water

Article
Nature-Based Solutions for Agriculture in Circular Cities:
Challenges, Gaps, and Opportunities
Alba Canet-Martí 1,† , Rocío Pineda-Martos 2, *,† , Ranka Junge 3 , Katrin Bohn 4 , Teresa A. Paço 5 ,
Cecilia Delgado 6 , Gitana Alenčikienė 7 , Siv Lene Gangenes Skar 8 and Gösta F. M. Baganz 9,10

1 Institute for Sanitary Engineering and Water Pollution Control (SIG),

University of Natural Resources and Life Sciences Vienna (BOKU), Muthgasse 18, 1190 Wien, Austria;
alba.canet@boku.ac.at
2 Urban Greening and Biosystems Engineering Research Group (NatUrIB), Departamento de Ingeniería
Aeroespacial y Mecánica de Fluidos-Área de Ingeniería Agroforestal,
Escuela Técnica Superior de Ingeniería Agronómica (ETSIA), Universidad de Sevilla, Ctra. de Utrera km. 1,
41013 Seville, Spain
3 Ecological Engineering Centre, Institute of Natural Resource Sciences, Zurich University of Applied Sciences,
Grüentalstrasse 14, 8820 Waedenswil, Switzerland; ranka.junge@zhaw.ch
4 Centre for Spatial, Environmental and Cultural Politics, School of Architecture and Design,
University of Brighton, Mithras House, Lewes Road, Brighton BN2 4AT, UK; k.bohn@brighton.ac.uk
5 Linking Landscape, Environment, Agriculture And Food (LEAF), Instituto Superior de Agronomia,
Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal; tapaco@isa.ulisboa.pt
6 Interdisciplinary Center of Social Sciences (CICS.NOVA), Faculdade de Ciências Sociais e Humanas,



Universidade Nova de Lisboa, Av. de Berna 26-C, 1069-061 Lisboa, Portugal; ceciliadelgado@fcsh.unl.pt

 7 Food Institute, Kaunas University of Technology, Radvilenu 19C, LT 50524 Kaunas, Lithuania;
gitana.alencikiene@ktu.lt
Citation: Canet-Martí, A.;
8 Division Food Production and Society, Department Horticulture,
Pineda-Martos, R.; Junge, R.; Bohn,
Norwegian Institute of Bioeconomic Research (NIBIO), Reddalsveien 215, 4886 Grimstad, Norway;
K.; Paço, T.A.; Delgado, C.;
siv.skar@nibio.no
Alenčikienė, G.; Skar, S.L.G.; Baganz, 9 Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Müggelseedamm 301, 12587 Berlin,
G.F.M. Nature-Based Solutions for Germany; g.baganz@igb-berlin.de
Agriculture in Circular Cities: 10 Faculty of Architecture, RWTH Aachen University, Schinkelstr. 1, 52062 Aachen, Germany
Challenges, Gaps, and Opportunities. * Correspondence: rpineda@us.es
Water 2021, 13, 2565. https:// † A.C.-M. and R.P.-M.: These authors contributed equally to this work.
doi.org/10.3390/w13182565
Abstract: Urban agriculture (UA) plays a key role in the circular metabolism of cities, as it can use
Academic Editor: water resources, nutrients, and other materials recovered from streams that currently leave the city
Marie-Christine Gromaire as solid waste or as wastewater to produce new food and biomass. The ecosystem services of urban
green spaces and infrastructures and the productivity of specific urban agricultural technologies
Received: 29 July 2021
have been discussed in literature. However, the understanding of input and output (I/O) streams of
Accepted: 13 September 2021
different nature-based solutions (NBS) is not yet sufficient to identify the challenges and opportunities
Published: 17 September 2021
they offer for strengthening circularity in UA. We propose a series of agriculture NBS, which,
implemented in cities, would address circularity challenges in different urban spaces. To identify
Publisher’s Note: MDPI stays neutral
the challenges, gaps, and opportunities related to the enhancement of resources management of
with regard to jurisdictional claims in
published maps and institutional affil-
agriculture NBS, we evaluated NBS units, interventions, and supporting units, and analyzed I/O
iations. streams as links of urban circularity. A broader understanding of the food-related urban streams
is important to recover resources and adapt the distribution system accordingly. As a result, we
pinpointed the gaps that hinder the development of UA as a potential opportunity within the
framework of the Circular City.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
Keywords: urban agriculture; nutrient streams; urban food systems; urban circularity challenges;
This article is an open access article resources management; urban sustainability
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).

Water 2021, 13, 2565. https://doi.org/10.3390/w13182565 https://www.mdpi.com/journal/water

Water 2021, 13, 2565 2 of 22

1. Introduction
In the face of growing concerns about resource constraints and the need to act on the
global climate emergency, many countries intend to move towards a greener, competitive,
and “resourceful” urban circular economy (CE) [1–3]. Food and biomass production can
significantly contribute to closing of material cycling, thus maximizing the reuse of re-
sources in the urban environment itself while reducing the need for external resource inputs
(I) [4–7]. The primary production of food and biomass within the city has environmental,
social, and economic benefits depending on how the nature-based solutions (NBS) are
implemented. The COST Action CA17133 Circular City “Implementing nature-based solu-
tions for creating a resourceful circular city” (https://circular-city.eu, accessed on 28 July
2021) defines NBS as “concepts that bring nature into cities and those that are derived from
nature. As such, within this definition, we achieve resource recovery using organisms (e.g.,
microbes, algae, plants, insects, and worms) as the principal agents. However, physical
and chemical processes can be included for recovery of resources, as they may be needed
for supporting and enhancing the performance of NBS” [6,8,9]. This definition is used as a
reference concept in the present study.

1.1. Advantages and Challenges in the Contribution of Urban Agriculture towards Circularity
in Cities
Placing food production in the city offers ample potential to improve the sustainability
of the urban food systems. One aspect of urban placement is the shorter distance between
food-production sites and consumers or stores, enabling faster delivery and reduction
of storage capacity. Short food-supply chains are easier to supervise regarding quality
and origin [10,11] and can contribute to food security [10,12]. They enable reduction of
response reaction times to consumer demands and adaptation of cultivation programs to
the needs of consumers [10]. Shortening distances decreases the use of fossil fuels in food
transportation and, consequently, decreases the emissions of carbon dioxide [13], thereby
contributing to climate change mitigation. Since these and other advantages/benefits—
i.e., food security, economic, social, and environmental dimensions—of urban agriculture
(UA) can lead to cleaner and more sustainable cities [4], it is important to consider the
environmental impacts of any urban food production.
Introducing circular processes into the city offers opportunity to increase sustainability,
and in this respect, Atanasova et al. [5] formulated a set of urban circularity challenges
(UCC) [6,7]. Closing of the key cycles (i.e., water, nutrients, materials . . . ) as much
as possible [5–7,9] optimizes the utilization of urban resources [14,15]. Addressing the
UCC3 on “Nutrient recovery and reuse” [5] comprises areas of great concern in UA, e.g.,
nutrient streams—especially when phosphorous (P) is involved. Furthermore, issues arise
concerning resilience and resource efficiency of urban food systems towards a CE approach:
security and safety, transport and economic activities, food loss and waste management,
and more, especially in relation to unexpected events and/or crisis, such as the COVID-19
pandemic and its lockdown measures [16].
If implemented to a high standard, UA can respond to several of the UCC [5–7], and it
will cover a range of scales—from small scale, such as domestic food growing [17], to large
scale, such as in peri-urban farming. Urban agriculture addresses primarily the UCC5 of
“Food and biomass production”; however, it touches on most other UCC as well [5–7]. Its
primary production sites are located within the city boundaries or in transitional urban
hinterland zones. Conceptual solutions for such zoning were already suggested in the
nineteenth century by von Thünen in 1827 [18] and Howard in 1898 [19] in order to improve
urban sustainability and further developed in the twenty-first century, e.g., within the
Continuous Productive Urban Landscape concept [20].
Urban agriculture also requires a joint adaptation of other UCC within the urban-
rural nexus, such as nutrient recovery and reuse (UCC3 ), urban water management and
treatment (UCC1,2 ), and improved energy efficiency (UCC6 ) [5–7]. The geographical
locations, complex networks, and individual characteristics of each UA project— including
Water 2021, 13, 2565 3 of 22

its input (I) and output (O) resources—are of great importance for the project’s success.
However, for many sites and designs, only limited information is available about the type
and interaction of food-focused NBS and their I/O streams, such as water or nutrients [7].
These missing site-resource inventories are one of the main gaps that prevent the circularity
of UA. The present study aims to address this gap.

1.2. What Does Circularity Imply for Urban Agriculture?

According to the CE concept [9,21] cities can work towards three ambitions for a CE
regarding food: (1) “sourcing food grown regeneratively and locally where appropriate”,
e.g., implementing circular urban farming systems, such as aquaponics [7,22,23]; (2) “mak-
ing the most of food” by reducing food waste and/or transforming it into new products;
and (3) “designing and marketing healthier food products”, such as novel plant-based
proteins, as alternatives to meat and dairy.
The current global food system has a notable environmental impact. Agriculture uses
85% of global water resources [24] and is responsible for about a quarter of all greenhouse
gasses released by human activity. Food system analysis reveals that natural resource use
and emissions associated with modern systems can be substantially reduced by shifting
towards a circular system [25,26]. The aim is to reduce resource consumption and emissions
to the environment, e.g., by closing the loop of materials. Moving towards a food system
that sources and produces locally will prevent the leakage of elements, such as carbon (C),
nitrogen (N), and phosphorous (P) and stimulate the reuse and recycling of resources in a
way that adds value to the system [5,7].
According to de Boer and van Ittersum [27], circularity in agricultural production
comprises three principles: (1) “plant biomass is the basic building block of food and
should be used by humans first”; (2) “by-products from food production, processing, and
consumption should be recycled back into the food system”; and (3) “use animals for what
they are good at”, i.e., from “low-opportunity-cost feeds” to valuable outputs and products.
While 10% of the world’s population lives in hunger, a third of the food produced in
the world is wasted every year, together with an increasing trend of population intensifica-
tion [3,28]. Edible food surpluses can be redistributed, and products that are no longer edi-
ble could be turned into new products—from organic fertilizers to biomaterials, medicines,
and bioenergy, thus boosting new sources of income in the bioeconomy [4,9,29,30].
The food production in a CE minimizes or eliminates waste, emulating natural pro-
cesses in ecosystems where waste is transformed into resources that feed other processes.
To be safely returned to the soil as compost or fertilizer, recovered resources must be
free of contaminants. This implies separate treatment of waste streams to avoid cross-
contamination [21,31]. The resulting cycles can contribute to the regeneration of ecosystems,
which in turn provide renewable resources and support biodiversity. Investments in im-
plementation and efficiency improvements are necessary for long-term success in the
transition from linear to circular food systems [7,32].

1.3. The Objectives of This Study

The present study addresses the significance, roles, opportunities, and threats of
UA within urban sustainability and climate resilience. It places UA as a key activity of
any future city that decisively impacts on urban circularity measures and, at the same
time, is itself impacted by urban circularity. Aiming to understand and make visible the
necessary resource streams in relation to urban food, we discuss selected UA typologies in
their complex interactions with other aspects of a circular city, namely the water, nutrient,
material, waste, and building system cycles as well as energy flows [5–7].
Following the framework proposed by Langergraber et al. [6] to address UCC using
NBS, this research aims to:
1. Evaluate NBS units (NBS_u), NBS interventions (NBS_i), and supporting units (S_u)
addressing UCC on food and biomass production [6,33];
Water 2021, 13, 2565 4 of 22

2. Define the input and output (I/O) streams, analyzing the inputs (I) necessary for the
operation and the outputs (O) generated by UA related NBS (hereinafter UA-NBS);
3. Summarize the main circularity aspects that are relevant for UA; and
4. Pinpoint the gaps that currently hinder the efficient development and implementation
of UA-NBS within the Circular City framework [8,9].

2. Materials and Methods

To answer the research questions above, four elicitation workshops with a multidisci-
plinary team of experts were held between January and April 2021 within the framework
of the COST Action CA17133 Circular City. The workshops were based on the IDEA proto-
col, which stands for “Investigate”, “Discuss”, “Estimate”, and “Aggregate” [34,35]. The
workshops’ participants were divided into four working groups (WG) formed within the
COST Action Circular City and corresponding to the sectors of “Built Environment” (WG1);
“Sustainable Urban Water Utilization” (WG2); “Resource Recovery” (WG3); and “Urban
Farming” (WG4) [8,9]. The total number of participants in the Circular City workshops
ranged from 70 to 81, and the UA expert group (WG4) was run by 6–11 members [7].
The WG4 comprised experts in agronomy, food science, urban planning and architecture,
aquaponics systems, water-food-energy nexus, agricultural water management, participa-
tory systems, and governance (further details can be found in Langergraber et al. [6,7]).
According to the methodology reported by Castellar et al. [33] and Langergraber
et al. [6], the NBS were classified into NBS units (NBS_u), differentiating between spatial
units (NBS_su) and technological units (NBS_tu), and NBS interventions (NBS_i), including
soil interventions (NBS_is) and river interventions (NBS_ir), following the classification
of Castellar et al. [33]. The list of NBS corresponded to that extended by Langergraber
et al. [6], in which supporting units (S_u) were also considered to improve the functioning
of the NBS. All these units addressed at least one UCC [5–7].
During the workshops, the following questions were posed to the UA expert group
(i.e., WG4 members):
• How do the NBS_u, NBS_i, and S_u contribute to food and/or biomass production?
• Which NBS_u, NBS_i, and S_u are relevant to UA?
• How is food and biomass production (UCC5 ) related to the other UCC?
• What are the main I/O streams of UA-NBS?
• What are the key opportunities and challenges for achieving circularity in UA?

2.1. Identification of Nature-Based Solutions Relevant for Urban Agriculture

The following steps were taken to identify the most relevant NBS regarding UA, i.e.,
selected from UA-NBS, and classify them according to the urban space in which they are
located (implementation):
• Evaluation of food and biomass production (UCC5 ): From a list of fifty-one NBS_u and
NBS_i and ten S_u proposed by Langergraber et al. [6] and based on their contribution
to the UCC5 [5], a separate evaluation regarding food and/or biomass production was
made for each NBS_u/i and S_u. To have a more accurate categorization adapted to
the UCC5 , the rating was based on the relevance of either food or biomass inputs (I)
or outputs (O) (Table 1). Thus, the proposed categories were as follows (Table 1):
1. Food and/or biomass production with relevant I and/or O: UA-NBS whose main
purpose is food and/or biomass production or that, due to its characteristics,
produce a relevant amount of food and/or biomass and/or consume it for their
operation;
2. Usable for food and/or biomass production: UA-NBS that may produce food
and/or biomass, even if it is not their primary purpose, contributing to the UCC5 ;
and
3. Food and/or biomass production with no relevant production levels: these UA-
NBS can produce plant material or food in small quantities. They are considered
as potential contributors that can be scaled up or designed for that purpose.
Water 2021, 13, 2565 5 of 22

• Urban agriculture-related NBS-composed list: The NBS_u/i and S_u considered

relevant for food and/or biomass production were those addressing, contributing,
and/or potentially contributing to the food and/or biomass production (UCC5 ).
• Classification according to typologies and urban space (implementation): The NBS_u/i
related to UA were grouped according to the type of urban space they are associated
with: (A) as urban blue infrastructure (urban water); (B) as green infrastructure (GI)
in buildings (including containers); (C) as GI on buildings; (D) as GI for parks and
landscape; and (E) as GI for the urban farm. NBS_u/i can be located in one or multiple
urban spaces. The classification was based on the defining characteristics of the NBS,
the expert knowledge of workshop participants, and literature references.
• Selection of representative UA-NBS: To narrow the list and focus on food and biomass
production (UCC5 ), eight UA-NBS were selected as relevant representatives to assess
the I/O streams and identify circularity challenges. The selection was made according
to the available references, considering that all typologies and urban spaces were
covered, and upon the experience of the participants in the workshops. In order to
gather information on the selected UA-NBS, a literature search was carried out using
the names and synonyms given in Langergraber et al. [6].

Table 1. Marking system for urban circularity challenges (UCC) addressed by nature-based solutions units (NBS_u),
interventions (NBS_i), and supporting units (S_u), following Langergraber et al. [6,7] and categorization used for food and
biomass production (UCC5 ).

Mark General Category (UCC) Food and Biomass Production Category (UCC5 )
• Addresses directly the UCC Food and/or biomass production with relevant I and/or O
• Contributes to the UCC Usable for food and/or biomass production
# Contributes potentially depending on specific design Food and/or biomass production with no relevant production

2.2. Linkages between Food and Biomass Production and Other Urban Circularity Challenges
An evaluation of the NBS_u, NBS_i, and S_u in relation to the UCC was conducted to
identify the existing gaps and opportunities to approach circular UA successfully based
on the general assessment presented by Langergraber et al. [6]. The relationships revealed
whether the UA-NBS implementation facilitates addressing other UCC, i.e., an opportunity,
or whether it is a challenge to be considered.

2.3. Urban Agriculture-Related Nature-Based Solutions Circularity: Input and Output Streams
To identify the gaps in resource management within circular cities, I/O streams were
defined using NBS_u, NBS_i, and S_u as CE entities, following the methodology defined
by Baganz et al. [36]. General I/O streams were identified by all the WG participants from
the COST Action Circular City based on an interdisciplinary approach [6], and the “Urban
Farming” group (WG4) was focused on those streams directly related to UA. Following the
framework proposed by Langergraber et al. [6], we used a systematic approach to describe
in detail the resource streams (i.e., I/O streams) participating in the food and biomass
production (the food system) by means of UA-NBS [36]. By using this approach, it was
possible to determine the connection between the different sectors represented by the WG
and food and biomass production for better resource optimization (Figure 1) [7].
Water 2021, 13,
Water 2021, 13, 2565
x FOR PEER REVIEW 66 of
of 22
22

Figure 1. An urban agriculture centric view of the input and output

output streams studied within the working groups defined
by the COST Action CA17133 Circular City on “Built Environment” (WG1),
by the COST Action CA17133 Circular City on “Built Environment” (WG1), “Urban
“Urban Water”
Water” (WG2),
(WG2), “Resource
“Resource Recovery”
Recovery”
(WG3), and “Urban Farming” (WG4)
(WG3), and “Urban Farming” (WG4) [7].[7].

2.4. Identification of Key Challenges and Opportunities of Agricultural Nature-Based Solutions

2.4. Identification of Key Challenges and Opportunities of Agricultural Nature-Based Solutions in
in Circular
Circular Cities
Cities
A
A SWOT
SWOT analysis
analysis was
was used
used by the team
by the team of
of experts
experts of
of “Urban
“Urban Farming”
Farming” (Working
(Working
Group
Group 44ofofthethe COST
COST Action)
Action) participating
participating in the in the workshops
workshops to internal
to pinpoint pinpoint(strengths
internal
(strengths and weaknesses)
and weaknesses) and externaland(opportunities
external (opportunities andfactors
and threats) threats) factors influencing
influencing UA-NBS
UA-NBS while addressing
while addressing UCC, with UCC, with particular
particular attentionattention
to mattertoand
matter and flows
energy energyasflows as
well as
well as space, social, and economic
space, social, and economic effects. effects.

3. Results and Discussion

3.1. Nature-Based
3.1. Nature-Based Agricultural
Agricultural Solutions
Solutions for
for Food
Food and
and Biomass
Biomass Production
Production towards
towards Urban
Urban
Circularity
Circularity
The fifth UCC proposed by Atanasova et al. [5] on “Food and biomass production”
The fifth UCC proposed by Atanasova et al. [5] on “Food and biomass production”
was rated separately both for food and for biomass production by using the methodology
was rated separately both for food and for biomass production by using the methodology
reported by [6] and, specifically for the UCC , following the criteria presented in Table 1,
reported by [6] and, specifically for the UCC55, following the criteria presented in Table 1,
i.e., addressing the UCC , contribution to the UCC , and potential contribution, depending
i.e., addressing the UCC55, contribution to the UCC55, and potential contribution, depending
on specific design (see also Table 2 and Figure 2). Those UA-NBS not addressing the
on specific design (see also Table 2 and Figure 2). Those UA-NBS not addressing the UCC5
UCC5 (i.e., without food and/or biomass production) are not presented in Table 2 for
(i.e., without food and/or biomass production) are not presented in Table 2 for not being
not being considered within the objectives of this study (related details can be found in
considered within the objectives of this study (related details can be found in Langergra-
Langergraber et al. [7]).
ber et al. [7]).
In total, 43 UA-NBS (i.e., 40 NBS_u/i and 3 S_u) were selected to address UCC5 as
those implemented/designed to produce food and/or biomass; the match between food
Table 2. Selected urban agriculture related NBS units and interventions (UA-NBS_u/i) and supporting units (UA-S_u),
and biomass production was the final rate addressing UCC5 (Table 2, Figures 2 and S1).
addressing the fifth urban circularity challenge on “Food and biomass production” (UCC5) [5,6]: ● addressing the UCC5
We propose a S_u on Chemical and biological methods (S11) to be considered as ad-
by food production, biomass production,
dressing and food
UCC5, sinceand
it biomass
was notproduction
previously(score ● contribution
= 1.00); in
reported and ο proposed
the framework potential by
contribution to the UCC5 depending on specific design (scores = 0.66 and 0.33, respectively). Empty
Langergraber et al. [6]. This S_u would include those enzymatic and fermentation cells are those UA- pro-
NBS_u/i and UA-S_u not addressing the UCC5 via food or biomass production.
cesses involving UCC5 —mainly biomass production/transformation [37] (Table 2).
Classification 1,2 (#) UA-NBS_u/i and UA-S_u 3 Food Biomass UCC5 Implementation 4
(1) Infiltration basin ○ ○ A
● NBS_tu (5) (Wet) Retention pond ○ ○ A
(7) Bioretention cell ○ ○ A
Water 2021, 13, 2565 7 of 22

Table 2. Selected urban agriculture related NBS units and interventions (UA-NBS_u/i) and supporting units (UA-S_u),
addressing the fifth urban circularity challenge on “Food and biomass production” (UCC5 ) [5,6]: • addressing the UCC5
by food production, biomass production, and food and biomass production (score = 1.00); • contribution and o potential
contribution to the UCC5 depending on specific design (scores = 0.66 and 0.33, respectively). Empty cells are those
UA-NBS_u/i and UA-S_u not addressing the UCC5 via food or biomass production.

Classification 1,2 (#) UA-NBS_u/i and UA-S_u 3 Food Biomass UCC5 Implementation 4
(1) Infiltration basin # # A
(5) (Wet) Retention pond # # A
(7) Bioretention cell # # A
(8) Bioswale # # A
• NBS_tu
(9) Dry swale # # A
(10) Tree pits # # # A,D
(11) Vegetated grid pavement # # A,D
(12) Riparian buffer • • A
(13) Ground-based green facade • • • B,C
(14) Wall-based green facade • • • B,C
(15) Pot-based green facade • • • B,C
(16) Vegetated pergola • • • B,C
• NBS_tu
(17) Extensive green roof • • • C,D
(18) Intensive green roof • • • C,D,E
(19) Semi-intensive green roof • • • C,D
(20) Mobile green and vertical mobile
• • • B,C
garden
NBS_tu (21) Treatment wetland • • A,D
(23) Composting • • • C,E
NBS_is
• (25) Phytoremediation • • B,C
(S6) Biochar/Hydrochar production • • —
S_u (S7) Physical unit operations for
• • —
solid/liquid separation
(S11) Chemical and biological methods • • —
(28) River restoration • • A,D
• NBS_ir (29) Floodplain • • A,D
(32) Coastal erosion control # # A,D
(33) Soil improvement and conservation # • • D,E
• NBS_is (34) Erosion control # # D,E
(36) Riverbank engineering # # A,D
(37) Green corridors # • • D,E
(38) Green belt # • • A,D
(39) Street trees • • • D
• NBS_su (40) Large urban park • • • D,E
(41) Pocket/garden park • • • D,E
(42) Urban meadows # • • D
(43) Green transition zones # • • D
(44) Aquaculture • # • A
(45) Hydroponic and soilless technologies • • • A,B,C,E
NBS_tu (46) Organoponic/Bioponic • • • A,B,C,E
(47) Aquaponic farming • • • A,B,C,E
•
(48) Photo Bio Reactor • • B,C
(49) Productive garden • • • D,E
NBS_su (50) Urban forest • • • D
(51) Urban farms and orchards • • • D,E
1 • Rainwater Management, • Vertical Greening Systems and Green Roofs, • Remediation, Treatment, and Recovery, • (River) Restoration,
• Soil and Water Bioengineering, • (Public) Green Space, • Food and Biomass Production, following color legend presented at Langer-
graber et al. [6,7]. 2 NBS_tu, nature-based solution technological unit; NBS_is, soil intervention; S_u, supporting unit; NBS_ir, river
intervention; NBS_su, spatial unit [6,7]. 3 Numbered (#) according to Langergraber et al. [6,7]. 4 Typology and urban space where the rated
UA-NBS_u/i would be implemented: (A) as urban blue infrastructure; (B) as green infrastructure (GI) in buildings; (C) as GI on buildings;
(D) as GI for parks and landscape; and (E) as GI for the urban farm.
 Water 2021, 13, x FOR PEER REVIEW 8 of 22

We propose a S_u on Chemical and biological methods (S11) to be considered as address-

Water 2021, 13, 2565 ing UCC5, since it was not previously reported in the framework proposed by Langergra-
8 of 22
ber et al. [6]. This S_u would include those enzymatic and fermentation processes involv-
ing UCC5—mainly biomass production/transformation [37] (Table 2).

Figure2.2. Colum
Figure Colum chart
chart representing
representing the
theselected
selected43
43NBS_u/i
NBS_u/iand
andS_u and
S_u categorized
and as as
categorized those ad-
those
dressing, contributing, and potentially contributing to the UCC5 on “Food and biomass production”,
addressing, contributing, and potentially contributing to the UCC5 on “Food and biomass produc-
respectively.
tion”, respectively.

Theselected
The selectedUA-NBS_u/i
UA-NBS_u/i and and S_u
S_u were
were grouped
grouped withinwithin thethe three
three main
main groups
groups pre- pre-
sented in Table 1 as those categorized as follows: (1) Food and/or
sented in Table 1 as those categorized as follows: (1) Food and/or biomass production biomass production with
relevant
with I and/or
relevant O, addressing
I and/or the UCC
O, addressing -11 NBS_u/i
the 5UCC and 1 S_u;
5 -11 NBS_u/i and 1(2) usable
S_u; for food
(2) usable forand/or
food
biomassbiomass
and/or production, contribution
production, to the UCC
contribution to the5-19
UCC NBS_u/i
5 -19 and
NBS_u/i 2 S_u;
and and
2 S_u;(3) food
and and/or
(3) food
biomassbiomass
and/or production with nowith
production relevant production,
no relevant representing
production, potential
representing contribution
potential contribu- de-
pending on specific design-10 NBS_u/i (cf. Tables 1 and 2, Figure
tion depending on specific design-10 NBS_u/i (cf. Tables 1 and 2, Figure 2) [6,7]. The later2) [6,7]. The later classi-
fication refers
classification to those
refers NBS_u/i
to those intrinsically
NBS_u/i intrinsicallycomposed
composed of vegetation
of vegetation andand notnotprimary
primary de-
signed forfor
designed food
food and/or
and/orbiomass
biomassproduction
production as theas ones
the onescategorized
categorizedfor “Rainwater
for “Rainwater Man-
agement” [6,7].[6,7].
Management” However, the actions
However, and infrastructures
the actions and infrastructures wouldwouldbe designed
be designed and imple-and
implemented
mented as food as food
and/orand/or
biomass biomass
systems systems and technologies
and technologies [7]. [7].
AAsecond
secondclassification
classificationconcerns
concernstotothe theimplementation
implementationofofthe therelevant
relevantUA-NBS_u/i
UA-NBS_u/i
and
and S_u according to their typology and typical urban site (Table 2). In thissense,
S_u according to their typology and typical urban site (Table 2). In this sense,18 18
NBS_u/i
NBS_u/i were classified as urban blue infrastructure (A); 10 as green
classified as urban blue infrastructure (A); 10 as green infrastructure (GI) infrastructure (GI)in
in buildings
buildings (B);1414asasGI
(B); GIon
onbuildings
buildings(C); (C);22 22 as
as GIGI for parks and and landscape
landscape (D); (D); and
and12 12
specifically
specificallyforforGI GIasasurban
urbanfarms
farms(E).
(E).Pearlmutter
Pearlmutteretetal. al.[38]
[38]presented
presentedthe thestate
stateofofthetheart art
on
onNBS
NBSin inthe
thebuilt
builturban
urbanenvironment
environmentas asthe
thelevel
levelofofgreen
greenbuilding
buildingmaterials,
materials,systems,systems,
and
andsites
sites[3].
[3].Similarly,
Similarly,somesomeofofthe theselected
selectedUA-NBS_u/i
UA-NBS_u/iand andS_uS_upresented
presentedin inthis
thisstudy
study
are
are classified following two of the three scales of described NBS implementationin
classified following two of the three scales of described NBS implementation inthe
the
built
builtenvironment
environmentbybyPearlmutter
Pearlmutter et et
al. al.
[38][38]
at green
at greenbuilding
buildingsystems
systems(i.e., (i.e.,
in/on buildings’
in/on build-
greening) and sites
ings’ greening) and(e.g.,
sitesparks
(e.g., and
parkslandscape
and landscapeand urban farms).farms).
and urban
3.2. Relevance of Nature-Based Solutions Related to Urban Agriculture to Address the Fifth Urban
3.2. Relevance of Nature-Based Solutions Related to Urban Agriculture to Address the Fifth
Circularity Challenge
Urban Circularity Challenge
We highlighted and analyzed eight UA-NBS_u (NBS_tu and NBS_su) from the pre-
viouslyWeselected
highlighted
groupand analyzed
of 40 eight
units and UA-NBS_u indicated
interventions (NBS_tu and NBS_su)
in Section 3.1from the pre-
as relevant
viously selected group of 40 units and interventions indicated in Section
representatives regarding the UCC5 [6,7]. Among them, two belonged to the category of 3.1 as relevant
representatives
“Vertical Greening regarding
System the UCC5Roofs”:
& Green [6,7]. Among them,green
wall-based two belonged
facade (14)to the
andcategory
intensiveof
green roof (18); one to the “(Public) Green Space” category: green corridors (37); intensive
“Vertical Greening System & Green Roofs”: wall-based green facade (14) and and five
green
to roof (18);
the “Food andone to theProduction”
Biomass “(Public) Green Space” category:
classification: hydroponicgreen
andcorridors (37); and five
soilless technologies
to theorganoponic/bioponic
(45), “Food and Biomass Production” classification:
(46), aquaponic farminghydroponic and soilless
(47), productive garden technologies
(49), and
(45), organoponic/bioponic (46), aquaponic farming (47), productive garden
urban farms and orchards (51) (Table 2, Figure S1). The selected representatives UA-NBS_u, (49), and ur-
ban farms and orchards (51) (Table 2, Figure S1). The selected representatives
were clustered as both general categories on addressing and contribution to the UCC5 , with UA-NBS_u,
food and/or biomass production with relevant I/O and as usable for food and biomass
production, respectively (Table 2, Figure S1).
Particular attention was given to describe their main characteristics and capacity for
food and/or biomass production, with an emphasis on their contribution to circularity in
Water 2021, 13, 2565 9 of 22

cities, identifying potential I/O streams, and how they relate to the city’s resource flows (cf.
Sections 3.3–3.5):
1. Wall-based green facade (14): Wall-based green facades, as “Vertical Greening Sys-
tems”, are known for their ability to mitigate urban heat island (UHI) effect and
to enhance building energy savings in the urban environment, e.g., increasingly,
the possibilities for crop production and wastewater treatment, particularly grey-
water [39–41]. They mostly consist of a modular vertical support structure with
vegetation, substrate, irrigation, and drainage systems. Depending on the purpose of
the system, different plants are used, with low-maintenance plants being the most
common option to minimize costs. This NBS_tu can produce ornamental plants (low
maintenance) as well as horticultural crops. When designed for food production, they
are generally used for self-consumption and local supply (e.g., restaurants, schools, or
hospitals) [42]. The yield depends on the crop/plant, type of substrate, management,
irrigation and drainage systems, and the climate and orientation when it is placed
outdoors. Indoor, wall-based green facades under controlled conditions at buildings
or greenhouses are mostly used to produce high-yielding crops. In order to address
circularity, it is relevant to characterize drainage water, which can be reused since it is
rich in nutrients. Additionally, wall-based green facades can be designed as modular
treatment systems when irrigated with wastewater, resembling constructed wetlands,
where plant matter can be harvested and used as biomass [39,43].
2. Intensive green roof (18): Green roofs can be used to cultivate agricultural products,
and their importance for this purpose has increased in recent years, as they provide
additional land space in urban centers [44,45]. Intensive green roofs are character-
ized by a substrate depth between 15 and 70 cm, which requires more maintenance
than extensive ones and allows for a wider choice of plants [46]. As an engineered
structure, a green roof requires prefabricated materials to be constructed, such as
protection and drainage layers, substrate, etc. Such structures may be built in residen-
tial buildings but also in commercial ones. For example, a supermarket in Brussels,
Belgium (Delhaize chain), has a 360 m2 urban farm on the rooftop for greenhouse
and open-air vegetables, with a certified organic production system [47]. The aim
is to control the production system and sell the products in the supermarket on the
ground floor, avoiding transportation and the need for a cold chain. The residual
heat from the refrigeration systems is used to heat the greenhouse, improving energy
efficiency (UCC6 ). Since the farm is small, and the impact is thus limited, it serves as
a demonstrator of possibilities for professional UA.
3. Green corridors (37): According to Castellar et al. [33], green corridors aim to re-
naturalize areas along derelict infrastructure, such as railways or along waterways
and rivers, by transforming them into linear parks. Green corridors can play an
important role in urban GI networks and can offer shelter, food, and protection for
the urban wildlife while enabling migration from one green patch to another. Back-up
irrigation may be provided by reclaimed wastewater, and the biomass produced can
be used for energy generation and composting. As for the vegetation planted, it
depends on the site and the objectives set. Forest species, fruit trees, and fruiting
shrubs or ornamental species are generally used. Lisbon (European Green Capital
2020) is an example of a network of nine green corridors that are part of the urban
GI. They cover an area of about 200 ha and contribute to ecological connectivity,
create spaces for UA, revalorize abandoned spaces by increasing soil permeability,
and improve the connection to other NBS specialized in rainwater retention and
infiltration [48]. Other cities, such as Montreal, Mexico City, Seoul, London, or
Singapore, also have green corridors that provide ecosystem services to the city [49].
At each site, this NBS_su is adapted to the local context, from the use of plant species
and the reuse of the available resources to the using of space according to social needs.
4. Hydroponic and soilless technologies (45): In hydroponics, plants grow in water con-
taining necessary macro- and micronutrients that are supplied by mineral fertilizers
Water 2021, 13, 2565 10 of 22

dissolved in water according to the plant-specific recipe. In ebb and flow systems and
in grow beds, the plants grow in different media, like mineral-/rockwool, vermiculite,
sand, gravel, etc., which also offer mechanical support [4,50,51]. Other soilless tech-
nologies, such as nutrient film technique, aeroponics, and deep flow technique, do not
involve media. Recently, soilless technologies are being innovated by implementing
artificial intelligence to learn the best way of composition of synthetic, mineral, or
organic fertilizers to grow the crop, often together with artificial light in greenhouses
or plant factories.
5. Organoponic/Bioponics (46): In contrast to hydroponic that relies on mineral fertil-
izers, bioponics is an emerging soilless technology for nutrients recovery that links
(organic) vegetable production to organic effluent remediation or organic waste recy-
cling [52]. The plants in growing media derive nutrients from natural animal, plant,
and mineral substances that are released by the biological activity of microorgan-
isms [53]. Bioponics allows closing nutrient cycles by using organic waste streams,
such as urine [54], biogas digestate [55], chicken manure [52,56], and others, thus
reducing the use of mineral fertilizers and the greenhouse gas emissions. Aquapon-
ics [57] could also be considered as a form of bioponics, as it utilizes waste streams
(process water, sludge) from an aquaculture. Synonyms used for bioponics are “or-
ganic hydroponic” [58,59], digeponics [60], or anthroponics [61]. Beside the source of
nutrients, the key difference between organic and conventional soilless culture is the
active promotion of microorganisms in bioponics to enhance nitrification, mineraliza-
tion, and disease suppression and thus contribute to productivity and plant quality
similar to soil-based systems [62].
6. Aquaponic farming (47): Aquaponics is a technology that couples tank-based ani-
mal aquaculture with hydroponics by using water from aquaculture for plant nu-
trition and irrigation. Trans-aquaponics extends this technology to tankless aqua-
culture and/or non-hydroponic plant cultivation. Aquaponic farming comprises
both aquaponic types, whereby aquaponic farming does not imply a specific size
but the fact that such this generic NBS_tu can embody both aquaponics types [22].
The NBS_tu can be established in very different setups: while aquaponics is often
implemented as controlled environment agriculture, trans-aquaponics includes, e.g.,
pond-aquaponics [63,64], outdoor aquaponics [65,66], aquaculture using constructed
wetland for sludge removal [22], and other integrated aqua-agriculture systems [67]
that exploit the aquaponics principle. Both technologies are often used for food pro-
duction, but aquaponics cannot be eco-certified because it exploits hydroponics and
is thus not soil-based, a precondition for eco-certification—at least in the European
Union. However, it is possible to meet a large city’s demand for tomatoes, fish, and
lettuce through aquaponic production, as shown in a case study related to Berlin [23].
7. Productive garden (49): Productive gardens are found around the world and con-
tribute significantly to food security. Vegetables, fruits, herbs, and, occasionally, small
livestock are produced in reduced spaces for the market, private consumption, or
educational purposes. The productivity of urban gardens depends on climate con-
ditions and type of crops and can exceed that of rural farms [68]; if correct cultures
are selected and machine-based crop treatment technologies are replaced by manual
work, it results in higher cropping density and higher biodiversity of crops to be
grown together [69]. Different types of cultivation can be selected for horticultural
crops, both in open fields and/or under cover.
8. Urban farms and orchards (51): Urban farms and orchards are part of the city’s
GI and are intended for food and biomass production. They are large enough to
grow cereal crops, fruit and vegetables, and even big livestock [6]. This NBS_su
can seek an economic profit or have social and educational purposes. It is common
to find urban farms located in public areas and managed by a community (e.g.,
neighborhood). While other NBS_u are more specific, with a defined configuration,
this unit encompasses a wide range of possibilities that make it very versatile. It is the
 cereal crops, fruit and vegetables, and even big livestock [6]. This NBS_su can seek
an economic profit or have social and educational purposes. It is common to find
Water 2021, 13, 2565 urban farms located in public areas and managed by a community (e.g., neighbor- 11 of 22
hood). While other NBS_u are more specific, with a defined configuration, this unit
encompasses a wide range of possibilities that make it very versatile. It is the NBS_su
that mostthat
NBS_su resembles the ruralthe
most resembles farms,
ruralwith thewith
farms, advantage of having
the advantage of the urban
having thestreams
urban
nearby to tap into. For example, food waste—which has a high nutritional
streams nearby to tap into. For example, food waste—which has a high nutritional value—
can be usedbeto
value—can feed
used to animals and and
feed animals lower thethe
lower production
production costs;
costs;on
onthe
the other hand,
other hand,
treated water can be used for irrigation [27]. Within its boundaries, several
treated water can be used for irrigation [27]. Within its boundaries, several other NBS other NBS
can be
can be implemented
implemented to to close
close loops,
loops, such
such asas composting
composting (23)
(23) or
or constructed
constructedwetlands
wetlands
for wastewater or runoff water treatment for on-farm resource recovery
for wastewater or runoff water treatment for on-farm resource recovery and reuse. and reuse.
Once the
Once the selected
selected UA-NBS_u
UA-NBS_u were were analyzed
analyzed individually,
individually, we wecompared
comparedtheir theirjoint
joint
contributions to the UCC (cf. Figure 3). Their contribution to food
contributions to the UCC5 (cf. Figure 3). Their contribution to food and biomass production,
5 and biomass produc-
tion,
as as defined
defined in Table in Table 1, resulted
1, resulted in scores
in scores of 1.00of(•1.00 (●),(•0.66
), 0.66 ), and(●),0.33
and(#),
0.33corresponding
(○), correspond- to
each representative NBS_u (cf. Table 2, Figure S1). Some UA-NBS_u have anhave
ing to each representative NBS_u (cf. Table 2, Figure S1). Some UA-NBS_u an im-
important
portant
share in share
both foodin both
andfood and production,
biomass biomass production, as isof
as is the case the case of productive
productive garden (49) garden
and
(49) and
urban urban
farms and farms and orchards
orchards (51). Other (51).UA-NBS_u
Other UA-NBS_u were specifically
were specifically designeddesigned
for foodfor
food production,
production, so thesocontribution
the contribution to food
to food production
production is higher
is higher thanthanthatthat of biomass,
of biomass, as inas
in the
the casecase of hydroponic
of hydroponic andand soilless
soilless technologies
technologies (45),
(45), organoponic/bioponic(46),
organoponic/bioponic (46),and
and
aquaponic farming
aquaponic farming (47).
(47). However,
However, they they can
can also
also bebeused
usedfor forbiomass
biomassproduction
production[7]. [7].InIn
contrast, green
contrast, green corridors
corridors (37)
(37) generally
generally produce
produce large
large amounts
amounts of of biomass;
biomass; however,
however,the the
capacityto
capacity to produce
producefood foodisis lower,
lower,generally
generallyattributed
attributedto toberries
berriesand andfruits
fruitsfrom
fromtrees
treesand
and
shrubs. IfIfdesigned
shrubs. designedto to include
include production
production sitessites
(e.g.,(e.g., productive
productive gardens) gardens) or specific
or specific plants,
they canthey
plants, contribute to food production.
can contribute A wall-based
to food production. green facade
A wall-based green (14) can also
facade (14)produce
can also
food and food
produce biomass,
and although
biomass, its main purpose
although its mainispurpose
often UHI mitigation
is often and building
UHI mitigation andenergy
build-
savings. Finally, an intensive green roof (18) encompasses different
ing energy savings. Finally, an intensive green roof (18) encompasses different types of types of herbaceous
and shrub species,
herbaceous and shrubincluding
species,trees, producing
including biomass;
trees, producing however,
biomass;it can also contribute
however, it can alsoto
food production.
contribute to food production.

Figure3.3.Evaluation
Figure Evaluationfor
forfood
food
andand biomass
biomass production
production of nature-based
of nature-based agricultural
agricultural solutions
solutions se-
selected
lected
as as urban
urban agriculture
agriculture representatives.
representatives. Numbers
Numbers of technological
of technological andand spatial
spatial units(NBS_tu
units (NBS_tuand
and
NBS_su)corresponds
NBS_su) correspondstotothat
thatgiven
givenby
byLangergraber
Langergraberetetal.
al.[6]
[6](cf.
(cf.Table
Table2).
2).

3.3. Interfaces between Food and Biomass Production and the other Six Urban Circularity
3.3. Interfaces between Food and Biomass Production and the Other Six Urban
Challenges Challenges
Circularity
TheUCC
The UCCon on“Food
“Foodand
andbiomass
biomassproduction”
production”(UCC
(UCC55))[5]
[5]seeks
seeksto
to close
close the
the production
production
loop, maximizing the use of available resources while reducing the need
loop, maximizing the use of available resources while reducing the need for external for external
re-
source inputs. The UA-NBS_u addressing the UCC5 are closely related to other UCC [5–7].
Urban food production faces challenges, such as nutrient and water supply, urban planning,
and energy efficiency. Conversely, the urban environment also offers opportunities for
farming different to those in rural environments. Figure 4 indicates whether the imple-
mentation of UA-NBS is an opportunity to address other UCC or whether an UCC poses
Water 2021, 13, 2565 12 of 22

a challenge for food and biomass production in order to close material loops, improve
energy efficiency, and make use of urban spaces.
(1) “Restoring and maintaining the water cycle (by rainwater management)”—UCC1 :
Several NBS_u/i and S_u identified as relevant for the UCC5 also address the UCC1 .
The nature of the NBS_u/i, with a significant vegetation component and located in
different urban spaces, such as the UA-NBS classified as “Vertical Greening Systems
and Green Roofs” and “(Public) Green Space”—e.g., green corridors (37) and large
urban parks (40) —, enable the restoration and maintenance of the water cycle at dif-
ferent scales. These NBS_u/i facilitate processes, such as water retention, infiltration,
transport, treatment, and evapotranspiration [70]. The UA-NBS_su from the category
of “Food and Biomass Production”, i.e., productive garden (49), urban forest (50),
and urban farms and orchards (51), are also relevant for the UCC1 , as they enable the
same processes as the above. The implementation of these UA-NBS_u is seen as an
opportunity to regulate the water cycle and not a barrier to be overcome in the sector
of UA.
(2) “Water and waste treatment, recovery, and reuse”—UCC2 : NBS_u/i and S_u ad-
dressing the UCC2 are crucial for UA, as water is a continuous input stream to most
UA-NBS. In general, a minimum quality is required to use reclaimed water for ir-
rigation and fertigation. In addition, some UA-NBS may require a certain quality
depending on the crop or culture. Furthermore, the effluent water from UA-NBS, e.g.,
aquaculture (44) and photo bio reactor (48), needs to be treated, and for this purpose,
other NBS_u/i and/or S_u, such as circular systems like aquaponic farming (47), can
be implemented [71].
(3) “Nutrient recovery and reuse”—UCC3 : Nutrient recovery, reuse, and recycling is key
to achieving a circular metabolism of cities [31,72]. For this purpose, it is necessary to
identify and analyze the nutrient-rich flows generated in the city, such as wastewater
or organic waste. Urban agriculture harnesses the recovered nutrients and keeps
them in the urban system. Besides, the NBS_u and S_u from the category of “Remedi-
ation, Treatment and Recovery” [6,7] (cf. Table 2) comprise anaerobic treatment (26),
phosphate precipitation (for P recovery) (S3), and ammonia stripping (for N recovery)
(S4), and they are not considered as relevant for food and biomass production because
they do not generate food and/or biomass to a significant extent nor require it to
operate. However, they may be crucial for nutrient recovery from urban streams to be
used in UA-NBS. In addition, the recovered nutrients must be able to meet the needs
of crops or living organisms, considering the macro- and micronutrients required for
production. It is therefore seen as both a challenge and an opportunity to recover and
reuse nutrients.
(4) “Material recovery and reuse”—UCC4: Material recovery is seen as an opportunity for
UA. Urban agriculture can provide a considerable amount of biomass that can be used
for several purposes, e.g., building materials, soil amendment, or energy production.
For example, biochar/hydrochar production (S6) and composting (23), classified
as S_u and NBS_is, respectively (“Remediation, Treatment and Recovery”), can be
obtained from the biomass produced in vertical greening systems and agricultural
waste. Biodegradable materials, such as wood, can be used directly to build structures.
One challenge would be to replace stable insulating materials, such as plastic and
glass, or materials used in irrigation pipes. This could be accomplished by using
recovered and/or recycled materials.
(5) “Energy efficiency and recovery”—UCC6: Mitigation of UHI effect is one of the
strengths of UA-NBS in urban outdoor spaces such as infrastructure, i.e., NBS_u/i
and S_u located in/on buildings, in parks and landscape, and/or urban farms. At the
building scale, green roofs or vertical greening systems can improve energy efficiency
by reducing rooftop and walls’ surface temperature during summer, improving
insulation and decreasing heat losses during the cold season [42]. On the other hand,
NBS_u that include greenhouses or are located indoors may require energy to regulate
Water 2021, 13, 2565 13 of 22

room temperature and to provide artificial lighting. However, high-yield crops or

indoor urban vertical farming using hydroponics and soilless technologies (45) can
substantially increase energy efficiency [73,74]. In addition, within the urban system,
Water 2021, 13, x FOR PEER REVIEW there is the possibility of recovering heat sources for food and biomass production 12 of 22
that would otherwise be lost.
(6) “Building system recovery”—UCC7: An urban system is multi-stakeholder and space-
constrained;
resource inputs. Thetherefore,
UA-NBS_uthe essential planning
addressing the UCC to5 are
achieve circularity
closely related toisother
challenging.
UCC [5–
Both the design of new spaces and the retrofitting and adaptation
7]. Urban food production faces challenges, such as nutrient and water supply, of old ones require
urban
planning for the effective implementation of the NBS_u/i. By using UA-NBS,
planning, and energy efficiency. Conversely, the urban environment also offers opportu- urban
spaces
nities can bedifferent
for farming revalorized, although
to those the
in rural complexity ofFigure
environments. the urban system
4 indicates makes the
whether it
a challenge for food and biomass production, as there are different
implementation of UA-NBS is an opportunity to address other UCC or whether an UCC ownerships,
available spaces, and regulations to consider. Achieving circularity may require
poses a challenge for food and biomass production in order to close material loops, im-
new approaches.
prove energy efficiency, and make use of urban spaces.

Figure4.4.Relationships
Figure Relationshipsamong
amongfood foodand
andbiomass
biomassproduction
productionand
andother
otherurban
urbancircularity
circularitychallenges
challenges
describedby
described byAtanasova
Atanasova et et al. [5–7]. The
Thethickness
thicknessofofthe
thearrows
arrowsindicates
indicatesthethe
relevance
relevance of of
thethe
op-
portunity ororchallenge,
opportunity respectively.
challenge, respectively.UCC:
UCC:(1) (1)
Restoring andand
Restoring maintaining
maintainingthe the
water cyclecycle
water (by rain-
(by
water management);
rainwater management);(2) (2)
water
waterandand
waste treatment,
waste recovery,
treatment, andand
recovery, reuse; (3) (3)
reuse; nutrient recovery
nutrient recoveryand
reuse; (4) material recovery and reuse; (5) food and biomass production; (6) energy efficiency and
and reuse; (4) material recovery and reuse; (5) food and biomass production; (6) energy efficiency
recovery; and (7) building system recovery [5].
and recovery; and (7) building system recovery [5].

(1) Contribution
3.4. “Restoringofand maintaining
Input and Outputthe water
Streams to cycle
Urban(by rainwater
Circularity management)”—UCC1:
in Nature-Based
SeveralSolutions
Agricultural NBS_u/i and S_u identified as relevant for the UCC5 also address the UCC1.
The nature
From of the NBS_u/i,
a CE perspective, UA iswith
seena as
significant vegetation
an opportunity component
to counteract theand located
linear “take-in
different urban spaces, such as the UA-NBS classified as “Vertical
make-waste” economy [16]. Urban agriculture can be designed to minimize the need for Greening Systems
andinputs
external GreentoRoofs”
produce andfood
“(Public) Green Space”—e.g.,
and biomass to be consumed green corridors
in the (37)emerging
city. This and large
urban parks
and inclusive (40) —,
approach enable of
consists themaking
restoration and maintenance
the most of the materials of the
and water
wastecycle at dif-
streams
ferent
used for scales. These
production, NBS_u/i
closing waterfacilitate processes,
and nutrient loops,such
andas water retention,
reducing discharges infiltration,
into the
transport,
environment treatment,
[7,16,75]. and sense,
In this evapotranspiration [70].to
circularity refers The
theUA-NBS_su
connectionfrom the category
between urban
of and
streams “Foodtheand Biomass
streams Production”,
needed in UA. From i.e., productive garden
the standpoint (49),
of the urban forest
NBS_u/i (50),
and S_u,
and urban farms and orchards (51), are also relevant for the UCC
an urban stream of matter or energy with the appropriate characteristics becomes an 1 , as they enable the
input same processes
stream. In turn,asthetheoutput
above.stream
The implementation
of one NBS_u/i of or
these
S_uUA-NBS_u
can become is the
seeninput
as an
streamopportunity to regulate
of another, thus tappingthe water
into urbancycle and notBased
resources. a barrier to be overcome
on system analyses in of the sector
resource
of and
streams UA. a corresponding streams information model to describe inputs and outputs
(2) “Water and waste treatment, recovery, and reuse”—UCC2: NBS_u/i and S_u address-
ing the UCC2 are crucial for UA, as water is a continuous input stream to most UA-
NBS. In general, a minimum quality is required to use reclaimed water for irrigation
and fertigation. In addition, some UA-NBS may require a certain quality depending
on the crop or culture. Furthermore, the effluent water from UA-NBS, e.g., aquacul-
Water 2021, 13, 2565 14 of 22

(I/O) [7], a practical solution to enhance circularity by concrete streams is presented

(Table 3, Figure S2).

Table 3. Biomass and living organisms as resource streams related to nature-based solutions units (NBS_u) associated with
urban agriculture while address the fifth urban circularity challenge on “Food and biomass production” [5–7].

Stream Type Category Subcategory I in UA-NBS_u 1 O from UA-NBS_u 1

Compost
Organic fertilizer (18) (37) (49) (51) 2
Manure (types)
Mulch
Biomass Organic crop protection Woodchips (18) (37) (49) (51)
Biochar
Food waste Vegetables, fruits — (18) (37) (49) (51)
(14) (18) (37) (45) (46)
Crop residues
(18) (37) (49) (51) (47) (49) (51)
Pruning remains (14) (18) (37) (49) (51)
Edible
Plants Ornamental (14) (18) (37) (45) (46) (47) (49) (51)
Seedlings
Algae (45) (46) (47)
Marketable — (47)
Fish
Fingerlings (47)
Poultry (18) (37) (49) (51)
Living organisms
Livestock (37) (49) (51)
Edible (51)
Worms
Other (18) (37) (49) (51)
Edible (51)
Insects Auxiliary (14) (18) (37) (45) (46) (47) (49) (51)
Aquatic, larvae (47)
Mushrooms (51)
Mycorrhiza, Bacteria (18) (37) (47) (49) (51)
Microorganisms Fungi (18) (37) (49) (51)
Aquatic (47)
1 I: input to NBS_u, required for its operation and maintenance; and O: output from NBS_u. 2 (#): number assigned according to Table 2 [6,7].

According to Langergraber et al. [6], the main types of input and/or output streams
of UA-NBS were analyzed following the categories below:
• Biomass and Living organisms (cf. Table 3): Biomass refers to the total mass of
all living organisms in an area. In a circular city, that means all organic materials
derived from produced plant mass together with all microorganism and animals,
important in a CE point of view [76]. Biomass is an important resource for technologies
like pyrolysis—conversion of biomass to biochar—heat transfer [77], Fe2/biocarbon
composite derived from a phosphorous-containing biomass [78], and several other
biomass-derivate methods. Biomass concerns to materials including soil conditioners,
such as wood chips or biochar; organic fertilizers, such as manure or compost; different
types of organic waste, ranging from food waste to crop residues or pruning residues;
and to organic crop-protection products (Table 3, Figure S2). Cultivation of plants,
mushrooms, and insects may positively influence the air and soil quality. Plants take
up essential nutrients from the soil; however, they can also absorb metals like lead
(Pb), cadmium (Cd), arsenic (As), tin (Sn), chromium (Cr), and nickel (Ni). This makes
certain plants, together with other living organisms, effective phytoremediators [79].
Water 2021, 13, 2565 15 of 22

• Water: Irrigation water is required whenever precipitation is not sufficient. Using

tap water may lead to competition with other urban users [80]; therefore, alternative
water sources should be preferred. These could be subterranean water, stored rainfall
water, or treated wastewater. Urban agriculture provides an opportunity to reuse
(waste)water wherever it is generated, as opposed to rural agriculture, because there
are no or less costs associated with transport. The use of water is minimized in
soil-independent production systems with a closed circuit for water, as exemplified
by Rufí-Salís et al. [14], who found daily water savings up to 40% for such systems.
However, soilless systems mostly require higher energy inputs [81].
• Nutrients: Nutrient-rich urban waste for the primary production can be recycled
from wastewaters of different provenance, e.g., domestic wastewater, urine, feces,
greywater; wastewaters from food production, e.g., milk, tea, coffee, brewery; and
nutrient-rich solid waste streams, e.g., composting, biogas, biochar. The nutrient-rich
streams usually need to be subjected to one or several stages of treatment before use
in UA. As Jurgilevich et al. [82] pointed out, the demand for nutrients, especially
phosphorous (P), is growing drastically faster than the human population. This
is coupled to large nutrient losses on one side [83] and increasing global nutrient
imbalance [82] on the other. While the soils of rich countries accumulate nutrients, the
soil in developing countries experience P deficit [84]. Schoumans et al. [84] argued
that the European P cycle could be completely closed if imported chemical P fertilizers
were replaced by P fertilizers recovered from waste streams.
• Energy: Energy flows can be optimized, too. Mohareb et al. [85] proposed co-
location strategies of agricultural operations and waste streams in order to increase
energy efficiency; this is the sixth urban circularity challenge (UCC6 ) proposed by
Atanasova et al. [5] on “Energy efficiency and recovery” that is mainly addressed. Such
a strategy can be, for instance, to locate greenhouse food production next to waste heat
or waste nutrient sources, such as from biogas or refrigeration equipment. Another
possibility would be using phase-change technologies [86] to mediate between the
locations that emit waste heat and locations that require heat, therefore obliterating
the need for close proximity of these operations.
Urban agriculture must adapt the field agronomic production methods to smaller
areas in urban spaces, as the available surface is restricted. Therefore, crop production in
the urban environment tends to intensify in the direction of high edible biomass per surface
unit, e.g., green leafy vegetables, legumes, using plants with short life cycle; therefore,
annuals are preferred over fruit trees [87]. This intensification optimizes the use of soil,
while soil-independent systems, either horizontal or vertical, further enable increased
production rates per area. This is the case of hydroponics or aquaponic systems, where
multilayer or multilevel systems can be used to enlarge cultivation surface. However,
soil-based UA is more adequate for nutrient recycling, as the most used method for this
process is the composting of solid organic waste [88]. The substrate to use in UA might be
soil resulting from a natural process or fabricated, sometimes recurring to waste as main
structural components or just as amendments from different urban waste streams [89].
Soils previously occupied with industrial facilities may have elevated toxicity levels, but
several techniques can be used to overcome this problem [89,90].

3.5. Challenges of Circular City Resource Flows

The current research highlights the role of resource streams to close loops within
the urban metabolism, thus creating a Circular City [4–7,36]. For two types of resource
streams (i.e., food and biomass production) stream categories are shown in Table 3. Each
stream is attributed as output (O) from and/or input (I) to appropriate NBS units (NBS_u);
other non-NBS-endpoints, such as biogas plants, private gardens, or the “market”, are also
possible. With this qualitative representation, a part of a Circular City resource network
could already be constructed. However, the challenges should not be underestimated, as
both the qualitative and quantitative properties of the NBS_u must be matched. A good
Water 2021, 13, 2565 16 of 22

example to demonstrate possible difficulties with Circular City resource flows is NBS_u
aquaponic farming (47) [6,7].
Aquaponic farming can be configured very differently internally and thus integrated
flexibly within the Circular City, but that impacts input (I) resource demands and output (O)
resource provisions significantly. For example, an aquaponic system that uses a combined
heat and power unit may produce electrical energy instead of consuming it. Another
example is the externalization of the most important internal resource stream, the transfer
water, that cannot optimally supply the hydroponics. It requires targeting of the plant
needs by the addition of fertilizer depending on the fish species, stocking density, and plant
species [91,92]. This can be controlled since aquaculture and hydroponics are operated
jointly by one operator. If aquaponic elements are split into separate units as distinct NBS
with potentially different owners or operators [36], this coordination process becomes
more difficult. Additionally, in extended aquaponics, the plants may grow in soil rather
than hydroponically, involving another nutrient source to be considered. This concerns
the qualitative side of the material flow, but the quantitative aspects of coupling can also
pose problems. For example, young tomato plants need considerably less water than
mature plants, but aquaculture provides a constant fish-water output. This mismatch can
be countered with staggered crop production, which in turn requires a greenhouse and
year-round operation [93].
But even if these problems are solved, there is still the site question, and the proposal
to use the roofscape must be critically questioned due to the prevailing usage competition
in cities.

3.6. SWOT Analysis of Urban Agriculture Related Nature-Based Solutions

A SWOT analysis was used to determine the strengths, weaknesses, opportunities,
and threats of UA-NBS implementing the UCC5 in practice (cf. Figure 5). Internal factors
are attributes of the UA-NBS that represent either a strength or a weakness, and they
depend on the objective to be achieved, in this case, addressing the UCC5 . Opportunities
and threats are external factors that depend on the studied context, i.e., an urban environ-
ment with great potential for resources recovery due to the large volume of waste and
wastewater generated.
• Strengths of UA are the reduction of the environmental footprint by using sustainable
production methods, enabling organic certification, and increasing profitability [94]
(Figure 5).
• Weaknesses identified in UA-NBS are the lack of professional experience that can lead
to inappropriate use of phytosanitary products, thus aggravating pollution problems
in the city. In addition, the risk of contamination is higher when treated water or
materials obtained from waste are used instead of sources, such as mainstream water
or freshwater. Traceability of products by means of regular monitoring and digital
tools, e.g., internet of things (IoT) and blockchain technology (BCT), would facilitate
both food safety and environmental risk mitigation (Figure 5).
• Opportunities include that the implementing UA-NBS as part of a sustainable bioecon-
omy in cities facilitates the reuse of resources stemming from urban metabolism, e.g.,
building materials, water, and nutrients reduce the environmental footprint of the final
products [95]. For this purpose, Langergraber et al. [6,7] proposed supporting units
that enable nutrients and carbon to be recovered and directed back into the system. In
this regard, regulations like the recently approved European Union Circular Economy
Fertilizing Products Regulation (EU 2019/1009) may facilitate the use of fertilizers that
are produced in the same city, fostering circularity. Urban metabolism and industrial
synergy provide multiple streams of different characteristics that can be harnessed for
food and biomass production.
• Threats are, as noted above, the mismatch between the supply of nutrients and tracer
elements recovered from wastewater and waste streams and the nutritional demand
of crops, which can create a surplus or a deficiency [31,72], which has to be considered.
 of fertilizers that are produced in the same city, fostering circularity. Urban metabo-
lism and industrial synergy provide multiple streams of different characteristics that
can be harnessed for food and biomass production.
• Threats are, as noted above, the mismatch between the supply of nutrients and tracer
Water 2021, 13, 2565 elements recovered from wastewater and waste streams and the nutritional demand 17 of 22
of crops, which can create a surplus or a deficiency [31,72], which has to be consid-
ered.

Figure 5. Strengths,
Figure weaknesses,
5. Strengths, opportunities
weaknesses, andand
opportunities threats of nature-based
threats of nature-basedagricultural solutions
agricultural solutions
addressing the fifth urban circularity challenge to achieve circularity in cities.
addressing the fifth urban circularity challenge to achieve circularity in cities.

TheThe safety of food

safety of food grown
grown ininthetheurban
urbanenvironment
environmentremains remains a concern
concern in interms
termsofofsoil,
soil,water,
water,andandairairpollution
pollution[96]. [96].Although
Althoughresearch researchon onthe theeffects
effectsofofpollution
pollutionon onUA UAisisstill
stillscarce,
scarce,there
thereareareseveral
several studies
studies that
that assessed
assessed thethe feasibility
feasibility andand safety
safety of vegetables
of vegetables grown
grown in different
in different urbanurban
spaces spacesusing using
UA-NBS,UA-NBS, suchsuch as intensive
as intensive greengreen
roofsroofs
(18)(18)
andandurban
urban farms
farms andand orchards
orchards (51),(51), concluding
concluding that,
that, in general,
in general, thethe concentrations
concentrations of of contam-
contaminants
inants
andandtracetrace metals
metals foundfound in the in plants
the plants werewerebelow below the European
the European regulatory
regulatory thresh-[97].
thresholds
oldsThe[97]. The following
following factorsfactors
should should be considered
be considered in orderin order to determine
to determine the exposure
the exposure of UA
of UAto pollutants
to pollutants in in
cities:
cities:(i)(i)
growing
growinglocation,
location,e.g., e.g.,indoor
indooror or outdoor
outdoor UA, soil-based,
soil-based, or
soillesstechnologies;
or soilless technologies; (ii)(ii)
type type of crop,
of crop, e.g., e.g.,
leafyleafy
vegetablesvegetables
with awithlargealeaf
large leaf area
surface surface
are area
moreare more exposed
exposed to atmospheric
to atmospheric particles,particles, and root vegetables
and root vegetables are moretoexposed
are more exposed soil
to soil contamination
contamination compared to compared to fruiting and
fruiting vegetables; vegetables;
(iii) soil and contaminant
(iii) soil and character-
contaminant
characteristics,
istics, e.g., ground-borne e.g., ground-borne
(root system) and (root system)pollution
air-borne and air-borne (plant pollution
above ground (plant above
level)
[97].ground level) [97].
Climate
Climate change
change is a is a threat
threat as well
as well as an as opportunity
an opportunity to the
to the circularity
circularity of UA-NBS.
of UA-NBS. As As
climatic
climatic conditions
conditions maymay determine
determine the the availability
availability of resources,
of resources, e.g.,e.g., extreme
extreme precipitation
precipitation
events
events posepose a challenge
a challenge for rainwater
for rainwater harvesting,
harvesting, whilewhile even even distribution
distribution of rainfall
of rainfall facil- fa-
itates more efficient irrigation of green and productive areas, reducing dependence on on
cilitates more efficient irrigation of green and productive areas, reducing dependence
external
external sources.
sources. Moreover,
Moreover, vegetated
vegetated areas areas enhance
enhance evapotranspiration
evapotranspiration processes,
processes, whichwhich
mitigates the duration of high air temperatures in cities
mitigates the duration of high air temperatures in cities [98]. Consideration of alternative [98]. Consideration of alternative
sources
sources maymay be necessary
be necessary to ensure
to ensure a successful
a successful operation
operation andand maintenance.
maintenance. Furthermore,
Furthermore,
citycity fragmentation
fragmentation andandurbanurban sprawl
sprawl increase
increase thethe heterogeneityofofurban
heterogeneity urbanspaces
spacesthat
thatcan
can be
used for UA at different scales, enhancing the value of
be used for UA at different scales, enhancing the value of fragmented spaces (e.g., roof- fragmented spaces (e.g., rooftops)
tops)andandexpanding
expanding thethemanagement
management options
options ofof urban
urban areas
areas [42].
[42].The
Theco-design
co-designofofthe theUA-
UA-NBS in multidisciplinary teams would minimize uncertainty and provide insight into the
NBS in multidisciplinary teams would minimize uncertainty and provide insight into
the city’s
city’s potential.
potential.
Some circularity challenges were also recognized by Williams et al. [99] when identi-
fying challenges to implementing looping actions, including technical constraints, linear
resource systems, or the lack of circular planning and design in cities. Finally, in addition
to the environmental benefits that UA-NBS provide, it is worth noting that they can also
relieve societal challenges, such as food security, improved human health and well-being,
sustainable urban development, or disaster-risk management [100,101].

4. Conclusions
Urban agriculture plays a key role in terms of a Circular City, as it can use recovered
resources to produce new food and biomass. Thus, food and biomass production can
contribute significantly towards closing the urban cycle, maximizing the reuse of resources
in the urban environment while reducing the need for external resource inputs.
Water 2021, 13, 2565 18 of 22

Greater commitment with urban agriculture would help to address urban circularity
challenges. In this regard, nature-based solutions for food and biomass production con-
tribute to address at least one urban circularity challenge. Certain nature-based solutions
for food and biomass production can be circular in themselves, while others need nearby
nature-based solutions or are strategically located to address other urban circularity chal-
lenges. In future, this descriptive approach can be underpinned by mathematical models,
which would make it possible to support the theoretical approach with statistical data.
We analyzed how input and output resource streams related to food and biomass
production are located as part of other resource streams to close the cycles within the urban
metabolism, i.e., into and out of a Circular City. Design solutions geared towards closing
loops, such as aquaponic farming, are targeted by urban agriculture in circular cities. A
broader understanding of the food-related urban streams is important to recover resources
and adapt the distribution system accordingly. For it, essential knowledge of the input and
output streams is required in order to design, adapt, or couple urban agriculture-related
nature-based solutions units and interventions and supporting units.
Additionally, the need for better knowledge, transversal research networks, gover-
nance, regulations, and policy strategies and dialogues to improve nature-based agricul-
tural solutions in circular cities should be highlighted.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10

.3390/w13182565/s1, Figure S1: Potential of selected representatives urban agriculture-related NBS
units and interventions (NBS_u/i) to address the fifth urban circularity challenge (UCC5 ) on “Food
and biomass production” [5–7], according to the score range (0.33, 0.66, 1.00) (cf. Tables 1 and 2).
Numbers refer to those of NBS_u/i from Table 2. Color legend refers to the categories of NBS_u/i (cf.
Table 2) [6,7]; Figure S2: Overview of input and output streams in urban agriculture with focus on
water.
Author Contributions: Overall conceptualization, A.C.-M. and R.P.-M.; conceptualization, R.J. and
G.F.M.B.; writing—original draft preparation, A.C.-M., R.P.-M., R.J., K.B., T.A.P. and G.F.M.B.;
writing—review and editing, A.C.-M., R.P.-M., R.J. and G.F.M.B.; writing— specific (sub-)chapters
and review, K.B. and T.A.P.; writing—specific (sub-)chapters, C.D., G.A. and S.L.G.S.; illustrations,
A.C.-M., R.P.-M., R.J. and K.B; final editing, R.P.-M. All authors have read and agreed to the published
version of the manuscript.
Funding: The APC was funded by the COST Action CA17133 Circular City.
Institutional Review Board Statement: Not applicable.
Acknowledgments: The work was carried out within the COST Action CA17133 Circular City (“Im-
plementing nature-based solutions for creating a resourceful circular city”, https://circular-city.eu/|
https://www.cost.eu/actions/CA17133/, duration 22 Oct 2018—21 Oct 2022). COST Actions are
funded within the EU Framework Programmes for Research and Technological Development (cur-
rently: Horizon Europe). The authors are grateful for the support. T.A.P. acknowledges FCT (Por-
tuguese Foundation for Science and Technology) through the research unit UID/AGR/04129/LEAF.
Additionally, the authors would like to acknowledge all participants of the Circular City work-
shops that contributed during the discussions to the development of the urban agriculture-related
nature-based solutions framework, especially to Celestina M.G. Pedras, G. Adrian Peticilă, and
Mart Külvik.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Halog, A.; Anieke, S. A review of circular economy studies in developed countries and its potential adoption in developing
countries. Circ. Econ. Sustain. 2021, 1, 209–230. [CrossRef]
2. Nikolaou, I.E.; Jones, N.; Stefanakis, A. Circular economy and sustainability: The past, the present and the future directions.
Circ. Econ. Sustain. 2021, 1, 1–20. [CrossRef]
3. Pineda-Martos, R.; Calheiros, C.S.C. Nature-based solutions in cities—Contribution of the Portuguese National Association of
Green Roofs to Urban Circularity. Circ. Econ. Sustain. 2021. [CrossRef]
Water 2021, 13, 2565 19 of 22

4. Skar, S.L.G.; Pineda-Martos, R.; Timpe, A.; Pölling, B.; Bohn, K.; Külvik, M.; Delgado, C.; Pedras, C.M.G.; Paço, T.A.; Ćujić, M.;
et al. Urban agriculture as a keystone contribution towards securing sustainable and healthy development for cities in the future.
Blue-Green Syst. 2020, 2, 1–27. [CrossRef]
5. Atanasova, N.; Castellar, J.A.C.; Pineda-Martos, R.; Nika, C.E.; Katsou, E.; Istenič, D.; Pucher, B.; Andreucci, M.B.; Langergraber, G.
Nature-based solutions and circularity in cities. Circ. Econ. Sustain. 2021, 1, 319–332. [CrossRef]
6. Langergraber, G.; Castellar, J.A.C.; Pucher, B.; Baganz, G.F.M.; Milosevic, D.; Andreucci, M.-B.; Kearney, K.; Pineda-Martos, R.;
Atanasova, N. A Framework for Addressing Circularity Challenges in Cities with Nature-based Solutions. Water 2021, 13, 2355.
[CrossRef]
7. Langergraber, G.; Castellar, J.A.C.; Andersen, T.R.; Andreucci, M.-B.; Baganz, G.F.M.; Buttiglieri, G.; Canet-Martí, A.;
Carvalho, P.N.; Finger, D.C.; Bulc, T.G.; et al. Towards a Cross-Sectoral View of Nature-Based Solutions for Enabling Circular
Cities. Water 2021, 13, 2352. [CrossRef]
8. COST (European Cooperation in Science and Technology) Action CA17133 (2018) Memorandum of Understanding for the
Implementation of the COST Action “Implementing Nature Based Solutions for Creating a Resourceful Circular City” (Circu-lar
City Re.Solution) CA17133; COST 044/18. Brussels, Belgium. Available online: https://e-services.cost.eu/files/domain_files/
CA/Action_CA17133/mou/CA17133-e.pdf (accessed on 27 July 2021).
9. Langergraber, G.; Pucher, B.; Simperler, L.; Kisser, J.; Katsou, E.; Buehler, D.; Garcia Mateo, M.C.; Atanasova, N. Implementing
nature-based solutions for creating a resourceful circular city. Blue-Green Syst. 2020, 2, 173–185. [CrossRef]
10. Pölling, B.; Mergenthaler, M. The location matters: Determinants for “deepening” and “broadening” diversification strategies in
Ruhr metropolis’ urban farming. Sustainability 2017, 9, 1168. [CrossRef]
11. Aubry, C.; Kebir, L. Shortening food supply chains: A means for maintaining agriculture close to urban areas? The case of the
French metropolitan area of Paris. Food Policy 2013, 41, 85–93. [CrossRef]
12. Song, S.; Goh, J.C.L.; Tan, H.T.W. Is food security an illusion for cities? A system dynamics approach to assess disturbance in the
urban food supply chain during pandemics. Agric. Syst. 2021, 189, 103045. [CrossRef]
13. Viljoen, A.; Bohn, K. Second Nature Urban Agriculture–Designing Productive Cities: Ten Years on from the Continuous Productive Urban
Landscape (CPUL City) Concept, 1st ed.; Routledge: London, UK, 2014; p. 312.
14. Rufí-Salís, M.; Petit-Boix, A.; Villalba, G.; Sanjuan-Delmás, D.; Parada, F.; Ercilla-Montserrat, M.; Arcas-Pilz, V.; Muñoz-Liesa, J.;
Rieradevall, J.; Gabarrell, X. Recirculating water and nutrients in urban agriculture: An opportunity towards environmental
sustainability and water use efficiency? J. Clean. Prod. 2020, 261, 121213. [CrossRef]
15. Stillitano, T.; Spada, E.; Iofrida, N.; Falcone, G.; De Luca, A.I. Sustainable agri-food processes and circular economy pathways in a
life cycle perspective: State of the art of applicative research. Sustainability 2021, 13, 2472. [CrossRef]
16. Food and Agriculture Organization of the United Nations, FAO. Land & Water–Circular Economy: Waste-to-Resource &
COVID-19. Available online: http://www.fao.org/land-water/overview/covid19/circular/en/ (accessed on 27 July 2021).
17. Lobillo-Eguíbar, J.; Fernández-Cabanás, V.M.; Bermejo, L.A.; Pérez-Urrestarazu, L. Economic sustainability of small-scale
aquaponic systems for food self-production. Agronomy 2020, 10, 1468. [CrossRef]
18. Clark, C. Von Thünen’s isolated state. Oxford Econ. Pap. 1967, 19, 370–377. [CrossRef]
19. Howard, E.; Osborn, F.J. Garden Cities of To-Morrow, 1st ed.; Routledge: London, UK, 2013; 170p.
20. Viljoen, A.; Bohn, K.; Howe, J. CPULs–Continuous Productive Urban Landscapes: Designing Urban Agriculture for Sustainable Cities;
Architectural Press: Oxford, UK, 2005; p. 295.
21. Ellen MacArthur Foundation. Cities and Circular Economy for Food; Ellen MacArthur Foundation: Cowes, UK, 2019; pp. 1–66.
22. Baganz, G.F.M.; Junge, R.; Portella, M.; Goddek, S.; Keesman, K.; Baganz, D.; Staaks, G.; Shaw, C.; Lohrberg, F.; Kloas, W. The
Aquaponic Principle–It is all about Coupling. Rev. Aquacult. 2021, in press. [CrossRef]
23. Baganz, G.F.M.; Schrenk, M.; Körner, O.; Baganz, D.; Keesman, K.; Goddek, S.; Siscan, Z.; Baganz, E.; Doernberg, A.; Monsees, H.;
et al. Causal relations of upscaled urban aquaponics and the Food-Water-Energy Nexus–A Berlin case study. Water 2021, 13, 2029.
[CrossRef]
24. Shiklomanov, I.A.; Rodda, J.C. World Water Resources at the Beginning of the Twenty-First Century; Cambridge University Press:
Cambridge, UK, 2004.
25. Van Zanten, H.H.E.; Herrero, M.; Van Hal, O.; Röös, E.; Muller, A.; Garnett, T.; Gerber, P.J.; Schader, C.; De Boer, I.J.M. Defining a
land boundary for sustainable livestock consumption. Glob. Change Biol. 2018, 24, 4185–4194. [CrossRef]
26. Van Zanten, H.H.E.; Van Ittersum, M.K.; De Boer, I.J.M. The role of farm animals in a circular food system. Glob. Food Sec. 2019,
21, 18–22. [CrossRef]
27. de Boer, I.J.M.; van Ittersum, M.K. Circularity in Agricultural Production; Wageningen University & Research: Wageningen, The
Netherlands, 2018; p. 74.
28. United Nations, Department of Economic and Social Affairs–Population Division. World Population Prospects 2019–Highlights,
ST/ESA/SER.A/423; United Nations: New York, NY, USA, 2019; p. 46. Available online: https://population.un.org/wpp/
Publications/Files/WPP2019_Highlights.pdf (accessed on 27 July 2021).
29. Food and Agriculture Organization of the United Nations, FAO. FAO Framework for the Urban Food Agenda–Leveraging Sub-national
and Local Government Action to Ensure Sustainable Food Systems and Improved Nutrition; FAO: Rome, Italy, 2019; p. 44. [CrossRef]
30. Pascucci, S. Building Natural Resource Networks: Urban Agriculture and the Circular Economy; Burleigh Dodds, Burleigh Dodds
Series in Agricultural Science; Burleigh Dodds Science Publishing Limited: Cambridge, UK, 2020; pp. 1–20.
Water 2021, 13, 2565 20 of 22

31. Wielemaker, R.C.; Weijma, J.; Zeeman, G. Harvest to harvest: Recovering nutrients with New Sanitation systems for reuse in
Urban Agriculture. Resour. Conserv. Recycl. 2018, 128, 426–437. [CrossRef]
32. Dorr, E.; Koegler, M.; Gabrielle, B.; Aubry, C. Life cycle assessment of a circular, urban mushroom farm. J. Clean. Prod. 2021, 288,
125668. [CrossRef]
33. Castellar, J.A.C.; Popartan, L.A.; Pueyo-Ros, J.; Atanasova, N.; Langergraber, G.; Sämuel, I.; Corominas, L.; Comas, J.; Acuña, V.
Nature-based solutions in the urban context: Terminology, classification and scoring for urban challenges and ecosystem services.
Sci. Total Environ. 2021, 779. [CrossRef]
34. Hemming, V.; Burgman, M.A.; Hanea, A.M.; Mcbride, M.F.; Wintle, B.C. A practical guide to structured expert elicitation using
the IDEA Protocol. Methods Ecol. Evol. 2018, 9, 169–180. [CrossRef]
35. Hemming, V.; Walshe, T.V.; Hanea, A.M.; Fidler, F.; Burgman, M.A. Eliciting improved quantitative judgements using the IDEA
Protocol: A case study in natural resource management. PLoS ONE 2018, 13, 1–34. [CrossRef]
36. Baganz, G.F.M.; Proksch, G.; Kloas, W.; Lorleberg, W.; Baganz, D.; Staaks, G.; Lohrberg, F. Site resource inventories—A missing
link in the circular city’s information flow. Adv. Geosci. 2020, 54, 23–32. [CrossRef]
37. You, C.; Chen, H.G.; Myung, S.; Sathitsuksanoh, N.; Ma, H.; Zhang, X.Z.; Li, J.Y.; Zhang, Y.H.P. Enzymatic transformation of
nonfood biomass to starch. Proc. Natl. Acad. Sci. USA 2013, 110, 7182–7187. [CrossRef]
38. Pearlmutter, D.; Theochari, D.; Nehls, T.; Pinho, P.; Piro, P.; Korolova, A.; Papaefthimiou, S.; Garcia Mateo, M.C.; Calheiros, C.;
Zluwa, I.; et al. Enhancing the circular economy with nature-based solutions in the built urban environment: Green building
materials, systems and sites. Blue-Green Syst. 2020, 2, 46–72. [CrossRef]
39. da Cunha, J.A.C.; Arias, C.A.; Carvalho, P.; Rysulova, M.; Canals, J.M.; Perez, G.; Gonzalez, M.B.; Morato, J.F. “WETWALL”-an
innovative design concept for the treatment of wastewater at an urban scale. Desalin. Water Treat. 2018, 109, 205–220. [CrossRef]
40. Specht, K.; Siebert, R.; Hartmann, I.; Freisinger, U.B.; Sawicka, M.; Werner, A.; Thomaier, S.; Henckel, D.; Walk, H.; Dierich, A.
Urban agriculture of the future: An overview of sustainability aspects of food production in and on buildings. Agric. Hum. Values
2014, 31, 33–51. [CrossRef]
41. Specht, K.; Zoll, F.; Siebert, R. Application and evaluation of a participatory “open innovation” approach (ROIR): The case of
introducing zero-acreage farming in Berlin. Landsc. Urban Plan. 2016, 151, 45–54. [CrossRef]
42. Thomaier, S.; Specht, K.; Henckel, D.; Dierich, A.; Siebert, R.; Freisinger, U.B.; Sawicka, M. Farming in and on urban buildings:
Present practice and specific novelties of Zero-Acreage Farming (ZFarming). Renew. Agric. Food Syst. 2015, 30, 43–54. [CrossRef]
43. Addo-Bankas, O.; Zhao, Y.; Vymazal, J.; Yuan, Y.; Fu, J.; Wei, T. Green walls: A form of constructed wetland in green buildings.
Ecol. Eng. 2021, 169. [CrossRef]
44. Walters, S.A.; Midden, K.S. Sustainability of urban agriculture: Vegetable production on green roofs. Agriculture 2018, 8, 168.
[CrossRef]
45. Baganz, G.F.M.; Baganz, E.; Baganz, D.; Kloas, W.; Lohrberg, F. Urban rooftop uses: Competition and potentials from the
perspective of farming and aquaponics–A Berlin Case Study. In Proceedings of the REAL CORP 2021, Vienna, Austria, 7–10
September 2021.
46. Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL). Guidelines for the Planning, Construction and Mainte-
nance of Green Roofing–Green Roofing Guideline; FLL: Berlin, Germany, 2018.
47. Bellamy, D. Belgian Supermarket Grows its Greens on the Roof. Available online: https://www.euronews.com/2018/07/05
/belgian-supermarket-grows-its-greens-on-the-roof (accessed on 13 April 2021).
48. European Union. LISBON–European Green Capital 2020; EU: Bietlot, Belgium, 2020. [CrossRef]
49. Natural Walking Cities. Green Corridors–Essential Urban Walking and Natural Infrastructure. Available online: http://
naturalwalkingcities.com/green-corridors-essential-urban-walking-and-natural-infrastructure/ (accessed on 21 May 2021).
50. Maucieri, C.; Nicoletto, C.; Junge, R.; Schmautz, Z.; Sambo, P.; Borin, M. Hydroponic systems and water management in
aquaponics: A review. Ital. J. Agron. 2018, 13, 1–11. [CrossRef]
51. Maucieri, C.; Nicoletto, C.; Van Os, E.; Anseeuw, D.; Van Havermaet, R.; Junge, R. Hydroponic technologies. In Aquaponics Food
Production Systems; Goddek, S., Joyce, A., Kotzen, B., Burnell, G.M., Eds.; Springer: Cham, Switzerland, 2019; pp. 77–110.
52. Wongkiew, S.; Hu, Z.; Lee, J.W.; Chandran, K.; Nhan, H.T.; Marcelino, K.R.; Khanal, S.K. Nitrogen recovery via aquaponics–
bioponics: Engineering considerations and perspectives. ACS ES&T Eng. 2021, 1, 326–339.
53. National Organic Standards Board. Organic Hydroponic and Aquaponic Task Force Report. Available online: https://www.ams.
usda.gov/content/organic-hydroponic-and-aquaponic-task-force-report (accessed on 28 July 2021).
54. Pradhan, S.K.; Nerg, A.M.; Sjoblom, A.; Holopainen, J.K.; Heinonen-Tanski, H. Use of human urine fertilizer in cultivation
of cabbage (Brassica oleracea)–Impacts on chemical, microbial, and flavor quality. J. Agric. Food Chem. 2007, 55, 8657–8663.
[CrossRef] [PubMed]
55. Lind, O.P.; Hultberg, M.; Bergstrand, K.J.; Larsson-Jonsson, H.; Caspersen, S.; Asp, H. Biogas digestate in vegetable hydroponic
production: pH dynamics and pH management by controlled nitrification. Waste Biomass Valori. 2021, 12, 123–133. [CrossRef]
56. Wongkiew, S.; Koottatep, T.; Polprasert, C.; Prombutara, P.; Jinsart, W.; Khanal, S.K. Bioponic system for nitrogen and phosphorus
recovery from chicken manure: Evaluation of manure loading and microbial communities. Waste Manag. 2021, 125, 67–76.
[CrossRef] [PubMed]
57. Schmautz, Z.; Espinal, C.A.; Smits, T.H.M.; Frossard, E.; Junge, R. Nitrogen transformations across compartments of an aquaponic
system. Aquac. Eng. 2021, 92, 102145. [CrossRef]
Water 2021, 13, 2565 21 of 22

58. Chinta, Y.D.; Eguchi, Y.; Widiastuti, A.; Shinohara, M.; Sato, T. Organic hydroponics induces systemic resistance against the
air-borne pathogen, Botrytis cinerea (gray mould). J. Plant Interact. 2015, 10, 243–251. [CrossRef]
59. Fujiwara, K.; Aoyama, C.; Takano, M.; Shinohara, M. Suppression of bacterial wilt disease by an organic hydroponic system.
J. Gen. Plant Pathol. 2012, 78, 217–220. [CrossRef]
60. Stoknes, K.; Wojciechowska, E.; Jasińska, A.; Gulliksen, A.; Tesfamichael, A.A. Growing vegetables in the circular economy;
cultivation of tomatoes on green waste compost and food waste digestate. Acta Hortic. 2018, 1215, 389–396. [CrossRef]
61. Sánchez, H.J.A. Lactuca sativa production in an Anthroponics System. 2015. Available online: https://www.hemmaodlat.se/
research/lactuca%20sativa%20production%20in%20an%20anthroponics%20system.pdf (accessed on 28 July 2021).
62. Gartmann, F.; Julian Hügly, J.; Brinkmann, N.; Schmautz, Z.; Smits, T.H.M.; Junge, R. Bioponics, an Organic Closed-Loop Soilless
Cultivation System: Management, Characteristics, and Nutrient Mass Balance Compared to Hydroponics and Soil Cultivation.
2021; (manuscript in preparation).
63. Pantanella, E. Pond aquaponics: New pathways to sustainable integrated aquaculture and agriculture. 2008. Available online:
http://wptest.backyardmagazines.com/wp-content/uploads/2009/08/pondaquaponics.pdf (accessed on 28 July 2021).
64. Salam, M.A.; Asadujjaman, M.; Rahman, M.S. Aquaponics for improving high density fish pond water quality through raft and
rack vegetable production. WJFMS 2013, 5, 251–256. [CrossRef]
65. Food and Agriculture Organization of the United Nations, FAO. Integrated Agriculture-Aquac: A Primer; FAO/IIRR/WorldFish
Center: Rome, Italy, 2001.
66. Food and Agriculture Organization of the United Nations, FAO. Small-Scale Aquaponic Food Production–Integrated Fish and Plant
Farming; FAO: Rome, Italy, 2014.
67. Zajdband, A.D. Integrated agri-aquaculture systems. In Genetics, Biofuels and Local Farming Systems. Sustainable Agriculture Reviews;
Lichtfouse, E., Ed.; Springer Nature: Geneve, Switzerland, 2011; Volume 7, pp. 87–127.
68. McDougall, R.; Kristiansen, P.; Rader, R. Small-scale urban agriculture results in high yields but requires judicious management
of inputs to achieve sustainability. Proc. Natl. Acad. Sci. USA 2019, 116, 129–134. [CrossRef]
69. Morel, K.; San Cristobal, M.; Leger, F.G. Small can be beautiful for organic market gardens: An exploration of the economic
viability of French microfarms using MERLIN. Agr. Syst. 2017, 158, 39–49. [CrossRef]
70. Oral, H.V.; Radinja, M.; Rizzo, A.; Kearney, K.; Andersen, T.R.; Krzeminski, P.; Buttiglieri, G.; Cinar, D.; Comas, J.; Gajewska, M.;
et al. Management of urban waters with nature-based solutions in circular cities. Water 2020, 2, 112–136.
71. Turcios, A.E.; Papenbrock, J. Sustainable treatment of aquaculture effluents–What can we learn from the past for the future?
Sustainability 2014, 6, 836–856. [CrossRef]
72. Wielemaker, R.; Oenema, O.; Zeeman, G.; Weijma, J. Fertile cities: Nutrient management practices in urban agriculture.
Sci. Total Environ. 2019, 668, 1277–1288. [CrossRef]
73. Avgoustaki, D.D.; Xydis, G. Indoor vertical farming in the urban nexus context: Business growth and resource savings.
Sustainability 2020, 12, 1965. [CrossRef]
74. Körner, O.; Bisbis, M.B.; Baganz, G.F.M.; Baganz, D.; Staaks, G.B.O.; Monsees, H.; Goddek, S.; Keesman, K.J. Environmental
impact assessment of local decoupled multi-loop aquaponics in an urban context. J. Clean. Prod. 2021, 313, 127735. [CrossRef]
75. Rossi, L.; Bibbiani, C.; Fierro-Sañudo, J.F.; Maibam, C.; Incrocci, L.; Pardossi, A.; Fronte, B. Selection of marine fish for integrated
multi-trophic aquaponic production in the Mediterranean area using DEXi multi-criteria analysis. Aquaculture 2021, 535, 736402.
[CrossRef]
76. Sherwood, J. The significance of biomass in a circular economy. Bioresour. Technol. 2020, 300, 122755. [CrossRef] [PubMed]
77. Li, J.-S.; Zhu, L.-T.; Luo, Z.-H. Effect of geometric configuration on hydrodynamics, heat transfer and RTD in a pilot-scale biomass
pyrolysis vapor-phase upgrading reactor. Chem. Eng. J. 2022, 428, 131048. [CrossRef]
78. He, Z.; Zheng, W.; Li, M.; Liu, W.; Zhang, Y.; Wang, Y. Fe2 P/biocarbon composite derived from a phosphorus-containing biomass
for levofloxacin removal through peroxymonosulfate activation. Chem. Eng. J. 2022, 427, 130928. [CrossRef]
79. Yadav, S.; Yadav, A.; Bagotia, N.; Sharma, A.K.; Kumar, S. Adsorptive potential of modified plant-based adsorbents for sequestra-
tion of dyes and heavy metals from wastewater–A review. J. Water Process Eng. 2021, 42, 102148. [CrossRef]
80. Lupia, F.; Pulighe, G. Water use and urban agriculture: Estimation and water saving scenarios for residential kitchen gardens.
Agric. Agric. Sci. Proc. 2015, 50–58. [CrossRef]
81. O’Sullivan, C.A.; Bonnett, G.D.; McIntyre, C.L.; Hochman, Z.; Wasson, A.P. Strategies to improve the productivity, product
diversity and profitability of urban agriculture. Agr. Syst. 2019, 174, 133–144. [CrossRef]
82. Jurgilevich, A.; Birge, T.; Kentala-Lehtonen, J.; Korhonen-Kurki, K.; Pietikäinen, J.; Saikku, L.; Schösler, H. Transition towards
circular economy in the food system. Sustainability 2016, 8, 69. [CrossRef]
83. Cordell, D.; Drangert, J.-O.; White, S. The story of phosphorus: Global food security and food for thought. Glob. Environ. Change
2009, 19, 292–305. [CrossRef]
84. Schoumans, O.F.; Bouraoui, F.; Kabbe, C.; Oenema, O.; van Dijk, K.C. Phosphorus management in Europe in a changing world.
AMBIO 2015, 44, 180–192. [CrossRef] [PubMed]
85. Mohareb, E.; Heller, M.; Novak, P.; Goldstein, B.; Fonoll, X.; Raskin, L. Considerations for reducing food system energy demand
while scaling up urban agriculture. Environ. Res. Lett. 2017, 12, 125004. [CrossRef]
86. Kürklü, A. Energy storage applications in greenhouses by means of phase change materials (PCMs): A review. Renew. Energy
1998, 13, 89–103. [CrossRef]
Water 2021, 13, 2565 22 of 22

87. Drescher, A.W. Food for the cities: Urban agriculture in developing countries. Acta Hort. 2004, 643, 227–231. [CrossRef]
88. Goldstein, B.; Hauschild, M.; Fernandez, J.; Birkved, M. Testing the environmental performance of urban agriculture as a food
supply in northern climates. J. Clean. Prod. 2016, 135, 984–994. [CrossRef]
89. Salomon, M.J.; Watts-Williams, S.J.; McLaughlin, M.J.; Cavagnaro, T.R. Urban soil health: A city-wide survey of chemical and
biological properties of urban agriculture soils. J. Clean. Prod. 2020, 275, 122900. [CrossRef] [PubMed]
90. Watts-Williams, S.J.; Cavagnaro, T.R. Arbuscular mycorrhizas modify tomato responses to soil zinc and phosphorus addition.
Biol. Fertil. Soils 2012, 48, 285–294. [CrossRef]
91. Suhl, J.; Dannehl, D.; Kloas, W.; Baganz, D.; Jobs, S.; Scheibe, G.; Schmidt, U. Advanced aquaponics: Evaluation of intensive
tomato production in aquaponics vs. conventional hydroponics. Agric. Water Manag. 2016, 178, 335–344. [CrossRef]
92. Nozzi, V.; Graber, A.; Schmautz, Z.; Mathis, A.; Junge, R. Nutrient management in aquaponics: Comparison of three approaches
for cultivating lettuce, mint and mushroom herb. Agronomy 2018, 8, 27. [CrossRef]
93. Baganz, G.; Baganz, D.; Staaks, G.; Monsees, H.; Kloas, W. Profitability of multi-loop aquaponics: Year-long production data,
economic scenarios and a comprehensive model case. Aquac. Res. 2020, 51, 2711–2724. [CrossRef]
94. Tokunaga, K.; Tamaru, C.; Ako, H.; Leung, P.S. Economics of small-scale commercial aquaponics in Hawai‘i. J. World Aquac. Soc.
2015, 46, 20–32. [CrossRef]
95. Rosemarin, A.; Macura, B.; Carolus, J.; Barquet, K.; Ek, F.; Järnberg, L.; Lorick, D.; Johannesdottir, S.; Pedersen, S.M.; Koskiaho, J.;
et al. Circular nutrient solutions for agriculture and wastewater–A review of technologies and practices. Curr. Opin. Environ. Sust.
2020, 45, 78–91. [CrossRef]
96. Wortman, S.E.; Lovell, S.T. Environmental challenges threatening the growth of urban agriculture in the United States.
J. Environ. Qual. 2013, 42, 1283–1294. [CrossRef]
97. Aubry, C.; Manouchehri, N. Urban agriculture and health: Assessing risks and overseeing practices. Field Actions Sci. Rep. 2019,
20, 108–111.
98. Choi, Y.; Lee, S.; Moon, H. Urban physical environments and the duration of high air temperature: Focusing on solar radiation
trapping effects. Sustainability 2018, 10, 4837. [CrossRef]
99. Williams, J. Circular cities: Challenges to implementing looping actions. Sustainability 2019, 11, 423. [CrossRef]
100. Albert, C.; Schroter, B.; Haase, D.; Brillinger, M.; Henze, J.; Herrmann, S.; Gottwald, S.; Guerrero, P.; Nicolas, C.; Matzdorf, B.
Addressing societal challenges through nature-based solutions: How can landscape planning and governance research contribute?
Landsc. Urban Plan. 2019, 182, 12–21. [CrossRef]
101. European Commission. Towards an EU Research and Innovation Policy Agenda for Nature-Based Solutions & Re-Naturing
Cities –Final Report of the Horizon 2020 Expert Group on “Nature-Based Solutions and Re-Naturing Cities” (Full Version).
Directorate-General for Research and Innovation. Publications Office of the European Union, Luxembourg. 2015. Available
online: https://ec.europa.eu/newsroom/horizon2020/document.cfm?doc_id=10195; https://op.europa.eu/en/publication-
detail/-/publication/fb117980-d5aa-46df-8edc-af367cddc202 (accessed on 19 January 2021).