The multi-faceted nature of Blue-Green Systems coming to light

Cities are facing more challenges in the way they manage water, especially under rapid urbanisation and climate change.One of the solutions for our cities is to adopt green, vegetated infrastructure into water systems, i.e., so called 'Blue-Green Systems'.'Blue' here stands for water, while 'green' refers to resource and energy efficient, recovery and reuse water systems.They are distributed treatment systems labelled as nature friendly engineered approaches, ranging from, but not limited to, constructed wetlands, ponds, large bioretentions, and swales to porous pavements, infiltration trenches and small raingardens, etc.They are not new and have been increasingly adopted in urban areas under various concepts all over the world, e.g., water sensitive urban design (WSUD) in Australia, nature-based solutions (NBS) in Europe, sustainable urban drainage systems (SuDS) in UK, low impact developments (LIDs) in the USA, and more recently, sponge cities (SCs) in China.Such Blue-Green Systems are ecologically beneficial and already valued for their beautifying presence and are highly utilised design features in the cities.In addition to their water treatment functions, Blue-Green Systems provide multiple benefits, e.g., frequent flood mitigation, waterway health protection, microclimate improvement, and improvement of amenity values in urban landscapes.The first issues of Blue-Green Systems:• present the latest studies in this area.Firstly, the multiple benefits of Blue-Green Systems are recog- nised, not only for water pollution management (Ao et al. 2019), flood resilience improvement (O'Donnell et al. 2020), but more broadly their services to society and contributions to the United Nations Sustainable Development Goals (Sørup et al. 2019).Researchers have also started to identify NBS as a viable solution for resource recovery (Kisser et al. 2020).• present studies with regard to the technology enhancement, by looking into the pollution retention capacity of media used (Tedoldi et al. 2019), and employment of real time control schemes (Persaud et al. 2019) in specific blue-green technologies, for improving system performance.• deal more broadly with a wider spectrum of BGS topics.