Part-day ozonation for nitrogen and organic carbon control in recirculating aquaculture systems

Recent work on ozone application in recirculating aquaculture systems (RAS) gives guidelines as to the daily ozone dose as a function of feed rates, but does not indicate whether the ozone should be added continuously throughout the day or in shorter, more intense doses. This study examined the effect of adding the same total amount of ozone (15 g ozone per kg feed) 24, 12, and 6 h per day, compared with a control (no ozone). The ozone treatments significantly reduced (P < 0.05) total organic carbon (TOC), turbidity, and total ammonia nitrogen (TAN) compared with the control, but surprisingly did not significantly reduce average nitrite concentrations. Nitrite, TAN, and TOC were tracked hourly for several days during each run. TOC drops by about 2-4 mg/l during the first 6 h alter feeding, and then rises steadily for the next 12 h. The different ozonation regimes did not significantly affect this trend. Ammonia concentrations rise to a peak of almost 1 mg/l higher than pre-feeding about 7 h after feeding, before falling back to baseline concentrations about 12 h after feeding. Ozonation extended the duration (P<0.1) of the elevated ammonia concentrations. Nitrite concentration peaks about 9 h after feeding. The amplitude of the peak is about 0.25 mg/l for the control and the continuous ozone treatment, but only 0.15 mg/l for the part-day treatments. Sharp decreases in TOC and nitrite were noticeable (though not statistically significant) during the periods of most intense ozonation (2 g ozone per h during the 6 h per day treatment).