NC0083909_Fact Sheet (Draft)_20250909

NC DEQ / DWR / NPDES
EXPEDITED FACT SHEET - NPDES PERMIT RENEWAL
NPDES Permit NCO083909
Sara Bassett / Compliance & Expedited Permitting Unit / sara.bassett(crdeq.nc.gov / July 2025
FACILITY INFORMATION
Facility Name
Rodanthe/Waves/Salvo RO WTP
Applicant Name
Dare County
Applicant Contact Email
Patrick Irwin, Public Utilities Director
[e-mail]
Applicant Mailing Address
600 Mustian St Kill Devil Hills NC 27948
Facility Physical Address
23697 Hwy 12 Rodanthe NC 27689
Permitted Flow (MGD)
Limited: 0.211 MGD
Type of Waste
RO reject
Facility Class
PCNC
County
Dare
Permit Status
Renewal
Regional Office
WaRO
STREAM CHARACTERISTICS
Receiving Stream
Blackmar Gut
Stream
Classification
SA;HQW
Stream Segment
30-22-13
Outfall
Coordinates
35035'44.2"N
75028'13.1"W
7Q 10 (cfs) g,u, me,/wi ter
tidal
Drainage basin
Tar -Pamlico
30Q2 (cfs)
tidal
Subbasin
03-01-55
Average Flow (cfs)
tidal
HUC
030102051505
IWC (%) sttminer/winter
100
303(d) List
Yes; Shellfish Growing
Area -Prohibited (Fecal, SH,
SA)
FACILITY SUMMARY
Facility will continue to operate a Reverse Osmosis water treatment plant with a discharge of
reverse osmosis concentrate reject water. This water treatment plant currently has a design
potable flowrate of 1.25 MGD. This facility is located at 23697 Highway 12 in Rodanthe in
Dare County. Water and wastewater treatment consists of -
Replaceable cartridge filters
• Inhibitor addition system
• Reverse osmosis unit
• Submerged discharge
APPLICATION NOTES
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NCO083909 - Page 1 of 6

• In the 2017 permit renewal, a Discharge Alternatives Evaluation special condition was
added to the permit due to WET testing failures. Dare County submitted this DAE with
their application in 2022, however, the only implemented action was to decrease the anti -
sealant usage as this was suspected to be contributing to the failures. As the facility is still
failing tests, the DAE condition will remain in the permit. The permittee is encouraged to
further consider the following option as identified in the 2022 DAE:
o "Increase sample dilution based on dilution modeling or dilution/mixing of
effluent prior to Discharge"
• RO plant processes ground water from three brackish wells and has a 75% recovery rate.
• This facility was original permitted in 1994 and has not been expanded nor changed its
primary operation since receiving its original NPDES permit. In 1994 the HQW class did
not exist and was not applicable until after 1995. As such, stream classification postdates
discharge therefore the HQW classification does not apply to RPA (per last issued permit
in 2017).
• Application noted the following additives:
o Antiscalant- Vitec 5100
o Cleaning chemical- Avista RoClean P111 and P903.
RENEWAL SUMMARY
This renewal contains the following changes:
• The Supplement to Permit Cover Sheet has been updated to include chemical additives
noted by the permittee.
• A monthly average flow limit has been added to Section A. (1.) based on the facility's
WET compliance history and receiving waters characteristics.
• Nitrogen monitoring in Section A. (1.) has been updated as the Division has determined
that the components of TN (Total Kjeldahl Nitrogen and Nitrate plus Nitrite Nitrogen)
should also be reported in eDMR. The sample type for Total Nitrogen has been changed
to calculated.
• Analysis of eDMR for Total Arsenic did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria (no detections). The limits have been
removed from Section A. (1.). Monitoring will be maintained.
• Analysis of eDMR for Total Nickel did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria. The limits have been removed from
Section A. (1.). Monitoring will be maintained.
• Analysis of eDMR for Total Zinc did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria. The limits have been removed from
Section A. (1.). Monitoring will be maintained.
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NC0083909 - Page 2 of 6

• Analysis of eDMR for Total Copper demonstrated reasonable potential to exceed
applicable NC water Quality Standards/criteria. Limits have been added to Section A. (1.)
and the monitoring frequency has been increased to monthly. A compliance schedule has
been added as Section A. (4.).
• Section A. (2.) regarding Whole Effluent Toxicity (WET) Testing has been updated to
the most current version.
• Section A. (3.) regarding Discharge Alternatives Evaluation is included in this renewal as
no sufficient alternative was identified in the last evaluation and WET failures are still
occurring.
• Section A. (4.) has been added as a compliance schedule for the new Total Copper limits.
• Section A. (5.) has been updated regarding electronic submission of effluent data. Federal
regulations require electronic submittal of all discharge monitoring reports (DMRs).
eDMR SUMMARY
11 /2020-5/2025
The table below summarizes eDMR from 11/2020 through 5/2025. DMR values reported as
"Less Than <" were included in the data analysis at 1/2 the less than value. Minimum values
shown may also be 1/2 reported value.
Effluent:
Parameter _
00070 - Turbidity
00094 - Conductivity
00300 - Oxygen, Dissolved (DO)
00400 - pH
00480 - Salinity
01002 - Arsenic, Total (as As)
01067 - Nickel, Total (as Ni)
01092 - Zinc, Total (as Zn)
50050 - Flow, in conduit or thru treatment
plant
70295 - Solids, Total Dissolved
C0610 - Nitrogen, Ammonia Total (as N) -
Concentration
01042 - Copper, Total (as Cu)
01147 - Selenium, Total (as Se)
CO600 - Nitrogen, Total - Concentration
CO665 - Phosphorus, Total (as P) -
Concentration
Upstream:
Maxj Min Average Count UoM
5.5
0.2
1.4
54
ntu
13500
6380
10552.8
54
umhos/cm
1.82
0.89
1.3
54
mg/l
8.37
8.05
8.2
108
su
7.82
3.48
6.0
54
ppth
5
0.0005
2.0
54
ug/l
7
0.5
1.8
54
ug/l
51
5
20.9
54
ug/l
0.236
0.0013
0.1
1628
MGD
10200
3400
6563.1
54
mg/1
17.6
1.4
8.5
54
mg/l
20.5
0.5
5.6
18
ug/l
12.3
0.5
4.2
18
ug/l
15.3
7.1
10.7
18
mg/l
1.01
0.19
0.7
18
mg/l
Parameter Max Min Average Count UoM
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NC0083909 - Page 3 of 6

00094 - Conductivity
00300 - Oxygen,
Dissolved (DO)
00400 - pH
00480 - Salinity
Downstream:
Parameter
00094 - Conductivity
00300 - Oxygen, Dissolved
(DO)
00400 - pH
00480 - Salinity
COMPLIANCE HISTORY
43300
12800
33118.52
54
umhos/cm
12.76
3.36
8.05
54
mg/1
8.63
7.49
7.95
54
su
27.8
7.3
20.86
54
ppth
Max
Min
Average
Count
UoM
41900
10600
32116.67
54
umhos/cm
12.66
2.6
8.01
54
mg/1
8.51
7.59
7.95
54
su
26.9
6.01
20.10
54
ppth
WET — facility has failed 4/12 most recent tests including the last two (July 2022-April 2025)
No other significant compliance concerns
NCG59 GENERAL PERMIT ELIGIBILITY (FOR CONVENTIONAL & GREENSAND
ONLY)
This facility utilizes Reverse Osmosis technology and is not conventional nor greensands.
• Conclusion: Ineligible for NCG59 General Permit.
FLOW
The Division has begun reviewing the need for effluent flow limits in all water treatment plant
NPDES permit renewals. Flow limits aim to avoid instances of abnormally increased volumes of
discharge. Additionally, WET testing and effluent parameter limits are calculated using an
assumed flow. If the discharged flow is significantly higher than the value used for this
calculation, it is possible that water quality standards may be exceeded or that WET tests will be
inaccurate.
Because this facility:
• Discharges to tidal, unmodeled receiving waters designated as HQW,
• Has a history of failed WET tests,
• And has Total Copper limits,
A flow limit will be applied to this outfall. This limit has been calculated from the past three
years of flow eDMR (May 2022-May 2025) plus a 15% reserve capacity.
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NCO083909 - Page 4 of 6

0.183 MGD X 115% 0.211 MGD
WHOLE EFFLUENT TOXICITY (WET) TESTING
WET testing will be maintained as:
"The permittee shall conduct acute toxicity tests on a quarterly basis using protocols defined in
the North Carolina Procedure Document entitled "Pass/Fail Methodology for Determining Acute
Toxicity In A Single Effluent Concentration" (Revised -December 2010, or subsequent versions).
The monitoring shall be performed as a Mysid Shrimp (Mysidopsis Bahia) 24-hour test. The
effluent concentration at which there may be at no time significant acute mortality is 90%
(defined as treatment two in the procedure document). The tests will be performed during the
months of January, April, July, and October."
The Discharge Alternatives Analysis condition will remain to address failures.
REASONABLE POTENTIAL ANALYSIS (RPA)
The need for toxicant limits is based upon a demonstration of reasonable potential to exceed
water quality standards, a statistical evaluation that is conducted during every permit renewal
utilizing the most recent effluent data for each outfall. The RPA is conducted in accordance with
40 CFR 122.44 (d) (i). The NC RPA procedure utilizes the following: 1) 95% Confidence
Level/95% Probability; 2) assumption of zero background; 3) use of/2 detection limit for "less
than" values; and 4) streamflows used for dilution consideration based on 15A NCAC 2B.0206.
Effective April 6, 2016, NC began implementation of dissolved metals criteria in the RPA
process in accordance with guidance titled NPDES Implementation of Instream Dissolved Metals
Standards, dated June 10, 2016. A copy of this guidance document and a spreadsheet of the RPA
results is attached.
A reasonable potential analysis (RPA) was performed for a discharger going to saltwater. A flow
(Qw) of 0.211 MGD was used based on the maximum, monthly average flow between May 2022
and May 2025 plus a 15% reserve capacity. Data was evaluated between 12/2020 and 5/2025.
RPA was not designated as HQW (see notes section above).
Based on this analysis, the following permitting actions are proposed for this permit:
• Total Arsenic — This parameter did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria (no detections). The limits have been
removed. Monitoring will be maintained as this is a parameter of concern for RO
facilities.
• Total Nickel - This parameter did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria. The limits have been removed.
Monitoring will be maintained.
• Total Selenium - This parameter did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria. Monitoring will be maintained as there
were detections.
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NCO083909 - Page 5 of 6

• Total Zinc - This parameter did not demonstrate reasonable potential to exceed
applicable NC water Quality Standards/criteria. The limits have been removed.
Monitoring will be maintained.
• Total Copper — This parameter demonstrated reasonable potential to exceed applicable
NC water Quality Standards/criteria. Limits have been added and the monitoring
frequency has been increased to monthly. A compliance schedule has been added.
COMMENTS ON DRAFT PERMIT
Comments Requested from:
Washington Regional Office
Aquatic Toxicology Branch
Operator Certification Program
US Fish and Wildlife Service
NC Wildlife Resources Commission
Fact Sheet for Permit Renewal
July 2025 - NPDES Permit NCO083909 - Page 6 of 6