Combating planktonic algae with benthic algae

• We tested artificial stream beds for nutrient removal from eutrophicated ponds.
• Photoautotrophic biofilms developed within a few days – up to a peak dry mass of 250 g m−2 after 3-4 weeks.
• We achieved phosphorus removal rates of 19 mg m−2 d−1.
• The cost-effective water treatment proved to be highly suitable for removing nutrients.
• Biomass is produced as a by-product and can further be used for industrial purposes.

Anthropogenic eutrophication of surface waters may cause mass development of algae, including cyanoprokaryotes, with serious effects on ecosystem health. In the current study, drastic hydrological changes initiated a eutrophication process of a former backwater of the river Danube (Austria). As a first restoration measure, a gravel filter followed by a phosphate trap was installed in 2007. Because this purification system was not effective enough, we constructed an additional biological treatment in 2011, using the self-purification process in streams, particularly through photoautotrophic biofilms. We installed artificial stream beds (algae turf scrubber = ATS) at the gravel filter surface, which were supplied with water from the backwater. The study aimed to estimate growth, productivity and composition of the algal biomass to test the potential of ATSs for nutrient removal and to examine the biomass with regard to further applications. Measurements of biomass, nutrients and fatty acids were done during three runs. Maximal phosphorus removal rates were observed with about 19 mg total phosphorus m−2 d−1; peak biomass of approximately 250 g m−2 dry mass was achieved. In total, approximately 18.8 kg of phosphorus ha−1 could be removed within five months. Further use of the harvested biomass as fertilizer, fermentation feedstock or combustion fuel is possible. The pilot study showed that the ATS technology has great potential and provides an effective and ecologically sustainable way to remove nutrients from surface waters, with the positive effect of producing biomass.