NC0038377_Modification_20210326

Mayo Steam Plant
DUKE
Energy Progress
i". <Ca: DUKE 10660 Boston Road
Roxboro,NC 27574
ENERGY®
File: Mayo12520-R March 12, 2021
Sergei Chernikov, PhD.
North Carolina Division of Environmental Quality
1617 Mail Service Center
Raleigh, NC 27699-1617 REGE
Subject: Duke Energy Progress, LLC.
Mayo Electric Generating Plant ®E�DWRINPDES
NPDES Permit NC0038377 NC
Permit Modification Request
Dear Dr. Chernikov:
Duke Energy Progress, LLC. is requesting a modification of the Mayo Steam Station's NPDES
permit to make the following changes:
1. Duke Energy requests that landfill leachate from the CCR landfill being constructed to
comply with terms of the settlement agreement in the matter of Superior Court 13 CVS
11032 be identified as a contributing flow to the lined retention basin. Potential leachate
data based on Mayo samples from wells located in the vicinity of the ash pond is included
as Attachment 1, to illustrate the expected leachate characterization. The leachate volume
generated by the new landfill will vary over time but is expected to be approximately
50,000 gallons per day during the initial phases of development and reducing over time.
Only water conditioned ash will be placed in the new CCR landfill.
2. Duke Energy requests an increase in the flow limit from the dewatering system
during dewatering activities and requests establishment of an internal outfall to allow
reporting of the dewatering system flow limit specified in the permit. The flow limit
from the dewatering system currently is 2.0 MGD daily maximum. However, the ash
basin is influenced by a large drainage area during rain events. Duke Energy requests
that the flow limit be increased to 3.0 MGD and expressed as a monthly average
similar to the flow requirement at our Belews Creek facility. Additionally, the permit
currently states that this flow limit is applicable to interstitial water only but does not
provide a mechanism to report that data and demonstrate compliance.
3. Duke requests removal of Outfalls 004, 005, 006c, 006d and 006e from the permit.
Outfalls 004 and 005 have been re-routed and now discharge as a part of Outfall 002.

Permit NC0038377 Modification Request
March 12, 2021
Outfalls 006c, 006d and 006e have been grouted closed and no longer have the potential
to discharge.
4. Duke Energy requests a permit application update to include addition of ammonia
conversion maintenance drainage as a contributing flow to the lined retention basin. The
Mayo Station has two Selective Catalytic Reactors (SCR) for NO), control. An ammonia
stripping column will be added per SCR utilizing aqueous ammonia as the reagent.
Periodic maintenance and or unit shutdown periods, will require the column contents to
be drained. Each column could have a maximum of 740 gallons of water with ammonia
concentration less than 1.0 ppm with the pH being controlled in the system with caustic.
The column contents would be drained to the lined retention basin for treatment
discharging at internal Outfall 002A and then ultimately to Outfall 002 to Mayo
Reservoir. A process narrative description is attached and hereby included in our
application on file.
5. Duke Energy requests the sampling frequency for the domestic package plant (internal
Outfall 011) be updated to quarterly to better align with system design and the permit
requirements at the Roxboro Steam Plant. The Mayo package plant discharges an
estimated 12,000 gallons per day through the internal outfall and has demonstrated
compliance through weekly sampling since the permit's current effective date.
6. Duke Energy requests the addition of industrial stormwater from the new CCR landfill to
be added as a contributing flow to the lined retention basin before discharging to Mayo
Reservoir via Outfall 002.
A check in the amount of$1,030 is included for the permit modification review fee.
Should you have any questions please contact Ms. Lori Tollie at Lori.Tollie@duke-
enerQy.com or at (336)-854-4916.
Sincerely,
jk1C-0(14 -
Tom Copolo
General Manager—Mayo Steam Station