Efficacy of H2O2 on the removal kinetics of H2S in saltwater aquaculture systems, and the role of O2 and NO3−

Abstract

The occurrence of hydrogen sulfide (H2S) represents a challenge for recirculating aquaculture systems (RAS) under saline conditions. Even low concentrations of the toxic gas can result in sudden mass mortalities of fish, leading to large economic losses. There is an urgent need for efficient strategies to remove H2S, which can be applied effectively with a short response time, to prevent the risk of H2S-induced casualties. This study examines the kinetics of the two common oxidants applied to rearing water in a RAS facility; oxygen (O2) and hydrogen peroxide (H2O2) and evaluates their efficiency and applicability for the removal of H2S in an industrial RAS. Furthermore, we tested whether nitrate (NO3−) can be an oxygen donor in the chemical oxidation of H2S. The baseline oxidation rates of H2S by O2 were determined in air-equilibrated seawater (SW) and RAS water (RASW). The feasibility of using H2O2 as a practical treatment was evaluated by testing increasing H2O2 to H2S ratios in SW. In addition, RASW dilutions that yielded different concentrations of NO3− and total chemical oxygen demand (TCOD) were tested to identify their effects on H2S removal. The half-lives (t½) of H2S, derived from O2 oxidation rates, were considerably shorter in SW (118.5 ± 28.6 min) compared to RASW (168.0 ± 18.7 min). The addition of a 1:1 mole ratio of H2O2 to H2S, significantly increased the removal rate and decreased the half-life (t½) of H2S in SW to 29.5 ± 6.6 min. Further increasing H2O2:H2S ratios to 2:1 and 4:1, reduced t½ to 21.7 ± 5.2 and 17.4 ± 6.1 min, respectively. Similarly, a dosage of H2O2 at a ratio of 1:1 in RAS water resulted in a considerably shorter t½ of 86.1 ± 10.1 min. The influence of organic matter on the required H2O2 dose was demonstrated by the t½, which were reduced by 49% in RAS water and 75% in SW. NO3− was not found to be involved in the chemical removal of H2S. The results provide an improved understanding of the influence of RAS water chemistry and quality on H2S kinetics and the direct applicability of the kinetics for treating acute H2S levels in RAS to avoid mass mortalities. In conclusion, the addition of H2O2 is an efficient water treatment technology for H2S removal, and by adjusting H2O2 dosages accordingly to the concentrations of H2S and specific systems water parameters, a t½ <30 min can be achieved. Thus the technology is applicable in an industrial RAS, as a treatment process for acute levels of the hazardous gas H2S that is easily implemented, and safe for the fish.