Types and Characteristics of Landfill Leachate
As urbanization continues to advance and develop in many countries around the world, the population is increasingly concentrated in cities. This has led to a significant rise in the amount of municipal solid waste (MSW) being transported over recent years. The increase in waste generation has also resulted in a continuous rise in leachate production. With many countries imposing stricter requirements on surface and groundwater environments, the proper disposal of leachate has become an important environmental issue. Although there has been a significant amount of research on leachate treatment methods in academia, there are still not many new technologies applied in engineering projects. Leachate is particularly challenging to treat due to its high pollutant content and variety, especially as many places' leachates are mixed with kitchen waste, cosmetics, pharmaceuticals, and even industrial waste. These multiple factors contribute to the difficulty and high cost of leachate treatment.
Types of Leachate
Leachate can be categorized into three types based on its source of generation:
1. Landfills
Leachate from landfills primarily originates from the water content of the waste itself, the water produced by decomposition, rainfall, and groundwater infiltration. For landfills with well-maintained drainage systems, good bottom liner systems, and proper coverage, the amount of leachate generated from rainfall and groundwater infiltration is relatively small. The leachate primarily consists of the water content of the waste and the water generated from decomposition.
2. Transfer Stations
At waste transfer stations, mixed household waste is compressed during the transfer process to reduce volume. The compression equipment squeezes the waste, causing some of the moisture in the leachate to be separated from the waste material, resulting in high-concentration leachate. Additionally, transfer stations need to wash the ground to maintain cleanliness, and the wash water generated during this process mixes with the high-concentration leachate produced by the compression.
3. Incineration Plants
The leachate from waste incineration plants mainly consists of the moisture contained in the waste that is squeezed out after being stored in the pit, as well as the wastewater generated from its acidic fermentation.
Leachate Characteristics
(1) Leachate contains various types of pollutants, including organic pollutants (aromatic hydrocarbons and cyclic hydrocarbons, linear and cyclic ethers, alcohols, and lipids) and inorganic pollutants (nitrogen compounds, silica, phosphorus-containing compounds, and sulfur compounds), heavy metals, and toxic substances such as chlorobenzene and its derivatives, toluene and its derivatives, styrene, and absorbable halogenated compounds (AOX). Humic substances are the main component of water-soluble organic matter in leachate, accounting for more than 60%, and are the main components of non-degradable organic matter as well as the primary substance causing color.
(2) For landfill leachate, the concentration of various pollutants changes over time. Researchers have collected landfill waste and leachate from different ages, extracted and prepared water-soluble organic matter, and found that early-stage leachate predominantly contains aliphatic substances, while the content of aromatic substances in water-soluble organic matter decreases over time. Functional groups such as carbonyl, carboxyl, and hydroxyl groups on benzene rings also reduce. In the middle to late stages of landfill, water-soluble organic matter primarily consists of carbohydrates, organic amines, and the content of aromatic substances and their functional groups increases. Studies on leachate at different stages show that early-stage leachate contains more easily degradable organic matter, while late-stage leachate contains more difficult-to-degrade substances and highly humified humic substances, leading to differences in biodegradability. Young landfill leachate has a biochemical oxygen demand (BOD) >10000 mg/L, a BOD/COD ratio of 0.5-1.0, and good biodegradability. Middle-aged leachate has a BOD of 4000-10000 mg/L and a BOD/COD ratio of 0.1-0.5. Old landfill leachate has a BOD <4000 mg/L and a BOD/COD ratio <0.1, indicating poor biodegradability. Young landfill leachate has an ammonia nitrogen concentration of less than 400 mg/L, while old landfill leachate has an ammonia nitrogen concentration greater than 400 mg/L.
(3) The water quality of landfill leachate varies greatly depending on the region and season. Researchers analyzed the leachate quality characteristics of a landfill and an incineration plant in Chongqing, China, during the winter, and compared the results with international studies. The results showed that the leachate from the landfill and incineration plant had significant differences. However, the leachate characteristics from the landfill were more similar to those of leachate from a landfill around ten years old. The COD (4173 mg/L) and volatile fatty acid (VFA) content (384 mg/L) were low, while NH-N concentration (3885 mg/L) was high. The DH value was 7.83, with a very low BOD/COD ratio (0.09). In contrast, the incineration plant leachate's water quality was more similar to that of early-stage landfill leachate. The COD content in the incineration plant leachate was much higher than in the landfill, but the proportion of humic acid (HA) was higher in the landfill leachate.
