Diverse reactions of aquaculture biofilter biofilms following acute high-dose peracetic acid

Abstract

Peracetic acid (PAA) is an effective disinfectant in aquaculture systems to reduce pathogen loads and improve water quality. However, its effectiveness in disinfecting biofilm in recirculating aquaculture systems (RAS) and resetting biofilters between productions remains unknown. This study evaluated the effects of acute PAA exposure on biofilter biofilms from freshwater RAS. Identical types of bioelements were collected from a pilot-scale RAS (without prior PAA treatment) and a commercial RAS (with PAA treatment), and exposed to PAA concentrations of 0, 1, 2, 4, 8, and 16 mg/L for 1 h. Microbial activity and viability of the exposed biofilms were evaluated using respirometry and flow cytometry. Results showed dose-dependent inhibition of biofilm activity and viability in the pilot-scale RAS. Nitrite oxidation was the most sensitive process to PAA, with an IC₅₀ of 1.27 mg/L (the concentration at which PAA inhibited biofilm metabolic activity by 50 %), followed by ammonia oxidation (IC₅₀ = 1.59 mg/L) and endogenous respiration (IC₅₀ = 2.67 mg/L). Microbial activity linked to H₂O₂ decomposition was least affected (IC₅₀ = 4.68 mg/L). Live cell counts decreased from 9.1 × 10⁷ counts/cm² to 2.4 × 10⁷ counts/cm² of bioelement surface, with dead cells proportion increasing from 15 % to 54 %. In contrast, biofilter biofilms from the commercial RAS exhibited significantly lower sensitivity to PAA dosage, with reductions in nitrite oxidation (39 %) and ammonia oxidation (51 %) observed only at 16 mg/L compared to control. These findings suggest that routine PAA exposure, as part of the other operating conditions on the commercial RAS, can enhance the biofilm's sensitivity to PAA. The study provides new insight into the sensitivity of aquaculture biofilm to PAA treatment and its effect on associated microbial processes.

Keywords: Disinfection; Flow cytometry; Microbial inhibition; Nitrification; RAS; Respirometry.

Fig. 1

Profiles of dissolved oxygen (DO) concentrations in 350 mL respirometric chambers with 20 pieces of bioelements, collected from (a) pilot-scale RAS and (b) commercial RAS. First, the bioelements were exposed to different PAA concentrations for 60 min. Then, four sets of substrate spikes were made: first with tap water (I, Time = 0–40 min), then with addition of NaNO₂ (II, Time = 40–55 min), NH₄Cl (III, Time = 100–115 min), and H₂O₂ (IV, Time = 160–225 min). DO values are represented as mean ± SD (n = 3). Dark shaded areas represent the pump-on period when substrate injected into the chambers, light dark shaded areas represent the pump-off period when oxygen consumption/release rates were calculated, and white areas with breaks represent intervals of 45 min when the draining and refilling of the reservoir takes place.

Fig. 2

The metabolic activity of biofilm attached on bioelements collected from pilot-scale RAS (red square) and commercial fish farm (blue circle) towards increasing PAA exposure. (a) OCR_endo, (b) OCR_NOB, (c) OCR_AOB, and (d)k_or are response parameters indicating biofilm metabolic activities related to endogenous respiration, nitrite oxidation, ammonia oxidation, and enzymatic process for H₂O₂ decomposition, respectively. Values are reported as mean ± SD (n = 3). The letters denote post-hoc Tukey test results within the PAA concentration level. The same letter represents no significant difference and vice versa (p < 0.05).

Fig. 3

The abundance of (a) total cells and (b) live cells, and (c) the proportion of dead cells (%) in the biofilm that are attached on bioelements collected from pilot-scale RAS (red square) and commercial RAS (blue circle) towards increasing PAA concentration exposure. The biofilm samples for the measurement are dislodged from the surface of a single bioelement covering a surface area of 448 mm².