Power Plant Water Management
Fate of As, Se, Hg in a Passive Integrated System for Treatment of Fossil Plant Wastewater - Tennessee Valley Authority (TVA)
TVA, in cooperation with EPRI and the American Electric Power (AEP), is installing a demonstration passive treatment system at the Paradise Fossil Plant near Drakesboro, Kentucky, to treat ammonia in the flue gas desulfurization (FGD) effluent stream. The passive system is used to convert ammonia in the wastewater to nitrate in an aerobic component, followed by denitrification in anaerobic wetlands. In addition to NH 4, the wastewater stream also contains other hazardous pollutants captured from flue gas emissions, including arsenic (As), selenium (Se), and mercury (Hg).
Biogeochemical reactions could affect the retention and speciation of As, Se, and Hg in the wastewater as it moves through the treatment system. A more thorough understanding of these transformations is needed. For this project, an extraction trench component is being used for removal of As, Se, and Hg. This extraction trench is integrated into the passive system components described above, which is installed and operated by TVA with support from EPRI.
The objectives of the proposed study are:
- Design and install an extraction trench in the planned TVA/EPRI passive treatment system
- Monitor the movement of As, Se, and Hg through the treatment system
- Assess the removal efficiency of As, Se, and Hg from fossil plant wastewater by each component of the treatment system
- Determine the effect of each component of the treatment system on the speciation of As, Se, and Hg
Construction of the passive wetlands treatment system demonstration is at Paradise Fossil plant. The wetlands are designed to treat an influent of 0.5 MGD. Installation of the extraction trench is incorporated into the construction of the TVA passive treatment system. A portion of the fossil plant wastewater stream is diverted into the system for the demonstration. This on-site demonstration is operated for three years. Water samples are collected every four weeks from the inlet to the treatment system, and from the outlet of each component of the treatment system. Samples are analyzed for As, Se, Hg, and methyl Hg. Additional analysis is conducted to determine the chemical speciation for As, Se, and Hg. These results are used to assess the removal rates and speciation changes of As, Se, and Hg due to passage through each component of the treatment system. Water quality parameters (pH, conductivity, temperature, dissolved oxygen, and oxidation-reduction potential) are continuously monitored as part of the TVA demonstration project, and provide information on conditions when As, Se, and Hg sampling is conducted.