Zero-Outbreak RAS System Practices: Engineering a Dual Ozone–Biofilter Control System for Disease-Free Aquaculture

Stabilization of a recirculating aquaculture system (RAS) as a zero-outbreak system has become a fundamental objective in modern aquaculture systems engineering, especially in a high stocking rate and low water exchange rate intensive commercial production system where microbial growth conditions are optimal. As aquaculture systems expand at a global level, maintaining water quality, stabilizing microbial populations, and eliminating pressure of pathogens inside highly controlled systems has become a key economic consideration and viability in the long term(Li et al., 2023). Zero-outbreak facility is the one that can maintain the well-being of fish and the environmental balance with the absence of disease incidents that interrupt the cycles of production and cause a high level of mortality. This stability cannot be accomplished through mere water exchange but rather a rigorous water treatment scheme that is scientifically based. The dual ozone biofilter method is one of the most effective methods employed in modern aquaculture and it is a synergistic process comprising of both advanced oxidation and biological nitrification to ensure the water quality, prevent pathogens, and achieve consistent environmental conditions, which is vital to the success of long-term systems (Preena et al., 2021).

Recirculating aquaculture systems recycle over 95 percent of water contained in culture tanks, mechanical filters and treatment chambers. Although this will decrease the environmental discharge and enhance sustainability, it will also cause the concentration of dissolved organic carbon, suspended solids, mucus, fecal particles, uneaten feed, and diverse microbial communities (MAT, 2025). When such compounds build up beyond the optimum levels, they limit the penetration of light, elevate biochemical oxygen requirements, promote the growth of detrimental bacteria and add stress to the fish. Stress suppresses the immune system, destroys feeding performance, and predisposes Vibrio, Aeromonas, Flavobacterium, parasites, viruses, and other opportunistic pathogens. Because of these reasons, high performance RAS design is focused on effective water treatment mechanisms which can constantly regulate organic load and microbial activity (Fossmark et al., 2020).

Ozone plays a central role in addressing this challenge. As one of the strongest oxidants used I aquaculture water treatment, ozone rapidly breaks down dissolved organic matter, color pigment, fine colloids, and microbial contaminants. Numerous aquaculture studies, including those in salmonid, tilapia, and marine finfish production, have shown that ozone application can significantly improve water clarity, increase ultraviolet transmittance, depresses heterotrophic bacterial population, and reduces concentration of ozone sensitive pathogens. Because ozone decomposes into oxygen, it avoids leaving harmful chemical residues in the system. This is its distinctive feature from chlorine-based disinfectants, which leave persistent byproducts incompatible with recirculating systems. Ozone thus functions as a rapid, residue-free oxidant capable of clarifying water and decreasing pathogen pressure upstream of the biofilter(Xue et al., 2023).

However, ozone alone cannot maintain a stable RAS environment. Fish release ammonia continuously through their gills and metabolic waste, and even low concentration of ammonia impairs gill function, suppress appetite and inhibit growth. Due to this fact, biological filtration is the second key pillar of the dual-treatment approach. In the biofilter, Nitrosomonas, Nitrobacter and Nitrospira are specific nitrifying bacteria that will turn ammonia to nitrite and subsequently to nitrate via the nitrification process (Oshiki et al., 2022). This bio-chemical conversion is necessary in preserving a safe environment in high-density aquaculture plants. Due to ozone being sensitive to these bacteria, physical separation between ozone contact and biological filtration must be maintained during system design. In contemporary RAS, ozone is sprayed into a separate chamber where it combines with water then flows through a degassing unit that removes all the remaining ozone. This step is only done after which treated water can be admitted into the biological filtration process(Xiao et al., 2019).

Nitrifying bacteria are very sensitive to oxidative stress and thus, any remaining ozone must not be released into the biofilter. Modern RAS engineering fulfils this need by ensuring practical system layout. This involves injection of ozone in a special contact chamber which is then combined with water over a controlled duration. An off-gas or degassing unit is provided downstream which removes any residual ozone and the water is then passed into the biofilter. This will avoid exposing nitrifying bacteria to reactive oxidative molecules which have the potential of destroying their metabolic pathways(Mahmoodi & Pishbin, 2025). With a well-designed system, the biofilter has the advantage of cleaner, clearer, oxygen-rich water with a much lower organic load. This will enhance the stability of nitrifying colonies and efficiency of ammonia conversion leading to more effective control of water-quality(Pumkaew et al., 2021).

The synergy of ozone treatment and biological filtration scientists is supported by scientific studies. Comparative studies on the water entering biofilters with ozone and non-ozone water indicate that ozone water enhances the efficacy of nitrification by decreasing the heterotrophic fight over oxygen and surface area. Ozonated water also causes a lower biofouling, more stable nitrifying biomass and faster recovery following stress events like feeding spikes or temperature changes in biofilters fed ozonated water. With effective functioning of biofilters, levels of ammonia and nitrite are maintained at a low and constant level, lowering the stress levels in fish, and lowering the chances of disease outbreaks. The basis of a zero-outbreak RAS strategy is this synergy whereby the ozone clears the water and the pathogens, and the biofilter keeps the nitrogen steady (Pumkaew et al., 2021).

To ensure the success of the dual ozone-biofilter system, it is important to maintain the right operation parameters. The values of oxidation-reduction potential in the ozone contact chamber are normally 275 to 320 millivolts (mV). This spectrum aids in efficient reduction of organic matter without generating any undesirable reaction byproducts (Davidson et al., 2021). Before the ozone unit, mechanical drum filters of sixty to one hundred microns in size are used to remove large, suspended solids to enhance ozone efficiency by decreasing the organic load. Optimal values of dissolved organic carbon are four milligrams per liter because beyond this level, the water fails to be clear and promotes the growth of microbes. The concentration of dissolved oxygen below the ozone chamber is usually more than nine milligrams per liter since ozone decomposes naturally to produce oxygen. Having high dissolved oxygen levels greatly improves fish metabolism as well as the rate of nitrification. Most importantly, the amount of residual ozone entering the biofilter should also be zero, this is achieved through constant monitoring to ensure that the nitrifying bacteria is not damaged.

The table below summarizes typical operational values in a functional dual ozone–biofilter RAS:

Table: Operational Parameters in Dual Ozone–Biofilter Recirculating Aquaculture Systems

Even with these guidelines, challenges can arise during system operation. Ozone demand varies based on the growth of biomass, the intensity of feeding, temperature variation, and other unforeseen activities like mortalities. Excessive ozone may lead to irritation of the gills, oxidative stress or immunosuppression of fish (Han et al., 2023). Under-ozonation permits the dissolved organic carbon to build up, moving the microbial communities to a state of instability and susceptible to disease. Mechanical failures in ozone injectors, contact chambers, or degassing systems can cause ozone leakage into culture tanks, resulting in acute stress responses. Many producers therefore rely on automated ORP-controlled ozone dosing systems using real-time monitoring to maintain consistent performance.

Ozone effects on the ecology of microbes are not confined to the inhibition of pathogenicity. Although ozone is a more effective method to eliminate the concentrations of harmful microorganisms, over-oxidation can destroy the positive microbial communities involved in degrading organic matter and maintaining biofilter stability. Under extreme oxidation conditions some microbial strains are ozone resistant and therefore may grow out of proportion, changing ecological equilibrium undesirably. To prevent these imbalances, effective RAS operators use moderate, managed doses of ozone that focus on reliability in the quality of water and not the aggressive treatment of water (Botondi et al., 2023).

The dual ozone-biofilter system does not only favor the quality of water, but also the sustainability of the entire farm. Disease-free conditions reduce the usage of antibiotics and minimize losses in operations. Constant water quality enhances efficiency of feed-conversion, growth rates and predictability of harvest. As pressures mount on the world aquaculture to produce high quality seafoods with minimum effect on the environment, zero-outbreak RAS operations are a feasible way forward to sustainable intensification.

At WOLIZE, our team is specialized in the designing of advanced RAS aquaculture systems that are scientifically optimized to have ozone-biofilter integration. We assist manufacturers in delivering consistent, disease-resistant and high-performance operations via professional engineering, technical assistance and continuous evaluation of the systems.

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