Inactivation of bacteria using ultraviolet irradiation in a recirculating salmonid culture system

The objective of this research was to determine the ultraviolet (UV) irradiation dosages required to inactivate bacteria in a commercial-scale recirculating salmonid culture system. Research was conducted in the commercial-scale recirculating system used for Arctic char growout at the Conservation Fund Freshwater Institute (Shepherdstown, West Virginia). This recirculating system uses a UV channel unit to treat 100% of the 4750 L/min recirculating water flow with an approximately 100-120 mW s/cm2 UV irradiation dose. However, a second UV irradiation unit was operated at a constant intensity to treat a side-stream flow of water pumped from the commercial-scale recirculating system's low head oxygenator (LHO) sump. The side-stream water flow ranged from 0.15-3.8% (i.e., 7-180 L/min) of the entire recirculating flow so as to regulate the water retention time (i.e., from 3-70 s) within the UV irradiation unit and thus produce a range of UV irradiation doses (mW s/cm2). UV irradiation doses of approximately 75, 150, 300, 500, 980, and 1800 mW s/cm2 were applied to determine the dose required to inactivate total heterotrophic bacteria and total coliform bacteria. Total heterotrophic bacteria counts and total coliform bacteria counts were measured immediately before and immediately after the side-stream UV irradiation unit. Total heterotrophic bacteria in the recirculating system required a UV dosage in excess of 1800 mW s/cm2 to achieve a not quite 2 LOG10 reduction (i.e., a 98.0 ± 0.4% reduction). In contrast, total coliform bacteria were more susceptible to UV inactivation and complete inactivation of coliform bacteria was consistently achieved at the lowest UV dose applied, i.e., at approximately 77 mW s/cm2. These results suggest that: (1) the UV dose required to inactivate total heterotrophic bacteria-and thus disinfect a recirculating water flow-was nearly 60 times greater than the 30 mW s/cm2 dose typically recommended in aquaculture and (2) inactivating 100% of bacteria in a given flow can be difficult, even at excessive UV doses, because UV irradiation cannot always penetrate particulate matter to reach embedded bacteria. We present a hypothesis that the recirculating system provided a selection process that favors bacteria that embed within particulate matter or that form bacterial aggregates that provides shading from some of the UV irradiation, because the bacteria in the recirculating water were exposed to approximately 100-120 mW s/cm2 of UV irradiation every 30 min.