Landfill Leachate at 12,000 mg/L COD: Where Heterogeneous Ozone Catalysis Fits

The stubborn disease of wastewater

Landfill leachate, and the leachate from incineration plants, is the high-strength organic stream produced as waste decomposes. Waste generation keeps rising, and leachate output in China has approached 80 million tonnes a year. Its composition has become more complex along the way, with more than 70 pollutants identified, including aromatic hydrocarbons, halogenated compounds, heavy metals and various cancer promoters and co-carcinogens.

The indicator values explain why conventional trains struggle. COD can reach tens of thousands of mg/L and ammonia nitrogen exceeds 1,000 mg/L, hundreds of times municipal sewage levels. As landfills age, biodegradability falls: the BOD5/COD ratio plunges below 0.1 and refractory organics exceed 60% of the total. A biological process cannot degrade what it cannot take up, and by the time a site is ten years old the biological stage is mostly idle.

Why the three mainstream options all leak

Membrane treatment, usually nanofiltration or reverse osmosis, gives stable effluent in the short term. It also fouls severely, needs frequent element replacement, and produces 15-30% hazardous concentrate that is difficult to treat again, which converts a water problem into a disposal problem.

Fenton oxidation relies on large chemical doses and generates iron sludge classified as hazardous waste. The reaction needs harsh conditions, and the effluent readily regains colour once the residual peroxide is gone.

Biological treatment, MBR included, is limited by biodegradability. Effluent COD often stays above 1,000 mg/L, which is not a discharge limit any modern standard will accept.

What all three share is that they move the pollutant rather than destroy it, or they destroy it at the cost of a secondary waste stream. Deep oxidation that combines efficient degradation, a low carbon footprint and stable operation is the direction the industry has moved toward.

How the heterogeneous catalyst changes the reaction

SINOKLE developed a heterogeneous ozone catalytic oxidation technology that addresses the low efficiency and poor selectivity of plain ozonation through catalyst-ozone synergy, aiming at precise deep mineralisation rather than partial conversion. Transition metal ions such as Cu, Fe and Mn are fixed onto a dedicated solid support by atomic deposition modification, forming a composite catalyst that is both highly active and highly stable, and avoiding the metal-ion leaching and heavy sludge that homogeneous catalysis brings.

In operation the catalyst activates ozone to generate large quantities of hydroxyl radicals with a redox potential as high as 2.8 V. That is high enough for non-selective attack on refractory organics, breaking complex structures down to carbon dioxide and water rather than to smaller but still refractory fragments. The catalyst surface, with a specific surface area above 250 m2/g, also adsorbs pollutants and holds them in contact with ozone, which raises the reaction rate 2-5 times over ozonation alone and shortens retention time to 5-15 minutes, only 1/8 to 1/4 of conventional oxidation.

Four processes side by side

COD removal reaches 85% or more on mature leachate, against 40%~60% for conventional ozonation, 60%~75% for Fenton, and 90% or more for membranes, with the caveat that the membrane figure comes with a concentrate stream attached.

Retention time tells a similar story: 5-15 min for the catalytic route, against 60-120 min for ozonation, 30-60 min for Fenton and 20-40 min for membrane treatment. Time is vessel volume, and vessel volume is capital cost.

Chemical consumption is ozone only, with no other reagents. Ozonation alone needs a high ozone dose; Fenton needs very large quantities of ferrous salt and hydrogen peroxide; membranes need regular scale inhibitor and biocide dosing.

Secondary pollution separates the options most clearly. The catalytic route produces zero hazardous waste and zero concentrate. Fenton generates 10-15% hazardous iron sludge. Membranes generate 15-30% hazardous concentrate. Ozonation produces no obvious secondary pollution but is inefficient, which is its own kind of cost.

Operating cost per tonne runs CNY 30-50/t against CNY 60-80/t for ozonation, CNY 50-70/t for Fenton and CNY 100-150/t for membrane treatment. Catalyst service life is 5 years or more, against heavy equipment wear for ozonation, severe corrosion for Fenton, and a membrane element life of 1-3 years.

Three practical details

The process runs stably across pH 3-11 without complex acid or alkali adjustment, which removes an entire dosing and control loop. Intelligent O&M keeps the whole train pressurised and enclosed, with an AI monitoring system adjusting ozone dosing in real time and cutting manual intervention by 70%; that is also what makes the unit tolerable on a site with a thin operations team. The skid-mounted integrated design occupies one-third of the area of conventional processes and allows rapid installation and start-up.

Two projects that have run long enough to count

At an ageing landfill in Hubei, influent COD is 8,500-12,000 mg/L, ammonia nitrogen 1,200-1,500 mg/L and biodegradability only 0.08. With the SINOKLE heterogeneous ozone catalytic oxidation unit as the tertiary step, effluent COD falls steadily below 100 mg/L, ammonia nitrogen removal reaches 98% and colour is 30 or below, colourless and odourless, with every indicator better than the Class I limits of GB 16889-2008. Operating cost is CNY 42/t, 58% lower than the membrane process it replaced, and the hazardous concentrate disposal burden is gone entirely.

At a waste incineration plant in Guangdong, the leachate carries high concentrations of refractory organics and heavy metals, and effluent COD after a conventional Fenton plus MBR train still exceeded 500 mg/L. After the catalytic oxidation stage was introduced, final effluent COD settled below 80 mg/L, suspended solids removal reached 99% or more and heavy metals met limits. The system ran 18 months without failure, catalyst activity stayed stable, and no secondary pollution was produced.

Where it belongs in a train

This is a destruction step, not a concentration step, so it replaces the membrane stage that would otherwise generate concentrate, or it sits after biology to polish what biology cannot reach. It does not remove ammonia by stripping and it does not soften water; where nitrogen limits are tight, a dedicated biological or stripping stage is still needed. Used in that position, it converts the leachate problem from a waste-disposal problem back into a water-treatment problem, which is the only version of the solution that scales.