NC0087840_Fact Sheet_20240216 (2)

Fact Sheet Renewal – REDRAFT 20Feb2024
NPDES NC0087840
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DEQ / DWR / NPDES
EXPIDITED FACT SHEET FOR PERMIT RENEWAL
NPDES PERMIT NC0087840
Facility Information
Applicant/Facility
Name: Town of Middlesex - Middlesex Well No.4 WTP
Applicant Address: Town of Middlesex; P.O. Box 69; Middlesex, NC 27557
Facility Address: 10921 East Finch Street; Middlesex, NC 27557
Permitted Flow: Maximum monthly average 0.0034 MGD (not limited)
Type of Waste: Treated backwash wastewater from greensand filters
Facility/Permit Status: WPCS Class PC-1 /Active; Renewal
County: Nash County
Miscellaneous
Receiving Stream: UT to Turkey Creek Stream Class: C; NSW
Subbasin: 03-04-07 Stream Index No: 27-86-3-(1)
Drainage Area (mi2): ~ HUC: 030202030104
Summer 7Q10 (cfs) ~ 303(d) Listed? No
Winter 7Q10 (cfs): ~ Regional Office: Raleigh
30Q2 (cfs): ~ USGS Quad: Middlesex, NC
Average Flow (cfs): ~ Permit Writer: Joe R. Corporon, P.G.
IWC (%): 100% Date: Re-DRAFT 20Feb2024

BACKGROUND
The Town of Middlesex operates a designed-rate 0.133 MGD potable-water treatment plant
(WTP) to serve its surrounding community. Currently operated by Envirolink, Inc., the WTP
applies greensand filtration technology generating a filer-backwash averaging 0.005 MGD [for
~15 minutes, one day per week]. Backwash wastes discharge to an unnamed tributary (UT) to
Turkey Creek, a waterbody currently classified C; NSW within the Neuse River Basin.
It is however noteworthy that the effluent travels via culverts, and a system of unlined ditches
(providing subsurface infiltration) such that the effluent seldom, if ever, reaches the unnamed
tributary (RRO Inspection Staff Report, 2013).

CURRENT TREATMENT PROCESSES
1. Well water is chlorinated then introduced to two greensand filter units.
2. filtered water continues through air stripper treatment then
3. pumped into the potable water distribution system
4. operator manually backwashes/rinses each sand filter as needed [typ. once (1) per week],
5. process backwash/rinsate passes through a meter, sand filter beds,
6. tablet dechlorinator
7. discharged into a UT to Turkey Creek.
Permittee determines when to remove solids based on their accumulation in drying beds; solids
removed and disposed by licensed contractor, per state regulations.

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PERMITTING HISTORY
2008 – Renewal permit issued with no changes to the previous permit [Greensand Filter 2004
WTP Strategy limiting Flow, TSS, TRC, and pH].
2013 – Applied 2009 WTP Strategy for Greensand filter technology and BPJ:
• flow limit removed; monitoring changed to 2/month; footnote added;
• corrected outfall location; IWC corrected to 100%;
• fluoride and zinc monitoring removed (neither used by the [Permittee);
• iron and manganese - reduced monitoring to Quarterly
• added Turbidity monitoring, 2/month (EPA requirement);
• per Neuse River 2013 Nutrient Guidelines, semi-annual monitoring required
for nutrients as Total Nitrogen (TN) and Total Phosphorus (TP)
• TN requires Total Kjeldahl Nitrogen (TKN) plus Nitrate and Nitrite Nitrogen.
2018 – Renewal continued 2009 WTP Strategy for Greensand filter technology
1. Updated site map and narrative for Supplement to Permit Cover Sheet.
2. Discontinued monitoring for Total Iron [no standard/no longer a
parameter of concern (POC)].
3. Continued quarterly monitoring for Total Manganese as required for
greensand, but not limited, per WTP strategy [not discharging to WS waters];
4. Total Hardness [for calculating dissolved-fraction limits] – Because manganese cannot
be limited (limit applies only to WS), and because manganese is the only remaining
metal POC, Total Hardness monitoring (U&E) is not required;
5. Current Neuse River Guidelines classify this facility as non-nitrogen
producing; however, TN/TP semi-annually monitoring remains in the
permit per strategy; no changes recommended.
6. Because TN/TP monitoring is required [class NSW], this facility is not
eligible for WTP General Permit NCG590000.

COMPLIANCE -- Since 2020 BIMS records no violations / no penalty assessments issued
by the DWR.

PERMITTING STRATEGY -- RATIONALE FOR 2023 RENEWAL
The following will be incorporated into the permit.
1. Updated site map and narrative for Supplement to Permit Cover Sheet.
2. Discontinued monitoring for Total Iron [no standard/no longer a
parameter of concern (POC)].
3. Continued quarterly monitoring for Total Manganese as required for
greensand, but not limited, per WTP strategy [no discharge to WS waters].
4. Total Hardness [to evaluate dissolved-fraction limits] – Because manganese
cannot be limited (applies only in WS waters), and because manganese is the only
remaining metal POC, Total Hardness monitoring (U&E) is not required.
5. Current Neuse River Guidelines classify this facility as non-nitrogen
producing; however, TN/TP semi-annually monitoring remains in the
permit per Neuse River strategy; no changes recommended.
6. Because TN/TP monitoring is required [class NSW], this facility is not
eligible for WTP General Permit NCG590000.
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PROPOSED SCHEDULE FOR PERMIT ISSUANCE

Re-Draft to Public Notice: February 20, 2024
[Tentative] Submittal for final signature: March 22, 2024
[Tentative] Permit Issue date: April 26, 2024
[Tentative] Effective date June 1, 2024
Note: permit issuance was delayed due to personnel scheduling and State signatory issues.

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: Re-DRAFT 20Feb2024

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/Aquatic Life Protection
Parameter Acute FW, µg/l
(Dissolved)
Chronic FW, µg/l
(Dissolved)
Acute SW, µg/l
(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

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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
The Water Effects Ratio (WER) is equal to one unless determined otherwise under 15A NCAC
02B .0211 Subparagraph (11)(d)

Metal NC Dissolved Standard, µg/l
Cadmium, Acute WER*{1.136672-[ln hardness](0.041838)} ∙ e^{0.9151 [ln hardness]-
3.1485}
Cadmium, Acute Trout
waters
WER*{1.136672-[ln hardness](0.041838)} ∙ e^{0.9151[ln hardness]-
3.6236}
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]-1.700}
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-[ln hardness](0.145712)} ∙ e^{1.273[ln hardness]-
4.705}
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.

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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
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 1Q10 using the formula 1Q10 = 0.843 (s7Q10, 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 (s7Q10, cfs *Avg. Upstream Hardness, mg/L)
(Permitted Flow, cfs + s7Q10, cfs)

The Combined Hardness for acute is the same but the calculation uses the 1Q10 flow.
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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 = (s7Q10 + Qw) (Cwqs) – (s7Q10) (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)
Qw = permitted effluent flow (cfs, match s7Q10)
s7Q10 = 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:
1Q10 = 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.
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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)
[Total as, CaCO3 or (Ca+Mg)] N/A No metals monitored
Average Upstream Hardness (mg/L)
[Total as, CaCO3 or (Ca+Mg)] N/A
7Q10 summer (cfs) ~ N/A
1Q10 (cfs) ~ N/A
Permitted Flow (MGD) ~ Not limited