Evaluation of Disinfectants in RAS Water

Recirculating aquaculture systems (RAS) are being adopted by many Norwegian aquaculture companies because they are considered more sustainable, provide high biosecurity, and have a more controlled environment. One of the key characteristics of RAS is water treatment and reuse. Despite having high biosecurity, if a pathogen gains access to the system or an opportunistic pathogen emerges, the water circulation can spread the pathogens which can lead to development of fish disease. The Norwegian Food Safety Authority (Mattilsynet) enlisted several disinfectants that are approved for use in aquaculture. Seven approved disinfectants were assessed in this experiment. These include three peracetic acid (PAA) based disinfectants (Addi Aqua, Aqua Des, and Perfectoxid), two quaternary ammonium compound (QAC) based disinfectants (Virkon Aquatic and Virocid), one hydrogen peroxide (H2O2) based disinfectant (Free Bac ®35) and, one chlorine dioxide (ClO2) based disinfectant (Life Clean). The efficacy of these disinfectants has been demonstrated in aquaculture surfaces and auxiliary equipment but their effectiveness in water disinfection is not fully understood yet. Therefore, the objective of this study was to test whether the above seven disinfectants had an impact on the ammonia, pH, and bacterial load of RAS biofilter water. Six small-scale biofilters were set up to simulate the MBBR in RAS. The cultured water and biomedia were collected from the research facility, Havbruksstasjonen i Tromsø, Kårvika. 8 groups of samples were created, 1 control (no disinfectants), and the other 7 were designated to treatment with the aforementioned disinfectants. The samples were taken in triplicates. The test was carried out in 35 days (1 replicate of each disinfectant carried out each week). The recommended dosage and contact time of the respective manufacturers were used in the experiment. Ammonium and pH levels were measured before and after the disinfectants were administered. Trypticase Soya Agar (TSA) and Cefsulodin-Irgasan-Novobiocin (CIN) plates were used to count bacterial colonies (CFU) before and after disinfection. The results of the study were that all three PAA-based disinfectants caused a 50% decrease in pH and a 100% reduction in ammonium levels and Perfectoxid seemed to have the strongest effect. QAC-based disinfectants also caused a considerable drop in the water's pH (Virocid 12%, Virkon: 69% reduction) and ammonium levels (Virocid: 93%, Virkon: 66% reduction). Contrastingly, H2O2-based disinfectant (Free Bac ®35) and ClO2-based disinfectant (Life Clean) seemed to have no considerable impact on ammonium (Free Bac ®35: 4% increase, Life Clean: 6% decrease) or pH (Free Bac ®35: 2% and Life Clean: 1% reduction). All PAA-based and QAC-based disinfectants have caused a 100% reduction of bacterial colonies on both TSA and CIN plates. However, Free Bac ®35 and Life Clean did not cause any reduction in the bacterial load on either of the plates. In conclusion, 5 out of 7 approved disinfectants (Addi Aqua, Aqua Des, Perfectoxid, Virkon Aquatic, and Virocid) were effective in disinfecting the water whereas Free Bac ®35 and Life Clean were ineffective. These findings can be useful in future research to determine what factors could affect their inefficacy.