This project tackles one of the greatest challenges to water management in the Baltic Sea Region – eutrophication, but more specifically historical phosphorous loads in lakes. Excessive algal blooms in eutrophic lakes decrease ecosystem services and have negative impacts on human health. Many lakes still suffer from phosphorous due to historically mistreated catchments. If not addressed, phosphorous hot spots will continue to eutrophicate the Baltic sea. We will build trust for the ALUM water treatment method that is able to combat eutrophication hot spots in the Baltic states rapidly and cost-effectively by providing long-lasting positive effects on lakes. ALUM method is widely used in water treatment, including our expertise partner Sweden which has more than 60 years of experience and ~250 positive application examples. Yet, ALUM has never been used in Baltic countries due to identified barriers: lack of know-how, skepticism about outcomes, belief it is too expensive, and fear of aluminum itself. ALUM is a safe method often mistaken as a chemical treatment while in reality, we are adding phosphorus-binding mineral that is commonly found on Earth to permanently capture phosphorous. The project will build trust in target groups and thus foster ALUM application on a wider scale through targeted activities: ALUM treatment demonstration, co-creation of missing supportive tools (technical regulations, recommendation, roadmap), and initiation of expertise linking hub.