Symbiotic microalgal–bacterial biofilms can be very attractive for potato wastewater treatment. Microalgae remove nitrogen and phosphorus and simultaneously produce the oxygen that is required for the aerobic, heterotrophic degradation of organic pollutants. In this study, symbiotic microalgal–bacterial biofilms were grown in flow cells with ammonium and phosphate, and with acetate as a simulated biodegradable organic pollutant. The symbiotic biofilms removed acetate without an external oxygen or carbon dioxide supply, but ammonium and phosphate could not be completely removed. The biofilm was shown to have a considerable heterotrophic denitrification capacity. The symbiotic relationship between microalgae and aerobic heterotrophs was proven by subsequently removing light and acetate. In both cases, this resulted in the cessation of the symbiosis and in increasing effluent concentrations of both acetate and the nutrients ammonium and phosphate.
