Introduction
Water treatments and therapeutants applied in recirculating aquaculture systems (RAS) are complicated by potential impacts on critical nitrifying bacterial populations in biofilters (Noble & Summerfelt, 1996), specifically reduced oxidation of total ammonia nitrogen (TAN) and nitrite-nitrogen. Hydrogen peroxide (H2O2), while considered environmentally benign due to non-toxic end-products (Block, 1991), is known to disrupt biofiltration when applied at therapeutic concentrations in RAS (Fredericks, 2015). Previous research has assessed low-dose H2O2 applications in RAS to avoid biofilter impacts (Møller et al., 2010; Pedersen et al., 2012); however, the ability for RAS producers to use therapeutic or disinfection-level H2O2 concentrations in culture tank static bath treatments, followed by decomposition of H2O2 prior to resumption of recirculating water flow through biofilters, would be advantageous. We therefore sought to assess H2O2 decomposition using a novel enzymatic product following static bath applications of H2O2 at both therapeutic- and disinfection-level concentrations.
Materials and methods
Two single-day experiments were conducted to assess the effectiveness of a new catalase enzyme product, BioRas® Balance, in the decomposition of H2O2 into H2O and O2, applied as a static bath at either therapeutic- or disinfection-level concentrations. The first trial (Trial 1) employed simulated static bath treatments for bacterial gill disease in replicated culture tanks, with live fish present, followed by application of BioRas® Balance in a range of concentrations to assess dose-response decomposition of H2O2 over time. Trial 2 was a similar dose-response experiment, but employed disinfection-level H2O2 concentration with no live fish present. Trial 1. Diploid Atlantic salmon Salmo salar were received as fertilized eggs and hatched onsite using a temperature-controlled incubation system, and then cultured to market-size in freshwater RAS. To begin the study, 216 fish (5 kg mean weight) were stocked randomly into 12 replicated partial recirculating aquaculture systems (PRAS). Each PRAS consisted of a 5.0 m3 dual-drain tank, a gas conditioning column with counter current forced air ventilation, and a low head oxygenator (LHO) with a sump. A 0.4 horsepower pump, located in the sidewall box of each tank, continuously recirculated 379 L/min of water. Biomass densities at the start of the experiment were approximately 20 kg/m3. PRAS were operated at a 90% recirculation rate (on a flow basis), and under these conditions typical water quality parameter values for both studies were as follows: temperature (14 oC), pH (7.8), alkalinity (277 mg/L), CO2 (4.5 mg/L), TAN (0.2 mg/L), and total suspended solids (1.0 mg/L). Following acclimation, 60-minute static bath tank treatments were carried out, simulating therapeutic treatment for bacterial gill disease (100 mg/L H2O2 applied for approximately 60 minutes’ duration). BioRas® Balance was then added to the culture tanks to create concentrations of 1.5, 2.5, 5.0, and 10.0 mg/L (target concentrations were selected based on previous unpublished research); each tank was randomly assigned one of these BioRas® Balance concentrations, for a total of three replicates per treatment concentration. Following the addition of the catalase enzyme, water samples were collected and H2O2 concentrations analyzed at 10 minute intervals, until H2O2 concentration within a given tank was less than 0.5 mg/L. Trial 2. The second trial was carried out to simulate a typical disinfection event using a higher concentration of H2O2 (i.e., 250 mg/L initial target concentration). Dosing of H2O2 and BioRas® Balance, as well as H2O2 concentrations assessments, were carried out in an identical manner to the procedures used in Trial 1. The only major differences with Trial 2 were that i) water continued to recirculate within the PRAS (as opposed to a culture tank static bath), and ii) no fish were present in the culture tank. Initial and final H2O2 concentrations during the pre-catalase, disinfection phase were 261.0 ± 15.3 mg/L and 244.9 ± 14.1 mg/L, respectively.
Results
In both trials, enzymatic decomposition of H2O2 followed a dose-response relationship, catalyzing a first order decay reaction. First-order decay constants and kinetic reaction formulae for each BioRas® Balance application concentration were calculated using least-squares regression of the log-transformed H2O2 concentration data; reaction rate constants calculated for Trial 1 were in close agreement to those calculated in Trial 2, indicating that the reaction rate constant may be independent of the initial concentration of H2O2. Overall, the results of these experiments are encouraging for RAS farmers who may require utilizing H2O2 at concentrations that would normally compromise biofilter nitrification processes. Under the conditions of our studies, BioRas® Balance was effective at quickly reducing H2O2 to safe concentrations, in either post-static bath therapeutic application or post-system disinfection scenarios.
References
Block, S.S. (1991). Peroxygen compounds. In: Block, S.S (Ed.), Disinfection, Sterilization, and Preservation (4th ed.). Lea & Febiger, Philadelphia, ISBN0-683-30740-1.
Fredricks, K.T. (2015). Literature Review of the Potential Effects of Hydrogen Peroxide on Nitrogen Oxidation Efficiency of the Biofilters of Recirculating Aquaculture Systems (RAS) for Freshwater Finfish. Open-File Report, Reston, VA, p. 30.
Møller, M.S., Arvin, E., & Pedersen, L.-F. (2010). Degradation and effect of hydrogen peroxide in small-scale recirculation aquaculture system biofilters. Aquaculture Research, 41, 1113–1122.
Noble, A.C., & Summerfelt, S. T. (1996). Diseases encountered in rainbow trout cultured in recirculating systems. Annual Review of Fish Diseases, 6, 65-92.
Pedersen, L.-F., Good, C.M., & Pedersen, P.B. (2012). Low-dose hydrogen peroxide application in closed recirculating aquaculture systems. North American Journal of Aquaculture, 74, 100–106.