Landfill leachate accumulation liquid refers to the highly polluting liquid that can not be treated in time due to the damage of landfill seepage control system, overload of treatment facilities, extreme weather (such as rainstorm), etc. Its composition is complex, containing high concentrations of COD (up to tens of thousands of mg/L), ammonia nitrogen (thousands of mg/L), heavy metals, recalcitrant organic matter (such as humic acid), and pathogens. If leaked, it will seriously pollute soil, groundwater, and surface water. Therefore, emergency response needs to focus on "rapid pollution control, risk reduction, and temporary compliance", while balancing timeliness and safety.
Water quality characteristics and treatment difficulties of leachate from garbage
Core features:
The concentration of organic matter is high (COD can reach thousands to tens of thousands of mg/L), and it contains a large amount of difficult to degrade substances such as humus;
High ammonia nitrogen concentration (often up to 1000-3000mg/L) inhibits microbial activity;
Large fluctuations in water quality and quantity (affected by rainfall, garbage composition, and landfill years);
High salt content (especially in old landfill leachate) can easily lead to microbial poisoning.
Difficulties in processing: high concentration, strong toxicity, significant differences in biodegradability (young filtrate has better biodegradability, old filtrate has poor biodegradability), and high processing costs.
Core treatment process of leachate from garbage
The processing flow requires targeted removal of pollutants in stages, with a typical process of "pretreatment → biological treatment → deep treatment". The specific techniques are as follows:
(1) Pre treatment: Reduce load, stabilize water quality
The core of pretreatment is to remove suspended impurities, regulate water quality and quantity, and create conditions for subsequent treatment.
Grille and screening: By using mechanical grids (to remove particles with a diameter>10mm) and fine screens (to remove impurities of 2-5mm), the risk of subsequent equipment blockage is reduced.
Regulating tank: Balance the amount and quality of leachate (such as pH and pollutant concentration) to avoid subsequent process failures due to impact loads; Some regulating tanks will be equipped with mixing devices (mechanical or aeration) to prevent impurities from settling.
Precipitation and coagulation: For leachate with high suspended solids content, natural precipitation can be carried out through sedimentation tanks, or coagulants (such as PAC, PAM) can be added to form flocs, removing colloids and some COD (removal rate of about 10% -30%).
Ammonia nitrogen pretreatment: If the concentration of ammonia nitrogen is too high (>1500mg/L), it is necessary to first use the blow off method (adjust the pH to 10-11, and introduce air to remove ammonia nitrogen) or the inflection point chlorination method (add chlorine gas to oxidize ammonia nitrogen to nitrogen gas) to reduce the concentration and avoid inhibiting microorganisms in biological treatment.
(2) Biological treatment: degradation of organic pollutants and ammonia nitrogen
Biological treatment is the use of microbial metabolism to decompose organic matter and ammonia nitrogen in leachate, which is the core process of treatment. The process should be selected based on the biodegradability of the leachate.
Anaerobic biological treatment: Suitable for young leachate with high concentration of organic matter (COD>5000mg/L), the organic matter is decomposed into methane and carbon dioxide by anaerobic microorganisms (such as methanogens), and the COD removal rate can reach 60% -80%, while reducing the subsequent aerobic treatment load. Common processes:
Upflow anaerobic sludge blanket (UASB): high efficiency and small footprint;
Anaerobic membrane bioreactor (AnMBR): Combining anaerobic and membrane separation, the effluent quality is more stable.
Aerobic biological treatment: Suitable for low to medium concentration organic matter (COD<5000mg/L) or anaerobic treated leachate, organic matter is oxidized to CO ₂ and H ₂ O by aerobic microorganisms (such as bacteria and activated sludge), while achieving ammonia nitrogen nitrification (conversion to nitrate). Common processes:
Activated sludge process: a traditional process with low cost but large sludge volume;
Sequential Batch Reactor (SBR): Flexible adjustment of operating cycle to adapt to water quality fluctuations;
Membrane bioreactor (MBR): The combination of activated sludge and membrane separation results in good sludge water separation efficiency, with COD and ammonia nitrogen removal rates of over 90%, but the membrane is prone to fouling.
Combination process: Due to the complexity of the leachate, a single process is difficult to meet the standard, and a combination of "anaerobic+aerobic" is required, such as:
UASB+A/O (anaerobic/aerobic): Anaerobic COD reduction, A/O denitrification (anaerobic denitrification, aerobic nitrification);
AnMBR+MBR: Suitable for high concentration, difficult to degrade leachate, resulting in better effluent quality.
(3) Deep processing: Ensure compliance with emission standards or reuse
After biological treatment, there may still be a small amount of recalcitrant substances (such as humic acid, heavy metals), salts, etc. in the effluent, which need to be deeply treated to meet the requirements of the "Pollution Control Standards for Landfills of Municipal Solid Waste" (GB 16889-2008) or reuse.
Membrane separation technology: Selective interception and removal of pollutants through membranes is a mainstream advanced
Treatment Process
Ultrafiltration (UF): removes colloids and large organic molecules as a pretreatment for subsequent nanofiltration/reverse osmosis;
Nanofiltration (NF): intercepts small molecule organic matter and divalent salts, with a desalination rate of 70% -80%;
Reverse osmosis (RO): intercepts almost all pollutants (including monovalent salts), and the effluent COD can be reduced to below 10mg/L. It is suitable for reuse (such as greening and road cleaning), but it will produce high salt concentration liquid (requiring further treatment).
Advanced Oxidation technology: By generating hydroxyl radicals (· OH) to oxidize recalcitrant substances, suitable for treating membrane concentrate or biologically recalcitrant leachate:
Fenton oxidation (Fe ² ⁺+H ₂ O ₂): Low cost, can remove some COD and chromaticity;
Ozone oxidation: Strong oxidation ability, no secondary pollution, but high energy consumption;
Photocatalytic oxidation: Combining ultraviolet light with catalysts (such as TiO ₂), suitable for low concentration and difficult to degrade substances.
Adsorption method: using activated carbon, zeolite and other adsorbents to adsorb residual organic matter and heavy metals, commonly used in deep treatment of the end, to compensate for the shortcomings of other processes, but the adsorbent needs to be regularly regenerated or replaced.
Evaporation/Crystallization Technology: For high salt concentration solutions (such as RO concentration solutions), "zero emissions" can be achieved by evaporating water and crystallizing to remove salt. However, the energy consumption is extremely high, making it suitable for low water volume scenarios.
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