NC0020605_Wasteload Allocation_19880628

NPDES DOCUMENT SCANNIMO COVER SHEET
NPDES Permit:
NC0020605
Tarboro WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Speculative Limits
Correspondence
Re:
Instream Assessment (67B)
Environmental Assessment (EA)
Permit
History
Document Date:
June 28, 1988
Thies document is printed on reuse paper - ignore Mazy
content on the reverfte wide

I
DIVISION OF ENVIRONMENTAL MANAGEMENT
June 28, 1988
TO: Allen Wahab
FROM: Ruth Clark 'RC
THROUGH: Randy Dodd-7��>
Trevor Cleme
Steve Tedder V
SUBJECT: AT Checklist: Tarboro WWTP
NPDES No. NC0020605
Edgecombe County
A review of the water quality modeling analysis used to
develop NPDES permit limits for the Town of Tarboro is attached.
Please contact me if there are any questions.

TOWN OF TARBORO WWTP A.T. REVIEW
NPDES No. NC0020605
Edgecombe County
Model Summary
Introduction
A Level B analysis was completed in 1987 to determine efflu-
ent limits for the Town of Tarboro's WWTP. The facility dis-
charges to the Tar River, a class C stream. At the outfall, the
receiving stream drains an area of 2180 square miles and is char-
acterized by an average flow of 2230 cfs, a summer 7Q10 of 90 cfs,
and a winter 7Q10 of 210 cfs.
The facility is currently designed at 3.0 MGD (78% domestic,
22% industrial) and will be expanded to 5.0 MGD (87% domestic, 13%
industrial). The Level B model was run to determine effluent lim-
its at both flows. The resulting limits were:
3.0 MGD
3.0 MGD
5.0 MGD
5.0 MGD
Summer
Winter
Summer
Winter
BODS (mg/1)
30
30
30
30
NH3-N (mg/1)
--
--
9
18
DO (mg/1)
--
--
5
5
TSS (mg/1)
30
30
30
30
Fecal Coliform (/100 ml)
1000
1000
1000
1000
pH (SU)
6-9
6-9
6-9
6-9
This A.T. Review will describe the methods used to obtain the
permit limits for the 5.0 MGD allocation.
Model Reaches
The model extended 15.3 miles below the Tarboro WWTP to SR
1400 (Figure 1). The model was divided into two reaches. The
first was 6.7 miles in length and ended at the mouth of Town
Creek. The second was 8.6 miles long and ran from Town Creek to
SR 1400.
Flow
Flow estimates were obtained from the lJSGS. Three estimates
were obtained on the Tar River, and one estimate was obtained for
Town Creek (Figure 1). The estimates were as follows:
USGS # 02.0835.0000 at Tarboro WWTP
DA 2180 mi'
Qavg 2230 cfs
7Q10g 90 cfs
7Q10w 210 cfs

iJSGS # 02.0836.4400 at NC 42
DA 2420 mi.,
Qavg 2470 cfs
7Q10, 100 cfs
7Q10 240 cfs
USGS # 02.0836.9200 at SR 1400
DA 2490 mi'
Qavg 2540 cfs
7Q1% 102 cfs
7Q10,,, 242 cfs
USGS # 02.0836.4300 at mouth of Town Creek
DA 199 mi,
Qavg 2.29 cfs
7Q10- 0.4 cfs
7Q10, 2.0 cfs
The flows and drainage area at the mouth of Town Creek were
subtracted from those at NC 42 to obtain flow and drainage area
estimates on the Tar River above the confluence with Town Creek.
These estimates were used. to calculate runoff in Reach 1, and the
results were as follows:
DA= 2221 mi'
Qavg= 2241 cfs
7Q10, = 99.6 cfs
7Q10-, 238 cfs
Runoff in Reach 1 was calculated as:
Runoff
Qavg=
(2241-2230 cfs)/6.7
mi = 1.64 cfs/mi
Runoff
7Q10-;=
(99.6-90 cfs)/6.7
mi = 1.43 cfs/mi
Runoff
7Q10w=
(238-210 cfs)/6.7
mi = 4.18 cfs/mi
Runoff in Reach 2 was calculated as:
Runoff
Qavg=
(2540-2470 cfs)/8-6 mi
= 8.1.4 cfs/mi
Runoff
7Q10,=
(102-100 cfs)/8.6 mi =
0.23 cfs/mi
Runoff
7Q10,,,=
(242-240 cfs)/8.6 mi =
0.23 cfs/mi
Town Creek was modeled as tributary flow into Reach 2.
Slopes
The slopes were measured from USGS topographical maps
(1:24000) and were estimated as 0.70 ft/mi in both reaches.
Velocity
Time of travel studies were performed in the Tar River in
1975 and 1977 at Rocky Mount approximately 32 miles upstream. The
results of these studies were used to develop a power function for
velocity. The equation was:
V=(0.0228)Q-66

The summer 7Q10 flow at the Tarboro WWTP (90 cfs) was used to det-
errn-ine the velocity in both reaches. The resulting value was
0.444 ft/sec.
Design Temperature
Data from STORET station 04150000 located at Highway 42
(Figure 1) were used to determine the summer design temperature.
Temperature values for July through October were obtained for the
years 1981 to 1987. The values were ranked from highest to low-
est, and the value at the 75th percentile was used in the model.
The value was 26°C.
CBOD Decay Rate (Kd)
A modified version of the Bosco (1966) equation was used to
estimate the CBOD decay rate. The equation is:
Kd = K,. + 'n(V/H) (at 20'C)
where n = coefficient of bed activity
exp(-2.8105 ln(slope))
exp(-2.8105 ln(0.70))
0.0486
K, = CBOD bottle decay rate
0.2 for instream CBOD <_ 50
H = depth
(Q/15V)'-5 where Q is 7Q10s
(90/(15(0.444)))O 5
3.676 ft
Kd = 0.21 /day
mg/1
and V= velocity
The value was then adjusted to reflect design temperature by
the following:
Kd(T) = Kd(200C) * 1.047`20
= 0.21 * 1.0472-6-2.0
= 0.28 /day
NBOD Decay Rate (Kn)
The EPA default value for Kn of 0.3 /day for streams with
slope less than 20 ft/mi was used. This value is for a tempera-
ture of 20°C and therefore had to be adjusted for design temper-
ature by the following:
Kn(T) = Kn(20°C) * 1.080T-"
= 0.3 ( 1. 080"-")
= 0.48
Reaeration Rate (Ka
The reaeration rate was based on the O'Connor Dobbins formula
for slow moving streams and estuaries. The formula is Ka=2/H
where H is depth in feet. The values obtained were:
Reach 1: Ka = 2/3.92 = 0.51 /day
Reach 2: Ka = 2/4.04 = 0.50 /day
These estimates were for temperatures of 20°C and were adjusted
according to the formula:

Ka(T) = Ka(20°C) * 1.022"'
to obtain 0.58 in Reach 1 and 0.57 in Reach 2.
Headwater DO
The headwater DO was obtained from temperature and DO
data for the months of July through October from 1981 to 1987 at
STORET station 64150000 (Figure 1). The saturation DO level at
each temperature was determined from the Standard Methods table at
a chlorinity level. of zero (American Public Health Association,
1985). The percent saturation was then calculated as
(DO observed)/(DO saturation value from table) for each value.
Values which were supersaturated were eliminated. The remaining
values were then used to compute the average percent saturation.
The value was 81%. The saturation DO at design temperature (8.113
mg/1) was then multiplied by 0.81 to obtain 6.57 mg/l headwater
DO. This value was also used for the tributary flow of Town
Creek.
Headwater CBOD and NBOD
STORET station 04150000 was used to obtain headwater CBOD and
NBOD values. The average CBOD value (May to September data) was
1.71 mg/l while the NBOD value was 1.16 mg/l. The DEM default
values are 2 mg/1 CBOD and 1 mg/l NBOD. Standard wasteload proce-
dure is to choose the higher value of observed and default esti-
mates; therefore, a headwater CBOD value of 2 mg/1 and an NBOD
value of 1.16 mg/1 were used in the model.
Tributary CBOD and NBOD
The flow from Town Creek was
2. The default values of 2 mg/l
and NBOD.
Winter Model
modeled as a tributary in Reach
and 1 mg/l were assigned for CBOD
All model inputs were assumed to be the same as in the summer
model with the exception of design temperature and headwater DO.
A temperature of 16°C was used based on the location of the stream
in the state. The headwater DO was computed as 90% of the satura-
tion DO at 16°C found in the Standard.Methods manual (American
Public Health Association, 1985). Kd, Kn, and Ka were the same
for 20°C, but were corrected to reflect design temperature using
the formulas cited earlier.

Model Input Summary
Headwater Conditions
7Q10.,
90
cfs
7Q1Ow
210
cfs
Qavg
2230
cfs
Sumner Temp.
26
°C
Winter Temp.
16
°C
CBOD
2
mg/1
NBOD
1.16
mg/1
DO summer
6.37
mg/l
DO winter
8.88
mg/1
Reach Conditions
S1R1
S1R2
Length (mi)
6.70
8.62.
Qw (MGD)
5.0
0
Slope (ft/mi)
0.70
0.70
7Q10y RO (cfs/mi)
1.43
0.23
7QIOw RO (cfs/mi)
1.64
0.23
Qavg RO (cfs/mi)
4.18
8.14
Trib 7Q10 (cfs)
0
0.40
Trib 7Q10w (cfs)
0
2.00
Trib Qavg (cfs)
0
229
Allocation
The summer model run resulted in limits of 30 mg/l BODs, 9
mg/l NH,-N, and 5 mg/1 DO. This corresponds to a BOD„ of 85.5
based on the assumption that CBOD = 1.5*BODS and NBOD = 4.5*NH3-N.
Division procedure does not allow BODS to exceed 30 mg/l or a
given winter limit to be greater than twice the corresponding
summer limit. Therefore, the winter model was run at 30 mg/1 BODS
(45 mg/l CBOD) and 18 mg/l NE,-N (81 mg/1 NBOD). The DO stream
standard was protected, and these limits were assigned.
CC: Jim Greenfield `

Literature Cited
American Public Health Association. 1985. Standard Methods for
the Examination of Water and Wastewater. Sixteenth Edition.
Bosko, K. 1966. Advances in Water Pollution Research. Interna-
tional Association on Water Pollution Research. Munich.

STORET Station
04150000
USGS 02.0835.0000 01
Town
Creek
USGS 02.0836.4300 —0
USGS 02.0836.4400
USGS 02.0836.9200
0.6 mi.
Hwy 64
SR 44
0.3 mi Tarboro WWTP
6.7 mi.
m
relpWIN
SR 42
SR 1400
Figure 1: Tarboro Model Schematic
Reach 1
Reach 2

--- — — mew RESULTS
Discharger :lawmen m»
Receiving Stream :+„ RIVER
------------------------------------------------
Them. co.;s 5.07 mgzc
» e End qoo Is 2.B4 mzl.
The End moo is c«a mzc
-----------------------------------
segment !
Reach I
Reach e
m m,
wno, m mnamt Reach!
5.00 11.60 z
SUMMER'
am+e
SAVED AS iPmwoowrq»m c
--------------------
m
,I -A
nmo
eo
Waste Flow
imuw,
(mg/1)
w,
0.00
0.
0.0 00

* MODEL SUMMARY DATA ***
Discharger . TARBORO WWTF Sut,!_anin : O3O303
Receiving Steam : TAR RI'VE.R Stream Class: C
Summer 7010 . 90. Winter 7010 : 210.
Design Temperature: 26. - SumAr
1LENGTW SLOPE! 4ELOCIT'i ; DEPTH: Kd , Kd , K3 , A , KN , KN 1 KNR 1 KNR
mile : tt/mil fps , ft !d2sigri{ 3201 i eslgri d20' Idesiga! 3201 ;design; 3201 ,
_____________________________________________________________________________________________________
3egaent ! 0.70' 0.70: 0.444 332 , 0.23 1 021 1 0.58 , v5!l M48 I W30 I p.43 1 MOO ,
-----------------------------------------------------------------------------------------------------
SegRent 1 8.521 0.70! 0.444 434 1 013 1 0.21 1007 1 ME: 1t,48 1000 10.48 I MOO
Read, 2
_____________________________________________________________________________________________________
F Iow
CDOI:
MOOD
cfs 1
mgil 1
mgtl ,
Segment 1.
Reach 1.
Waste
1 7.750 50
45.000 1
40.000 ,
Tributary
0..000 ,
2.��'00 ,
1.000 ,
.. Runoff �
1 1.430 ,
2.000 ,
1 . 1.6 ,
Segment. I Reach 2
Runoff flow is in Cfs/mile
5. 000
6. 570
6.570
6.57
1.160 1 6.570

WINTER
WINTER
SAVED ON LBDATA
11
---------- MODEL RESULTS
�
----------
Discharger
;
TARBORn WHTP
Receiving Stream
�-------------------------
:
TAR RIVER
w
The End D.O.
is 6.86
--------------------------------------------
my/l.
The End C8OD
is 3.51
mg/l.
The End NGOD
______________________________________________________________________
is 4.30
mg/l.
WLA WLA
WLA
DO rlin
CGOD NBOD
CO
Waste Flow
(mg/1)
Milepnint Reach #
(mg/1) (mg/1)
____ ____
(mg/l}
__ '
(mgd)
... .... ... _______
segment l
6.86
_______ _______
15.20 2
Reach 1
45.00 81.00
5.03
5,00000
Reach 2
0.00 0.00
0.00
().00000

> MODEL SUMMARY DATA >
Discharger : TARBORO WWTP Subbasin : moma
Receiving stream :!m RIVER Stream Class: c
Sumer Sum :90. maw ,mo:aw.
Design Tempme !c-JgW
;wSLOPE ee:we:e;ura;KIN ; KIT :m;m;
:m.:;Gu m ; e :wm: ». »ono m. »uw a. »uw m1;
-----------------------------------------------------------------------------------------------------
m !1 z/ c/ 0.444 c,.wl;gl. 6/0.e10-30.0.2210-00-----------------------------------------------------------------------------------------------------
w ,.z «/ «w .4,04.0.18.0. L!0.46 0/0.e;0.3010.e.0.001
h 2 1
-- -- - -- - - - - - --- - -- - -- -- -- - -- - -- - -
1 Flaw 1 c m ; Sum ;
1 cfs 1 G 3 ; q w ;
Segment IReach !
Waste 1 7.750 1 45.000:y Su001
Headwaters! ;mmo:e.i,: 1.601
Tributary 1 0.0 ; xom ; cem 1
* Runoff ; «am : 2.000 ; 1.160
Segment IReach e
Waste ; 0.000 1
Tributary 1 woo;
* Runoff ; 0.230 ;
Sum ;
GZI :
m Sun; 1.160 ; 8.890
m mo: 1.160 1 m qo
* Runoff mew is incfw,ce

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