NC0023965_Report_20050211

NPDES DOCUHENT !iCANNINC COVER SHEET
NC0023965
Wilmington Northside WWTP
NPDES Permit:
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
February 11, 2005
This document in printed on reuse paper - ignore any
content on the resrerrae side

Iriel'aiEvE
lb
WLk9I9N iI�'�ttj ; FEB t 41111
February 11, 2005
Mr. Alan W. Klimek, PE, Director/Division of Water Quality
North Carolina Department of Environment and Natural Resources
Division of Water Quality
1617 Mail Service Center
Raleigh, NC 27699-1617
Re: NPDES Permit NC 0023965
Special Condition A7-Effluent Mixing Model
Dear Mr. Klimek:
The issuance of the referenced permit was accompanied by Special Condition A.7 which
stated that:
"Prior to renewing this permit, the City of Wilmington shall submit a CORMIX
Model (or equivalent) providing 'additional information regarding end -of -pipe
dilution. .The model shall address dilution at the approved 16.0 MGD increased
flow rate into the Cape Fear River for both the existing and the new discharge -line
diffusers."
DWQ offered the following explanation to support its imposition of Special Condition
A.7.
"The Division has reviewed Permittee submittals and modeling efforts to evaluate
effluent -mixing conditions in the Lower Cape Fear River. To date, these appear
insufficiently detailed to define end -of -pipe dilution. Due to the lack of detail and
the permitted discharge increase, the EPA has voiced concerns about potential
future impact."
The City has gone through significant effort and expense to produce the referenced
documentation. Specifically, the City has caused. the preparation and submittal of "3-Diminional
EFDC Water Quality Model of the Lower Cape Fear River and its Estuary", Tetra Tech Inc. May
2001, in support of its NPDES permit application and proposed effluent limitations. Chapter 4
specifically addressed the Discharge Dilution Analysis.
Public Utilities Department
Administration Division • P.O. Box 1810 • Wilmington • North Carolina • 28402-
E810 .
[phone] phone • [phone] fax

Mr. Alan W. Klimek
February 11, 2005
Page Two
Chapter 4 of the referenced document is being submitted herewith in fulfillment of
Special Condition A.7. Please advise of it's acceptance for this purpose or advise of specific
actions required by DWQ which would result in its acceptance.
If you or your staff have any questions or need additional information please feel free to
contact me.
Yours very truly,
'/
-77aatewol,
Hugh T. Caldwell; P.E.
'Director of Public Utilities
cc: ✓Kenneth L. Vogt, Jr., Superintendent of wastewater Treatment
JeffCermak, Wastewater Supervisor - Northside Plant
Wyatt Blanchard, New Hanover County Engineering
Greg Thompson, New Hanover County Engineering
Tony Boahn, McKim & Creed
Ron Taylor, Hazen & Sawyer
Trevor Clements; Tetra Tech

4 - DISCHARGE DILUTION ANALYSIS
The calibrated and verified hydrodynamic model was applied to evaluate near field and far
field mixing and transport of the Wilmington Northside and Southside Wastewater Treatment
Plant (WWTP) effluents. The concept of dilution was used to quantify the degree of mixing and
the transport of the effluent in the Cape Fear system. For each treatment plant, a conservative
tracer having concentration Ce was introduced into the plant effluent. The hydrodynamic and
transport model was -then used to simulate the distribution of the tracer in the system for a two
month period encompassing June and July 1998. The July monthly mean flows in the Cape Fear,
Black and Northeast Cape Fear Rivers were 29.42 cms (1036 cfs), 1.93 cms (68 cfs), and 2.01
cms (71 cfs), respectively. Tracer concentrations were predicted to reach a quasi -steady state,
i.e., the concentration time variation in all cells of the model repeated with each subsequent tidal
cycle,. by the last week of July. The dilution of the effluent during the last tidal cycle in each
model cell was then determined by
(4.1)
D=�
C
where D is the dilution and C is the concentration in the cell of interest. The following two
sections summarize the results of the near and far field dilution analyses.
4.1 Near Field Dilution
The near field dilution analysis was conducted to determine the mixing and dilution of the
effluents in the immediate vicinity of the plant discharges. For the Northside treatment plant, two
approaches were used. The first simple approach assumed that the effluent is instantaneously
mixing in the horizontal model cell in which the discharge is located. Since the Northside
discharge has a two -nozzle submerged diffuser, the second approach used a jet -plume sub -model.
The EFDC.model includes an embedded version of the JETLAG (Lee and Cheung, 1990,
Hamrick, 1998) jet -plume model. The JETLAG model is based on the Lagrangian formulation
used in the UM component of the US EPA's PLUMES model (Baumgartner, Frick, and Roberts,
1994). The JETLAG model has been extensively tested against CORMIXI (Jirka, Doneker, and
Hinton, 1996) by Davidson and Pun (1998) who conclude that the two models provide
quantitatively similar predictions of mixing and dilution from single port discharges over a wide
range of conditions. The JETLAG model embedded in EFDC allows the two-way interaction
between nearfield and far field processes, with the EFDC far field model providing dynamic
ambient conditions for JETLAG, with JETLAG appropriately transferring the equivalent far field
source to EFDC. For the jet -plume based analysis, the two port Northside diffuser head is
represented by a dynamically equivalent single port discharge. Since the Southside treatment
plant does not have a submerged discharge structure, only the first approach was used.
The results of the near field dilution analysis are summarized in Table 4.1. For the Northside
discharge, accounting for the mixing dynamics of the submerged discharge structure results in
approximately 30 percent greater dilutions in the model cell where the discharge is located. The
Northside discharge is located in a high energy region of the river system with tidal mixing
dominating the dilution process. The Southside discharge is located is a less energetic region and
has correspondingly lower near field dilutions. The model cell in which the Northside discharge
is located has a surface area of approximately 0.07 square kilometers and a mean depth of
figTetra Tech, Inc.
4-1

-4-- 01 Chapter 4
DISCHARGE DILUTION ANALYSIS
approximately 10-meters. The model cell in which the Southside discharge is located has a
surface area of approximately 0.11 square kilometers and a mean depth of approximately 1.meter.
Thus the modelgrid constrained volume of the Northside discharge cell is approximately 6 times
that of the Southside discharge cell which is consistent with the results of the simple dilution
approach. Figures 4.1 and 4.2 illustrate the Northside and Southside near field dilutions over two
tidal cycles.
4.2 Far Field Dilution
Dilution of the Northside and. Southside treatment plant discharges were determined in all
model cells. Table 4.2 summarizes dilutions at a number of selected locations of interest. At
Navassa, the dilutions from both treatment plants are lower at the bottom of the water column
than at the surface. During the relatively low flow in the Cape Fear River during the simulation
period, there is upstream intrusion of salinity well past Navassa. This upstream salinity intrusion
is accompanied by a tidally averaged, density driven transport upstream in the bottom portion of
the water column and downstream in surface portion (Hamrick,1979). The upstream average
transport is responsible for an increased upstream transport of material from the treatment plants
near the bottom and a compensating decreased in the upstream transport near the surface. The
strength of this two -layered, upstream -downstream net transport is approximately proportional to
the salinity gradient. A different situation is observed 6 miles upstream in the Northeast Cape
Fear River at station NCF6. Although, salinity is present at this station, the much lower .
freshwater discharge from the Northeast Cape Fear River results in a weaker salinity gradient and
net two layer circulation. As a result, dilutions at the bottom and surface of the water column are
approximately the same.
Dilution of the Northside effluent is greater in the bottom layer at the downstream main
estuary stations, M61, M54, and M42, due to a compensating effect of the smaller bottom layer
dilution upstream in the Cape Fear River. For the Southside effluent, the higher surface dilution
at the upstream M61 station is consistent with the salinity intrusion effect. At the two
downstream stations, M54 and M42, higher bottom dilutions of the Southside effluent occur,
consistent with that observed for the Northside effluent at downstream stations. Figures 4.3-4.5
show two tidal cycle plots of the Northside effluent dilution at Navassa, NCF6, and Marker 61.
Figures 4.6-4.9 show two tidal cycle plots of the Southside effluent dilution at Markers 61, 54,
and 42.
The far field dilution analysis can be used to make a very simplified estimation of the impacts
of the Wilmington treatment plant effluents on low dissolved oxygen near Navassa and NCF6. If
ultimate biochemical oxygen demand was simplistically represented as conservative tracer, the
contribution of the Northside effluents BOD. to the oxygen deficit at Navassa could be crudely
estimated as
BODu +(BODu
DOd, = _..._—
D norduside D sosahside
(4.2)
Using minimum dilutions of 300 and 391 for the Northside and Southside effluents at
Navassa (from Table 4-2), and ultimate BOD's of 35 mg/liter for both effluents (from Figures 5-2
and 5-3), equation (4.2) gives a contribution to the dissolved oxygen deficit of approximately 0.2
rag/liter during summer low flow conditions. A similar calculation for six miles up the Northeast
Cape Fear River gives a value of 0.17 rag/liter. These simple estimates can be viewed as
conservative since the minimum bottom layer dilution rather than the tidally average dilution over
4-2 ® Tetra Tech, Inc.

)
i
1
Chapter 4 May 2001
DISCHARGE DILUTION ANALYSIS
both layers has been used, and the effects of oxidation between the discharge and the points of
interest have been neglected. It is also noted that the value of 0.2 mg/liter is near typical field
instrument sensitivity.
The general conclusion that can be drawn from the discharge dilution analysis is that both of
the Wilmington treatment plant discharges are well diluted by natural physical mixing processes
in the river system. Model predicted dilutions at various locations in the river system are very
consistent with classical estuarine circulation patterns which lend credence to the results. Using
the dilution predictions in conjunction with Equation 4.2 as the basis for a simple analysis, the
two Wilmington discharges in combination are estimated to be responsible for approximately 0.2
mg/liter or less of dissolved oxygen deficit in dissolved oxygen impaired regions of the river
system.
® Tetra Tech, Inc 4-3

May 2001 Cttap r 4
DISCHARGE DILUTION ANALYSIS
• FIGURE 4-1, NEARFIELD NORTHSIDE EFFLUENT DILUTION, BASED ON JET -PLUME ANALYSIS APPROACH.
Dilution Facto
900
800
700
600
500
400
300
200
100
0
1
—A--Bottom
—6—Surface
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 t 9 20 21 22 23 24
Hour
FIGURE 4-2. NEARFIELD SOUTHSIDE EFFLUENT DILUTION, BASED SIMPLE ANALYSIS APPROACH.
DBudon Factor
120
100
80
60
40
20
0
—a-Bottom
--Surface
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
4-4
STetra Tech, Inc.

Chapter 4 May 2001
DISCHARGE DILUTION ANALYSIS
FIGURE 43. DILUTION OF NORTHSIDE EFFLUENT AT NAVASSA.
,wv
3500
#••Bottom
—6—Surface
3000
t500
A00
1500
•
1000
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
STetra Tech, Inc. .4-5

May 2001 Chapter 4 .
DISCHARGE DILUTION ANALYSIS
FIGURE 4-4. DILUTION OF NORTHSIDE.EFFLUENT 6 MILES UP THE NORTHEAST CAPE FEAR RIVER.
800
700
—A—Bottom
—0—Surface
600
.J
500
400
330
200
.
100
.
4 -5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
• Hour
ti
4-6 S Tetra Tech, Inc.

Chapter 4 May2001
DISCHARGE DILUTION ANALYSIS
FIGURE4-5. DILUTION OF IVORTHSIDE EFFLUENT AT CHANNEL MARKER 81.
600
500
1
400
0
u.
0 300
0
200
100
0
mile —Bottom
—Surface
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
STetra Tech, Inc.
4-7

Chapter 4
may 200/ DISCHARGE DILUTION ANALYSIS
FIGURE 4-6. DILUTION OF SOUTHSIDE EFFLUENT AT CHANNEL MARKER 61.
450
400
350
300
-
250 -
200
150
100
.
.
—A—Bottom
—40—Surface
50
•
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 ' 18. 19 20 21 22 23 24
• Hour
4-8 S Tetra Tech, Inc.

}
1
Chapter 4 I May 2001
DISCHARGE DILUTION ANALYSIS
FIGURE 4-7. DILUTION OF SOUTHSIDE EFFLUENT AT CHANNEL MARKER 54.
Dilution Facto
400 -
350
300
260
200
160
100
50
0
—di—Bottom
-A-•S udac e
1 •
1 2 3 .4
5 6 7 8 9 10 11 12 13 14 16 18 17 18 19 20 21 22 23 24
Hour
STetra Tech, Inc 4-9

May 2�01 1chapter4
DISCHARGE DILUTION ANALYSIS
FIGURE 4-8. DILUTION OF SOUTHSIDE EFFLUENT AT CHANNEL MARKER 42.
6 6 7 8 9 10 11 12 13 14 16 16 17 16 19 20 21 22 23 24
Hour
4-10
Tetra Tech, Inc.

Chapter 4
DISCHARGE DILUTION ANALYSIS
TABLE 4.1. SUMMARY OF NEAR FIELD DILUTION ANALYSIS
May 2001
Dilution Criteria
Minimum
Northside Plant
Dilution
Maximum
Northside Plant
Dilution
Minimum
Southside Plant •
_ Dilution
Maximum
Southside Plant
Dilution
Near Surface Dilution in Discharge
Cell using Simple Approach
248
613
36
109
Near Bottom Dilution in Discharge
Ce11 using Simple Approach
241
332
331
797
33
63
.
Near Surface Dilution in Discharge
Ce11 Using Jet -Plume Approach
Near Bottom Dilution in Discharge
Cell Using Jet -Plume Approach
303
• 420
Dilution at Maximum Plume Rise
4.5
25.4
-2. FAR FIELD DILUTION AT SELECTED LOCATIONS
Minimum
Northside Plant
Dilution •
791
(799)
. 300
(325)
Maximum
Northside Plant
Dilution
3103 •
(3372)
448
(500)
• Minimum
Southside Plant
Dilution
1160
391
Maximum
Southside Plant
Dilution
4054
609
Dilution Criteria and Location
Near Surface Dilution at Navassa
(jet -plume approach)
Near Bottom Dilution at Navassa
(jet -plume approach)
Near Surface Dilution at NCF6
359
803
464
1105
(jet -plume approach)
(356)
(813)
•
Near Bottom Dilution at NCF6
357
799
464
1099
(jet -plume approach)
(356)
(809)
Near Surface Dilution at M61
301
375'
327
474
(jet -plume approach)
(307)
(374)
291
Near Bottom Dilution at M61
443
489
269
(jet -plume approach)
(455)
(495)
Near Surface Dilution at M54
364
439
• 184
355
Near Bottom Dilution at M54
.400
493
203
255
Near Surface Dilution at M42
418
650
222
338
Near Bottom Dilution at M42
543
787
274
400
STetra Tech, Inc.
4-11