Leachate

Leachate is the polluting liquid produced when water, from rainfall, the moisture of the waste itself or internal decomposition processes, percolates through solid waste and carries away dissolved or suspended substances.

In waste management the term is associated mainly with landfills, although it can also occur in composting piles, industrial facilities, mining operations and agricultural activities. Leachate is one of the main environmental risks in waste management because it can contaminate soil and surface or groundwater, as well as causing odours and hazardous gas emissions.

Legal definition

Law 7/2022 on waste and contaminated soil for a circular economy requires landfill operators in Spain to install systems to collect and treat leachate so as to prevent environmental impacts. At European level, the Landfill Directive (1999/31/EC) sets mandatory technical requirements for lining, capture and treatment of leachate.

Characteristics of leachate

• High organic load: decomposing matter, with elevated COD and BOD.
• Heavy metals: such as cadmium, lead, mercury and zinc.
• Nitrogen compounds: ammonium, nitrates and nitrites.
• Salts and chlorides from inorganic waste.
• Emerging contaminants: microplastics and pharmaceutical compounds.

How leachate forms in landfills

The volume and composition of leachate depend on:

1. The amount of water entering the landfill (rain, snow and waste moisture).
2. The type of waste deposited (organic, industrial or hazardous).
3. The age of the landfill: young leachate has a higher organic load, while older leachate contains more salts and metals.
4. Local climatic and geological conditions.

Environmental risks

• Groundwater contamination through infiltration.
• Pollution of rivers and lakes if leachate reaches them untreated.
• Soil degradation from the build-up of salts and metals.
• Gas emissions from the volatilisation of compounds.
• Health risks from direct or indirect exposure.

Leachate treatment

Physico-chemical treatment

• Metal precipitation, pH neutralisation, and coagulation and flocculation.

Biological treatment

• Activated sludge to degrade organic matter.
• Sequencing batch reactors (SBR) and membrane bioreactors (MBR).

Advanced treatment

• Reverse osmosis, nanofiltration, advanced oxidation (ozone, hydrogen peroxide) and activated carbon.

Controlled recirculation

• In bioreactor landfills, leachate is recirculated to accelerate the degradation of the waste.

Prevention strategies

1. Reduce the amount of biodegradable waste sent to landfill, favouring composting and anaerobic digestion.
2. Improve lining in older landfills.
3. Increase leachate capture with redundant collection systems.
4. Support research into more energy-efficient advanced treatments.

Relationship with the circular economy

Although leachate is an unwanted by-product of waste management, controlling it is essential for landfills to remain a safe disposal option within the waste hierarchy. The circular economy aims to minimise leachate generation by limiting landfilling and promoting prevention, recycling and material and energy recovery.

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