Nitrate Removal in a Constructed Wetland

Located in Albany, Oregon, ATI Wah Chang is a reactive and refractory metals producer and has been making zirconium mill products since 1956 on 110 acres near the Willamette River. Today ISO 9001:2000 and AS9100-certified ATI Wah Chang produces an expanded line of metal products that includes hafnium, niobium, tantalum, titanium, and vanadium. During the production, metal is treated with nitric acid to remove impurities and contaminants, and this process is termed pickling. The nitric acid is brought to a neutral pH, but then must be disposed of as wastewater that contains nitrate. Many metal industries must deal with this high nitrate wastewater stream. When nitrate is released to natural waters, it ‘fertilizes’ the water and causes excess algae growth, which results in oxygen depletion from the water. Low oxygen levels are harmful to fish and the natural water ecosystem. This project will investigate constructed wetland systems to remove nitrate from metal industry pickling wastewater.

ATI Wah Chang, the City of Albany, and the City of Millersburg are partnering to develop an innovative wastewater project using constructed wetlands to improve the water quality of the Willamette River. The project employs new techniques in modeling treatment wetlands to reduce the temperature of the effluent of the municipal wastewater treatment plant (WWTP). Adding warm water to the Willamette River also harms the natural ecosystem. The wetlands, named The Talking Water Gardens, have been specifically designed to cool the water before it is emptied into the river by flow through a series of shallow and deep zones and associated cascade waterfalls. ATI Wah Chang’s wastewater will be combined with the WWTP effluent in the wetland and enter the Willamette River through the same outfall diffuser.

Research Team

This collaborative project will result in a research partnership that will be leveraged for future federal funding opportunities and support undergraduate and graduate research and education at OSU. The project will employ two graduate and several undergraduate students in the investigation and design of nitrate removal in constructed wetlands. The collaboration has already become a unique resource for the development and enrichment of courses at OSU. This spring (2010) students in the senior capstone environmental engineering course designed a wetland to cool and remove nutrients from the combined effluent of the Albany WWTP and ATI Wah Chang.

The Talking Water Gardens wetland plan, which is currently under start-up, does not incorporate features specifically designed to remove nitrate from the wastewater. The Talking Water Gardens design incorporates extensive contact between the water and the atmosphere to mitigate temperature which results in a generally aerobic system. The dominant mechanism for the removal of nitrate nitrogen within treatment wetlands is through microbial conversion to nitrogen gas which can escape to the atmosphere. However, microbial conversion of nitrate, or denitrification, occurs in the absence of oxygen under anoxic conditions. Information gathered from the operation of multiple wetland systems treating municipal waste throughout Europe indicate that the most effective nitrate treatment occurs in horizontal subsurface flow (HF) wetlands due to their ability to provide anoxic conditions. HF beds are constructed using a gradation of gravel and sand about 4 feet deep with plants on the surface, allowing saturated, essentially horizontal subsurface flow. Alternatively, vertical subsurface flow systems were found to effectively convert ammonia nitrogen to nitrate, while both bed types were effective at organic carbon treatment. Organic carbon waste is referred to as biological oxygen demand (BOD). The best total nitrogen removal occurred in systems combining both types of subsurface flow beds allowing for the oxidation of ammonia to nitrate and the subsequent conversion of nitrate to nitrogen gas.

We are characterizing the Talking Water Gardens system with respect to hydraulic pathways, microbial transformation potential, and developing approaches that, if incorporated, could lead to enhanced nitrate removal.

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