Strengthening international collaboration: enhanced natural treatment solutions for water security and ecological quality in cities Actions should identify and review examples of innovative natural water treatment systems in Europe and beyond to develop understanding of their functioning, the ecological processes involved and their capacity and performance in treating pollution under diverse and uncertain conditions, such as maximum/minimum loads, uncertainties on composition of the pollution entering the system, necessary time for the treatment, capacity to cope with temporal variation in rainfall, etc. They should develop methodologies and guidance for the design and implementation of urban enhanced natural treatment systems and their integration into the urban water cycle, the urban landscape and the receiving waters to enhance the circularity and hence sustainability of the overall system. They should develop new business models for their construction, operation and long-term management and maintenance and standards for the treatment processes and the different uses for which the effluent may be used within different regulatory frameworks.Actions should include pilots/demonstrations for testing innovative approaches or the use of established solutions under new conditions and monitoring from baseline through construction and for a period of time, to establish the functionality of the system and assess the physical, social and economic benefits of the deployed solutions. Appropriate methodologies for public/social engagement in the implementation of such solutions should be developed.Actions should envisage clustering activities with other relevant ongoing and future nature-based solutions relevant projects funded under previous and current H2020 Work Programmes for cross-projects co-operation, consultations and joint activities on cross-cutting issues and share of results as well as participating in joint meetings and communication events. To this end, proposals should foresee a dedicated work package and /or task and earmark the appropriate resources accordingly.In line with the strategy for EU international cooperation in research and innovation (COM(2012)497), international cooperation is strongly encouraged. International participants should explore the possibility to apply for co-funding under their national governments[[See http://ec.europa.eu/research/participants/docs/h2020-funding-guide/cross-cutting-issues/international-cooperation_en.htm#support-non-eu-countries]].To ensure coverage of geographic, socio-economic and cultural diversity (including possible gender differences in the use/management of water) as well as sharing innovative solutions across the EU, pilot actions/demonstrations must be implemented in at least 3 cities situated in different Member States or Associated Countries that are committed to implement the proposed innovative actions/schemes during the project and assess their impacts and cost-efficiency.The Commission considers that proposals requesting a contribution from the EU in the range of EUR 5 million would allow this specific challenge to be addressed appropriately. Nonetheless, this does not preclude submission and selection of proposals requesting other amounts. It is expected that, by 2050, half of the human population will live in cities[[For the purposes of this topic, the definition of a 'city' is to be understood according to the harmonised definition of a city established by the OECD and the European Commission, which can be found at: http://ec.europa.eu/regional_policy/sources/docgener/focus/2012_01_city.pdf]] and possibly half of them will live in informal settlements without appropriate waste water collection infrastructure and treatment facilities. In view of the increasing pressure on water resources world-wide, collection, recycling and safe reuse of polluted water is considered to be an essential component for sustainable water resources management. Surface and groundwater in cities and downstream urban areas may suffer serious pollution from point and diffuse sources from upstream and in-catchment which might have a negative impact on the ecology, quality of life and land values in the city. Furthermore, urban run‐off, storm water and waste water represents a threat for water quality because of the pollutant load it conveys. Enhanced nature-based treatment solutions (such as artificial wetlands and lakes, bio-filtration, etc.) have the potential to remove pollutants from water (e.g. storm water, urban run‐off, river water, wastewater) that will lead to improved water quality and water use efficiency. Such natural treatment measures, when well planned and integrated into the overall urban planning and design, can also contribute to climate adaptation by reducing flood risk and heat island effects and constitute attractive components of the urban landscape. The innovation challenge is therefore how to design enhanced natural treatment systems that will provide effective ecological services of water purification and storage, as well as valuable habitats, constitute integral part of the overall urban water cycle and attractive components of the urban landscape and ensure that in closing the water cycle challenges associated with chemical and biological hazards are properly addressed through well-defined and validated risk assessment methodologies and implemented in relation to the final water use. The project results are expected to contribute to:increased use of innovative natural water treatment measures as Nature-based solutions, integrated into the overall urban water cycle and constituting attractive components of the urban landscape for more sustainable urban water management that enhance the overall urban metabolic processes and mitigate impact on receiving waters;enhanced water availability with reduced pressure on existing freshwater resources through treatment, remediation, reclamation and re-use of polluted water and wastewater steams;increased investments into natural water treatment solutions from urban authorities, water companies or property developers through evidence of the benefits for deploying such ""systemic"" approaches as opposed to alternative water treatment systems;sharing and cross-fertilization of capacity, expertise and know-how among European and international partners on new innovative natural water treatment concepts and solutions for enhanced opportunities for up-taking, upscaling and business in the European and global markets;increased business opportunities for the design, development and installation of natural water treatment measures in cities.