Ozone in Aquaculture: An Efficient Tool for Water Quality and Biosecurity H2_

 

Ozone production in aquaculture

 

Aquaculture faces constant challenges in maintaining optimal water quality and protecting animal health. Pathogens, organic waste, and dissolved contaminants can accumulate in both flow-through systems and Recirculating Aquaculture Systems (RAS), impacting growth, survival rates, and product quality.

Ozone (O₃) is increasingly being adopted as a powerful solution to these challenges. Recognized as one of the strongest oxidants available for water treatment, ozone offers a wide spectrum of action, from pathogen control to organic load reduction, while leaving no chemical residue other than oxygen.

What is Ozone?

Ozone is a triatomic form of oxygen (O₃) with a much higher oxidation potential than molecular oxygen (O₂). It is a naturally unstable gas that readily reacts with organic and inorganic compounds, breaking down complex molecules and inactivating microorganisms. Because of its strong oxidative properties, ozone is widely used in water treatment across industries, and aquaculture.

How Ozone is Produced

In aquaculture, ozone is typically produced on-site using an ozone generator supplied in pure oxygen from liquid oxygen or from an oxygen generator. The process involves:

• Pure oxygen feed gas.
• Passing the oxygen through an electrical discharge (corona discharge) inside the generator.
• This electrical energy splits O₂ molecules into individual oxygen atoms, which then recombine with other O₂ molecules to form ozone (O₃).

Most commercial ozone generators for aquaculture are designed to produce ozone at concentrations of around 10% by weight in the oxygen stream. This on-demand production eliminates storage concerns and ensures the gas is used immediately, as ozone rapidly decomposes back into oxygen.

Mode of Action

Ozone works primarily through oxidation, attacking:

• Pathogens such as bacteria, viruses, and parasites.
• Organic matter, including dissolved and particulate organics.
• Color and odor compounds that degrade water quality.

Its advantages over other oxidants like chlorine include:

Higher oxidative strength (1.52 times more potent than chlorine).

On-site production from oxygen and electricity (reducing logistics and chemical handling).

The final product of ozone decomposition is oxygen.

Applications in Aquaculture

Inlet Water Disinfection

Ozone can be applied to disinfect incoming water before it enters tanks or ponds, significantly reducing the risk of introducing pathogens. This is applicable to all species and systems, including flow-through farms and RAS facilities.

In shrimp farming, ozone is increasingly used as a replacement for chlorine, particularly in areas where electricity is cheaper than chemical supply chains. It is also effective against recurrent viral diseases such as White Spot Syndrome.

Caution for seawater use: When treating seawater, Oxidation-Reduction Potential (ORP) should be carefully monitored. Exceeding 800 mV of ORP can oxidize bromide ions into bromine, which is toxic to aquatic species.

RAS: Recirculating Water Disinfection & Organic Load Control

In RAS opérations, ozone serves multiple roles:

• Pathogen load reduction: Comparable to UV disinfection but with the added benefit of oxidizing organics.
  
  
• Elimination of organic matter: By oxidizing dissolved organic matter, ozone reduces the nutrient supply for heterotrophic bacteria. In a RAS, this pre-oxidation reduces the organic load reaching the biofilter, which promotes the activity of nitrifying bacteria and improves the overall efficiency of biological filtration.
  
  
• Foam fractionation enhancement: Ozone is often injected via a protein skimmer. The oxidation process improves bubble formation, enabling the removal of fine suspended solids (Protein skimming and ozone work synergistically: ozone partially oxidizes organics, reducing surface tension and making them easier to remove, while the skimmer physically extracts these particles from the system. This combination lowers bacterial load, improves water clarity, and supports overall system stability.
  
  
• Prevention of NO₂⁻ spikes: In RAS systems, nitrites (NO₂⁻) can quickly reach concentrations that are toxic to fish, particularly after a high organic load or a temporary imbalance in the biofilter. Ozone helps limit these spikes by partially oxidizing NO₂⁻ into nitrate (NO₃⁻), which reduces nitrite accumulation and helps stabilize the nitrogen cycle.
  
  
• Off-Flavor Prevention: Off-flavors such as geosmin and MIB (2-methylisoborneol) can severely reduce the market value of harvested fish. Ozone does not directly destroy these compounds at the concentrations typically used in aquaculture. However, by oxidizing dissolved organic matter and fine particulates before bacteria have the opportunity to convert them into geosmin or MIB, ozone indirectly reduces the conditions that lead to their formation. This proactive control of precursors helps maintain product quality and flavor, minimizing the risk of costly depuration delays before harvest.

 

Protein skimmers from @CMAQUA

 

Dosing Guidelines & Operational Considerations 

The correct ozone dose in RAS depends largely on feed load.

Common industry guidelines suggest:

• 10–20 g O₃ per kg of feed (some systems may go up to 25 g/kg).

The feed-to-ozone ratio is commonly used as a reference for sizing both the ozone generator and the protein skimmer in RAS systems. However, the most efficient way to control ozone dosing in operation is by monitoring the ORP (Oxidation-Reduction Potential). A target of around 700-750 mV at the outlet of the protein skimmer is generally a good indicator that organic matter and pathogens are being effectively oxidized without excessive residuals downstream.

Since skimmers generally operate in parallel with the main water recirculation system, this allows the ORP to be maintained at around 200-300 mV so as not to pose a risk to the animals.

 

Operational dosing should also consider:

• Water flow rate through the treatment unit.
• Retention time in the contact chamber or skimmer.
• Continuous ORP monitoring to avoid overdosing, especially in seawater.
• The extent of mechanical filtration; better particle removal reduces ozone demand.

For inlet water ozone treatment, the required ozone dose will mostly depend on the initial organic load. A practical rule of thumb is to target 1 g O₃ per m³ of water. Above this level, it becomes difficult to dissolve more ozone efficiently in water under normal conditions.

A simple estimation method can be used to approximate the ozone requirement based on chlorine tests:

1. Add a chlorine source (e.g., calcium hypochlorite) to a defined volume of the target water while measuring ORP.
2. Record the quantity of free chlorine required to reach 700 - 750 mV.
3. Convert this chlorine requirement to the equivalent ozone dose, knowing that both ozone (O₃) and “active chlorine” (as Cl₂) consume 2 electrons per mole:

• Equivalent weight (MW / n):
  • O₃: 48 / 2 = 24 g per equivalent
  • Cl₂: 70.9 / 2 = 35.45 g per equivalent
• Therefore, 1 kg of active chlorine (Cl₂-equivalent) ≈ 0.68 kg of O₃ for equal oxidizing capacity.
  
  
• Considering an ozone dissolution efficiency of ~85%, this represents ~0.80 kg of ozone to match the effect of 1 kg of free chlorine.

In all cases, it is recommended to remove as much suspended solid material as possible before ozone disinfection. This increases the efficiency of ozone oxidation and reduces overall oxidant demand.

Benefits & Limitations

Benefits

• Broad-spectrum disinfection (bacteria, viruses, parasites).
• Stronger oxidant than chlorine.
• Is produced on site (delivery free).
• Reduces organic load and improves water clarity.
• Enhances dissolved oxygen levels after decomposition.
• Supports better feed conversion, growth rates, and product quality.

Limitations

• Overdosing in seawater can produce toxic bromine compounds.
• Equipment costs and operational control requirements.
• Requires skilled operators and well-synchronized automatic dosing to manage ozone dosing based on redox potential (ORP).

Conclusion

Ozone is a proven, versatile tool for improving water quality and biosecurity in aquaculture. Its ability to disinfect, control organics, and support better system performance makes it valuable for both RAS and flow-through systems.

While careful dosing and monitoring are essential to avoid risks, the trend toward on-site ozone generation from oxygen offers logistical, economic, and environmental advantages over chemical disinfectants. As the industry continues to optimize system design, the combination of ozone and advanced mechanical filtration like protein skimming is likely to become even more widespread in the years ahead.