Researchers studied using willow trees to clean industrial and city wastewater. They started a project in 1998 across France, Greece, the UK, and Sweden to see how well willow plantations could treat different types of wastewater in various climates. They wanted to see if watering willow plants with this wastewater would have good effects. They also checked for any problems and figured out solutions. After four years, they will finish the project in 2002. This project will help understand if using willow trees to clean wastewater is effective and what issues need to be solved.