Inside the Enclosed Pressurised Tank: Where CDOF Puts the Ozone

Ozone is a strong oxidant and a poor team player. Introduced into a tank through a diffuser, most of it rises, coalesces and leaves the water within a metre of the inlet, which is why ozone systems acquire a reputation for high power bills and modest COD removal. The design question is therefore not whether to use ozone but how to keep it in contact with the pollutant long enough, and with enough radical conversion, to be worth the energy.

Three technologies in one vessel

CDOF is an integrated unit that combines advanced ozone oxidation, swirl separation and dissolved-air flotation in a single pressurised envelope. The combination is not decorative: oxidation breaks the refractory fraction, flotation lifts the resulting solids and oils, and swirl separation provides the internal hydraulics that make both happen in a compact volume. The unit is built for comprehensive purification of a range of refractory wastewaters rather than one narrow duty.

What sits inside the shell

The structure is an enclosed pressurised vertical tank, comprising a main reaction tank, a secondary reaction tank, a jet ozone injection device, homogeneous and heterogeneous catalysts, a pressure-reducing hydrodynamic cavitation device, a tail-gas treatment unit and an ozone generator. Every one of those items exists to serve a specific step in the contact problem.

The path the water takes

After pressurisation, wastewater enters the jet ozone injection device and is mixed with the ozone-containing gas. Under the cutting action of the high-velocity water flow the gas is dispersed into a large number of microbubbles rather than a coarse bubble swarm. The microbubble-rich liquid is then rapidly re-pressurised so the ozone dissolves into the water quickly, and undissolved oxygen microbubbles are compressed further into micro and nano bubbles before entering the main reaction tank.

Inside the tank, dissolved ozone contacts the catalyst and generates hydroxyl radicals. Those radicals react quickly with the organic load measured as COD, breaking macromolecules into small molecules that end up as carbon dioxide and water. The micro and nano oxygen bubbles do a second job while they are there: they continuously scour and agitate the catalyst, which strengthens the contact reaction and raises the rate between ozone and catalyst, and also keeps the bed from caking and blocking. The same bubbles adsorb and remove suspended solids, colloids and oils, which reduces the non-dissolvable share of COD and, in turn, lowers ozone consumption.

Treated water then passes through the pressure-reducing hydrodynamic cavitation device and is released into the secondary reaction tank. The pressure drop forces a large number of microbubbles out of solution, and the ozone that did not react in the main tank precipitates here as new microbubbles that continue contacting catalyst and further reduce COD. Those bubbles also keep removing residual non-dissolvable pollutants — oils, suspended solids, colloids — so the water arriving at downstream processes is already within specification.

Keeping the catalyst alive

A catalytic ozone system is only as good as its catalyst after six months in service, which is why the skid carries a periodic automatic backwash and activation program, roughly one to two times per week, plus an acid-washing regeneration program at about once a month. The schedule is the difference between a unit that holds its performance and one whose removal curve sags quietly until somebody notices the power consumption has not changed but the effluent has.

The engineering case, item by item

Reaction speed and consumption. Ozone is dispersed in dissolved form and as micro-nano bubbles, so reaction rates are fast, contact with catalyst is thorough and radical conversion is high. Ozone consumption runs at 1/4-1/3 of conventional systems and operating cost at 1/3-1/4 of conventional ones.

Enclosed safety. The main reaction tank operates at 0.3-0.6 MPa and the secondary tank at 0.03-0.1 MPa. Sealed pressurised operation means zero ozone leakage, which changes the site environment from a monitored hazard zone into an ordinary equipment area.

Anti-clogging behaviour. The reaction is a floating-bed catalytic process, so the catalyst does not settle into a cemented mass; backwashing, activation and regeneration keep it effective over long cycles and hold treatment performance consistent.

Control. Fully automatic DCS/PLC control with automatic optimisation and early warning needs no manual intervention, which matters on a plant where the oxidation stage is not the operator's main job.

Deployment. Reaction time is only 15 minutes, one sixth of conventional processes. The skid-mounted design has a small footprint, few auxiliary facilities and a short construction period — the last of which is often the deciding factor in a retrofit where the treatment plant has to fit inside an existing battery limit.

Where the design earns its cost

The economics are clearest where the pollutant is refractory and the discharge limit is tight: landfill leachate concentrate, refinery wastewater after biological treatment, chemical effluent with a high proportion of non-biodegradable COD. Where the load is largely biodegradable, a conventional biological train will beat this unit on operating cost every month, and the honest thing to do is say so before the quotation stage.