Ozonation followed by ultraviolet irradiation provides effective bacteria inactivation in a freshwater recirculating system

Authors: SHARRER, MARK - FRESHWATER INSTITUTE; SUMMERFELT, STEVEN - FRESHWATER INSTITUTE

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2007
Publication Date: 9/1/2007
Citation: Sharrer, M.J., Summerfelt, S.T. 2007. Ozonation followed by ultraviolet irradiation provides effective bacteria inactivation in a freshwater recirculating system. Aquacultural Engineering. 37:180-191.

Interpretive Summary: Fish culture systems that recirculate water can require an internal disinfection process to control the accumulation of pathogens and other microbial populations. The objective of this work was to determine the levels of bacteria disinfection that can be achieved using ozonation alone or using the cumulative effect of ozonation followed by ultraviolet irradiation. Results indicate that when only ozone was applied, the total heterotrophic bacteria counts and total coliform bacteria counts in the water exiting the contact basin could be reduced but not eliminated. However, when ozonation was followed immediately with a modest dose of ultraviolet irradiation, the bacteria counts were reduced to almost zero. Thus, combining ozone with UV irradiation can be used to provide nearly complete bacteria inactivation within a water recirculating system, which would significantly reduce the risk of spreading fish disease.

Technical Abstract: Recirculating aquaculture systems may require an internal disinfection process to control population growth of pathogens and heterotrophic bacteria. Ozonation and ultraviolet (UV) irradiation are two technologies that have been used to treat relatively large aquaculture flows, including flows within freshwater systems that recirculate water. The objective of the present study was to evaluate the effectiveness of ozone application alone or ozone application followed by UV irradiation to reduce abundance of heterotrophic and total coliform bacteria in a water reuse system. Results indicate that when only ozone was applied at dosages (defined by the product of the ozone concentration times the mean hydraulic residence time (C*t) ) that ranged from 0.10 to 3.65 min*mg/L,the total heterotrophic bacteria counts and total coliform bacteria counts in the water exiting the contact basin were reduced to, respectively, 3-12 cfu/ml (1.1-1.6 LOG10 reduction) and 2-18 cfu/100 ml (1.9-3.1 LOG10 reduction). Bacteria inactivation appeared to be just as effective at the lowest ozone C*t dosage (i.e., 0.1 mg/L ozone after a 1 min contact time) as at the highest ozone C*t dosage (i.e., 0.2 mg/L ozone after a 16.6 min contact time). As with our previous research on UV inactivation of bacteria, we hypothesize that the recirculating system provided a selection process that favors bacteria that embed within particulate matter or that form bacterial aggregates that provides shielding from oxidation. However, when ozonation was followed by UV irradiation, the total heterotrophic bacteria counts and total coliform bacteria counts in the water exiting the UV irradiation unit were reduced to, respectively, 0-4 cfu/ml (1.6-2.7 LOG10 reduction) and 0-3 cfu/100 ml (2.5-4.3 LOG10 reduction). Thus, combining ozone dosages of only 0.1-0.2 min-mg/L with a UV irradiation dosage of approximately 50 mJ/cm2 would consistently reduce bacteria counts to near zero. These findings were orders of magnitude lower than the bacteria counts measured in the system when it was operated without disinfection or with UV irradiation alone. These findings indicate that combining ozonation and UV irradiation can effectively disinfect recirculating water before it returns to the fish culture tank(s).