NC0028037_Fact Sheet_20240723

FACT SHEET FOR PERMIT RENEWAL
City of Lexington NCO028037 - Lexington WTPs #1 & #2
Division of Water Resources (DWR) - Basic Information for Permit Renewal
Permit Writer / Date:
Joe R. Corporon, P.G. / 23Jul2024
Permittee / Permit Number:
City of Lexington / NCO028037
Facility Name / System WPCS Class:
Lexington WTP #1 & #2 / Class PC-1
Mailing Address:
28West Center Street Lexin on 27292
Facility Address:
2979 Old Greensboro Street Extension,
Lexington, Davidson County
Facility Contacts:
Tom Johnson, Water Resources Director
[[e-mail]]
ORC: R.S. Walser rswalser lexin onnc. ov
Basin Name / Subbasin Number:
Yadkin Pee -Dee / 03-07-07
Receiving Stream / Hydrological Unit HUC:
Abbotts Creek / 030401020306
Stream Classification / Stream Segment:
WS-III• CA / 12-119- 4.5
Daily Maximum NH3 limits Needed?
N/A to this effluent
Does permit need TRC limits/language?
Already resent
Does Permittee perform WET Testing?
Yes — Chronic [IWC = 52%
Does permit have Special Conditions?
Yes — High Rock Lake reo ever
Does permit have instream monitoring?
Yes Hardness — Effluent & U stream
Is the stream impaired on 303 d list)?
No not at the point of discharge)
Any obvious compliance concerns?
None
Any permit mods since lastpermit?
None
Expiration dates - Previous / New
30A r2024 / 30A r2029
Comments on Draft Permit?
Yes (See Section 7 below)
Facility Overview -- The City of Lexington WTPs #1 and #2 (Lexington or the Permittee)
operates two (2) Conventional WTPs [flocculation, sedimentation, disinfection, filtration], and
they combine their effluent into one (1) discharge. Lexington confirmed the use of 10 media
filters and 8 drying beds (see previous Fact Sheet). The system produces at a design flowrate for
potable water is 9.3 MGD [currently averaging 3.2 MGD] and their wastewater discharges
intermittently [one day/ week] at a flow rate averaging 0.193 MGD.
For renewal, the Permittee reported a maximum monthly average flow of 1.994 MGD (database
Jan2021 thru Apr2024). DWR used this figure for reasonable potential analyses (RPAs) and to
calculate an instream waste concentration (IWC) of 52%.
Treatment Operation Details -- Lexington draws raw water from downstream of Thom-A-Lex
Reservoir. Raw water is chlorinated, fluorinated, and treated with zinc oxide (corrosion
inhibitor), then recycled intermittently to backwash and rinse filter beds. The resulting
wastewater (sedimentation and backwash) is combined and flows to a splitter box where flow is
directed toward two settling lagoons and drying pads to generate solids.
A sand and gravel filter system underlies each storage pad with its effluent directed to a v-notch
weir to measure flow. The Permittee removes solids from drying beds and transports them
offsite, as needed, to Davidson County Landfill [permit #29-061 for disposal.
To dewater, or to maintain a safe operating levels in the lagoons, Lexington uses a gravity decant
system (telescoping siphon) to intermittently extract wastewater and discharge it to the sanitary
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sewer. Sludge accumulating in the settling lagoons is transferred by a pump to the drying beds. If
needed, the Permittee may add polymer to a lagoon to accelerate sludge thickening. A combined
wasteflow (lagoons and drying beds) is dechlorinated (liquid calcium thiosulfate), before passing
through an effluent flow meter to Outfall 001.
Receiving Stream — Abbotts Creek is currently classified WS-III; CA [stream segment 12-119-
(4.5)]. Abbotts Creek is not impaired for turbidity, therefore Turbidity monitoring is
discontinued for renewal, per current Permitting Guidance. Stream Impairment does exist
downstream for Chlorophyll -a but does not exist at the point of discharge.
Compliance History — [May 2021 to June 2024]
• One (1) NOD for late / missing DMR, Apr2022
• Twelve (12) TRC violations (no action BPJ), all reported below 50 ug/L.
• One (1) TSS violation - Monthly Average exceeded by 5% (no action BPJ).
• Twelve (12) Quarterly WET Tests— All Pass [May 2021 to June 2024].
Reasonable Potential Analyses (RPAs) — RPAs were calculated using a reported maximum
monthly average flow of 1.994 MGD (database Jan2021 through Apr2024). Results:
• Copper — RP = Yes for both acute and chronic; applied limits and monitoring 2/Month.
New limits [21.9 mg/L MA / 28.6 mg/L DM] were compared to the previous Total
Copper database to evaluate future compliance. Only two (2) samples in 41 exceed these
limits in a database with considerable variability (see DMR reports and RPA).
• Zinc - RP = no; [max predictable < 50% allowable]; monitoring discontinued.
Changes / Updates to Previous Permit
• Total Zinc — RP = no; [max predictable < 50% allowable]; monitoring discontinued.
• Total Copper — updated limits from previous 41.45 MA / 47.85 DM to 21.9 MA / 28.6 DM,
monitoring updated from Monthly to 2/Month. [NCAC 2B .0505]
• Turbidi — discontinued; receiving stream not impaired for Turbidity.
• updated Schedule of Compliance for Total Copper limits [see permit section A. (4.)]
• undated footnote for Upstream Total Hardness monitoring [see A. (1.), #5
• undated eDMR footnote [see A. (L), footnote #1 and language in Special Condition in A. (5.)
• updated Facility Location Map
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Eligibility for General Permit NCG590000:
• Lexington uses conventional treatment technology; therefore, they are eligible.
• Lexington has passed last 12 WET tests therefore by TOX standards, they are eligible.
• Renewal adds limits for Total Copper therefore they are not eligible.
• Conclusion: Lexington WTP is not eligible for the NCG59.
PROPOSED SCHEDULE FOR PERMIT ISSUANCE
Draft Permit to Public Notice:
Submitted for final signature [estimated]:
Permit Issue [estimated]:
Effective Date [estimated]:
NPDES DIVISION CONTACT
July 24, 2024
August 16, 2024
August 30, 2024
October 1, 2024
If you have questions about any of the above information, or about the attached permit, please
email Joe R. Corporon, P.G. Doe.corporongdeq.nc.gov].
NAME
23JUL2024
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.
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Table 1. NC Dissolved Metals Water Quality Standards/Aquatic Life Protection
Parameter
Acute FW, µg/l
(Dissolved)
Chronic FW,
µg/1
(Dissolved)
Acute SW, µg/1
(Dissolved)
Chronic SW,
µg/l
(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 1 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 213.0200 (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 2. Dissolved Freshwater Standards for Hardness -Dependent Metals
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](0.041838)) • e^{0.9151 [In hardness]-3.1485)
Cadmium, Acute Trout waters
WER*(1.136672-[ln hardness](0.041838)) eA(0.9151[in hardness]-3.6236)
Cadmium, Chronic
WER* { 1.101672-[/n hardness](0.041838)) e^ (0.7998[in hardness]-4.4451)
Chromium III, Acute
WER*0.316 • e^ {0.8190[ln hardness]+3.72561
Chromium III, Chronic
WER*0.860 • e^{0.8190[ln hardness]+0.6848)
Copper, Acute
WER*0.960 - e^{0.9422[ln hardness]-1.700)
Copper, Chronic
WER*0.960 - eA(0.8545[in hardness]-1.7021
Lead, Acute
WER*{1.46203-[ln hardness](0.145712)) - e^{1.273[in hardness]-1.460)
Lead, Chronic
WER*{1.46203-[ln hardness](0.145712)) • eA(l.273[ln hardness]4.705)
Nickel, Acute
WER*0.998 • eA(0.8460[in hardness]+2.255)
Nickel, Chronic
WER*0.997 • eA10.8460[In hardness]+0.0584)
Silver, Acute
WER*0.85 - e^{1.72[ln hardness]-6.59)
Silver, Chronic
Not applicable
Zinc, Acute
WER*0.978 • eA(0.8473[ln hardness]+0.884)
Zinc, Chronic
WER*0.986 - eA(0.8473[ln hardness]+0.884)
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General Information on the Reasonable Potential Analvsis (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 Permittine Guidance/WOBELs 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 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, 7Q10 (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
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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 (s7Q10, cfs *Avg. Upstream Hardness, mgL)
(Permitted Flow, cfs + s7Q10, cfs)
The Combined Hardness for acute is the same but the calculation uses the 1 Q 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.
EPA default partition coefficients or the "Fraction Dissolved" converts the
value for dissolved metal at laboratory conditions to total recoverable metal
at in -stream ambient conditions. This factor is calculated using the linear
partition coefficients found in The Metals Translator: Guidance for
Calculating a Total Recoverable Permit Limit from a Dissolved Criterion
(EPA 823-B-96-007, June 1996) and the equation:
Cd;ss = 1
Ctotal I + { [Kpo] [ss(]+a)] [10-6] }
Where:
ss = in -stream suspended solids concentration [mg/1], minimum of 10 mg/L
used, and
Kpo and a = constants that express the equilibrium relationship between
dissolved and adsorbed forms of metals. A list of constants used for each
hardness -dependent metal can also be found in the RPA program under a
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 = (s7O10 + Ow) (Cwgs) — WOW) (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)
Cb = background concentration: assume zero for all toxicants except NH3*
(µg/L or mg/L)
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Qw = permitted effluent flow (cfs, match s7Q 10)
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 s7Q10 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:
Parameter
Value
Comments(Data Source
Average Effluent Hardness (mg/L)
25 mg/L
Default value used (no
Total as, CaCO3 or Ca+M
monitoring data available
Average Upstream Hardness (mg/L)
25 mg/L
Default value used (no
Total as, CaCO3 or Ca+M
monitoring data available
7 10 summer cfs
2.90
BIMS
1 10 cfs
2.43
RPA
Maximum Monthly Average
Permitted Flow (MGD)
1.994
(previous 36 months at time of
renewal
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