Ozone and UV in public aquariums: treating water without exposing animals

In a public aquarium, ozone and ultraviolet treatment can support water clarity and reduce the circulation of selected microorganisms. They do not turn a complex life-support system into a sterile environment. Performance depends on what actually reaches the reactor: flow, particles, dissolved organic matter, colour, salinity and delivered dose. Safety also depends on what leaves it. A useful treatment barrier must therefore be measured as a complete chain, from water preparation through to the protection of animals and staff.

Separate two complementary processes

Ozone is a powerful oxidant. It can alter dissolved organic matter, promote microflocculation, improve colour and contribute to microbial inactivation. Its chemistry continues after injection while reactive species remain. That persistence can support treatment but creates risk if water returns to animals with harmful residual oxidants.

UV acts within an irradiation chamber. Its effect depends on fluence—the intensity received over exposure time. Water does not retain a disinfectant residual once it leaves the unit. Downstream contamination, pipe biofilm or an untreated bypass can therefore defeat the barrier.

The technologies do not automatically compensate for one another. Ozonation may improve UV transmission by changing coloured or particulate material, while also generating reaction products. Process order and injection points must be selected for the individual system. An ozone–UV combination needs validation as one operating train, not as two nominal ratings added together.

Validate what the UV reactor actually receives

An illuminated lamp does not demonstrate delivered fluence. Sleeve fouling, lamp ageing, poor hydraulics, turbidity and declining UV transmittance all reduce performance. Maximum flow is especially important because faster passage shortens exposure. Credible validation therefore links flow, transmittance, lamp condition and a defined microbiological objective.

Work by Summerfelt and colleagues illustrates how full-flow disinfection requires deliberate hydraulic and operational design. In an aquarium, teams should identify priority circuits: return to a sensitive collection, quarantine water, make-up water or effluent. Treating a small side stream may gradually improve the overall water body without ensuring that an agent shed near one animal is intercepted before reaching another enclosure.

Daily records can include flow, lamp status, operating hours, alarms and UV transmittance where instrumentation is available. Periodic microbiological measurements may confirm a trend but should not be overextended. One indicator count cannot represent every virus, bacterium, fungus and parasite relevant to a mixed collection.

Control ozone and its residual chemistry

Oxidation–reduction potential is useful operational information, but it is neither a direct ozone concentration nor proof of disinfection. It responds to multiple redox couples and changes with the water matrix. A universal ORP setpoint cannot be transferred safely from freshwater to marine systems or between facilities.

In seawater, ozone reacts with naturally occurring bromide and can generate brominated oxidants and bromate. Tango and Gagnon examined this chemistry in a marine recirculating system. Rebelo and colleagues reported genotoxic and haematological alterations in turbot under their experimental exposure conditions. These studies do not provide one safe dose for all species. They show why the absence of immediate mortality is insufficient validation.

Design should prevent gas and residual oxidants from reaching animals: adequate contact, degassing, off-gas destruction, polishing where necessary, and interlocks tied to water flow. Sampling points should represent untreated water, post-contact water and the final return to livestock. An alarm must stop ozone delivery and move the system toward a safe state.

Design around the most sensitive animals

Large aquariums may hold fish, elasmobranchs, invertebrates, corals, larvae and essential biofilter communities. Their tolerance of oxidants is not uniform. Treatment should not be set solely for the most robust species or the visual clarity expected by visitors.

Operational surveillance can include ventilation rate, avoidance of return flows, gill irritation, appetite and unusual behaviour. These signs are non-specific, so they need alignment with life-support records, maintenance activity and ozone operating periods. A fouled filter, stopped pump, replaced carbon bed or new UV lamp can change exposure even when the nominal setpoint remains unchanged.

Biofilters also require protection. Excess oxidation or an abrupt change in organic loading may affect microbial communities that perform nitrogen transformations. Ammonia, nitrite, oxygen, pH and temperature should be followed closely after major treatment changes.

Make operation auditable

A robust procedure defines normal operation, maintenance, failure and restart modes. It states who may change setpoints, how sensors are checked, how often UV sleeves are cleaned, how off-gas destruction is verified and which limits trigger diversion or shutdown. Historical data allow an animal event to be compared with the actual equipment state rather than a remembered routine.

Biosecurity never relies on these processes alone. Quarantine separation, dedicated equipment, diagnostic sampling, carcass management and controlled water transfers remain essential. Ozone and UV reduce probability; they cannot repair a hidden hydraulic connection or an unmanaged introduction.

Commissioning should include challenge or surrogate testing suited to the objective, verification under worst credible flow and water quality, and confirmation that bypass valves and alarms behave as documented. Revalidation is appropriate after hydraulic changes, lamp or reactor replacement, and major changes to the collection or feed load.

Vetofish can help define biological requirements, prioritise treatment circuits, place meaningful monitoring points and connect maintenance records with health indicators. The aim is not maximum treatment power. It is a documented, stable and fail-safe barrier whose microbiological benefit remains compatible with every collection’s health and welfare.