NC0026441_Instream Assessment_19880523

NPDES DOCUWENT SCANNIMO COVER SHEET
NPDES Permit:
NCO026441
Siler City WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Engineering Alternatives Analysis
201 Facilities Plan
I�tream Assessment
Correspondence re:
Permit History
Date Range:
Document Date:
May 23, 1988
THIS DOCUMENT IS PRINTED ON REUSE PAPER - ICNORE ANY CONTENT ON THE
REVERSE SIDE

DIVISION OF ENVIRONMENTAL MANAGEMENT
May23, 1988
MEMORANDUM
TO: Robert Van Tilburg
THRU: David Vogt
Trevor Clement
Tedder
Steve Tedder
FROM: Michael Scoville 115
SUBJECT: Instream Assessment for the Town of Siler City WWTP
NPDES No. NCO026441
Chatham County
Summary
.The -Town of Siler.`City has requested a JOG to increase their winter -waste
flow by 0.15 MGD. Siler-Cfty has had two SOCs in the past with granted add'
. tiona1;fIow;totaling-0.034'185-MGD-::- This-JOChas been '.requested because='.the--.'., --
facility is unable to meet. the final effluent limitations for flow. BOD5, and pH ..
contained.,in -their NPDES permit. ..The Region wishes-to'give the facility inter _
limits while it improves it's treatment capabilities by adding equalization and
pH stabilization. Repair of sewer lines to eliminate-inf-iltration and inflow
.will also.be required to reduce unnecessary flow.
A Level-B analysis was performed to evaluate the impact of the requested
wintertime flow increase. The analysis included the comparison of three waste -
flows and their effects on water quality: the average flow prior to the SOCs
(1.372el MGD), the design flow (1.800 MGD), and the design flow plus the
requested additional 0.15 MGD (1.950 MGD). The results of the analysis indicate
that the additional flow will have no additional impact on the quality of the
receiving waters. In fact, at greater discharges, the model predic,rt�s,ara�Pa,gher
minimum DO concentration due to predicted increased channel veloc,i`ty,tand--r•c?¢eY.a-
.n.,'. is ti� .
tion. Since the greater flows are not predicted to have a negat,i;ye impact
the water quality, the EMC criteria is thereby met, and there 1s no"barbs;for
denying the JOC for u to 1.950 MGD wintertime wasteflow.
1. �Q
Y � 9 P yz
Background
.
The most recent wasteload allocation for Siler City, completed-�:`n.;Augus;t`�e
1986, was modeled based on ultimate 130D (BODu) rather than on the separate com-
ponents of CBOD and NBOD. A BODu of 110 mg/1 was determined to be the limit for

this facility, and at that time was generally considered to represent secondary
treatment. Now that CBOD and-NBOD"are calculated -separately, however-, i-t is
evident that a BODu-of 110 mg/1 is less than secondary limits.
The new Level-9 model was used to perform this analysis. Siler City's cur-
rent BOD5 limit of 30 mg/I was used i-. the model. For NH3-N, the assumed sec-
ondary limit of 20 mg/1 was not used because in the past the facility has not
discharged NH3-N at this concentration. A value of 17.0 mg/1 was used, repre-
senting the highest monthly NH3-N average between 4/07 and 3/ee. Since the.
effluent at this facility is 25% industrial, a BOD5 multiplier of 2.0 was
applied. The NH3-N multiplier is 4.5. The wasteflow CBOD and NBOD input to the
model, then, were 60 mg/1 and 76.5 mg/l, respectively.
Other modifications of this model include it's extension to a total distance
of 10.09 miles and the calculation of new runoff values. In this analysis the
minimum DO was predicted to occur further downstream than in the previous anaiy-
sis, and the predicted DO concentration was less than 5.0 mg/1 for 3.75 miles.
Thus, a downstream extension of the model's reach was necessary. Also, runoff
values, previously set equal to zero according to USGS estimates, were recalcu-
lated using yields (cfs/mi sq) derived from drainage areas at different. point -
along the Rocky River. The winter 7010 runoff was estimated to be 0.048 rfs/mi.
The runoff was assumed to be the same for the entire section of the Rocky Rive,
that was included in -this analysis. .
The headwater char-acter-istics ar.e=; 1'i'=_,ti ed''in. T.ab l:e-;. TreL model-wi-iih:
these headwater characteristics'an& the* -•above inputs while. varyino_the.facii-
ity's wasteflow. A1'1 other model. par.'ameters".were kept.
evaluated are discussed in the Summary,sectYan. - -
Results and Discussion
The results of this analysis indicate.that an increase of wasteflow from the
Town of Siler City WWTP will not have any additional impact on the water cualitp
of Loves Creek and Rocky River. The greater flows are expected to increase
velocity and reaeration and actually -lessen the extent of the DO sag. 'In accor-
dance with EMC criteria, there is no basisfordenying the JOC.
At the average pre-SOC flow of 1.3728 MGD, the model predicted a minimum DO
of 3.06 mg/1 and a 3.75 mile section of the Rocky River with a DO less than 5.0
mg/1. The design flow (1.800 MGD) produced a minimum DO of 3.23 mg/1 and a
river section with DO less than 5.0 mg/1 that lasts .for 3.70 miles. �P�otin
imum with a wasteflow of 1.950 MGD is predicted to be 3.37 mg/l, old tie a
of the river predicted to have a substandard DO is 3.75. miles loff:rf R S!J�r to
Table I for a summary of model output.
Recommendations
As a result of t�-,is
Town of Siler City WWI"P
of 1.95 MGD is an 8.3/..
greater than the desian
a nemd J
alys:s it is rcomende_':a� .':e -,. - �t Itcm the`
not be Permitted to exceed !.95 MGD.A was't
increase of the plant's design capacity. Any
capacity of the plant could possibly cause a
oiscnarge
reduction
w

of plant efficiency and the failure of the facility to meet it's effluent lim-
The Town of Siler City is currently operating under less stringent limits
than would ordinarily be recommended. This analysis has shown that the impact
of this facility on the local surface waters may be greeter than it was first
thought to be. The degradation of water quality in Loves Creek and Rocky River
as a result of this discharge is an issue that needs to be addressed. It is
recommended that a reconnaisance study be done in this area to assess the true
impact of the discharge and the extent of the water quality damage. Such a
study would also al-leviate some of the uncertainty of the model's predictions
and aid in determining if more stringent limits need to be imposed on the Town
of Siler City WWTP.
cc: George Everett
Kent Wiggins
Steve Reid
C: QS S

TABLE 1. Instream Assessment Summary for the Town of Siler City
WINTER
a..; Wasteflow Assumptions
Design Capacity
Average Wasteflow (4/87-3/88)
Total of Previous SOC wasteflows
Wasteflow for this JOC
Total Allowable Wasteflow
Mode, Input Summary
1.8000 MGD
1.4070 MGD
0.0342 MGD
0.5430 MGD
1.9500 MGD
Headwater Conditions:
Drainage Area
7.9 sq-._mi.
Winter 701.0-
0:.-4 cfs,.
-
', Q a v g
8. 7' c.f s-.
Design.-Tempera:ture_.
1.4.0_.C:..
- ..
-
,.. NBOD
DO
9.. 28- mg./-t-
Wastewater Inputs:
Flow
See Above
CBOD (2.0*Recommended
BOD5)'
60-.0 mg/1
- NBOD (4.5*Recommended
NH3-N)
76.5 mg/1
'. Dissolved Oxygen
5.0 mg/.1
Model Output- Summary
4`'
p1h•','�
1.v
Flow DO min.
Net Change
Distance
< 5.0 mg/l
`Net & ge
(MGD) (mq/1)
(m /1)
(mi)----
;'.-(mI
Pre-SOC 1.3729 3.06
NA
75
NA
Design Capacity 1.8000 3.20
0.22
3.70
-0.05
W/Current JOC 1.9500 3.37
0.31
3.75
0.00

Request Form for In -stream Assessment for JOC
Name of Faciltiy: Town of Siler. City Subbasin: 03:06:12
County: Chatham Design Flow: 1.8mgd (existing)
Receiving Stream: Loves Creek
Background Data :
A. Why is JOC needed? The Town is unable to meet
0
U
t. Additional treatment facilitles
B. History of SOC requests:
1. Monthly Average waste flow prior to any SOC?
No flow data is available prior to.EMC'WQ NO.
8310.''
Period:.8601 through 8603,.Avg ':1.4244,-'mgd'.
Period: 8604 through.8603 Avg.:.i. l *4745.: rigd.
Period: .8Z04-'through-:8803. Avg: == `l'. 4070 mgd'.
2.. Previously.approved SOC's:
EMC *WQ NO. '83 10-` •Date-,12/8/83 `�:' flow. 0.0154'
EMC WQ NO. 83-10 Ad Date: -unknown flow: 0.018785 mgd
Total of previously
- . approved: SOC flow: 0:034185' mgd "
3. Flows lost from.plant-flow: 0 mgd
(facilities off line)
4. Current JOC request flow: 0 mgd
5. Total Plant flow post-SOC
(sum of original flow and
SOC minus losses) flow: 1.441185 mgd

I
2 -
6. Is this an accurate flow balance for plant?
Why or why not? Yes. The flow is fairly
consistent with peaks in the winter months.
C. A copy of the data is attached for your review
(January 1986 through March 1987).
CURRENT SOC REQUEST:
A. Request is for domestic or industrial waste? If it is a
combination, please specify percentages. none
Domestic gpd a
Industrial gpd %
B. What type of industry? Please attach any pertinent data.
n/a
C. The region proposes.the following JOC:limits:
Winter_ .Summer
BODS 30 mg/1- - 15 mg-/1
NH31
DO:.-, 5' ''rmg/.'l: 5-,mg/1
TSS- -30 mg/l'- 30 mg/1.
Fecal_Coliform. 1000. #/100iml 1000 #1100m1
pH -'. 6.0-9.O s.u. 6:0-9.0 s'.u.'
other -parameters:- flow- :1.•95-'mgd ': _: 1.80 ingd .
D. What is the basis for these limits?
These are limits which the RRO staff believes that
the plant can meet -if properly operated and maintained,
A
y

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