Farmers in western France are testing a bio-based fertiliser made from algae grown on wastewater, which can reduce mineral fertiliser use by up to 25% when combined with them. The algae feed on organic matter in wastewater, turning it into nitrogen-rich biomass. While it doesn't match mineral fertilisers in yield alone, it shows promise when used together. The WALNUT project aims to treat various wastewaters and recover nutrients for agriculture. In Ourense, Spain, advanced water treatment is recovering nitrogen from sludge, providing an alternative to conventional fertiliser production.