water Review Nature-Based Units as Building Blocks for Resource Recovery Systems in Cities Eric D. van Hullebusch 1, * , Aida Bani 2 , Miguel Carvalho 3 , Zeynep Cetecioglu 4 , Bart De Gusseme 5,6 , Sara Di Lonardo 7 , Maja Djolic 8 , Miriam van Eekert 9 , Tjaša Griessler Bulc 10,11 , Berat Z. Haznedaroglu 12 , Darja Isteniˇ c 10,11 , Johannes Kisser 13 , Pawel Krzeminski 14 , Sanna Melita 15 , Dolja Pavlova 16 , El ˙ zbieta Plaza 17 , Andreas Schoenborn 18 , Geraldine Thomas 13 , Mentore Vaccari 19 , Maria Wirth 13 , Marco Hartl 13 and Grietje Zeeman 15   Citation: van Hullebusch, E.D.; Bani, A.; Carvalho, M.; Cetecioglu, Z.; De Gusseme, B.; Di Lonardo, S.; Djolic, M.; van Eekert, M.; Griessler Bulc, T.; Haznedaroglu, B.Z.; et al. Nature-Based Units as Building Blocks for Resource Recovery Systems in Cities. Water 2021, 13, 3153. https://doi.org/10.3390/w13223153 Academic Editor: Carmen Teodosiu Received: 30 September 2021 Accepted: 30 October 2021 Published: 9 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institut de Physique du Globe de Paris, Université de Paris, UMR 7154 CNRS, F-75238 Paris, France 2 Department of Environment and Natural Resources, Faculty of Agronomy and Environment, Agricultural University of Tirana, Rruga Paisi Vodica, 1029 Tirana, Albania; aida_alushi@hotmail.com 3 Associação CECOLAB, Collaborative Laboratory towards Circular Economy, R. Nossa Senhora da Conceição, 3405-155 Oliveira do Hospital, Portugal; miguel.carvalho@cecolab.pt 4 Department of Chemical Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden; zeynepcg@kth.se 5 Center for Microbial Ecology and Technology (CMET), Ghent University, 9000 Gent, Belgium; bart.degusseme@farys.be 6 Center for Advanced Process Technology for Urban REsource Recovery (CAPTURE), 9000 Gent, Belgium 7 Research Institute on Terrestrial Ecosystems-National Research Council (IRET-CNR), Via Madonna del Piano 19, 50019 Sesto Fiorentino, Italy; sara.dilonardo@cnr.it 8 Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia; mirkovic.maja@gmail.com 9 Department of Environmental Technology, Wageningen University, 6707 Wageningen, The Netherlands; miriam.vaneekert@wur.nl 10 Faculty of Civil and Geodetic Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia; tjasa.bulc@zf.uni-lj.si (T.G.B.); darja.istenic@zf.uni-lj.si (D.I.) 11 Faculty of Health Sciences, University of Ljubljana, 1000 Ljubljana, Slovenia 12 Institute of Environmental Sciences, Bogazici University, Istanbul 34342, Turkey; berat.haznedaroglu@boun.edu.tr 13 Alchemia-Nova, Institute for Circular Economy & Nature-Based Solutions, 1140 Vienna, Austria; jk@alchemia-nova.net (J.K.); geraldine.thomas@alchemia-nova.net (G.T.); maria.wirth@alchemia-nova.net (M.W.); marco.hartl@alchemia-nova.net (M.H.) 14 Norwegian Institute for Water Research, Økernveien 94, N-0579 Oslo, Norway; pawel.krzeminski@niva.no 15 LeAF BV, Bornse Weilanden 9, 6708 Wageningen, The Netherlands; sanna.melita@wur.nl (S.M.); zeemangrietje@gmail.com (G.Z.) 16 Departmentof Botany, Faculty of Biology, University of Sofia, 1164 Sofia, Bulgaria; pavlova@biofac.uni-sofia.bg 17 Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden; elap@kth.se 18 Institute of Natural Resources Science, Zurich University of Applied Science, 8400 Waedenswil, Switzerland; andreas.schoenborn@zhaw.ch 19 Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, Via Branze 43, 25123 Brescia, Italy; mentore.vaccari@unibs.it * Correspondence: vanhullebusch@ipgp.fr Abstract: Cities are producers of high quantities of secondary liquid and solid streams that are still poorly utilized within urban systems. In order to tackle this issue, there has been an ever-growing push for more efficient resource management and waste prevention in urban areas, following the concept of a circular economy. This review paper provides a characterization of urban solid and liquid resource flows (including water, nutrients, metals, potential energy, and organics), which pass through selected nature-based solutions (NBS) and supporting units (SU), expanding on that characterization through the study of existing cases. In particular, this paper presents the currently implemented NBS units for resource recovery, the applicable solid and liquid urban waste streams and the SU dedicated to increasing the quality and minimizing hazards of specific streams at the Water 2021, 13, 3153. https://doi.org/10.3390/w13223153 https://www.mdpi.com/journal/water