Ozone (O₃) is one of the strongest oxidants used in aquaculture and water treatment. It is widely applied in Recirculating Aquaculture Systems (RAS), hatcheries and marine production systems to improve water quality, reduce dissolved organic matter, improve water clarity and support microbial control. Ozone is also widely used to sterilise supply and effluent water in aquaculture systems.
Unlike chlorine, ozone leaves no long-term disinfectant residual because it naturally decomposes back into oxygen. This makes it attractive for aquaculture, but it also means ozone must be generated on-site and used immediately. Ozone is highly unstable and therefore generated locally rather than stored.
When properly engineered and operated, ozone can significantly improve water quality and production stability. When poorly applied, it can damage fish, destroy biofilter performance, create toxic residual oxidants and present serious safety risks to personnel. Higher ozone concentrations can cause tissue damage, stock mortalities and risks to biofilters.
Why ozone is used
As production intensity increases, organic wastes accumulate within the system. While drum filters and biofilters remove solids and nitrogen compounds, they do not efficiently remove all dissolved organics and fine colloidal particles. Ozone is often used as an additional process step to manage these compounds. Ozone is used to remove organic carbon, turbidity, algae, colour, odour and taste, and to improve system stability.
Potential benefits include:
- Oxidation of dissolved organic matter.
- Reduction of water colour.
- Improved water clarity.
- Improved UV transmittance.
- Reduction of fine colloidal particles.
- Support of foam fractionation and protein skimming.
- Reduced microbial pressure.
- Improved overall water quality stability.
Many operators notice ozone first as “clearer water”, but the underlying benefits often extend throughout the entire treatment train.
How ozone works
Ozone is a highly reactive molecule consisting of three oxygen atoms. The third oxygen atom is weakly bound and readily reacts with other compounds. This makes ozone a powerful oxidising agent. The instability of ozone is what gives it its treatment effectiveness.
When ozone enters water it reacts with:
- Dissolved organic compounds.
- Fine particles.
- Bacteria, viruses and fungi.
- Some protozoan pathogens.
- Colour-causing and odour-causing compounds.
Many of these reactions occur very rapidly. In addition to direct oxidation, ozone may initiate secondary oxidation reactions that further improve water quality.
Typical ozone system layout
Ozone should generally not be applied directly into culture tanks. Instead, it is normally used in a dedicated treatment loop. Water is usually treated in a side-stream and returned only after ozone has reacted or been removed. Side-loop treatment and dedicated contact systems are commonly used to avoid direct fish exposure.
Typical components include:
- Oxygen concentrator or liquid oxygen supply.
- Ozone generator.
- Venturi injector or gas transfer system.
- Contact chamber.
- Protein skimmer or foam fractionator.
- Degassing unit.
- Residual ozone destruction.
- ORP monitoring.
- Safety systems and alarms.
Ozone and protein skimmers
One of the most important practical benefits of ozone is its interaction with foam fractionation systems. Ozone can alter dissolved organic compounds, improve particle aggregation, increase skimmer efficiency, improve water clarity and reduce yellowing compounds. Many marine RAS facilities combine protein skimmers and ozone treatment specifically because the two technologies complement each other.
Ozone and UV systems
Ozone and UV are often treated as competing technologies, but they are frequently complementary. Ozone can improve UV performance by reducing dissolved organics that absorb ultraviolet light. Improved UV transmittance is a recognized outcome of ozonation.
A common approach is: mechanical filtration → ozone treatment → foam fractionation → UV treatment → culture system. This sequence can improve overall treatment efficiency and biosecurity.
Ozone and water clarity
Water colour and turbidity are common challenges in intensive production systems. Over time dissolved organics accumulate and create yellow-brown water colour, lower UV penetration, reduced visibility and increased organic loading. Ozone breaks down many of these compounds and improves visual water quality. The result is often better visibility, improved UV effectiveness and reduced organic load.
Ozone and biofilters
Ozone can support biofilter performance indirectly by reducing organic loading. However, excessive ozone exposure can damage nitrifying bacteria. Potential problems include loss of nitrification capacity, reduced ammonia and nitrite conversion and biofilter instability.
For this reason, ozone is usually applied upstream of residual removal and not directly to biofilters. The goal is improved water quality without exposing biofilter bacteria to harmful residual oxidants.
Ozone in freshwater systems
Freshwater applications are generally more straightforward. Ozone primarily reacts with organic matter and microorganisms. Risks still exist, but chemistry is usually easier to manage than in marine systems. Even in freshwater systems: residual ozone must not reach fish, ORP should be monitored, ozone demand varies with water quality, and dosing should be adjusted gradually.
Ozone in seawater systems
Marine ozone treatment requires significantly greater caution. Seawater contains naturally high levels of bromide. Ozone reacts rapidly with bromide and forms secondary oxidants, often referred to as ozone-produced oxidants. These compounds may persist longer than ozone itself and can create biological risks if not properly controlled. Fish can sometimes be affected even when no measurable ozone remains.
These secondary oxidants may include:
- Hypobromous acid.
- Bromamines.
- Bromate.
- Other brominated oxidants.
Because of this chemistry, seawater ozonation is more complex, marine species may be exposed to secondary oxidants, conservative operation is essential and residual destruction is critical. Safe seawater ozone treatment depends on controlled contact time, off-gas handling, oxidant management and careful monitoring.
ORP control
ORP (Oxidation Reduction Potential) is commonly used for process control. When oxidation increases, ORP generally rises. However, ORP is not a direct ozone measurement. ORP responds to many chemical conditions within the water and should be interpreted as an indicator rather than an ozone concentration measurement.
ORP is best viewed as a process control parameter, an early warning indicator and a trend measurement — not a direct ozone sensor or a replacement for good system design. See ORP.
Safety
Ozone is highly effective but must be treated as an industrial oxidant. Ozone gas can be hazardous to personnel and should never be allowed to accumulate in enclosed spaces. High atmospheric ozone concentrations are hazardous to health.
- Ozone leak detection.
- Ventilation systems.
- Off-gas destruction.
- Emergency shutdown systems.
- Personnel training.
- Oxygen system safety.
- Alarm systems and routine maintenance.
When ozone is most valuable
Ozone often provides the greatest benefit in intensive RAS, hatcheries, smolt facilities, marine RAS, high-density production, systems with low water exchange and facilities requiring high water clarity. The technology becomes increasingly attractive as water reuse rates increase.
Practical rule of thumb
Ozone is not simply a disinfection tool. It is a water quality management technology. In most successful aquaculture applications, ozone improves system performance by reducing dissolved organics, improving water clarity, supporting foam fractionation and lowering microbial pressure. The goal is not maximum oxidation. The goal is controlled oxidation.
Fish should never be exposed to free ozone, marine systems require special attention due to bromide chemistry, and residual destruction should always be part of the treatment train. When properly designed and conservatively operated, ozone can be one of the most powerful tools available for maintaining stable water quality in intensive aquaculture systems.
Orientation — not a dosing protocol or species-specific limits. Verify design against supplier guidance, ORP practice and local water chemistry (especially bromide in seawater).
See also