From 1,300 mg/L to Surface-Water Class III: A CDOF Leachate Project Broken Down

Landfill leachate has a habit of humiliating treatment trains: high COD, deep colour, and an organic load that shrugs at conventional biology. A project at a waste-to-energy plant in Shiyan, Hubei shows what advanced catalytic ozonation does when it is engineered properly — and the numbers are worth reading closely.

The Chemistry That Makes It Possible

Advanced catalytic ozonation uses ozone and the reactive radicals formed as it decomposes to oxidise and break down organic pollutants. The hydroxyl radicals produced carry a redox potential of 2.8 V — against 2.07 V for the ozone molecule itself — second only to fluorine, and they act on impurities in every physical form: dissolved, emulsified, suspended, colloidal and microbial. Complex organics convert into low-molecular-weight organics, acids and aldehydes, while turbidity, colour and odour improve markedly alongside efficient disinfection. Early direct ozonation was slow, pronouncedly selective, wasteful of ozone and costly; advanced catalytic ozonation fixes utilisation, drops selectivity, avoids secondary pollution and has consequently spread through landfill leachate, petrochemical and pharmaceutical wastewater.

The Family of Advanced Oxidation Routes

The technology family includes O3/H2O2 and other advanced catalytic oxidation combinations: O3/UV, O3/ultrasonic, O3/activated carbon and O3/metal catalyst advanced oxidation. Each generates hydroxyl radicals by a different mechanism; the engineering choice comes down to wastewater character and cost per unit of COD removed.

Inside the CDOF Process

Building on existing advanced ozone oxidation technology, SINOKLE developed the CDOF process — a circulating dissolved-ozone flotation system combining advanced catalytic ozonation, cyclonic technology and dissolved-air flotation, driven by independently developed supercritical catalytic materials and high-efficiency heterogeneous catalysts to achieve cavitation-driven supercritical catalytic reaction, all inside a fully enclosed, automatically pressurised reactor.

• First to integrate multiple catalytic ozonation, cyclonic dissolved-air flotation and coagulation in one technology, removing COD, suspended solids, colloids, bacteria and viruses together.
• Multi-stage, multi-dimensional, multiple catalytic ozonation reactions: pressurised catalytic reaction, homogeneous and heterogeneous catalytic oxidation, hydrodynamic cavitation catalysis, low-pressure catalytic oxidation and cyclonic fluidised reaction, with stable and reliable results.
• A patented reactor design with residence time under 15 min against 90 min conventionally — sealed, pressurised, no ozone leakage, safe and environmentally sound.
• Ozone utilisation above 99.98%, dosing ratio 0.5–1.2 versus 2–4 conventionally — one fifth to one half of the conventional figure — and operating costs at 1/3–1/2 of a conventional process.
• DCS/PLC automatic control precisely regulates ozone dose, reaction level, pressure and tail-gas concentration, self-optimising against water quality to hold operating cost down.
• A dedicated catalyst with strong activity, high strength, large specific surface area and strong fouling resistance, effective over a wide pH range and over the long term, kept stable by a unique backwash, activation and regeneration process.

The Shiyan Case, Number by Number

Why Sealed and Pressurised Matters

Conventional contact basins vent unreacted ozone to atmosphere — wasted oxidant and a tail-gas treatment problem in one. The CDOF reactor holds the reaction under pressure instead, which does three things at once: ozone stays dissolved longer, the gas-liquid mass transfer rate climbs, and the cyclonic field keeps fresh catalyst surface in contact with the flow. The pressurised catalytic reaction, hydrodynamic cavitation and cyclonic fluidised stages each attack the same recalcitrant organics from a different angle, so the overall conversion does not depend on any single mechanism having a good day. That redundancy is why the results hold stable across the seasonal swings that make leachate notoriously inconsistent as a feed.

The Shiyan leachate project adopted a CDOF advanced ozone catalytic oxidation dissolved-air flotation integrated unit at 300 m³ per day. Its influent is upstream MBR effluent with COD of about 1,300 mg/L. After the CDOF unit, effluent COD falls below 400 mg/L, colour drops below 30 units and odour disappears entirely. Downstream biological treatment then polishes the stream to final effluent of COD at 20 mg/L or less, ammonia nitrogen at 1 mg/L or less and total phosphorus at 0.2 mg/L or less — every index stably meeting the Class III surface water standard for compliant discharge.

Where This Fits the Broader Plant

A CDOF unit upstream of biology changes the economics of the whole site. Because the reactor strips the fraction that inhibits microorganisms, the biological stage downstream runs without the shock loading that forces dilution or extended aeration elsewhere. Colour and odour removal ahead of biology also removes the feedback loops in which residual colour compounds hide underneath the oxygen demand readings and mislead operators. For waste-to-energy plants facing tighter discharge limits, the sequencing — catalytic ozonation first, biology second — is becoming the default answer rather than the experiment.

Two engineering lessons stand out. First, residence time is the real currency: cutting it from 90 min to under 15 min shrinks reactor volume, land take and capital in the same stroke. Second, utilisation governs operating cost; pushing ozone use above 99.98% while halving the dosing ratio is what makes catalytic ozonation affordable at municipal scale. Projects choosing between advanced oxidation routes should price both numbers before anything else.