A Wetland in Every Greenhouse

Susan Wood, graduate student in agricultural engineering, uses a carefully controlled growth chamber to quantify how efficiently her artificial wetlands remove nitrate from wastewater.

As water quality regulations tighten at both the state and federal levels, concerns about the release of wastewater from greenhouses are increasing. Greenhouse crops require a lot of water, and the effluent contains nitrates and other contaminants people do not want in their drinking water. Many greenhouses are located near large population centers, so the release of untreated or poorly treated wastewater could affect a large number of people. However, these operations often are small, family-owned businesses that don't have the capital to use conventional water treatment methods.

Graduate student Susan Wood is conducting research that may provide greenhouse operators with an economical and environmentally friendly solution to wastewater treatment. After earning a bachelor's degree in natural resources from Cornell University, Wood was employed as a water quality technician with the Soil and Water Conservation District in Gennessee County, New York."Working with farmers made me want to tackle agricultural pollution issues, but I was more interested in what the engineers did than in my role as technician," she says."I decided to satisfy both interests by pursuing a master's degree in agricultural and biological engineering at Penn State."

Wood and adviser Eileen Wheeler, an expert on controlled atmosphere environments for plants and animals, are now exploring the use of artificial wetlands to remove nutrients and contaminants from greenhouse wastewater."Artificial wetlands have become popular in recent years because they are relatively inexpensive compared to conventional wastewater treatment systems, and they provide effective and reliable treatment," Wood says."They may be a great solution for the greenhouse industry. But how large should a constructed wetland be? How will cooler winter temperatures affect the wetland's performance? Unfortunately, a lack of design specifications for artificial wetlands makes it difficult for operators to adopt the technology. It also means that existing systems are not working as well as they could."

Wood's goal is to take the high amounts of nutrients present in greenhouse wastewater and modify them naturally to meet clean water standards, so the effluent can be safely discharged into the environment, or reused on the farm or in the greenhouse. Her research involves a"subsurface flow" wetland, which keeps water levels below the top of a permeable growth medium such as gravel. Wetland plants are then established in the top layer of the gravel."This kind of wetland has many advantages," Wood says."Gravel has a lot of suitable surface area available for microbial activity, so the wetland can be smaller than one with surface water. Since a subsurface-flow wetland has no exposed water, mosquito and potential odor problems also are avoided."

Plants are a vital component of this engineered system because they provide an enhanced micro-environment around the roots for microbial activity, in addition to taking up pollutants."For nitrogen, we use cattails, bulrush, or common reeds," Wood says."Some plants, such as water hyacinth, can take up heavy metals like zinc. Others may be able to take up organic chemicals in pesticides. Most people associate agricultural engineering with tractors and machinery, but I'm studying agricultural and biological engineering. By quantifying exactly what happens in this biological system–the relationships between temperature, plant growth, and nitrate removal–I hope to make the wetlands as efficient as possible. For instance, we know that temperature can affect the rate of nitrogen removal in an artificial wetland system, so that's one factor I'm looking at."

Using a growth chamber in the headhouse behind Tyson Building, Wood carefully controls the environment in her experiments. To make her artificial wetlands, she took 10 PVC pipes measuring 4 inches across and 12 inches high, filled them to the top with gravel, and installed tubes at the bottom of each pipe from which to take water samples."I used washed pea gravel as a rooting medium because it doesn't affect nitrogen uptake," she says."In five of the pipes, I planted iris, a natural wetland plant with potential commercial value. This is important because greenhouse space is at a premium." The remaining five tubes were not planted, so they could serve as controls in the experiment.

For each round of experiments, Wood selected a temperature and let the plants acclimate for one week. She then added a nitrate solution and studied how long it took the wetlands to remove enough nitrogen to meet clean water standards. Because plant growth affects nutrient uptake, Wood measured the length of plant shoots before and after each experiment. At 12 and 24 hours, she sampled water from the tubes, then tested it for nitrate content. She also checked pH as well as conductivity, or salt content."These factors indicate whether something else is affecting nitrate removal," she says.

Wood found that removal of nitrates was greater in the planted systems at all temperatures."Nitrate reduction occurs very quickly in planted systems, although it takes longer at lower temperatures," she says."After 48 hours, planted systems remove from 2 to 11 times as much nitrate as unplanted systems. Over 72 hours at 64 degrees, planted systems removed 1.51 millimols of nitrate. The unplanted systems removed only 0.04 millimols of nitrate." Wood also confirmed that temperature does make a difference."Greater removal rates of nitrate occurred at 73 degrees than at 64 degrees for both types of systems," she says."Higher temperatures may quicken the process by helping the microbes that assist denitrification to grow."

After she completes her studies, Wood plans to put her skills to work helping to solve larger wastewater treatment problems."I'd like to take my research to a much larger scale," she says,"perhaps by helping farmers to use artificial wetlands to solve some of their nutrient management problems."

–Eston Martz