Landfill leachate is the liquid formed when water filters through waste in a landfill. This runoff, sometimes called “landfill tea,” gathers dissolved and suspended materials from the refuse it passes through. The contaminated liquid poses a risk to groundwater and surrounding surface water ecosystems if not properly collected and managed. Treating leachate is necessary to prevent environmental pollution and protect public health.
Leachate Composition and Characteristics
The makeup of leachate is highly variable and depends on several factors. The type of waste, such as municipal solid waste or industrial byproducts, determines the available contaminants. Climatic conditions like rainfall influence the volume and pollutant concentration of the leachate. These variables mean that no two landfills produce identical leachate, requiring a tailored treatment approach.
A primary determinant of leachate’s characteristics is the landfill’s age. Leachate from young landfills (less than five years old) has high concentrations of readily biodegradable organic matter, like volatile fatty acids. As a landfill matures into the methanogenic phase (after about 10 years), the composition shifts. Older leachate contains more refractory substances like humic and fulvic acids, which are harder to break down biologically.
Leachate also contains a mixture of other pollutants. Inorganic macro-components are present in high levels, including ammonia, chloride, sodium, potassium, and calcium. Heavy metals like lead, cadmium, chromium, and mercury are also present, originating from items like batteries and electronics. A range of xenobiotic organic compounds, which are synthetic chemicals like pesticides and plastics components, add to the liquid’s toxicity.
Biological Treatment Processes
Biological treatment uses microorganisms to decompose organic contaminants in leachate. These processes are most effective for young leachate with easily biodegradable pollutants. The two primary categories are aerobic, which requires oxygen, and anaerobic, which functions without it. Aerobic processes break down organic matter into carbon dioxide and water and convert ammonia into nitrate.
The activated sludge process is a widely used aerobic method. Leachate is mixed with microorganisms in an aeration tank where air is continuously supplied, allowing the microbes to consume organic pollutants. The mixture then flows to a clarifier, where the solid microbial mass (sludge) settles. A portion of this sludge is recycled back to the aeration tank to maintain the microbial population.
Another aerobic system is the sequencing batch reactor (SBR), which completes all treatment steps in a single tank operating in a timed sequence. The steps include fill, react, settle, decant, and idle. This design offers flexibility and control, making it effective for removing organic content and nitrogen.
Anaerobic treatment is suitable for high-strength organic loads, using processes like the upflow anaerobic sludge blanket (UASB) reactor. In a UASB reactor, leachate flows upward through a dense bed of granular sludge. Microbes digest the organic matter to produce biogas, a mixture of methane and carbon dioxide.
Biological methods can achieve high removal rates for organic pollutants, measured as chemical oxygen demand (COD), with efficiencies exceeding 90%. Their effectiveness is limited by toxic or non-biodegradable compounds in mature leachate. For this reason, biological treatment is often the first step in a larger treatment strategy.
Physical and Chemical Treatment Processes
Physical and chemical processes are used when biological methods are insufficient, particularly for mature leachate with low biodegradability. These technologies physically separate contaminants or use chemical reactions to transform them into less harmful substances. They are used as a “polishing” step after biological treatment to meet discharge regulations.
Phase transfer methods move pollutants from a liquid to a gaseous or solid state. Air stripping, for example, removes volatile compounds like ammonia by passing air through the leachate. The dissolved ammonia volatilizes into the air, which is then collected and treated.
Adsorption is another phase transfer technique. In this process, leachate is passed through a medium like activated carbon, which traps organic molecules on its surface.
Membrane filtration provides a physical barrier to contaminants. Technologies like reverse osmosis (RO) and nanofiltration (NF) use high pressure to force water through a semi-permeable membrane. This barrier blocks particles, dissolved salts, heavy metals, and organic molecules, with RO being capable of producing a high-quality effluent.
Chemical reactions are also used to treat leachate. In chemical precipitation, added chemicals react with dissolved metals to form insoluble solid particles that can be filtered out. Advanced oxidation processes (AOPs) use powerful oxidizing agents like ozone or hydrogen peroxide to destroy persistent organic pollutants.
Combined Treatment Approaches
A single treatment method is rarely sufficient to address the contaminants in landfill leachate. To meet discharge standards, facilities employ a combination of processes in a “treatment train.” This approach sequences different technologies to target specific pollutants at each stage for a more effective system.
A common strategy uses biological treatment as the initial step to cost-effectively remove the bulk of the organic load and ammonia. This pre-treatment makes subsequent physical or chemical processes more effective by lowering the contaminant concentration they must handle.
Following the biological stage, a physical or chemical process is used for “polishing.” For instance, effluent from a biological reactor might be sent to a reverse osmosis system. The RO membrane removes the remaining refractory organics, inorganic salts, and heavy metals, ensuring a high-quality final effluent.
The configuration of a treatment train is designed based on the leachate’s characteristics and purification requirements. For example, a system might use chemical precipitation to remove heavy metals before biological treatment to prevent microbial toxicity. This could be followed by activated carbon adsorption to remove any residual organic compounds.
Management of Treated Effluent and Residuals
Leachate treatment results in two primary outputs: the cleaned liquid (effluent) and concentrated waste byproducts (residuals or sludge). Managing both streams is the final part of the process, aiming to minimize environmental impact.
Treated effluent that meets regulatory standards has several outlets. It can be discharged into a municipal sanitary sewer for further treatment or, if high quality, directly into surface waters like rivers. The water can also be reused on-site at the landfill for dust control or irrigation.
Residuals, like biological sludge and reverse osmosis brine, contain the removed pollutants in a concentrated form. This hazardous concentrate requires careful disposal. The most practical solution is to re-deposit the sludge into a lined cell within the landfill. Liquid concentrate may be treated through evaporation to reduce its volume before disposal.