Minimization of THM formation in seawater-fed recirculating aquaculture systems operated with electrochemical NH₄ ⁺ removal

Replacing bio-filtration with an electro-chemical method for treating water within recirculating aquaculture systems (RAS) has the potential to solve various obstacles stemming from the very existence of the bio-filter. A recently developed RAS operational approach relies on electrooxidation of seawater for in-situ production of chlorine, which is used for simultaneous ammonia oxidation and water disinfection. While being the most commonly used disinfectant, the reactions between chlorine and organic matter have the potential to result in high amount of carcinogenic by-products, particularly in seawater, which contains a high Br⁻ concentration. Trihalomethanes (THM) are the major by-products of seawater chlorination. More specifically, highly-brominated THM have been reported to bio-accumulate in fish tissues. The current work focused on the implications of operating the new approach on the formation and accumulation of THM both in the rearing water and the fish tissues. The role of bromine/chlorine/organic matter interactions leading to THM formation the under unique RAS conditions are described and quantified. THM concentrations developing under the system's conditions were found low, likely due to the particular characteristics of the organic matter present in the system. Low concentrations of THM precursors were recorded in the physicochemical RAS operating in the absence of a biofilter, stemming from the low microbial activity and the continuous breakdown of the organic matter by chlorine. A method based on SPME separation followed by GC/MS analysis was developed for quantifying THM in the fish tissues. The bioconcentration factor (BCF) of bromoform and dibromochloromethane in the tissues amounted to 1.35 and 0.0 (mg/kg)/(mg/l) in the fish muscle and 0.23 and 0.91 (mg/kg)/(mg/l) in the liver, respectively. Assessment of the THM content in the fish tissues revealed that fish grown in the system meet both EPA regulation for edible fish and recommendations for THM oral exposure.