Innovative materials could boost the performance of RAS biofilters

An experimental study conducted at the North Sea Research Centre, part of the Technical University of Denmark, has revealed that conventional plastic beads, commonly used as biofilter media in Recirculating Aquaculture Systems (RAS), are significantly less effective in removing ammoniacal nitrogen and promoting bacterial activity compared to more innovative alternatives such as recycled polyurethane foam, coconut shells, and ceramic beads.

As mentioned in the study, plastic-based biofilter media have traditionally been used due to their durability and ease of handling. However, concerns regarding cost, environmental impact, and the generation of microplastics have driven the search for more sustainable and accessible alternatives. In developing countries, where the installation of RAS often faces economic barriers, the high cost of importing commercial biomedia can severely limit the adoption of this technology.

Among the materials tested, recycled polyurethane foam proved to be the most efficient, capable of removing up to 310 grams of ammoniacal nitrogen per cubic metre per day—more than double the performance of plastic beads. Coconut shells and ceramic beads also outperformed plastic beads, ranking second and third, respectively.

These findings highlight the urgent need to explore more sustainable and effective alternatives to optimise biofilters in aquaculture systems. In RAS, biofilters play a critical role as the central component responsible for maintaining water quality by removing toxic nitrogen compounds.

Recycled polyurethane foam also stood out for its stability and efficiency, even under high nutrient loads, requiring less maintenance than coconut shells and ceramic beads, which tend to be more prone to blockages.

About the study

The researchers employed a 20-cubic-metre recirculating aquaculture system designed for rainbow trout farming. Over a eight-week period, four types of biomedia were evaluated: recycled polyurethane foam, coconut shells, ceramic beads, and commercial plastic beads. The system was equipped with biofilters specifically designed to house the biomedia, maintaining a constant flow of water and air supply to ensure optimal conditions for bacteria activity.

During the initial weeks, a supplementary biofilters was used to facilitate the early colonization of nitrifying bacteria on the different biomedia. Throughout the experiment, key variables were measured, including ammoniacal nitrogen and nitrite removal rates, microbial activity (assessed via hydrogen peroxide degradation), and nitrification kinetics. The study also examined the impact of each material on water quality, noting any release of organic matter during use.

The experimental design included higher nutrient loads to assess the ability of the biomedia to maintain efficiency under varying levels of stress. Data collected enabled a comparative analysis of the performance, durability, maintenance requirements, and sustainability of each biomedia, resulting in practical recommendations for real-world aquaculture production systems.