Marshall Steam Station

Acknowledgement: A Water’s Edge Hydrology project by Terry Rightnour, ARM’s Senior Hydrologist

The Marshall project is a full-scale application of vertical flow organic cells to flue gas desulfurization wastewater. Working with others, the coal-fired utility had previously installed nine large surface flow wetlands on the site in an attempt to remove selenium from this discharge, but these cells did not achieve the desired reduction in concentration.  Under a tailored collaboration with the Electric Power Research Institute, a pilot study was undertaken by Water’s Edge using a VFC test cell, which demonstrated selenium and mercury removal rates in excess of 90%.  Six large VFC cells were then installed in replacement of three of the existing wetland cells below the site equalization basin.  This system retrofit has easily exceeded selenium and mercury reduction goals since construction, and results indicate that VFCs are effective for removing nitrate and mercury as well.  Oxidation/reduction potential (ORP) probes monitor various substrate depths in the cells to remotely monitor the reduction states of the organic beds.  No significant maintenance has been necessary for the system since startup.

Owner: Duke Energy
Facility: Marshall Steam Power Station
Location: Catawba County, NC
Year Constructed: 2011
Construction Cost:1 $4,850,000
Annual O&M Cost: $25,000
10-yr Treatment Cost:2 $0.019/gal
Design Flow: 1587 GPM

Compliance Parameter Results 3

pH 6.94 SU
Arsenic <0.002 mg/L
Mercury 0.000017 mg/L
Nitrate < 5 mg/L
Selenium 0.0115 mg/L
TSS < 5 mg/L

1 Construction cost in 2012 dollars – VFC cells only,
2 Estimated total site construction cost including wetland cells is $15,000,000 + 10 x $25,000 annual O&M divided by 10 year design flow.
3 Average effluent value for system life following startup.

Marshall Constructed Wetland FGD Wastewater Treatment System