Using Ozone with UV Disinfection for Effective Water Treatment - Tech Talk 136
Ozone is a powerful oxidant and is very beneficial with improving water quality in Aquatic Life Support Systems (ALSS). Ozone oxidizes NO2 & NO3, helping to remove color, dissolved organic wastes, and micro-flocculates fine organic particulate, as well as improving dissolved oxygen levels.
Ozone delivers a noticeable benefit in water quality, especially when applied to Recirculating Aquaculture Systems (RAS), which is why many system designers and operators are attracted to it.
At the same time, ozone also optimizes conditions for UV disinfection. Yes, both ozone and UV disinfection can work together within the ALSS to offer superior results, specifically, improved UV light transmittance, which increases the UV system’s effectiveness—boosting the germicidal disinfection capacity.
Ozone is not Instant Treatment, rather it requires that the target pathogen be exposed to specific concentrations for an extended period of time. For example: successful ozone disinfection in freshwater, targeting IPNV (Infectious Pancreatic Nacreous Virus), requires an ozone concentration of 0.15 mg/l for a period of 15 minutes (Cryer, 1992 #670). If this is the case, it means a Flow-Through or RAS with a water flow of 3,000 GPM 11,355 LPM) would require an ozone contact area large enough to effectively complete disinfection, and for most RAS, this is simply not practical.
UV disinfection is Instant Treatment and at a fluence (UV dose) of 246 mJ/cm², it achieves a 4-log (99.99%) reduction of the pathogen IPNV. This means that a UV system with a disinfection capacity large enough to treat 3,000 GPM (11,355 LPM) will successfully achieve germicidal disinfection on a single-pass inside the UV vessel. Therefore, UV disinfection is the best choice as a primary germicidal disinfectant.
An Effective Combination
OZONE (clarifier) and UV (disinfectant) well work together! Ozone, being the powerful oxidizer it is, improves the UV Transmittance (UVT) in culture water,helping the UV system, positioned downstream of the ozone treatment, to perform more effectively. For instance, testing an RAS for UVT may reveal a 75%T. With appropriate ozone treatment, that 75%T could be increased to 90%T. The increased %T, from 75%T to 90%T dramatically increases the efficiency of the downstream UV system, and in many cases, reduces the size of the required UV system. All ALSS are unique, and so we are forced to apply ozone and UV differently based on the specific conditions and requirements of each application.
TRO (Total Residual Oxidant) Alert
If bromide (Br) is present in the water (ground water or seawater), a reaction is possible with the by-product being hypobromous acid and hypobromite, which are harmful to aquatic life. Controlling ozone addition in seawater applications is necessary. There, ozone should only be used as a clarifier; enough to clarify (0.001 mg/l – 350 mv), and yet even with this super low concentration the formation of TRO’s (Total Residual Oxidants) remains possible.
The accepted method among the Aquaculture community to measure ozone concentrations, especially super-low concentrations (0.001 mg/l – 350 mv) is to use an ORP (Oxygen Reduction Potential) Meter. It must be mentioned that due to the unstable characteristics of ozone, ORP meters are not as reliable at measuring ozone concentrations as Ozone Analyzers. Unfortunately, few analyzers are available that can measure such super-low concentrations (below .05 mg/l). Therefore, in seawater applications (especially RAS), both an ORP meter/controller, to measure and maintain the low ozone concentration, and a Total Residual Oxidant (TRO) Meter, to measure and detect dangerous TRO’s, should be employed.
For more information regarding seawater and ozone see: “Ozone Application and Brominated By-Products: Monitoring, Formation, and Destruction in Water Recirculating Systems for Fish Culture” (Steven T. Summerfelt, Susan Clements, and Michael Gearheart).
As mentioned previously, all ALSS are unique; however, there is one important similarity, in that a residual concentration of ozone greater than 0.001 mg/l can be harmful to fish and invertebrates. Safety protocols must be established and closely followed. For example, an ozone concentration of 0.03 mg/l is effective at controlling Saprolegnia, but allowing that same ozone concentration to reach the eggs or fish would cause them harm. Reliable ozone control is required. Placing a properly sized UV system downstream of the 0.03 mg/l ozone treatment can provide reliable ozone destruct.
Ozone Destruct using UV:
Fluence of 60 mJ/cm² destructs ozone concentration of 0.1 mg/l
90 mJ/cm² destructs 0.3 mg/l
Automating ozone control to achieve reliability is recommended. Integrating the ozone generator operation with the UV system establishes a safety-firewall, protecting vulnerable livestock from potentially dangerous ozone. Applying a quality TRO Meter and implementing these control measures are especially important when applying ozone treatment to seawater systems.
LHO – O3 – UV
The greatest challenge working with ozone in ALSS is getting it into the culture water. Various methods exist that include: venturi injection, bubble contactors, and pressurized reactors. Larger systems with high bio-densities, and high flows, create additional challenges; many associated with cost. Shown here is a stacked CO2 stripper with an LHO (Low Head Oxygenator) tower. This arrangement, with the substitution of ozone for oxygen, combined with UV disinfection creates a powerful and efficient filtering combination.
- Mechanically filtered (minimum 60 µm) water enters the CO2 stripper where it is de-gassed, increasing pH and gas-exchange.
- Ozone injection takes place at the LHO; concentrations of up to 0.3 mg/l are possible.
- The tower’s sump provides additional contact-time for the ozone to work on remaining organic material in the culture water.
- Culture water exiting the tower is at its highest water quality within the ALSS.
- The properly sized UV system destructs any residual ozone while providing reliable and effective germicidal disinfection.
In this ALSS scenario, the ozone generator controls are interfaced into the UV system’s PLC. The UV system PLC’s integrated ORP Meter measures ozone concentration and, with set-points inputted, controls the operation of the ozone generator. Ultimately, if the UV system was to fail or an ozone (ORP) measurement exceeded the safety set-point, the UV’s PLC would shut down the ozone generator.