Dissolved ozone destruction using ultraviolet irradiation in a recirculating salmonid culture system

The objective of this research was to determine the ultraviolet (UV) irradiation dosages required to destroy dissolved ozone in a commercial-scale recirculating salmonid culture system operated at a constant 13–15 °C. Research was conducted in the recirculating system located at the Conservation Fund Freshwater Institute (Shepherdstown, West Virginia), which contains a UV channel unit to treat 100% of the 4750 L/min recirculating water flow with an approximately 90 mW s/cm2 UV irradiation dose. However, the majority of ozone destruction data was collected using a second UV irradiation unit that was used to treat a side-stream flow of water pumped from the commercial-scale recirculating system’s LHO sump. The water flow in this side-stream was adjusted to 85, 170, 255, and 330 L/min (i.e., approximately 1.8–7.4% of the entire recirculating flow) so as to produce a range of different water retention times within the UV irradiation unit (i.e., 6.7, 3.3, 2.2, and 1.7 s, respectively) and thus produced UV irradiation doses of 153.3 ± 2.1 mW s/cm2, 80.4 ± 2.6 mW s/cm2, 49.3 ± 0.6 mW s/cm2, and 35.6 ± 0.3 mW s/cm2, respectively. The results show that dissolved O3 removal across the UV irradiation unit could be modeled using first order kinetics and was dependent upon the inlet O3 concentration and the retention time within the irradiation chamber. At a temperature of 13–15 °C, UV irradiation doses of 80.4 ± 2.6 mW s/cm2 and 153.3 ± 2.1 mW s/cm2 consistently removed 100% of the dissolved O3 when the inlet O3 concentration was ≤0.30 mg/L. A UV irradiation dose of 49.3 ± 0.6 mW s/cm2 consistently removed 100% of the dissolved O3 when the inlet O3 concentration was ≤0.10 mg/L. A UV irradiation dose of 35.6 ± 0.3 mW s/cm2 could not remove 100% of the dissolved O3 even at inlet O3 concentration of ≤0.10 mg/L. When dissolved O3 data was averaged for each UV dosage applied, then approximately 91 ± 2%, 81 ± 5%, 77 ± 1%, and 58 ± 5% of the dissolved O3 was removed when passed through UV dosages of 153.3 ± 2.1 mW s/cm2, 80.4 ± 2.6 mW s/cm2, 49.3 ± 0.6 mW s/cm2, and 35.6 ± 0.3 mW s/cm2, respectively. Note that the mean dissolved O3 concentrations entering the UV unit were 0.64 ± 0.09 mg/L, 0.51 ± 0.10 mg/L, 0.41 ± 0.06 mg/L, 0.43 ± 0.07 mg/L, respectively, for the conditions just described.