Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2004
Publication Date: 8/11/2005
Citation: Sharrer, M.J., Summerfelt, S.T., Bullock, G.L., Gleason, L.E., Taeuber, J. 2005. Inactivation of bacteria using ultraviolet irradiation in a recirculating salmonid culture system. Aquacultural Engineering. 33:135-149. Interpretive Summary: Recirculating aquaculture systems may require an internal disinfection process to control the amplification of pathogens and other microbial populations. We evaluated use of UV irradiation for controlling and mitigating microbial populations and certain water quality parameters within intensive fish farming systems. Research results determined the UV irradiation dose required to inactivate total heterotrophic and total coliform bacterial populations using a side-stream of water flow from an operating high-density aquaculture recirculating system. The results are significant in observing that the UV irradiation dose apparently required for effective inactivation of certain bacterial populations in recirculating aquaculture system water may be nearly two orders of magnitude higher than conventional design recommendations for non-reuse applications. This finding is important for biosecurity and product quality planning.
Technical Abstract: 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 4,750 L/min recirculating water flow with an approximately 90 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 waterflow 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 sec) 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 1,800 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 1,800 mW.s/cm2 to achieve a not quite 2 LOG10 reduction (i.e., a 98.00.4 % 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/cm2dose 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 minutes.