Oxygen and Ozone Transfer into Water: What You Need to Know for Efficient System Design
Introduction
"I bought an ozone generator but I'm not getting the results I expected" — this complaint is heard far more often in the sector than one might think. In most cases the problem is not the device itself, but how efficiently the system transfers the gas into water. Producing ozone or oxygen is not enough; effectively transferring — dissolving — the produced gas into water should be the system's primary performance criterion.
In this post we cover the fundamental concepts of gas-liquid transfer, the factors that affect transfer efficiency, and how to select the right transfer method for different application contexts.
Why the CT Value Underpins Everything
In water treatment and disinfection literature, the CT value — concentration (mg/L) × contact time (minutes) — defines the minimum parameter required to achieve a target inactivation level. To achieve 3-log (99.9%) inactivation of Cryptosporidium parvum oocysts in water at 10°C, approximately 5 mg·min/L CT is required, whereas 0.02 mg·min/L suffices for the same inactivation of E. coli (US EPA Disinfection Guidance).
To achieve these values, sufficient dissolved ozone or oxygen must be present in the water for a defined period. Even if the dose is correct, if contact time is insufficient or the gas has not dissolved adequately into the water, the CT value cannot be reached. Transfer inefficiency therefore directly equates to loss of the target CT.
Factors Affecting Transfer Efficiency
Bubble size: Gas transfer is directly proportional to the gas-liquid interfacial area. A bubble of 1 mm diameter offers 10 times more surface area than a 10 mm bubble — dissolution rate increases proportionally. Microbubble and nanobubble technologies (50–200 μm range) are preferred for this reason in aquaculture and high-efficiency applications in particular.
Water temperature: The solubility of ozone and oxygen in water is inversely proportional to temperature. Ozone solubility in water at 10°C is approximately 2 times higher than in water at 25°C. The drop in transfer efficiency seen in summer months in uncooled systems is due to this physical phenomenon.
Water pH: Ozone stability in water is higher at lower pH; ozone half-life is longer in the pH 6–7 range. Above pH 8, however, ozone rapidly converts to hydroxyl radicals and spontaneously decomposes, making CT accumulation more difficult.
Oxygen purity: Oxygen at 90–95% purity produced by a PSA oxygen generator delivers 2–3 times higher ozone production efficiency compared to an air-fed system. Higher ozone concentration means much higher dissolved ozone levels with the same transfer method.
Salt and organic matter content: High salt concentration (seawater, brackish water) reduces gas solubility. High organic matter (COD) causes injected ozone to be rapidly consumed; in this case a higher ozone dose is required to reach the target CT value.
Transfer Methods: Which One, When?
Venturi Injector: Draws gas into the system via the pressure drop created by water flow; no mechanical moving parts. Easy to maintain and reliable. Transfer efficiency is generally in the 80–90% range. Widely used in medium and large-scale water treatment, wastewater, drinking water, and aquaculture applications. Requires sufficient flow rate and pressure differential to operate effectively.
Porous Diffuser (Fine Bubble): Delivers gas into the liquid as small bubbles through a porous stone or membrane. Transfer efficiency in the 60–80% range, dependent on bubble size; finer pores → smaller bubbles → better transfer. Carries clogging risk and requires regular maintenance. In deep tanks (aquaculture, bioreactors), water column pressure improves transfer efficiency.
Side Stream Injection: Concentrated gas is injected into a small side stream separated from the main flow, then returned to the main line. Used together with a venturi, high dissolved gas concentrations can be achieved. Preferred in high-flow-rate systems to optimise the flow/capacity balance.
Microbubble and Nanobubble Generators: Produce bubbles in the 10–200 μm range; dissolution efficiency can exceed 95%. Bubbles remain suspended in water for extended periods; with neutral surface tension they do not rise spontaneously. Ideal for aquaculture, high-precision water treatment, and ozone washing lines, but with higher upfront investment costs.
Reactor Design and Contact Time
No matter how good the transfer method, insufficient contact time leads to loss of the target CT value. Ideal reactor design should include: a baffled structure preventing short-circuit flow (plug flow), a correctly sized contact chamber for the water volume, and a dissolved ozone/oxygen measurement point at the outlet. In high-flow-rate systems in particular, commissioning without a hydraulic retention time (HRT) calculation is one of the most common design errors that frequently results in disappointment.
A Special Case in Aquaculture: TRO Management and the Degasser
In aquaculture systems, efficient transfer of oxygen and ozone into water simultaneously makes TRO (Total Residual Oxidant) management mandatory. Ozone decomposes rapidly after being introduced to water; however, the TRO residuals it generates can damage fish gills. The use of a degasser (stripping column) after ozonation is therefore essential: the degasser reduces TRO to safe levels (≤ 0.01 mg/L) while preserving dissolved oxygen. This balance sits at the heart of the system design in the 50 Nm³/hour O₂ + 400 g/hour ozone integrated system OCS Ozone installed at the Gümüşdoğa Keban facility.
The OCS Ozone Approach
At OCS Ozone, we calculate transfer efficiency, reactor hydraulics, and CT accumulation from the outset in every system design. As the only Turkish manufacturer producing its own oxygen generator, reactor, and transformer in-house, we offer designs that approach gas transfer from an integrated systems perspective. Let us work together to determine the optimum transfer method and reactor configuration for your facility's water quality, flow rate, and target CT values.