Landfill leachate is the liquid that drains out of a solid waste mass after rain, infiltration, and the natural compaction of decomposing refuse. It is dark, heavily odorous, and carries one of the most difficult contaminant loads found in any industrial wastewater. Plants that handle it usually find that biological treatment removes the easy fraction and then stalls, leaving residual colour, a stubborn COD tail, and ammonia nitrogen that will not fall to the discharge limit. Ozone is increasingly deployed at exactly that point in the train.

Why Leachate Resists Conventional Treatment

Fresh leachate from a young landfill is high in volatile fatty acids, so its BOD to COD ratio is favourable and biology performs reasonably well. As a site ages the character shifts: readily degradable organics are consumed inside the waste mass, and what leaves the site is dominated by humic and fulvic-like substances and complex aromatic compounds. These molecules are large, recalcitrant, and intensely coloured. The BOD to COD ratio drops, kinetics slow down, and the plant ends up with effluent that is biologically stable but still fails on colour and residual COD.

Two complications make leachate unusual. Ammonia nitrogen is often present far above what municipal biological systems are designed for, and it competes with organic substrate for oxygen and alkalinity. Leachate composition also moves with the season: a dry month concentrates the liquid, a wet month dilutes it and adds flow. Any treatment step chosen for leachate must tolerate that swing, not just a single design sample.

What Ozone Does in Leachate

Ozone attacks leachate through two routes at the same time. Molecular ozone reacts selectively with electron-rich structures such as aromatic rings and double bonds. In parallel, ozone decomposes in water to form hydroxyl radicals, which are far less selective and react with almost any organic molecule. At neutral to alkaline pH, and especially when hydrogen peroxide or a catalyst is added, the radical pathway dominates and the process becomes an advanced oxidation step.

Four Effects That Matter to an Operator

• Colour removal. Chromophoric groups in humic substances are cleaved quickly. Colour fall is usually the first visible result and often the most dramatic one.
• COD reduction. Ozone does not mineralise everything it touches. It converts large recalcitrant molecules into smaller, more biodegradable fragments, so part of the COD is oxidised directly and another part becomes removable downstream.
• Ammonia nitrogen. Ozone oxidises ammonia to nitrate slowly at low pH and much faster above pH 8.5. Biological nitrification remains the economical route for bulk ammonia removal, with ozone used for polishing.
• Odour and disinfection. Ozone oxidises the reduced sulfur compounds responsible for the landfill smell and inactivates bacteria, which matters for plants near housing.

Where Ozone Fits in the Treatment Train

Ozone is rarely the whole answer. Its value comes from being placed where the other unit processes have already done the work they are good at.

Table 1. Typical position of an ozone stage in a leachate treatment train

Pairing ozone with a downstream biological step is the most useful configuration in leachate work. Ozonation judged immediately after the contact tank can look disappointing on COD; when the ozonated water then passes through an aerated biological stage, overall COD and colour removal is substantially better than either stage achieves alone.

Sizing: Dose, Contact Time, and pH

Required ozone demand depends on the residual COD and aromatic content of a particular liquid, so it cannot be read off a generic table. What can be given is a design envelope plus a testing method to narrow it.

Table 2. Indicative design envelope for an ozone stage on landfill leachate

Because the envelope is wide, treatability testing is not optional. A bench-scale run on a genuine sample, measuring COD, colour, and ammonia against applied dose, produces a dose-response curve a quotation can be built on. A supplier who quotes without asking for a sample is quoting a guess.

Oxygen-Fed Generation, and What Drives Operating Cost

Leachate duty is continuous, the dose is high, and the treated water often goes to a sensitive receptor. That combination points to oxygen-fed generation. Oxygen feed produces two to three times the ozone concentration of dried air, raising the driving force for dissolution, cutting the off-gas volume that must be destroyed, and lowering specific energy per kilogram of ozone. The trade-off is an oxygen supply: PSA generation on site, or liquid oxygen delivered to a tank. Above roughly one kilogram of ozone per hour, on-site PSA is normally the cheaper route over the equipment life.

Cost per cubic metre is dominated by three items:

• Electrical energy. Industrial corona discharge systems consume roughly 6 to 10 kWh per kilogram of ozone, and oxygen-fed units sit at the favourable end of that band. Energy is usually more than half of the oxidant cost.
• Oxygen supply. Either PSA power draw or delivered liquid oxygen, both proportional to ozone output and to how much ozone escapes in the off-gas.
• Reagent and maintenance. Hydrogen peroxide where AOP is used, plus dielectric and seal replacement on a planned interval rather than a reactive one.

Two operating levers cut this cost without new equipment. Run the ozone stage when the biological effluent actually needs it rather than at a fixed dose around the clock, and keep the injector and contactor clean, because transfer efficiency decays quietly as scale and biofilm build up inside the column.

Design Details That Decide Reliability

• Off-gas destruction. Leachate contactors vent ozone and volatile organics. A thermal or catalytic destruct unit is part of the safety envelope, not an accessory.
• Foaming. Leachate surfactants foam heavily in a bubble column. Contactors need foam control, freeboard, and a defined foam return route to the head of the plant.
• Cooling water. The dielectric cooling circuit is a separate loop and should run on soft or demineralised water; leachate-derived water will scale the tubes.
• Turndown. Load swings seasonally, so the generator should modulate to roughly 20 to 30 percent of rated output instead of cycling on and off.
• Instrumentation. Dissolved ozone or ORP at the contactor outlet plus a generator output signal are the minimum feedback needed to run to demand rather than to a timer.

Five Questions When Comparing Proposals

• Has the supplier asked for a leachate sample and produced a dose-response curve, or only a flow rate?
• Is quoted ozone capacity stated at a defined concentration, pressure, and cooling water temperature?
• Is transfer efficiency specified, and is the contactor type named?
• Is off-gas destruction in scope, with the destruct technology identified?
• Are wetted materials listed against the chloride level of the actual leachate?

Ozone will not turn leachate into clean water on its own, and no honest supplier will say it will. Placed correctly behind a biological stage and followed by a polishing step, it removes the colour and refractory COD tail that nothing else in the train reaches economically. Getting that placement right is worth more than any single equipment upgrade.