**1. Introduction**

The landfilling of solid waste is still a significant problem in the solid waste managemen<sup>t</sup> systems of all countries worldwide [1,2]. However, landfill leachate is a complex liquid generated from rainwater penetration through landfills that often includes high-strength contaminant resistance, such as humic acids, ammonia nitrogen, heavy metals, xenobiotics, and inorganic salts, which are important to avoid due to their adverse effects on the environment [3,4]. The factors that affect the composition of landfill leachate include the composition of waste, the level of compaction, the absorptive capacity of solid waste and age of solid waste, weather variations, precipitation, landfilling temperature, size of landfilling, hydrogeological conditions, factors of the landfill operation, pH, and chemical and biological activities in the process of landfilling [5,6]. In general, young leachate produced from new landfills (<5 years old) has a large biodegradation of comparatively low-molecular-weight materials, such as volatile organic acids, chemical oxygen demand (COD), total organic carbon (TOC), biological oxygen demand (BOD5), and biodegradability (BOD5/COD) [7]. On the other hand, old leachate (>10 years old) has high organic content of relatively high-molecular-weight materials, for instance, humic and fulvic substances that are refractory and not rapidly degradable. Generally, old leachate has a lower concentration of COD, TOC, BOD5, and BOD5/COD [6,7]. Leachate is stabilized and has low biodegradability since most landfills are old [8–11]. As a landfill gets older, a change from a relatively shorter initial aerobic to a longer anaerobic decomposition period takes place. Due to the biological breakdown of organic compounds and precipitation of soluble components, such as heavy metals, the strength of leachate generally lowers over time. Because of its biodegradable nature, organic compounds decrease faster than inorganic compounds with the increasing age of leachate production.

Several treatment techniques, such as physico-chemical processes, are used to treat leachate (coagulation precipitation, activated carbon adsorption [12,13], membrane filtration, activated carbon adsorption [14,15], and/or other separation techniques) [16], in addition to biological treatment methods, such as aerobic and anaerobic processes. In the past two decades, many studies reported that biological processes in the treatment of young leachate are e ffective but are comparatively insu fficient in the treatment of old leachate because of the existence of bio-refractory substances [6,7]. Currently used procedures often involve mixed methods intended as modular or multi-stage units that are capable of treating pollutants that change in concentration over time. The appropriate treatment of leachate is applied to enhance and develop an appropriate technique of treatment that meets the relevant quality standards and regulations [5].

In particular, the palm oil sector in Malaysia adopts four types of treatment technologies: waste stabilization ponds, activated sludge systems, closed anaerobic digesters, and land application systems [17]. However, many questions remain unanswered about the cost of treatment, the production of sludge, and chemical residues in treated wastewater. Usage of low-cost and natural materials of wastewater treatment are currently gaining increased attention from a series of studies. Palm oil mill effluent (POME) is classified as a pollutant which is known for its ability to enhance the biodegradability of pollutants due to the relatively high content of organic matter [18,19]. An aeration process supplies the oxygen to the wastewater and acts as an oxidation of organics. Moreover, oxygen can also enhance the biodegradation of organics by bacteria which use oxygen to break down the organic matter into the form of CO2 and H2O [20]. Stripping of ammonia is a simple desorption method utilized to reduce the ammonia concentration of leachate. Ammonia reacts with water to form ammonium hydroxide [21], as shown in Equation (1).

$$\text{NH}\_4^+ + \text{OH}^- \leftrightarrow \text{H}\_2\text{O} + \text{NH}\_3^-. \tag{1}$$

The alkaline wastewater flows downward when the air enters through the bottle. As the air continues to flow, the wastewater moves in the opposite direction and ascends to the top of the aeration bottle. NH3 is stripped from dropping water droplets into the air flow, and then released into the atmosphere [22]. In this research, the e ffectiveness of aeration processes used in the treatment of a leachate/POME mixture was evaluated. The optimum experimental conditions for POME dosages were designed, and the ability to remove the di fferent medicines was studied. In addition, the e fficacy of POME in the leachate removal of heavy metals was also investigated. The removal of heavy metals by POME during aeration may be attributed to the high level of suspended solids in POME, which may act as a natural coagulant [23].
