The use of biological traps for water treatment in Recirculating Aquaculture Systems

Abstract

Increasing global demand for fish and the depletion of natural fish stocks has stimulated the development of aquaculture all over the world. One of the main restrictions for future growth and sustainability of the aquaculture lies in its waste discharge. The main concern includes dissolved or particulate organics, nitrogen and phosphorus, which may lead to eutrophication of aquatic ecosystems receiving the wastewaters. Therefore, nutrient removal is essential for aquaculture wastewater treatment for protection of the surrounding environment.

Recirculating aquaculture systems (RAS), enable fish production in relative isolation from the surrounding environment, and they offer advantages in terms of reduced water consumption, improved opportunities for waste management and nutrient recycling. In RAS a microbial biofilter is used for converting the ammonia excreted by fish into nitrate, and due to the high recirculation of water, the RAS's wastewater is concentrated with nutrients that can be used for microalgae cultivation.

This project will test the hypothesis that by the combination of microalgae and filter feeder cultivation in RAS's effluent, the nutrient release into the environment can be significantly reduced. The objectives are to evaluate (1) the effectiveness of microalgae to remove dissolved nutrients from the RAS's wastewater; (2) the effects of environmental conditions on the algal biomass production. As harvesting of microalgae is a challenging and expensive process, this project will evaluate (3) the potential of Daphnia magna & lake mussel Anodonta anatina to filter the algae from the water. This project will produce new knowledge on nutrient recycling and thus supports the concept of circular economy, and conservational and sustainable management.

Our preliminary results are promising: the tested green microalgae can remove more than 90% of dissolved nitrogen and phosphorus from laboratory-scale RAS's wastewater effluent after 4 days, while still in the exponential growth phase; and more than 99% after 9 days while they are in the saturation growth phase.

RAS production is growing worldwide but also in Finland, thus the potential for re-using valuable nutrients in RAS's wastewater should be investigated to improve the sustainability of aquaculture.