Leachate Treatment: Landfill Leachate Treatment Processes and Technologies

A very wide range of leachate treatment processes has been applied to landfill leachate, with varying degrees of success.

That remains as true today as when this page was first written in 2011.

However, one point deserves emphasis from the outset:

There is no single landfill leachate treatment process which is automatically the best solution for every site.

The most appropriate treatment system depends upon the composition and age of the leachate, flow variation, discharge route, permit limits, available land, power, operator skills, sludge and concentrate disposal routes, and the whole-life cost of operating the plant.

For many UK municipal landfill leachates, well-designed biological treatment remains one of the most reliable and sustainable starting points. But modern leachate treatment increasingly combines biological processes with physical, chemical or membrane treatment where tighter discharge standards or difficult contaminants require additional removal.

This page provides an overview of the principal landfill leachate treatment technologies, how they work, where they are most useful, and some of the limitations which are easily overlooked during process selection.

What Is Landfill Leachate Treatment?

Landfill leachate treatment is the process of reducing contaminants in collected landfill leachate sufficiently to allow the treated liquid to be safely discharged, transferred for further treatment, reused where permitted, or otherwise managed in accordance with the site’s environmental permit.

Leachate treatment is only one part of the wider management system.

Before treatment is considered, operators should also minimise leachate generation, collect and extract it reliably, provide adequate storage and equalisation, and understand how leachate quality changes with landfill age and season.

For the wider subject, see our guide to Leachate Management Techniques.

For the engineering of collection wells, pumps and transfer systems, see Landfill Leachate Collection and Extraction.

Why Landfill Leachate Is Difficult to Treat

Leachate is not a uniform wastewater.

Its characteristics vary according to:

• the age of the landfill;
• the types of waste deposited;
• rainfall and infiltration;
• degree of waste degradation;
• landfill temperature;
• recirculation practices;
• the effectiveness of capping;
• the presence of industrial or unusual wastes; and
• the amount of dilution or mixing occurring within the collection system.

Typical contaminants may include:

• ammoniacal nitrogen;
• biodegradable and refractory organic matter;
• chemical oxygen demand (COD);
• biochemical oxygen demand (BOD);
• suspended solids;
• iron and other metals;
• chloride and other dissolved salts;
• alkalinity;
• sulphides and odorous compounds;
• dissolved methane; and
• a wide range of trace organic contaminants.

As landfills mature, the readily biodegradable fraction generally falls, while ammonia and more refractory organic compounds can remain important for many years.

That is one reason why a treatment plant designed simply as a conventional sewage works can perform poorly when applied to landfill leachate.

For more detail on typical leachate chemistry, see Landfill Leachate Composition.

The First Step: Define the Required Treated Leachate Quality

Process selection should start with the required discharge quality rather than with a preferred piece of equipment.

In England, landfill operators must ensure that leachate meets the required standard before it is discharged.

Treatment may therefore be required to:

• meet a consent or permit for discharge to surface water;
• satisfy the statutory water undertaker’s conditions for discharge to sewer;
• produce a quality suitable for transfer to another treatment facility; or
• meet the requirements of a permitted recovery or reuse scheme.

The Environment Agency’s current guidance is available at Landfill Operators: Manage Leachate.

The discharge destination often determines the treatment process just as much as the raw leachate does.

Biological Leachate Treatment

In the UK, landfill leachate treatment has for many years been strongly associated with biological nitrification systems.

That is for a good reason.

Many mature landfill leachates contain substantial concentrations of ammoniacal nitrogen, and biological nitrification can convert ammonia to nitrate reliably when the plant is properly designed, heated where necessary, supplied with sufficient oxygen and alkalinity, and operated by people who understand the process.

Biological systems also remove a substantial proportion of biodegradable organic matter.

The principal biological options include:

• sequencing batch reactors (SBRs);
• conventional activated sludge systems;
• membrane bioreactors (MBRs);
• moving bed biofilm reactors (MBBRs);
• fixed-film and attached-growth systems; and
• various hybrid biological processes.

Sequencing Batch Reactor (SBR) Leachate Treatment

Sequencing batch reactor treatment has a long and successful history in UK landfill leachate treatment.

In an SBR, biological treatment, settlement and decanting take place sequentially within the same reactor or a linked group of reactors.

A typical operating cycle may include:

1. filling;
2. aeration and biological treatment;
3. settlement;
4. decanting of treated effluent; and
5. sludge wasting or an idle period before the next cycle.

For leachate, SBRs offer several useful advantages:

• good control over reaction time;
• ability to retain a large nitrifying biomass;
• flexibility when flows vary;
• relatively straightforward process control;
• the possibility of incorporating anoxic phases for denitrification; and
• less dependence on a separate final settlement tank.

For many municipal landfill leachates, an extended-aeration SBR remains a very effective treatment option.

However, the plant must be designed specifically for leachate. Simply taking a sewage-treatment SBR design and applying it to high-ammonia leachate is unlikely to provide a robust result.

Nitrification: Removing Ammoniacal Nitrogen

One of the main purposes of biological treatment at many UK landfill sites is nitrification.

Nitrification is the biological oxidation of ammoniacal nitrogen, ultimately converting it largely to nitrate.

The process requires:

• a healthy population of nitrifying microorganisms;
• sufficient dissolved oxygen;
• suitable pH;
• adequate alkalinity;
• sufficient reactor residence time;
• appropriate sludge age; and
• a suitable operating temperature.

Nitrifying organisms grow significantly more slowly than the heterotrophic organisms which remove readily biodegradable organic matter.

For that reason, temperature and sludge age become particularly important during cold UK winters.

A treatment plant which operates comfortably in summer can lose ammonia-removal capacity if the biological reactor becomes too cold.

Heating, insulation and adequate process volume therefore deserve serious consideration during design.

Denitrification: When Nitrate Must Also Be Removed

Nitrification solves the ammonia problem by converting ammoniacal nitrogen to nitrate.

But it does not remove nitrogen from the liquid.

If the discharge permit requires a low total nitrogen or nitrate concentration, a further denitrification stage may be needed.

Denitrification uses bacteria operating under anoxic conditions to convert nitrate ultimately to nitrogen gas.

In some leachates there may not be enough readily biodegradable carbon remaining after nitrification to support efficient denitrification. An external carbon source may then be necessary.

This adds process complexity and operating cost, which is why it is important not to specify denitrification unless the receiving environment or discharge standard genuinely requires it.

Biological Treatment and COD Removal

Biological treatment can remove a substantial proportion of the COD in younger and intermediate-age landfill leachates.

However, as a landfill ages, an increasing proportion of the residual COD may comprise refractory organic compounds which conventional biological treatment cannot readily degrade.

This is one of the fundamental limitations of biological treatment.

A perfectly healthy nitrification plant can produce an effluent with very low ammoniacal nitrogen while still containing appreciable residual COD and colour.

If tighter COD or colour limits apply, a polishing process may therefore be needed.

Membrane Bioreactor (MBR) Leachate Treatment

A membrane bioreactor combines biological treatment with membrane separation.

Instead of relying solely on gravity settlement to separate biomass from treated water, an MBR uses microfiltration or ultrafiltration membranes.

This allows:

• very low suspended solids in the treated effluent;
• high biomass concentrations;
• long sludge ages;
• a relatively compact plant footprint; and
• a consistent feed to subsequent polishing processes such as reverse osmosis.

MBRs can be highly effective, but they introduce additional energy use, membrane cleaning, membrane replacement and fouling considerations.

They should therefore be selected on whole-life performance rather than simply because membrane technology appears more advanced.

For a detailed cost discussion see MBR Leachate Treatment System Pricing.

Reverse Osmosis (RO) for Leachate Treatment

Reverse osmosis has been widely applied to landfill leachate, sometimes as a principal treatment technology and sometimes as a polishing stage following biological treatment.

RO can produce a very high-quality permeate because dissolved contaminants are retained by the membrane.

This can be especially attractive where extremely stringent discharge standards apply.

However, the apparent simplicity of the process conceals an important engineering issue:

reverse osmosis separates contaminants; it does not destroy them.

The process produces:

• a relatively clean permeate stream; and
• a much smaller, highly concentrated reject or concentrate stream.

What happens to that concentrate is therefore a central part of the treatment design.

Why Returning RO Concentrate to the Landfill Can Be Problematic

This was one of the concerns raised in the original version of this article, and it remains valid.

If RO concentrate is continuously returned to a contained landfill, many non-degradable dissolved contaminants can simply be recycled.

Water leaves as permeate while salts and refractory contaminants are returned.

Over a long period this can contribute to increasing concentrations in the leachate.

RO should therefore never be selected without a credible long-term concentrate management strategy.

Possible routes may include off-site treatment or disposal, further concentration, evaporation, crystallisation or some other appropriately permitted residuals-management process.

For more on this issue see Reverse Osmosis vs Evaporation for Leachate Volume Reduction.

Activated Carbon Treatment

Activated carbon can be useful as a polishing stage where residual dissolved organic compounds remain after biological treatment.

Granular activated carbon can adsorb a range of refractory organic contaminants and may assist with colour and trace-organic removal.

Its effectiveness depends heavily on the leachate composition.

High concentrations of competing organic matter can exhaust carbon rapidly, so activated carbon is normally more attractive after the bulk pollution load has already been removed.

As with membrane treatment, the contaminants are not simply made to disappear. The spent carbon must eventually be regenerated or replaced and properly managed.

Ion Exchange

Ion exchange resins can selectively remove particular dissolved ions and may have a role where a specific contaminant has to be reduced to a low concentration.

However, resin fouling, regeneration chemicals and management of the resulting regenerant can make ion exchange expensive for untreated high-strength landfill leachate.

It is therefore generally more suited to targeted polishing applications than to bulk primary treatment.

Advanced Oxidation Processes

Advanced oxidation processes (AOPs) use highly reactive oxidising species to break down difficult organic contaminants.

Examples include combinations involving:

• ozone;
• hydrogen peroxide;
• ultraviolet light;
• Fenton-type oxidation; and
• other catalytic oxidation systems.

AOPs can be valuable where refractory COD, colour or particular trace organic contaminants remain after conventional treatment.

However, chemical and energy consumption can be substantial.

For most leachates, oxidation is therefore more likely to be justified as a polishing stage than as the first process applied to raw leachate.

Physical and Chemical Pre-Treatment

Many successful treatment plants include comparatively simple pre-treatment stages ahead of the main biological or membrane process.

These may include:

• screening;
• settlement;
• flow equalisation;
• pH adjustment;
• coagulation and flocculation;
• metal precipitation;
• air stripping in appropriate applications;
• dissolved methane management; and
• antifoam or scale-control dosing.

Good pre-treatment can significantly improve the reliability of the more expensive processes downstream.

Equalisation Is Part of Treatment Design

One of the least glamorous but often most valuable pieces of a leachate treatment plant is the balance or equalisation tank.

Landfill leachate flows can vary considerably with rainfall and site operations.

Leachate strength can also change rapidly where different cells, sumps or storage tanks are brought online.

Equalisation can:

• smooth peak flows;
• reduce shock ammonia loads;
• blend leachates of different strengths;
• provide a stable feed to biological treatment;
• provide operational buffer capacity during maintenance; and
• allow abnormal leachate to be isolated rather than immediately entering the treatment plant.

A treatment process is usually easier to operate when the feed is made as boring and predictable as possible.

Good equalisation helps achieve exactly that.

Leachate Treatment to Sewer

Not every landfill needs to produce an effluent suitable for direct discharge to a river or stream.

Where sewer access is available, the treatment objective may instead be to meet the trade effluent conditions imposed by the statutory water undertaker.

Pre-treatment may therefore focus on contaminants which would otherwise make the leachate unsuitable for the sewer or receiving sewage treatment works.

In some cases this can be significantly simpler and more economical than treating all the way to a direct surface-water discharge standard.

However, sewer discharge must be authorised by the relevant statutory water undertaker.

Direct Discharge to Surface Water

Where treated leachate is discharged directly to the water environment, treatment requirements can be much more demanding.

The required permit limits will depend upon the receiving environment and environmental risk assessment.

The process may consequently need to remove:

• ammonia;
• BOD;
• COD;
• suspended solids;
• nitrate or total nitrogen;
• metals;
• specific organic contaminants; or
• other site-specific substances.

The benefit of the UK approach has historically been that discharge requirements can be related to the receiving environment rather than imposing one arbitrary numerical standard irrespective of local conditions.

That principle remains important because treatment taken beyond what is environmentally necessary carries both financial and environmental costs.

Best Available Techniques (BAT) and Modern Leachate Treatment

The original version of this page stated that SBR biological treatment with nitrification and denitrification was usually the Best Available Technique.

Our view remains that well-designed biological treatment is frequently one of the best available technical solutions for municipal landfill leachate.

However, the regulatory use of the term BAT – Best Available Techniques is now more formal and should not be reduced to one process technology.

Where on-site leachate treatment is above the relevant thresholds in Schedule 1 of the Environmental Permitting Regulations, qualifying installations must comply with the applicable Waste Treatment BAT requirements and associated emission levels.

Even where the activity falls below those thresholds, the Environment Agency states that leachate must still be stored and treated safely and that BAT principles should be followed.

In practice, BAT is therefore a combination of:

• appropriate process selection;
• good containment;
• energy efficiency;
• emissions control;
• monitoring;
• reliable operation;
• appropriate residuals management; and
• avoidance of transferring pollution from one environmental medium to another.

See the Environment Agency’s current leachate management guidance.

Secondary Containment and Pollution Prevention

A modern treatment plant must also be designed so that a failure of tanks, pipework, pumps or chemical-storage equipment does not simply create a new pollution incident.

Current Environment Agency guidance requires secondary containment for leachate storage and treatment facilities unless other suitable measures provide equivalent protection.

This is especially important around:

• raw leachate tanks;
• high-strength intermediate liquors;
• treatment chemicals;
• sludge storage;
• RO concentrate;
• chemical-cleaning solutions; and
• final effluent storage where recontamination is possible.

Sludge and Treatment Residues Must Be Considered

• Every treatment process produces something.
• Biological plants produce excess sludge.
• Reverse osmosis produces concentrate.
• Activated carbon eventually becomes spent carbon.
• Ion exchange produces regenerant.
• Chemical precipitation creates sludge.
• Evaporation produces a concentrated residue.

A treatment technology should therefore never be assessed solely on the quality of the liquid effluent.

The cost, environmental impact and legal route for managing all residual streams must be included in the assessment.

Choosing the Right Leachate Treatment Process

A rational process-selection study should begin with representative analytical data and realistic flow information.

The designer should then consider:

• average and peak leachate flow;
• ammoniacal nitrogen concentration and load;
• BOD and COD;
• biodegradability;
• salinity and chloride;
• metals;
• alkalinity and pH;
• suspended solids;
• seasonal temperature;
• specific contaminants of concern;
• the required treated-water quality;
• residuals and concentrate disposal;
• energy availability;
• operator skill requirements;
• plant redundancy;
• capital cost;
• operating cost; and
• expected duration of treatment during landfill aftercare.

That final item is especially important.

A landfill leachate treatment plant may have to operate for decades after waste disposal has stopped.

A technology which appears economical based only on initial capital cost can become extremely expensive when energy, membranes, chemicals, sludge disposal and specialist maintenance are considered over the full operating life.

Leachate Treatment Plant Costs

There is no meaningful universal price per cubic metre for landfill leachate treatment.

Costs depend upon both flow and contaminant load, and the required final effluent standard can completely change the treatment train.

A relatively simple biological nitrification system discharging to sewer is a fundamentally different project from a biological plant followed by membranes, carbon and reverse osmosis producing a high-quality direct-discharge effluent.

For more detail see our dedicated guide: Leachate Treatment Plant Pricing: Setup, Operating and Long-Term Costs.

Biological Treatment Versus Reverse Osmosis

The choice between biological treatment and RO is sometimes presented as though one technology must be better than the other.

That is not really the correct comparison.

Biological treatment destroys or converts biodegradable pollution.

Reverse osmosis physically separates dissolved contaminants into a clean permeate and a concentrated reject stream.

Those are fundamentally different functions.

For many leachates, the most effective treatment train may therefore use biological treatment first to remove ammonia and biodegradable organics, followed by membrane or adsorption processes only where the discharge requirement justifies them.

This reduces the pollutant loading reaching the expensive polishing stage and can improve the sustainability of the whole plant.

Do Not Over-Treat Without Good Reason

There is an understandable tendency in wastewater treatment to assume that a lower final concentration must always represent a better environmental result.

That is not necessarily so.

Very advanced treatment may require substantial:

• electricity;
• chemicals;
• membrane replacement;
• carbon replacement;
• sludge transport;
• concentrate transport; and
• off-site disposal.

If a simpler biological treatment process can demonstrably protect the receiving environment and comply with the permit, adding unnecessary process stages may increase cost and carbon emissions without delivering a proportionate environmental benefit.

The objective should be appropriate treatment, not treatment complexity for its own sake.

Frequently Asked Questions About Leachate Treatment

What is the most common landfill leachate treatment process?

Biological treatment, particularly activated-sludge and sequencing batch reactor processes designed for nitrification, has been widely used for municipal landfill leachate. However, the most appropriate process depends on the leachate composition and required discharge quality.

Can biological treatment remove ammonia from landfill leachate?

Yes. Properly designed aerobic biological treatment can convert ammoniacal nitrogen to nitrate through nitrification. Reliable operation requires adequate oxygen, alkalinity, sludge age, temperature and process control.

Does nitrification remove nitrogen completely?

No. Nitrification converts ammonia into nitrate. Where total nitrogen or nitrate must also be reduced, a subsequent denitrification stage may be required.

Can reverse osmosis completely treat landfill leachate?

RO can produce very high-quality permeate, but it also produces a concentrated reject stream containing most of the contaminants removed from the water. A sustainable concentrate management route is therefore essential.

Is returning RO concentrate to the landfill a permanent solution?

Not necessarily. Repeatedly returning concentrate can recycle non-degradable salts and refractory contaminants into the landfill and may increase their concentration over time. The long-term mass balance needs to be considered.

What is an MBR leachate treatment plant?

A membrane bioreactor combines biological treatment with membrane filtration. It can retain high biomass concentrations and produce a low-solids effluent, but involves membrane energy, cleaning and replacement costs.

Can landfill leachate be discharged directly to a sewer?

Only where the statutory water undertaker has authorised the discharge and the leachate meets its trade effluent conditions. Pre-treatment may be required before sewer discharge.

What determines the cost of leachate treatment?

The principal factors are flow, contaminant load, leachate composition, final discharge standard, treatment technology, energy consumption, chemical use, sludge or concentrate disposal, maintenance and the expected operating life of the plant.

Conclusion

A wide range of processes is available for landfill leachate treatment, but treatment technology should never be selected simply because it is fashionable or capable of producing the lowest laboratory result.

For many UK municipal landfill leachates, well-designed biological treatment remains a robust and highly effective core technology, particularly where the principal challenge is ammoniacal nitrogen.

Where biological treatment alone cannot meet the required discharge quality, it can be supplemented by denitrification, membranes, activated carbon, ion exchange, advanced oxidation or other polishing technologies.

Reverse osmosis can achieve excellent separation, but the concentrate it produces remains a real waste stream requiring a sustainable long-term solution.

The best treatment scheme is therefore one which considers the complete mass balance:

what enters the plant, what leaves as treated water, what becomes gas or biological solids, and what remains as sludge, concentrate or another residual waste.

That, together with reliable operation, appropriate containment, permit compliance and whole-life cost, is the basis of sound modern leachate treatment design.

First published in 2011 and substantially reviewed and updated in 2026 to reflect current landfill leachate treatment practice and Environment Agency permitting guidance.