Landfill leachate: what a field test of a sorption bed showed

Landfill leachate can rarely be brought to discharge quality by a single unit. We ran a field test of a sorption bed on real leachate: how the bed works, what the test showed, and what data are needed to move from a test to a design.

How leachate differs from ordinary wastewater

Leachate forms when precipitation passes through the body of a landfill and carries out the products of waste decomposition. It differs from municipal or industrial wastewater in four ways, and each one affects the choice of treatment train.

Ammonium nitrogen. Its concentration in mature leachate is orders of magnitude higher than in domestic sewage, while biological oxidation is slow and requires oxygen.

Dissolved organics. In mature leachate these are largely humic in nature: large molecules that enter the micropores of sorbents poorly and degrade biologically at a slow rate.

High dissolved solids. Membrane methods are used to reduce them substantially, and this determines the make-up of the train if the treated water is to be discharged.

Variability. Two samples taken on the same day at different points of one site differed in dissolved solids by more than a factor of two. Designing from a single sample is not possible.

How the bed is built

The bed is made up of three components, each with its own task.

The ion-exchange component handles ammonium. It is a natural framework mineral: its crystal lattice carries a permanent negative charge, balanced by mobile sodium, potassium and calcium cations. On contact with leachate, the ammonium cation takes the place of an exchangeable cation and is held in the lattice. The material is selective: it takes up ammonium more readily than sodium, calcium and magnesium, which are considerably more abundant in leachate.

Two features important for design follow from this. Capacity is finite: as the bed saturates, it is replaced or regenerated. The rate is limited by diffusion of the ion into the grain, so the result depends on contact time.

How the test was arranged

The arrangement is simple and reproducible at any site. A vessel with a sampling tap cut into its lower part. Inside, a filter cloth so that material is not carried into the tap. On top of the cloth, 4 litres of the bed; over that, 10 litres of leachate — a liquid-to-bed volume ratio of 2.5 to 1.

Sixty minutes of static contact, without stirring and without forced feed. Six samples were then drawn in sequence through the tap, with one litre drained between them.

Six samples rather than one is essential. A single sample cannot distinguish a result from chance, whereas six show how stable the effect is.

Ammonium nitrogen, pH, conductivity, dissolved oxygen and temperature were determined.

What the test showed

Ammonium nitrogen fell by 70–75 %. The result was reproduced in all six samples with a spread of a few per cent — this is the main outcome of the test.

The pH stayed at its initial level, 7.6–7.9. This matters more than it seems. Ammonium exists in water in equilibrium with ammonia, and the share of volatile ammonia rises with pH. Had the bed raised the pH, part of the ammonia would have left for the air, and the instrument would have shown a fall in concentration unrelated to treatment. An unchanged pH means the ammonium genuinely passed into the bed by ion exchange.

Dissolved oxygen did not change significantly: the contact regime was stable across all samples.

On standing, a precipitate formed in the treated leachate. This indicates that some oxidation and precipitation processes did not have time to complete in the bed within one hour and continued in the collected water. With longer contact these processes largely take place within the bed, which also acts as a filter. The conclusion for a full-scale unit is simple: a clarification stage is needed after the sorption stage.

Contact time matters more than the composition of the bed

In one hour it is mainly the surface layer of the grain that is used, while reaching equilibrium takes from several hours to a day. The 70–75 % obtained is the result of one hour, not the limit of what the bed can do.

Confirmation came from within the test itself. Two samples were drawn at an increased rate, meaning the liquid spent less time in the bed. In both, residual ammonium was higher than in the other four.

The result was limited by rate, not by capacity. At a ratio of 2.5 to 1, little ammonium passed through the bed relative to the exchange capacity of materials of this class. Longer contact will therefore raise efficiency, and the bed itself can handle a considerably larger volume of leachate.

For design this is the key parameter. The design contact time sets the required bed volume, and the bed volume sets the cost of the unit and how often the material is replaced. It is determined by a separate test on the same leachate: one bed, samples at 1, 4, 8 and 24 hours. No site access is needed for this, a canister of leachate is enough.

What to do next

If you are assessing leachate treatment at your own site, collect the data before choosing equipment:

• ✓ammonium nitrogen, preferably across different seasons
• ✓COD and BOD, to establish the share of biodegradable organics
• ✓dissolved solids and conductivity at several points, not at one
• ✓pH and temperature across seasons
• ✓daily leachate volume and how it rises after rainfall
• ✓where the treated water is to go and who sets the limit

A sorption stage removes ammonium nitrogen and part of the organics and works within a multi-stage train: together with clarification and, where water is discharged, with a biological stage or membranes. To put together a train for a specific effluent: industrial wastewater treatment.