Why Use Ozone in Aquaculture?
Use Ozone in aquaculture water treatment projects for the following purposes:
Removal of fine and colloidal solids
Fine and colloidal solids consist of particles 1-30 microns (m m) and 0.001-1 m m respectively. The small size of the particles enables the solids to remain in suspension and avoid most mechanical methods of separation. The accumulation of fine and colloidal solids can impair
bio-filter nitrification efficiencies and stress fish stocks.
Ozone removes fine and colloidal solids by causing clumping of the solids (microflocculation), which facilitates removal by foam-fractionation, filtration and sedimentation.
Removal of dissolved organic compounds
Dissolved organic compounds (DOC's) or refractory organics, give the water a characteristic tea-colored stain. DOC's are non-biodegradable and accumulate according to feed input, water exchange rate and the rate of solids removal. High levels of DOC's can stress fish and reduce nitrification efficiencies of the
bio-filter. Ozone removes dissolved organics by:
- Oxidation into products that are more readily nitrified in the bio-filter
- Including precipitation, which enables removal of waste particles by conventional filtration or sedimentation
Removal of Nitrite
Nitrite can accumulate as production intensifies and organic loadings on the bio-filter increase. Bacteria that process ammonia into nitrite (Nitrosomonas spp) operates more efficiently under high organic loadings than bacteria that process nitrite to nitrate (Nitrobacter) and levels of nitrite rise accordingly. High levels of nitrite can be toxic to fish. Data available for silver perch, Bidyanus bidyanus indicates levels of nitrite as low as 2.8 parts per million (ppm) can reduce growth of fingerlings by 5%. Ozone removes nitrite by:
- Direct oxidation to nitrate
- Reducing organic loading, which improves bio-filtration efficiency and nitrification
Disinfection
The high stocking densities, associated fish stress and increased nutrient loads found in RAS create an ideal environment for fish pathogens. An important step in reducing the risk of disease outbreaks in RAS is the use of standard quarantine procedures for any fish introduced. Facilities using surface waters, including RAS and flow-through hatchery systems, are also interested in reducing the pathogen load introduced via the source water. The disinfection of effluent waters before introduction to the environment is also crucial to prevent the translocation of exotic diseases.
Ozone can effectively inactivate a range of bacterial, viral, fungal and protozoan fish pathogens. The effectiveness of ozone treatment depends on ozone concentration, length of ozone exposure (contact time), pathogen loads and levels of organic matter. If high levels of organic matter are present, the demand created by oxidizing the organic matter can make it difficult to maintain enough residual ozone for effective disinfection.
