Plants with purpose

Growing up I heard about the sunflower and its ability to remove heavy metals from the ground, a process called phytoremediation. Sunflowers, and many other organisms, have adapted to survive removing contaminants from water and soil. Bioremediation (using both plants and microbes to remove contaminants from water and soil) has been used over the years in many forms. Nursery growers can implement a bioremediation strategy to effectively treat irrigation water.

Biological and ecological treatment

A natural wetland can be viewed as a living water filter. As water enters a wetland, it slows down and the contaminants within the water are removed through a series of physical and chemical processes. Wetland plants, or macrophytes, serve many functions within this wetland system. The shoots and sometimes roots can cause water to slow, which help suspended sediment to settle or fall out of the water column. Some contaminants, especially nutrients including nitrogen and phosphorus, are removed by plants and microorganisms. Wetland plants often promote the establishment of microbial colonies, which can help transform or inactivate contaminants. By understanding how these natural systems work, they can then be developed into cost-effective options for ornamental growers. Many constructed system designs have been developed including surface flow wetlands, sub-surface flow wetlands, floating treatment wetlands, and vegetated buffers or channels, all of which will be discussed below.

Surface flow wetlands

Free water surface (FWS) wetlands (also called surface flow wetlands) closely mimic natural wetlands such as bogs, swamps, and marshes. Water flows over a planted soil surface and through plants from an inlet to an outlet, allowing the water to contact plant shoots, the exposed surface of the bed, and to a limited degree the roots and subsurface material of the wetland. FWS wetlands are often lined with clay, a membrane liner, or an alternative impermeable material to prevent leaks. Leaks can short-circuit a treatment system and reduce treatment effectiveness. Typical plants used to establish FWS include cattail (Typha spp.), bulrush (Scirpus spp.), and reeds (Phragmites spp.), though many other plant species colonize wetlands over time. Much of the biological treatment of the system occurs at the point where sediment and water meet, where plant roots, soil, debris, and microbes interact. Wetland hydrology is designed so that sediment (total suspended solids or TSS) can fall out of the water column. Sediment often carries phosphorus, as well as trace levels of metals, pesticides and other organic chemicals. Over time, plants and microorganisms can take up or break down these chemicals, with typical reductions of 22 percent of TSS, 25 percent of nitrogen and 50 percent of phosphorus. Construction costs for FWS wetlands are generally lower than conventional wastewater treatment systems. Due to their passive nature, they also minimize mechanical equipment, energy, and maintenance requirements. However, FWS wetlands require a relatively large land area and annual maintenance (pump maintenance, removal of invasive woody plant species), especially when working to reduce nitrogen and phosphorus levels. Wildlife also use FWS as habitat — birds, amphibians, and reptiles all use FWS for food and shelter, with positive (species diversity and habitat restoration) and negative (humans v. snakes) results.

Subsurface flow wetlands

Subsurface flow wetlands (SF) move water through a substrate (all water is below the surface of a coarse substrate); making substrate choice very important. Coarse rock, gravel, sand and other soils have been used, but a gravel medium is most common in the U.S. The substrate is typically planted with similar plants to the FWS wetlands described above, although the shoot portion of the plant is typically completely above the water as the flow is designed to remain below the substrate surface. The main advantage of this system is the reduced size needed to treat the same volume of water because of the increased volume where treatment can take place. This system also reduces mosquito populations, odors and human and animal contact with the water. There are some disadvantages to be aware of. These systems tend to have low oxygen levels. While this is ideal for removal of nitrate, it drastically reduces the removal of ammonia. Introducing aeration into a portion of the wetland can help solve this problem. The substrate can also clog over time, particularly if sediment is not removed before water is introduced for treatment.

Horizontal vs vertical flow wetlands

Most SF wetlands operate as horizontal flow (HF) wetlands, where water flows horizontally from the inlet to the outlet, directing the water across the system. These systems typically have a 1- to 3-percent slope, which must be maintained. Vertical flow (VF) wetlands place influent water on the substrate surface. The water filters down through the substrate to the bottom of the basin where it is collected in a drainage pipe. These systems typically are cyclically filled and drained, which helps with aeration, and have been shown to remove double the ammonia of the typical SF wetland. Vertical flow wetlands can be installed either above or below ground. The use of multiple substrates and plants has resulted in variable treatments and results. Pumps are used in VF wetlands, to control the water level, while HF wetlands simply have a single pump that introduces water into the system (like the FWS). Whether using a VF or HF wetland, water movement is necessary to ensure subsurface flow and avoid stagnation. Furthermore, these systems are susceptible to clogging within the porous substrate if proper pre-treatment does not occur.

Floating treatment wetlands

A more recent form of constructed wetland treatment has been floating treatment wetlands (FTWs). A FTW consists of a floating mat (think yoga mat with holes) that suspends the plants on the surface of the water. This allows the roots to have direct access to the water, while maintaining foliage above the water surface. Physical entrapment of particulate pollutants by the root system is a significant removal pathway and is efficient at reducing TSS, particulate zinc, copper and nitrogen and phosphorus. While removal of TSS, N and P can be substantial (12 percent nitrogen reduction and 10 percent phosphorus), the lack of contact with a substrate reduces the efficacy of FTW systems in comparison with FWS and SF wetlands since the plants are only able to remove nutrients from the water, and the limited sediment they capture in their roots. These FTW systems, do allow for a greater variety of plants, including some ornamental plants. In a FTW system, plants must be in water at least three feet deep, so roots do not grow into the pond sediment. Worker safety is an important consideration when installing and removing plants, particularly in ponds that are deep or have steep sides.

Researchers are currently assessing these systems as a potential hydroponic production system, in which the plants grown in the FTW are removed and sold. A major benefit of FTWs is that current infrastructure (ponds and water retention basins) can be used, without having to add space. Furthermore, FTW systems are free floating, allowing them to adjust to variable water levels and flows and cannot be clogged. Performance of FTWs depends heavily on coverage, pond volume, climate/season, and the plant selection.

Vegetative swales and buffers

A final option for nursery water management is vegetative swales and buffers. Most growing operations have these biological treatment systems in place, but may not realize the function or how to maximize their potential as a bioremediation technology. Vegetative swales or ditches are broad shallow channels designed to convey water runoff to ponds or wetland areas. The swales are vegetated along the bottom and sides of the channel. Vegetative swales serve to stabilize the soil, reduce the volume and velocity of the water runoff and increase the flow path length and roughness. They are commonly used to redirect surface water at an operation. Vegetative buffers are a variant on this concept and have a similar function. Vegetative buffers have vegetation along the sides and above the channel, rather than planting the channel bottom. This slows down and filters the water prior to entering the channel and allows a faster, free flow of the water once it has entered the channel. Depending on the operation, plant selection can focus on one of two broad groups of plants. The first group of plants are those that can handle both wet and dry conditions, for example if you have variable amounts of runoff, including dry periods over the course of the year. The second group of plantings would be those that prefer to remain consistently wet (think plants in the FWS or SF). Vegetative swales and buffers often become wildlife habitat and can also be colonized by weeds. Maintenance is required to ensure that plantings do not become too dense to restrict the flow of water through the swale.

With each of these systems, it is important to remember that both plant growth and senescence is cyclic. Nutrient removal will be highest during active growth, and in some situations, plants can release nutrients back into the system at the end of the growing season. For free water surface wetlands, this can be partially avoided by removing either the above-ground portion, or the whole plant.

While perhaps not as “magical” as sunflowers, constructed wetlands and plant remediation systems have a promising role in our nursery industry. Using these natural systems reduces potential problems with chemical treatment or mechanical treatment techniques which can be expensive and require training and maintenance. Using nature to better perform water treatment practices can result in surprisingly effective and low-cost remediation practices.

Lauren Garcia Chance is an PhD candidate in Environmental Toxicology at Clemson University; Dr. John Majsztrik (jmajszt@clemson.edu ) is a research assistant professor and Dr. Sarah White (swhite4@clemson.edu ) is an associate professor in the Department of Plant & Environmental Sciences at Clemson University, Clemson, S.C. Acknowledgments: Funding for this material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2014-51181-22372.