Ozone in RAS: Nitrite, Colour, and Pathogen Control
Introduction
Recirculating aquaculture systems (RAS) treat and reuse most of their water, making them strong on water saving and biosecurity. But continuous recirculation also means fine particles, colour, organic load, and pathogens can accumulate. Ozone is one of the most widely used tools in RAS to manage this build-up, but when designed incorrectly it can harm both the fish and the biofilter.
This post looks at what ozone does in RAS, the risks it carries, and how a safe system is laid out.
Benefits of Ozone in RAS
Ozone is a strong oxidant and has several different effects in RAS water. It can help fine suspended particles clump together (microflocculation), increasing the amount mechanical filters can capture. It can oxidise the dissolved organic compounds that colour the water, clarifying it. It can help reduce nitrite, which is more toxic to fish, by oxidising it to nitrate. It also helps inactivate bacteria, viruses, and other pathogens, supporting biosecurity.
Risks: Why Isn't It Simply Added to Every RAS?
Ozone and its reaction by-products can damage fish gills even at very low concentrations. The ozone dosed into the water must therefore be fully consumed or removed before it reaches the fish tanks. Likewise, the nitrifying bacteria in the biofilter are sensitive to oxidants; if residual ozone reaches the biofilter, nitrification can be disrupted.
Seawater RAS carries an additional risk: ozone can react with bromide in the water to form toxic compounds such as hypobromous acid and bromate. In saltwater systems, ozone dosing and monitoring must therefore be managed much more tightly.
Safe Design: Side Stream, Contact, and Degassing
The common approach is to apply ozone not to the whole water flow but to a side stream making up only part of the system flow. A typical sequence: a portion of the water that has passed mechanical filtration is taken to an ozone contact line, ozone is dissolved via a venturi or similar injector, sufficient contact time is provided, then degassing and, where needed, activated carbon remove residual ozone before the water returns to the system.
Whether ozone is applied before or after the biofilter depends on system design and goals; in every case, though, the rule is the same: residual ozone must not reach the biofilter or the fish tanks.
Control: ORP and Dissolved Ozone Measurement
The safety of ozone in RAS depends on monitoring. ORP (oxidation-reduction potential) is a practical indicator that continuously shows the water's oxidation state and is commonly used to adjust ozone dose automatically, with the target value kept below the limit set for the species and water type (fresh or salt). Dissolved ozone measurement at the contact line outlet is an additional way to confirm residual ozone has really been removed.
Why Oxygen-Fed Ozone Is an Advantage
RAS facilities already have an oxygen generator. Using the same oxygen source to feed the ozone generator gives a higher and more stable ozone concentration than air feed and simplifies the equipment. Oxygen-enriched gas leaving the ozone contact line can also be put to use in the system.
The OCS Ozone Approach
At OCS Ozone, we design oxygen and ozone generators together, and with OCS AQUA PRIME we address aquaculture facilities' oxygenation and water quality needs in a single system. In RAS projects, we plan side-stream design, ozone dose, degassing, and ORP control specifically for the facility and species.
If you would like to evaluate ozone for your RAS facility, get in touch for a free technical assessment.