A comprehensive review of environmental and operational issues of constructed wetland systems

Journal Pre-proof A comprehensive review of environmental and operational issues of constructed wetland systems Carlo Ingrao, Sabina Failla, Claudia Arcidiacono PII:

S2468-5844(19)30061-3

DOI:

https://doi.org/10.1016/j.coesh.2019.10.007

Reference:

COESH 147

To appear in:

Current Opinion in Environmental Science & Health

Received Date: 4 October 2019 Revised Date:

23 October 2019

Accepted Date: 25 October 2019

Please cite this article as: Ingrao C, Failla S, Arcidiacono C, A comprehensive review of environmental and operational issues of constructed wetland systems, Current Opinion in Environmental Science & Health, https://doi.org/10.1016/j.coesh.2019.10.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

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A comprehensive review of environmental and operational issues of constructed

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wetland systems

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Carlo Ingrao a, Sabina Failla b, Claudia Arcidiacono b

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a.

Enna, Italy

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Faculty of Engineering and Architecture, Kore University of Enna, Cittadella Universitaria - 94100

b.

Department of Agriculture, Food and Environment, University of Catania, Via S. Sofia, 100 – 95123 Catania, Italy

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(*) Corresponding author: Dr. Eng. Carlo Ingrao ([email protected])

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Abstract

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Constructed Wetlands (CWs) are increasingly gaining ground for the treatment of domestic and

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agricultural wastewaters, coal mine drainage, and storm-water runoff, mainly due to a set of

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beneficial features, including environmental quality preservation, landscape conservation, and

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economic convenience.

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harvesting that can significantly contribute to both pollutant removal efficiency and sustainability of

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CWs.

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This review investigates this field of research by touching those themes and close-related ones, to

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contribute to enhancing the state-of-the-art and the knowledge on CWs.

These would not be possible without activities like monitoring and

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Keywords: Constructed wetland; Environment protection; Landscape conservation; Monitoring;

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Harvesting

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1. Introduction

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Survival and sustainability of life on this planet largely depend upon the quality of encompassing

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environment around the globe. For a very long time, Earth has been considered as an endless

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storehouse of natural resources such as soil, water and air. This belief has led to the unscrupulous

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usage of those resources, thereby resulting in a so high depletion that the consumption rate has

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overcome that of the natural replacement. For instance, during the last century, industrial and

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military practices have contaminated huge surface-areas within all of the developed and most of the

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underdeveloped countries, thus causing both degradation and abandonment of waters and soils. In

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this regard, environmental and socio-economic problems arising from contaminated land sites and

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polluted waters are causing havoc worldwide (Saier Jr. and Trevors, 2010; Singh, 2012). In

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particular, the high organic anthropogenic pollution loads, which are often released into water

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bodies close to urban areas, usually cause changes in their quality, altering their physical, chemical,

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and biological features (Souza et al., 2013). Most of the domestic effluents are discharged raw into

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receiving water bodies, since only a small percentage of them is usually subjected to adequate

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purification treatments before discharge. Those water bodies, mainly represented by ponds and

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lakes located near to large urban or industrial areas, suffer: reduction in the availability of Dissolved

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Oxygen (DO); and increase in nutrient concentration, such as nitrogen and phosphorus, leading to

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eutrophication and acidification (Souza et al., 2013). Therefore, wastewater treatment is needed

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through efficient alternative technologies aimed at lowering the pollutant content to levels being

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tolerable by the recipient water bodies, so avoiding serious injuries to the environment and to the

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health of humans (Wu et al., 2015). In this regard, Constructed Wetlands (CWs) represent a

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reasonable option for this purpose and, in fact, are increasingly gaining grounds mainly owing to

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lower cost, less operation and maintenance requirements (Rai et al., 2013). Application of CWs for

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water pollution control has a long-standing tradition in urban, peri-urban, rural, agricultural and

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mining environments (Avellan et al., 2017). CWs may be utilised in a wide range of settings: from

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arid to tropical; from relatively high to low nutrient loads; and from a vast variety of pollutants

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(Avellan et al., 2017). In fact, it has recently gained significant acceptability, with special regard to

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emerging economies, mainly due to their versatility, economic and environmental benefits, wildlife

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habitats, walking routes, land preservation, erosion control, and usability in touristic facilities

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(Ezeah et al., 2015). Moreover, they have developed rapidly over the last three decades being

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established worldwide as an alternative to conventional and more technically-equipped treatment

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systems (Garcia et al., 2010).

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The study wishes to contribute to this field of research and explores environmental and operational

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issues of CWs. It is true that the literature is rich of research and review studies in this area, but the

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idea of this review article was to touch some of the main issues related to CWs, including those that

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are not often addressed, in a way to create a tool for rapid knowledge dissemination.

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This study is the first step of a research that the authors are currently involved in with regard to

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sustainability in the CW field, and was developed for reasons of contextualisation and

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backgrounding.

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Besides this introductory piece for study back-grounding and contextualisation, the paper was

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structured as shown in Fig. 1.

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Fig.1. The figure shows the framework that has been followed for the study development

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2. Constructed wetlands

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Wetlands are highly diverse and specific ecosystems that play a vital role in the environment. They

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are identified as areas that are covered by water or that have waterlogged soils for a significant part

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of the vegetation growing period (Mejáre and Bülow, 2001; Herath and Vithanage, 2015). Wetlands

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are land transitional between terrestrial and aquatic systems, and are characterised by the water

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table being usually at or near the land, or by the land being covered by shallow water (Mejáre and

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Bülow, 2001; Herath and Vithanage, 2015). Two types of wetlands can be acknowledged: the

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natural; and the constructed ones. Natural wetlands usually purify and improve the quality of water

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passing through the system, because they act as ecosystem filters (Cheng et al. 2002; Herath and

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Vithanage, 2015). By contrast, CWs are artificially engineered systems that are designed and

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constructed to utilise natural processes that are suitable to remove pollutants from contaminated

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water within a more controlled environment (Faulwetter et al., 2009; Vymazal, 2011a; Herath and

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Vithanage, 2015). Both natural and constructed wetlands can be considered as feasible alternatives

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to conventional systems for the treatment of wastewaters (Herath and Vithanage, 2015). However,

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natural wetlands were documented as resulting in lower removal efficiency than the constructed

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ones, mainly because of short water circuits, with subsequent little time retention (Herath and

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Vithanage, 2015). Both systems are generally comprised of vegetation, substrates, soils,

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microorganisms and water, and utilise complex processes involving physical, chemical, and

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biological mechanisms to remove various contaminants: the quality of the outlet water is so

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improved (Davis, 1994; Vymazal, 2011b; Saeed and Sun, 2012). Vegetation contributes to

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increasing the aesthetics of the site and to enhancing landscape by creating significant wildlife

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habitat for a variety of animals such as songbirds, insects, amphibians and so on (Herath and

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Vitanage, 2015). Wetland plants can be classified based upon their capacity of adapting to life in

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water as: emergent; submerged; and floating. Arundo donax L. is one emergent plant species that is

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highly used in CW systems, along with others like Vetiveria zizanioides L., Miscanthus x giganteus

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L., and Phragmites australis L (Herath and Vitanage, 2015; Toscano et al., 2015). Arundo donax

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L., in particular, is a highly versatile crop that can find a number of other applications, including

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utilising them for production of panels with building enveloping function (Barreca et al., 2019).

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There exist several differences between natural and constructed wetlands but, amongst them, the

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most important is the isolation of the water regime from natural patterns and boundary conditions

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(Herath and Vithanage, 2015). CWs, in particular, are robust systems, that are low-energy

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demanding, cost-effective, and are easy to operate and maintain with only periodic on-site labour

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(Davis, 1994). They can be classified based upon hydrological features into two divisions: free-

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water-surface; and subsurface systems. In turn, the latter can be horizontal or vertical, depending

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upon the flow direction (Vymazal, 2010; Herath and Vithanage, 2015). A simplification of these

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CW types was shown in Fig. 2.

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Fig.2. The figure shows CW classification, based upon hydrology and flow direction (Source: Ghermandi et al., 2007)

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CWs are suitable for decentralised wastewater treatment in those areas that have no access to public

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sewage systems or that are economically underdeveloped, and where water reuse and recycling are

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important objectives for the economy of scale (Brix, 1999; Vymazal, 2009; Pérez-Salazar et al.,

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2019). However, compared to conventional Waste Water Treatment Plants (WWTSs): CWs require

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larger spaces, thereby being economical relative to other options only where land is available and

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affordable; and their pollutant removal efficiency may be less consistent, as it may vary seasonally

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in response to changing environmental conditions, including rainfall and drought (Davis, 1994).

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Furthermore, the literature has proven that CW biological components are sensitive to toxic

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chemicals, such as ammonia and pesticides (Davis, 1994). In this regard, Solano et al. (2004)

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reported of toxic effects on the aquatic plants due to the high organic load of the influents, when

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CWs were used for primary treatment, and so were the actual treatment provided. Additionally,

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lushes of pollutants or surges in water flow may temporarily cause reduction in treatment

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effectiveness (Davis, 1994). Therefore, design is essential to be made to take into account site-

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specific conditions and requirements, so avoiding risks of compromising CW functioning due to the

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unexpected encountering of those limitations.

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CWs have, however, several benefits mainly related to environmental quality preservation,

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landscape conservation, and economic convenience, which is why they are so widely spreading.

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Those benefits would not be possible without activities like monitoring and harvesting that – under

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conditions of proper application and management - significantly contribute to both pollutant

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removal efficiency and sustainability of CWs.

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These and close-related themes have been investigated in the text following, to favour enhancement

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of the state-of-the-art and knowledge on CWs.

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2.1 Environment-related issues

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In the last few decades, CWs have been widely designed and installed to treat various kinds of

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wastewaters such as, for instance, domestic sewage, agricultural wastewater, industrial effluent, and

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polluted river water to remove the pollutants contained like, mostly, organic matter, nutrients, trace

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elements, pharmaceutical contaminants, and pathogens (Cui et al., 2010; Saeed and Sun, 2012; Wu

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et al., 2015). In this context, CW treatment performances are critically dependent upon optimal

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operating parameters (i.e., water depth, hydraulic retention time and load, feeding mode and design

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of setups), which could generate variations in the removal efficiency of contaminants (Wu et al.,

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2014).

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Recently, numerous studies have been focussed upon the design, development, and performance

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test of CWs, also via comparison amongst different systems and plant species. By way of example,

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Toscano et al. (2015) analysed the load removal efficiency of four emergent plant species (Vetiveria

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zizanioides L., Miscanthus x giganteus L., Arundo donax L., and Phragmites australis L.)

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considering the evapotranspiration effect on removal processes with regard to weather conditions

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and vegetation types. Similarly, Shelef et al. (2012) documented the capability of Bassia indica L.

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for salt remediation in the following three systems: a hydroponic system with mixed salt solutions;

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a recirculated vertical flow constructed wetland (RVFCW) with domestic wastewater; and a vertical

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flow constructed wetland (VFCW) for treating goat farm effluents. Recently, Vo et al. (2019)

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conducted a review of the literature on shallow constructed wetlands, which enabled deepening the

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knowledge on their applications as wetland roofs, as well as on their influence factors, benefits,

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challenges, and improvement potentials for future applications.

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Several other studies have been conducted over the course of the last decades, with special focus

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upon the associated environmental issues. Lower Greenhouse Gas (GHG) emissions and higher

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environmental sustainability rates of CWs, compared to conventional Wastewater Treatment Plants

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(WWTPs) were highlighted in those studies. For instance, Fuchs et al. (2011) applied Life Cycle

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Assessment (LCA) to compare the environmental impacts of VFCWs and horizontal flow

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constructed wetlands (HFCWs) calculating, also, the emissions of those GHGs being mostly

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connected to such systems, namely dinitrogen monoxide (N2O), carbon dioxide (CO2), and methane

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(CH4). Emissions of these three GHGs were also estimated by Pan et al. (2011) who carried out a

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life-cycle based study to compare a vertical subsurface flow constructed wetland (VSSF-CW) and a

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cluster of conventional WWTPs in the city of Changzhou (China). Similarly, Chen et al. (2011)

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quantified the overall GHG emissions embodied in the construction and operation stages of a

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typical CW and compared results to a typical conventional WWTP for a low-carbon assessment of

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WWTP engineering. In the field of GHG-emission assessment, de Klein and van der Werf (2014)

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documented that wetland can act as a sink of CO2, with net values in the range 0.27 – 2.4 kg m−2

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y−1, depending upon the plant settings and the operation conditions. Based upon findings of the

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studies reviewed, it can be asserted that CW systems for wastewater treatment cause less

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environmental impacts and less GHG emissions than conventional WWTPs. In particular, Mander

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et al. (2014) highlighted significantly low emissions of CO2 and CH4 in free water surface (FWS)

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compared to horizontal subsurface flow (HSSF) by performing a review of 158 papers published

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from 1994 to 2013.

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Several other LCAs and related evaluations have been conducted since then, like that of Resende et

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al. (2019) and Flores et al. (2019). The former investigated coupling CWs with two decentralised,

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small-scale, wastewater treatment systems (with and without aeration) and performed a combined

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assessment of the related economic and environmental issues. Based upon results from Resende et

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al. (2019), it can be highlighted that operation of the aerated CW is the activity causing the greatest

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environmental impact for all analysed impact categories, with results ranging from 64% for human

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toxicity to 100% for freshwater eutrophication. By contrast, the life cycle cost per cubic meter of

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treated sewage for the aerated system was almost two times smaller than that of the system without

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aeration. Therefore, through their study, the authors documented that aeration is cost-effective for

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small-scale WWTPS, when they are coupled to CWs.

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In a recent study related to agroindustry effluents, Flores et al. (2019) assessed the environmental

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performance of CW systems for winery wastewater treatment by comparing a set of scenarios

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providing different technology- and management-based options. Through their study, the authors

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highlighted CWs as an environmentally friendly technology leading to reduction of the

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environmental impacts associated with winery wastewater treatment, with the latter being made on-

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site with low consumption rates of energy and chemicals.

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Carrying capacity is another important environmental issue to be considered in CWs. It is a

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quantitative concept recognising the limit of the ability of natural ecosystem to support increasing

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population growth in respect of the availability of resources and the tolerance of environmental

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degradation (Ali, 2013). That limit is, however, difficult to be assessed, mainly owning to: the

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complexity of water environmental systems; and the diversity of influencing factors (Wang et al.,

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2017). Carrying capacity is important, because it enables understanding the mutual relationships

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between social sustainable development and water resources. It is often used to explore the

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interconnection between social economy coordinates and water environmental systems, and can

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play crucial roles in the comprehensive development of a country or a region, as well as its

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development scale (De la Sen and Alonso-Quesada, 2009; Wang et al., 2017).

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Engineered water systems like CWs may contribute to enhancing the ecological carrying capacity

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of the global ecosystem, through sustainable treatment of wastewaters as part of the disposal of

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global waste flows. In this regard, CWs can support local authorities in achieving the balance

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between watershed ecosystem protection and economic development, so contributing to formulation

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of sustainable policies (Ali, 2013; Ni et al., 2016).

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Encouraging results on CW systems, in terms of pollutant removal efficiency, environmental

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sustainability, and economic convenience as documented by the subject literature, have been

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stimulating the attention of researchers and scientists worldwide to further explore and improve

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such a valid wastewater treatment technology. This could include, also, application of CWs to treat

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wastewaters from touristic, restaurants, and recreational facilities that – as is known - are

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characterised by huge variability rates in terms of guests’ presence (Calheiros et al., 2015). This

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aspect has led to high fluctuations of wastewater production over time, which have made traditional

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treatment systems often inadequate in terms of efficiency, smell abatement and digestion process.

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Furthermore, when those facilities are placed in areas of high naturalistic interest great attention

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should be paid upon effluent water treatment, and the impact that the related plant may cause to

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both environment and landscape (Calheiros et al., 2015; Toscano et al., 2015). Therefore, a solution

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could be the use of CW systems because they answer all the aforementioned needs, whilst being no

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invasive for natural landscapes and causing highly reduced impacts to human health and to the

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environment (Calheiros et al., 2015).

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In this context, sustainable water usage practices and management systems may include treatment

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of wastewaters in CW systems and their re-usage for irrigation purposes, in a circular economy

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perspective, thereby avoiding water overexploitation and pollution. Fig. 3 shows how it would be

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like.

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Fig.3. The figure shows wastewater treatment and recirculation within the given facility, for irrigation purposes1.

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2.2 Landscape conservation

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The increasing construction of new buildings, on one hand, meets the demand for dwellings of a

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growing population but, on the other hand, narrows the city's green space, often leading to

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suffocation and discomfort for human beings. Actually, the current green space densities in some

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cities are significantly lower than the average green space index, that the Economist Intelligence

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Unit has quantified in 39 m2 per person (Vo, et al., 2019).

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In this context, green infrastructure can play a vital role, as it is increasingly emerging as a planning

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and design concept to provide a framework for conservation and development, and arises from the

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need to create places for people to live and enjoy nature (Sleegers, 2010). Green infrastructure may

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help communities to plan development in a way that it optimises land use to meet the needs of

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people and nature, so resulting in a new shape of the built environment (Sleegers, 2010). It is

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principally structured by a hybrid hydrological drainage network, complementing and linking relict

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green areas with built environment, to provide ecological services (Sleegers, 2010). Green 1

Personal elaboration, using pictures extrapolated from; https://www.thatsfarming.com/news/irish-water-s-strategic-and-sustainablewastewater-management-plan; https://www.agriturismoboltei.com/; https://durwest.com/portfolio-item/oak-bay-beach-hotel; https://casertaweb.com/notizie/pasqua-gia-sould-out-negli-agriturismo-caserta-della-campania/; https://agronotizie.imagelinenetwork.com/agricoltura-economia-politica/2016/03/25/pasqua-gli-agriturismi-sperano-nel-beltempo/48088; https://www.toro.com/en/about/corporate-responsibility/product-safety-sustainability/irrigation-efficiency

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infrastructure applies key principles of landscape ecology to built environments in a multi-scale and

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multi-layered approach (Sleegers, 2010).

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Following the green infrastructure principles as a planning and design concept, phytoremediation

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can become one significant and complimentary element to favour creation of the future

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development framework in sustainable manners (Sleegers, 2010).

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Phytoremediation is an integrated economically viable technology that is based upon usage of green

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plants for the degradation, removal, and detoxification of a set of different pollutants from

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contaminated soils, sediments, or waters (Clayton, 2007; Herath and Vithanage, 2015).

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In the past two decades, phytoremediation of wetlands has been documented to be a significant

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technology for wastewater remediation, since wetland sites act as a sink of various toxic

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contaminants that include heavy metals, radionuclide, pesticides, organic carbon, particulate matter,

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and nutrients. CWs for wastewater treatment are one current practice that is increasingly

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representing a vital part of phytoremediation (McCutcheon and Schnoor, 2003; Herath and

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Vithanage, 2015). They are viewed today as a popular type of green infrastructure that is intended

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to collect, treat, and store wastewaters (including the storm ones) from urban watersheds. As a

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large-scale landscape, they have the potential to perform not just hydrologic functions, but also

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those oriented to urban design and ecosystem services. In this regard, when CWs are installed

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within urban regions, they may take on human-related values, as they provide some contact with

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nature, and some opportunities for tourism and recreation that are otherwise rare in the urban

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landscape (Ehrenfeld, 2000; Ezeah et al., 2015).

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In this context, having recognised the restorative nature of vegetated landscapes, it has been

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progressively understood that the contact with nature in different types of landscapes have positive

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effects upon the health and the well-being of individuals. Recent studies have demonstrated the

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benefits on the mental and behavioural conditions, with the result that applications, such as healing

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gardens and horticultural therapy, have started spreading (Senes and Toccolini, 2013). Designed

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and easily accessible landscapes are preferred compared to more wild and near-nature settings.

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Additionally, amongst green spaces, it has been found that people perceive the capacity of a

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protected area to enable reduction of negative physical symptoms (e.g., muscular pain or tension),

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as well as of the negative emotional ones (e.g., depression or stress) (Jiricka-Pürrer et al., 2019).

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In the urban context, landscape urbanism has built upon pioneering concepts of design with nature

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practiced by Olmstead, Spirn, McHarg and others, and embraced ecological principles with Weller

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(Smith, 2015), conducting to applications in remediated landscapes (Sleegers, 2010).

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In rural areas, Integrated Constructed Wetlands (ICWs) has been developed in Ireland to explore

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diversity issues in pollution problems, such as the treatment of sewage effluent from small villages

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and wastewater from livestock (Harrington et al., 2013). Recent studies have analysed the aesthetic

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and environmental changes of the rural landscape due to intensive agricultural production (Picuno

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et al., 2011; Arcidiacono and Porto, 2012a, 2012b), with the aim of supporting sound policies of

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landscape and environmental planning.

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Although awareness of the importance of ecological landscape planning has grown in urban and

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rural areas, phytoremediation systems have been viewed differently from the conventional

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horticulture or landscape architecture process; traditionally, a landscape architect is involved in the

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project after the remediation process is complete (Sleegers, 2010). Recently, there has been

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increased interest and support to combine urban planning and design with phytoremediation, as a

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holistic system-based approach to improve the quality of the environment and the health of humans

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at the local and regional scale. In two phytoremediation application experiences in Germany

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(Sleegers, 2010), the remediation infrastructure was proposed as a multi-layered, multi-functional

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green infrastructure, as it: was part of the street and pedestrian circulation network; constituted a

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habitat for wildlife and a source of building materials or a wood-based fuel; and was suitable to be

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transformed into a surface storm-water treatment system after remediation.

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Remediation and self-sustaining systems for storm- and waste-water treatment proposals, using

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wetland vegetation (e.g., Phragmities, and Iris) and creating an urban greenway, introduce new

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landscape paradigms, i.e., unique landscapes of remediation. Thus, phytoremediation can be viewed

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as a process-oriented tool for an evolving green infrastructure network that defines new landscapes.

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In this direction, outdated conceptions of landscape beauty should be reconsidered in this field

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where transformative remediation as a systematic design tool provides conceptual bridges between

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aesthetics and ecological design (Sleegers, 2010). This should be done, however, considering

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planning- and location-related issues which cannot be neglected for the correct functioning of the

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given phytoremediation system. Planning consists of several steps, including: characterising the

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quantity and quality of the wastewater to be treated; determining the discharge standards that need

297

to be met; and selecting the type and configuration of the most suited system (Davis, 1994). CWs

298

should be designed to take advantage of many of the same processes developing in natural wetlands

299

(Vymazal, 2010; Herath and Vithanage, 2015), but have to be modified to fit the needs of the

300

project and the specifics of the site, making way to avoid its disturbance (Davis, 1994).

301

Site selection is an important part of the planning, and should be done by considering several

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important related issues like: land use and access; land availability; and site topography. Wetlands

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should be placed in a way to allow water to flow by gravity. If odours or insects are problematic, as

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happens with some agricultural wastewaters, wetlands should be placed the farthest possible from

305

dwellings. The site should be of easy access to personnel, delivery vehicles, and equipment for

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construction and maintenance activities (Davis, 1994). Its site should be selected to favour water

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time-retention and, in turn, the CW effectiveness, and to accommodate present and future

308

requirements. A CW can be built almost anywhere, but selection of a site with gradual slopes that

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can be easily altered to collect and hold water simplifies design and construction, and minimise

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environmental and economic burdens (Davis, 1994), in line with the sixth sustainable-development

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goal. In doing so, in line with Ingrao et al. (2018), application of holistically sustainable CWs rather

312

than conventional WWTPs can contribute to the transition to truly equitable, sustainable, post-fossil

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carbon societies.

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2.3 Monitoring

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Monitoring is an important tool for the correct operating of any CW, as it: provides data for

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treatment performance improvement; identifies problems as the base for solving; documents the

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accumulation of toxic pollutants before they bio-accumulate; and finally determines compliance

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with regulatory requirements (Davis, 1994).

320

Monitoring is needed to measure whether the wetland is meeting the objectives that it has been

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designed to achieve, and to indicate its biological integrity. Wetlands monitoring should start well

322

before the work commences, when intervention is most effective (Davis, 1994; Davidsson et al.,

323

2000). When planning a monitoring program, it should be taken into due account that its level of

324

intensity is strongly dependent upon the size and the complexity of the wetland system (Davis,

325

1994). In general, it is recommended to start on a high intensity level during the very beginning of

326

the wetland working, and then to reduce it when the main patterns of water flow and processes

327

become well-known (Davidsson et al., 2000). Since many parameters undergo heavy fluctuations

328

within and between years, it is important that monitoring programs proceed during longer periods

329

(at least several years) to identify trends in wetland performance (Davidsson et al., 2000). The

330

wetland should, however, be checked periodically, to observe general site conditions and to detect

331

major adverse events that may occur, like erosion or growth of undesirable vegetation. Vegetation

332

in CWs can be considered as a massive biofilm, which is required to assess its health and abundance

333

(Brix, 1994; Davis, 1994; Herath and Vithanage, 2015). Different environmental parameters,

334

including wind velocity, light intensity and insulation of snow, should be controlled as part of

335

monitoring activities (Herath and Vithanage, 2015). In addition to this, species composition and

336

plant density can be easily determined by inspecting quadrats within the wetland at selected

337

locations (Davis, 1994).

338

Finally, pollutant removal efficiency can be assessed by determining hydraulic load rates, inflow

339

and outflow volumes, water quality changes between water input and output flow, and excursions

340

from normal operating conditions. In particular, the removal effectiveness can be calculated as the

341

difference between the influent and the effluent loads (Davis, 1994).

342

2.4 Harvesting

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When CW vegetation is harvested, the nutrients captured are exported subsequently. The harvested

344

biomass can be utilised for soil amendment or fertilisation or as livestock feed, offering a

345

complementary approach to aquatic remediation, which could provide several ecosystem benefits

346

(Quilliam et al., 2015). Since aquatic plants are a reservoir of both energy and nutrients, after

347

harvesting they can be suitable candidates for application in biogas plant for production of both

348

electric and thermal energy, as solid and liquid digestate (O’Sullivan et al., 2010). By doing so,

349

compared to the initial option, the harvested plant value is amplified and the outputs from the

350

transformation stage are multiplied. The frequency of harvesting usually affects submerged biomass

351

production, biomass nutrient content and the resulting amount of nutrients removed (Luo et al.

352

2018), as well as both composition and structure of the vegetation. Great care needs to be taken

353

when performing a harvesting treatment, as it may reduce competitive strength of the plants and,

354

additionally, may negatively affect the nutrient removal efficiency (Verhofstad et al., 2017; Zheng

355

et al., 2018).

356

Verhofstad et al. (2017) implemented and monitored experimental ecosystems of two shallow

357

ponds of 30 × 15 m and a water depth of approximately 75 cm located near Bemmel, in the

358

Netherlands. The experiment allowed them to understand that harvesting frequency strongly affects

359

the amount of biomass harvested during one growing season. The study has additionally

360

documented that a frequency of either once or five times per season removes around 32% and 27%

361

less biomass than when harvesting is performed twice, respectively. Also nutrients as nitrogen,

362

phosphorous, and potassium, which are eliminated with biomass, followed a similar trend.

363

Therefore, an intermediate frequency was suggested by the authors, when aiming to remove the

364

maximum amount of nutrients. By contrast, major frequency rates may be a proper practice to

365

lessen dominance of some species that could cause issues due to excessive growth. As concerns the

366

harvesting method, in their case the submerged vegetation was manually cut at ∼20 cm above the

367

sediment using hedge trimmers to maintain viable shoots and to reduce the risk of losing the entire

368

vegetation. In the light of the above, the authors concluded that the frequency of harvesting and the

369

way the latter is done could significantly affect several indicators of the correct CW working,

370

namely: submerged biomass production; biomass nutrient content; the resulting amount of nutrients

371

removed; as well as the vegetation composition and structure.

372

In another experimental study, Zheng et al. (2018) showed exponential increases in P. australis L.

373

density and biomass with continuous harvesting more than three times compared to unharvested

374

CW. Moreover, plants harvested by cutting their stalks at about 20 cm above the substrate surface

375

was documented by the authors as enhancing the microbial community diversity and richness

376

significantly, which can be considered as an indicator of the CW performance. In particular, the

377

Chemical Oxygen Demand (COD) removal rate in post-harvest CW was about 4.3 times higher

378

than that in the unharvested ones, as well as lower organic N removal efficiencies were

379

documented, at the end of the experimental activities. From a point of view of plant physiology, the

380

plant heights in the harvested CW increased slightly over those in the unharvested CWs and this

381

difference was more evident after five years of continuous harvesting, at a nearly 45cm height.

382

From an environmental point of view, Zhu et al. (2007) found that plants harvesting affects

383

methane emissions because, several days after harvesting, they decreased in free water surface flow

384

systems, but remained high in the subsurface flow ones. According to the authors, this should be

385

attributed to the root activity of the plants, which changed after harvest.

386

In the study of Luo et al. (2018), a sustainable multiple harvesting management was implemented at

387

a 50cm height (Luo et al., 2017) above the surface water level, in order to support shoot

388

regeneration. Results from the study showed a high N-removal efficiency, due to the harvesting

389

strategy, which stimulated plant productivity. Findings from this research also demonstrate that

390

harvesting is essential for the correct operating of wetland systems. However, the most suitable

391

method to do it and its relative cost were proven by this author team to be strongly affected: by the

392

type of CW considered, as depicted in Fig. 2 of this manuscript; and by background conditions,

393

such as the geographical location of the CW and the organic load of the water influent.

394

Furthermore, the interaction between biomass yield and nutrient concentration determines the

395

optimum time of harvests, which can occur in summer, autumn or winter (rarely). In this regard,

396

Yang et al. (2016) highlighted summer harvesting as a proper way to enhance pollutant removal.

397

Several methods are available to cut submerged plants and remove the biomass from aquatic

398

ecosystems, such as manual cutting when mechanical harvesting is not feasible or utilisation of

399

harvester boats, which cuts the shoots at a pre-set depth and transports the cuttings into a hold on

400

the boat, using a conveyor belt. Mowing boats, harvesters and also cutters with blades are widely

401

utilised to reduce the biomass of submerged species. However, attention should be paid when such

402

an operation is carried out, because a large number of plant fragments may be generated, with a

403

subsequent risk for further, involuntary spread of the plant species. Other methods using a V-shaped

404

blade pulled behind a boat and dragged across the sediment like a shovel, cut the plants on, or just

405

below, the sediment. Submerged plants could also be mechanically managed using rotovators,

406

which rescind also the stem bases (Hussner et al., 2017).

407

These methods allow for cutting a larger area and faster than manual harvesting, but is generally

408

less precise and only suitable for larger ecosystems. By contrast, manual harvesting is a very

409

governable and accurate method to remove plant biomass but it is only sustainable at a small scale

410

because it is very labour-intensive, thereby leading to increase in economic costs.

411

Because harvesting is costly, identifying the optimal harvesting frequency for nutrient removal is a

412

very good approach to lessen costs, without compromising CW behaviour in terms of pollutant

413

removal efficiency. Using the harvested biomass for useful applications, such as bioenergy

414

production and agricultural fertiliser, may reduce the net cost of harvesting management even

415

further (Verhofstad et al., 2017). Additionally, logistics limitations, such as access, offloading and

416

disposal sites for harvested biomass should be considered as they can extend both working time and

417

management costs.

418

Finally, considering the universal applicability of CWs, it is important to utilise the harvested

419

biomass extensively to enhance the multi-purpose use of all those sites in which they get to be

420

located (Avellan et al., 2017). Additionally, that extensive utilisation would give added-value to

421

that biomass, as the latter would turn out into a zero-burden resource for several applications, with

422

its life cycle being extended to a new ‘cradle’. Several routes can be followed for harvested

423

biomass application, and are well documented in the literature; they are mainly about utilising it for

424

production of fodder, energy, and fuels (Solano et al., 2004; Avellan et al., 2017). According to this

425

author team, tools like LCA may help to find the most sustainable of those routes or sustainable

426

combinations of them, thereby favouring sustainability-oriented planning in biomass utilisation at

427

the regional and the global scale.

428 429

3. Conclusions and future perspectives

430

The study attained the proposed goal of highlighting technological and environmental issues of

431

CWs, as well as issues related to monitoring, landscape conservation, and harvesting, so outlining

432

the background of the research that the authors are currently in the process of developing.

433

Based upon findings of the studies reviewed, it can be asserted that, though having a few

434

limitations, these systems cause less GHG emissions and less environmental impacts than

435

conventional WWTPs, and contribute to sustainable enhancement of the ecological carrying

436

capacity of the global ecosystem through wastewater treatment as part of global waste disposal.

437

Moreover, thanks to their advantages in terms of pollutant removal efficiency, environmental

438

sustainability, and economic convenience, these systems have stimulated the attention of

439

researchers and scientists worldwide to extend their application also in other contexts as those of

440

touristic and recreational facilities for water re-usage in a circular economy perspective. In this

441

direction, there has been increased interest to join landscape planning and design with CW systems,

442

as a holistic system-based approach to improve the quality of the environment and the health of

443

humans at the local and regional scale. Other environmental advantages are linked to harvested

444

biomass that could offer several ecosystem benefits being utilised for soil amendment or

445

fertilisation or as livestock feed, but also for energetic purposes.

446

Finally, future perspectives of this review have to do with doing research in the CW field through

447

sustainability assessments, with the final aim of contributing to meeting the requirements from the

448

sixth sustainable development goal, and to feeding the specialised literature and knowledge.

449 450

Acknowledgements

451

Dr. Carlo Ingrao would like to thank his co-authors for having supported and advised him in the

452

design and development of study, as well as in the writing of this document by contributing section

453

2.2. (Prof. Claudia Arcidiacono), and section 2.4 (Prof. Sabina Failla). Additional thanks to Prof.

454

Claudia Arcidiacono, for her in-depth revision of the final version of this paper before submission.

455

Furthermore, the whole team of authors wish to thank the anonymous reviewers who, through their

456

comments, have contributed to improvement and upgrade of the paper.

457

Finally, thanks to Prof. Jacopo Bacenetti for having invited us to publish in the journal issue

458

“Environmental monitoring assessment 2020”, and for so kindly and promptly handling this paper

459

submission.

460 461

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of the wastewaters outlet from the winery industry. The authors have found this article to

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phytoremediation and constructed wetlands, and on the close-related aspects. This

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publication has been of outstanding utility and interest for this author team to develop the

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further deepen their knowledge on constructed wetlands, because it interestingly

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highlighted that their correct functioning and the sustainability, especially under aeration

608

conditions, can be positively influenced by combination of them with conventional

609

wastewater treatment plants.

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This article represents, to the eyes of these authors, a significant contribution to the growth of

625

the knowledge in the field of constructed wetlands. Together with Herath and Vithanage

626

(2015), it has been particularly interesting and relevant for development of the landscape

627

issue related section, as it nicely highlighted phytoremediation system as a green

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dominated by submerged aquatic plants. Ecological Engineering 106, 423–430. (**)

644

This paper has been of outstanding interest for this study development because it allowed to

645

understand that the frequency of harvesting and the way vegetation is harvested can

646

negatively affect the correct functioning of the constructed wetland system, with special

647

regard to its pollutant removal efficiency.

648

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651

This study has been of special interest for this author team, because it was proven as useful

652

and valid for contextualisation of this review study. In addition to this, since it deals with

653

constructed wetlands, it allowed the authors to understand that the COESH journal is not

654

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655

journal itself.

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681

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682