Achieved improved water quality

The Filter Ecosystem project, completed in 2007, aims to improve water quality and enhance the ecological health of the Molentargius wetland area in Sardinia, Italy, also known as the Molentargius Saline Regional Nature Park. This diverse territory includes freshwater basins such as Bellarosa Minore and Perdalonga, saline water basins like Bellarosa Maggiore and Stagno di Quartu, and the predominantly dry area known as Is Arenas.
The Molentargius ecosystem is a crucial European site for various aquatic birds and features diverse flora, including endemic species and endangered plants on the "Red List." It is one of the few globally recognized wetlands of international importance for aquatic birds located entirely within a large urban center, protected as a biotope by the RAMSAR Convention in Sardinia's major urban area. Since the 1980s, it has suffered environmental degradation due to inadequate infrastructure for wastewater management and human pressures.
Implemented by the city of Cagliari, the project employs a phytoremediation technique to supply water to the ponds. This involved constructing a phytoremediation system between the Bellarosa Maggiore and Bellarosa Minore ponds, functioning as a surface flow filter ecosystem resembling a marsh or wetland.
The system enhances natural self-purification processes, promoting prolonged contact between water, organic matter, and plants. This setup creates microhabitats that support microbial flora crucial for purification. The project's primary goal is to treat wastewater from the Is Arenas treatment plant, ensuring that the treated water meets optimal conditions for sustaining the wetlands' biodiversity. Additionally, the project will expand reed bed areas, vital habitats for avian species, and is designed to purify up to 300 liters of water per second to meet the area's needs (Refs. 1, 2, 3).

In the 1980s, rapid industrialization, urban population growth, and increased water usage led to severe pollution in Dianchi Lake, exceeding its environmental capacity and significantly degrading its water quality (Ref 1). This deterioration had serious effects on nearby ecosystems, including Cuihu Lake (Ref 2). Cuihu Lake (翠湖), also known as Green Lake Park, is a well-known park and body of water in the center of Kunming, Yunnan Province, China. In Cuihu lake the decline in water quality caused a sharp reduction in native species like the golden line barb, which depend on the local ecosystem for survival, pushing them to near extinction and destabilizing the entire ecosystem (ibid.).
To address this, several effective initiatives have been launched to restore Cuihu Lake’s ecosystem in recent years (Ref 2, 3). One of them was started in 2020 when a restoration project was initiated by the site's administration in collaboration with Dr. Li Weiwei’s team from the Kunming Institute of Zoology, Chinese Academy of Sciences. The project focused on creating micro-habitats and restoring the aquatic ecosystem (ibid.). Ecosystem and biodiversity restoration efforts were conducted across a 5,000 m² area on the east side of Green Lake Park, covering approximately 1/27th of the lake (ibid.).
As a result, more than 20 aquatic animal species are now thriving and contributing to water purification, while three submerged plant species cover over 50% of the project area (Ref 2, 3, 4). Bird diversity and numbers are also increasing, with species like white-breasted kingfishers, green-winged teals, and crested guinea-fowls returning to the area (ibid.). Additionally, there has been a significant improvement in water quality (ibid.).

The "Recuperación del Laguito" project aims to restore El Laguito, a coastal water body in Cartagena, Colombia, severely affected by sedimentation that blocks natural water exchange with the Caribbean Sea. This blockage has led to stagnant, low-oxygen water, degrading water quality and aquatic ecosystems (ref 1). The project seeks to restore El Laguito’s environmental and social functions by reestablishing seawater flow, reducing sediment impact, improving water quality, and reviving the ecosystem. Additionally, it aims to provide data to guide long-term solutions for sustainably managing this community resource (ref 1).
The project is a response to recurring ecological crises, such as frequent fish die-offs due to low oxygen levels. Local residents initially took matters into their own hands, manually attempting to reopen the channel. The formal intervention started following a 2019 directive from the Ministry of Environment and Sustainable Development, outlining a phased strategy: short-term installation of an axial pump for water exchange, medium-term studies for a permanent fix, and long-term construction plans (ref 3). Currently, the project involves using heavy machinery to clear sediment and allow seawater inflow, supported by the Gobernación de Bolívar, Cardique, and the Cartagena Mayor's Office. This phase serves as a transitional step towards a more sustainable, permanent solution (ref 2). The project’s impacts focus on ecological restoration. Short-term benefits include better oxygenation, reduced harmful algal blooms, and revived marine life. Long-term goals encompass sustainable water management, ecosystem stabilization, tourism enhancement, and the development of recreational and sports facilities in harmony with environmental conservation (ref 4).

The Ciénaga de las Quintas is a coastal wetland located alongside one of Cartagena’s main roads, adjacent to the public market square and situated between the Jiménez and Bazurto Bridges. It is home to two predominant mangrove species (Ref 1). The wetland currently faces pollution problems because it is used for the disposal of solid waste, such as plastics and construction materials, and it receives wastewater and household garbage from nearby residences. Las Quintas is also the main waste disposal area for the Bazurto market, which handles 43,430 tons of food products each month (ref 3). The recovery initiatives for the Ciénaga de las Quintas are led by the Environmental Public Establishment of Cartagena (EPA Cartagena) in collaboration with other public institutions, the private sector, and citizens, with the main goal of achieving the ecological restoration of the city's water bodies and to maintain and preserve our mangrove ecosystems (ref 2). Since 2020, key implementation activities have included the removal of solid waste, planting of native tree species, dismantling of informal structures within the mangrove zone, and the rehabilitation of areas previously occupied by debris (Refs 4, 5).

The ROOF WATER-FARM is an innovative demonstration site located in the heart of Berlin, near Potsdamer Platz, showcasing sustainable urban infrastructure that integrates wastewater treatment and food production. Situated in the "Block 6" quarter of the 1987 International Building Exhibition in Berlin-Kreuzberg, the site transforms wastewater and rainwater collected from nearby buildings into usable resources for farming and fertilizer production (Ref. 1). This project provides a real-world example of how sustainable systems can be incorporated into everyday urban design to enhance resilience and functionality.

At the ROOF WATER-FARM, water from rooftops and surrounding households is channeled into a treatment plant, where it is analyzed for micropollutants and purified. The treated water is then used for both fish farming in aquaponics systems and plant irrigation in hydroponic farming. The nutrient-rich water from the fish tanks nourishes plants, while also contributing to the production of food. This integrated approach actively supports climate protection by managing rainwater runoff and using plants for CO2 storage, helping to mitigate the urban heat island effect (Ref. 4).

This demonstration site embodies how cities can move toward a circular economy, where wastewater is reused for both farming and water management. The project also serves as a hub for learning and experimentation in sustainable urban design, with the greenhouse acting as a space for collaboration and innovation in infrastructure development (Ref. 2; Ref. 5). By incorporating these technologies into urban environments, the ROOF WATER-FARM demonstrates a future where buildings and neighborhoods efficiently recycle water, produce food, and reduce environmental impact.

The Floating University was initiated in 2018 at the rainwater retention basin of the former Tempelhof airport in Berlin. Though not an official university, it serves as a space for transdisciplinary, communal learning, transforming an abandoned urban site into a vibrant community resource. The project reclaims the basin, hosting workshops and events that range from ecological lectures to cultural and art programs, fostering collaboration between diverse disciplines and communities.
The architecture collective raumlaborberlin originally envisioned the project as a six-month initiative. Artists, universities, and seminar groups contributed to designing the space and organizing workshops. However, due to its success, an NGO, Floating e.V., was established to sustain the project. Today, the NGO manages the site and continues its mission of engaging communities with urban and ecological themes.
After Tempelhof airport closed in 2008, the basin was infrequently cleaned, leading to contamination and sediment accumulation. Over time, a wetland ecosystem emerged, attracting plants, insects, birds, mammals, and amphibians. This natural development complements the Floating University’s infrastructure, which consists of open timber structures like a kitchen, art installations, communal spaces, vertical gardens, gardening beds, and an auditorium. These structures integrate seamlessly with the basin’s existing technical framework and wetland environment.
Floating e.V. aims to make the basin accessible to Berlin’s urban population while sparking discussions about urban climate, resource management, and water cycles through diverse and artistic programs. Smaller initiatives focus on specific ecological themes, such as plant diversity and soil health, through projects like a seed archive and soil lab.
The Floating University exemplifies how abandoned urban spaces can be revitalized into hubs for learning, creativity, and ecological awareness. (1-3, 8, 9)

The Detroit RiverWalk is an urban development project along the Detroit River, designed to reconnect residents with their waterfront and revitalize a historically industrial area. Once characterized by abandoned buildings and industrial debris, the riverfront between the MacArthur and Ambassador Bridges had long been inaccessible to the public. To address this, the Detroit Riverfront Conservancy was established in 2003 to lead the redevelopment of this space.
The RiverWalk currently stretches along a 3.5-mile section, with plans to expand to 5.5 miles, integrating ecological restoration, public access, and recreational opportunities. The esplanade features native landscaping, artistic elements, and amenities, including four pavilions, a custom carousel, and areas for fishing, walking, biking, and inline skating.
Environmental remediation has been a crucial part of the project. In collaboration with the U.S. Environmental Protection Agency’s Great Lakes National Program Office, the Detroit Riverfront Conservancy successfully cleaned up 13,000 cubic yards of contaminated sediment, significantly reducing pollution and contributing to the removal of the Detroit River from the list of the most polluted sites in the Great Lakes. Furthermore, the project also includes greenways, park creation and revitalization.
The RiverWalk is not just a public space; it symbolizes Detroit’s transformation from a Rust Belt city to a sustainable urban environment. It exemplifies the power of public-private partnerships in revitalizing urban areas. The RiverWalk continues to expand, with plans to connect various parks and extend to the Ambassador Bridge, further enhancing the city’s waterfront. Its success was recognized in 2021 when it was named the Best Riverwalk in America by USA Today, cementing its status as a model for urban renewal and community engagement. (Ref. 1, 2, 3, 4)

The Gårda pilot rain garden has been constructed as part of the research project Innovative Rain Gardens at Chalmers University of Technology in Gothenburg. In addition to causing flooding, runoff rain water in urban environments can be contaminated with microplastics, organic pollutants and metals which can spread into the natural environment. Yet, most of the storm water in urban environments is not treated. The purpose of the Innovative Rain Gardens project is to research the ability of a pilot rain garden to treat such pollution. The project acknowledges the function of a rain garden to prevent flooding, but in this study only the pollution treatment potential is investigated. The pilot rain garden was constructed next to the E6 highway in central Gothenburg [Ref. 1-4]. It includes several different bioretention filters where selected plants are grown in filters containing different materials such as biochar, ash and soil [Ref. 2, 3]. Results so far show that the use of rain gardens can significantly reduce pollution. However, further research is needed to determine the long term function of the bioretention filters and the potential of scaling up the NBS. The project is funded by the Swedish Research Council for Sustainable Development (Formas), IMMERSE - Implementing Measures for Sustainable Estuaries, an Interreg project supported by the North Sea Programme of the European Regional Development Fund of the European Union, and COWIfonden (private foundation) [Ref. 1-3].

Sourced from Northern Worcestershire, the River Stour has played a significant role in the industrial heritage of Birmingham and the Black Country (Ref 1). It consists of 11 Water Framework Directive water bodies and multiple tributaries, spanning 200 km in length (Ref 2, 3). Once a pristine Salmonid river that powered the Industrial Revolution, rapid industrialization severely impacted the river, leaving it heavily polluted with sewage (ibid.).
Initiated by the Birmingham and Black Country Wildlife Trust (BBCWT) in 2021, the Love Your River Stour (LYRS) project focuses on Dudley and Wolverhampton, aiming to transform the River Stour and its surrounding green spaces into a beautiful, wildlife-rich source of pride for local communities and to create a corridor for wildlife to thrive through the heart of the Black Country (Ref 4).
Phase 1 of the project was completed in June 2023, bringing significant improvements along the river (Ref 5, 6, 7, 8). In Phase 2, the project will extend to a stretch running through Cradley Heath and Lye (Ref 7, 8). This initiative is funded by the UK Government's Green Recovery Challenge Fund.

The Juan Angola River is an urban channel included in the system of rivers and channels of Cartagena, and it connects the Ciénaga de la Virgen wetland and the Bay of Cartagena, facing similar issues as the Cienaga de la Virgen such as illegal occupation of its shores, illegal waste dumping, deforestation of mangroves, and pollution. The recovery plan for the Juan Angola River is led by the EPA Cartagena (Environmental Protection Agency), with the support of other institutions, aiming to restore the Juan Angola’s ecosystem. This includes targeting the mangrove forest, dredging the natural channel, and delineating buffer zones with urban infrastructure to create public recreational spaces. The activities involve relocating people illegalling cupping informal settlements from the river shore, moving an existing bridge (the Benjamín Herrera bridge) which obstructs the current, recovering the low tide and riverbed areas, building pedestrian paths, clean the riverbank, recover mangrove areas and establish programs of education and environmental awareness (ref 1 and 2).