- Ozone transfer efficiency
- Leak-free design and construction
- Construction with ozone resistant materials
Materials used in an ozone treatment system must be highly resistant or inert to ozone. Use of improper materials can lead to erosion of the unit and cause dangerous and costly leakages. Such systems are not suitable for the long-term application of ozone and require on-going, high replacement costs. The generation of ozone in systems with substandard materials is also less efficient as ozone is lost as the materials of the reactor are oxidised. The use of some plastics, such as polyvinyl chloride (PVC) and polycarbonate is not recommended for long-term applications for this reason. Galvanised steel is also not recommended.
Stainless steel contact chambers and piping are recommended for use with ozone. Valves should be made of stainless steel, with gaskets and
membranes of Teflon® or similar.
Ozone can be applied continuously, as a series of treatments per day or as a single batch treatment per day. Application in most situations can be linked to the feeding strategy employed in the culture system. Three to four hours after feeding fish, the concentrations of ammonia, dissolved organics and other wastes products reach a maximum. If fish are fed several times during the day, a series of ozone treatments can be introduced after each feed to target the associated rise in waste levels. If feed is introduced 24 hours per day, water quality degrades continuously and so ozone application should be continuous. A single batch ozone treatment can be used to target rises in waste levels in the system associated with a moderate feed event or to treat batches of exchange or inlet water from the supply source.
Continuous ozonation is beneficial when compared to batch and serial treatments because water quality remains relatively stable. However, the lower costs of serial and batch ozonation make these treatments regimes viable management options.
The required amount of ozone for treatment in an RAS (Re-circulating Aquaculture Systems) is usually calculated according to the daily feed rate. Rates of 10-15 g of ozone per kilogram of feed are generally recommended to reduce accumulated organics. Any background organic loadings of the source water used for the RAS should also be taken into account.
If disinfection is the primary goal of ozonation, the amount of ozone necessary is largely dependent on the background organic loading of the water to be treated. In pure water, residual concentrations of 0.01-0.1 ppm ozone for periods as short as 15 seconds can be effective in reducing bacterial loads. However, in water with organic loadings the residual ozone concentration and/or contact time of ozone must be increased to produce significant disinfection. Natural waters (seawater, brackish and freshwaters) generally require residual concentrations of between 0.1-0.2 ppm ozone and contact times of 1-5 minutes for disinfection. Aquaculture effluent generally requires between 0.2-0.4 ppm residual ozone for 1-5 minutes for significant disinfection to occur after oxidation of organics.
The optimum rate of ozone for disinfection is highly variable and represents the sum of ozone demands from dissolved organics, colloidal solids, nitrate and disinfection. In many situations in RAS, the cost of production of sufficient residual ozone for complete disinfection after all other ozone demands are met is prohibitive. However, some reduction in pathogen loads can be achieved using moderate levels of ozone, and water quality improvements are considerable.
Disinfection of exchange and effluent water is more cost effective than treating the entire system due to the relatively small volumes treated. Disinfection of source water with ozone, in combination with quarantine procedures for incoming stock, reduces the risk of disease outbreak within the system.
Site of Application:
Ozone is reported to be toxic to a wide range of fresh and salt-water organisms at residual concentrations between 0.01 ppm and 0.1 ppm. When
deciding where to introduce ozone the effect of residual concentrations from the reactor on either the bio-filter or fish stocks should be
carefully considered. There are several locations in a RAS where ozone may be added depending on the desired outcome.