Climate change has increasingly become a critical concern due to the continual increase of greenhouse gas (GHG) emissions such as methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). But, what are the greenhouse gas implications of employing natural strategies such as constructing wetlands?
Constructed Wetlands
Although we hear about CO2 more frequently than other GHGs, CH4 is a very potent GHG being roughly 86 times worse that CO2 over a 20 year period (International Panel on Climate Change (IPCC), 2021)[14].
According to the IPCC, to achieve the global temperature rise control target of 1.5 °C, a reduction of CH4 emissions by one-third is needed by 2030 and nearly half by 2050 [6,14]. Like CO2, CH4 emissions originate from both anthropogenic and natural sources with wetlands being one of the largest natural sources of CH4 emissions [35].
Wastewater treatment is a source of GHG emissions and can be very resource intensive. One way we can reduce the impact of wastewater treatment on the environment is through the use of natural solutions referred to as a Constructed Wetlands (CW).
These are human made wetlands that are designed to make use of nature’s ability to purify water coming from industrial and residential waste and converting it into water that returns safely to the ecosystem.
Whenever we consider green solutions to any problem, it is critical that we understand it from multiple perspectives so that we can make educated decisions and take the necessary trade-offs into account. While CWs can be better for the environment overall, we must remember that wetlands, including constructed wetlands, can be either sources of GHGs or sinks depending on various factors. In this article we will have a brief look at constructed wetlands, their uses, benefits and their potential impact on GHG emissions. Despite the importance of understanding GHG emissions from human activities, there is a gap in our understanding of the impact of CWs requiring much needed further research.
What are Wetlands?
Wetlands are areas with a water table near or above the land surface, seasonally or permanently, throughout the year [26]. Depending on definitions and estimates, wetlands cover only 5-8% of the earth’s land surface yet contribute a disproportionate amount to the carbon pool of 20-30% [23].
Wetlands also provide many beneficial functions and ecosystem services [26], among which is the capacity of wetlands to purify water, modulate atmospheric concentrations of GHGs and act as a carbon sink for climate change mitigation [26].
Many factors influence wetland function (e.g., pollution, urbanization, and land use changes). Among these factors is the major threat of climate change [25,26] through the effects of rising temperature, changes in rainfall intensity and frequency, and extreme climatic events (e.g., drought and flooding)[26]. As a result, these factors can change a wetland from a carbon sink to a source and reduce its water purification abilities [19,9]. The warmer conditions associated with climate change also potentially mean the release of more N2O emissions from wetlands [12].
What are Constructed Wetlands and do they come at a Price?
One of the most resource-intensive industrial practices is wastewater treatment [34]. Constructed wetlands (CWs) are a nature-based solution specially designed wastewater treatment systems that mimic the structure and function of a natural wetland for treating various wastewater, including domestic, industrial, and municipal wastewater, storm water runoff, agricultural runoff and wastewater, and landfill leachate [15,16,8,29]. These are used throughout Canada and the world and even in parts of Nunavut despite being frozen for many months every year.
When we think of a wastewater treatment facility, we usually do not picture a plant filled pond or marsh area, and yet a constructed wetland is exactly that. Compared to wastewater treatment plants, CWs have many advantages: lower GHG emissions [17,25], lower construction and operation costs; and provide social values by representing landscape elements [7]. Unfortunately, extensive global use of CWs could cause significant GHG emissions due to the vast amount of wastewater generated [34]. The benefits of CWs as an alternative nature-based solution for wastewater treatment, however, may still come at a price in the context of climate change. While CH4 emissions originate from anthropogenic (human) and natural sources, wetland ecosystems are the largest natural source [35,34] with CWs emitting GHGs at a rate 2-to-10 times higher than natural wetlands [21, 34].
Thus, while CWs with emergent vegetation can sequester large amounts of carbon and treat wastewater[4] their widespread use as a low-cost, simple, and highly efficient pollutant removal method means CWs may have consequences [11]. As several studies have indicated that CWs can be a carbon sink or a source [2,3,28] balancing carbon sequestration and GHG emissions for each wetland is necessary to assess the consequences [3]; however, this is no easy feat as the dynamics of GHG emissions are highly variable. The GHG emissions from CWs vary spatially and temporally and are further influenced by other environmental factors, directly or indirectly, through changes in vegetation, water availability, and temperature [20,36,31]. An effect of higher temperatures might be an increase in CO2 27 through CW water losses from evapotranspiration (evaporation and transpiration) that exposes the upper sediment layers to oxygen where the organic matter can be transformed into CO2 [18]. Thus, the higher risk of future droughts makes it essential to select more suitable plant species with an adequate density to tolerate water shortage in CWs [26].
The age of the CW may also impact GHG emissions; however, studies are still inconclusive. Some research indicates that with age, CWs can function as carbon sinks [32] whereas others report a higher rate of CO2 and CH4 because of more accumulation of organic matter [1,5]
Plants are an essential component of CWs, as they affect the transformation of pollutants and the emission of GHGs 11. In CWs, plants fix and convert inorganic carbon from the atmosphere and water into organic carbon and sequester carbon through the substrate, resulting in wetlands as carbon sinks [34]. In CWs, CH4 is generated from the degradation of organic matter, in the absence of oxygen, either present in the influent into the CW or from plant litter accumulation [22] that gradually converts organic matter into CH4 and CO2, through the metabolic activity of microorganisms which is then released into the atmosphere, resulting in wetlands also serving as a carbon source [33]. Since CH4 is a much stronger GHG than CO2, understanding this process is critical to designing CWs that maximize the conversion of CH4 to CO2 so that the impact on GHG emissions can be reduced.
The structure of CWs can also impact GHG emissions [26] The main types of CWs are:
(1)Free water surface (FWS) or surface flow—resemble natural swamp regions where plants are rooted in a submerged layer of sand or gravel, giving them a similar appearance mainly used for the tertiary treatment of domestic and municipal wastewater, mine drainage waters, and for storm water and agricultural runoff [15,16].
(2)Subsurface flow constructed wetland – this wetland can be either with the vertical flow (VSSF), where the effluent moves vertically, from the planted layer down through the substrate and out or with the horizontal flow (HFFS), where the effluent moves horizontally, parallel to the surface [22,29].
An assessment of CWs showed that the potential contribution of FWS CWs to future global warming is small as they have been linked to the lowest CO2 and N2O emissions compared to other types of constructed wetlands [26]. However, FWS CWs can have high CH4 emissions compared to other types of CWs [26] The type of CW also influences CH4 emissions [26], as according to available literature, the presence of plants reduces CH4 emissions from HSSF CWs. However, it increases emissions from FWS and VSSF CWs [11]
Final Thoughts
How do we move forward?
CH4 has become essential to global atmospheric carbon reduction due to its potentially considerable role in future global warming. Achieving the global temperature rise control target of 1.5 °C requires effectively controlling CH4 emissions from CWs [34] The design of a CW must consider both wastewater treatment and ensuring that it is designed with GHG emissions in mind. Although there will be emissions related to human activities, intelligently designing these highly effective waste treatment wetlands can minimize their potential impact. There is a lack of understanding about the design criteria for CWs regarding CH4 emissions. The uncertainty of whether CW functions are positive or negative to climate change necessitates further research on the relationship between CWs and climate change to allow for high pollutant removal performance to meet water quality objectives while minimizing CH4 emissions in CWs [34]
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Contributors
Researchers
Katarina Duke
Authors
Katarina Duke
Mauro Aiello, Ph.D.
Lark Scientific Financial Support
Axel Doerwald
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