NC0023965_Instream Assessment_19890110

NPDES DOCUMENT SCANNING COVER SHEET
NPDES Permit:
NCO023965
Wilmington - Northside WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Engineering Alternatives Analysis
201 Facilities Plan
nstream Assessme
Correspondence re: draft permit
Permit History
Date Range:
Document Date:
January 10, 1989
THIS DOCYNENT IS PRINTED ON REUSE PAPER - IGNORE ANY CONTENT ON THE
REVERSE SIDE

DIVISION OF ENVIRONMENTAL MANAGEMENT
January 10, 1909
i1EMOmANDUM
10: Pre —ton Howard
r
'u J THRU: !)avid Voqt CG/
Trevor Clemer:ts / w�
Steve TedderNL
FRCP?: M i c h a e I S c G v eMYJ
SUBJECT: instream Asses -,meat
NPDES No. NC0023965
New Hanover County
SOC Case No. 88-61.
Summa=
for the City of Wilmington Northside I42WTP
The City of Wilmington has requested a first-time SOC for effluent HODS lim-
its that approximate the current level of operation of their Northside WWTF. The
exis ing facility frequently exceeds_- its IPDES permit limitation of 30 moil SOD5.
Presently, the facility's trickling filters cannot provide effecti,✓e treatment
for the 8.0 MGD design capacity and the pant is currently :-elying on chemical
feed to reduce it'., effluent.BOD. Wilmi-:;'on has committed to upgradinq their
facility by March 31, 1991. and has also agreed to limit -additional interim was-
tewater flows to no more than a total of=:00;000 gpd. A summary of the fa
ity's wasteflow information is shown belc'.�.
Design
Capacity
8.0000
MGD
Pre-SOC
Flow (11/e7-10/eS)
5.3066
MGD
Additional
SOC =low
0.4000
MGD
Maximum
Allowable SOC Fl.cw
2.6934
MGD
The Wilmington Northside WWTP currently has a monthly average BOD5 limit of
30 mo/l (from November 1987 to October 1939 this limit has been exceeded four
times: the average monthly BOD5 concentration during this 12 month period was
2B.9 mg/l, with individual monthly averages ranging from 25.06 mg/1 to 35.92
mg/1). The City reports that 63% of the 5OD5 monthly averages between December
1986 and October 198E have been 2e mg/i o- greater. Until the completion of
their plant upgrade, the City has requested 9OD5 limits of 40 mg/i during the
months of April through September and 45 mg/1 from October to March.

Analvsis and Discussion
The City of Wilmington Northside WWTP discharges into the Northeast Cape Fear
River near its confluence with the Cape Fear River. Althouol, the impact of this
discharge on the water ouality in the Cape Fear River Estuary is uncertain, this
lower section of the river is _trongiy influenced by tidal action. The rela--
tively open accesstothe Atlantic Ocean results in tidal patterns that create a
significant dispersal mechanism, influencing the river's capacity to assimilate
wastewater loads. The tidal amplitude at Wilmington is approximately 3.0 feet
and the net tidal velocity is approximately 2.0 ft/sec.
DEM does not presently have a water quality model that can reliably predict
the impact of the proposed increase in 30D leading by the Northside W61T". How—
ever, a comprehensive rncdel of the Cape Fear Estuary System is currently heino
developed by t:he Technical Suoport Unit for evaluating discharges into this
tidaliv-affected area. Due to the documented substandard summertime DO conren-
trations (see attached). itcconceivable that the final NPDES permitted limits
c
for this facility tas well _,_others) uray change at the ne::t renewal r,hcr the nv:q
estuary model is employed as predictive tool for a WLA anal;psis. For WiLning-
ton's Northside IJWTP it may therefor= be more economical in tha lone rL:n to
incorporate in the current upgrade i:nc- treatment capabilit,V to meet limit_ r:ore
stringent than secondary limit_.
Finally, while it is expected that the increased 301) loading proposed by 'Wil-
minoton will not violate the EMC criteria for SOC approval, the impact o` the
additional 6OD cannot be reliably predicted.
cc: Chuck Wakild
Kent Wiggins
Steve Reid
Wilminaton Northside WLA File

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