Landfill Leachate Treatment: Composition, Ammonia and Treatment Trains

What is landfill leachate and why is it so hard to treat?

Leachate is the liquid generated when rainfall percolates through a waste mass and dissolves soluble and suspended contaminants. It is one of the most concentrated and variable effluents an engineer meets: chemical oxygen demand (COD) can range from a few hundred to tens of thousands of mg/L, ammoniacal nitrogen from 200 to well over 3,000 mg/L, plus chloride, heavy metals, xenobiotic organics and colour.

Three features make it difficult. First, the very high ammoniacal-N load, which does not attenuate with landfill age and dominates the nitrogen duty. Second, a large fraction of recalcitrant COD — humic and fulvic substances that resist biological attack. Third, high salinity (chloride, conductivity) that inhibits biology and drives operators toward membranes. A robust design has to address all three, not just bulk BOD.

How does leachate composition change as a landfill ages?

Leachate chemistry tracks the microbial phases inside the cell. In the early acetogenic (young) phase, fermentation produces volatile fatty acids, so the liquor is acidic (pH 5–6.5), rich in biodegradable BOD, and has a high BOD:COD ratio (often 0.5–0.8). As the cell matures into the methanogenic (old) phase, VFAs are consumed to methane, pH rises above 7.5, and the residual COD is dominated by recalcitrant humics. The BOD:COD ratio falls below 0.1, while ammoniacal-N stays high or rises.

The practical consequence: a train designed for a fresh cell will be mis-matched a decade later. Young leachate rewards biological treatment; old leachate needs membranes or oxidation to reach the same discharge quality, because the remaining COD is intrinsically non-biodegradable.

Mechanistically, the shift is a change in the electron-donor pool available to microorganisms. Acetogenic leachate offers short-chain fatty acids — high-energy, readily assimilated substrates that support fast heterotrophic growth (high observed biomass yield). Methanogenic leachate has already exported that energy as CH₄, leaving condensed aromatic humics whose C–C and C–O bonds sit outside the substrate range of common heterotrophs. No amount of extra aeration or retention time will mineralise them at a useful rate, which is the fundamental reason polishing must switch from biological to physical or oxidative as the cell ages.

Young vs old leachate: a comparison

The table contrasts the two end-members and the treatment that fits each. Real sites usually sit somewhere on this continuum, and a single landfill migrates from left to right over its life.

Note that the ammonia load, the one contaminant that biology alone struggles to eliminate without a dedicated nitrogen route, remains high across the whole life cycle. That is why nitrogen removal, not COD removal, usually sets the reactor size.

How do you remove ammonia from leachate?

Ammoniacal nitrogen is the defining challenge. Three routes are used. Conventional nitrification–denitrification oxidises ammonia to nitrate (autotrophic nitrifiers) then reduces nitrate to nitrogen gas (heterotrophic denitrifiers, using leachate carbon or a dosed source such as methanol). Partial nitritation–anammox (deammonification) shortcuts the pathway: about half the ammonia is oxidised to nitrite, then anammox bacteria combine ammonium and nitrite directly to N₂, cutting aeration by roughly 60% and eliminating the external carbon demand — well suited to old, carbon-poor leachate. Physical ammonia stripping (raising pH above 10.5 with lime or caustic and air-stripping free NH₃) is used as pre-treatment on very strong liquors.

The kinetics also constrain the design. Autotrophic nitrifiers grow slowly, with a maximum specific growth rate around 0.5–0.8 d^-1 at 20 °C and a strong temperature dependence (roughly halving for every 8–10 °C drop), so the aerobic solids retention time must be held well above the nitrifier washout point — commonly 10–20 days on cold leachate. Free ammonia and free nitrous acid are themselves inhibitory at the concentrations leachate reaches, which is precisely the window a nitritation route exploits to stall oxidation at nitrite and starve the nitrate-oxidisers.

Site-specific selection between these routes is a core part of our leachate treatment engineering, because the choice cascades into aeration power, chemical dosing and sludge production. For the biological nitrogen stage itself, see our note on biological nutrient removal.

Worked example: oxygen and alkalinity demand for nitrification

Take a methanogenic leachate treated at 120 m³/day carrying 1,800 mg/L ammoniacal-N, with a target effluent of 50 mg/L, i.e. 1,750 mg/L to be nitrified. Work the nitrogen load and its consequences.

• Nitrogen load: 120 m³/d × 1,750 g/m³ = 210,000 g/d = 210 kg N/day to nitrify.
• Oxygen demand: 210 kg N/d × 4.57 kg O₂/kg N = 960 kg O₂/day (≈ 40 kg O₂/h), before any carbonaceous BOD demand.
• Alkalinity destroyed: 210 kg N/d × 7.14 kg CaCO₃/kg N = 1,500 kg/day as CaCO₃. Leachate alkalinity rarely covers this, so pH would crash without recovery.
• Alkalinity recovered by denitrification: at 3.57 kg CaCO₃/kg N, denitrifying the full load returns ≈ 750 kg/day — roughly half — sharply cutting caustic dosing.

The result: a nitrification–denitrification loop on this leachate must supply about 960 kg O₂/day and make up roughly 750 kg/day of residual alkalinity (as CaCO₃). This is why deammonification, which needs far less oxygen and no added carbon, becomes attractive as loads climb.

What treatment train handles recalcitrant COD and salinity?

Once ammonia is dealt with biologically, the residual recalcitrant COD, colour and dissolved salts still block discharge. Two polishing philosophies dominate. A membrane bioreactor (MBR) followed by reverse osmosis uses biology for nitrogen and biodegradable COD, then RO as an absolute barrier to salinity, humics and metals. Alternatively, an advanced oxidation process (AOP) — ozone, ozone/H₂O₂, UV/peroxide or Fenton — chemically cleaves the humic COD into biodegradable fragments, either as a standalone polish or to raise biodegradability ahead of a further biological pass.

RO is often the deciding barrier for consented discharge; its selection and staging are covered in our guide to reverse osmosis system design. Where oxidation is the better fit for breaking down recalcitrant molecules, see advanced oxidation processes. A well-configured membrane stage, using the right membrane configuration, is frequently the difference between meeting and missing a chloride or COD consent.

Why is RO concentrate the real problem — a worked recovery example?

RO does not destroy contaminants; it concentrates them into a reject stream that must go somewhere. Managing that concentrate is the single hardest economic and environmental problem in leachate treatment. The concentrate volume follows directly from the recovery.

For the 120 m³/day plant above at r = 0.75: permeate = 90 m³/day, concentrate = 120 × (1 − 0.75) = 30 m³/day, and the dissolved solids are concentrated by CF = 1/0.25 = 4×. That 30 m³/day of 4×-strength brine is now the disposal duty — commonly returned to the landfill (recirculation), evaporated, or driven toward zero liquid discharge. Every point of extra recovery shrinks this stream but raises osmotic pressure and scaling risk, so the recovery target is really a concentrate-management decision in disguise.

How do you select and stage the overall train?

Sequencing follows the leachate's position on the age continuum and the discharge route (sewer under a trade-effluent consent, or direct to controlled waters). A defensible logic:

• Characterise first: COD, BOD, BOD:COD, ammoniacal-N, chloride, conductivity and metals across seasons — leachate is highly variable.
• Young leachate: lead with robust biology (SBR or MBR) sized on nitrogen load; biodegradable COD follows for free.
• Old leachate: nitrify/deammonify for ammonia, then polish recalcitrant COD and salinity with RO and/or AOP.
• Plan the concentrate route before fixing RO recovery — it usually governs whole-life cost.
• Buffer flow and load with adequate balancing, because rainfall events swing both hydraulic and contaminant loads.

Because discharge quality is legally binding, tie every design figure to a consent limit and confirm it with pilot data on the actual leachate before procurement.