What Is Leachate & How Do We Treat It?

Leachate is the liquid that percolates through waste in a landfill, picking up dissolved and suspended contaminants along the way. It is widely regarded as one of the most difficult wastewaters to treat: highly variable in composition, laden with ammonia and refractory organics, and capable of contaminating groundwater if released untreated. This article explains what leachate is, why its composition changes over time, and how engineers design treatment systems to handle it.

What Exactly Is Leachate?

Leachate forms when water-from rainfall, snowmelt, or the waste itself-infiltrates a landfill and comes into contact with decomposing solid waste. As the liquid moves through the waste mass, it dissolves organic compounds, salts, heavy metals, and other contaminants, emerging as a dark, malodorous, and toxic wastewater . The term also applies to liquid released from other waste storage or disposal facilities, but municipal solid waste landfills are the most common source.

The volume and quality of leachate depend on several factors: the amount of precipitation, the age and composition of the waste, the design of the landfill liner and cover system, and the stage of waste decomposition . A landfill that is still accepting fresh waste behaves very differently from one that has been closed for twenty years.

Why Leachate Is a Serious Environmental Problem

Untreated leachate poses a direct threat to groundwater and surface water. If it escapes through a liner failure or is discharged without adequate treatment, its contaminants can migrate into aquifers and water bodies . The environmental concern is not limited to acute toxicity. Leachate contains substances that are mutagenic, genotoxic, and capable of disrupting endocrine systems in wildlife .

Ammonia is a particular concern. Unlike many organic pollutants that degrade over time as a landfill stabilizes, ammonia concentrations remain elevated for decades and often become the dominant long-term pollutant in mature leachate . This is why leachate treatment cannot rely on biological degradation alone for complete treatment-ammonia must be addressed through dedicated processes.

The Composition of Leachate Changes with Landfill Age

Leachate composition is not static. As waste decomposes, the landfill passes through distinct phases, and the leachate chemistry shifts accordingly. Engineers classify leachate as young, intermediate, or mature based on landfill age, and treatment strategies must account for this evolution .

Young leachate, from landfills less than about five years old, is characterized by high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), a BOD/COD ratio typically above 0.3, and a pH near neutral . The organic matter is largely biodegradable, making biological treatment effective. Mature leachate, from landfills older than ten years, contains mostly refractory organics-humic and fulvic substances that resist biological breakdown-while ammonia remains high . The BOD/COD ratio drops well below 0.1, and biological treatment alone becomes inadequate.

The table below summarizes the practical differences.

Biological Treatment: The First Line of Defense

For young and intermediate leachate, biological treatment is the most cost-effective option. Aerobic processes-activated sludge, sequencing batch reactors, and moving bed biofilm reactors-can remove a substantial fraction of BOD and COD when the organic matter is biodegradable . Anaerobic digestion is also used, particularly where energy recovery from methane is feasible and where the leachate has high organic strength .

The limitation is clear: biological treatment works well on the biodegradable fraction but leaves behind ammonia and refractory organics. A moving bed hybrid bioreactor study reported COD removal of 77–80% and ammonia removal of 60–94% under optimized conditions, but the effluent still required further polishing to meet discharge standards . This is why modern leachate treatment almost always employs a treatment train rather than a single process.

Membrane and Advanced Treatment for Refractory Leachate

When biological treatment reaches its limits, membrane filtration and advanced oxidation take over. Reverse osmosis is highly effective at rejecting both organics and salts, achieving COD and total nitrogen removals above 98% in pilot-scale applications . Nanofiltration and microfiltration are used as polishing steps or as pretreatment for subsequent processes.

Advanced oxidation processes (AOPs)-including Fenton oxidation, ozonation, and electrochemical oxidation-are specifically deployed to break down recalcitrant organics that biological systems cannot touch. A study combining Fenton oxidation with submerged microfiltration achieved COD removal of 73.7% and UV254 removal of 85.6% from mature leachate, while also reducing ecotoxicity to compliant levels . These processes are more expensive than biological treatment, but they are often the only way to meet discharge limits for mature leachate.

The table below compares representative treatment performance from published studies.

Designing a Leachate Treatment Train

No single treatment method is sufficient for leachate . The engineering challenge is assembling a sequence of processes that matches the specific leachate’s characteristics and the required discharge standard. A typical train for mature leachate might begin with biological pretreatment to reduce biodegradable load, followed by AOP to break refractory compounds and improve biodegradability, then a membrane step for final polishing. For young leachate, a simpler biological system with ammonia removal may suffice.

Two practical considerations shape the design. First, leachate volume and quality fluctuate seasonally and over the landfill’s lifespan; treatment systems must be robust to shock loads and long-term compositional drift. Second, concentrate streams from membrane processes require management-they cannot simply be discharged and must be recirculated, evaporated, or treated further . A treatment train that solves the liquid problem by creating a concentrated waste problem is not a complete solution.

FAQ

Is leachate the same as wastewater?
No. Leachate is far more concentrated and variable than typical municipal wastewater, with much higher ammonia and refractory organic content. It requires dedicated treatment processes rather than discharge to a conventional sewage plant without pretreatment .

Can leachate be treated on-site?
Yes, and it usually should be. On-site treatment avoids the cost and risk of transporting hazardous liquid. Engineered landfills are required to have leachate collection and treatment systems in place before waste acceptance begins .

Why is ammonia so difficult to remove from leachate?
Ammonia persists long after biodegradable organics have decomposed. It does not degrade biologically under anaerobic conditions and can only be removed through nitrification-denitrification, air stripping, or advanced oxidation-each of which adds cost and operational complexity .

Facing a leachate treatment challenge at your landfill or industrial site? Our process engineers can help you evaluate treatment options matched to your leachate chemistry and discharge requirements-contact us to discuss your project.