What Is O₃/H₂O₂ Advanced Oxidation?

Ozone is a powerful oxidising agent capable of reacting with cell membranes, proteins, enzymes and certain components of the genetic material of microorganisms. It also reacts with numerous organic and inorganic substances present in water.

When ozone is combined with hydrogen peroxide, H₂O₂ accelerates certain ozone decomposition reactions and promotes the formation of reactive oxygen species, including:

• • hydroxyl radicals, •OH;

• • superoxide radicals, •O₂⁻;

• • singlet oxygen, ¹O₂;

• • other short-lived oxidative intermediates.

The hydroxyl radical is particularly reactive. Unlike molecular ozone, which retains a degree of chemical selectivity, hydroxyl radicals react rapidly with a very broad range of biological molecules and organic contaminants.

Why use the Peroxone process in Aquaculture?

In aquaculture water, ozone is rapidly consumed by dissolved organic matter, fine particles, nitrite, biofilms and microorganisms. A significant proportion of the applied ozone may therefore be consumed before it reaches the intended treatment target.

Combining ozone with hydrogen peroxide may be particularly relevant when the treatment objectives extend beyond improving water clarity or reducing general bacterial counts.

The process can be investigated for:

• • reducing the concentration of certain waterborne parasites;

• • inactivating free-floating bacteria;

• • controlling bacteria associated with particles or biofilms;

• • enhancing the treatment of certain viruses;

• • oxidising compounds that react slowly with molecular ozone;

• • degrading certain veterinary pharmaceutical residues;

• • reducing part of the dissolved organic load;

• • limiting selected taste and odour compounds;

• • improving water quality before UV disinfection or advanced filtration.

O₃/H₂O₂ treatment may be considered for incoming water, a side-stream recirculation loop, quarantine units, hatcheries, treatment tanks or aquaculture effluent before discharge.

However, the oxidation stage should normally remain physically separated from the fish or shellfish tanks until ozone, hydrogen peroxide and any secondary oxidants have been reduced to safe concentrations.

How effective is O₃/H₂O₂ against Pseudocohnilembus persalinus?

A parasitic ciliate rather than a bacterium

Pseudocohnilembus persalinus is a scuticociliate parasite associated with scuticociliatosis in marine aquaculture. It may affect the skin, gills and muscles of infected animals and, in systemic cases, may spread to internal organs or nervous tissue.

Because the parasite can circulate through the water, water treatment may provide an additional biosecurity barrier alongside quarantine, veterinary management, hygiene procedures and stock control.

Effectiveness against other Aquaculture bacteria and viruses

It is important to distinguish studies that have directly assessed O₃/H₂O₂ advanced oxidation from those that have investigated ozone alone.

Current scientific evidence does not support the use of a single universal ozone or hydrogen peroxide dose for every aquaculture pathogen. Treatment efficiency depends on the target organism and the characteristics of the water.

Why does sensitivity vary between microorganisms?

Oxidation efficiency depends on the structure of the organism, the composition of its membrane or viral capsid, its repair mechanisms, its ability to aggregate and the protection provided by particles or dissolved organic matter.

Viruses do not all have the same sensitivity to ozone or hydroxyl radicals. Similarly, a free-floating bacterium in filtered water may be easier to inactivate than a bacterium embedded within a biofilm or protected by suspended particles.

Additional benefits for Aquaculture water quality

The potential benefits of advanced oxidation extend beyond microbiological inactivation. In a recirculating aquaculture system, the process may also help:

• • oxidise part of the dissolved organic matter;

• • reduce water colour;

• • improve UV transmittance;

• • promote the microflocculation of certain colloidal particles;

• • transform selected persistent organic compounds;

• • reduce certain pharmaceutical residues;

• • control selected taste and odour compounds;

• • reduce compounds such as geosmin and 2-methylisoborneol.

Research conducted in rainbow trout recirculating aquaculture systems showed that intensive O₃/H₂O₂ or H₂O₂ treatments could reduce geosmin and 2-methylisoborneol concentrations in both the water and the fish.

However, the researchers concluded that further process optimisation was required to achieve sufficiently high removal rates under commercial operating conditions.

O₃/H₂O₂ should therefore be considered as a tailored engineering process designed around a measurable treatment objective rather than as a universal solution to every water quality issue.

How can O₃/H₂O₂ be integrated into a RAS facility?

In most aquaculture projects, the advanced oxidation process should be installed on a separate treatment loop or side-stream. This configuration allows the water to be treated within a controlled reactor before it is returned to the culture tanks.

Example treatment sequence

1. 1. Mechanical filtration: removal of suspended solids before ozone treatment.

1. 1. Protein skimming or foam fractionation: reduction of part of the organic load and improvement of ozone transfer efficiency.

1. 1. Side-stream extraction: diversion of a controlled proportion of the recirculation flow.

1. 1. Hydrogen peroxide dosing: controlled injection using a dosing pump linked to the water flow rate.

1. 1. Ozone injection: transfer using a venturi injector, diffuser, static mixer or another suitable gas-to-liquid transfer system.

1. 1. Contact reactor: maintenance of the required hydraulic and oxidative contact time.

1. 1. Off-gas destruction: treatment of residual gaseous ozone using a catalytic or thermo-catalytic ozone destructor.

1. 1. Residual oxidant removal: degassing, activated carbon, UV, retention time or another suitable quenching process.

1. 1. Analytical monitoring: verification of water quality before the treated water is returned to the animals.

The position of the biological filter must be considered carefully. Excessive exposure to ozone, hydrogen peroxide or secondary oxidants can disrupt nitrifying bacteria and reduce the biofilter’s capacity to remove ammonia and nitrite.

In many installations, the oxidation stage is therefore positioned after mechanical treatment and outside the main biological filtration loop, or on a controlled side-stream with appropriate safety interlocks.

Monitoring requirements

Ozone demand of the water

Organic matter, nitrite, suspended solids and various dissolved ions consume ozone. Two aquaculture facilities operating at the same water flow may therefore require very different ozone production capacities.

Water analysis and ozone demand testing should be carried out before equipment is sized. Results obtained using synthetic or highly filtered water cannot be applied directly to heavily loaded aquaculture water.

The H₂O₂-to-O₃ ratio

Insufficient hydrogen peroxide may limit radical formation. Conversely, excessive H₂O₂ may consume reactive species, reduce the molecular ozone residual too rapidly or leave residual peroxide in the treated water.

The molar H₂O₂-to-O₃ ratio should be optimised according to:

• • the target microorganism or contaminant;

• • water pH;

• • temperature;

• • salinity;

• • dissolved organic carbon;

• • ozone demand;

• • required contact time.

Specific considerations for seawater

In seawater and brackish water, ozone reacts rapidly with naturally occurring bromide ions. These reactions can produce hypobromous acid, bromine-based residual oxidants and, under certain conditions, bromate.

Oxidation-reduction potential, or ORP, does not always distinguish ozone from other oxidants. Depending on the facility and the species being farmed, monitoring may therefore include:

• • total residual oxidants, or TRO;

• • dissolved ozone;

• • residual hydrogen peroxide;

• • bromide and bromate;

• • pH, temperature and salinity;

• • chemical oxygen demand, COD;

• • total organic carbon, TOC;

• • UV254 absorbance;

• • biofilter performance.

O₃/H₂O₂ engineering and integration by OXYTRADING Ltd

OXYTRADING Ltd supports aquaculture operators, hatcheries, research centres and RAS system manufacturers with the assessment, design and integration of ozone and advanced oxidation systems.

Our services may include:

• • analysis of the water quality and sanitary objectives;

• • assessment of ozone demand;

• • definition of a laboratory or pilot testing protocol;

• • on-site or laboratory treatment trials;

• • selection of the H₂O₂-to-O₃ ratio;

• • sizing of the ozone generator;

• • design of the ozone injection system;

• • design of the contact reactor;

• • ozone off-gas treatment;

• • residual oxidant removal;

• • instrumentation and automated safety systems;

• • commissioning, training and maintenance support.

Validation trials can focus on target microorganisms, microbiological indicators, COD, TOC, nitrite, taste and odour compounds, pharmaceutical residues or selected micropollutants.

The objective is to establish whether the process is technically relevant, determine the actual oxidant consumption and provide a safe basis for full-scale system design.