What Are the Physical and Chemical Methods for Efficient Landfill Leachate Treatment?
Physicochemical treatment technologies are mainly used to remove ammonia nitrogen and inorganic components from landfill leachate, creating favorable conditions for the core biological treatment stage; they are a key part of leachate pretreatment. The main methods include coagulation-sedimentation, adsorption, and ammonia stripping.
1. Coagulation-sedimentation
Coagulation-sedimentation is notably effective for treating refractory wastewater, removing suspended solids and colloidal impurities from leachate more efficiently than other processes. By dosing coagulants, fine suspended particles and colloidal substances in the water are destabilized, aggregate with each other, and grow in size until they settle naturally. Many factors affect coagulation-sedimentation performance; the mechanisms can be grouped into four: double-layer compression, adsorption and charge neutralization, sweep flocculation (enmeshment), and adsorption bridging.
Different types of coagulants differ in leachate treatment performance, so a suitable coagulant must be selected according to the specific characteristics of the leachate. Zhang Haixia et al. applied coagulation-sedimentation to advanced leachate treatment and systematically compared the treatment performance of polyferric sulfate (PFS), polyferric chloride (PFC), and polyacrylamide (PAM). The study showed that although the treatment efficiency of PFS was slightly lower than that of PAM, the gap was limited, and PFS was less affected by pH fluctuation; considering overall economics, it is more suitable as a coagulant. Tang Hongyan et al. explored the application of magnesium ammonium phosphate (MAP) for leachate treatment at the Luoyang landfill, and through orthogonal experiments involving Mg/P/N dosing ratios, settling time, and pH, determined the optimal reaction conditions as pH = 9.5, Mg:N = 1.8:1, P:N = 1.8:1. Under these conditions, the NH4+-N removal rate exceeded 97%, while the COD removal rate was about 20%. Coagulation alone has a limited effect; studies indicate that combining it with other processes can significantly improve leachate purification. For example, Li et al. combined a coagulant with activated carbon adsorption for landfill leachate pretreatment, significantly reducing the color, turbidity, and COD of the water sample, removing some heavy metals, and effectively reducing its biological toxicity.
2. Adsorption
Adsorption uses porous solid materials with a large specific surface area to retain pollutants from the leachate on their surface. Through mechanisms such as van der Waals forces, chemical bonding forces, and ion exchange, adsorbents concentrate pollutants on their surface. Commonly used adsorbent materials include activated carbon, clay, fly ash, and zeolite. Developing new low-cost, high-performance adsorption materials, or modifying existing adsorbents to enhance their adsorption capacity, is an important research direction in this field.
To improve the adsorption performance of fly ash, Li Zhangliang et al. modified fly ash by microwave activation over different time periods and applied it to leachate treatment. Experimental results showed that the modified fly ash significantly outperformed raw fly ash in removing CODCr and color from the leachate, with maximum removal rates of 46.05% and 81.16% respectively. Liu Hui et al. produced an activated carbon material from air-dried, crushed, and high-temperature carbonized corn cob to treat leachate. This activated carbon is not only made from widely available raw materials but also shows good treatment performance.
Summary and Outlook
Although various physicochemical technologies are used in leachate treatment, they generally suffer from high operating costs and potential secondary pollution. Current research focuses on improving existing processes to reduce energy consumption, such as mechanical vapor compression (MVC) evaporation technology. This technology exploits the fact that the pressure and temperature of steam rise progressively during compression: the high-temperature steam enters the heat-exchange tubes of the evaporator, cold water is sprayed outside the tubes, and the steam condenses inside the tubes, releasing heat that continuously evaporates the sprayed water. The energy consumption of the system mainly comes from the compressor motor and a small-power immersion heater that maintains stable operation, so it is relatively low. The low-energy MVC/MVR evaporation technology can be directly used for landfill leachate treatment, producing effluent that meets the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste (GB 16889-2008) at relatively low energy consumption.
