Beyond water quality: Micro particles in Recirculation Aquaculture Systems

Micro particles accumulate in recirculating aquaculture systems (RAS) and with the fast expansion of the RAS industry in recent years, more focus has been put on understanding the impacts of micro particles on fish and systems alike. Micro particles are partly responsible for bacterial activity within systems due to their high surface area to volume ratio.

Their small size challenges their removal, and technologies that can reduce micro particles in RAS are still lacking. The overall aim of this PhD was to resolve the implications of micro particles on RAS operation including water quality and fish performance, and to acquire methods and technologies for controlling micro particle development in RAS.

This thesis is accompanied by four scientific manuscripts as well as unpublished data collected over the last three and a half years. The first manuscript ( Paper I ) examines the distribution of micro particles and bacterial activity in seven Danish model trout farms (MTF). Twenty separate RAS units were sampled over a short period and water samples accessed for micro particles and bacterial activity.

The results revealed large variations in micro particle loads across, as well as within, farms suggesting that system specific conditions predominate. A strong correlation (r = 0.92) between micro particle surface area and bacterial activity furthermore supported the hypothesis that particle surface area is important in controlling bacterial activity in lower intensity RAS.

The second manuscript ( Paper II ) assesses the potential of ultraviolet radiation (UV) and micro filtration (1 µm) for controlling micro particle levels in rainbow trout RAS. At the same time, an indirect assessment of the amount of micro particles composed by microorganisms was carried out by examining the reduction caused by UV treatment alone.

A two-by-two factorial trial was conducted over a 13 week period in 12 replicated pilot scale RAS. The results showed that both micro filtration and UV had large impacts on the micro particles present in the systems, with large reductions in both numbers and volume. Micro filtration resulted in a significant reduction of particle volume (89%) and a significant reduction in particle numbers and bacterial activity (50 and 54%, respectively).

Ultraviolet radiation, on the other end, lead to significant reductions in particle numbers (74%) and bacterial activity (89%). The combination of both methods reduced the presence of micro particles by approximately 88% (all metrics) and reduced bacterial activity by 95%. It was also estimated that at least 64% of all particles by numbers and 30% by volume were composed of living microorganisms.

The third manuscript ( Paper III ) tests the effects of sample storage (temperature and duration) on micro particle and bacterial activity analysis. This was done by storing samples from two different RAS at room temperature and at 4ºC over a total of 72 hours, and tracking changes in particle numbers, volume and surface area, as well as bacterial activity.

In addition, some samples were store at -20ºC and subjected to similar analysis following de-frosting. Results showed different dynamics in samples from either system, with large increases in bacterial activity and micro particles within the first 3 hours in samples stored at 4ºC from one of the systems.

In comparison, micro particle numbers and bacterial activity remained stable for the first 6 hours in samples obtained from the other system before starting to decline. In both cases, the results suggested that changes in micro particle numbers were bacterial driven. The results also showed that samples for bacterial activity and micro particle assessment should not be frozen.

The last manuscript ( Paper IV ) focuses on the effects of foam fractionation and ozonation in freshwater, rainbow trout RAS on the control of micro particles, bacterial activity and other water and biofilter quality parameters, with focus on the build-up of organic matter. A two-by-two factorial trial was conducted in 12 replicated pilot scale systems over 8 weeks to determine the individual and combined effects of both treatments.

The results showed large reductions for the individual treatments and the highest removals for the combined treatment. Ozonation by itself reduced the number of particles by more than 83% and bacterial activity by 48%. Foam fractionation, on the other end, resulted in 54% less particles in numbers and 62% less particle volume, while it reduced bacterial activity by more than 54%.

The combination of both treatments resulted in approximately 90% reduction of particle numbers and bacterial activity, as well as 75% removal of organic matter (BOD5). The results obtained supported previous findings on ozone’s effect in RAS. Furthermore, the results showed that foam fractionation has similar removal efficiency as that typically found in saltwater, suggesting that foam fractionation could become a tool for controlling organic matter build-up in RAS, especially when combined with ozone.

The changes in physicochemical water quality parameters deriving from the treatments in Papers II and  IV did not show any effect on the fish, sustaining that rainbow trout have a high degree of tolerance to micro particles. In conclusion, the results of this thesis corroborate that rainbow trout is highly tolerant to high levels of micro particles, while the control of micro particles through different methods leads to significant improvements in different physicochemical water quality parameters in RAS.

Furthermore, micro particles in RAS are intrinsically connected to bacteria. In low intensity systems, surface area provided by micro particles seem to partly control the amount of bacterial activity in the system, while in higher intensity systems most micro particle dynamics appears to be the result of changes in bacterial populations.

The main driver behind the large fluctuations in micro particles seems to be organic matter build-up. These results highlight new possibilities for controlling micro particles in RAS, including disinfection. Together with reports from the industry, the results of Papers II and IV maintain that disinfection is an efficient way of controlling micro particles.

However, while disinfection can be used to control micro particles in RAS, it does not deal with the underlying cause of bacteria, which is organic matter. As seen in  Papers II and IV , control of organic matter in RAS will not only control the level of micro particles, but will also control the build-up of organic matter which is the direct cause of micro particles.