Aged Landfill Leachate: Six Quality Traits That Set the Process Train

Landfill leachate is usually described as strong wastewater, which is true of a young site and progressively less true of an old one. What changes with age is not only the load but the ratio between the pollutants, and that ratio is what decides whether a biological process can carry the plant or whether something else has to be bolted on in front of it.

What governs composition in the first place

The makeup and strength of leachate respond to a long list of variables: the type and characteristics of the waste received, the landfilling method, the leaching behaviour of the pollutants themselves, chemical reactions inside the mass, rainfall, the age of the site and the structure of the landfill. Among those, site design and service life dominate. Two landfills taking similar waste can produce leachates that behave quite differently simply because one has been operating for fifteen years and the other for three.

The aged-site signature

At 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 stays low. Ammonia nitrogen has declined somewhat, but by far less than COD and BOD, so the carbon-to-nitrogen ratio becomes unbalanced. The pollution load is distinctly lighter at this stage, and yet the effluent still cannot meet direct discharge standards and is, in many ways, harder to treat than the strong leachate of a young cell — because the easy carbon that once fed a biological system is gone while the nitrogen remains.

Trait one: complex composition, pronounced fluctuation

In the older cells the B/C ratio of the leachate sits below 0.3 while ammonia nitrogen stays high. Leachate from newer cells in the same site typically shows a BOD/COD ratio of 0.4-0.6 with a comparatively low ammonia nitrogen concentration. Seasonal weather adds an obvious swing within the same area. This pattern of variation goes well beyond what conventional wastewater experience prepares you for, and it places two demands on the design: strong resistance to shock loading, and safeguard measures that assume the feed will misbehave.

Trait two: organic concentration varies by an order of magnitude

Leachate generated by freshly placed waste carries 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 the site ages, COD in leachate from the older cells falls to a few thousand milligrams per litre, the B/C ratio drops below 0.3, biodegradability deteriorates and ammonia nitrogen rises. A biological process is still appropriate for this project, but the nutrient ratios have to be adjusted before the biological stage rather than inside it — by the time the mixed liquor sees the water, the carbon source is no longer there to be balanced.

Trait three: 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 the nitrogen present 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. Working backwards, that implies an ammonia nitrogen removal rate above 99% and a total nitrogen removal rate above 98%. A process train that delivers 90% nitrogen removal is not a partial success here; it is a non-compliance.

Trait four: colour

The leachate at this site is dark brown with a high colour value. Colour is easy to dismiss as an aesthetic issue until it becomes the parameter that fails the permit. The train therefore needs a unit with a pronounced decolourising effect, and in practice that usually means an oxidation step ahead of, or integrated with, the biological stage rather than a polishing filter at the end.

Trait five: salt content

Conductivity of the leachate is as high as 15,700 mg/L, which indicates a large amount of dissolved salts and heavy-metal ions. Two consequences follow. Process selection has to account for the system's salt tolerance, since a biological population that is healthy at one salinity can collapse at another. And the design needs a high recovery rate so that the volume of concentrate produced is minimised — concentrate is the cost that keeps arriving after commissioning, and a membrane stage sized without that constraint will generate a disposal stream nobody budgeted for.

Trait six: scaling tendency and fouling behaviour

Calcium and magnesium concentrations are high, and combined with severe organic pollution this readily produces scaling and organic fouling. Where membrane processes are used, the design has to focus on mitigating or eliminating the effect of organic fouling and inorganic scaling on the membrane system: pretreatment that removes hardness and organics, cleaning regimes that are written for the actual foulant, and instrumentation that detects fouling before it becomes irreversible.

What the six traits add up to

None of these observations is exotic on its own. Taken together they describe a plant that must remove nitrogen at a rate above 99% from a stream with almost no biodegradable carbon left, strip colour from a dark brown liquid, tolerate salinity that varies with rainfall, and avoid concentrating a brine it cannot dispose of. That combination is why aged leachate designs converge on a sequence rather than a single unit — biological nitrogen removal with an external carbon source, an oxidation stage for colour and residual organics, and a membrane stage whose recovery is set by the salt balance rather than by the water balance.