Ozone in RAS: Water Polishing, Residual Toxicity and ORP Control

What ozone does in recirculating aquaculture: fine solids, colour and nitrite, residual oxidant toxicity to fish, ORP as a control signal, and staff safety with off-gas destruction.

What ozone does in a recirculating system

SRAC Publication 453 describes ozone (O3) as a form of oxygen that is a very powerful oxidant, stronger than chlorine, and notes that UV is the more common internal disinfection step, while ozone tends to be used in larger facilities with technically trained staff. Stiller et al. (2020) list its water-quality effects as reductions in organic matter, chemical oxygen demand, dissolved organic carbon, fine particulates, nitrite, colour and even hydrogen sulphide. The FAO guide explains the micro-flocculation effect: very fine particles are broken down and recombine into larger particles that the filters can then capture, a process called water polishing that is especially useful in hatchery and fry systems. Ozone can also disinfect intake water.

In six replicated 9.5 m³ rainbow trout systems, Davidson et al. (2011) found that ozone significantly reduced total suspended solids, colour and biochemical oxygen demand, raised UV transmittance and lowered dissolved copper and iron. Ammonia and nitrite were only slightly lower, and ozone did not prevent nitrate accumulation. Growth, survival and feed conversion generally improved in low- and near-zero-exchange systems.

Generation and dosing

According to SRAC 453, corona discharge is the most powerful generation method; its output depends on the oxygen content of the feed gas, and the gas must be dry, so it is normally oxygen-enriched and passed through a dryer. Ozone is dosed either through a packed column or, more often, through a venturi that draws gas into the recirculating line as fine bubbles. It reacts first with dissolved organics and only then with bacteria, viruses and protozoa, so disinfection requires a residual held for some seconds, which is hard to maintain in grow-out water rich in fine solids. A 2017 University of New Brunswick thesis, based on a commercial salmon smolt hatchery, calculated that dosing should not exceed 21.9 ± 2.8 g of ozone per kg of feed to avoid residuals in that system. Sharrer and Summerfelt (2007) reported that a low ozone exposure, expressed as concentration × contact time of 0.1–0.2 mg·min/L, followed by about 50 mJ/cm² of UV reduced bacteria counts to near zero.

Residual ozone toxicity

SRAC 453 warns that overdosing damages fish, that accidental releases into the tank can kill fish and crustaceans, and that many installations follow ozonation with a neutralisation step such as an activated carbon column. Lazado et al. (2026) add that ozone breaks down quickly in water, but residual ozone left uncontrolled can be harmful or lethal. In brackish water and seawater the risk is greater: Stiller et al. explain that ozone reacts with bromide to form free bromine and other total residual oxidants (TROs), and SRAC 453 cautions against toxic by-products such as bromate. Gill damage, including lamellar fusion, necrosis and hypertrophy, is the main lesion described at high doses.

Turbot: TRO above 60 µg/L as Cl2 (no ORP reported) impaired performance (studies cited by Stiller et al.).
European seabass: 30–50 µg/L as Cl2, about 320–350 mV ORP.
Atlantic halibut: 16–23 µg/L as Cl2, about 330 mV.
European lobster larvae: 14–20 µg/L as Cl2, about 400 mV.

ORP as a control parameter

Oxidation-reduction potential (ORP) is the voltage between an inert sensing electrode, usually platinum, and a reference electrode, expressed in millivolts. Wikipedia notes that temperature, pH, mixed redox couples and electrode poisoning limit how readings can be interpreted, so ORP is most useful for tracking changes and for process control rather than as an absolute value. Stiller et al. report that ozone levels on farms are usually expressed as ORP in mV, as transparency or turbidity, or as TROs in µg/L as Cl2, that methods are not standardised, and that ORP and TRO values are rarely reported together. A 2026 review in Frontiers in Microbiology says ozonation requires monitoring of ORP, contact time, off-gassing and species-specific tolerance.

Published thresholds are specific to species and salinity. In a flow-through trial with Atlantic salmon post-smolts of about 100 g in brackish water (12 ppt), Stiller et al. tested ORP levels of 250, 280, 350, 425 and 500 mV; 425 mV and above caused at least 33 percent cumulative mortality within 10 days, and gill damage was more marked from 350 mV. They concluded that exposure above 350 mV carries significant welfare consequences for brackish-water salmon. Lazado et al. (2026) ran ozonated brackish RAS units at 300–350 mV ORP, a range previously identified as safe for brackish-adapted Atlantic salmon, without loss of growth.

Staff safety and off-gas destruction

The FAO guide warns that in indoor farms staff may inhale too much ozone, so correct dosing, monitoring and proper ventilation are essential. Wikipedia notes that most people can smell ozone at about 0.01 ppm, that 0.1–1 ppm causes headaches, burning eyes and airway irritation, and that low concentrations degrade rubber and plastics. Corona discharge fed with humid air also forms nitrogen oxides. Undissolved ozone leaving contact chambers is destroyed in the gas phase mainly by catalytic decomposition over manganese dioxide or other transition metals; thermal decomposition is slow below 250 °C, and carbon-based adsorbent or catalyst filters can also remove ozone from air.