Effects of two different ozone doses on seawater recirculating systems for black sea bream Acanthopagrus schlegeli (Bleeker): Removal of solids and bacteria by foam fractionation

Foam fractionators, with and without additional ozonation, were evaluated for their effect on solids removal (suspended solids, SS; volatile suspended solids, VSS; dissolved organic carbon, DOC), particle size distribution of the foam in seawater recirculating systems during a 44-day experimental period. The effect of ozone on heterotrophic bacteria was also quantified in the entire system. Three separate but identical recirculating systems (4.5 m(3) system volume) with foam fractionators (300 mm in diameter, 3 m in height) were used in this study. One system (Control: CS) did not receive ozone, while the other two systems were ozonated at a rate of either 20 g ozone/day (T 20) or 40 g ozone/day (T 40) per kg of feed applied, respectively. A total of 107 kg of black sea bream Acanthopagrus schlegeli (Bleeker) with an average weight of 334.5 g was stocked into each system. Daily feeding rate was 1% of total body weight. The solids enrichment factor (EF = C-c/C-i, where C-c = concentration in foam condensate, C-i = concentration in inlet water) in T 40 was 10 times more dilute than the factors in CS and T 20. However, due to the higher volume of the foam (>10 times) in T 40, the removal rates of SS, VSS and DOC were the highest in T 40. but were not significantly different from T 20 (P > 0.05). The mean particle sizes at the 90% cumulative removal point decreased with ozonation, 71.2 +/- 15.9, 57.9 +/- 10.2 and 48.0 +/- 10.2 mu m in CS, T 20 and T 40, respectively. The overall mean particle diameter of solids in the foam decreased as ozonation increased, with values of 29.4 +/- 4.4, 23.9 +/- 3.8 and 20.5 +/- 3.7 mu m in CS, T 20 and T 40. Numbers of heterotrophic bacteria in the inlet were 6.21 +/- 4.93 x 10(5) CFU/mL, 0.29 +/- 0.19 x 10(5) CFU/mL and 0.30 +/- 0.29 x 10(5) CFU/mL in CS, T 20 and T 40, declining sharply with the addition of ozonation. As the number of the bacteria in the inlet decreased, the bacteria in the foam and the removal rate greatly decreased with increasing ozonation. However, EFs in T 20 (76.4) and T 40 (14.5) were higher than that in CS (12.2), and the T 20 showed significantly higher EF (P < 0.05). Therefore, based on the EF, ozonation improved the removal efficiency of heterotrophic bacteria, even at the lowest concentration. (C) 2010 Elsevier B.V. All rights reserved.