NC0004987_Modification Request_20200608

DUKE Marshall Steam Station
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ENERGY® 8320 East Hwy 150
Terreell,, NC 28682
a:828.478.7700
June 8, 2020
Ms. Julie Grzyb
North Carolina Division of Water Resources �, r- rTh
1617 Mail Service Center REC' L)
Raleigh, NC 27699-1617 JUN 11
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Subject: NPDES Wastewater Permit Modification Request NC0i, ?
Duke Energy Carolinas, LLC.
Marshall Steam Electric Plant
NPDES Permit #: NC0004987
Catawba County
Dear Ms. Grzyb:
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,
Rick Roper
General Manager III
Marshall Steam Station
cc: Wes Bell (electronic filing)
Attachment:
Update to April 29, 2020 Permit Modification Request

Attachme
nt
Marshall Steam Station
Ash Basin Closure and Remediation Activities
Page 1 of 2

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
ID Fan Control House NPDES Permit#NC0004987
- Non-contact Cooling I Catawba County
Water
internal Outfall 003 Permit Modification Request
0.2 MGD
Intake Screen April 2020
Backwash
1 V Outfall001 Once-through Cooling WaterLake
1093 MGD } Norman
-j Ash Sluice '( 1 Submerged Flight Conveyor i Domestic WastewaterSanitary POTW
II
Evap.1.7 MGD
Lake
Norman Flue Gas Desulfurization(FGO) ` Water Supply ( Wastewater
Groundwater Outfall002 Lake
Wet Scrubber Wastewater from Lake Norman Extraction Wells Treatment —j
} _ /� System 3.0 MGD Norman
FGD Wastewater Treatment SystemEl
Internal Dudall006
(Phys/Chem&Rio)
0.8 MG0
> Heat Exchanger Cooling Water a 1.15 MGD (
Storm Water J
IAir Preheater Water, Precipitator&Boiler Wash Waters,
Misc.Cleaning Wastewaters
> Fly Ash Silo Sump
Ash Basin
( Dewatering
I > Misc. Equipment Cooling 0.53 MGD 0.04
-> Raw Water MGD
1
3 Treatment i Stormwater /
4 V VL
› Boiler and Turbine I Yard Drain Sumps I Outfalls 002A and 002E
Room Sumps INGD (2 flows to 1A) __- _ _ Yard Sump Overflows i Lake
(no flow expected) : Norman
Retention Basin Overflow
>j Coal Pile Alternate -_-.- 2.43 (no flow expected)
i Route MGD
`I V
Stormwater I > Holding Basin 0.5
I ___ MGD > Retention Basin
Outfall 005 Lake
5 MGD Norman
Landfill Leachate
0.04 MGD