Notes on Conventional Tank-Type Ozone Catalytic Oxidation Technology
Ozone catalytic oxidation is a wastewater treatment process that uses ozone, a highly efficient oxidant (oxidation potential E0 = 2.07 V), to degrade and remove organic and inorganic pollutants in water. With the continuous progress of ozone generation equipment and catalytic oxidation processes, this technology has been widely applied in wastewater treatment in recent years.
In terms of construction form, this conventional technology usually consists of two main parts: an ozone contact oxidation tank and an intermediate tank. From bottom to top, the ozone contact oxidation tank contains: the ozone gas inlet and backwash water inlet, the aeration device, the supporting layer, the heterogeneous fixed catalyst packing layer, the backwash water outlet, the wastewater inlet, and the ozone gas overflow zone. The intermediate tank, from bottom to top, is mainly divided into: the air inlet and vent, aeration discs, the wastewater stripping section, the ozone overflow zone, and the off-gas decomposition treatment device.
The process flow is briefly as follows: wastewater is first introduced into the ozone contact oxidation tank. Gas produced by the ozone generation system forms fine bubbles through the aeration discs, then passes through the supporting layer and contacts the catalyst layer. As the bubbles keep rising, the microbubbles continuously coalesce and grow, eventually floating to the surface and escaping. The initial bubble diameter produced by the aeration discs is generally below 50 microns, but by the time they reach the catalyst layer they have grown to about 3 mm. The excessively large bubbles mean only a small amount of ozone can penetrate into the catalyst interior to generate highly reactive hydroxyl radicals (·OH). This not only results in low ozone utilization efficiency but also significantly extends the required residence time of wastewater in the oxidation tank—usually more than 90 minutes. The oxidized wastewater then flows into the intermediate tank, where air is fed through the aeration discs for stripping to remove residual ozone before proceeding to subsequent treatment.
To prevent scaling and clogging of the catalyst layer, the backwash frequency and duration of this tank-type system must be determined based on the actual operating conditions of the tank, water quality monitoring results and historical experience data. As operating time accumulates, the backwash interval gradually shortens while the time required for each wash tends to lengthen.
The main characteristics of this technology include:
The ozone bubbles are relatively large (usually greater than 1-3 mm), so ozone utilization is low and the bubbles easily float to the liquid surface and escape; therefore ozone consumption is high and operating costs are high. In addition, the large bubbles contact the catalyst insufficiently, resulting in low ·OH conversion efficiency.
Heavy ozone consumption combined with untimely off-gas destruction can easily lead to leakage, threatening plant equipment and personnel safety.
Fixed-bed catalysis: the catalyst bed is prone to caking, and powder shedding easily blocks the aeration discs; backwashing or acid washing is ineffective. On the one hand, stable and consistent treatment performance cannot be guaranteed over the long term; on the other, ozone consumption increases, raising leakage risk.
Semi-automatic or manual operation increases labor costs.
The tank-type design has a large footprint, long residence time (generally around 90 min), many auxiliary facilities and a long construction period.
