NC0055786_Pretreatment_HWA_Letter_20200616

ROY COOPER
Governor
MICHAEL S. REGAN
Secretary
S. DANIEL SMITH
Director
NORTH CAROLINA
Environmental Quality
June 16, 2020
(sent via email: [e-mail])
Eglantina Minerali, Pretreatment Program Coordinator
City of Lexington
28 W. Center Street
Lexington, NC 27292
Subject: Pretreatment Review of Headworks Analysis (HWA)
City of Lexington (NPDES Number: NC0055786)
Davidson County
Dear Ms. Minerali:
The PERCS Unit ofthe Division of Water Resources has reviewed the Headworks Analysis (HWA) submitted
by the City of Lexington. The HWA was initially received by the Division on January 30, 2020, followed by
additional information received on May 15, 2020, and June 12, 2020.
The Division concurs with the HWA calculations for all pollutants of concern, with corrections below.
These approved Maximum Allowable Headworks Loadings (MAHLs), Maximum Allowable Industrial
Loadings (MAILS), and the basis for these values are found on the last page of the attached HWA
spreadsheet which has the corrections marked. The results of the HWA are also listed in the bottom
section of the attached allocation table (AT).
1. HWA Corrections: Please do not hesitate to contact PERCS if you have any questions or concerns
regarding the changes made outlined below, or if the POTW does not agree these modifications. All
corrected spreadsheets were emailed to you for your files, with the changes highlighted. Please
ensure the POTW's paper and electronic HWA, AT, and other supporting documents contain these
corrected documents, especially the allocation table (AT).
a) POTW Average Flow: POTW average flow was corrected based on DMR monthly averages
data pull from the Division's compliance database (see updated removal rate spreadsheet).
b) NPDES Limits:
i. BOD — Used more stringent Summer limit of 5 mg/l.
ii. Ammonia — Used more stringent Summer limit of 1 mg/l.
iii. Total Phosphorus - Used Summer seasonal total of 4,910 Ibs and converted to
concentration -based limit (0.43 mg/1), by dividing 210 days (April through October),
WWTP permitted flow, and 8.34 (conversion factor).
c) Stream Standards: Stream standards for several parameters were updated based on the
dissolved water quality standards, site -specific stream information, and effluent and instream
hardness, submitted by the POTW (see attached calculator spreadsheet).
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d) Removal Rates:
i. Total Suspended Solids (TSS) —Updated TSS removal rate based on DMR effluent data.
Please note that, while recalculating DMR monthly effluent averages using %2
detection level (DL), it was noticed that DL was not consistent throughout the HWA
sampling period. Please ensure that laboratory meets the correct DL consistently as
listed in the Division -approved LTMP for all DMR and LTMP samplings in order to
be able to calculate correct removal rates.
ii. Total Phosphorus (TP) — Recalculated removal rate using DMR monthly average
effluent as the POTW samples weekly effluent TP, required by NPDES permit.
e) WWTP Design Criteria:
i. BOD, TSS — PERCS approves POTW's using of 1.5 multiplier option to currently
available design criteria, since the WWTP has been in full NPDES permit compliance
for the previous two years (2018-2019) for BOD and TSS, per PERCS 2007-2008 design
memorandum.
Ammonia — Calculated NH3 design value, 13.125 mg/I, using TKN value of 21 mg/I and
by applying NH3/TKN ratio of 25/40 (Metcalf and Eddy). PERCS chose to select design
loading as AHL as it appears to be more conservative than the calculated NPDES
loading. This selection did not cause ammonia overallocation. See more discussion
below in item 2.
f) Silver Inhibition Concentration: Silver inhibition concentration in the Inhibition Loading
Calculation table was corrected as this appears to be a typographical error.
g) Uncontrollable Concentrations: Uncontrollable concentrations for several parameters were
updated on the Mass Balance spreadsheet (see attached) and carried over to HWA
spreadsheet. PERCS chose not to use zero for silver uncontrollable concentration as it was
noticed that not all the samples were measured using best available detection label (currently
1 ug/1). POTW may be able to use uncontrollable as zero if all the influent sample data is less
than 1 ug/I.
2. WWTP Design Criteria: WWTP design values for BOD and TSS, provided by the POTW, were calculated
in 2002 by Pease architect & engineers. PERCS was notified that there were no design changes since
then. Please keep PERCS informed regarding any WWTP design changes or plant upgrades in future.
PERCS highly recommends POTW to obtain design values from design consultants for ammonia as it
was calculated by PERCS this time, as well as TP, and use the design numbers in the next HWA
calculation.
3. Overallocation:
a) Influent Loadings and TSS Overallocation — During the review, it was noted that influent BOD
ant TSS concentrations were much higher than the ones from previous HWA period, causing
TSS overallocation on the AT, even though the POTW has not had any BOD or TSS permit
violations during the previous two years. According to POTW, for the last several years some
of the sludge had been recirculated through the influent sampling point, which appears to be
the reason for increased loadings at the influent. Per POTW, sludge recirculation has been
stopped as of end of 2019. PERCS is approving this HWA with the following
recommendations:
I. POTW should not recirculate return sludge at the influent. Influent should be true
influent without mixing with any side stream or return activated sludge, per your LTMP.

II. In order to resolve TSS overallocation, calculate DMR monthly average TSS as well as BOD
influent and effluent from January through December 2020, and overall removal rates
and submit updated HWA by April 1, 2021.
b) Silver Overallocation — Silver appears to be overallocated based on dissolved metals stream
standard. PERCS is approving this HWA with following recommendations:
i. Start all LTMP samplings using PERCS recommended lower DL of 1 ug/l.
ii. Review IUP limits and compare with discharge data to lower silver limits.
4. Detection Levels (DLs): During the review of removal rates, it was noticed that detection levels for
several parameters, such TSS (5 mg/l, 2.5 mg/l, 1mg/1), As (5 ug/l, 3 ug/1), Cr (5 ug/l, 2 ug/1), Se (5 ug/l,
3 ug/1), and Ag (5 ug/l, 1 ug/1) were not consistent throughout the sampling period. Please review
currently recommended lower PQLs (see attached) with your LTMP DLs, follow-up with your contact
lab, and submit updated LTMP by September 30, 2020.
5. Allocation Table: Allocation table (AT) has been updated, per IUP limits. Please ensure to keep the
AT updated as the IUPs get modified or renewed.
6. Next HWA Due: The HWA approved today is primarily based on DMR/LTMP data for the period of
February 2018 through August 2019. Unless conditions at the POTW change significantly and thus
warrant an earlier submittal (see Comprehensive Guide, Section e), the POTW must submit the next
updated HWA due by July 15Y, 2024. Please note that, permittee must submit updated HWA to the
Division within 180 days of the effective date of a new permit, per Part IV, Section DA, of your permit.
Federal and State pretreatment regulations require the local delegated pretreatment program to
effectively control and document the discharge of wastewater from Significant/Categorical Industrial
Users to the POTW. It is the POTW's responsibility to ensure that these objectives are consistently met.
Thank you for your continued cooperation with the Pretreatment Program. If you have any questions or
comments, please contact Monti Hassan at [phone] [email: Monti. [e-mail]].
Sincerely,
DocuSig ned by:
"' 4 H -
866374CA4137482_
for S. Daniel Smith, Director
Division of Water Resources
mh/Lexington. HWA.review.005
attachments:
HWA AT Approved
HWA Removal Rate
Dissolved Metals Calculator
Uncontrollable Mass Balance
PERCS Recommended PQLs
ec:
Laserfiche
Jim Gonsiewski (WSRO)
PERCS Unit