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

摘要

a- 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. a- Application of ozone at 20g ozone/day improved solids removal and caused a significant decrease in bacteria counts. a- Foam generation was suppressed during the day because of oils in the fish diets so foam generation peaked several hours after the last daily feeding. 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 (4.5m3 system volume) with foam fractionators (300mm in diameter, 3m 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 20g ozone/day (T 20) or 40g ozone/day (T 40) per kg of feed applied, respectively. A total of 107kg of black sea bream Acanthopagrus schlegeli (Bleeker) with an average weight of 334.5g 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, 71.2A plus or minus 15.9, 57.9A plus or minus 10.2 and 48.0A plus or minus 10.2I14m 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 29.4A plus or minus 4.4, 23.9A plus or minus 3.8 and 20.5A plus or minus 3.7I14m in CS, T 20 and T 40. Numbers of heterotrophic bacteria in the inlet were 6.21A plus or minus 4.93A-105 CFU/mL, 0.29A plus or minus 0.19A-105 CFU/mL and 0.30A plus or minus 0.29A-105 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.