To meet the challenge of accumulated particles in RAS, this study investigated the effect of an advanced system for enhanced particle separation and nutrient reduction by assessing a membrane filtration unit combined with a biofilter unit in the water treatment system. The effect of the membrane treatment on number of colloidal particles, number of bacteria, turbidity and nutrient concentration was investigated, and how this may have impacted the cod larvae growth and survival. By filtrating the whole circulating water 2 times per day (or 8.5% at any time), the membrane filtration unit managed to remove colloidal particles and lower the turbidity to levels much lower than typically obtained in conventional recycling systems (e.g. up to 44% and 77% reduction for number of colloidal particles and turbidity, respectively). Bacteria concentrations were also significantly lower (up to 80%) and more stable in the membrane filtration system compared to the conventional system. These findings could explain the observed higher survivaland growth rates in the intensive rearing of cod larvae found in thisstudy. The ammonia level was up to 56% lower in the mRAS compared to the cRAS configurations, however, the ammonia values in both treatment schemes were well below toxic levels. The dissolved organic carbon (fTOC) level was up to 52% lower in the mRAS compared to the cRAS configurations and correlates to lower number of bacteria measured and a lower level of biomass growth in the mRAS alternative. The membrane filtration unit performed sat-isfactorily with relatively low fouling development and chemicallycleaning only being necessary two times during the experiment.Further investigations in design and operation of the membranefiltration unit are required to optimize this component of the treatment scheme