NC0036269_Pretreatment_HWA_Additional_Information_Requested_20240513

North Carolina Department of Environmental Quality Division of Water Resources
512 North Salisbury Street 1611 Mail Service Center Raleigh, North Carolina 27699-1611
919.707.9000

Via Electronic Mail ([e-mail])

Beverly Metcalf
Environmental, Health & Safety Compliance Manager
Water & Sewer Authority of Cabarrus County
6400 Breezy Lane
Concord, NC 28025

Subject: Pretreatment Review of Head Works Analysis
Request for Corrections and/or More Information
Program: Water & Sewer Authority of Cabarrus County (WSACC)
Wastewater Treatment Plant: Rocky River Regional WWTP
NPDES #: NC0036269, Cabarrus County

Dear Beverly Metcalf:

The Municipal Unit of the Division of Water Resources has reviewed the updated Headworks Analysis (HWA) and
the Allocation Table (AT) for WSACC’s Rocky River Regional WWTP (NC0036269). The submission was
received by the Division on 05/31/2022. The Division focused on reviewing the 30 MGD version of WSACC’s
HWA as requested.
The review indicates that corrections and/or more information are required before the HWA and AT will be
adequate. Please see the enclosed HWA and AT spreadsheet which has the related items highlighted.
 HWA and AT Corrections and Observations
o Pass-Through Loading Calculations
 It appears as though two different tests were used for mercury sampling methods (one for
influent and one for effluent); By using two methods, with different PQLs, the calculation
for the removal rate yields a number higher than normal (see “Removal Rate calc. 2019 –
2022” spreadsheet). The Division does not require influent samples to be tested using
Method 1631 for mercury, but at the same time two different methods cannot be used to
calculate a removal rate for the same pollutant. The recommendation is to use the literature
value for the type of treatment used at the wastewater treatment plant.
 For the Silver removal rate, it is not clear where the value is taken from. The data in the
Removal Rate spreadsheet are below detection levels, and a literature value for the removal
rate is not given in the spreadsheet. The removal rate value for silver cannot be determined
from the information provided by WSACC.
 The values for the instream calculated dissolved metals values should not be rounded up
or down. Please record them in the spreadsheet exactly as they are in the Fact Sheet RPA
table. Pollutants needing correction are: Cadmium, Copper, Lead, Molybdenum, and Zinc.
o Inhibition Loading Calculations
 Can you please provide the data used to arrive at the Activated
Sludge/Nitrification/Trickling Filter (A.S./Nit/T.F.) Inhibition Concentration values for
Cyanide and Zinc?
DocuSign Envelope ID: D626DE28-7B52-4539-BB10-8F7593E8A534
5/13/2024

North Carolina Department of Environmental Quality Division of Water Resources
512 North Salisbury Street 1611 Mail Service Center Raleigh, North Carolina 27699-1611
919.707.9000

o Maximum Allowable Industrial Loadings Calculations
 WSACC provided a spreadsheet with data sampling collected from an uncontrolled source
(“Uncontrollable data 2019-2022”). During the review of the HWA, the Division found
that this data is not being used to calculate uncontrolled loading. Instead, the uncontrolled
loading is derived by using the influent data and subtracting the industrial contribution.
This data calculation can be found in “SIU and Uncontrollable Mass Balance Spreadsheet
2019-2022”. Please provide the assumption and rationale for using the calculated data
instead of the sampled data.
The Division requests that WSACC proposes a timeline to respond to and meet the corrections and observations
outlined above. WSACC is requested by the Division to reply within 90 days of the date of this letter to set up a
MS Teams meeting to discuss a plan to update the HWA to align with National and State Pretreatment Standards.
Thank you for your continued cooperation with the Pretreatment Program. If you have any questions or comments,
please contact Natalie Gorensek [email: [e-mail]] at [phone] or Octavio Henriquez at
[phone] [email: [e-mail]].

nhg/cab_rocky_hwa_req_2024
Ec: Municipal Unit File
Wes Bell, MRO
Central Files (Laserfiche)
Sincerely,

Michael J. Montebello, NPDES Branch Chief
Division of Water Resources

DocuSign Envelope ID: D626DE28-7B52-4539-BB10-8F7593E8A534