Effects of drum filters, swirl separators and foam fractionation on water quality in pilot scale recirculating aquaculture systems

Abstract

This study investigated the impact of different filtration configurations on water quality in pilot scale recirculating aquaculture systems (RAS), focusing on the removal efficiency of particulate and dissolved organic matter. The experiment was conducted over six weeks using 12 pilot-scale RAS units, each stocked with juvenile rainbow trout (Oncorhynchus mykiss) in freshwater. Four treatment configurations in triplicate systems were evaluated: drum filter only, a swirl separator combined with a drum filter (Swirl + Drum), a drum filter combined with a foam fractionator (Drum + FF), and a swirl separator combined with a foam fractionator (Swirl + FF). The results showed that the combination of a swirl separators and a drum filter achieved the highest removal of organic matter per day. Despite the high organic matter removal in the Swirl + Drum treatment, it did not translate to the best water quality outcomes, highlighting the importance of addressing dissolved organic matter and fine particulates. Pilot RAS with foam fractionation showed the most significant improvements in water quality parameters, particularly in reducing total BOD, COD, and microbial activity. The Swirl + FF treatment exhibited a large decrease in total BOD (40 % reduction) and total COD (27 %), with microbial activity reduced by over 55 % compared to the RAS with drum filters only. The study also identified that Swirl + FF, led to more total phosphorus and orthophosphate , suggesting that particles smaller than the swirl separator's threshold but too large for foam fractionation contributed to increased phosphorus levels. This study underscores the effectiveness of combining swirl separators and drum filters for particulate removal and highlights the crucial role of foam fractionation in managing dissolved organic matter and enhancing overall water quality in RAS. These findings provide new knowledge of solids removal and water quality and may have important implications for the design and optimization of RAS systems.