State Water Resources Research Institute Program

Project ID: 2007WA196B
Title: Quantifying and Enhancing Nitrogen Removal in Constructed Wetlands
Project Type: Research
Start Date: 3/01/2007
End Date: 2/28/2008
Congressional District: Washington, Fifth
Focus Categories: Wetlands, Water Quality, Nutrients
Keywords: natural treatment systems, constructed wetlands, ammonia, nitrate, nitrification, denitrification
Principal Investigators: Beutel, Marc; Peters, Troy; Watts, Rick
Federal Funds: $ 24,000
Non-Federal Matching Funds: $ 48,107
Abstract: Nitrogen pollution of aquatic ecosystems is a growing concern throughout the Pacific Northwest. In Washington, a detailed review of the 2002/2004 State Water Quality Assessment List yields over 250 water bodies that exhibit ammonia contamination, and of the 65 aquatic systems with approved TMDLs in Washington sixteen are listed for ammonia. The environmental impacts of nitrogen pollution of surface waters are wide ranging and can be broken down into three broad categories: (1) eutrophication - ammonia and nitrate are critical phytoplankton nutrients that exacerbate eutrophication particularly in coastal waters; (2) hypoxia - nitrogen pollution can cause low DO conditions in surface waters directly through the biological oxidation of ammonia and indirectly via the decay of phytoplankton blooms stimulated by nitrogen pollution; and (3) toxicity - ammonia and nitrite are extremely toxic to aquatic biota, while at elevated levels nitrate is toxic to infants.

Natural treatment systems (NTS), which commonly include a combination of constructed wetlands and ponds, are unique in that they contain a range of aerobic and anaerobic environments that facilitate the biotransformation of a range of pollutants. In addition, NTS provide auxiliary benefits such as wildlife habitat and recreational and educational uses, while promoting environmental sustainability through the minimal use of energy and chemicals. Constructed wetlands are especially good at removing nitrate via denitrification, the biological conversion of nitrate to harmless nitrogen gas. However, case studies documenting the effects of environmental parameters on the removal of nitrogen in NTS, particularly treating agricultural runoff, are uncommon. In addition, while wetlands are good at removing nitrate, they are not good at removing ammonia. This is a critical limitation on the use of NTS to remove nitrogen from ammonia-rich effluent from domestic wastewater treatment plants.

This project includes two main components. In the first component a student will examine three years of water quality data from an existing NTS that treats irrigation return flows in the Yakima Valley. The data set provides an exceptional opportunity to examine and document how a number of factors (e.g., nitrate loading rate, temperature, and wetland age) affect nitrogen removal rates in a large-scale NTS treating agricultural runoff. In the second component bench-scale experimental wetland mesocosms will be used to examine how two novel technologies, oxygenation and immobilized cells, may be used to enhance rates of nitrification in treatment wetlands. Oxygen is a key environmental parameter that limits the ability of NTS to process ammonia. Oxygenation, the addition of dissolved oxygen to surface waters using pure oxygen gas instead of air as an oxygen source, is currently being used to add oxygen to a number of lakes and rivers. Liquid oxygen can be purchased and stored on site very economically and safely. Immobilized cell biotechnology has been used to enhance nitrification rates in wastewater treatment plants that treat ammonia-rich wastewaters. Cell immobilization involves entrapment and encapsulation of the nitrifying microbe cells in small beads or pellets of a support media that are easily retained in bioreactors. Both of these innovative treatment strategies have yet to be evaluated in the framework of NTS.

It is anticipated that this project will yield important technical results needed to implement, manage and optimize NTS for the removal of nitrogen from point and non-point sources. In addition, the project will stimulate future collaboration and proposal development with a number of regional researchers and water quality managers.

Progress/Completion Report, PDF

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