A systemic literature review was performed using the Scopus database from 2015-to 2021 through peer-reviewed journal papers and conference articles. This discussed MCWPs’ background and available assessment methods. The following keywords were used: wetlands, constructed wetlands, and wastewater treatment, and limited to the field of engineering. A total of 612 publications were extracted, noting that the topics of constructed wetlands scored the highest contribution. This showed an increasing number of studies throughout the past years as shown in Fig. 2. Almost half of them were more concerned with environmental science (46%) but with the least focus on landscape and the ecological impact of wetland ecosystems. Also, few numbers of local studies discussed their environmental engineering aspects e.g., pollution, environmental chemistry, waste management and disposal, water science and technology, management, monitoring, policy and law, general environmental science, ecology, nature and landscape conservation, health, toxicology and mutagenesis, ecological modelling, and global and planetary change. Though, it was noted that previous studies lacked discussing their environmental assessment during the design and construction phases which might compromise the sustainability of the entire process. This formulates the justification of this study as an endeavour to attain Egypt’s sustainability goals for 2030.
3.1 Background on MCWPs
The application of MCWPs started experimentally in the 1950s in Germany in The Max Planck Institute, whereas the first systems to be practically constructed were in the 1960s in Europe and the US in the 70s. However, until the late 80s, the technical aspects of constructed wetlands were not widely discussed. During the past 20 years, due to the increase in environmental awareness, there has been a significant increase in applications and research in this regard [8]. However, the utilization of constructed wetlands in urban settlements has not reached its full potential yet [5].
While conventional water treatment plants are usually associated with high energy consumption, elevated construction and operation costs, as well as their unattractive, industrial visual appearance, MCWPs offer an environmentally friendly, cost feasible and aesthetic approach to water treatment processes [16]. They primarily serve as water management systems offering diverse and visually rich environments that combine water elements and landscape transforming them into constructed wetland parks [17]. These offer a wide range of urban social, economic, educational, recreational functions besides enhancing biodiversity [7]. Their environmental benefits include treatment of certain pollutant loads, preserving ecosystems and wildlife, climate regulation, and reduced dependence on chemicals. Socially, MCWP parks allow for social interaction with the water treatment process hence increasing social connectivity and awareness of prevailing water management problems. Economic benefits are demonstrated as 1/3 of the construction costs of conventional treatment systems and approximately 1/4 of the operation and maintenance costs, besides being more durable with a minimum of 15 years lifecycle [18]. Moreover, the high construction costs of conventional water treatment systems necessitate their construction as centralized systems with extended water pipes. This makes constructing water treatment plants in peri-urban areas an economically unfeasible option. On contrary, MCWPs can be widely implemented onsite as decentralized or centralized treatment systems for domestic, agricultural as well as industrial wastewater. This is in addition to their ability to mitigate storm-water runoff [7]. Also, they can be implemented on different scales ranging from household to neighborhood and community scale. The only major constriction to the application of MCWPs is concerned with land availability as they require a greater area per person equivalent [16].
The water treatment property of constructed wetlands employs the interconnections between certain plants and vegetation macrophytes, micro-organisms and the soil in a systematic process. This is reliant on factors such as the natural context, local climate, project design, types of plants, and microbial functions [19]. During the purification process, vegetation macrophytes absorb different pollutants from the wastewater accumulating them in their tissues. Simultaneously, this maintains a suitable environment for the growth of microorganisms which play a significant role in pollutants removal [20]. Moreover, the roots of vegetation macrophytes transfer oxygen through the water enhancing the aerobic conditions required for the purification process [21]. As a result of these combined processes, the wastewater quality is enhanced to meet the standards of water reuse.
Constructed wetlands are either categorized according to water levels into surface flow or subsurface flow constructed wetlands, or according to the direction of water flow as horizontal flow, vertical flow or hybrid systems. They are also sometimes classified according to their primary function e.g. habitat preservation, flood control, storm water retention or water treatment [22].
3.2 Environmental sustainable assessment methods
There are several assessment criteria for the environmental performance of buildings established and implemented worldwide. Examples are the Building Research Establishment Environment Assessment Method (BREEAM) of the UK, Japan’s Comprehensive Assessment System for Built Environment Efficiency (CASBEE), Australia’s Green Star, the US’s Leadership in Energy and Environmental Design (LEED), as well as Egypt’s Green Pyramid Rating System (GPRS). These mainly focus on assessing the performance of individual buildings [23, 24]. However, with the increasing awareness of the importance of environmental issues occurring at the city/neighbourhood levels, sustainability assessment systems worldwide have established distinct landscape assessment systems. Among the first tools for assessment of the sustainability aspects of green spaces is the “Green Flag Award” originally developed in Britain in 1996 Müller and Elsner ,2016) and Sustainable Sites Initiative (SITES) developed in the US in 2012. Also, recently in Germany, a research project was concluded by the German Federal Government and the German Research Platform for Landscape (FLL) in 2015 to evaluate outdoor facilities and develop a certification system [25]. These systems support an integrated design approach, during the development/planning phase of the project site, or throughout the design and management phases [26, 27].
3.3 Assessment indicators and weighting methods
Indicators are proposed parameters set to inform or describe the state of a certain system in relation to a specific concept, therefore they must be easily applied, clearly formulated, and relevant to the broad concept targeted by the assessment [28]. Sustainability indicators provide tools to evaluate how the system fulfils sustainability criteria through quantitative and qualitative assessments [28, 29]. Moreover, assigning specific indicators and their respective metrics must fulfil a set of basic criteria, e.g. 1) the effect and the function of the process, 2) temporal changes over specific time frames, 3) the feasibility of the measurement process in terms of costs, duration, number and qualification of required personnel, and 4) the interpretation of results and relevant presentation to end-users [29].
Following the broad conceptualization of sustainability, sustainable landscape projects should attempt to efficiently make use of available ecosystem services and meet socio-economic aspects while considering futuristic needs [30].This indicates that sustainable landscape indicators must simultaneously consider the three pillars of sustainability: environmental, economic, and social. Another pillar to be considered is the aesthetic value of the project [31]. However, sustainability assessment is a context-driven process that needs to be tailored on a case-by-case basis [32].
Since MCWPs are recently considered as an alternative nature-based method for wastewater treatment, it is increasingly imperative to understand and investigate how they function. It is also important to develop systems to assess and monitor their performance for an informed decision-making process [17]. In this regard, economic sustainability requires balancing the input cost with the expected benefits with no losses [33]. Relevant indicators are related to the cost of project development as well as required costs to operate and maintain the project e.g. labour and equipment ria(Rai, 2012). In developing countries, a project’s feasibility and operational cost are of high importance to be considered for the success of the project [34].
Environmental indicators include nutrients, biodiversity, soil, and energy. Other sets of indicators are set to measure the impact of constructed wetlands on the quality of air, sludge quality and gas emissions rate. Social and cultural indicators are as important as the economic and environmental ones, despite not being widely mentioned in literature as they are difficult to assess. These target the end-user linked to such projects either directly or indirectly e.g. local governance and regulation frameworks. Also, it is crucial to assess the community acceptance of such technologies and the familiarity with their operation. For example, a study among the local community conducted by Zakaria et. al. [34] showed that the acceptance of two case study MCWPs in rural sites in Egypt scored high value for several reasons, whereas conventional water treatment plants did not receive the same acceptance score in another rural site [34]. Further, assessing the efficiency of the installation and operation phase of such projects requires experts’ opinions, and the availability of operation training programs [33].
Weighting methods were categorized into 3 main categories; equal weighting methods, statistical-based methods, and expert / public opinion-based methods [35]. The Equal weighting method implies that all indicators are equally important, and no other statistical or empirical data supports other options; however, it is highly questionable in terms of the transparency and validity of its results. The Statistical based methods e.g., factor analysis is mainly used to examine correlations between indicators rather than weighting them. Expert opinion methods rely on extensive knowledge. An example of such a method is the Budget Allocation Method (BAL) where higher points “n” for an indicator represents higher budget allocation. Its main advantage is that it is a straightforward approach besides being transparent. However, it is often criticized because sometimes weights were assigned according to public and political concerns rather than indicators’ actual contribution to sustainability. Public opinion weighting methods depend on stakeholders’ concerns about various sustainability dimensions. It is characterized by its explicit nature and its short and simple execution, its main disadvantage is that results are more localized and not transferable across various locations [4, 36].
3.4 Leopold matrix
A matrix is an evaluation method designed to assess the impact of different activities on a set of indicators. In the simple form of matrices, indicators are arranged vertically while the different impacts are arranged horizontally. A checkmark is used to mark the impact of any activity on the corresponding indicators. One of the earliest matrix methods to be developed is the Leopold Matrix, suggested in 1971 [13, 15, 37]. Later, it was followed by the Component Interaction Matrix in 1974. Other forms of matrices further developed e.g. Modified Graded Matrix, Loran Matrix, and the Impact Summary Matrix [14, 15, 38]. One of the most prominent advantages of a1 matrix tool is its flexibility and adaptability to several types of projects, especially medium and large-scale projects and its efficiency in presenting data in a simple and easily comprehended form.
The Leopold Matrix is a simple analysis of the impacts of a project through many cells representing the magnitude and significance of different actions under several factors [11]. The Leopold Matrix assesses projects through comprehensively managing the project's challenges, impacts as well as the mitigation actions assigned to reduce negative impacts and improve positive impacts. Moreover, this methodology links various impacts to their respective project phase(s), either preliminary design, final design, construction, or operation phases, and in some cases the demolition phase is included as well. The linkage of the impacts to the projects’ phases indicates areas and phases of mitigation actions [15].
The construction phase of a project could have a significant impact on the environmental, social, and economic aspects of a project even though these impacts usually end after construction completion, however, in some cases construction extends for a prolonged period. Consequently, evaluating impacts arising during the construction phase should not be overlooked to suggest and discuss alternative construction methods and mitigation procedures. The operation phase, on the other hand, contributes most of the project’s impacts and is therefore considered the key purpose for the assessment process [15].In the case of MCWP, the system operation lifetime is dependent on the degree of pollutants contamination of the wetland cells and their removal and storage ability of accumulated wastes. A review of several constructed wetland projects shows that they have been efficiently operating for an extended period of 20 years. The monitoring and periodic removal of wetland deposits and the reintroduction of new substrates to the cells are essential procedures to extend their efficient performance [39].