Ozone Destruction 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.)

Creating an adequate level of residual ozone at the end of the contact chamber to ensure kill-off of bacteria will also necessitate that this same ozone be removed prior to the water reaching the aquatic organisms. Ozone residuals can be lethal to fish at ozone concentrations as low as 0.01 mg/L, but the actual concentration depends upon species and life stage, Table 12.2.(Shown Below)

Due to the acute toxicity of residual ozone to aquatic animals (Wedemeyer et al. 1979), a de-ozonation unit has to be included. In many cases, residuals are eliminated by water retention within tanks immediately after ozonation, Fig. 12.4, or by applying small doses of a reducing agent, e.g., sodium thiosulphate. Dissolved ozone can also be stripped into air when passed through a forced-ventilation packed aeration column, Fig. 12.4.

Air stripping will also remove dissolved oxygen concentrations that are in excess of saturation, which may or may not be desirable. Dissolved ozone can also be destroyed by passing the water through a biofilter or bed activated carbon, reaction with low levels of hydrogen peroxide, or contact with high intensity UV-light (catalyzing the conversion of O3 to O2).

Achieving ozone destruction with UV electromagnetic radiation depends on the wavelength of the UV light source and the quantity of energy transmitted (Rodriguez and Gagnon, 1991; Hunter et al. 1998). Ozone residuals are destroyed at UV light wavelengths ranging from 250 to 260 nm. Ironically, UV wavelengths of 185 nm can be used to generate ozone. The primary ozone treatment components are located on the process pipeline immediately after ozone gas has been transferred into the water. First, the right hand vessel provides 10 minutes (at 1600 L/min flow) of plug flow contacting to achieve disinfection. Next, the middle vessel provides 20 minutes (at 1600 L/min flow) of plug flow contacting to achieve further disinfection and ozone residual destruction. Finally, the left hand vessel (a counter-current, forced-ventilation column) strips any minor ozone residuals and elevated dissolved oxygen levels immediately before the flow is piped to the fish culture systems

Ozonation by-products and their toxicity towards aquatic animals are not well described, especially in seawater. More long-lived reaction products are formed when brackish and seawaters are ozonated. Here, ozone reacts with bromide ions, and to a lesser extend with chloride ions, to form oxidants toxic to fish and shellfish. The most important are hypobromous acid (HOBr) and hypobromite ion (OBr-). Both have strong biocidal effects. By prolonged ozonation hypobromite ion can be further oxidized to bromate (BrO3-), which is a persistent compound. In addition, small amounts of halogenated organic compounds like bromoform will be formed. Activated carbon filtration has been successfully used for removal of residual ozone and other oxidants in ozonated seawater (Ozawa et al. 1991).

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.

Species Ozone Concentration mg/l Effect
Rainbow Trout 0.0093 96-h LC50a
Rainbow Trout 0.01-0.06 Lethal
Bluegill 0.01 60% Mortality after 4 Weeks
Fathead minnow 0.2-0.3 Lethal
White Perch 0.38 24-h LC50
Bluegill 0.06 240h LC50
Striped Bass (larvae) 0.08 96-h LC50