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

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.