Bauer Nimr LLC, PO Box 1186, PC114 Al Mina, Muscat, Oman
4.12.1 Introduction
Large-scale TWs are considered as distinct applications of wetland technology due to their size. The term
“large-scale”refers to wetland sizes much higher than the average wetland system, which is wetland beds with surface area starting from a few hectares up to a few thousands of hectares. Such facilities are built to deal with large flows, hence the higher area demand. As is easily understood, large-scale wetlands can be constructed only in areas where there is available land, e.g., in rural and/or remote areas, in the desert, etc.
Since the main limitation of wetland technology is in any case the higher area demand compared with traditional/conventional treatment methods, the number of large-scale wetlands is small. The fact that the construction and operation/maintenance costs increase with increasing wetland size also contributes to the small number of large-scale wetlands, when compared with the several thousands of wetland plants operating around the world. However, some of these large facilities are unique and are even considered as milestones for wetland technology, demonstrating its treatment capacity and the scaling-up possibilities.
4.12.2 Design objectives
The main goal of large-scale wetlands is, as for all wetland plants, water quality improvement. The large size of such wetland systems allows for the receiving and treatment/polishing of high volumes up to hundreds of thousands of m3per day. Large-scale wetlands have been designed for the following main applications:
• The majority of large wetland systems (with a surface area of 40–2,600 hectares) receive stormwater and urban runoff, and function to control floods and to remove excess phosphorus from agricultural drainage (Dunneet al., 2012; Kadlec, 2016; Pietro & Ivanoff, 2015; Simet al., 2008).
• Other systems (with a surface area of up to 900 hectares) have been designed as tertiary treatment stages, receiving and polishing secondary effluents from domestic/municipal and/or industrial wastewater treatment plants (Kadlec, 2016; Kadlecet al., 2010; Wuet al., 2017).
• Eutrophicated river or lake water treatment to remove nutrients (e.g., nitrogen, phosphorus) and improve the water quality of the final receiving water body is also another common application (Dunneet al., 2013).
• A TW with 2,400 hectares has also been designed to remove nitrate from the municipal drinking water supply in southern California, USA in order to protect human health and to reduce eutrophication and algal clogging in deep groundwater recharge ponds (Reillyet al., 2000).
• A large wetland system (360 hectares) has been designed to treat produced water contaminated with oil hydrocarbons from an oil field under desert climatic conditions (Stefanakiset al., 2018).
• A few applications also exist for wetland systems in secondary treatment of municipal wastewater, serving populations from 3,000 (Morvannouet al., 2015) up to 20,000 p.e. (Masiet al., 2017b).
These figures are considered unusual for TWs for secondary treatment of municipal wastewater, since TWs are generally viewed as best choice for small and medium communities, but are indicative of the potential to design wetland systems even for thousands of inhabitants.
Large-scale wetlands also provide a series of additional ecosystem services, which are usually integrated in their function and operation. TWs with a surface area of several hectares are in practice a new habitat for
wildlife that attracts birds, fish and reptiles. For example, it is reported that a large wetland system built in the desert of Oman is used by thousands of birds during their migration as a stopover to rest and feed (Stefanakis et al., 2018). The same is also observed in treatment wetlands in Florida, USA (Kadlec, 2016). Moreover, many of these systems are designed as polycultures, i.e., they are planted with more than one plant species, promoting in this way vegetation biodiversity. Additionally, considering that these systems are large vegetated areas they are also designed to provide an aesthetical upgrade of the site, while many systems are used for recreational and educational purposes.
4.12.3 Processes required and TW type to be used
Due to their large size and the associated high costs, the most frequent wetland type used for large-scale applications are FWS wetlands and only a few case studies of large-scale subsurface-flow wetlands exist (e.g., Masiet al., 2017b). FWS wetlands are simpler and easier (and, thus, cheaper) to build, compared to subsurface-flow systems filled with gravel media. The FWS type is widely used for stormwater and runoff treatment, to improve urban water quality and to polish effluents from wastewater treatment plants. The main target in these applications is nutrient (i.e., nitrogen and phosphorus) removal, hence biological processes are mostly required (such as microbial degradation), as well as physical/chemical processes (e.g., sedimentation, filtration, adsorption) and plant uptake/assimilation. Solids removal can also be a target (filtration). FWS wetlands are also used for produced water treatment at oilfields. In this case, oil hydrocarbons are the target pollutants and their removal mainly occurs through bacteria biodegradation.
4.12.4 Specific considerations during design and for construction
The design and construction of large-scale wetlands obviously includes a larger variety of technical and economic challenges, in order to successfully develop such a wetland project. The main issues that should be taken into account are as follow:
• Land availability is crucial for the financial sustainability of a large-scale wetland project. An area with relatively cheap (or even free) and adequate land should be selected for the wetland siting.
• There is an economy of scale for large-scale wetlands that for large-scale FWS wetlands reduces the cost per hectare in comparison to smaller systems. Use of large pumps to send water to a large wetland should, however, be avoided as it will offset this benefit.
• For large-scale FWS wetlands, installation of plastic impermeable liner is usually avoided due to cost implications. Natural minerals (e.g., clay) are often used to construct a sealing layer, but this is not always technically and financially feasible for large wetlands systems.
• Water flow path and depth variations may occur over time, owing to flow resistance by vegetation roots and stems, which could render it difficult to control water depth and could risk the stability of the embankments.
• Planting and establishing plants in large wetlands is an expensive task due the large number of seedlings and labour required and the potential initial need for nutrients supply.
• Maintaining a healthy vegetation cover can be a challenge; usually large wetlands are polyculture systems (i.e., with many different plant species) presenting changes with time. Although implemented in some cases, plant harvesting is usually unfeasible, as it can be expensive and technically challenging.
• Some large wetland systems (e.g., for stormwater treatment) may have seasons with no water inflow, which can result in complete dry-out and the subsequent risk of releasing pollutants stored in the
organic sediments of the bed. In such cases, the design should make provision to keep the wetland system saturated to prevent the drying of the vegetation.
• Short-circuiting, preferential flow, stagnant water or dead zones without vegetation within the wetland bed could all affect the transformation/removal processes and, thus, the treatment efficiency, as well as creating nuisance issues (mosquito breeding, odour). Vegetation management to maintain plant coverage and tracer tests to identify flow paths are often necessary.
• Longer start-up periods may be required for large wetlands.
• Multiple wetland cells, which can be isolated from the water flow, provide flexibility during the operation and maintenance period.