NC0002305_Report_20061212

NPDES DOCUMENT SCANNIN`i COVER SHEET
NPDES Permit:
NC0002305
Guilford Mills WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
.. /7a 0/t)
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
December 12, 2006
Thies d eumerit is printed on retinae paper - ignore any
content on the re-srernae wide

IT3 Engineering, P.C.
2401 Research Drive
Raleigh, NC 27606
[phone]
MEM
Water Quality Modeling of the
Cape Fear River
Prepared for:
Guilford Mills
Kenansville, North Carolina
— Prepared by:
Wu Seng Lung, Ph.D., P.E.
Professor of Environmental Engineering
University of Virginia
and
W. Gilbert O'Neal, Ph.D., P.E.
President
IT3 .Engineering, P.C.
December 12, 2006
Draft

PAUL R. BRUESCH, P.E., R.E.M.
CORPORATE
ENVIRONMENTAL MANAGER
GUILFORD MILLS, INC.
[phone] • FAX: [phone]
CELL: [phone] • HOME: [phone]
5644 HORNADAV ROAD • GREENSBORO. NC 27409
E-MAIL. [e-mail] • www.guilfordmdls.com
ITT Technologies, Inc.
W. Gilbert O'Neal, Ph.D.,P.E.
President and CEO
2410 College of Textiles 919-513-7581 Phone
Raleigh, NC 27695-8301 919-882-9410 fax
USA 919-413-1126 cell
www.itt.edu wgoneal@a itt.edu
GUILFORD
OS9000
FM 85468
ISO 14001
738443
BRENT TURNER
DIRECTOR OF ENGINEERING
AUTOMOTIVE PRODUCTS
[phone] • MAIN: [phone] • FAX: [phone]
(P. O. BOX 509) • 1754 NC 903/11 NORTH • KENANSVILLE, NC 28349
E-MAIL: bturneregfd.com • www.gullfordproducts.com
1019

Pal
Introduction
Water Quality Modeling of the
Cape Fear River at Guilford Mills, NC
12/12/06
f=, The Guilford Mills Plant located in Kenansville, North Carolina manufactures fabrics used
primarily in automotive interiors. Dyeing and finishing operations result in the discharge of
around 1.0 mgd of wastewater. The wastewater is biologically treatment using the extended
aeration process and then chemically treated using aluminum salts, and polymers for
coagulation and precipitation of colloidal and particulates remaining after secondary
sedimentation. Chemical, or tertiary treatment, is required for compliance with a average
and maximum CBOD of 10 mg/L. While the discharge from the secondary clarifier generally
1=1averages below this limit, compliance with the 10 mg/L maximum limit can not be
consistently obtained without tertiary treatment.
f.' The operating cost for chemical treatment presently exceeds $400,000 per year and is a
significant factor affecting the economic competitiveness of Guilford Mills. Low cost
imported fabrics continue to threaten the U.S. textile industry and margins are minimal.
Thus, every manufacturing cost, including those associated with environmental control,
must be minimized to maintain market viability and manufacturing jobs in North Carolina.
r1
It is understood that water quality must not be compromised by cost reduction efforts.
However, only those measures and associated costs required for protection of the receiving
stream should be required.
The Guilford NPDES permit stipulates water quality limiting permit conditions for CBOD.
These limits were established based on desktop models of the receiving stream. These
models were established using best professional judgment; however, they were not based on
extensive field data. Thus, in an effort to verify the need for the additional cost of compliance
using tertiary treatment, a water quality model was developed to simulate carbonaceous
biochemical demand (CBOD), nitrogenous biochemical oxygen demand (NBOD), and
dissolved oxygen (DO) concentrations in the Cape Fear River following the discharge of the
Guilford Mills wastewater treatment plant. Data collected from two water quality surveys
conducted in May 2002 and September 2005 were used to support the modeling analysis.
The field program also included a time -of -travel study for the modeling area and sample
collections for water quality parameter analyses. Figure 1 shows the water quality sampling
stations.
Modeling Approach and STREAM Model
The basic principle of a water quality model is mass balance. The following physical and
biochemical processes related to BOD and DO are considered in the model: deoxygenation
of organic material from the Guilford Mills wastewater treatment plant; resupply of oxygen
from the atmosphere to the reservoir; algal photosynthesis and respiration; sediment oxygen
demand; wastewater characteristics; and water temperature, which affects all biochemical
reaction and reaeration rates.

V1
STREAM, a steady-state, 1-D BOD and DO model for streams and rivers, was configured for
the Cape Fear River. Three water quality parameters are modeled: ultimate CBOD (CBODu),
NBOD, and DO (Lung, 2001). The model has been widely used in wasteload allocations to
support regulatory permitting work. Recent applications of STREAM include a modeling
study of the Roanoke River for assimilative capacity calculations (Lung and Sobeck, 1999)
and Walnut Creek in Alabama.
The following processes are included in the STREAM model: removal of CBOD in the water
`—' column, biological oxidation of CBOD and NBOD, stream reaeration, algal photosynthesis
and respiration, and sediment oxygen demand (SOD). Essentially, the model tracks the
mass balance of CBOD, NBOD, and DO along the stream. The complete model formulation
is listed in the Appendix.
Key Technical Considerations
The key to running the STREAM model is assigning the kinetic coefficients such as CBOD
removal and deoxygenation rates, nitrification rate, and stream reaeration rate. To achieve
full calibration of the model for the Cape Fear River, model tuning should be conducted such
that the model calculations would capture the key water quality trend(s) using the kinetic
coefficient values consistent with literature reported bounds for BOD and DO models of
streams.

Min
Guilford Mills
NE Cape Fear TMDL Study
Station Map
4.
Grid Lines are 1000 by 1000 meters.
Station dX Descriptor
1 -50 Above outflow
2 200 Complete Mix
3 8200 Pig Farm
4 -- —
5 15200 River Section
6 20200 Sarecta Bridge
NOTE: Reference is discharge point (X = 0)
Station 4 not used.
Measurements in Feet
S1/S2 (marked at outflow)
S3 (Pig Farm)
S5
S6 (Under Sarecta Bridge)
Figure 1. Water Quality Sampling Stations for the Cape Fear River near Guilford, NC
In this study, the ambient stream flow above the Guilford Mills wastewater treatment plant
is small and the plant often provides a significant portion of the stream flow in the vicinity of
the discharge. A unique feature of such low -flow stream receiving point source discharges is
that the first half of the classic DO spatial profile is missing. That is, the DO sag is usually at
the wastewater outfall. The DO level recovers progressively from that point on (Lung, 2001).
The wastewater CBOD„ to CBOD5 ratio is extremely important in this modeling study. It is
widely known that this ratio is closely related to the characteristics of the wastewater and

n
MIR
thereby to the CBOD deoxygenation rate in the stream. As the treatment level is increased,
the deoxygenation coefficient is reduced, resulting in a higher ratio. This ratio is also needed
to convert the model calculated CBOD„ loads to CBOD5 loads for developing the allowable
permit loads for the regulatory agency. In this study, recent lab results from the long-term
CBOD runs of the wastewater and the receiving water samples provided data for the
modeling analysis.
Another key model parameter is the stream reaeration coefficient. In lieu of field
measurements, an empirical equation is usually used. For small streams such as the Cape
Fear River in the study area, the equation by Tsivoglou and Neal (1976) is recommended
(Lung, 2001). A comprehensive analysis by Grant (1976) indicated that the Tsivoglou
Equation is most accurate for small, shallow streams. The STREAM model offers this
equation as one of the options for the users:
Ka = 0.88US for 10 < Q < 300 cfs (3a)
KQ= 1.88US for 1<Q<10cfs (3b)
Where K . is the reaeration coefficient (day-1) at 20°C, S is the stream channel bottom slope in
ft/mile, and LI is averaged stream velocity in ft/sec.
Mass Transport Modeling
The first step of the modeling analysis is to track the mass transport in the receiving water
using a conservative substance. The STREAM model was first configured to simulate
specific conductivity in the Cape Fear River following the wastewater treatment plant
discharge. Since the specific conductivity levels of the effluent from the Guilford Mills
wastewater treatment plant are much higher than the ambient conductivity concentration, it
serves as an excellent conservative tracer for this purpose. Another purpose of the mass
transport modeling is to verify the time -of -travel results from the dye study in the field.
The specific conductivity model was configured for both water quality surveys: May 2002
and September 2006. Results of the analysis are presented in Figure 2, showing model
17-7' calculated and measured conductivity concentrations along the Cape Fear River in the study
area. Note the sharp increase of conductivity levels following the discharge of the treatment
plant effluent in the September 2005 survey due to the extremely low river flow. In general,
the mass transport model results match the spatial trend of the specific conductivity levels
quite well, substantiating the mass transport of and flow balance in the model.

n
r,
r
s.
400.
5.300
:.52
=100
0 0
Flow above Guilford Mills = 24.9 cfs May 28-30, 2002)
—1
"500
0 Flow above Guilford Mills = 4.01;cfs (Sept 6-8, >400-
.c
L3co- f
20o-co
E
locio
CD
o to
0 0 �. t
:2 3
,River Miles below :Guilford Mills
Legend: Data (Average and Range)
Model Results
Figure 2. Conductivity Model Results vs. Data — Checking Mass Transport and Time -of -Travel
Figure 3 shows the plot of time -of -travel for both flow conditions. The lower flow in
September 2005 contributes to a much higher time -of -travel. The measured travel times to
reach Station 6 (at 3.827 miles below the wastewater treatment plant) are matched by the
calculated values closely, i.e., approximately at 15 hours and 43 hours for the May 2002 and
September 2005 flows, respectively.

r-�
r-i
I1
1I
n
60
55
50
• 45co
40-
v 35 -
�a
.530
0 25-
L
• 20
m 15
it- 10-
5
0
-1
Legend: ONO 4111. Ole IMO
Sept 2005
May 2002 •
/
/.
.
1
•
•
•
.
•
1 2 3
River Miles below Guilford .Mills
Figure 3. Calculated Time -of -Travel for May 2002 and September 2005 Conditions
Analysis of BOD Data
To accurately characterize the deoxygenation rate in the Cape Fear River following the
discharge of the Guilford Mills wastewater treatment plant, long-term BOD analyses of
samples from the plant effluent and at locations upstream and immediately downstream of
the discharge was conducted in early 2006. Figure 4 shows the results of the BOD analysis of
the plant effluent over a period of 104 days from January 31, 2006 to May 15, 2006. The plots
rx-0 in Figure 4 show a significant level of NBOD at 24.52 mg/L in the effluent, due to addition of
ammonia at the plant. A corresponding production of nitrite/nitrate is shown in Figure 4.
Results also show that nitrification in the effluent is complete by the end of approximately 30
days due to the exhaustion of ammonia. Then the breakdown of dissolved carbon in the
effluent took place in the remaining period of the incubation. While Figure 4 suggests
nitrification in the effluent, plots of the long-term BOD analysis of the receiving water
n samples show very insignificant nitrification the Cape Fear River as shown in the long-term
BOD results of the receiving water samples (Figure 5).
It should be noted that ammonia is added as a nutrient supplement to enhance performance
of the biological system. Thus, the addition of nitrogen can be control to eliminate or
minimize nitrification in the effluent.
7
Mal

I1
50
45-
40-
35-
cn
30 -
E 25-
O 20-
m
15.-
10 -
5-
o- ' •
0 10 20 30 40 50 60 70 80 90 100 110
Nitrite/Nitrate (mg/L)
20
18-
16-
14-
12
10-
8-
6- Ammonium Oxidized = 5.33 mg/L
4- Nitrite/Nitrate Produced = 5.10 mg/L
Measured Total BOD
Calculated NBOD
e -o- 0
Total NBOD = 24.52 mg/L
0
2-
0 , , , •
0 10 20 30 40 50 60 70 80 90 100 110
• ,
Incubation Time (Day)
Figure 4. Long -Term Plots of BOD and Nitrite/Nitrate Data of the Effluent Sample
As shown in Figure 5, the BOD levels in the downstream samples are slightly higher than
those from the upstream sample. Lab results also show extremely low levels (almost zero) of
ammonia in the ambient river samples. The long-term BOD data from the sample collected
.71 immediately below the treatment plant discharge are re -plotted in Figure 6.

rzcl
n
n
f=,
12
10
8
vn
E s
m 4
0
2
Upstream of Treatment Plant
Qom_' I • ► ► , ► ►
O 5 10 15 20 25 30 35 40 45 50 55 60
12
Incubation Time (Day)
10-
8-
cn
0
CO 4
0
2-
;WIOb_' i
O 5 10 15 20 25 30 35 40 45 50 55 60
Incubation Time (Day)
Figure 5. Data Plots of Long -Term BOD Analysis of the Receiving Water Samples
Downsteam of Treatment Plant
r,
n
A simple regression analysis of the BOD data yielded a BOD bottle rate, ki of 0.035 day-1
(Figure 6), characterizing a slow bio-oxidation process in the receiving water and reflecting
that the treatment plant effluent is highly stabilized in terms of carbon. Such a low carbon
deoxygenation rate in the Cape Fear River is associated with a high CBOD„/ CBOD5 ratio of
6.23. [This ratio is needed to convert the model results in CBOD„ to CBOD5.]

12
10-
8-
6-
2-
0
Downsteam of Treatment Plant
k, = 0.035 day'
•
0 5 10 15 20 25 30 35 40 45 50 55 60
Incubation Time (Day)
Figure 6. Regression Analysis of the BOD Data for Sample at Immediately Below the Discharge
BOD/DO Model Results
The calibrated mass transport model was then used to simulate the BOD and DO
concentrations in the river under both flow conditions. Based on the BOD data analysis, a
deoxygenation rate of 0.035 day-1 was used in the modeling analysis. While the long-term
BOD data of the ambient river samples show insignificant nitrification, a small nitrification
rate of 0.05 day-1 (as a conservative assumption) was adopted in the modeling analysis.
Figure 7 shows the model calculated vs. measured CBOD5, NBOD, and DO concentrations
under the May 2002 flow condition. Similar results for CBOD5, NBOD, and DO
concentration profiles for the September 2005 flow condition are presented in Figure 8. The
model is capable of closely reproducing the dissolved oxygen data in both surveys, thereby
calibrating and verifying the key model coefficients such as the in -stream carbon
deoxygenation and nitrification rates. Note that the depression of dissolved oxygen levels
in a classic DO sag curve is missing in both data sets - a typical dissolved oxygen profile
observed in many low flow streams these days (Lung, 2001).
10

r-t
n
10
9
8
E 7
River Flow above WWTP = 24.9 cfs & Temp = 24.5°C
0
Alb
-1 0 1
10
9-
8-
p s-
.� 5 -
0 4-
co 3-
z 2-
1-
0
3 4 5
•
—1
1 2 3 4 5
fsi 12 1 1
11-
10- Saturation DO 8.42 mg/L
9
Dissolved Oxyge
7-
6-
4-
3-
2-
1-
0
—1 0 1 2 3 4
Copefear River Miles
Guilford Mls
E
0
ca
0
L
0
u)
Legend Data (Average and Range: May 28-30, 2002)
Model Results
Figure 7. Model Calculated CBODu, NBOD, and DO concentrations vs. Measured for May 2002
11

10
-j 9
8
River Flow above WWTP = 4.0 cfs & Temp = 24.5°C
E 7- -
' m 5 -
c0 4-
>% 3-
g., -° _ 2
in 1- -
0
—1 0 1 2 3 4 5
tI
n
10
9-
8-
•
5-
0 4
m 3- E.
Z 2_ o`
1-
5 Q'
a
0
-1
Sarecta Br a
-
• 12
-
• 11-
?1-
g
c 8-
al 7-
x
>" g_
0 5-
-c 4-
> 3-
2-
N 1-
0 0
—1
O 1 2 3 4 5
Saturation DO = 8.42 mg/L
O 1 2 3 4 5
Capefeor River Miles below Guilford Mills
Legend: Data (Average and Range: Sept 6-8, 2005)
Model Results
Figure 8. Model Calculated CBOD5, NBOD, and DO concentrations vs. Measured for September 2005
12

ry
n
n
Model Projection Analysis
Following the model calibration and verification analysis, a number of CBOD loading
scenarios were developed to evaluate their impacts on dissolved oxygen in the Cape Fear
River immediately below the treatment plant discharge:
1. Determine the maximum CBOD5 concentration in the effluent to meet a DO level of 5
mg/L in the Cape Fear River under the September 2005 low flow conditions,
assuming a nitrification rate of 0.05 day-1 in the river.
2. Same as above but without nitrification in the river.
3. Same as Scenario 1 but to meet a DO level of 4 mg/L in the Cape Fear River.
4. Same as Scenario 3 but without nitrification in the river.
5. Determine the minimum stream flow rate to be maintained to meet a DO level of 5
mg/L with a CBOD5 concentration of 20 mg/L in the effluent and nitrification in the
river.
6. Same as Scenario 6 but without nitrification in the river.
7. Same as Scenario 5 but with a CBOD5 concentration of 30 mg/L in the effluent and
nitrification in the river.
8. Same as Scenario 7 but without nitrification in the river.
A water temperature of 28°C is assumed for all the projection model runs.
Model results for the projection runs are summarized in Tables 1 and 2.
Table 1. Model Projection Results - Maximum Effluent CBOD5 Concentration
Scenario
Flow (cfs)
DO (mg/L)
Nitrification
Max. CBOD5 (mg/L)
1
4.01
5
Yes
12.5
2
4.01
5
No
17.0
3
4.01
4
Yes
24.0
4
4.01
4
No
29.4
Table 2. Model Projection Results - Minimum River Flow
Scenario
DO (mg/L)
Nitrification
Effluent CBOD5 (mg/L)
Minimum Flow (cfs)
5
5
Yes
20
5.21
6
5
No
20
4.41
7
5
Yes
30
6.25
8
5
No
30
5.62
A range of CBOD5 levels from 12.5 mg/L to 17 mg/L in the treatment plant effluent is
expected to meet the DO standard of 5 mg/L under a very low flow rate of 4 cfs in the Cape
Fear River upstream of the treatment plant discharge. This range of CBOD5 levels is
increased by approximately 12 mg/L if the DO standard is lowered to 4 mg/L in the
13

ambient water. Table 2 suggests that stream flow rates above a range from 4.41 cfs to 6.25 cfs
would be required to maintain a DO standard of 5 mg/L, depending on the CBOD5
concentration (20 mg/L or 30 mg/L) in the treatment plant effluent and the nitrification rate
in the river. Under the most conservative condition, if the stream flow rate is above 6.25 cfs,
the DO standard of 5 mg/L can be met with a 30 mg/L of CBOD5 in the plant effluent.
Current permit limits are based on the following flow conditions:
Summer Low Flow (7Q10): 6.5 cfs
Winter Low Flow: 18 cfs
Average Flow: 398 cfs
Based on these conditions and the model results, CBOD5 as high as 30 mg/L can be
assimilated while maintaining 5 mg/L or upstream DO conditions. Tertiary limits appear to
be required only under flow conditions below the 7Q10 flow.
References
Grant, R.S, 1976. Reaeration Coefficient Measurements of Ten Small Streams in Wisconsin
Using Radioactive Tracers. U.S. Geological Survey Water Resources Investigations, pp.76-
r 79.
Lung, W.S. 2001. Water Quality Modeling for Wasteload Allocations and TMDLs, John
Wiley & Sons, New York, NY, 333p.
Lung, W.S. and Sobeck, R.G., 1999. Renewed Use of BOD/DO Models in Water Quality
,, Management. Journal of Water Resources Planning and Management, 125(4):222-227.
rmi
1I
Tsivoglou, E.C. and Neal, L.A., 1976. Tracer Measurements of Reaeration: III. Predicting the
Reaeration Capacity of Inland Streams. Journal of Water Pollution Control and Federation,
48(12): 2669-2689.

APPENDIX - The STREAM Model Formulation
Under steady-state conditions, a 1-D BOD/DO STREAM model includes the following
equations (Lung, 2001):
rat
where
r=,
x x
D — K°L-K•
° (e-Kr U — ea U) CBOD (la)
Ka — Kr
x x
+ Kn N° (e-Ka U — e-Ka U) NBOD (1b)
Ka — Kn
-Ka
+ D°e U Initial DO Deficit (lc)
-Ka x
— P (1— e U) Algal Photosynthesis (Id)
Ka
R _Ka X
+ K (1— e U) Algal Respiration (1e)
Kr,
-Ka x
+ SOD (1— e U) Sediment Oxygen Demand (10
HKa
D = Dissolved oxygen deficit (mg/L)
KD = In -stream CBOD deoxygenation rate (day-1)
L° = Initial stream CBOD concentration below the wastewater outfall (mg/L)
Ka = In -stream reaeration coefficient (day-1)
Kr = In -stream CBOD removal rate (day-1)
x = Stream distance downstream from the point source (mile)
LI = Average stream velocity (mile/day)
Kn = In -stream nitrification rate (day-1)
Na = Initial stream NBOD concentration below the wastewater outfall (mg/L)
D° = Initial stream DO deficit concentration below the wastewater outfall (mg/ L)
15

r'
P = Algal photosynthesis rate (mg 02 L-1 day-1)
R = Algal respiration rate (mg 02 L-1 day-1)
SOD = Sediment oxygen demand (gm 02 m-2 day-1)
H = Average depth of the water column (ft)
The initial stream CBOD concentration, Lo, in Eq. la must be expressed as ultimate oxygen
demand. Because of zero -order and first -order kinetics formulated in the model, the
dissolved oxygen deficit terms due to different sources and sinks are added, i.e.,
superimposed. The dissolved oxygen concentration C may be determined from the
computed deficit using the following equation:
C=Cs -D
where Cs is the saturated dissolved oxygen concentration (mg/L). The following equation is
recommended by EPA (1995 to calculate the saturated dissolved oxygen concentrations as a
function of temperature for freshwater streams:
468
CS _ 31.6+T
where T is water temperature in °C. This equation is accurate to within 0.03 mg/L
compared with the Benson -Krause equation on which, the Standard Methods tables are
based (Lung, 2001).
(2)

Sec Clar & Final Effluent Tot BOD Concs
6120106
718106
7/26 06
8r13/06
911/06
9119)06
1017106
10/25106
11112)06
1211106
5.7
Secondary Clarifier Et t -Tot 000 sPt20)
0
Final Effluent • Tot B00 IP521
Permit Limit 0aily Nair - 10 mg1L
•
A-$L{oo, o0DAK.

IT3 Engineering, P.C.
2401 Research Drive
Raleigh, NC 27606
[phone]
Water Quality Modeling of the
Cape Fear River
Prepared for:
Guilford Mills
Kenansville, North Carolina
Prepared by:
Wu Seng Lung, Ph.D., P.E.
Professor of Environmental Engineering
University of Virginia
and
W. Gilbert O'Neal, Ph.D., P.E.
President
IT3 Engineering, P.C.
December 12, 2006
Draft

Water Quality Modeling of the
Cape Fear River at Guilford Mills, NC
12/12/06
Introduction
The Guilford Mills Plant located in Kenansville, North Carolina manufactures fabrics used
primarily in automotive interiors. Dyeing and finishing operations result in the discharge of
around 1.0 mgd of wastewater. The wastewater is biologically treatment using the extended
aeration process and then chemically treated using aluminum salts, and polymers for
coagulation and precipitation of colloidal and particulates remaining after secondary
sedimentation. Chemical, or tertiary treatment, is required for compliance with a average
and maximum CBOD of 10 mg/L. While the discharge from the secondary clarifier generally
averages below this limit, compliance with the 10 mg/L maximum limit can not be
consistently obtained without tertiary treatment.
The operating cost for chemical treatment presently exceeds $400,000 per year and is a
significant factor affecting the economic competitiveness of Guilford Mills. Low cost
imported fabrics continue to threaten the U.S. textile industry and margins are minimal.
Thus, every manufacturing cost, including those associated with environmental control,
must be minimized to maintain market viability and manufacturing jobs in North Carolina.
It is understood that water quality must not be compromised by cost reduction efforts.
However, only those measures and associated costs required for protection of the receiving
stream should be required.
The Guilford NPDES permit stipulates water quality limiting permit conditions for CBOD.
These limits were established based on desktop models of the receiving stream. These
models were established using best professional judgment; however, they were not based on
extensive field data. Thus, in an effort to verify the need for the additional cost of compliance
using tertiary treatment, a water quality model was developed to simulate carbonaceous
biochemical demand (CBOD), nitrogenous biochemical oxygen demand (NBOD), and
dissolved oxygen (DO) concentrations in the Cape Fear River following the discharge of the
Guilford Mills wastewater treatment plant. Data collected from two water quality surveys
conducted in May 2002 and September 2005 were used to support the modeling analysis.
The field program also included a time -of -travel study for the modeling area and sample
collections for water quality parameter analyses. Figure 1 shows the water quality sampling
stations.
Modeling Approach and STREAM Model
The basic principle of a water quality model is mass balance. The following physical and
biochemical processes related to BOD and DO are considered in the model: deoxygenation
of organic material from the Guilford Mills wastewater treatment plant; resupply of oxygen
from the atmosphere to the reservoir; algal photosynthesis and respiration; sediment oxygen
demand; wastewater characteristics; and water temperature, which affects all biochemical
reaction and reaeration rates.
2

STREAM, a steady-state, 1-D BOD and DO model for streams and rivers, was configured for
the Cape Fear River. Three water quality parameters are modeled: ultimate CBOD (CBODu),
NBOD, and DO (Lung, 2001). The model has been widely used in wasteload allocations to
support regulatory permitting work. Recent applications of STREAM include a modeling
study of the Roanoke River for assimilative capacity calculations (Lung and Sobeck, 1999)
and Walnut Creek in Alabama.
The following processes are included in the STREAM model: removal of CBOD in the water
column, biological oxidation of CBOD and NBOD, stream reaeration, algal photosynthesis
and respiration, and sediment oxygen demand (SOD). Essentially, the model tracks the
mass balance of CBOD, NBOD, and DO along the stream. The complete model formulation
is listed in the Appendix.
Key Technical Considerations
The key to running the STREAM model is assigning the kinetic coefficients such as CBOD
removal and deoxygenation rates, nitrification rate, and stream reaeration rate. To achieve
full calibration of the model for the Cape Fear River, model tuning should be conducted such
that the model calculations would capture the key water quality trend(s) using the kinetic
coefficient values consistent with literature reported bounds for BOD and DO models of
streams.
3

Guilford Mills
NE Cape Fear TMDL Study
Station Map
Station dX Descriptor
1 -50 Above outflow
2 200 Complete Mix
3 8200 Pig Farm
4 -- --
5 15200 River Section
6 20200 Sarecta Bridge
NOTE: Reference is discharge point (X = 0)
Station 4 not used.
Measurements in Feet
N S1/S2 (marked at outflow)
Grid Lines are 1000 by 1000 meters.
S3 (Pig Farm)
S5
S6 (Under Sarecta Bridge)
Figure 1. Water Quality Sampling Stations for the Cape Fear River near Guilford, NC
In this study, the ambient stream flow above the Guilford Mills wastewater treatment plant
is small and the plant often provides a significant portion of the stream flow in the vicinity of
the discharge. A unique feature of such low -flow stream receiving point source discharges is
that the first half of the classic DO spatial profile is missing. That is, the DO sag is usually at
the wastewater outfall. The DO level recovers progressively from that point on (Lung, 2001).
The wastewater CBOD„ to CBOD5 ratio is extremely important in this modeling study. It is
widely known that this ratio is closely related to the characteristics of the wastewater and
4

thereby to the CBOD deoxygenation rate in the stream. As the treatment level is increased,
the deoxygenation coefficient is reduced, resulting in a higher ratio. This ratio is also needed
to convert the model calculated CBOD„ loads to CBOD5 loads for developing the allowable
permit loads for the regulatory agency. In this study, recent lab results from the long-term
CBOD runs of the wastewater and the receiving water samples provided data for the
modeling analysis.
Another key model parameter is the stream reaeration coefficient. In lieu of field
measurements, an empirical equation is usually used. For small streams such as the Cape
Fear River in the study area, the equation by Tsivoglou and Neal (1976) is recommended
(Lung, 2001). A comprehensive analysis by Grant (1976) indicated that the Tsivoglou
Equation is most accurate for small, shallow streams. The STREAM model offers this
equation as one of the options for the users:
Ka = 0.88US for 10 < Q < 300 cfs (3a)
Ka=1.88US for 1<Q<10cfs (3b)
Where Ka is the reaeration coefficient (day-1) at 20°C, S is the stream channel bottom slope in
ft/mile, and 11 is averaged stream velocity in ft/sec.
Mass Transport Modeling
The first step of the modeling analysis is to track the mass transport in the receiving water
using a conservative substance. The STREAM model was first configured to simulate
specific conductivity in the Cape Fear River following the wastewater treatment plant
discharge. Since the specific conductivity levels of the effluent from the Guilford Mills
wastewater treatment plant are much higher than the ambient conductivity concentration, it
serves as an excellent conservative tracer for this purpose. Another purpose of the mass
transport modeling is to verify the time -of -travel results from the dye study in the field.
The specific conductivity model was configured for both water quality surveys: May 2002
and September 2006. Results of the analysis are presented in Figure 2, showing model
calculated and measured conductivity concentrations along the Cape Fear River in the study
area. Note the sharp increase of conductivity levels following the discharge of the treatment
plant effluent in the September 2005 survey due to the extremely low river flow. In general,
the mass transport model results match the spatial trend of the specific conductivity levels
quite well, substantiating the mass transport of and flow balance in the model.
5

E500 ,
Flow above Guilford Mills = 24.9 cfs (May 28-30, 2002)
>400 -
= 300 -
e-2200-
:1100-
c) 0
—1 0 1 2 3 4 5
•
500 i s
o Flow above Guilford Mills = 4.01 cfs (Sept 6-8, 2005)
,I)400
300 - 0 i Q i%'200 - ..
m m
100 -
U 0 • •a t°n
4•
—1 0 1 2 3 4 5
River Miles below Guilford Mills
Legend: Data (Average and Range)
Model Results
Figure 2. Conductivity Model Results vs. Data — Checking Mass Transport and Time -of -Travel
Figure 3 shows the plot of time -of -travel for both flow conditions. The lower flow in
September 2005 contributes to a much higher time -of -travel. The measured travel times to
reach Station 6 (at 3.827 miles below the wastewater treatment plant) are matched by the
calculated values closely, i.e., approximately at 15 hours and 43 hours for the May 2002 and
September 2005 flows, respectively.
6

60
55
In' 50-
L
45-
S- 40-
Tv
0 35-
= Legend:
30-
0 25-
L
�
20-
15
>
v
it 10
5
-1 0 1 2 3 4 5
River Miles below Guilford Mills
Figure 3. Calculated Time -of -Travel for May 2002 and September 2005 Conditions
Sept 2005
May 2002
iee
I
ee
te
/
.
o
e do
Analysis of BOD Data
To accurately characterize the deoxygenation rate in the Cape Fear River following the
discharge of the Guilford Mills wastewater treatment plant, long-term BOD analyses of
samples from the plant effluent and at locations upstream and immediately downstream of
the discharge was conducted in early 2006. Figure 4 shows the results of the BOD analysis of
the plant effluent over a period of 104 days from January 31, 2006 to May 15, 2006. The plots
in Figure 4 show a significant level of NBOD at 24.52 mg/L in the effluent, due to addition of
ammonia at the plant. A corresponding production of nitrite/nitrate is shown in Figure 4.
Results also show that nitrification in the effluent is complete by the end of approximately 30
days due to the exhaustion of ammonia. Then the breakdown of dissolved carbon in the
effluent took place in the remaining period of the incubation. While Figure 4 suggests
nitrification in the effluent, plots of the long-term BOD analysis of the receiving water
samples show very insignificant nitrification the Cape Fear River as shown in the long-term
BOD results of the receiving water samples (Figure 5).
It should be noted that ammonia is added as a nutrient supplement to enhance performance
of the biological system. Thus, the addition of nitrogen can be control to eliminate or
minimize nitrification in the effluent.
7

50
45-
40-
,.
35-
i 30
E 25 -
0 20'
ao
15-
10-
5-
Nitrite/Nitrate (mg/L)
0
20
18
16
14
12
10
8
6
4
2-
p 1 1 I 1 1
0 10 20 30 40 50 60 70 80 90 100 110
Measured Total BOD
Calculated NBOD
-o- o
Total NBOD = 24.52 mg/L
10 20 30 40 50 60 70 80 90 100 110
Ammonium Oxidized = 5.33 mg/L
Nitrite/Nitrate Produced = 5.10 mg/L
Incubation Time (Day)
Figure 4. Long -Term Plots of BOD and Nitrite/Nitrate Data of the Effluent Sample
As shown in Figure 5, the BOD levels in the downstream samples are slightly higher than
those from the upstream sample. Lab results also show extremely low levels (almost zero) of
ammonia in the ambient river samples. The long-term BOD data from the sample collected
immediately below the treatment plant discharge are re -plotted in Figure 6.
8

12
10
• 8
as
E 6
0
m 4
U
2
Upstream of Treatment Plant
0. _" 1 . I 1 1 1 • 1 1 1 1
0 5 10 15 20 25 30 35 40 45 50 55 60
Incubation Time (Day)
12
10
..
co
8
E 6
0
m 4
U
2
Downsteam of Treatment Plant
•
- -- i./44A-
difilitib67
kity-0".
01 1 1 1 • •
0 5 10 15 20 25 30 35 40 45 50 55 60
Incubation Time (Day)
Figure 5. Data Plots of Long -Term BOD Analysis of the Receiving Water Samples
A simple regression analysis of the BOD data yielded a BOD bottle rate, ki of 0.035 day-1
(Figure 6), characterizing a slow bio-oxidation process in the receiving water and reflecting
that the treatment plant effluent is highly stabilized in terms of carbon. Such a low carbon
deoxygenation rate in the Cape Fear River is associated with a high CBODu/CBOD5 ratio of
6.23. [This ratio is needed to convert the model results in CBODu to CBOD5.]
9

Downsteam of Treatment Plant
12
10-
8-
6-
0.035 day'
2-
0 �aMIMI
0 5 10 15 20 25 30 35 40 4 : 50 5 60
•
•
NIP
Incubation Time (Day 4 vt144/1/
Figure 6. Regression Analysis of the BOD Data for Sample at Immedi - ely Below the Discharge
Gr-lam nip
BOD/DO Model Results
rodmme4if
The calibrated mass transport model was then used to simulate the BOD and DO
concentrations in the river under both flow conditions. Based on the BOD data analysis, a
deoxygenation rate of 0.035 day-1 was used in the modeling analysis. While the long-term
BOD data of the ambient river samples show insignificant nitrification, a small nitrification
rate of 0.05 day-1 (as a conservative assumption) was adopted in the modeling analysis.
Figure 7 shows the model calculated vs. measured CBOD5, NBOD, and DO concentrations
under the May 2002 flow condition. Similar results for CBOD5, NBOD, and DO
concentration profiles for the September 2005 flow condition are presented in Figure 8. The
model is capable of closely reproducing the dissolved oxygen data in both surveys, thereby
calibrating and verifying the key model coefficients such as the in -stream carbon
deoxygenation and nitrification rates. Note that the depression of dissolved oxygen levels
in a classic DO sag curve is missing in both data sets - a typical dissolved oxygen profile
observed in many low flow streams these days (Lung, 2001).

10
9
0 8
E 7
6
0
00 5
0 4
3
2
1
0
—1
10
9
8
-J 7
0'
6
5
0 4
m 3
z 2
1
River Flow above WWTP = 24.9 cfs & Temp = 24.5°C
0
0
0
0 1 2 3 4
0
—1
MOP
0
1
yvtAARLA4e-
5 4464,,,4,
80° t
0 1 2 3 4 5
3 12 I
11 • -
E 10g_-____.
Saturation DO = 8.42 mg/L--------------------------c-
v, 7-
x i i--- *
---4
0 5-
-v 4- m P. &_
> 3- c « -
0 2• o
N v�
N 1- 5 a
0 0 ,
-1 0 1 2 3 4 5
Capefear River Milesriz,:dAywy .5
Legend: i Data (Average and Range: May 28-30, 2002)
Model Results
Figure 7. Model Calculated CBODu, NBOD, and DO concentrations vs. Measured for May 2002
11

10
River Flow above WWTP = 4.0 cfs & Temp = 24.5°C
9-
cp
8-
E 7-
0 6- -
0 5- -
0 4- -
3- -
1v • 2- -
,n 1-
- 1 0 1 2 3 4 5
AM
10
9-
, 8-
7_
� 6-
5-
0 4-
co 3- E
Z 2 _ `0 4°.
1- P? .S
0
- 1
2 12
11-
E 910 _ Saturation DO = 8.42 mg/L
Sorecto Br c
Mr
•
•
410
•
•
0 1 2 3 4 5
dw
•
c-
01
O 5-i_______i_---ir--
-0 4-
> 3-
5 2-
O O-
-1
----8
•
dlo
•
•
eD
r
MP
N.
AD
0 1 2 3 4 5
Capefear River Miles below Guilford Mills
Legend: § Data (Average and Range: Sept 6-8, 2005)
Model Results
Figure 8. Model Calculated CBOD5, NBOD, and DO concentrations vs. Measured for September 2005
12

Model Projection Analysis
Following the model calibration and verification analysis, a number of CBOD loading
scenarios were developed to evaluate their impacts on dissolved oxygen in the Cape Fear
River immediately below the treatment plant discharge:
1. Determine the maximum CBOD5 concentration in the effluent to meet a DO level of 5
mg/L in the Cape Fear River under the September 2005 low flow conditions,
assuming a nitrification rate of 0.05 day-1 in the river. `� J4117
2. Same as above but without nitrification in the river. r
3. Same as Scenario 1 but to meet a DO level of 4 mg/L in the Cape Fear River.
4. Same as Scenario 3 but without nitrification in the river.
5. Determine the minimum stream flow rate to be maintained to meet a DO level of 5
mg/L with a CBOD5 concentration of 20 mg/L in the effluent and nitrification in the
river.
6. Same as Scenario 6 but without nitrification in the river.
7. Same as Scenario 5 but with a CBOD5 concentration of 30 mg/L in the effluent and
nitrification in the river.
8. Same as Scenario 7 but without nitrification in the river.
A water temperature of 28°C is assumed for all the projection model runs.
Model results for the projection runs are summarized in Tables 1 and 2.
Table 1. Model Projection Results - Maximum Effluent CBOD5 Concentration
Scenario
Flow (cfs)
DO (mg/L)
Nitrification
Max. CBOD5 (mg/L)
1
4.01
5
Yes
12.5
2
4.01
5
No
17.0
3
4.01
4
Yes
24.0
4
4.01
4
No
29.4
Table 2. Model Projection Results - Minimum River Flow
i
FBess -14.«-.
Scenario
DO (mg/L)
Nitrification
Effluent CBOD5 (mg/L)
Minimum Flow (cfs)
5
5
Yes
20
5.21
6
5
No
20
4.41
7
5
Yes
30
6.25
8
5
No
30
5.62
A range of CBOD5 levels from 12.5 mg/L to 17 mg/L in the treatment plant effluent is
expected to meet the DO standard of 5 mg/L under a very low flow rate of 4 cfs in the Cape
Fear River upstream of the treatment plant discharge. This range of CBOD5 levels is
increased by approximately 12 mg/L if the DO standard is lowered to 4 mg/L in the
13

ambient water. Table 2 suggests that stream flow rates above a range from 4.41 cfs to 6.25 cfs
would be required to maintain a DO standard of 5 mg/L, depending on the CBOD5
concentration (20 mg/L or 30 mg/L) in the treatment plant effluent and the nitrification rate
in the river. Under the most conservative condition, if the stream flow rate is above 6.25 cfs,
the DO standard of 5 mg/L can be met with a 30 mg/L of CBOD5 in the plant effluent.
Current permit limits are based on the following flow conditions:
Summer Low Flow (7Q10): 6.5 cfs
Winter Low Flow: 18 cfs
Average Flow: 398 cfs
Based on these conditions and the model results, CBOD5 as high as 30 mg/L can be
assimilated while maintaining 5 mg/L or upstream DO conditions. Tertiary limits appear to
be required only under flow conditions below the 7Q10 flow.
References
Grant, R.S, 1976. Reaeration Coefficient Measurements of Ten Small Streams in Wisconsin
Using Radioactive Tracers. U.S. Geological Survey Water Resources Investigations, pp.76-
79.
Lung, W.S. 2001. Water Quality Modeling for Wasteload Allocations and TMDLs, John
Wiley & Sons, New York, NY, 333p.
Lung, W.S. and Sobeck, R.G., 1999. Renewed Use of BOD/DO Models in Water Quality
Management. Journal of Water Resources Planning and Managernent,125(4):222 227.
Tsivoglou, E.C. and Neal, L.A., 1976. Tracer Measurements of Reaeration: III. Predicting the
Reaeration Capacity of Inland Streams. Journal of Water Pollution Control and Federation,
48(12): 2669-2689.
14

APPENDIX - The STREAM Model Formulation
Under steady-state conditions, a 1-D BOD/DO STREAM model includes the following
equations (Lung, 2001):
where
I x
D — KDL° (e-KrU—e-Kau) CBOD (1a)
Ka — Kr
+ K" N° (e —K„ U — e —Ka U) NBOD (lb)
Ka —K"
—Ka —
+ D°e U Initial DO Deficit (1c)
P -Ka x
— K (1— e U) Algal Photosynthesis (1d)
Ka
-K, x
+ R (1— a U) Algal Respiration (1e)
Ka
—KQ x
+ SOD (1— e U) Sediment Oxygen Demand (1f)
HKa
D = Dissolved oxygen deficit (mg/L)
KD = In -stream CBOD deoxygenation rate (day-1)
L° = Initial stream CBOD concentration below the wastewater outfall (mg/L)
Ka = In -stream reaeration coefficient (day-1)
Kr = In -stream CBOD removal rate (day-1)
x = Stream distance downstream from the point source (mile)
LI = Average stream velocity (mile/day)
• = In -stream nitrification rate (day-1)
No = Initial stream NBOD concentration below the wastewater outfall (mg/L)
D° = Initial stream DO deficit concentration below the wastewater outfall (mg/L)
15

P = Algal photosynthesis rate (mg 02 L-1 day-')
R = Algal respiration rate (mg 02 L-1 day-')
SOD = Sediment oxygen demand (gm 02 m-2 day-1)
H = Average depth of the water column (ft)
The initial stream CBOD concentration, Lo, in Eq. la must be expressed as ultimate oxygen
demand. Because of zero -order and first -order kinetics formulated in the model, the
dissolved oxygen deficit terms due to different sources and sinks are added, i.e.,
superimposed. The dissolved oxygen concentration C may be determined from the
computed deficit using the following equation:
C=Cs-D
where CS is the saturated dissolved oxygen concentration (mg/L). The following equation is
recommended by EPA (1995 to calculate the saturated dissolved oxygen concentrations as a
function of temperature for freshwater streams:
468
C_
S 31.6+T
where T is water temperature in °C. This equation is accurate to within 0.03 mg/L
compared with the Benson -Krause equation on which, the Standard Methods tables are
based (Lung, 2001).
(2)
16