In the field of environmental protection, waste is referred to as Municipal Solid Waste (MSW). Currently, MSW treatment in China is divided into two methods: incineration and landfilling. Landfilled waste does not undergo sterilization and contains a large amount of bacteria and viruses; it occupies a significant amount of land and poses potential risks such as biogas and heavy metal pollution; its leachate can cause long-term pollution of groundwater resources. However, due to its low treatment cost, landfilling remains the primary method for centralized disposal of municipal waste in China. This method carries enormous potential harm. If leachate is not properly treated, it will bring endless consequences for future generations.

Landfill leachate is the wastewater that permeates out during the stacking and landfilling of waste due to fermentation, scouring by rainwater, and saturation by surface and groundwater. Its sources mainly include four aspects: moisture contained within the waste itself; water generated from biochemical reactions of the waste; back seepage of groundwater; and atmospheric precipitation. Among these, atmospheric precipitation constitutes the majority of the total leachate volume.

Leachate is a complex, high-concentration organic wastewater. Its properties depend on waste composition, compaction degree, climate, hydrological conditions, and landfill age. Generally, it has the following characteristics: complex water quality, high hazard; high concentrations of COD and BOD5; high ammonia nitrogen content; relatively high metal content.

Based on the age of the landfill, leachate is divided into two types:

Young Landfill Leachate, also known as "young leachate," with a landfill age of less than 5 years. Its characteristics are: high COD and BOD5 concentrations, good biodegradability.

Example Landfill Leachate:

COD: 20,000–30,000 mg/L;

Biodegradability: Good;

Effluent COD after biological treatment: around 1000 mg/L;

Non-biodegradable substances: Humic acids, Fulvic acids, Dissolved Organic Matter (DOM);

Treatment requirements: COD ≤100 mg/L.

Old Landfill Leachate, also known as "old leachate," with a landfill age of over 5 years, where the waste has gradually aged. Its pH is close to neutral. COD and BOD5 concentrations decrease, the BOD5/COD ratio decreases, and the ammonia nitrogen concentration increases. It presents as high NH3-N, high TN, low COD, poor biodegradability, and most of the COD consists of hard-to-degrade humic substances, making sludge proliferation difficult. As landfill operation time extends, the quantity and quality of leachate change. Process systems designed for treating young leachate are not suitable for treating old leachate.

Example Old Landfill Leachate:

Influent Quality:

COD content: 1200 mg/L;

Ammonia nitrogen content: 1700 mg/L;

Effluent Quality:

COD ≤200 mg/L;

Ammonia Nitrogen ≤45 mg/L;

Total Nitrogen ≤70 mg/L;

Heavy metal discharge standard according to GB/T31962-2015.

The landfill age of most landfills in China exceeds 10 years. The quality of raw landfill leachate water deteriorates year by year. Conventional "pretreatment + biological treatment" is not suitable for treating old landfill leachate.