Treatment of landfill leachate with different techniques: an overview

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.

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. ) as well as contaminants of emerging concern (Eggen et al. ). 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. a).
Minimising risks to the environment and human health is a serious concern in open dumping and sanitary landfills (Xaypanya et al. ). Appropriate key techniques for landfill leachate treatment consist of biological methods and chemical and physical processes. However, a comprehensive assessment on 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. ).
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. a).
Based on its age, landfill leachate may be divided in three key groups (Table 1), namely, young, intermediate and old (Aziz ; Tejera et al. ). Aziz () and Vaccari et al. () stated that in 'young' landfills (i.e. the acid phase),  However, there is a slightly difference in some other studies (Wang et al. a, b) due to the wastes 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.

Colour and TDS
Colour is a common pollutant in landfill leachate. The

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. a). Urban solid waste and landfill leachate contain a wide variety of organic compounds (Scandelai et al. ). 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.  and approximately 520% are organic. Inorganic ions contain chloride (Cl À ), nitrites and nitrates, cyanide (CN À ), sulphides (S À ) and sulphates (SO 2À 4 ). Moreover, inorganic cations contain ammonia and ferrous (Tałałaj ).
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. ); therefore, several heavy metals in high concentrations have been reported in the landfill leachates (Chuangcham et al. ). Removal of heavy metals is a difficult task; consequently, we make more attention on removal of metals from landfill leachate in this study. Dan et al. (a) 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. a).

LANDFILL LEACHATE TREATMENT METHODS
The different landfill leachate treatment methods are shown in Figure 1 and Table 3.

Biological treatment methods
The biological degradation of contaminants results from the metabolic activities of microorganisms (Gotvajn &

Bioreactors
Bioreactors have been applied for treating wastewaters during several years because these methods are simple and   Anaerobic ammonium oxidation (anammox). Anammox bacteria transform ammonium (an electron donor) and nitrite (an electron acceptor) into nitrogen gas, using CO 2 as carbon source for growth (Torreta et al. ). The most commonly applied mechanism of the anammox process is presented by the following equation (Gamoń et al. ): Nitrification and denitrification process. The denitrification and nitrification process involve the microbial elimination of ammonium. Ammonia is transformed into nitrate under an aerobic condition, which in turn is reduced to N 2 by an anoxic condition during a conventional nitrification-denitrification process (Thakur & Medhi ). In the process, firstly, ammonia is oxidised by ammonia-oxidising bacteria into nitrite (NO À 2 ). Secondly, NO À 2 is converted into nitrate by nitrite-oxidising bacteria. Finally, the denitrification of nitrate into N 2 is performed by heterotrophic bacteria    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. ). Different adsorbents and their performance are shown in Table 5.

Adsorption and ion-exchange
In adsorption, the pollutants can adhere to the surface of the adsorbent over several mechanisms The EO of organics in metal oxide anodes was described by Ukundimana et al. () as follows (Equations (4)- (6)).
Water is electrolysed via anodic catalysis to generate adsorbed hydroxyl radicals.
Adsorbed hydroxyl radicals at metal oxide (MO x ) electrodes (except for BDD and Pt) may form chemisorbed active oxygen.
Meanwhile, the hydroxyl radicals will react to one another to form molecular oxygen to complete the electrolysis of the water molecules.
Organic pollutants (R) in landfill leachate can be oxidised via the mechanisms illustrated in Equation (7) by reacting to the physiosorbed hydroxyl radicals MO x (•OH) formed by Equation (6).
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 À ) are vital ions responsible for the indirect oxidation of ammonium to nitrogen gas (Equations (10) and (11)) (Ghimire et al. ). EO has been deemed effective for ammonium elimination (Mandal et al. ).
In an EO procedure, the formation of metal oxide on an anode relies on the pH of the electrolyte and metal ion.
Yasri & Gunasekaran () indicated that a metallic hydroxide film might form on an anode in an alkaline media for transition metals (Equations (12) and (13)).
Ozone is a powerful oxidant, with a redox potential of 2.07 V in an alkaline solution. Consequently, O 3 can oxidise organic and inorganic substances. Gautam et al. () 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 ).
An indirect reaction by •OH is revealed in the following equation (Nilsson ): 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 O 2 and contaminant molecules into radicals (Equations (18) and (19)).

Membranes technology
The use of different membrane technology to treat wastewater has gained considerable attention (Dabaghian et al.  100% of sulfadiazine, 97% of total organic carbon, 94% of BOD 5 and 97% of COD were eliminated by ozonation and membrane bioreactor (Lastre-Acosta et al. ).

Constructed wetlands
Mojiri et al. (b) suggested that the CW system was engineered to increase water quality.

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.