Title Micro screens and micro-particles in replicated recirculating aquaculture systems
Authors & affiliations Paulo Fernandes ǂ Lars-Flemming Pedersen Per Bovbjerg Pedersen ǂ
Corresponding author:
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Technical University of Denmark DTU Aqua – National Institute of Aquatic Resources Section for Aquaculture Niels Juelsvej 30 9850 Hirtshals
Abstract Solids accumulation is a primary area of focus in recirculating aquaculture systems (RAS). Large solids are quickly removed from RAS by installing mechanical filters, which can, however, lead to high background concentration of micro-particles within the system. Although in such quantities, there is a severe lack of information on micro-particles, specifically towards potential system impairments they may induce. This study investigated the effects of 4 different mesh sizes (no mesh, 100, 60 and 20µm), in groups of replicated RAS, on dissolved and solid substances, nitrification kinetics and rainbow trout (Onchorhynchus mykiss) growth. The filters were backwashed three times a day, and operational conditions were kept at constant level for six weeks, at a daily feeding level of 250g per system, and resulting feed loadings of 3.1Kg feed·m-3 of make-up water. At the end of the six-week period, an intensive backwashing campaign and biofilter nitrification trials were also conducted. Microscreens were observed to remove particles and affect other parameters compared to initial conditions. Increased β-value, lower particulate surface area and solid organic substances, were all observed at the end of the trial in the filtered systems. Furthermore, they all reached a steady-state regarding particulate compounds accumulation, and the time to reach system equilibrium was reduced with decreasing mesh size. Most particulate parameters accumulated in the control group, and system equilibrium had not been reached by the end of the experiment. Data from an intensive backwashing campaign, performed after the experimental period, further support the steady-state hypothesis in filtered systems. 0’-order nitrification rates (k0a) were equivalent for all systems (0.15±0.022) and comparable to literature k0a levels, typically between 0.1-0. 2g·m-2·d-1 in RAS. Unfortunately, no information could be acquired on 1st-order kinetics, as the sampling ended too early and contained insufficient resolution. More information on 1st-order kinetics could have provided adequate evidence of the intrinsic interaction between organic micro-particles and biofilters in RAS. Microscreens induced a significant effect in removing particulate compounds, but only when compared to systems without a dedicated filter. Moreover, a 20µm mesh did not significantly improve water quality, as demonstrated by equivalent water chemistry and particulate compounds, compared to the 100µm treatment. The resulting leveling of filtered systems is hypothesized to be related to a high particle removal/production rate and constant operations and conditions, related to stable feed loading. Keywords: Microscreen, nitrification kinetics, water quality, micro-particles, particle size distribution, BOD5, COD.
