Nature-based solutions (NBS) can protect, manage and restore natural or modified ecosystems. They are a multidisciplinary, integrated approach to address societal challenges and some natural hazards effectively and adaptively, simultaneously providing human well-being and biodiversity benefits. NBS applications can be easily noticed in circular cities, establishing an urban system that is regenerative and accessible. This paper aims to offer a review on NBS for urban water management from the literature and some relevant projects running within the COST Action ‘Implementing nature-based solutions for creating a resourceful circular city’. The method used in the study is based on a detailed tracking of specific keywords in the literature using Google Scholar, ResearchGate, Academia.edu, ScienceDirect and Scopus. Based on this review, three main applications were identified: (i) flood and drought protection; (ii) the water-food-energy nexus; and (iii) water purification. The paper shows that NBS provide additional benefits, such as improving water quality, increasing biodiversity, obtaining social co-benefits, improving urban microclimate, and the reduction of energy consumption by improving indoor climate. The paper concludes that a systemic change to NBS should be given a higher priority and be preferred over conventional water infrastructure.

This paper provides an overview of current electric energy consumption of full-scale municipal MBR installations based on literature review and case studies. Energy requirements of several MBRs were linked to operational parameters and reactor performance. Total and specific energy consumption data were analysed on a long-term basis with special attention given to treated flow, design capacity, membrane area and effluent quality. The specific energy consumption of an MBR system is dependent on many factors, such as system design and layout, volume of treated flow, membrane utilization and operational strategy. Operation at optimal flow conditions results in a low specific energy consumption and energy efficient process. Energy consumption of membrane related modules was in the range of 0.5–0.7 kWh/m3 and specific energy consumption for membrane aeration in flat sheet (FS) was 33–37% higher than in a hollow fibre (HF) system. Aeration is a major energy consumer, often exceeding 50% share of total energy consumption. In consequence, coarse bubble aeration applied for continuous membrane cleaning remains the main target for energy saving actions. Also, a certain potential for energy optimization without immediate danger of affecting the quality of the produced effluent was observed.