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Book Review

Spotlight on Constructed Wetlands

An inland constructed wetland in Germany – The Sustainable Sanitation Alliance

Wetlands are one of the world’s powerhouses for ecosystem services, filtering our water, controlling coastal erosion, and providing feeding and nursery habitat for a huge variety of wildlife. They are super productive, containing plant species that grow fast and therefore contribute a huge influx of organic material to the system when they die and start to decompose. These dominant plant communities, typically grasses and sedges in most wetlands, also form dense mats of standing or floating material that stabilize an otherwise dynamic system. Water is strained through a network of alive and decaying plant matter that acts like a huge cheesecloth, slowing its movement and removing suspended sediment. Taken up by the plants, once water-borne nutrients eventually cycle their way down to macroinvertebrates and other organisms that consume the plants, all of course with the help of nutrient fixing soil microbes. The system is so effective, water at the end of the wetland comes out significantly cleaner than it went in, having been stripped of its sediment and most attached nutrients. In an attempt to mimic some of these processes, water quality managers are increasingly turning to constructed wetlands to filter urban, industrial, and agricultural runoff. When effective, they use the same biotic and abiotic processes of natural wetland vegetation, soils, and microbial communities to remove large amounts of sediment, organic contaminants, and heavy metals from water bound for human consumption or return to the ocean.

The pollutants water quality scientists seek to control include Nitrogen, Phosphorus, and heavy metals. The first two are naturally occurring chemicals necessary for plant growth but they can exist in harmful forms or in overly abundant concentrations. Some systems thrive in nutrient heavy conditions, but excessive nitrogen can cause eutrophication (nutrient loading) in lakes and coastal waters, leading to sometimes toxic algae blooms, mass fish deaths from lack of oxygen, and in drinking water can cause birth defects in human infants. Traditional water treatment technologies use a combination of physical and chemical processes to remove nitrogen and other pollutants, but these are usually costly. Wetland microbes, however, convert nitrogen into nitrate gas as part of their natural life cycle, which is eventually released into the atmosphere, AND they do it for free. This is called denitrification (turning Nitrite and Nitrate Nitrogen forms into the atmospheric form) and it’s important because it removes Nitrogen entirely from the system. There’s also a variety of microbes that fix nitrogen (take it from the atmosphere and turn it into forms useful for plants), but for our purpose we’ll focus on removing nutrients here.

The Nitrogen Cycle – Environmental Protection Agency. This process works in a similar way in the ocean, but the bacteria could be free floating in the water column.

Excess phosphorus also creates algae blooms, as it is naturally rare in most systems, and can therefore also lead to low oxygen conditions. You see these sometimes in the mouths of rivers or estuaries near large coastal cities. Phosphorus is what’s called a limiting nutrient in wetland systems, so when it’s available the organisms that take it up tend to explode in productivity. Algae blooms though can be hazardous to human health if they happen in our drinking or swimming water. They can also significantly harm coastal fisheries. Phosphorus can be incorporated into plant matter or converted to phosphates that get trapped in wetland soils. It’s tougher to remove than nitrogen though, and requires really effective plant species or microbe activity to appropriately sequester it.

 

Lake Erie algae bloom – NASA Earth Observatory

Metals accumulate in wetlands from mining activities, stormwater runoff in urban areas, and other sources and are often bound to other molecules, making them difficult to remove. Metals like Mercury, Cadmium, Chromium, and Lead are particularly troublesome. While naturally occurring and in some cases necessary for various biological functions, most metals exist naturally in very small concentrations, so its easy to accumulate too much. Mercury is interesting because it actually becomes harmful once its in the water, being turned into its bioavailable form by microbes that live at the sediment-water boundary. Mercury is notorious for bioaccumulation (increasing in concentration up the food chain) because it’s so persistent in animal tissues. Cadmium, Chromium, and Lead are also bad for plant species and can impair the photosynthetic processes in these organisms that form the basis of all food chains. Metal pollution can therefore very quickly become a serious problem in aquatic systems.

Wetlands clean water in a couple ways:

Wetland vegetation is pretty effective at slowing and collecting sediment from the murky waters typical of these systems. So potential contaminants, if they are sediment-bound, can be stored in wetland substrates. Some plants are naturally good at sequestering contaminants either via sorption to the plant surface (sticking to the contaminant) or via metabolization (uptake) within the plant tissues, and these are the ones targeted for use in constructed wetlands.

Grasses and sedges are particularly good at breaking down contaminants otherwise toxic to most plant life. Cattails are often used in North America because they’re so hardy over a range of conditions and are easy to plant. However, there are few wetland plants capable of completely biodegrading pollutants within their tissues. Even species that are really good at taking up aquatic contaminants cannot completely eliminate the contaminant in their tissues, which means when the plant decomposes or is eaten by an animal, those contaminants can re-enter the system. This is especially true for phosphorus and heavy metals, which some grasses can metabolize but never completely remove.

Recent efforts have looked more at the ability of sediment microorganisms to metabolize stubborn pollutants. Though plants remove a small proportion of pollutants and supply microorganisms with necessary carbon, microorganisms are collectively responsible for removing the vast majority of pollutants, by amount, in wetlands. These include bacteria, fungi, algae, nematodes and other tiny organisms that live in aquatic or wet sediments and eat a variety of living and dead organic matter (carbon-based matter). Some are also able to use metallic compounds. Because these food items tend to be rare in the sediment and because wetlands in particular are difficult environments for these organisms, most of these living sediment components exist for most of the year in dormant stages where they don’t eat much. Getting the desired effect out of a constructed wetland requires fairly precise control of these organisms’ lifecycles.

The two most common types of constructed wetlands are surface and subsurface flow wetlands. Each sends incoming water through a mix of different rock mediums to tease out large particles, much like a traditional water treatment facility, followed by a wetland filter, but they have slightly different designs.

Surface flow wetlands are designed similarly to natural wetlands, where sediment settles to the wetland floor and contaminants are taken up by plants and microbes. They typically have open water covering much of the wetland and are used as a final element in other water treatment processes. They take up a lot of space though and can encourage algae and mosquito growth, reducing the quality of the water at the end of the process and creating a health hazard in the adult mosquitoes produced.

Surface flow treatment wetland – Tilley, E., Compendium of Sanitation Systems and Technologies

Horizontal subsurface flow treatment wetland – Tilley, E., Compendium of Sanitation Systems and Technologies

 

 

 

 

 

 

 

 

Subsurface wetlands, on the other hand, are designed for more precise purposes and can be used to remove a wider range of contaminants. Vertical subsurface wetlands pump contaminants directly into the substrate, carefully controlling environmental variables to maximize contaminant uptake and breakdown. They require less space but can be expensive to construct and manage. Horizontal subsurface wetlands act under gravity alone and allow water to slowly filter through a slight slope.

Then of course, there are combination wetlands that use some or all of these designs. Picking a design for a constructed wetland ultimately comes down to the intended purpose and what environment the wetland is situated in. Done right, some wetlands have been shown to be more effective at removing pathogenic microbes and heavy metals from wastewater than traditional activate sludge techniques used in your typical waste treatment centre. What’s more, they can do it at low cost.

The effectiveness of microbe-mediated biodegradation, though, is dependent on a number of highly variable factors. Oxygen content, pH, temperature, and organic content can all have large influences on the microbial community structure of wetland sediments and these factors will vary greatly by the type of constructed wetland as well as the surrounding environmental conditions. For this reason the environmental and financial benefit conveyed by constructed wetlands will also vary widely by location.

On top of these considerations, planners need to consider the many other risks and benefits of wetland creation. Evidence suggests constructed wetlands with high biodiversity in both plant and microbial life have an increased pollutant removal success rate and are capable of removing multiple pollutants at one time. Highly diverse habitats are also more stable and attract wildlife, giving constructed wetlands great potential to double as urban wildlife parks. Surface and tidal flow wetlands designed for wastewater treatment can make great recreational areas and particularly large treatment wetlands, or those that join to natural wetlands or other green spaces, may also mitigate urban heat island effect in large cities. But again, some wetlands are heaven for disease-carrying mosquitoes, they take up valuable space (especially in urban areas), and not everybody sees wetlands as having the same recreational value as groomed parks.

Future research in constructed wetlands is therefore likely to revolve around two important themes: best practices in technique and social science perspectives in public acceptance. We still don’t have a satisfactory final destination for many of the pollutants constructed wetlands can’t entirely recycle. Metals stored in plant material can be removed from the system by harvesting the plants, but they need to go somewhere and this is really only moving the problem somewhere else. There are also still large gaps in our knowledge of soil microbe dynamics and how these processes could be used to our benefit. Furthermore, there’s great potential in supplementing further study of existing processes in the fate of organic and metal pollutants with the inclusion of genetically modified microbes. We also don’t have established best practices for incorporating constructed wetlands, for varying purposes, into the urban planning process. Water treatment wetlands are usually an afterthought to urban planning, incorporated into existing treatment facilities and, while this is an important first step, including wetlands in the earliest stages of the planning process could allow for maximization of benefits.

The Yuen Long Wetland Park in Hong Kong – K.C. Tang

 

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