UV irradiation and micro filtration effects on micro particle development and microbial water quality in recirculation aquaculture systems

Recent studies have focused on micro particle build-up in recirculation aquaculture systems (RAS), and a correlation between micro particles and microbial activity has been shown. This study evaluated how micro particle build-up and microbial activity are affected by UV irradiation and micro filtration. Using 12 identical pilot scale RAS stocked with rainbow trout (Oncorhynchus mykiss), a two-factor factorial experiment was carried out testing in triplicate systems the effect of UV irradiation (systems with or without) in combination with cartridge filtration (1 or 200 μm pore size) on selected water quality parameters. The trial ran for 13 weeks. Water samples were obtained once a week, and the number and size distribution of micro particles was analysed. Microbial activity was derived from the hydrogen peroxide degradation rate, and concentrations of total and dissolved organic matter were measured as chemical oxygen demand (COD).
Overall, both UV and cartridge filtration had significant effect (<0.05) on micro particle distribution and microbial activity in the systems. By the end of the trial, a two-way Anova showed that UV treated RAS, independently of cartridge filtration pore size, were significantly (p < .05) lower in micro particle numbers (74% reduction), micro particle surface area (54% reduction), and dissolved COD (34% reduction) compared to systems without UV. Similarly, microbial activity was reduced up to 89% independently of cartridge filtration. UV thus appeared to reduce micro particle numbers by destroying bacteria. In addition, the effect of UV on dissolved COD suggested a possible feedback mechanism between microbial activity and substrate release in the systems. For micro filtration, a 1 vs. 200 μm pore size significantly reduced the number of micro particles (by 50%), micro particle volume (by 83%), and micro particle surface area (by 73%) independently of UV treatment. This was accompanied by a significant reduction in particulate COD (80%) and microbial activity (approximately 54% reduction independent of the use of UV). Hence, cartridge filtration appeared to reduce a build-up of micro particle by directly removing bacteria and bacteria substrate. In conclusion the study sustains that combining UV and particle removal is a potential viable tool for managing microbial water quality in RAS.