Ozone Transfer in Aquaculture

(To obtain more information on recirculating aquaculture systems, a complete set of tables, figures and references shown on this and related pages please refer to the book "Recirculating Aquaculture Systems, 2nd Edition", M. B. Timmons, et al, 2002, Cayuga Aqua Ventures, Ithaca, NY. You can obtain a copy of the publication from Cayuga Aqua Ventures by visiting their website at www.c-a-v.net.)

Ozone is generated within either air or oxygen feed gas and the ozone gas must be transferred into water for microbiological inactivation or other oxidative purpose. The ozone gas flow can be transferred into the water using any of the typical oxygen transfer devices (Summerfelt and Hochheimer, 1997), which have been discussed in Chapter 8 of this book. Effective transfer of ozone gas into water is important because the cost of producing ozone is not insignificant, especially if the ozone is carried within purified oxygen feed gas that was either purchased or produced on site.

Ozone transfer efficiency and the subsequent rate of ozone decomposition depend upon the efficiency of the contacting system used and the rates that ozone reacts with constituents within the water. The rate that ozone reacts depends on the type and concentration of constituents within the water. Rapid reaction with oxidizable inorganics and organics will maintain a low apparent equilibrium concentration of ozone within the liquid film and increase the rate of ozone transfer. The driving force for ozone transfer is maximized when the ozone absorbed is rapidly consumed by reaction with constituents within water. In fact, when ozone reacts very fast, ozone decomposes at the gas surface and no molecular ozone is transferred into the water (Bablon et al. 1991).

Ozone transfer units that have a continuous liquid-phase, i.e., units that disperse gas bubbles within a liquid, -- such as U-tubes, Speece cones, Fig. 12.2, aspirators, bubble diffusers, and enclosed mechanical surface or subsurface mixers -- provide both ozone transfer and some reaction time. Ozone transfer units that have a continuous gas-phase, i.e., units that disperse liquid drops and films within a gas, such as spray columns, packed columns, and multi-stage low head oxygenators, Fig. 12.3, provide efficient transfer but very little time for reaction (Summerfelt and Hochheimer, 1997). Continuous gas-phase transfer units are best suited for use in situations that normally require the transfer of the maximum amount of ozone in the shortest time for economical fixed and variable costs. On the other hand, continuous liquid-phase transfer units are usually selected for situations where reaction is rate limiting and an ozone residual must be maintained for a specific length of time (Bellamy et al. 1991).

Most ozone contactors rely on continuous liquid-phase units that bubble ozone into the liquid (Bellamy et al. 1991). High column bubble diffusers are frequently used for aquacultural applications and can achieve more than 85% ozone transfer to the liquid phase (Helge Liltved, unpublished data). These units are particularly well suited to situations where reaction is rate limiting and an ozone residual must be maintained for a specific length of time, such as during disinfection. Speece cones, Fig. 12.2, and U-tubes are also being used to efficiently and rapidly transfer ozone/oxygen feed gas within RAS, where oxidation of nitrite and organic matter are the primary goals of ozonation (not disinfection).

Ozone transfer within continuous gas-phase units is not as common as within continuous liquid phase units (Bellamy et al. 1991). When ozone transfer has been reported within continuous gas-phase units, they are mostly packed columns. However, continuous gas-phase units can be designed to efficiently transfer ozone within relatively smaller vessels. Ozone transfer efficiency was 100% in the LHO (Low Head Oxygenator) units evaluated in the recirculating system at The Freshwater Institute. In this system, complete ozone transfer occurred because ozone is 13 times more soluble than oxygen in water according to Henry’s law; short circuiting in the gas phase within the LHO was prevented by breaking the chamber into eight separate compartments; gas residence times within the LHO chambers were about 45 min; and, there was nitrite and dissolved and suspended organic material in the water that rapidly reacted with the dissolved ozone (Summerfelt and Hochheimer, 1997).

Ozone transfer into RAS is sometimes accomplished using the same gas transfer unit that is used for oxygen supplementation. This can be done if the transfer unit is fabricated from ozone resistant material (Bullock et al. 1997). In these situations, adding ozone to a recirculating system that is already using purified oxygen only requires installation of an ozone generator and the accompanying ozone distribution, monitoring, and control mechanisms (Summerfelt and Hochheimer, 1997). All of the other necessary equipment (oxygen supply and distribution system, gas transfer units, and control mechanisms) would already be in place, Fig. 12.3.

The off-gas discharged from the transfer unit will contain some ozone if ozone transfer is not 100% efficient. These ozone containing off-gas discharges may require treatment to destroy remaining ozone. Gas phase ozone destruct systems are available for this type of application based on thermal or thermal catalytic methods. Ozone destroyers (decomposers) for gas phase ozone removal can be found by following this link.