Effects of Ozone Concentration on the Activity of Biofilm and Its Effectiveness of Purification in Recirculating Aquaculture Systems

Abstract:

Ozone (O3) is a strong oxidizing agent that has been widely used in factory recirculating aquaculture systems. The proper concentration is a key to the utilization of ozone. Ozone at inappropriately high concentration could impair the growth, behavior, and health of farmed fish. Furthermore, ozone at high concentration also affects the biofilm in the biofilter, which could reduce the efficiency of water treatment in the recirculating aquaculture systems (RAS). In this study, ozone was added in RAS at different concentrations, and the subsequent effects on the activity and purification effectiveness of biofilm were tested. It was shown that when the Oxidation-Reduction Potential (ORP) was lower than 450 mV, the efficiency of ammonia nitrogen treatment was increased along with the elevation in ozone concentration. Under this condition the highest removal rates of ammonia nitrogen and nitrite nitrogen reached 39.9% and 28.2% respectively, and the survival rate of bacteria in the biofilm was 88.1%. This suggested that ozone did not affect the efficiency of the biofilter as ORP was below 450 mV. When ORP was 500 mV, after 24-hour treatment with ozone the removal efficiency was decreased from 36.5% to 12.2% for ammonia nitrogen, and from 28.1% to 8.4% for nitrite nitrogen. When ORP was 550 mV, 4-hour ozone treatment reduced the efficiency from 47.5% to 5.0% for ammonia nitrogen, and from 32.1% to 3.3% for nitrite nitrogen. This implied that higher ORP significantly reduced the efficiency of water treatment. The survival rate of bacteria in the biofilm dropped from 88.1% to 31.5%. Our study indicated that the effectiveness of water treatment and the survival rate of bacteria could be greatly affected by ozone. To ensure the safety and cost efficiency of RAS, we recommended that the concentration of ozone added in RAS should be controlled to keep ORP in the biofilter under 400 mV.