Effects of different levels of ozone on ammonia, nitrite, nitrate, and dissolved organic carbon in sterilization of seawater

Abstract

Ozone is applied to the recirculation aquaculture system to reduce bacteria and parasites. Besides the sterilization effects, it is known that ozone has oxidizing effects on some water quality parameters. Therefore, oxidizing effects of ozone on ammonia (NH4-N), nitrite (NO2-N), nitrate (NO3-N), and dissolved organic carbon were tested in this study. During the test, ozone effects on pH, dissolved oxygen (DO) and bromination were also monitored. Ozone concentrations were originally set to 0.05, 0.1, 0.15, 0.2, and 0.25 ppm, but actual treatment concentrations were maintained at 0.04, 0.11, 0.15, 0.19, and 0.23 ppm. The 5 ppm of NH4-N was oxidized within 12 h in all concentrations of ozone treatments, with the average oxidizing rate of 0.65±0.28 mg NH4-N/L per h. The 5 ppm of NO2-N was oxidized within 1.5 h in all concentrations of ozone treatments at a rate of 4.5 mg NO2-N/L per h. One of 5 ppm NO3-N was oxidized by all concentration of ozone treatment after 24 h. In addition, ozone also oxidized dissolved organic carbon and maintained the concentration at about 2.9±0.77 ppm from the 15 ppm of initial concentration by 12 h. DO was increased from 5.9 to 9.4 ppm within 30 min in all ozone treatment and stabilized thereafter. Bromate concentrations increased sharply within the first 6 h of ozonation at the rate of 7.3±2.4 mg/L per h in almost all ozone treatments; the rate decreased to 2.5±0.15 mg/L per h thereafter. However, bromate concentration was not increased in the ammonia experiment until all ammonia was oxidized. Therefore, further studies are needed to determine the relationship between NH4-N concentration and bromate formation in seawater.