Ozonation of a recirculation rainbow trout culture system

II. Effects on microscreen filtration and water quality

Steven J. Summerfelt, Joseph A. Hankins, Amy L. Weber, Martin D. Durant

Abstract

Ozone was added to water in a recirculating rainbow trout (Oncorhynchus mykiss) culture system just prior to the culture tanks in order to oxidize nitrite and organic material, improve overall water quality, and assist removal of solids across the microscreen. Data from four 8-week studies on ozonation and an 8-week no ozone control indicated that adding ozone reduced the mean concentration of TSS by 35%, COD by 36%, DOC by 17%, and color by 82% within the water entering the culture tanks. Additionally, ozone reduced the mean nitrite concentration 82% within the culture tanks. Adding ozone did not affect turbidity. Changes brought on by ozonation, particularly as it affected the characteristics of the suspended solids, also improved suspended solids removal across the Triangle ™ filter by an average of 33%. In addition, adding ozone decreased plugging of the microscreen filter panels; as indicated by an average of 35% fewer filter wash cycles, 53% less filter sludge flow produced, and 79% more settled solids volume in the Triangle ™ filter effluents. Comparison of two different ozone dosing rates indicated that adding Ozone to our recirculation system at a rate of 0.025 kg ozone per kilogram feed was nearly as effective as adding ozone at a rate of 0.036-0.039 kg ozone per kilogram feed.

Figure 1: Illustration of Triangle ™ filter’s working mechanism.  Water passes through the sieve panel while filtrate solids are removed.  At a certain point, the water dripping through the sieve activates an optical switch that turns on the backwash mechanism that clear solids from the sieve and washes them into a collection trough.

Conclusion

The results indicate that addition ozone to our recirculating system at a rate of 0.025 kg ozone per kilogram feed improved water quality, supported microscreen filtration, and according to the data in the accompanying paper (Bullock et al., 1997), reduced bacterial gill disease associated mortalities and chemical treatments required to control BOD epizootics. Adding ozone at a higher rate (0.036-0.039 kg ozone per kilogram feed) produced similar results that was much more likely to produce fish mortality, when on occasion ozone accumulated to toxic levels (Bullock et al., 1997). Since ozonation equipment is expensive, it is rational to add ozone at the lowest effective rate necessary to achieve the desired results. Adding ozone at the lower rate is also justified to reduce potential for fish to be exposed to ozone, particularly when little hydraulic retention time is available between the fish culture tank and the ozone transfer point.

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