Landfill Leachate Water Quality: Key Characteristics and Their Implications for Process Selection

The composition and concentration of landfill leachate are governed by many factors, including the type and characteristics of the waste, the landfilling method, the leaching behaviour of pollutants, chemical reactions, rainfall conditions, the age of the site and the landfill structure. Of these, the design of the site and its service life are the dominant influences.

At aged sites that have been operating for more than ten years, COD and BOD in the leachate have already fallen to low levels and the B/C ratio remains low. Ammonia nitrogen has declined somewhat, but by far less than COD and BOD, so the carbon-to-nitrogen ratio becomes unbalanced. Although the pollution load is markedly lighter at this stage, the effluent still cannot meet direct discharge standards and is difficult to treat.

Leachate from this aged site shows the following typical characteristics:

(1) Complex composition with pronounced fluctuations

In the older landfill cells the B/C ratio of the leachate is below 0.3 and ammonia nitrogen remains high, whereas leachate from newer cells typically has a BOD/COD ratio of 0.4-0.6 and a comparatively low ammonia nitrogen concentration. Seasonal weather also causes obvious swings in leachate quality within the same area. This pattern of variation goes well beyond that of conventional wastewater and places higher demands on process selection and operation. A leachate treatment system therefore needs strong resistance to shock loading and appropriate safeguard measures.

(2) Marked variation in organic pollutant concentration

Leachate generated by freshly placed waste contains very high BOD and COD, up to tens of thousands of milligrams per litre, with a B/C ratio of 0.4-0.6, good biodegradability and relatively low ammonia nitrogen. As landfill age increases, COD in leachate from older cells falls to a few thousand milligrams per litre, the B/C ratio drops below 0.3, biodegradability deteriorates and ammonia nitrogen rises. On this basis a biological process is appropriate for this project, but nutrient ratios must be adjusted before the biological stage.

(3) High ammonia nitrogen and total nitrogen

Test data show that ammonia nitrogen in the leachate at this site is usually in the range of 800-1500 mg/L. It tends to rise as the site ages, and nitrogen in the leachate exists mainly as ammonia nitrogen. Under Table 2 of the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste (GB 16889-2008), effluent ammonia nitrogen must be no more than 25 mg/L and total nitrogen no more than 40 mg/L. That implies an ammonia nitrogen removal rate above 99% and a total nitrogen removal rate above 98%, so the selected process must deliver high-efficiency nitrogen removal.

(4) Colour

The leachate at this site is dark brown with a high colour value, so the process train must include a treatment unit with a pronounced decolourising effect.

(5) Salt content

The conductivity of the leachate is as high as 15,700 mg/L, indicating a large amount of dissolved salts and heavy-metal ions. Process selection must take the system's salt tolerance into account and require a high recovery rate so that the volume of concentrate produced is minimised.

(6) Scaling tendency and fouling behaviour

Calcium and magnesium ion concentrations in the leachate are high, and combined with severe organic pollution this readily causes scaling and organic fouling. If membrane processes are used, the design must focus on mitigating or eliminating the adverse effects of organic fouling and inorganic scaling on the membrane system.