CDOF Process vs. Conventional Methods: A Comparative Analysis
2026-10-22 10:49:52
1. Conventional tank-type ozone catalytic oxidation
(1) Ozone catalytic oxidation: with its strong oxidizing power (oxidation potential E0 = 2.07 V), ozone can effectively decompose organic and inorganic pollutants in water. As ozone generation and catalytic oxidation technologies have advanced, the process has been widely applied in wastewater treatment in recent years.
(2) Structural form: a conventional tank-type ozone catalytic oxidation system usually comprises an ozone contact oxidation tank and an intermediate tank. The contact oxidation tank is arranged with a wastewater inlet and ozone escape zone, backwash water outlet, heterogeneous fixed catalyst bed, support layer and aeration device, plus an ozone gas inlet and backwash water inlet at the bottom. The intermediate tank mainly includes an off-gas destruction device and ozone escape zone at the top, a wastewater stripping zone, and an aeration device, air inlet and drain at the bottom.
(3) Process description: wastewater flows into the ozone contact oxidation tank while ozone gas from the ozone generation system passes through the aeration device to form microbubbles, entering the support layer and then contacting the catalyst. During the rise, microbubbles continually coalesce and grow, eventually reaching the surface and escaping. Initial microbubble size is usually below 50 μm, but by the time bubbles reach the catalyst layer their diameter has grown to about 3 mm. The excessive bubble size means only a few ozone molecules can penetrate the catalyst's internal surface to produce reactive hydroxyl radicals (·OH). This results in low ozone utilization efficiency on one hand and significantly extends the required retention time in the oxidation tank on the other — typically more than 90 minutes. The treated wastewater then flows into the intermediate tank, where air is blown in for stripping to remove residual ozone before entering subsequent steps.
To prevent catalyst fouling and scaling, backwash frequency and duration must be determined comprehensively based on tank operating conditions, water quality monitoring results and historical experience. As operating time accumulates, backwashing tends to become more frequent and longer.
(4) Technical characteristics:
- Ozone bubbles are relatively large (usually 1-3 mm or more) and ozone utilization is low, since bubbles easily rise to the liquid surface and escape; ozone consumption is therefore high and operating costs are high. In addition, large bubbles contact the catalyst insufficiently, so ·OH conversion efficiency is low.
- With heavy ozone consumption, off-gas destruction devices cannot keep up, and leakage easily occurs, threatening plant equipment and personnel safety.
- Fixed-bed catalysis makes the catalyst bed prone to compaction, and fines can clog the aeration discs; backwashing or acid cleaning is not very effective. On one hand, stable compliance of treatment performance cannot be guaranteed over long periods; on the other, ozone consumption increases and leakage risk rises.
- Semi-automatic or manual operation increases labour costs.
- The tank-type design occupies a large area, requires long retention time (generally about 90 min) and many supporting facilities, and involves a long construction period.
2. Closed pressurized tank ozone catalytic oxidation
(1) CDOF (Cyclonic Dissolved Ozone Flotation unit): creatively combines advanced ozone oxidation, cyclone technology and dissolved gas flotation to achieve efficient, comprehensive removal of various difficult-to-treat wastewater pollutants.
(2) Structural form: a closed pressurized vertical tank comprising a main reaction vessel (0.6 MPa), secondary reaction vessel (0.05 MPa), jet ozone dosing device, heterogeneous/homogeneous catalyst, pressure-reducing hydrodynamic cavitation device, off-gas treatment device and ozone generator.
(3) Process description: after pressurization, wastewater enters the jet ozone dosing device and mixes with ozone gas mixture (about 10% ozone, about 90% oxygen). The high-speed water flow shears the gas into numerous microbubbles, and rapid pressurization of the wastewater promotes fast ozone dissolution. Undissolved oxygen microbubbles are compressed into micro/nano bubbles and enter the main reaction vessel. Dissolved ozone contacts the catalyst fully and rapidly generates hydroxyl radicals, reacting quickly with organic pollutants (COD), breaking long molecular chains into short ones and finally producing carbon dioxide and water. In this process, micro/nano oxygen bubbles clean and agitate the catalyst, intensifying contact reaction; this raises both reaction rate and effectiveness and prevents catalyst compaction and clogging. Meanwhile, these microbubbles remove suspended solids, colloids and oil from the water, cutting non-dissolved COD and reducing ozone consumption. Wastewater treated in the main reaction vessel is depressurized through the hydrodynamic cavitation device, generating large numbers of microbubbles that enter the secondary reaction vessel. Unreacted dissolved ozone comes out of solution to form microbubbles, continuing to contact the catalyst for deep catalytic oxidation and further COD reduction. Micro/nano bubbles continue to remove non-dissolved pollutants, ensuring the effluent meets standards before entering downstream steps.
To prevent catalyst compaction and clogging, the CDOF integrated unit performs automatic backwash activation at regular intervals (about 1-2 times per week) and acid regeneration (about once a month), safeguarding long-term stable catalyst operation.
(4) Technical characteristics:
- Ozone is highly dispersed in water in dissolved form and as micro/nano bubbles, giving fast reaction rates, full and intensified contact with the catalyst, and high hydroxyl radical conversion efficiency. Ozone consumption is only one-quarter to one-third of conventional processes, and operating costs drop correspondingly to one-third to one-quarter of conventional levels.
The main reaction vessel operates at 0.3~0.6 MPa and the secondary at 0.03~0.1 MPa. Closed pressurized operation achieves zero ozone leakage, creating a site-friendly, safe and environmentally sound working environment.
Floating-bed catalytic reaction keeps the catalyst from clogging or compacting. Combined with automatic backwash, activation and regeneration programs, it ensures long-term catalyst stability and reliable treatment performance.
A fully automatic DCS/PLC control system integrates intelligent optimization and early warning, requiring no staffed attendance; equipment operation and maintenance are simple, with high operational stability and safety.
Reaction time is significantly shortened to only 15 minutes, about one-sixth of conventional processes. Modular skid-mounted design gives a small footprint, simplified supporting facilities and a short construction period.
