NC0081621_Comments_20200212

WSACC WATER&SEWER AUTHORITY
OF CABARRUS COUNTY
232 Davidson Hwy.
Concord,NC 28027
Ago4g,+.4,_
704.786.1783 • 704.795.1564 Fax
www.wsacc.org
IS014001:2014•NC Star Public Sector•ONSAS 18001:2001
February 12, 2020
Mr. Qais Banihani
NCDEQ—Division of Water Resources
NPDES Complex Permitting Unit
1617 Mail Service Center RECEIVED
Raleigh, North Carolina 27699
FEB 1 B.2020
Subject: Draft NPDES Permit Modification NCDEQ/DwR/NPD
Permit No.NC0081621 -Muddy Creek WWTP ES
Water and Sewer Authority of Cabarrus County
Dear Mr. Banihani:
The Water and Sewer Authority of Cabarrus County (WSACC) has received the Draft NPDES
Permit for our Muddy Creek WWTP and your letter of January 15, 2020. We appreciate the
Division's acceptance of our request for a 1.0 MGD Permit, but have concerns related to the
ammonia limits in the draft Permit. Our current 0.3 MGD Permit allows for an ammonia discharge
of 4.0 mg/1 on a monthly average during the summer months. For our future 1.0 MGD facility,
the monthly average summer ammonia limit has been significantly reduced to 1.0 mg/l.
We understand the in-stream ammonia limit for protection of aquatic life is 1.0 mg/1,but discharge
from the 1.0 MGD Plant will comprise less than 4% of the flow during the 7Q10 event. It would
therefore seem reasonable to allow for some dilution to our discharge when establishing an
ammonia limit. Furthermore, justification for this limit, as documented in your Fact Sheet,
indicates that the limit was based on 2007 Speculative Limits, subsequently confirmed by a recent
QUAL2K model. As you know the Division previously identified the 2007 Speculative Limits as
problematic and recommended "extreme care" in interpreting the model results. WSACC
therefore requests the following additional information from the Division:
1. The Division's calculations related to ammonia nitrogen.
2. A copy of the updated QUAL2K model.
3. Any new in-stream data used to calibrate the QUAL2K model, if available.
4. An explanation why the ammonia limits at Muddy Creek WWTP are lower than limits
recently established for other downstream POTWs of similar size.
Given the complexity of this evaluation,WSACC requests delaying issuance of the NPDES Permit
until these questions have been resolved.

-2-
In addition, WSACC has the following concerns with the Cover Letter, Draft Permit, and Fact
Sheet, which in our opinion should be changed prior to issuing the Permit:
• On page 2 of the Draft Permit and on page 1 of the Fact Sheet, the treatment process
component list includes aerobic digesters. The existing Muddy Creek WWTP does not
have aerobic digesters but does have aerated sludge holding tanks. Sludge from these tanks
is hauled to our Rocky River Regional WWTP for incineration; therefore, digestion is not
required.
• On page 2 of the Fact Sheet, the process flow diagram implies that effluent from the
clarifiers will go through the RAS/WAS system and then to the filters. The effluent flow
from the clarifiers should be shown going directly to the filters. An additional arrow should
go from each clarifier to the RAS/WAS system. From the RAS/WAS system,there should
be separate arrows going to both the head of the aeration basins and to the sludge holding
tanks.
• Page 2 of the Cover Letter indicates that mercury should be detected to 0.5 ng/L, but
according to DWR's own website guidance the PQL for mercury using Low Level EPA
Method 1631 E is 1.0 ng/L. The permittee is willing to detect at 1.0 ng/L.
We would welcome the opportunity to meet with the Division to discuss the limits at Muddy Creek
WWTP. If you have any questions or need any additional information,please let me know.
Yours very truly,
Water and Sewer Authority of Cabarrus County
Chad VonCannon, PE
Engineering Director