NC0026271_Regional Office Historical File Pre 2018 (123)

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DIVISION OF ENVIRONMENTAL MANAGEMENT.
June,6, 1996 N.C. DEPT. OF
ENVIRONMENT, HEALTH,
& NATURAL RESOURCES
MEMORANDUM
JUN 10 1996
M. D. Rex GleaROM: Jacquelyn M. NoDIVISION OF ENY?80t1':tENTAI 41AId,16EtAENT
MOO�RESYttLE RE61ORAL OFFICE
TB RU: Donald L. Si;;
Ruth SwanekaC.
Carla Sanderson
SUBJECT: Instream Assessment for SOC 67(b) Request
Taylorsville WWTP (EMC WQ # 96-12) ,
NPDES Permit No. NCO026271
Alexander County
Summary and Recommendation
The Instream Assessment Unit has reviewed the request for an instream assessment
for the Taylorsville WWTP. The Town is requesting a Special Order of Consent (SOC.)
during improvements to the wastewater treatment plant. The existing design flow is 0.430
MGD, and there are future plans.for expansion to 0.830 MGD. The total requested SOC
flow is 170,000 GPD, which includes excess inflow/infiltration (IM occurring at the plant
for a total SOC flow requested -limit of 0%600 MGD. The SOC limits requested are BOD5
= 45 mg/1 and TSS of 45 mg/l.
The results -of the Level B'model analysis indicated that according to the EMC 67
(b) criteria, the predicted. dissolved oxygen concentration in the Lower Little River was not
significantly affected by the. additional flow. The EMC 67 (b) criteria, which states that the
discharge will not decrease the DO minimum by more than 0.5 mg/1 or will not extend the
stream lengthof the DO minimum below the standard by more than OS miles, was .not
violated. with the addition of 170,000 GPD. ;;Our analysis determined that the facility can
accept the additional flow under the requested, SOC -limits.
Background
The Town of Taylorsville:discharges into:the Lower Little River in the Catawba
River Basin and has a stream classification of C. The summer.7Q10 flow has been
estimated using USGS regression. equations tobe 14 cfs with an average flow of 88 cfs,..
A review of compliance evaluation data shows that -from January 1995 to February
1996, Taylorsville has had:.six contraventions of -the flow limit, three contraventions of the
BOD5 limits and one of the TSS -limit. This seems.to be a continuation of historical
problems the plant has had meeting these parameters. The yearly average wasteflow of the
plant is OA315 MGD, Which. exceeds the.design-capacity.

Instream Assessment for Spruce Pine SOC
page 2
Analvsis and Discussion
An instream assessment was performed using the Level B model framework The
Carolina Glove Co. (0.015 MGD, BOD5 = 30 mg/1), is located 1 mile upstream of the
Taylorsville WWTP and was included at the beginning of the .model. The current design
flow of the Taylorsville WWTP is 0.430 MGD, however the yearly average now of
0.4315 was used as the pre-SOC flow. The post SOC flow is 0.6015 MGD, which allows
for 170,000 GPD of I/L The SOC limits recommended at 0.6015 MGD are 45 mg/l of
BOD5 and 45 mg/l of T.SS.
Two models were run at the flows of 0.4315 and 0.6015 MGD. Waste inputs into
the models for the Taylorsville plant included 90 mg/l CBOD (45 mg/1 * 2.0) and NBOD of
80.1 mg/1(existing NH3 limit of 17.8 mg/l * 4.5). The model results predict the DO
minimum of 7.43 mg/1 to occur at the Carolina Glove outfall. However, the difference of
predicted DO at the Taylorsville. outfall at the SOC flows were 7.76 and 7.61 mg/l,
respectively. This is a difference of 0.15 mg/1 in -the instream DO levels and does not
demonstrate a significant depression of the instream DO level .per 67(b) criteria.
Recommendation
Based on the model results, the Instream Assessment Unit recommends approval
of the 170,000 GPD of additional wasteflow for the Taylorsville WWTP and the SOC
limits of 45 mg/l of BOD5 and 45 mg/l of TSS. All other permitted limitations, and
effluent and instream monitoring requirements shall remain the -same as in the current
NPDES permit.
cc: Dianne Wilburn
Central Files
WLA File

Table 1. Instream Assessment Summary for the Town of Taylorsville
Design Capacity
0.4300 MGD
Pre-SOC Flow
0.4315 MGD
SOC Flows Requested
0.1700 MGD
Pre-SOC + SOC Flows
0.6015 MGD
Model Input Summary
Headwater conditions:
s7Q10 flow
14 cfs
w7Q10 flow
25 cfs
Average flow
88 cfs
Design Temperature
25 °C
CBOD
2.0 mg/l
NBOD
1.0 mg/l
DO (90% saturation)
7.44 mg/l
Wastewater Inputs:
Pre- SOC Wasteflow 0.4315 MGD
Requested SOC Wasteflow(s) 0.1700 -MGD
Recommended SOC Wasteflow(s) Q.6015 MGD
CBOD '90 mg/l
NBOD 8.0.1 mg/l
Model Output Summary
Qw DO Net Distance DO Net
(MGD) min. Change <5.0 mg/l Change
(mg/1) (mg/l) (mi.) (mi.)
0.4315 7.76 NA 0.0 NA
0.6015 7.61 0.15 0.0 0.0