NC0025526_Fact Sheet_20230517

FACT SHEET
RENEWAL -NPDES PERMIT
NCO025526 — WPCS Grade WW-I [wetlands]
Table 1 -Basic Information for Permit Renewal
Permit Writer/Date Joe R. Corporon,P.G. 17Ma 2023
Permit Number NCO025526
Facility Name Town of Walnut Cove WWTP
Mayor: Nellie Brown
Cory Willoughby, (ORC) 336-591-4809
Facility Contacts [[e-mail]]
Mark Bowman,Public Works Director
[[e-mail]]
[backup ORC, CERT. WW-2
Basin Name/Sub-basin number Roanoke River/ 03-02-01
Receiving Stream Town Fork Creek[index 22-25]
Stream Classification in Permit C
Does permit need Daily Max NH3 limits? No summer limits exist
Does permit need TRC limits/language? Existing
Does permit have toxicity testing? No
Does permit have Special Conditions? No
Does permit have instream monitoring? Yes: Temperature&D.O.
Is the stream impaired on 303 d list)? No
Any obvious compliance concerns? Yes, fecal coliform weekly geometric mean exceeded
on multiple occasions during last permit cycle
Any permit mods since lastpermit? No
Current expiration date February 28,2022
New expiration date February 28,2027
Comments received on Draft Permit?
Facility-This is a Lagoon/Wetlands System. Walnut Cove WWTP treats 100%domestic wastewater
(flow 0.500 MGD) servicing a local population of 1,532 [2021].
Town WWTP History In 1994, the Town of Walnut Cove evaluated conventional WWTP
technology considering both mechanical and biological processes, but costs escalated above $2
million. So as alternative, they built a combined wetlands system for approximately $1 million
and began operating in 1996.
Naturally clean
MW 10Yeaa.IM19a,uaWeppoaa.aNlwe C ,DOemwdV—aes—hft
aovM W be M k198 mead a es erro Mae'E Mw rt wpiks-.
� e
�© ro m _
v-- — W✓rt..aar
- -(rj' - Votre ppH
✓J rw.n
A wetland systemr��
,onM vnhe^�
-
swqe.wow»w,: r
.an u..a.auaeo.aa®..
e„sece...warsoanseua.- ,
aoa.amr.nmac+wnae.w. .. .n gamnwcw
eea+amc.m.m.wa+aw... ,c .-n.uanw nnrq
con caer.
soaa sa.awoen co..
Page 1 of 6

Treatment System Narrative: Walnut Cove's sewerage arrives at the treatment plant lift station and is
pumped into a primary lagoon. Treatment then continues to a secondary lagoon and beyond to a
duckweed raceway.After the duckweed treatment,treated waste flows into a settling pond, and from
there is separated into two final basins. Each final basin then directs it's flow into separate wetlands.
After the wetland systems,treated wastewaters are again reunited for the last time in a chlorine-contact
chamber where chlorine gas is applied. Treatment then concludes with dechlorination by sulfur dioxide
prior to final discharge at Town Fork Creek.A map of the flow is provided.
Receiving Stream—discharge to Town Fork Creek: Streamflow: 30Q2=21cfs; 7Q10w=20cfs; 7Q10s=
8.9cfs; 7Q10 Ave= 130cfs [BIMS]; a tributary to the Roanoke River(IWC=8.01%).
Stream Evaluation(Staff Report 201 j—Early modeling indicates that the receiving stream may likely
accommodate Dissolved Oxygen(DO) loading at permitted flow levels. Town Fork Creek shows a
predictably seasonal variation in DO. Downstream(300 feet)of the WWTP discharge,DO appears to
retard somewhat,but data rarely show instream values below 5.0 mg/L. The DO sag is lowest about 3
miles downstream,but it recovers prior to the creek's confluence with the Dan River.
Compliance History/Data Review-DMRs from 2010 and 2011 indicated that summer effluent DO
levels may measure low, at times<3.0 mg/L. Low levels may likely result from the performance
efficiency of the lagoons and constructed wetlands. Effluent is not aerated prior to discharge.
In lieu of WET testing,the Permittee monitors Ammonia Nitrogen with Summer limits gnly(Winter
=>Monitor and Report)—no permit changes recommended for renewal.
BIMS data for Effluent parameters of concern (Jan202O-Mar2O23)reported values generally within
permit limits except for periods of sporadic violations of Fecal Coliform limits [four(4) in 2021 and four
(4)in 2022] proceeding to NOVs and assessments. In addition,DWR cited the Permittee for TSS
Monthly Average limit violations. Other minor violations(not assessed) include monitoring frequency
(Late/Missing DMRs) -no permit changes recommended for renewal.
Summary of Changes and Rationale for Renewal
1. Instream Monitoring-DO and Temperature--The 2016 permit requires seasonal instream
monitoring of Town Fork Creek(upstream and downstream). It further specifies Winter
monitoring at a Measurement Frequency of 3/Week and Summer monitoring Weekly. While it is
unlikely that this facility would discharge a heated effluent,instream Temperature is required to
evaluate DO—no changes recommended for renewal.
2. Effluent--The 2016 permit does not require effluent Dissolved Oxygen(DO)monitoring.
Renewal has therefore added effluent DO and Temperature (with seasonal Measurement
Frequencies) for comparison to receiving-stream values. While it is unlikely that this facility
would discharge a heated effluent, concurrent Temperature monitoring is required to evaluate DO.
3. Ammonia Nitrogen(NH3 as N)—see WLA,May2023; limits Summer= 10.0/30.0 mg/L
(MA/DM); Winter: Monitor and Report only [see A. (1.)].No changes recommended for renewal.
PROPOSED SCHEDULE OF ISSUANCE
Draft Permit to Public Notice: 30May2O23 (est.)
Permit Scheduled to Issue: Mun2023 (est.)
Effective Date 01 Ju12023 (est.)
Page 2 of 6

NPDES DIVISION CONTACT
If you have questions about any of the above information, or on the attached permit, please email
Joe R. Corporon, P.G. [e-mail]].
NAME: DATE: 17MAY2023
J
NPDES Implementation of Instream Dissolved Metals Standards — Freshwater
Standards
The NC 2007-2015 Water Quality Standard(WQS)Triennial Review was approved by the NC
Environmental Management Commission(EMC)on November 13,2014. The US EPA subsequently
approved the WQS revisions on April 6, 2016,with some exceptions. Therefore,metal limits in draft
permits out to public notice after April 6, 2016 must be calculated to protect the new standards -as
approved.
Table 2.NC Dissolved Metals Water Quality Standards/A uatic Life Protection
Parameter Acute FW, µg/l Chronic FW, µg/l Acute SW, µg/l Chronic SW, µg/1
(Dissolved) (Dissolved) (Dissolved) (Dissolved)
Arsenic 340 150 69 36
Beryllium 65 6.5 --- ---
Cadmium Calculation Calculation 40 8.8
Chromium III Calculation Calculation --- ---
Chromium VI 16 11 1100 50
Copper Calculation Calculation 4.8 3.1
Lead Calculation Calculation 210 8.1
Nickel Calculation Calculation 74 8.2
Silver Calculation 0.06 1.9 0.1
Zinc Calculation Calculation 90 81
Table 2 Notes:
1. FW=Freshwater, SW= Saltwater
2. Calculation=Hardness dependent standard
3. Only the aquatic life standards listed above are expressed in dissolved form. Aquatic life
standards for Mercury and selenium are still expressed as Total Recoverable Metals due to
bioaccumulative concerns(as are all human health standards for all metals). It is still necessary
to evaluate total recoverable aquatic life and human health standards listed in 15A NCAC
2B.0200 (e.g., arsenic at 10 µg/l for human health protection; cyanide at 5 µg/L and fluoride at
1.8 mg/L for aquatic life protection).
Table 3.Dissolved Freshwater Standards for Hardness-Dependent Metals
Page 3 of 6

The Water Effects Ratio(WER) is equal to one unless determined otherwise under 15A NCAC
02B .0211 Subparagraph(11)(d)
Metal NC Dissolved Standard, /1
Cadmium,Acute WER*{1.136672-[ln hardness](0.04183 8)) e^{0.9151 [ln hardness]-
3.1485}
Cadmium, Acute Trout WER*{1.136672-[ln hardness](0.041838)} e^{0.9151[ln hardness]-
waters 3.62361
Cadmium,Chronic WER*{1.101672-[ln hardness](0.041838)} e^{0.7998[ln hardness]-
4.4451{
Chromium III,Acute WER*0.316 e^{0.8190[ln hardness]+3.7256{
Chromium III, Chronic WER*0.860 e^{0.8190[ln hardness]+0.6848{
Copper,Acute WER*0.960 e^{0.9422[ln hardness]-1.7001
Copper, Chronic WER*0.960 e^{0.8545[ln hardness]-1.702{
Lead,Acute WER*{1.46203-[ln hardness](0.145712)} •e^{1.273[ln hardness]-1.460}
Lead, Chronic WER*{1.46203-[lnhardness](0.145712){ •e^{1.273[ln hardness]-4.7051
Nickel,Acute WER*0.998 e^{0.8460[ln hardness]+2.255{
Nickel, Chronic WER*0.997 e^{0.8460[ln hardness]+0.0584{
Silver, Acute WER*0.85 • e^{1.72[ln hardness]-6.59{
Silver,Chronic Not applicable
Zinc,Acute WER*0.978 e^{0.8473[ln hardness]+0.884}
Zinc, Chronic WER*0.986 e^{0.8473[ln hardness]+0.884}
General Information on the Reasonable Potential Analysis (RPA)
The RPA process itself did not change as the result of the new metals standards. However, application of
the dissolved and hardness-dependent standards requires additional consideration in order to establish the
numeric standard for each metal of concern of each individual discharge.
The hardness-based standards require some knowledge of the effluent and instream(upstream)hardness
and so must be calculated case-by-case for each discharge.
Metals limits must be expressed as `total recoverable' metals in accordance with 40 CFR 122.45(c). The
discharge-specific standards must be converted to the equivalent total values for use in the RPA
calculations. We will generally rely on default translator values developed for each metal(more on that
below),but it is also possible to consider case-specific translators developed in accordance with
established methodology.
RPA Permitting Guidance/WQBELs for Hardness-Dependent Metals-Freshwater
The RPA is designed to predict the maximum likely effluent concentrations for each metal of concern,
based on recent effluent data, and calculate the allowable effluent concentrations,based on applicable
standards and the critical low-flow values for the receiving stream.
If the maximum predicted value is greater than the maximum allowed value (chronic or acute),the
discharge has reasonable potential to exceed the standard,which warrants a permit limit in most cases. If
Page 4 of 6

monitoring for a particular pollutant indicates that the pollutant is not present(i.e. consistently below
detection level),then the Division may remove the monitoring requirement in the reissued permit.
1. To perform a RPA on the Freshwater hardness-dependent metals the Permit Writer compiles the
following information:
• Critical low flow of the receiving stream, 7Q 10(the spreadsheet automatically calculates
the 1 Q 10 using the formula 1 Q 10=0.843 (s7Q 10, cfs)0.993
• Effluent hardness and upstream hardness, site-specific data is preferred
• Permitted flow
• Receiving stream classification
2. In order to establish the numeric standard for each hardness-dependent metal of concern and for
each individual discharge,the Permit Writer must first determine what effluent and instream
(upstream)hardness values to use in the equations.
The permit writer reviews DMR's, Effluent Pollutant Scans,and Toxicity Test results for any
hardness data and contacts the Permittee to see if any additional data is available for instream
hardness values,upstream of the discharge.
If no hardness data is available,the permit writer may choose to do an initial evaluation using a
default hardness of 25 mg/L(CaCO3 or(Ca+Mg)). Minimum and maximum limits on the
hardness value used for water quality calculations are 25 mg/L and 400 mg/L,respectively.
If the use of a default hardness value results in a hardness-dependent metal showing reasonable
potential,the permit writer contacts the Permittee and requests 5 site-specific effluent and
upstream hardness samples over a period of one week. The RPA is rerun using the new data.
The overall hardness value used in the water quality calculations is calculated as follows:
Combined Hardness(chronic)
_(Permitted Flow, cfs *Avg. Effluent Hardness,mg/L)x(s7Q 10, cfs *Avg.Upstream Hardness,mg/L)
(Permitted Flow, cfs+s7Q10,cfs)
The Combined Hardness for acute is the same but the calculation uses the IQ 10 flow.
3. The permit writer converts the numeric standard for each metal of concern to a total recoverable
metal,using the EPA Default Partition Coefficients (DPCs) or site-specific translators,if any
have been developed using federally approved methodology.
4. The numeric standard for each metal of concern is divided by the default partition coefficient(or
site-specific translator)to obtain a Total Recoverable Metal at ambient conditions.
In some cases,where an EPA default partition coefficient translator does not exist(ie. silver),the
dissolved numeric standard for each metal of concern is divided by the EPA conversion factor to
obtain a Total Recoverable Metal at ambient conditions. This method presumes that the metal is
dissolved to the same extent as it was during EPA's criteria development for metals. For more
information on conversion factors see the June, 1996 EPA Translator Guidance Document.
5. The RPA spreadsheet uses a mass balance equation to determine the total allowable concentration
(permit limits) for each pollutant using the following equation:
Ca=(s7Q 10+Qw) (Cwgs)—(s7Q 10) (Cb)
Qw
Where: Ca=allowable effluent concentration(µg/L or mg/L)
Cwqs=NC Water Quality Standard or federal criteria(µg/L or mg/L)
Page 5 of 6

Cb=background concentration: assume zero for all toxicants except NH3* (µg/L or mg/L)
Qw=permitted effluent flow(cfs,match s7Q10)
s7Q 10=summer low flow used to protect aquatic life from chronic toxicity and human
health through the consumption of water, fish,and shellfish from noncarcinogens(cfs)
* Discussions are on-going with EPA on how best to address background concentrations
Flows other than s7Q 10 may be incorporated as applicable:
IQ 10=used in the equation to protect aquatic life from acute toxicity
QA=used in the equation to protect human health through the consumption of water,
fish, and shellfish from carcinogens
30Q2=used in the equation to protect aesthetic quality
6. The permit writer enters the most recent 2-3 years of effluent data for each pollutant of concern.
Data entered must have been taken within four and one-half years prior to the date of the permit
application(40 CFR 122.21). The RPA spreadsheet estimates the 95th percentile upper
concentration of each pollutant. The Predicted Max concentrations are compared to the Total
allowable concentrations to determine if a permit limit is necessary. If the predicted max exceeds
the acute or chronic Total allowable concentrations,the discharge is considered to show
reasonable potential to violate the water quality standard, and a permit limit(Total allowable
concentration)is included in the permit in accordance with the U.S. EPA Technical Support
Document for Water Quality-Based Toxics Control published in 1991.
7. When appropriate,permit writers develop facility specific compliance schedules in accordance
with the EPA Headquarters Memo dated May 10,2007 from James Hanlon to Alexis Strauss on
40 CFR 122.47 Compliance Schedule Requirements.
8. The Total Chromium NC WQS was removed and replaced with trivalent chromium and
hexavalent chromium Water Quality Standards. As a cost savings measure,total chromium data
results may be used as a conservative surrogate in cases where there are no analytical results
based on chromium III or VI. In these cases,the projected maximum concentration(95th%) for
total chromium will be compared against water quality standards for chromium III and chromium
VI.
9. Effluent hardness sampling and instream hardness sampling,upstream of the discharge,are
inserted into all permits with facilities monitoring for hardness-dependent metals to ensure the
accuracy of the permit limits and to build a more robust hardness dataset.
10. Hardness and flow values used in the Reasonable Potential Analysis for this permit included:
Table 4
Parameter Value Comments Data Source
Average Effluent Hardness(mg/L) N/A No metals monitored
[Total as, CaCO3 or(Ca+Mg)]
Average Upstream Hardness(mg/L) N/A
[Total as, CaCO3 or(Ca+Mg)]
7Q10 summer(cfs) 8.9 N/A
1 Q 10(cfs) ,. N/A
Permitted Flow(MGD) 0.500 N/A
Page 6 of 6