## 2 The Formation, Characteristics, and Composition of Landfill Leachate  
### Formation and Characteristics of Landfill Leachate  
Landfill leachate is generated through physical, chemical, and biological interactions between moisture and decomposing waste within landfills. The formation process begins when external water sources--such as precipitation, inherent moisture in waste, or groundwater intrusion--penetrate the landfill through its cover layer or cracks [44]. As water percolates through the waste strata, it initiates the aerobic degradation phase, where organic matter is broken down by microbial activity into carbon dioxide and water, accompanied by heat release. Once oxygen is depleted, the system transitions to an anaerobic phase, during which complex organic compounds are converted into small-molecule organic acids, ammonia nitrogen, and methane. Concurrently, DOM and heavy metals leach from components such as electronic waste and batteries. Environmental factors--including elevated temperatures and pH shifts from neutral to alkaline--further regulate pollutant transformation and migration. Ultimately, these processes yield leachate characterized by high COD, elevated ammonia nitrogen levels, and potential toxicity [35]. Landfill leachate exhibits significant temporal and spatial heterogeneity in composition, governed by a complex interplay of waste characteristics (type and composition), landfill engineering parameters (compaction density and cover system design), hydrogeological conditions (infiltration patterns and groundwater interactions), and temporal variables (landfill age and seasonal precipitation variations) [45]. From an analytical chemistry perspective, conventional leachate characterization typically employs these critical parameters: chemical oxygen demand (COD), total organic carbon (TOC), 5-day biochemical oxygen demand (BOD5), pH, ammonia nitrogen (NH\({}_{3}\)-N), and heavy metal profiles. The biodegradation potential of organic constituents is frequently assessed through two key indices: the BOD5/COD ratio, reflecting microbial metabolic accessibility, and the COD/TOC ratio, indicating organic carbon oxidation states [46]. A multitude of parameters influence the composition of leachate, including the type and composition of waste, operational mode, climatic conditions, hydrogeological factors, and the age of the landfill site [47]. Table 1 provides a summary of the typical characteristics of landfill leachate. It has been observed that the characteristics of landfill leachate vary, primarily due to factors such as the composition and moisture content of the waste, the age of the landfill facility, seasonal variation (including temperature and precipitation), and other contributing elements [48]. [PERSON] [49] discovered that the levels of pH, COD, TOC, total nitrogen (TN), and conductivity were remarkably higher in winter than in summer. However, there were no significant differences in redox potential and the concentrations of metals, total suspended solids, and volatile suspended solids between the two seasons. This seasonal fluctuation is primarily attributed to the inhibitory effects of low temperatures on microbial metabolic activity: during winter, reduced temperatures significantly decelerate the biodegradation rates of organic matter, resulting in the accumulation of refractory organic compounds and nitrogenous compounds (e.g., ammonia nitrogen) in leachate. Concurrently, the cold environment potentially enhances volatile fatty acid production through anaerobic fermentation processes, further elevating pH levels. Landfill age is also a key factor influencing leachate characteristics, and leachate age is typically classified into three stages, including young (<5 years), middle (5-10 years) and mature (>10 years). Young landfill leachate usually contains high concentrations of organic matter and high BOD/COD ratios (0.5-1.0), which is applicable for biological treatment.