NC0085359_Other Correspondence_20210209

Public Works
4600 Goldmine Rd
Monroe NC 28110
Union County
T. 704-289-1434 C E���
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www.unioncountync.gov
i FEB 092021
January 29,2021 I�PD��
NCDEQIDWR. . .
Mr.Mike Montebello
Water Quality Permitting Section—NPDES
Division of Water Resources
North Carolina Department of Environmental Quality
1617 Mail Service Center
Raleigh,NC,27699-1617
RE: Union County Public Works
Twelve Mile Creek Water Reclamation Facility
Modification Request to NPDES Permit No.NC0085359
Dear Mr.Montebello,
Union County Public Works Department(UCPW)received the Draft National Pollutant Discharge
Elimination System(NPDES)permit for our Twelve Mile Creek Water Reclamation Facility(WRF)on
December 7,2020. We provided comments on the draft permit on December 21,2021,which is within
the 30-day comment window. We understand that the draft permit has been released for public notice,but
the final permit has not yet been issued.
We have recently completed an in-depth review of our historical plant data for plant optimization.In
reviewing our historical data,we noted that the Twelve Mile Creek WRF has demonstrated slight
increases in the effluent five-day biochemical oxygen demand(BOD5)concentration in the past several
years.The Twelve Mile Creek WRF maintains compliance with effluent permit limits;however,we
suspect a nitrogen interference in the effluent BOD results.
BOD5 measures the oxygen consumed to biologically degrade organic matter,oxidize inorganic
chemicals,and oxidize reduced forms of nitrogen(e.g.,ammonia).The portion of the measured BOD5
associated with the organic oxygen demand is known as carbonaceous BOD5 (cBODs). The portion of the
measured BOD5 associated with oxidizing nitrogen is known as the nitrogenous BOD5(nBOD5).
Many plants with ammonia and/or total Kjeldahl nitrogen(TKN)limits have a cBOD5 limit instead of
BOD5. A cBODs measurement prevents ammonia from influencing the analysis.A concentration of
1 mg/L ammonia exerting a 4.6 mg/L oxygen demand results in an elevated BOD5 concentration. A
nitrification inhibitor is often added to suppress nitrification activity,resulting in more accurate
measurement of the cBOD5. Standard Methods for the Examination of Water and Wastewater(2017)
recommends that a nitrification inhibitor(as directed in Standard Method 5210 B.5e)be used for

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secondary effluent measurement to provide a more accurate estimate of cBOD5.
We respectfully request a change to our BOD5 effluent limit of 5 mg/L to a cBOD5 effluent limit. We
believe that cBOD5 effluent will more accurately portray the effluent quality of our facility. A cBODs
effluent limit will also avoid double-counting the impacts of ammonia on the receiving stream.
If you have any questions,please contact me at(704)296-4215 or [e-mail].
Sincerely,
(4-a--
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Andrew Neff, PE
Water and Wastewater Division Director
cc: Bart Farmer,Union County Public Works, Water Reclamation Facilities Superintendent
Jonathan Jordan,Union County Public Works, Supervisor Twelve Mile Creek WRF
Mary Sadler,PE,Hazen and Sawyer
Jim Struve,PE, Hazen and Sawyer
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