700 °C, without the addition of oxidants. Zeolite was used by following ScWO. Advanced oxidation process/Adsorption (ion-exchange) Scandelai et al. (2020)  
Nitrite 100% 
Nitrate 98% 
Colour 98% 
Turbidity 98% 
COD 74% 
COD 83.3% Kefir grains/Ag-doped TiO2 photocatalytic Biological pre-treatment was done in 250 mL beakers containing 50 mL of leachate inoculated with Kefir grains. Then, leachate was moved for treatment by using Ag-doped TiO2 photocatalytic. Advanced oxidation process/biological method Elleuch et al. (2020)  
Ammonia 70.0% 
Cd 100% 
Ni 94.0% 
Zn 62.5% 
Mn 53.1% 
Cu 47.5% 
COD 68% Coagulation/Photo-Fenton Ferric chloride in acidic condition and Alum in neutral condition were used as coagulant.
The photo-Fenton process was conducted using a high-pressure mercury immersion lamp of 450 W from ACE-Glass. 
Advanced oxidation process/Coagulation Tejera et al. (2019)  
Colour 97% 
HA (UV-254) 83% 
COD 97.8% Fenton process The Fenton reaction was done by adding powdered ferrous sulphate and an appropriate H2O2:Fe2+ ratio. Advanced oxidation process Roudi et al. (2018)  
COD 90.2% Coagulation-flocculation/ Microelectrolysis-Fenton processes Landfill leachate was treated by chemical flocculation with polyaluminium chloride (PAC) as flocculant, and subsequently purified by microelectrolysis-Fenton process. Concentration of H2O2 (mg/L) varied 2.66–4. Advance oxidation process/Coagulation-flocculation Luo et al. (2019)  
HA 93.7% 
COD 88.2% Electro-ozonation/adsorbent augmented SBR At first stage, the raw concentrated leachate was treated by electro-ozonation reactor. The electro-ozone reactor was reinforced by a cross-column ozone chamber to develop ozone gas diffusion. Furthermore, the ozone reactor was supported with anode and cathode plates (Ti/RuO2–IrO2, 18 cm × 8 cm). After that leachate was moved to the second reactor (SBR + Composite adsorbent). Advanced oxidation process/biological/adsorption Mojiri et al. (2017)  
Colour 96.1% 
Ni 73.4% 
Colour >90% EO/Coagulation Al2(SO4)3 with dosage of 50 g/L was added as coagulant. And two stainless steel plates were applied as electrodes. Sodium sulphate 0.1 mol/L was added to the leachate in order to improve the conductivity of the solution. Advanced oxidation process/coagulation de Oliveira et al. (2019)  
Turbidity >90% 
Ammonia >90% 
COD 36% UVsolar/O3/H2O2//Zeolite Ozone, hydrogen peroxide and UVsolar were considered in the same reactor with leachate to produce a high amount of hydroxyl radicals, which have a short life. The was added directly. Then, treated leachate was treated by zeolite. Advanced oxidation process /adsorption Poblete et al. (2019)  
Ammonia 99% 
COD 91% UV-based sulphate radical oxidation process/Coagulation-flocculation For coagulation-flocculation (pre-treatment), ferric chloride (FeCl3) was used, with COD:FeCl3 ratio = 1:1.3, as the coagulant. Then, leachate was treated by UV-based sulphate radical oxidation process (UV-SRAOP). For UV/SRAOP, the sulphate radical was produced using UV-activated persulphate (UV/PS) and peroxymonosulphate (UV/PMS). Advanced oxidation process/Coagulation-flocculation Ishak et al. (2018)  
Colour 100% Ozone/catalyst (ZrCl4Zirconium tetrachloride was added, dosage 1.2 g (COD/ZrCl4), as a catalyst to ozone reactor. Advanced oxidation process Abu Amr et al. (2017)  
COD 88% 
Ammonia 79% 
COD 16.5% Vermiculite/Ozonation Rotating packed bed reactor was used to provide greater gas diffusion to the medium. Optimum operation conditions were as follows: rotation of 915 rpm, pH of 5.8 and ozone flow of 3.9 L/min. Biodegradability was increased (BOD5/COD), from 0.13 to 0.49 by this treatment method. Advanced oxidation process Braga et al. (2020)  
Colour 40.5% 
COD 72% MAC/Ozonation MnCe-ACs were produced by impregnating Mn and Ce oxides onto granular activated carbon surfaces. MnCe-AC was added to a cylinder and ozone was added from bottom of the reactor. Advanced oxidation process/Adsorption Wang et al. (2015a, 2015b
HA 91% 
COD 100% Activated carbon (Oat hulls) Oat hulls adsorbents were activated with phosphoric acid and pyrolysed (N2 atmosphere) at 350 and 500 °C. Adsorption methods Ferraz & Yuan (2020)  
Colour 100% 
COD 51.0% Activated carbon (Coffee wastes) The washed coffee was oven-dried at 105 °C for 24 h prior to activation. And then it was activated via H3PO4Adsorption methods Chávez et al. (2019)  
Ammonia 32.8% 
Chlorine 66.0% 
Bromine 81.0% 
Copper 97.1% 
COD 93.6% Zero-valent iron nanofibers/reduced ultra-large graphene oxide (ZVINFs/rULGO) At the optimum condition, pH, dosage of ZVINFs/rULGO and reaction time were 3, 1.6 g/L and 45 min. Adsorption methods Soubh et al. (2018)  
Ammonia 84.8% 
COD 77.3% Silica nanoparticle At the optimum condition, pH and dosage of adsorbent were 6 and 90 min. Adsorption methods Pavithra & Shanthakumar (2017)  
Colour 82.5% 
COD 49% Zeolite Feldspar Mineral Composite Adsorbent Samples were shaken for 5 h with 200 rpm at pH 7. Adsorption methods Daud et al. (2016)  
Ammonia 45% 
COD 65.5–92.1% Amino acid modified bentonite Batch experiments were done under contact time 20–100 min, pH 2–11 and bentonite dosage of 10–40 g/L. Adsorption methods Hajjizadeh et al. (2020)  
Pb 99.2 MS@GG MS@GG was produced by modification of melamine sponge (MS) with polydopamine (PDA) and then coat with glutathione/graphene oxide. Adsorption methods Feng et al. (2019)  
COD 53.5% Tannin-Based Natural Coagulant Tannin dosage and pH were 0.73 g and 6, respectively. Coagulation/flocculation Banch et al. (2019)  
Ammonia 91.3% 
TSS 60.2% 
Fe 89.7% 
Zn 94.6% 
Cu 94.1% 
Cr 89.9% 
Cd 17.2% 
Pb 93.7% 
As 86.4% 
COD 61.9% Polyaluminium chloride and Dimocarpus longan Seeds as Flocculants A coagulation–flocculation process using a combination of Polyaluminium chloride (PACl) as a coagulant and Dimocarpus longan seed powder (LSP) as coagulant aid was done. Coagulation/flocculation Aziz et al. (2018)  
Colour 98.8% 
SS 99.5% 
COD 66.9% Red earth as coagulant The optimal pH and the optimal coagulant dosage were 5.0 and of 9,000 mg/L, respectively. Coagulation/flocculation Zainol et al. (2018)  
Ammonia 43.3% 
Turbidity 96.2% 
COD 45% Ferric chloride as coagulant and a cationic flocculant AN 934-SH polyelectrolytes as flocculant The pH was fixed at 6.3. Optimum condition was 7.2 g/L FeCl3 and 0.2 mL/L Flocculant. Coagulation/flocculation Taoufik et al. (2018)  
COD 94.6% Using membrane processes of NF and RO A working pressure and flow rate were set at 15 bar and 750 mL/min. The surface area of the membranes was 10.7 cm. Membrane Košutić et al. (2015)  
Ammonia Up to 88.9% 
COD
BOD Ammonia 
17.5–48.5% 45.4–81.6% 50–98.8% Using Aspergillus flavus The A. flavus strain were isolated form leachate contaminated soil. Bioremediation with the fungi Zegzouti et al. (2020)  
COD 40% Using Brevibacillus panacihumi strain ZB1 The pure colonies of B. panacihumi strain ZB1 were grown in sterile nutrient broth in the incubator shaker for 24 h. About 10% (v/v) of the B. panacihumi strain ZB1 was used to treat the raw leachate sample in the 200 mL conical flask. The leachate sample was treated anaerobically for 21 days and followed by 21-days aerobic treatment. Bioremediation Er et al. (2019)  
Ammonia 50% 
Mn 40% 
Cu 60% 
Se 52% 
Ammonia 90% Using Chlorella sp. After growing the Chlorella sp., it was inoculated for experimental studies. Bioremediation with microalgae Ouaer et al. (2017)  
COD 60% 
Ammonia 83% Using Chlamydomonas sp. SW15aRL The Chlamydomonas sp. strain SW15aRL, previously isolated from a sample of raw leachate in 2014 from a landfill site, was maintained in raw leachate or diluted raw leachate samples with a phosphate concentration adjusted to a molar N:P ratio ∼ 16:1 prior to the experiments. Bioremediation with microalgae Paskuliakova et al. (2018a)  
Leachate Pollution Index 74.7% Using garbage enzyme The garbage enzyme (fermented mixture of jaggery, organic waste and water in the ratio 1:3:10) was applied. Bioremediation/Enzyme Rani et al. (2020)  
COD 67% Using Colocasia esculenta, Gynerium sagittatum and Heliconia psittacorumPlants were transplanted in a constructed wetland with a gravity flow (Q = 0.5 m3/d). Phytoremediation/wetland Madera-Parra (2016)  
Cd 80% 
Pb 40% 
Hg 50% 
COD 75% Using Imperata cylindrica Contact time was ranged from 0 to 30 days. Phytoremediation Moktar & Tajuddin (2019)  
Pb 56.3% 
Cd 16.2% 
Zn 6.5% 
COD 81.0% Using Typha latifolia
Using Canna indica 
Flow rate of 5 L/day and a HRT of 22 days were used. Phytoremediation/wetland Yalçuk & Ugurlu (2020)  
Ammonia 60.0% 
COD 84.0% 
Ammonia 56.0% 
COD 86.7% Using Typha domingensis Plants in a reactor with two kinds of substrates including zeolite and ZELIAC. 20% of landfill leachate was mixed with 80% of domestic wastewater at optimum condition. Wetland/co-treatment Mojiri et al. (2016b)  
Ammonia 99.2% 
Colour 90.3% 
Ni 86.0% 
Cd 87.1% 
COD 93% Membrane bioreactor + Activated sludge
Membrane bioreactor + Indigenous leachate bacteria 
Membrane sequenced batch bioreactors were inoculated indigenous leachate bacteria or activated sludge. Bioreactor/Membrane Azzouz et al. (2018)  
Fe 71% 
Zn 78% 
COD 95% 
Fe 71% 
Zn 74% 
COD 63% Membrane bioreactor Organic load rate of 1.2 gCOD/L/day and sludge retention time of 80 days were selected. Bioreactor/Membrane Zolfaghari et al. (2016)  
TOC 35% 
Ammonia 98% 
Phosphorous 52% 
Ammonia >98% Membrane bioreactor DM filtration was conducted in a submerged configuration inside the aerobic bioreactor. Bioreactor/Membrane Saleem et al. (2018a)  
TN >90% 
COD 80% Air stripping, and aerobic and anaerobic biological processes For aerobic reactor, the activated sludge system was applied. And for anaerobic reactor, the upflow anaerobic fixed bed reactor was used. Bioreactor/Air Stripping Smaoui et al. (2020)  
Ammonia 78% 
Colour 85.8% SBR and coagulation Sequential treatment via SBR followed by coagulation was applied. Aluminium Sulphate was used as coagulant. Bioreactor/Coagulation Yong et al. (2018)  
COD 84.8% 
Ammonia 94.2% 
TSS 91.8% 
COD >70% Anaerobic Sequencing Batch Biofilm Reactor Biomass from the bottom of a landfill leachate stabilisation pond was immobilized in polyurethane foam cubes as inoculum. Bioreactor Contrera et al. (2018)  
COD 30% Aerobic sequencing batch reactor (ASBR) Air upflow velocity was set at 1.0–1.2 cm/s. Bioreactor Lim et al. (2016)  
Ammonia 65% 
TN 95.0% Partial-denitrification and Anammox Firstly, leachate diluted with municipal sewage. And two USB reactors were used. Integrated bioreactor Wu et al. (2018)  
TN 98.7% Partial nitrification, simultaneous anammox and denitrification During the aerobic phase, the DO was maintained below 0.5 mg/L. Integrated bioreactor Zhang et al. (2019)  
Ammonia 98% DM bioreactor DM filtration was conducted in a submerged configuration inside the aerobic bioreactor provided with a hydrostatic water head of 8 cm. And the initial inoculum was collected from the aerobic bioreactor in a municipal wastewater treatment plant. Bioreactor/Membrane Saleem et al. (2018b)  
TN 90% 
COD 99% Activated sludge process/RO Biological pre-treatments followed by RO. Bioreactor/Membrane Tałałaj et al. (2019
Ammonia 99% 
Fe 79.7% ZELIAC (a composite adsorbent), with dosage of 3 g/L, was augmented in SBR. Powdered ZELIAC was added to the SBR. Bioreactor/Adsorption Mojiri et al. (2016a)  
Mn 73.3% 
Cd 76.9% 
Ni 79.2% 
CompoundsRemoval (mg/L) or Removal efficiency (%)Treatment methodRemarksCategoryReferences
Ammonia 94.5% Adsorption/Photo-Fenton-Ozone Pre-treatment was done via activated carbon (Sawdust) activated by H3PO4. After the adsorption process, the leachate was moved to a solar photo-Fenton/O3 process. Advanced oxidation process/Adsorption Poblete & Pérez (2020)  
COD 95.1% 
Colour 95.0% 
HA (ABS25497.9% 
COD 94% Electrocoagulation/Fiber filtration Anodic electrodes were arranged in parallel. After electrocoagulation with aluminium or iron electrodes, the treated landfill leachate was applied to two stages of fiber filters. Advanced oxidation process/Coagulation/Adsorption Li et al. (2017)  
As 87% 
Fe 96% 
86% 
COD 3,381.9 mg/L Electro-catalytic ozonation The current density was 42.1 mA/cm2, and ozone concentrations varied 100–400 mg/h. This method increased biodegradability index from 0.27 to 0.45. Advanced oxidation process Ghahrchi & Rezaee (2020)  
BOD 1,521 mg/L 
Ammonia 90% Supercritical water oxidation (ScWO)/Zeolite ScWO was operated under a pressure of 23 MPa at 600 and 700 °C, without the addition of oxidants. Zeolite was used by following ScWO. Advanced oxidation process/Adsorption (ion-exchange) Scandelai et al. (2020)  
Nitrite 100% 
Nitrate 98% 
Colour 98% 
Turbidity 98% 
COD 74% 
COD 83.3% Kefir grains/Ag-doped TiO2 photocatalytic Biological pre-treatment was done in 250 mL beakers containing 50 mL of leachate inoculated with Kefir grains. Then, leachate was moved for treatment by using Ag-doped TiO2 photocatalytic. Advanced oxidation process/biological method Elleuch et al. (2020)  
Ammonia 70.0% 
Cd 100% 
Ni 94.0% 
Zn 62.5% 
Mn 53.1% 
Cu 47.5% 
COD 68% Coagulation/Photo-Fenton Ferric chloride in acidic condition and Alum in neutral condition were used as coagulant.
The photo-Fenton process was conducted using a high-pressure mercury immersion lamp of 450 W from ACE-Glass. 
Advanced oxidation process/Coagulation Tejera et al. (2019)  
Colour 97% 
HA (UV-254) 83% 
COD 97.8% Fenton process The Fenton reaction was done by adding powdered ferrous sulphate and an appropriate H2O2:Fe2+ ratio. Advanced oxidation process Roudi et al. (2018)  
COD 90.2% Coagulation-flocculation/ Microelectrolysis-Fenton processes Landfill leachate was treated by chemical flocculation with polyaluminium chloride (PAC) as flocculant, and subsequently purified by microelectrolysis-Fenton process. Concentration of H2O2 (mg/L) varied 2.66–4. Advance oxidation process/Coagulation-flocculation Luo et al. (2019)  
HA 93.7% 
COD 88.2% Electro-ozonation/adsorbent augmented SBR At first stage, the raw concentrated leachate was treated by electro-ozonation reactor. The electro-ozone reactor was reinforced by a cross-column ozone chamber to develop ozone gas diffusion. Furthermore, the ozone reactor was supported with anode and cathode plates (Ti/RuO2–IrO2, 18 cm × 8 cm). After that leachate was moved to the second reactor (SBR + Composite adsorbent). Advanced oxidation process/biological/adsorption Mojiri et al. (2017)  
Colour 96.1% 
Ni 73.4% 
Colour >90% EO/Coagulation Al2(SO4)3 with dosage of 50 g/L was added as coagulant. And two stainless steel plates were applied as electrodes. Sodium sulphate 0.1 mol/L was added to the leachate in order to improve the conductivity of the solution. Advanced oxidation process/coagulation de Oliveira et al. (2019)  
Turbidity >90% 
Ammonia >90% 
COD 36% UVsolar/O3/H2O2//Zeolite Ozone, hydrogen peroxide and UVsolar were considered in the same reactor with leachate to produce a high amount of hydroxyl radicals, which have a short life. The was added directly. Then, treated leachate was treated by zeolite. Advanced oxidation process /adsorption Poblete et al. (2019)  
Ammonia 99% 
COD 91% UV-based sulphate radical oxidation process/Coagulation-flocculation For coagulation-flocculation (pre-treatment), ferric chloride (FeCl3) was used, with COD:FeCl3 ratio = 1:1.3, as the coagulant. Then, leachate was treated by UV-based sulphate radical oxidation process (UV-SRAOP). For UV/SRAOP, the sulphate radical was produced using UV-activated persulphate (UV/PS) and peroxymonosulphate (UV/PMS). Advanced oxidation process/Coagulation-flocculation Ishak et al. (2018)  
Colour 100% Ozone/catalyst (ZrCl4Zirconium tetrachloride was added, dosage 1.2 g (COD/ZrCl4), as a catalyst to ozone reactor. Advanced oxidation process Abu Amr et al. (2017)  
COD 88% 
Ammonia 79% 
COD 16.5% Vermiculite/Ozonation Rotating packed bed reactor was used to provide greater gas diffusion to the medium. Optimum operation conditions were as follows: rotation of 915 rpm, pH of 5.8 and ozone flow of 3.9 L/min. Biodegradability was increased (BOD5/COD), from 0.13 to 0.49 by this treatment method. Advanced oxidation process Braga et al. (2020)  
Colour 40.5% 
COD 72% MAC/Ozonation MnCe-ACs were produced by impregnating Mn and Ce oxides onto granular activated carbon surfaces. MnCe-AC was added to a cylinder and ozone was added from bottom of the reactor. Advanced oxidation process/Adsorption Wang et al. (2015a, 2015b
HA 91% 
COD 100% Activated carbon (Oat hulls) Oat hulls adsorbents were activated with phosphoric acid and pyrolysed (N2 atmosphere) at 350 and 500 °C. Adsorption methods Ferraz & Yuan (2020)  
Colour 100% 
COD 51.0% Activated carbon (Coffee wastes) The washed coffee was oven-dried at 105 °C for 24 h prior to activation. And then it was activated via H3PO4Adsorption methods Chávez et al. (2019)  
Ammonia 32.8% 
Chlorine 66.0% 
Bromine 81.0% 
Copper 97.1% 
COD 93.6% Zero-valent iron nanofibers/reduced ultra-large graphene oxide (ZVINFs/rULGO) At the optimum condition, pH, dosage of ZVINFs/rULGO and reaction time were 3, 1.6 g/L and 45 min. Adsorption methods Soubh et al. (2018)  
Ammonia 84.8% 
COD 77.3% Silica nanoparticle At the optimum condition, pH and dosage of adsorbent were 6 and 90 min. Adsorption methods Pavithra & Shanthakumar (2017)  
Colour 82.5% 
COD 49% Zeolite Feldspar Mineral Composite Adsorbent Samples were shaken for 5 h with 200 rpm at pH 7. Adsorption methods Daud et al. (2016)  
Ammonia 45% 
COD 65.5–92.1% Amino acid modified bentonite Batch experiments were done under contact time 20–100 min, pH 2–11 and bentonite dosage of 10–40 g/L. Adsorption methods Hajjizadeh et al. (2020)  
Pb 99.2 MS@GG MS@GG was produced by modification of melamine sponge (MS) with polydopamine (PDA) and then coat with glutathione/graphene oxide. Adsorption methods Feng et al. (2019)  
COD 53.5% Tannin-Based Natural Coagulant Tannin dosage and pH were 0.73 g and 6, respectively. Coagulation/flocculation Banch et al. (2019)  
Ammonia 91.3% 
TSS 60.2% 
Fe 89.7% 
Zn 94.6% 
Cu 94.1% 
Cr 89.9% 
Cd 17.2% 
Pb 93.7% 
As 86.4% 
COD 61.9% Polyaluminium chloride and Dimocarpus longan Seeds as Flocculants A coagulation–flocculation process using a combination of Polyaluminium chloride (PACl) as a coagulant and Dimocarpus longan seed powder (LSP) as coagulant aid was done. Coagulation/flocculation Aziz et al. (2018)  
Colour 98.8% 
SS 99.5% 
COD 66.9% Red earth as coagulant The optimal pH and the optimal coagulant dosage were 5.0 and of 9,000 mg/L, respectively. Coagulation/flocculation Zainol et al. (2018)  
Ammonia 43.3% 
Turbidity 96.2% 
COD 45% Ferric chloride as coagulant and a cationic flocculant AN 934-SH polyelectrolytes as flocculant The pH was fixed at 6.3. Optimum condition was 7.2 g/L FeCl3 and 0.2 mL/L Flocculant. Coagulation/flocculation Taoufik et al. (2018)  
COD 94.6% Using membrane processes of NF and RO A working pressure and flow rate were set at 15 bar and 750 mL/min. The surface area of the membranes was 10.7 cm. Membrane Košutić et al. (2015)  
Ammonia Up to 88.9% 
COD
BOD Ammonia 
17.5–48.5% 45.4–81.6% 50–98.8% Using Aspergillus flavus The A. flavus strain were isolated form leachate contaminated soil. Bioremediation with the fungi Zegzouti et al. (2020)  
COD 40% Using Brevibacillus panacihumi strain ZB1 The pure colonies of B. panacihumi strain ZB1 were grown in sterile nutrient broth in the incubator shaker for 24 h. About 10% (v/v) of the B. panacihumi strain ZB1 was used to treat the raw leachate sample in the 200 mL conical flask. The leachate sample was treated anaerobically for 21 days and followed by 21-days aerobic treatment. Bioremediation Er et al. (2019)  
Ammonia 50% 
Mn 40% 
Cu 60% 
Se 52% 
Ammonia 90% Using Chlorella sp. After growing the Chlorella sp., it was inoculated for experimental studies. Bioremediation with microalgae Ouaer et al. (2017)  
COD 60% 
Ammonia 83% Using Chlamydomonas sp. SW15aRL The Chlamydomonas sp. strain SW15aRL, previously isolated from a sample of raw leachate in 2014 from a landfill site, was maintained in raw leachate or diluted raw leachate samples with a phosphate concentration adjusted to a molar N:P ratio ∼ 16:1 prior to the experiments. Bioremediation with microalgae Paskuliakova et al. (2018a)  
Leachate Pollution Index 74.7% Using garbage enzyme The garbage enzyme (fermented mixture of jaggery, organic waste and water in the ratio 1:3:10) was applied. Bioremediation/Enzyme Rani et al. (2020)  
COD 67% Using Colocasia esculenta, Gynerium sagittatum and Heliconia psittacorumPlants were transplanted in a constructed wetland with a gravity flow (Q = 0.5 m3/d). Phytoremediation/wetland Madera-Parra (2016)  
Cd 80% 
Pb 40% 
Hg 50% 
COD 75% Using Imperata cylindrica Contact time was ranged from 0 to 30 days. Phytoremediation Moktar & Tajuddin (2019)  
Pb 56.3% 
Cd 16.2% 
Zn 6.5% 
COD 81.0% Using Typha latifolia
Using Canna indica 
Flow rate of 5 L/day and a HRT of 22 days were used. Phytoremediation/wetland Yalçuk & Ugurlu (2020)  
Ammonia 60.0% 
COD 84.0% 
Ammonia 56.0% 
COD 86.7% Using Typha domingensis Plants in a reactor with two kinds of substrates including zeolite and ZELIAC. 20% of landfill leachate was mixed with 80% of domestic wastewater at optimum condition. Wetland/co-treatment Mojiri et al. (2016b)  
Ammonia 99.2% 
Colour 90.3% 
Ni 86.0% 
Cd 87.1% 
COD 93% Membrane bioreactor + Activated sludge
Membrane bioreactor + Indigenous leachate bacteria 
Membrane sequenced batch bioreactors were inoculated indigenous leachate bacteria or activated sludge. Bioreactor/Membrane Azzouz et al. (2018)  
Fe 71% 
Zn 78% 
COD 95% 
Fe 71% 
Zn 74% 
COD 63% Membrane bioreactor Organic load rate of 1.2 gCOD/L/day and sludge retention time of 80 days were selected. Bioreactor/Membrane Zolfaghari et al. (2016)  
TOC 35% 
Ammonia 98% 
Phosphorous 52% 
Ammonia >98% Membrane bioreactor DM filtration was conducted in a submerged configuration inside the aerobic bioreactor. Bioreactor/Membrane Saleem et al. (2018a)  
TN >90% 
COD 80% Air stripping, and aerobic and anaerobic biological processes For aerobic reactor, the activated sludge system was applied. And for anaerobic reactor, the upflow anaerobic fixed bed reactor was used. Bioreactor/Air Stripping Smaoui et al. (2020)  
Ammonia 78% 
Colour 85.8% SBR and coagulation Sequential treatment via SBR followed by coagulation was applied. Aluminium Sulphate was used as coagulant. Bioreactor/Coagulation Yong et al. (2018)  
COD 84.8% 
Ammonia 94.2% 
TSS 91.8% 
COD >70% Anaerobic Sequencing Batch Biofilm Reactor Biomass from the bottom of a landfill leachate stabilisation pond was immobilized in polyurethane foam cubes as inoculum. Bioreactor Contrera et al. (2018)  
COD 30% Aerobic sequencing batch reactor (ASBR) Air upflow velocity was set at 1.0–1.2 cm/s. Bioreactor Lim et al. (2016)  
Ammonia 65% 
TN 95.0% Partial-denitrification and Anammox Firstly, leachate diluted with municipal sewage. And two USB reactors were used. Integrated bioreactor Wu et al. (2018)  
TN 98.7% Partial nitrification, simultaneous anammox and denitrification During the aerobic phase, the DO was maintained below 0.5 mg/L. Integrated bioreactor Zhang et al. (2019)  
Ammonia 98% DM bioreactor DM filtration was conducted in a submerged configuration inside the aerobic bioreactor provided with a hydrostatic water head of 8 cm. And the initial inoculum was collected from the aerobic bioreactor in a municipal wastewater treatment plant. Bioreactor/Membrane Saleem et al. (2018b)  
TN 90% 
COD 99% Activated sludge process/RO Biological pre-treatments followed by RO. Bioreactor/Membrane Tałałaj et al. (2019
Ammonia 99% 
Fe 79.7% ZELIAC (a composite adsorbent), with dosage of 3 g/L, was augmented in SBR. Powdered ZELIAC was added to the SBR. Bioreactor/Adsorption Mojiri et al. (2016a)  
Mn 73.3% 
Cd 76.9% 
Ni 79.2% 

SBR, sequencing batch reactor; TSS, total suspended solids; SS, suspended solids.

Figure 1

Common landfill leachate treatment methods.

Figure 1

Common landfill leachate treatment methods.

Biological treatment methods

The biological degradation of contaminants results from the metabolic activities of microorganisms (Gotvajn & Pavko 2015). Owing to their cost effectiveness, biological techniques are commonly used to eliminate nutrients (e.g. ammonia) and organic compounds; however, such techniques may not be able to efficiently remove heavy metals and nonbiodegradable organics (Miao et al. 2019). Biological methods are classified into two main groups: (i) aerobic biological procedures and (ii) anaerobic biological procedures (Dabaghian et al. 2019).

Bioreactors

Bioreactors have been applied for treating wastewaters during several years because these methods are simple and reliable, and highly cost-effective (Gotvajn & Pavko 2015). But, the main drawbacks of bioreactor treatments involve temperature issues and leachate toxicity for microbial communities (Lippi et al. 2018).

Aerobic bioreactors

Aerobic treatments are the most commonly applied biological procedures. Aerobic reactors involve sustained aeration with large pre-established bacterial populations (i.e. activated sludge) (Torreta et al. 2017). The activated sludge process requires high concentrations of microorganisms, mainly bacteria, fungi and protozoa, to eliminate organic matter from wastewater (Rajasulochana & Preethy 2016). According to Wang et al. (2018a, 2018b), the activated sludge process may efficiently eliminate biodegradable organic material by completely transforming it into carbon dioxide and water. The sequencing batch reactor (SBR) is the most common method for treating landfill leachate. The SBR consists of several time-oriented periodic stages, and its batch operation may enhance process efficacy (Yong et al. 2018).

One of the main drawbacks of this technique involves the need for high concentrations of dissolved oxygen in biofilm reactors for denitrification (Payandeh et al. 2017).

Anaerobic bioreactors

Anaerobic methods generally demonstrate better landfill leachate treatment performance than aerobic treatment techniques owing to the high COD and high BOD/COD ratio of landfill leachates (Azreen & Zahrim 2018). Anaerobic approaches are effective biotechnological treatments for concentrated organic wastewater. Such methods are energy efficient and environmentally friendly owing to their low production of sludge and biogas (Gamoń et al. 2019). Anaerobic treatment involves the biological decomposition of organic or inorganic matter without oxygen molecules. Key drawbacks of this technique include long retention time, its sensitivity to temperature changes and low elimination efficiency (Azreen & Zahrim 2018). The anaerobic activated sludge process may require upflow anaerobic sludge blanket (UASB) and expanded granular sludge blanket (EGSB) reactors for the purification of landfill leachate. In a UASB reactor, wastewater flows through a sludge bed with high microbial activity (Gotvajn & Pavko 2015). Meanwhile, an EGSB is a third-generation anaerobic bioreactor that is characterised by high volumetric loading (Wang et al. 2018a, 2018b).

Anaerobic ammonium oxidation (anammox)
Anammox bacteria transform ammonium (an electron donor) and nitrite (an electron acceptor) into nitrogen gas, using CO2 as the carbon source for growth (Torreta et al. 2017). The most commonly applied mechanism of the anammox process is presented by the following equation (Gamoń et al. 2019):
formula
(1)

Anammox bacteria are considered monophyletic and comprise six candidate genera, namely, Candidatus jettenia, Candidatus anammoxoglobus, Candidatus brocadia, Candidatus scalindua, Candidatus anammoximicrobium and Candidatus kuenenia (Mojiri et al. 2020). Remarkably, other types of contaminants, such as high COD and heavy metals, can affect anammox activities. Therefore, the anammox reactors are often combined with other treatment methods (Kumar et al. 2016).

Nitrification and denitrification process

The denitrification and nitrification processes involve the microbial elimination of ammonium. Ammonia is transformed into nitrate under an aerobic condition, which in turn is reduced to N2 by an anoxic condition during a conventional nitrification–denitrification process (Thakur & Medhi 2019). In the process, firstly, ammonia is oxidised by ammonia-oxidising bacteria into nitrite (). Secondly, is converted into nitrate by nitrite-oxidising bacteria. Finally, the denitrification of nitrate into N2 is performed by heterotrophic bacteria during the anoxic step (Miao et al. 2019). Generally, this step is integrated into other treatment techniques owing to the effects of other pollutants on the process.

Phytoremediation

Phytoremediation methods employ the capability of plant-soil systems to degrade and inactivate potential toxic elements in leachate (Song et al. 2018). The benefits of phytoremediation include (1) low-cost installation and energy consumption and (2) the elimination of the pollutants from landfill leachate (Madera-Parra 2016).

Daud et al. (2018) used Lemna minor to treat landfill leachate. More than 70% of metals, 39% of COD and 47% of BOD are removed during a 15-day contact time. Daud et al. (2018) and Song et al. (2018) said that several aquatic plants, such as Colocasia esculenta, Pistia stratiotes, Eichhornia crassipes, Phragmites australis, Azolla filiculoides, Typha domingensis, Hydrilla verticillata, Azolla caroliniana, Salvinia Cucullata, Heliconia psittacorum, Azolla pinnata, L. minor, Lemna gibba, Lemna aequinoctialis, Gynerum sagittatum and Spirodela polyrhiza can be used to treat leachate. Plants with a remarkable metal-accumulating ability are categorised as hyperaccumulator (Tangahu et al. 2011). Hyperaccumulation is a vital factor for the success of phytoremediation (Alaboudi et al. 2018). Hyperaccumulator plants can be recognised by the translocation factor (TF) and the bioconcentration factor. TF (Equation (2)) is an indication of the plant's capability to translocate metals from its root to its shoot (Ndimele et al. 2014). BCF (Equation (3)) shows the accumulation of metals in plant tissues. Plants with BCF values of more than 2 or TF values more than 1 are considered as hyperaccumulator (Mellem et al. 2009). Table 4 illustrates the concentration of metals in roots and shoots of plants during removing metals by phytoremediation or constructed wetlands.
formula
(2)
formula
(3)
Table 4

TF and BCF during remediation of metals by plants

MetalPlantConcentration in influent (μg/L)Accumulation in root (μg/g)Accumulation in shoot/leaves (μg/g)TFBCFRemarksReferences
Zn Water hyacinth 1,420 1,100 600 0.58 1.3 Mixing ration of landfill leachate and tap water (75%) Abbas et al. (2019)  
Pb 770 600 360 0.68 0.7 
Cu 620 400 400 0.63 0.5 
Fe 1,120 800 650 0.53 
Ni 1,410 750 500 0.57 1.25 
Zn Water lettuce 1,420 1,300 660 0.6 1.2 Mixing ration of landfill leachate and tap water (75%) Abbas et al. (2019)  
Pb 770 650 350 0.5 0.6 
Cu 620 520 250 0.58 0.5 
Fe 1,120 1,000 500 0.5 
Ni 1,410 1,200 470 0.5 1.1 
Zn Lemna minor L. 1,470 NR NR NR 0.78 BCF reported after 3 days Daud et al. (2018)  
Pb 830 0.46 
Cu 690 0.63 
Fe 1,170 0.76 
Ni 1,210 0.58 
Zn S. globulosus 106–887 49.98 82.81 NR NR After 15 days Ujang et al. (2005)  
Ni 17–96 20.37 12.5 
Cu 8–31 11.11 12.78 
Cr 30–123 26.11 24.65 
Pb Jun-51 7.43 8.91 
Zn E. sexangulare 106–887 124.93 206.32 NR NR After 15 days Ujang et al. (2005)  
Ni 17–96 6.58 21.28 
Cu 8–31 5.99 12.06 
Cr 30–123 28.52 38.68 
Pb Jun-51 6.1 24.87 
Pb A. selengensis 4,080 404.79 (10365.37 (103NR NR – Wang et al. (2018a, 2018b
Cd 790 24.71 (1032.90 (103
Cr 6,120 765.59 (103127.99 (103
14,180 645.21 (103156.57 (103
Mn Vetiveria zizanioides 490 121.55 (10348.12 (103NR NR pH was set at 7. Roongtanakiat et al. (2007)  
Fe 16,150 1,430.07 (10362.31 (103
Cu 60 4.30 (1032.45 (103
Zn 4,090 82.31 (10314.27 (103
Pb 50 4.50 (1030.69 (103
Al Typha domingensis 6,560 303,910 NR 0.14 46.3 Industrial wastewater was treated by phytoremediation. Hegzay et al. (2011)  
Fe 10,460 154,680 NR 0.18 40.4 
Zn 3,870 117,640 NR 0.11 30.3 
Pb 990 14,870 NR 0.35 15.2 
Cu Echhornia crassipus 101.3 NR NR 5.08 0.61 Contaminated water was treated by phytoremediation. Pandey et al. (2019)  
Zn 259.4 NR NR 3.64 0.91 
Ni NR NR 7.63 1.83 
Pb 28.5 NR NR 1.73 0.88 
Fe 1,026.8 NR NR 1.04 0.92 
Cr Acorus calamus Linn. 11,390 64,480 7,980 NR NR – Sun et al. (2013)  
Fe 20,350 22,310 4,860 
Cu 45 1,590 650 
Zn 7,720 9,970 3,930 
Cr Juncus effusus L. 11,390 30,450 15,470 NR NR – Sun et al. (2013)  
Fe 20,350 77,290 14,090 
Cu 45 650 730 
Zn 7,720 13,290 540 
MetalPlantConcentration in influent (μg/L)Accumulation in root (μg/g)Accumulation in shoot/leaves (μg/g)TFBCFRemarksReferences
Zn Water hyacinth 1,420 1,100 600 0.58 1.3 Mixing ration of landfill leachate and tap water (75%) Abbas et al. (2019)  
Pb 770 600 360 0.68 0.7 
Cu 620 400 400 0.63 0.5 
Fe 1,120 800 650 0.53 
Ni 1,410 750 500 0.57 1.25 
Zn Water lettuce 1,420 1,300 660 0.6 1.2 Mixing ration of landfill leachate and tap water (75%) Abbas et al. (2019)  
Pb 770 650 350 0.5 0.6 
Cu 620 520 250 0.58 0.5 
Fe 1,120 1,000 500 0.5 
Ni 1,410 1,200 470 0.5 1.1 
Zn Lemna minor L. 1,470 NR NR NR 0.78 BCF reported after 3 days Daud et al. (2018)  
Pb 830 0.46 
Cu 690 0.63 
Fe 1,170 0.76 
Ni 1,210 0.58 
Zn S. globulosus 106–887 49.98 82.81 NR NR After 15 days Ujang et al. (2005)  
Ni 17–96 20.37 12.5 
Cu 8–31 11.11 12.78 
Cr 30–123 26.11 24.65 
Pb Jun-51 7.43 8.91 
Zn E. sexangulare 106–887 124.93 206.32 NR NR After 15 days Ujang et al. (2005)  
Ni 17–96 6.58 21.28 
Cu 8–31 5.99 12.06 
Cr 30–123 28.52 38.68 
Pb Jun-51 6.1 24.87 
Pb A. selengensis 4,080 404.79 (10365.37 (103NR NR – Wang et al. (2018a, 2018b
Cd 790 24.71 (1032.90 (103
Cr 6,120 765.59 (103127.99 (103
14,180 645.21 (103156.57 (103
Mn Vetiveria zizanioides 490 121.55 (10348.12 (103NR NR pH was set at 7. Roongtanakiat et al. (2007)  
Fe 16,150 1,430.07 (10362.31 (103
Cu 60 4.30 (1032.45 (103
Zn 4,090 82.31 (10314.27 (103
Pb 50 4.50 (1030.69 (103
Al Typha domingensis 6,560 303,910 NR 0.14 46.3 Industrial wastewater was treated by phytoremediation. Hegzay et al. (2011)  
Fe 10,460 154,680 NR 0.18 40.4 
Zn 3,870 117,640 NR 0.11 30.3 
Pb 990 14,870 NR 0.35 15.2 
Cu Echhornia crassipus 101.3 NR NR 5.08 0.61 Contaminated water was treated by phytoremediation. Pandey et al. (2019)  
Zn 259.4 NR NR 3.64 0.91 
Ni NR NR 7.63 1.83 
Pb 28.5 NR NR 1.73 0.88 
Fe 1,026.8 NR NR 1.04 0.92 
Cr Acorus calamus Linn. 11,390 64,480 7,980 NR NR – Sun et al. (2013)  
Fe 20,350 22,310 4,860 
Cu 45 1,590 650 
Zn 7,720 9,970 3,930 
Cr Juncus effusus L. 11,390 30,450 15,470 NR NR – Sun et al. (2013)  
Fe 20,350 77,290 14,090 
Cu 45 650 730 
Zn 7,720 13,290 540 

NR, Not Reported.

Bioremediation

Moris et al. (2018) stated that bioremediation involves biologically removing contaminants from the environment. Its benefits include cost-effective and environmentally-friendly techniques. The use of microalgae, algae and other fungi and bacteria for the bioremediation of landfill leachate has been reported in the literature (Moris et al. 2018; Spina et al. 2018). Paskuliakova et al. (2018a) claimed that algae can eliminate inorganic and simple organic compounds, whereas a few complex substances may undergo a certain degree of biotransformation. According to Paskuliakova et al. (2018b), microalgae that have been employed to treat landfill leachate include the Scenedesmus, Chlamydomonas and Chlorella genera as well as cyanobacteria and other phylogenetic. Moreover, major bacteria that have been utilised for landfill leachate treatment include Firmicutes, Actinobacteria, Proteobacteria, Brevibacillus panacihumi strain ZB1 and Pseudomonas putida (Moris et al. 2018; Michalska et al. 2020).

Co-treatment of landfill leachate and urban wastewater with biological methods

To enhance the biodegradability of landfill leachate and BOD/COD ratios, researchers have mixed domestic wastewater with landfill leachate before treatment (Mojiri et al. 2016a). Ranjan et al. (2016) used an SBR for the co-treatment of urban wastewater and landfill leachate. With a hydraulic retention time (HRT) of 6 days and a landfill leachate concentration of 20% v/v, 93, 83, 70 and 83% of ammonia, nitrite, COD and turbidity, respectively, were removed.

Mojiri et al. (2017) emphasised that owing to high COD and BOD/COD ratios, comparing landfill leachate treatments with methods used for domestic wastewater is difficult. Thus, a combined system should be applied to treat leachate. Li et al. (2020) employed denitrification/partial nitrification–anammox to eliminate nitrogen from intermediate landfill leachate. At optimum conditions, total nitrogen (TN) removal rate and TN elimination efficacy were 0.45 m3/d and 96.7%, respectively. The denitrification–nitrification–anammox process demonstrates two vital points, that is, the improvement of degradable COD in wastewater to realise nitrate removal and the improvement of autotrophic bacteria growth. Pirsaheb et al. (2017) utilised a combined aerobic–anaerobic/biogranular activated carbon SBR for landfill leachate treatment. This biodegradable landfill leachate treatment demonstrates high performance.

Physical and chemical treatment methods

Adsorption and ion-exchange

Erabee et al. (2018) expressed that adsorption has been broadly applied for the treatment of landfill leachate. Advantages of this method include its ease of operation, the simplicity of its design, its insensitivity to toxic substances and its ability to remove a variety of contaminants (Chávez et al. 2019). Different adsorbents and their performance are shown in Table 5.

Table 5

Adsorbents reported for landfill leachate treatment

Pollutants in landfill leachateAdsorbentAdsorption isothermAdsorption capacity (mg/g)RemarksReferences
TSS Activated carbon (AC) Langmuir 1.77 AC was derived from coconut shell. AC was modified by heating at 600 °C. Erabee et al. (2018)  
Ammonia 3.18 
Zn 0.02 
Mn 0.06 
Cu 0.07 
S2- 0.02 
COD AC Langmuir 272.75 AC was derived from walnut shell. Mahdavi et al. (2018)  
Colour AC Langmuir 555.55 AC was derived from sugarcane bagasse. Azmi et al. (2015)  
COD 126.58 
Ammonia 14.61 
Colour Freundlich 0.67 
COD 0.20 (10−2
Ammonia 3.0 (10−7
Pb AC Pseudo-second order 0.03 AC was derived from sugarcane bagasse. Salas-Enríquez et al. (2019)  
Cu 0.01 
Ni 0.01 
Zn 0.01 
Colour Biochar Langmuir 83.33 Biochar was derived from fallen mature fruits at 600 °C. Shehzad et al. (2016)  
COD Biochar 35.71 
Ammonia  500.00 
COD Biochar Pseudo-second order 490 Biochar was derived from coconut shell at high temperature, and it is activated via microwave heating. Lam et al. (2020)  
COD Biochar Freundlich 5.80 Biochar was derived from Miscanthus at 450. Kwarciak-Kozłowska et al. (2019)  
FA Magnetic graphene oxide Langmuir 82.16 – Zhang et al. (2016)  
HA 106.50 
Pb 45.50 
Bisphenol A Bentonite modified by hexadecyl trimethyl ammonium bromide (HTAB) Pseudo-second order 10.44 The HTAB-bentonite was synthesized by cation exchange with HTAB solution (20 mmol/L) over stirring. Li et al. (2015)  
Ni Red mud Langmuir 11.06 Batch experiments were done with neutral pH, adsorbent dosage of 10 g/L and shaking speed of 75 rmp. Ayala & Fernández (2019)  
Zn 12.04 
Cd 12.57 
Ni Freundlich 2.08 
Zn 4.40 
Cd 3.79 
Ammonia Zeolites (Clinoptilolite) Langmuir 17.45 – Pauzan et al. (2020)  
Bisphenol A High silica Y-type zeolite powder Pseudo-second order 141.0 Batch experiments were done in temperature room for 4 h at pH = 7. Chen et al. (2015)  
Colour Zeolites Langmuir 0.01 Activated zeolites were produced by heating to 250 °C. Aziz et al. (2020)  
COD 3.0 (10−4
Ammonia 8.9 (10−3
Colour Zeolites Langmuir 42.55 – Bashir et al. (2017)  
COD 0.22 
Ammonia 0.31 
Pb MS@GG Pseudo-second order 253.80 MS modified with PDA and then coated with glutathione/graphene oxide (GG) Feng et al. (2019)  
HA Aminated Magnetic Nanoadsorbent Langmuir 181.82 Amino-functionalized Fe3O4@SiO2 nanoparticles were produced by surface functionalization of Fe3O4@SiO2 nanoparticles using (3-aminopropyl) trimethoxysilane (APTMS) as the silylation agent. Batch experiments were done at neutral pH and shaken speed 150 rmp. Wang et al. (2015a, 2015b) > 
Pb Fe3O4@Mesoporous Silica-Graphene Oxide Composites Langmuir 333.33 – Wang et al. (2013)  
Cd 166.67 
Pollutants in landfill leachateAdsorbentAdsorption isothermAdsorption capacity (mg/g)RemarksReferences
TSS Activated carbon (AC) Langmuir 1.77 AC was derived from coconut shell. AC was modified by heating at 600 °C. Erabee et al. (2018)  
Ammonia 3.18 
Zn 0.02 
Mn 0.06 
Cu 0.07 
S2- 0.02 
COD AC Langmuir 272.75 AC was derived from walnut shell. Mahdavi et al. (2018)  
Colour AC Langmuir 555.55 AC was derived from sugarcane bagasse. Azmi et al. (2015)  
COD 126.58 
Ammonia 14.61 
Colour Freundlich 0.67 
COD 0.20 (10−2
Ammonia 3.0 (10−7
Pb AC Pseudo-second order 0.03 AC was derived from sugarcane bagasse. Salas-Enríquez et al. (2019)  
Cu 0.01 
Ni 0.01 
Zn 0.01 
Colour Biochar Langmuir 83.33 Biochar was derived from fallen mature fruits at 600 °C. Shehzad et al. (2016)  
COD Biochar 35.71 
Ammonia  500.00 
COD Biochar Pseudo-second order 490 Biochar was derived from coconut shell at high temperature, and it is activated via microwave heating. Lam et al. (2020)  
COD Biochar Freundlich 5.80 Biochar was derived from Miscanthus at 450. Kwarciak-Kozłowska et al. (2019)  
FA Magnetic graphene oxide Langmuir 82.16 – Zhang et al. (2016)  
HA 106.50 
Pb 45.50 
Bisphenol A Bentonite modified by hexadecyl trimethyl ammonium bromide (HTAB) Pseudo-second order 10.44 The HTAB-bentonite was synthesized by cation exchange with HTAB solution (20 mmol/L) over stirring. Li et al. (2015)  
Ni Red mud Langmuir 11.06 Batch experiments were done with neutral pH, adsorbent dosage of 10 g/L and shaking speed of 75 rmp. Ayala & Fernández (2019)  
Zn 12.04 
Cd 12.57 
Ni Freundlich 2.08 
Zn 4.40 
Cd 3.79 
Ammonia Zeolites (Clinoptilolite) Langmuir 17.45 – Pauzan et al. (2020)  
Bisphenol A High silica Y-type zeolite powder Pseudo-second order 141.0 Batch experiments were done in temperature room for 4 h at pH = 7. Chen et al. (2015)  
Colour Zeolites Langmuir 0.01 Activated zeolites were produced by heating to 250 °C. Aziz et al. (2020)  
COD 3.0 (10−4
Ammonia 8.9 (10−3
Colour Zeolites Langmuir 42.55 – Bashir et al. (2017)  
COD 0.22 
Ammonia 0.31 
Pb MS@GG Pseudo-second order 253.80 MS modified with PDA and then coated with glutathione/graphene oxide (GG) Feng et al. (2019)  
HA Aminated Magnetic Nanoadsorbent Langmuir 181.82 Amino-functionalized Fe3O4@SiO2 nanoparticles were produced by surface functionalization of Fe3O4@SiO2 nanoparticles using (3-aminopropyl) trimethoxysilane (APTMS) as the silylation agent. Batch experiments were done at neutral pH and shaken speed 150 rmp. Wang et al. (2015a, 2015b) > 
Pb Fe3O4@Mesoporous Silica-Graphene Oxide Composites Langmuir 333.33 – Wang et al. (2013)  
Cd 166.67 

In adsorption, the pollutants can adhere to the surface of the adsorbent over several mechanisms (Figure 2). The surface of the adsorbent has specific characteristics that allow the attachment of the adsorbate. Adsorption occurs under certain conditions, a reversible phenomenon which is named desorption, is applicable. In desorption, the adsorbates can be released from the surface of the adsorbent and got back to the liquid (Bello & Raman 2019).

Figure 2

Basic model of adsorption (Source: Bello & Raman 2019).

Figure 2

Basic model of adsorption (Source: Bello & Raman 2019).

Modified activated carbon (MAC), which is produced by immersing granular activated carbon (2.0 g) in a KMnO4 solution (30 mg/L) for 6 h, was created to treat landfill leachate. Approximately 99% of ammonia and 86% of zinc can be removed by MAC in a contact timespan of 120 min. The Langmuir adsorption capacity (mg/g) of this absorbent for the removal of ammonia and zinc is 0.16 (Erabee et al. 2018). Zamri et al. (2017) used an ion-exchange resin to treat landfill leachate, with a maximum adsorption capacity (mg/g) based on a pseudo second-order kinetic model of 13.4, 13.5, 14.2, 33,333.3, 10,000.0 and 50,000.0 for Cr6+, Al3+, Cu2+, COD, ammonia and colour, respectively.

Advanced oxidation processes

Advanced oxidation processes (AOPs) that apply a combination of oxidants and catalysts to produce hydroxyl radicals (•OH) in solutions, such as ultraviolet (UV), Fenton, ozonation and electrochemical oxidation (EO) methods, have garnered interest for the degradation of hazardous organic compounds or biorefractory in wastewater (Särkkäa et al. 2015). However, the main drawback of AOPs is high capital and operating costs.

In an EO process, contaminants are eliminated either by (a) direct EO in which organics are oxidised by moving electrons to an anode directly or (b) indirect EO in which certain electroactive species that act as mediators are produced to conduct the degradation procedure (Mandal et al. 2017). The EO of organics in metal oxide anodes was described by Ukundimana et al. (2018) as follows (Equations (4)–(6)).

Water is electrolysed via anodic catalysis to generate adsorbed hydroxyl radicals.
formula
(4)
Adsorbed hydroxyl radicals at metal oxide (MOx) electrodes (except for BDD and Pt) may form chemisorbed active oxygen.
formula
(5)
Meanwhile, the hydroxyl radicals will react to one another to form molecular oxygen to complete the electrolysis of the water molecules.
formula
(6)
Organic pollutants (R) in landfill leachate can be oxidised via the mechanisms illustrated in Equation (7) by reacting to the physiosorbed hydroxyl radicals MOx(•OH) formed by Equation (6).
formula
(7)
When electricity is applied to wastewater, oxygen gas derived from the breakup of water molecules and chlorine gas is produced in a chloride ion solution (Equations (8) and (9)). Hypochlorous acid (HOCl) and hypochlorite ion (OCl) are vital ions responsible for the indirect oxidation of ammonium to nitrogen gas (Equations (10) and (11)) (Ghimire et al. 2020). EO has been deemed effective for ammonium elimination (Mandal et al. 2017).
formula
(8)
formula
(9)
formula
(10)
formula
(11)
In an EO procedure, the formation of metal oxide on an anode relies on the pH of the electrolyte and metal ion. Yasri & Gunasekaran (2017) indicated that a metallic hydroxide film might form on an anode in an alkaline media for transition metals (Equations (12) and (13)).
formula
(12)
formula
(13)

EO, BDD, Ti/Pt, Ti/PbO2, Ti/SnO2, Ti/Pt/SnO2–Sb2O4, Ti/RuO2–IrO2 and graphite have been commonly applied as electrodes for the treatment of landfill leachate (Ukundimana et al. 2018). Among the benefits of EO, the breakdown of high molecular organic compounds, the absence of sludge and the complete mineralisation of organics are its most significant advantages (Mandal et al. 2017).

The Fenton process has been commonly employed for the oxidation of different organics from wastewater, as it exhibits a high oxidation potential of 2.72 V (Nakhate et al. 2018). Fe(II) ions are oxidised into Fe(III) in the presence of excess H2O2 (Equation (14)). This reaction mechanism displays the activation of H2O2 in the presence of Fe(II) ions to form hydroxyl radicals that can oxidise organic compounds (Gautam et al. 2019). This classic Fenton reaction may be assisted by electric currents (i.e. the electro-Fenton process) or UV irradiation (i.e. the photo-Fenton process), thereby considerably enhancing its efficacy (Seibert et al. 2019). Singa et al. (2018) argued that compared with other AOPs, the Fenton process includes benefits such as an easy implementation operation, high efficiency and the lack of an energy requirement for H2O2 activation.
formula
(14)

Ozone is a powerful oxidant, with a redox potential of 2.07 V in an alkaline solution. Consequently, O3 can oxidise organic and inorganic substances. Gautam et al. (2019) claimed that the key drawbacks of landfill leachate treatment through ozonation include the following. (1) Leachate is a complex wastewater with high organic compounds; hence, high amounts of ozone are required. (2) Ozone mass transfer from a gas to a liquid is low. The ozonation of pollutants may be performed by two techniques, namely, direct and indirect ozonation (Wang & Chen 2020).

A direct O3 molecule reaction with contaminants involves oxidation–reduction reactions (e.g. reactions between O3 and HO2/or ; Equations (15) and (16); Wang & Chen 2020).
formula
(15)
formula
(16)
An indirect reaction by •OH is revealed in the following equation (Nilsson 2018):
formula
(17)
UV treatment has been generally used to degrade aquatic organic compounds and kill microbes. During the absorption of UV light, electrons are transferred to oxygen molecules that convert O2 and contaminant molecules into radicals (Equations (18) and (19)).
formula
(18)
formula
(19)

UV treatment may result in the homolytic cleavage of the chemical bonds of contaminants, thereby causing the formation of two radicals (Mishra et al. 2017).

Approximately 99.9% of diethyl phthalate (DEP; organic pollutant) is removed from landfill leachate through the ozone/hydrogen peroxide process (O3/H2O2) at an initial concentration of 20 mg/L DEP and 120 min of ozonation (Mohan et al. 2019).

Membrane technology

The use of different membrane technology to treat wastewater has gained considerable attention (Dabaghian et al. 2019). Membrane separation involves the selective filtration of influent through different-sized pores (Warsinger et al. 2016). Microfiltration (MF), dynamic membranes (DMs), nanofiltration (NF), ultrafiltration (UF) and reverse osmosis (RO) are the main membrane processes employed in landfill leachate treatment (Dabaghian et al. 2019). The advantages of using membranes include low overall energy requirements, simplicity and high efficiency (Siyal et al. 2019).

DMs may provide a new approach by exploiting fouling as a means for solid–liquid separation. A DM is specified as a self-forming and regenerative fouling surface formed by the removal of colloids, suspended solids and microbial cell particles through a coarse underlying support material (Saleem et al. 2018b; 2019). For this purpose, cheap materials, such as filter cloths, have been applied as underlying support to develop DMs (Saleem et al. 2019).

MF and UF are categorised as low-pressure (<2 bar) processes. Separation by MF is primarily performed by sieving. However, this process is generally limited to the elimination of organic colloids, suspended solids or particles and bacteria owing to fairly large pore sizes (approximately 0.1–1.0 μm). UF membranes likewise operate mainly via sieving but contain a broader separation range compared with MF and rely on pore sizes between 0.01 and 0.1 μm to remove pathogens, particles and colloids (Warsinger et al. 2016).

Meanwhile, NF can eliminate ions that contribute substantially to osmotic pressure; thus, it allows operation pressures that are lower than those used in RO. Pre-treatment is required for heavily contaminated wastewater for NF to be effective (Nqombolo et al. 2018).

Among the new procedures for landfill leachate treatment, RO is one of the most promising and effective techniques (Yao 2017). The RO process separates contaminants into two streams, namely, permeate (filtrate) and highly polluted concentrates, which are often recirculated into the waste body (Tałałaj 2019). Pertile et al. (2018) removed 43% of COD and 63% of BOD from landfill leachate through MF, with a transmembrane pressure of 0.5–1.4 bar.

Coagulation and flocculation

Fundamentally, coagulation facilitates the destabilisation of fine particles (colloids) from wastewater to form a floc that can be settled simply (Achak et al. 2019). Coagulation/flocculation efficacy relies on selected coagulants/flocculants. Coagulants are generally trivalent-metal inorganic salts, such as aluminium sulphate, polyaluminium chloride and ferric chloride (Wei et al. 2018). Lippi et al. (2018) stated that the main advantage of this treatment is its high effectiveness in removing organic matter, suspended solids and humic acids. However, drawbacks include the cost of chemicals and the management of generated sludge.

Nascimento et al. (2016) utilised natural chitosan as a coagulant for landfill leachate treatment. The removal rate for colour and turbidity was 80 and 91.4%, respectively, with a chitosan dosage of 960 mg/L and a pH of 8.5. Nithya & Abirami (2018) removed 85.2% of turbidity from landfill leachate via pine bark as a natural coagulant, with a pH of 7 and a coagulant dosage of 4 g/mL.

Hybrid physical/chemical methods

To improve removal efficiency and decrease energy consumption, several physical/chemical treatment methods have been combined to treat landfill leachate. Xiang et al. (2019) posited that hybrid processes, especially AOPs, combined with other treatments may be promising approaches for saving energy. Four integrated systems for combined physical/chemical methods have been identified.

AOPs combined with membranes

The integration of membrane filtration with AOPs may efficiently mitigate membrane-fouling problems, thereby enhancing overall separation performance (Pan et al. 2019). Santos et al. (2019) removed 94–96% of COD and 96–99% of colour from landfill leachate by combining the Fenton, NF and MF processes. Santos et al. (2019) indicated that the concentration of dissolved solids may be high after an AOP–Fenton process owing to the presence of organic matter that has not been completely oxidised and the addition of salts and acid/basic agents. Thus, the use of membranes can resolve this issue.

AOPs combined with coagulation

According to Chen et al. (2019), this integrated method can reduce the concentration of organic pollutants and increase the biodegradability of wastewater by altering the molecular structure of residual organics. Gautam et al. (2019) identified energy intensiveness, electrode passivation and the formation of chlorinated organics as the main drawbacks of electrocoagulation methods. Integrated photoelectrooxidation and activated carbon can remove 70.3% of COD, 58.3% of ammonia and 58.4% of TN (Klauck et al. 2017). Chen et al. (2019) eliminated 88.3% of COD, 98.8% of colour and 94.3% of UV254 from landfill leachate by using a combined coagulation–ozonation process.

AOPs combined with adsorption

The integration of AOPs with adsorption has been suggested to improve pollutant removal efficiency, specifically, metals from landfill leachate. Bello & Raman (2019) stated that complex organic contaminant can be degraded by AOPs but complete mineralisation is not mostly practical and some intermediate contaminants are frequently generated. Therefore, combining AOPs and adsorption could remove these intermediates. Integrated H2O2-granular activated carbon can reduce 97.3% of COD and increase biodegradable ratio by 116% (Eljaiek-Urzola et al. 2018). Eljaiek-Urzola et al. (2018) stated that integrating H2O2 with activate carbon can improve the decomposition of peroxide in free radicals and enhance performance. Jafari et al. (2017) removed 99.8% of tetracycline, as emerging pollutants, from aqueous solution by Heterogeneous Fenton: activated carbon–Fe3O4.

Membrane filtration combined with coagulation or adsorption

According to Alimoradi et al. (2018), coagulants or adsorbents have been applied sequentially to membranes to eliminate suspended and colloidal substances from wastewater, thereby reducing organic load and hindering membrane fouling. Gkotsis et al. (2017) emphasised that the use of coagulants in MBR systems could contribute significantly to reducing transmembrane pressure. Apart from that, Alimoradi et al. (2018) stated that coagulation pre-treatment delays the reversible and irreversible fouling by improving sludge filterability and by eliminating soluble microbial products, respectively. Alimoradi et al. (2018) removed more than 90% of Al by integrated coagulation-membrane bioreactor. 99.2% of COD, 100% of suspended solids and 97.3% of total organic carbon were removed by combined coagulation and membrane (Boluarte et al. 2016). 100% of 4-chlorophenol, 78–100% of oxidation intermediates from wastewater by integrated catalytic oxidation and adsorption (Arsene et al. 2013).

Hybrid physical/chemical and biological methods

Biological ways are frequently employed to treat landfill leachate. However, a biological procedure alone is not efficient enough to eliminate the bulk of refractory contaminants in landfill leachate (Wu et al. 2010). Therefore, researchers (Mojiri et al. 2016b) have suggested integrated biological methods and physical/chemical techniques to improve biodegradability ratios and increase biological performance in treating landfill leachate. Five commonly applied combined treatment methods have been identified.

Integrated adsorption and biological treatment methods

Adsorption can be employed to diminish contaminants and leachate toxicity to provide favourable growth conditions for microbial growth (Er et al. 2018). Munz et al. (2007) listed the advantages of combination of adsorption, such as activated carbon, and biological methods as: protecting microorganisms from load pick of inhibiting organic and inorganic compounds, improving refractory organics, improving sludge settleability and dewaterability capacity. Besides, the application of the adsorption technique together with the biological method leads to a reduction of the quantity of adsorbent employed for the wastewater treatment process (Yi et al. 2018). Sawdust added to an SBR can remove 99% of COD and 95% of ammonia (Mohajeri et al. 2018). More than 60% of ampicillin was eliminated by integrating adsorption and biodegradation (Shen et al. 2010). Ammonia was removed at more than 70% from landfill leachate by integrated adsorption and biological treatment (Yi et al. 2018).

Integrated membrane and biological treatment methods

Generally, the membrane bioreactor is a vital innovation in treating wastewater treatments since it overcomes the disadvantages of the conventional activated sludge process, such as producing excess sludge, requiring secondary clarifiers, and limitations with elimination of recalcitrant (Iorhemen et al. 2016). Among anaerobic biological methods, the anaerobic membrane bioreactor (AnMBR) system, which decouples HRT from solid retention time (SRT), is feasible for treating heavy wastewater, such as leachate (Abuabdou et al. 2020). Regarding the drawbacks of membrane bioreactors, Abuabdou et al. (2020) argued that starting an AnMBR in temperatures below 20 °C may result in the reduction of biomass growth, thereby causing a long SRT for stabilisation. Xu et al. (2019) removed more than 90% of sulphonamides and tetracyclines by using a membrane bioreactor. More than 90% of COD was removed from landfill leachate by AnMBR (Zayen et al. 2010).

Integrated AOP and biological treatment methods

He et al. (2020) expressed that integrating AOP techniques, as a pre-treatment, leads to readily biodegradable intermediates for biological posttreatment. Therefore, it has a positive impact for treating wastewaters, such as landfill leachate. Researchers (He et al. 2020; Xia et al. 2020) reported that zone oxidation, photocatalyst and EO are promising pre-treatment methods to enhance biodegradability of refractory contaminants. A combined semiaerobic aged refuse biofilter and ozonation process can eliminate 92.1% of colour and 61.4% of UV254 from landfill leachate (Chen et al. 2019). More than 70% of aromatic pollutants, such as p-aminophenol, by hybrid reactor including ozone pre-treatment and bioreactor (Xia et al. 2020). COD concentration was decreased to less than 50 mg/L by combined photocatalytic pre-oxidation reactor with SBR (He et al. 2020). Integrated ozonation and membrane bioreactor removed up to 99% of pharmaceuticals, such as Etodolac (Kaya et al. 2017). 100% of sulfadiazine, 97% of total organic carbon, 94% of BOD5 and 97% of COD were eliminated by ozonation and membrane bioreactor (Lastre-Acosta et al. 2020).

Integrated coagulation and biological treatment methods

Coagulation/flocculation can be applied as pre-treatment and posttreatment with biological treatment methods (Niazi 2018; Güvenç & Güven 2019). Employed coagulation/flocculation as a pre-treatment leads to improvement of the biodegradability and reduces COD, colour and metals in landfill leachate. These advantages can enhance the treatment of landfill leachate with biological methods. The use of the coagulation/flocculation as a posttreatment can remove refractory pollutants, such as metals, COD and organics. Niazi (2018) expressed that biological treatment results the degrading dissolved and colloidal organics which transform to active biomass. The active biomass in reject water produced from the biological method can get more dissolved organics and colloidal solids from the wastewater which is eliminated by coagulation. An integrated coagulation and anaerobic bioreactor process can remove 72% of COD and 70% of total organic carbon (Yadav et al. 2016).

Constructed wetlands

Mojiri et al. (2016b) suggested that the constructed wetland (CW) system was engineered to increase water quality. A wetland system comprises permeable substrata, such as gravel, which is typically planted with emergent wetland plants, such as Schoenoplectus, Typha, Phragmites and Cyperus. Dan et al. (2017b) expressed that degradable organic carbon and ammonia can be efficiently removed from landfill leachate by CW systems. Nitrogen pollutants can be removed by adsorption through substrate, absorption through plant roots, volatilisation in ammonia forms, biological degradation and biochemical transformation into N2 (Gottshall et al. 2007; Badejo et al. 2018). Zhuang et al. (2019) expressed that more than 50% of nitrogen can be eliminated by microbial activities, such as the nitrification/denitrification process, while around 25% of nitrogen may be absorbed by plant roots. Up to 89% of ammonia removal using a CW was reported by Mannarino et al. (2006).

The majority of phenolic compounds are removed by microbial activities and adsorption through substrate (Rossmann et al. 2012). Dan et al. (2017a) removed 88–100% of phenols, 18–100% of 4-tert-butylphenol and 9–99% of bisphenol A by using a vertical flow-constructed wetland. Apart from organic contaminants, heavy metals can be removed by CW systems.

According to Dan et al. (2017b), various mechanisms, such as the adsorption of soil or substrates as well as particulates and soluble organics, the precipitation of insoluble salts and the uptake of aquatic plants and microorganisms, may affect metal removal via CW systems. Ujang et al. (2005) removed up to 92.2% of Zn, 96.8% of Ni, 99.5% of Cu, 87.5% of Cr and 98.1% of Pb by using a CW which contained E. sexangulare and media.

CONCLUSIONS

Landfill leachate often possesses significant pollution potential with high concentrations of organic and inorganic contaminants. Primary landfill leachate treatment techniques consist of physical, chemical and biological methods. Owing to high concentrations of contaminants in landfill leachate and its low biodegradability, integrated treatment methods and co-treatment with wastewater are strongly recommended. Membrane filtration and integrated biological methods (nitrification/denitrification/anammox) have demonstrated high performance in removing nitrogen and ammonia from landfill leachate. Moreover, coagulation/flocculation methods have exhibited high efficiency in removing suspended solids and turbidity, with a removal rate of more than 90%. Bioremediation has demonstrated varied removal efficiency for COD, ranging from 17.5 to 60% depending on bacteria or algae species, thereby failing to show high performance in reducing COD. Finally, physical/chemical treatments have exhibited high performance in removing heavy metals.

ACKNOWLEDGEMENT

We would like to thank the Japan Society for the Promotion of Science for their support and fellowship. This work was supported by JSPS KAKENHI, grant number JP17F17375.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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