Improved water quality in recirculating aquaculture systems (RAS) by applying a membrane bioreactor (MBR) concept for removal of colloidal and fine suspended solids

The world’s human population is growing, and there is an increased demand for food. Aquaculture production is increasing rapidly to meet these demands and there is a growing interest for recirculating aquaculture systems (RAS) to maximize production. A typical RAS facility consists of production tanks and a water treatment compartment for removal or conversion of accumulated nutrients, organic matter and particulates where the treated water is recycled to the production tanks. Reuse of water in RAS is high, typically > 90%. Benefits of RAS as compared to flow through systems are commonly cited as; reduction of water consumption, conservation of heat, better environmental control that can reduce the risk of disease and pollution problems, optimize the growth rates and health of certain aquatic species and life stages by surveillance of the water quality. Two of the most important water quality parameters to control in RAS are the content of solids and N-compounds as they can be very detrimental to the species being cultivated. This requires advanced treatment systems that include biological removal of nitrogen and efficient removal of the solids fractions, particularly the fine solids and colloidal fractions.In this study a membrane bioreactor (MBR) concept was investigated that consisted of a biofilter using the moving-bed-bioreactor with a submerged membrane filtration reactor. To assess the ability of MBR to improve water quality several investigations were conducted through two studies. The first study evaluated the production of the live feed organisms calanoid copepod (Acartia tonsa) and rotifer (Brachionus ‘Cayman’) in marine recirculating aquaculture system (RAS), and the second study evaluated RAS production of Atlantic cod larvae (Gadus morhua L).In the first study with live feed productions, the MBR concept was found to be a very efficient treatment process to remove the colloidal and fine suspended solids. However, the impact this had on the number of particles in the live feed production tanks was not readily apparent based on particle analysis. To benefit from this enhanced particle removal treatment in live feed production a higher recirculation rate and/or different operating conditions are necessary.In the second study with production of Atlantic cod larvae, the MBR concept improved water quality with respect to turbidity and colloidal material. This concept also appeared to give improved ammonia conversion in the biofilter. This was potentially due to reduced concentration of organic material and reduced heterotrophic competition. Results also show that the MBR concept lowered the number of bacteria, and affected the microbial composition in the fish tanks. Interestingly a higher cod larvae survival and significantly higher cod larvae size were observed when applying the MBR treatment scheme, and the cod larvae were more robust and had a higher tolerance to applied stress (i.e. higher survival) compared to the control group.One challenge with membrane filtration in general is the potential decrease in filtration flux resulting from accumulated materials on the surface of or within the membrane. Investigations of the membrane performance and change in transmembrane pressure (TMP) during the experiment revealed a clear correlation between change in TMP and the different feeding regimes applied. Algae paste, decaying rotifers and dry feed appear to contribute the most to membrane fouling.