NC0021717_Pretreatment_HWA_Review_20200427

ROY COOPER
Governor
MICHAEL S. REGAN
Secretary
S. DANIEL SMITH
Director
(sent via email: [e-mail])
Amber Garwood, Pretreatment Coordinator
Town of Wilkesboro
203 West Main Street
Wilkesboro, NC 28697
NORTH CAROLINA
Environmental Quality
April 27, 2020
Subject: Pretreatment Review of Headworks Analysis (HWA)
Town of Wilkesboro (NPDES Permit No. NC0021717)
Wilkes County
Dear Ms. Garwood:
The PERCS Unit of the Division of Water Resources has reviewed the Headworks Analysis (HWA) for the Town of
Wilkesboro. The HWA was initially received by the Division on October 12, 2019, followed by more information and
several revisions received on different dates since then.
The Division concurs with the HWA calculations for all pollutants of concern, with the corrections and observations
discussed 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 enclosed HWA and
Allocation Table (AT) spreadsheet.
HWA Calculations: Please free to contact PERCS if you have any questions or concerns about the changes
outlined below. All corrected spreadsheets were emailed to you for your files, with the changes highlighted.
Please ensure to replace or update POTW's HWA AT spreadsheets and supporting documents with these
approved ones.
a. POTW Average Flow: PERCS chose to use POTW average flow of 3.418 MGD based on the total sum of
monthly average flow of influent #1 and influent #2 calculated in the provided Removal Rate
spreadsheet. It appears that calculated DMR average effluent flow for the same period is 4.2 MGD
which is approximately 20% higher than the average influent flow. POTW was contacted regarding the
discrepancy of flow and needs to do research to find out whether there are additional influents that
are not being accounted for, including any inflow/infiltration issue within the collection systems.
Please update PERCS with your findings.
b. 7Q10 Stream Flow: 7Q10 stream flow was corrected to 126.62 MGD, per Pretreatment program info
database (see attached). You can also find this information on NPDES permit fact sheet.
c. WWTP Design TSS: PERCS was not able to use 1.5 times multiplier, per 2007-2008 PERCS memo, for
TSS design criteria, 250 mg/I, since the WWTP has had NPDES permit violations for TSS in February
2019, December 2019, and January 2020, per Division's DMR records. However, POTW should be able
to use 1.5 design multiplier in future HWA, if the WWTP remains in full compliance at least for two
years prior to HWA submission.
d. Average Influent Ammonia: PERCS removed average influent NH3 value from cell G49 as the value is
required only if the POTW has anaerobic digester.
e. TSS Uncontrollable Concentration: PERCS chose to use TSS uncontrollable concentration as WWTP's
average influent concentration calculated in Removal Rate spreadsheet as it appears to be more
representative than the calculated mass balance number (see updated mass balance spreadsheet).
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Land Application Sludge: It appears that the POTW continues to landfill sludge even though it holds active Land
Application Permit (WQ0000092). Please ensure to notify PERCS in future whether the situation changes or the
land application permit does not get renewed.
WWTP Design Criteria: PERCS used WWTP influent design values for BOD, TSS, TKN, and Total Phosphorus from
the submitted design document. Ammonia design concentration, 28 mg/l, was calculated using TKN value 45
mg/I, and NH3/TN ratio of 25/40 (Metcalf and Eddy). Please update PERCS if there are any design influent value
changes.
Allocation Table: It appears that the Allocation Table (AT) was not kept updated with IUP names, dates, and
permit limits. It is important to keep the AT updated on a regular basis to be able to get a snap shot of currently
available MAHLs/MAILs of the WWTP. Please ensure to update AT every time there are IUP renewals or IUP
modifications.
a. Also, it was noticed that throughout the HWA spreadsheet, formulas were inadvertently deleted in
several places. PERCS was able to restore missing formulas. Please use dark blue cells only to enter
data on spreadsheet as the rest of cells have built-in formula.
Mercury Effluent Sampling: It appears that POTW stopped running low level 1631 method for mercury effluent
sampling around August 2018. Per Division -approved LTMP, the POTW is required to sample quarterly mercury
effluents using low level (method 1631).
Available MAHLs: It appears that currently available MAHL for TSS is 5.8 percent, per approved AT. It is
recommended to have usually at least 20 percent reserve as MAHLs for all parameters at any given time. Please
review the industrial discharge data to see if permit limits can be lowered and raise percent MAHL available.
7. Next HWA Due date: The HWA approved today based on DMR/LTMP data for the period of January 2017
through December 2019. Unless conditions at the POTW change significantly and thus warrant an earlier
submittal (see Comprehensive Guide, Section 8), the POTW must submit the followings:
Updated LTMP due by September 30, 2020. Please note that the current Division -approved LTMP was
approved on June 16, 2010, and it needs to be updated. Please review the current pollutants of
Concern (POCs) and DLs/PQLs lists, as some of the detections levels have been recently lowered by the
Division (see attached). Also, submit an updated Flow diagram for the WWTP.
Updated site -specific HWA due by April 1, 2024.
i. Also, we noted that current NPDES permit has been expired as of September 30, 2019. Please note
that within 180 days of the receipt of NPDES permit renewal, submit an updated HWA using site -
specific water quality standards based on dissolved water quality standards, stream and effluent
hardness (see blank Calculator spreadsheet) and NPDES permit limits changes, if any.
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 do not hesitate to contact Monti Hassan at [phone], or email at [e-mail].
alyl°, ?' I,Ga by:
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Director, Division of Water Resources
mh/Wilkesboro.HWA.2019
attachments: Updated HWA AT, Recommended DL/PQL, Calculator spreadsheets
cc: Central Files (LF)
PERCS Unit (e-copy)
Paul DiMatteo (email)