Landfill leachate is a high-concentration organic wastewater with complex composition, high pollutant concentration, high chromaticity, and strong toxicity. It is a recognized challenge in the water treatment industry. It contains not only a large amount of organic pollutants but also various heavy metal pollutants. Improper handling can not only affect the quality of surface water but also endanger the safety of groundwater. So why is landfill leachate treatment so difficult?

1.Auxiliary equipment cannot meet the demand.Landfill leachate water quality is complex, highly hazardous, and is influenced by waste composition, site climate conditions, site hydrogeology and rainfall conditions, landfill conditions, and time. This leads to significant variations in leachate water quality and quantity, complex change patterns, and increased treatment difficulty. After a leachate treatment project is operational, the original auxiliary treatment process may fail to meet practical operational needs due to the treatment technology chosen, changes in water quality, effluent quantity, and other reasons.

2.Large variations in water quality increase treatment difficulty.Landfill leachate water quality changes significantly, production volume changes seasonally (significantly greater in the rainy season than in the dry season), pollutant composition changes seasonally, and their concentration changes with the extension of landfill age, increasing the treatment difficulty.

3.High metal content.Landfill leachate contains more than 10 types of metal ions. Iron and zinc levels are relatively high during the acid fermentation stage: iron concentration can reach around 2000 mg/L, zinc around 130 mg/L, lead up to 12.3 mg/L, and calcium even up to 4300 mg/L. These metal ions can severely inhibit the biological treatment process. Because the CO2 produced by waste degradation dissolves, landfill leachate is slightly acidic. This slightly acidic environment exacerbates the dissolution of water-insoluble carbonates, metals, and their oxides in the waste, so leachate contains high concentrations of metal ions, making treatment very difficult.

4.High CODcr and BOD5 concentrations. Landfill leachate is divided into two categories: one is fresh leachate (landfill time less than 5 years), characterized by high CODcr and BOD5 concentrations and strong biodegradability; the other is aged leachate (landfill time over 5 years), its pH is close to neutral, CODcr and BOD5 concentrations decrease, the BOD5/CODcr ratio decreases, ammonia nitrogen concentration increases, and biodegradability decreases, leading to higher project treatment costs. The highest CODcr and BOD5 in leachate can reach 90,000 mg/L and 38,000 mg/L respectively or even higher. Generally, CODCr, BOD5, and the BOD5/CODCr ratio decrease with the increasing "age" of the landfill, while the alkalinity content increases.

5.High ammonia nitrogen content,which increases with landfill time, reaching a maximum of 1700 mg/L. Nitrogen in leachate exists mostly in the form of ammonia nitrogen, accounting for about 40%-50% of TKN.