NC0038377_Fact Sheet_20240223

DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
RENEWAL
NPDES No. NC0038377, Duke Energy Progress, LLC
Mayo Steam Electric Generating Plant
Facility Information
Applicant/Facility Name:
Duke Energy Progress/Mayo Steam Electric Generating Plant
Applicant Address:
10660 Boston Road, Roxboro, NC 27573
Facility Address:
same
Permitted Flow
Not limited
Type of Waste:
99.8 %Industrial, 0.2%- domestic
Facility/Permit Status:
Existing/Renewal
County:
Person
Miscellaneous
Receiving Stream:
Mayo Reservoir
and Crutchfield
Branch
Regional Office:
RRO
Stream Classification:
WS-V and C
Quad
303 d Listed?:
No
Permit Writer:
Sergei Chernikov, Ph.D.
Subbasin:
030205
Roanoke
Date:
September 18, 2023
Drainage Area mil :
N/A
Summer 7Q 10 cfs
0
30Q2 (cfs):
0
Average Flow cfs :
0
IWC (%):
100%
(assumed, no
modeling info.
available
Primary SIC Code:
SUMMARY
This is a renewal for the Mayo Electric Generating Plant. The facility is a coal-fired electric
generating plant with one unit rated at a maximum dependable capacity of 745 mw.
Water for plant uses is withdrawn from the Mayo Reservoir as required to make up
evaporative losses from the cooling tower, boiler water and drinking water needs. This
facility is subject to EPA effluent guideline limits per 40 CFR 423- Steam Electric Power
Generating Point Source Category. The facility has a closed cycle cooling system (cooling
tower), actual intake flow and design intake flow is less than 125 MGD. The facility has
a dry fly ash handling system, dry bottom ash handling system, and one ash pond.
The facility is located in the Lower Piedmont area of the state, the applicable state
water quality temperature standard is 32oC (89.6 F).
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NPDES PERNHT FACT SHEET Duke Energy Progress - Mayo
NPDES No. NC0038377
In response to North Carolina's Clean Air Initiative (Clean Smokestacks Bill of 2002),
which requires the reduction of SOx and NOx from air emissions, the company installed
Flue Gas Desulfurization (FGD) system. The FGD is essentially a scrubber system to
remove SOx by mixing flue gas with a limestone slurry.
The FGD blowdown generates a flow of approximately 0.254 MGD, with relatively
elevated concentrations of metals and chloride. Duke Energy Progress treats the FGD
blowdown via VCE (vapor compression evaporator) whose purpose is to evaporate the
majority of the wastewater produced from the FGD scrubber system. The VCE became
operational in February, 2015. It produces two waste streams, both are utilized in the
plant processes. The concentrated wastewater is used for moisture conditioning of fly
ash prior to sending to the landfill. The second stream is a clean distillate that is
utilized to partially replace water withdrawal from Mayo Reservoir. The VCE system
eliminates the FGD blowdown stream from Outfall 002, except during severe rain
events.
The ash pond dam has two toe drains that are designed for the stability of the dam.
The average discharge of both drains is approximately 11,000 gpd, the discharge is
routed to the Crutchfield branch. The Crutchfield Branch does not discharge to the
Mayo Reservoir.
The facility built a new Retention Basin to reroute all waste streams that are currently
discharged to the ash basin. This change is necessary to decommission the existing
ash pond and meet the requirements of Coal Ash Management Act. The Retention
Basin has a cell where various vacuumed sediments and solids can be decanted prior
to disposal.
The facility also constructed a new FGD settling basin, the waste from the basin will be
treated by VCE.
The facility completed decanting in 2021 and initiated dewatering May 24, 2021
The facility operates the following outfalls:
• Outfall 001. Cooling Tower System (lat. - 360 31'28" long. - 780 52'S6"). Less
than once per year the cooling towers and circulating water system are
drained by gravity and discharged directly to Mayo Reservoir.
• Outfall 002. Ash Pond Treatment System (lat. - 360 32'03" long. - 780 53'27").
Outfall 002 discharges directly to Mayo Reservoir.
• Internal Outfall 008. Cooling tower blowdown is directly discharged to Mayo
Reservoir via the ash pond treatment system (Outfall 002).
• Outfall 002A. Discharge from the new lined retention basin. Retention basin
accepts wastes from holding cell (vacuumed sediments and solids), coal pile
runoff, stormwater runoff, landfill leachate from CCR landfill, industrial
stormwater from CCR landfill, ammonia conversion maintenance drainage,
cooling tower blowdown, and various low volume wastes such as boiler
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NPDES PERNHT FACT SHEET Duke Energy Progress - Mayo
NPDES No. NC0038377
blowdown, oily waste treatment, wastes/backwash from the water treatment
processes, including Reverse -Osmosis (RO) wastewater, plant area wash
down water, equipment heat exchanger water, groundwater, occasional
piping leakage from limestone slurry and FGD system, chemical metal
cleaning waste, and treated domestic wastewater. The wastewater from this
outfall discharges to Mayo Reservoir via Outfall 002.
Internal Outfall 002B. Yard sump overflows (contains all wastes routed to the
new retention basin). The wastewater from this outfall discharges to Mayo
Reservoir via Outfall 002.
Internal outfall 011. Domestic wastewater plant. The wastewater from this
outfall discharges to Mayo Reservoir via Outfall 002A.
ASH POND DAMS
Seepage through earthen dams is common and is an expected consequence of
impounding water with an earthen embankment. Even the tightest, best -compacted
clays cannot prevent some water from seeping through them. Seepage is not
necessarily an indication that a dam has structural problems, but should be kept in
check through various engineering controls and regularly monitored for changes in
quantity or quality which, over time, may result in dam failure.
REASONABLE POTENTIAL ANALYSIS (RPA)
The Division conducted EPA -recommended analyses to determine the reasonable
potential for toxicants to be discharged at levels exceeding water quality
standards/EPA criteria by this facility. For the purposes of the RPA, the background
concentrations for all parameters were assumed to be below detections level. The RPA
uses 95% probability level and 95% confidence basis in accordance with the EPA
Guidance entitled "Technical Support Document for Water Quality -based Toxics
Control." The RPA included evaluation of dissolved metals' standards, utilizing a
default hardness value of 25 mg/L CaCO3 for hardness -dependent metals. The RPA
spreadsheets are attached to this Fact Sheet.
RPA for Lined Retention Basin (LRB) (Outfall 002A).
The RPA was conducted for LRB, the calculations included: As, Be, Cd, Chlorides,
Total Phenolic Compounds, Cr III, Cr VI, Cu, CN, F, Pb, Hg, Mo, Ni, Se, Ag, Zn,
Sb, 5O4, and Tl (please see attached). The flow of 9.08 MGD (CCR leachate
included) was used for the analysis. The effluent data from BIMS was used for the
RPA. The analysis indicates reasonable potential to violate the surface water
quality standards or EPA criteria for the following parameters: Cd. The appropriate
limits were added to the permit.
The proposed permit requires that EPA methods 200.7 or 200.8 (or the most current
versions) shall be used for analyses of all metals except for total mercury.
CWA SECTION 316(a) TEMPERATURE VARIANCE
This section is not applicable since the facility has a closed cycle cooling system, which
is considered a BAT. Effluent temperature is monitored daily at the Outfall 001, 002,
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NPDES PERNHT FACT SHEET Duke Energy Progress - Mayo
NPDES No. NCO038377
and 002A, and instream temperature is monitored semi-annually to assure compliance
with the state temperature standard.
CWA SECTION 316(b)
The existing closed -cycle system at Mayo is one of the pre -approved compliance
alternatives for impingement in accordance with § 125.94(c) (1). EPA also considered it
as a pre -approved BTA for entrainment, but excluded it from the rule due to the cost
concerns. The Actual Intake Flow and Design Intake Flow for this station is less than
125 MGD.
Based on evaluation of the 316(b) study reports the Department concludes that the
existing Closed -cycle recirculating system represents BTA for meeting the impingement
and entrainment requirements of the Rule.
INSTREAM AND FISH TISSUE MONITORING- OUTFALL 002 (ASH POND)
The permit required quarterly monitoring for total arsenic, total selenium, total
mercury, total chromium, dissolved lead, dissolved cadmium, dissolved copper,
dissolved zinc, total bromide, total hardness (as CaCO3), turbidity, temperature, and
total dissolved solids (TDS). The data indicates that all collected values are either below
detection level or below water quality standards/criteria.
The permit also required annual fish tissue monitoring for arsenic, mercury, and
selenium. The collected data demonstrated that all monitored trace elements are below
State and Federal screening values.
TOXICITY TESTING-OUTFALL 002 (ASH POND)
Current Requirement: Outfall 002 - Acute P/F @ 90% using Pimephalis promelas
Recommended Requirement: Outfall 002 - Acute P/F @ 90% using Pimephalis promelas
This facility has passed all toxicity tests during the previous permit cycle, please see
attached.
For the purposes of the permitting, the long-term average flow was used in conjunction
with the 7Q 10 summer flow to calculate the percent effluent concentrations to be used
for WET.
COMPLIANCE SUMMARY
During the last 5 years, the facility had 6 limits violations: Total Thallium limit (Outfall
002), 4 BOD limits and 1 TSS limit (Internal Outfall 011). All limit violations for Outfall
011 occurred in 2018, please see attached.
The last compliance evaluation inspection conducted on 12/7/2022 concluded that "the
facility is compliant with the NPDES permit conditions".
PERMIT LIMITS DEVELOPMENT
• The Free Available Chlorine limits, Total Chromium Limits, Total Zinc Limits,
and Priority Pollutant Limits (Outfall 001 and Outfall 008) were established in
accordance with the 40 CFR 423.
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NPDES PERNHT FACT SHEET
Duke Energy Progress - Mayo
NPDES No. NC0038377
• The limits for Oil and Grease and Total Suspended Solids (Outfall 002, Outfall
002A, Outfall 002B, and Internal Outfall 011 (TSS only)) were established in
accordance with the 40 CFR 423.
• The pH limits (Outfall 001, Outfall 008, Outfall 002, Outfall 002A, and Outfall
002B in the permit are based on the North Carolina water quality standards (15A
NCAC 213.0200).
• The turbidity limit in the permit (Outfall 002) is based on the North Carolina
water quality standards (15A NCAC 2B .0200).
• The Whole Effluent Toxicity limit (Outfall 002) is based on the requirements of
15A NCAC 213.0500.
• The BOD and Fecal Coliform limits (Outfall 011) were established in accordance
with the 40 CFR 133.
• The Water Quality Based Effluent Limits for Total Arsenic, Total Cadmium, Total
Chlorides, Chromium (III), Chromium (VI), Total Copper, Total Lead, Total
Nickel, Total Zinc, Total Barium, and Total Thallium in the permit (Outfall 002 -
dewatering) are based on the North Carolina water quality standards (15A NCAC
2B .0200) and EPA water quality criteria.
• The Water Quality Based Effluent Limits for Total Arsenic, Total Cadmium, Total
Chlorides, Total Fluoride, Total Barium, and Total Antimony in the permit
(Outfall 002A - lined retention basin) are based on the North Carolina water
quality standards (15A NCAC 2B .0200) and EPA water quality criteria.
PROPOSED CHANGES
1. The description of Outfall 002 flows has been updated since all the process
wastewater has been rerouted to the Lined Retention Basin.
2. The Effluent Page for the decanting phase has been removed from the permit
since the facility moved to the dewatering phase in early October.
3. Internal Outfall 009 has been removed from the permit because all the FGD
wastewater is being evaporated by the VCE.
4. The sampling frequency for the instream monitoring has been reduced from
Monthly to Quarterly based on the review of the instream data.
5. The sampling frequency for the fish tissue monitoring has been reduced from
annual to once every five years based on the review of the fish tissue data.
6. The language for 316(b) condition has been updated to reflect the most recent
decision to recognize the existing intake structure as BTA for meeting
impingement and entrainment requirement of the CWA.
7. The Water Quality Based Effluent Limits for Total Cadmium were added to the
permit (Outfall 002A - lined retention basin) based on the results of the RPA.
The Division cannot grant the facility's request to remove 85% requirement to avoid
violation of the CWA anti -backsliding provision.
The Division cannot grant the facility's request to reduce WET frequency until the
dewatering phase is completed to ensure protection of the receiving stream.
PROPOSED SCHEDULE
Draft Permit to Public Notice: December 5, 2023
Permit Scheduled to Issue: February 9, 2024
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NPDES PERNHT FACT SHEET Duke Energy Progress - Mayo
NPDES No. NC0038377
STATE CONTACT
If you have any questions on any of the above information or on the attached permit,
please contact Sergei Chernikov at [phone] or [e-mail]
CHANGES IN THE FINAL PERMIT
• The Toxicity Test requirements were transferred from Outfall 002A to Outfall 002
to maintain consistency among the power plant permits.
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