Microalgal solutions to clean waste streams and produce biomass were evaluated in Nordic conditions during winter, spring, and autumn in Southeast Sweden. The study investigated nitrogen (N) removal, biomass quality, and safety by treating industrial leachate water with a polyculture of local microalgae and bacteria in open raceway ponds, supplied with industrial CO2 effluent. Total N (TN) removal was higher in spring (1.5 g−2d−1), due to beneficial light conditions compared to winter and autumn (0.1 and 0.09 g−2d−1). Light, TN, and N species influenced the microalgal community (dominated by Chlorophyta), while the bacterial community remained stable throughout seasons with a large proportion of cyanobacteria. Winter conditions promoted biomass protein (19.6–26.7%) whereas lipids and carbohydrates were highest during spring (11.4–18.4 and 15.4–19.8%). Biomass toxin and metal content were below safety levels for fodder, but due to the potential presence of toxic strains, biofuels or fertilizer could be suitable applications for the algal biomass. Practitioner points: Microalgal removal of nitrogen from leachate water was evaluated in Nordic conditions during winter, spring, and autumn. Total nitrogen removal was highest in spring (1.5 g−2d−1), due to beneficial light conditions for autotrophic growth. Use of local polyculture made the cultivation more stable on a seasonal (light) and short-term (N-species changes) scale. Toxic elements in produced algal biomass were below legal thresholds for upcycling. © 2024 The Author(s). Water Environment Research published by Wiley Periodicals LLC on behalf of Water Environment Federation.