NC0004987_Application_20200904

<& DUKE
IVENERGY®
June 8, 2020
Ms. Julie Grzyb
North Carolina Division of Water Resources
1617 Mail Service Center
Raleigh, NC 27699-1617
Subject: NPDES Wastewater Permit Modification Request
Duke Energy Carolinas, LLC.
Marshall Steam Electric Plant
NPDES Permit #: NC0004987
Catawba County
Dear Ms. Grzyb:
Marshall Steam Station
8320 East NC Hwy 150
Terrell, NC 28682
828.478.7700
With reference to the April 29, 2020 permit modification request, additional information
is available for the planned installation of a new wastewater treatment system for Outfall
002. The narrative attachment of the modification request has been updated to reflect
this information. This update is attached.
If you have questions, please contact Robert Wylie at [phone] or
[e-mail].
Sincerely,
r
Rick Roper
General Manager III
Marshall Steam Station
cc: Wes Bell (electronic filing)
Attachment:
Update to April 29, 2020 Permit Modification Request

Attachment
Marshall Steam Station
Ash Basin Closure and Remediation Activities
Page

Ash Basin Closure and Remediation Activities
Decanting of the ash basin is currently in progress. In the second half of 2020 the
decanting phase of the ash basin closure project will move into dewatering. The input from
rainfall run-off and dewatering activities will account for approximately 2 million gallons per
day (MGD) to Outfall 002. A pilot study for the groundwater remediation system associated
with the Corrective Action Plan (CAP) Update, submitted to NCDEQ on December 31, 2019,
plans to begin in the first half of 2021 with the full system being implemented shortly after.
The full-scale remediation project will be extracting and discharging an estimated 1 MGD of
groundwater to Outfall 002. Infiltration wells are also included in the remediation system, for
which water will be pumped from Lake Norman, treated, then allowed to infiltrate into
groundwater to increase remediation system effectiveness. The infiltration wells will
increase aquifer productivity and flush out constituents from the unsaturated zone.
Groundwater will then be pumped from the aquifer through the extraction wells and sent to a
Waste Water Treatment System (WWTS) prior to discharging from Outfall 002. Therefore,
the maximum daily discharge from Outfall 002 is projected to be 3 MGD during the future
ash basin closure activities.
An evaluation of projected concentrations determined that only two metal parameters were
to be above the calculated allowable limits without treatment. These two parameters were
arsenic (from ash basin pore water) and mercury (from CAP wells). Therefore, a WWTS is
being implemented to remove these two parameters to concentration levels below the
calculated allowable limits.
This WWTS will consist of a high rate clarifier followed by filtration when needed. Ahead of
the clarification the water will be chemically treated to assist in metal precipitation and
settleability. These chemicals may include, organo-sulfide, hydrogen peroxide, sodium
hydroxide, a coagulant and a polymer for a flocculant. Sodium Bisulfite may be used to
ensure there is no excess of oxidant leaving the treatment system. There will be a final pH
adjustment step in which sulfuric acid or sodium hydroxide is used to ensure the pH is within
the limits. The sludge generated by the clarifier and filter backwash is planned to be sent
back into the ash basin for removal by the excavation contractor.
The ash basin closure project will also result in the need to build a larger landfill footprint.
The leachate from the larger footprint is planned to be sent to a retention basin which will
then pump the leachate to the wastewater Lined Retention Basin that discharge under the
NPDES permitted outfall 005. This outfall was permitted based on an estimated flow of 5
MGD. The actual flow has been historically less than 5 MGD. It is estimated that the flow
from the additional leachate flow will not change the previously projected long-term average
flow or loading of significance.
The flow schematic has been updated as follows to reflect the upcoming changes that are
requested with this permit modification request.
Page 2 of 2

Marshall Steam Station Water Schematic
NPDES Permit #NC0004987
ID Fan Control House
Non -contact Cooling Catawba County
Water Internal Outfall W3 Permit Modification Request
0.2MGD
Intake Screen April 2020
Backwash
Once -through Cooling Water
Outfall 001 _ Lake
1093 MGD Norman
Domestic Wastewater Sanitary POTW
Ash Sluice Submerged Flight Conveyor
Evap. 1.7 MGD
Lake
Norman Water Supply Groundwater
Flue Gas Desulfurization (FGD)
Wastewater
Treatment Outfall 002 �
Lake
Wet Scrubber wastewater from Lake Norman Extraction Wells
System 3.0 MGD
Norman
FGD a
Wastewter Treatment System Internal Outfall 006
(Phys/Chem & Bio)
0.8 MGD
Heat Exchanger Cooling Water 1.15 MGD
Storm water
Air Preheater Water, Precipitator & Boiler Wash Waters,
Misc. Cleaning Wastewaters
Fly Ash Silo Sump
II
Ash Basin
Dewatering
Misc. Equipment Cooling 0.53 MGD 0.04
MGD
Raw water Stormwater
Treatment
`----------
Boiler and Turbine Yard Drain Sumps -----------
1'9
Outfalls 002A and 0028 ;
Yard Sump Overflows ---------
Lake
Sumps {2 flows to lA)
�
Norman
Norman
MGD
(no flow expected) i
Retention Basin Overflow
Alternate 2.43
(no flow expected)
Coal Pile --------- _ _ _
Route MGD
Stormwater Holding Basin MGD Retention Basin
----------------------------�
Outfall 005
Lake
5 MGD
Norman
I Landfill Leachate 0.04 MGD