Landfill Leachate Treatment: Ammonia, the COD Floor, and the Age Problem

Landfill leachate is one of the hardest effluents to treat and one of the longest-lived, because the landfill keeps producing it for decades after it stops taking waste. Leachate is defined by very high ammonia, a high and increasingly stubborn organic load, and a chemistry that shifts as the landfill ages, and the single most common mistake is designing a treatment plant for the leachate a young landfill produces rather than the recalcitrant, ammonia-heavy leachate the same site will produce ten years later. Ammonia is usually the binding parameter, the recalcitrant COD is usually the one that will not go away, and a plant built only around biology will hit a floor it cannot cross.

This guide is for the operators, engineers, and environmental leads who own a landfill's water. It covers what leachate is and why its age changes everything, what the published plants actually achieve, why ammonia usually drives the design, the treatment train and where biology hits its floor, and where these projects go wrong. It draws on a reference corpus that is honest about its own bias, so read the pond-based numbers as one well-documented tradition rather than global best practice.

Quick Navigation

• What landfill leachate is, and why age changes everything
• Benchmarks from 28 published landfill leachate cases
• Ammonia: the parameter that usually drives the design
• The treatment train, and where biology hits its floor
• Real installations
• Capital and operating cost, and where projects go wrong
• The CFO Hook
• Sources
• Related Articles
• FAQ

What landfill leachate is, and why age changes everything

Leachate is the liquid that forms when rain percolates through buried waste, dissolving and carrying whatever the waste releases. What it carries changes with the age of the landfill, and that age effect is the single most important thing to understand before designing a plant.

Young leachate, from a landfill in its first few years, is acidic and rich in readily biodegradable organic acids: high biochemical oxygen demand (BOD), a BOD-to-COD ratio around 0.5 to 0.7, and an organic load that biology removes efficiently. Mature or stabilised leachate, from an older or closed landfill, has lost the biodegradable fraction to the landfill's own internal digestion and is left with a recalcitrant, humic organic load: a BOD-to-COD ratio that falls to 0.1 or below, plus ammonia that keeps rising because the waste keeps releasing nitrogen. The published age series in the reference corpus shows this cleanly: a 1.7-year cell ran a BOD-to-COD ratio of 0.66, a 7.2-year cell around 0.08, and a 14.4-year cell carried ammonia at 1,764 mg/L that its lagoon train could not bring into compliance.

The design implication is severe and often missed: a treatment plant that clears the consent on a young landfill's biodegradable leachate will fail on the same landfill's stabilised leachate a decade later, when the organic load has gone recalcitrant and the ammonia has climbed. The defensible design anticipates the leachate the site will produce across its full life, including its long post-closure tail, not the leachate it produces at commissioning. One nuance the data adds: age is a guide, not a rule, one 2.1-year cell in the same study was already methanogenic and behaving like older leachate, so a real characterisation always beats an assumption from the landfill's age.

Benchmarks from 28 published landfill leachate cases

The numbers here come from a corpus of 28 published landfill leachate treatment cases that Aguato has transcribed from peer-reviewed journals and delivered-project reports, each with paired influent and effluent measurements and the actual treatment train. Two honest caveats first. The corpus is documented-heavy (about eight in ten cases at documented tier), which is good, but it is also Brazil-weighted and lagoon-dominated: half the cases are pond and lagoon systems, so the pond percentiles below describe one well-documented tradition rather than global best available technique, and membrane systems are under-represented. Read the distribution with that in mind.

Concentrations in mg/L. Three things stand out. First, ammonia is enormous, a median influent of 722 mg/L and a ninetieth percentile above 1,500, with individual young leachates reaching 4,700, which is why ammonia usually sets the design. Second, biology removes ammonia and BOD well but leaves a stubborn COD: the median effluent COD is 588 mg/L, and on the cases without a membrane it does not drop below roughly 250 to 1,600 mg/L no matter how long the lagoon retention. That is the recalcitrant-COD floor, and it is the defining limitation of biological-only leachate treatment. Third, total dissolved solids are high (individual plants report 7,000 to over 50,000 mg/L), and only a membrane or a thermal process genuinely removes them.

A caution the corpus itself raises: apparent pollutant removal in open lagoons is partly rainfall dilution, not treatment, and several of the source studies prove this using chloride and conductivity as conservative tracers. A landfill leachate plant should be judged on mass removal and on a conservative tracer, not on a concentration drop that a wet season can manufacture. Providers that build these trains are listed under ammonia removal suppliers and membrane filtration suppliers.

Ammonia: the parameter that usually drives the design

On most landfill leachate, ammonia is the binding parameter, both because it is present at hundreds to thousands of mg/L and because its discharge limit is tight. There are two routes to remove it, and the choice shapes the plant.

Biological nitrification and denitrification converts ammonia to nitrogen gas through the biology, and it works well where the leachate is warm enough and carries enough biodegradable carbon to drive the denitrification. The published plants prove it can reach near-total removal: one UK plant running a nitrifying sequencing-batch reactor and a reed bed took ammonia from 820 mg/L to 0.48 mg/L, a greater than 99.9% removal. The limitation is that mature leachate is often too carbon-poor to denitrify without an added carbon source, the same low carbon-to-nitrogen problem that shapes pharmaceutical and synthesis effluents.

Physical-chemical ammonia stripping raises the pH and drives the ammonia out of solution as a gas, which is then captured, and it does not depend on the biology being healthy. One published pond system in Brazil that reported 93% ammonia removal was shown to be stripping, not nitrification, because nitrite never appeared, and a full-scale groundwater plant in Italy strips ammonia and recovers it as ammonium sulphate fertiliser. Stripping is the robust route for the highest-ammonia and most inhibitory leachates, at the cost of energy, chemicals for pH control, and scaling on hard leachate.

The decision between the two turns on the leachate's temperature, its carbon-to-nitrogen ratio, and its ammonia concentration: biology for warmer, younger, carbon-richer leachate; stripping for the coldest, oldest, most concentrated streams.

The treatment train, and where biology hits its floor

The train that recurs follows the leachate's own chemistry. Equalisation first, because leachate flow and strength swing with rainfall. Biological treatment, anaerobic then aerobic on young high-strength leachate, or aerobic with nitrogen removal on stabilised leachate, carries the bulk of the ammonia and the biodegradable COD. The broad choice between aerobic and anaerobic treatment applies, with ammonia as the extra axis.

Then comes the floor. Biology cannot cross the recalcitrant-COD line, and the corpus shows biological and lagoon trains stalling at a median effluent COD near 590 mg/L, above most discharge consents. Two recourses cross it. Membranes, a membrane bioreactor followed by nanofiltration or reverse osmosis, take the effluent well below the floor: a Turkish plant with an MBR and nanofiltration reached 512 mg/L COD and cut colour from 8,800 to 175, and a Spanish plant with a reverse-osmosis stage reached 65 mg/L COD. The membrane path also removes the chloride and dissolved solids that biology leaves entirely untouched, which is why the membrane filtration and reverse osmosis route is the default for mature leachate on a tight consent. Advanced oxidation, ozone and related processes, breaks the recalcitrant molecules down instead of concentrating them, and is used where a membrane brine has nowhere to go. The advanced oxidation process stage is the alternative recourse when the reject stream from a membrane cannot be managed.

The membrane recourse creates its own problem, a concentrated reject stream, and on a landfill that reject often goes back onto the tip or into an evaporation or crystallisation step. This is the same zero liquid discharge tail that any high-salinity stream carries, and a leachate plant that adds membranes without a reject plan has moved the problem, not solved it.

Real installations, with the numbers and the train

Every row is a real facility with a published source in the Sources section. Concentrations in mg/L; the train reads in treatment order. The corpus is Brazil-weighted, which the country column reflects honestly.

The pairing tells the whole story. The young Brazilian cell and the young Moroccan leachate respond to biology, though the Moroccan leachate is so strong (COD 38,600) that even a 73% reduction leaves 10,520 mg/L. The 14.4-year cell shows the age failure directly: the same operator's mature leachate carries 1,764 mg/L ammonia and a lagoon-plus-UASB train cannot bring it into compliance. The Greek activated-sludge plant sits exactly on the recalcitrant-COD floor at 1,583 mg/L and fails its 125 mg/L limit, while the Spanish reverse-osmosis plant crosses the floor to 65 mg/L and takes chloride from 8,460 to 127, which biology never could.

Nepti benchmarks your own leachate against this corpus and against its age. Model your leachate and see where it sits against the real-plant distribution in Nepti before you accept a bidder's performance guarantee.

Capital and operating cost, and where projects go wrong

Leachate treatment cost spans an enormous range, because a lagoon system on a young leachate and a membrane-plus-evaporation plant on a mature one are barely the same category of project. Lagoon and biological systems are low in capital and operating cost but hit the COD floor; membrane and thermal systems clear the floor and remove salinity at several times the operating cost per cubic metre and with a reject stream to manage. The corpus offers real project economics only sparsely, so the honest guidance is that the cost is set by which side of the recalcitrant-COD floor the consent forces the plant onto, and that decision is set by the leachate's age and the discharge limit, not by the tender.

The failure modes recur across the published plants:

Designing for young leachate on an ageing site. A landfill in West Java, Indonesia ran an equalisation-plus-lagoon plant that removed only 14% of its BOD and let COD rise across the works, because the ponds were sized for a design leachate the ageing, overloaded site had long outgrown. The lesson is that leachate strength and recalcitrance climb with age, and a plant frozen at the design-year assumption falls behind.

Expecting biology to cross the COD floor. The Greek activated-sludge plant met its ammonia target but failed its COD limit, because stabilised leachate carries a recalcitrant COD that no amount of biological retention removes. The lesson is that a mature-leachate consent below about 500 mg/L COD needs a membrane or an oxidation stage, specified up front, not a longer aeration time.

Reading rainfall as removal. Several pond systems in the corpus show an apparent concentration drop that conservative-tracer analysis proves is dilution by rain, not treatment. The lesson is to judge a leachate plant on mass removal and a tracer such as chloride, because a concentration-only result flatters a lagoon in a wet season and hides an exceedance in a dry one.

The CFO Hook

Landfill leachate is a decades-long liability, not a one-time project, and the cost of getting the treatment wrong compounds across the landfill's post-closure life. The published plants show what is achievable: ammonia from hundreds or thousands of mg/L down to single digits, BOD down by more than 90%, and, with membranes, COD from thousands to under 100. But they also show the trap: a biological or lagoon plant hits a recalcitrant-COD floor near 590 mg/L that a tightening consent will eventually breach, and a plant designed for a young landfill's biodegradable leachate fails on the same site's stabilised leachate a decade later. The defensible move is to design for the leachate the site will produce across its full life, treat ammonia as the binding parameter it usually is, and decide up front whether the consent forces the plant across the COD floor onto membranes or oxidation, because retrofitting that capability onto a failing lagoon under an enforcement notice is the most expensive way to arrive at it.

Sources

The plant benchmarks and case studies in this guide are drawn from Aguato's landfill leachate reference corpus of published treatment studies, which is documented-heavy but Brazil and lagoon weighted, as this guide states. Featured facilities:

• Municipal landfill age series (four cells), Brazil: UFMG repository (2012)
• Municipal landfill ponds, Parana (Cianorte), Brazil: State University of Londrina repository (2015)
• Municipal landfill, Mohammedia, Morocco: study (2020)
• Medium municipal landfill activated sludge, Greece: study (2023)
• Municipal landfill MBR and nanofiltration, Kahramanmaras, Turkey: Journal of Membrane Science Research (2021)
• Hazardous-waste landfill freeze crystallisation, South Africa: study (2025)
• Municipal landfill ponds, West Java, Indonesia: IOP Conference Series (2018)

The Catalonia reverse-osmosis plant (Spain) and the Efford nitrifying reed-bed plant (UK) are drawn from a peer-reviewed journal and a conference proceeding in the same corpus.

Related Articles

• Aerobic vs Anaerobic Wastewater Treatment: Which Is Right for Your Site?
• Advanced Oxidation Processes for Industrial Wastewater
• MBR vs Activated Sludge: Cost, Footprint, and Effluent
• Industrial Wastewater Treatment Process: A Step-by-Step Engineering Walkthrough
• Zero Liquid Discharge: When ZLD Makes Sense and When It Doesn't

FAQ

What makes landfill leachate so hard to treat?

Leachate combines very high ammonia (hundreds to thousands of mg/L), a high organic load that becomes recalcitrant as the landfill ages, high dissolved solids and chloride, and a strength that swings with rainfall. Ammonia is usually the binding parameter, and the recalcitrant organic fraction sets a COD floor that biological treatment alone cannot cross. The chemistry also changes over the decades-long life of the landfill, so a plant has to be designed for a moving target.

How does landfill age change the treatment?

Young leachate is acidic and biodegradable, with a BOD-to-COD ratio around 0.5 to 0.7, and biology removes its load efficiently. Mature or stabilised leachate has lost the biodegradable fraction to the landfill's own internal digestion, leaving a recalcitrant organic load (BOD-to-COD often below 0.1) plus rising ammonia. A plant designed for young leachate will fail on the same site's stabilised leachate years later, which is why the design has to anticipate the full life of the landfill.

Why is ammonia the key parameter in leachate?

Because it is present at very high concentrations (median around 720 mg/L in published cases, up to 4,700 in strong young leachates) and its discharge limit is tight. Ammonia is removed either biologically, by nitrification and denitrification, which needs enough carbon and warmth, or physically, by pH-raising ammonia stripping, which is the robust route for the coldest, oldest, and most concentrated leachates. A leachate plant that does not treat ammonia deliberately will breach its nitrogen limit even if its COD is under control.

Can biological treatment alone meet a leachate discharge consent?

Often not, on mature leachate. Biological and lagoon systems remove ammonia and biodegradable COD well but hit a recalcitrant-COD floor, published cases without a membrane do not drop below roughly 250 to 1,600 mg/L COD. Where the consent is below that floor, a membrane stage (nanofiltration or reverse osmosis) or an advanced-oxidation stage is required. Young leachate on a looser consent can sometimes be handled biologically.

Do membranes solve landfill leachate?

They cross the COD floor and remove the chloride and dissolved solids that biology leaves untouched, taking effluent COD from thousands to under 100 mg/L in published reverse-osmosis plants. The catch is the concentrated reject stream, which on a landfill usually goes back onto the tip or into an evaporation or crystallisation step. A membrane plant without a reject plan has relocated the problem rather than solved it.

Why is rainfall a problem when judging a leachate plant?

Because rain dilutes leachate, and an open lagoon can show a drop in pollutant concentration that is dilution, not treatment. Several published studies prove this using chloride and conductivity as conservative tracers. A leachate plant should therefore be assessed on mass removal and on a conservative tracer, not on a concentration reduction that a wet season can manufacture and a dry season can unmask as an exceedance.