NC0025453_Winter Limits_20200921

TOWN OF CLAYTON
"SERVICE" OPERATIONS CENTER "ENVIRONMENT"
ELECTRIC SERVICE CAROB PUBLIC WORKS
[phone] Si• y [phone]
VEHICLE MAINTENANCE \o- WATER RECLAMATION
[phone] !;� [phone]
RECEIVED
SEP 2 5 2020
September 21, 2020
NCDEQ/DWR/NPDES
Michael Montebello
NC DEQ, DWR
Water Quality Permitting Section
1617 Mail Service Center
Raleigh, NC 27699-1617
Re: Winter Limits,Town of Clayton NC, NPDES NC0025453
Mr. Montebello:
The Town of Clayton is submitting a request to have winter limits for BODS and Ammonia-N added to
the permit limits to the current permit modification under review by the permitting group. A speculative
limits letter was issued by DWR on September 4, 2020 that indicated that winter limits would be
considered after a modeling effort indicated that seasonal limits would be protective of water quality.
The Town and Dewberry Engineers have completed the additional river modeling and by our
interpretation water quality would remain protected in the Neuse River.The additional model runs were
submitted to NC DEQ DWR modeling group on 09/18/2020.The model runs are carried out under the
most severe flow scenarios and are very unlikely to occur when the flow in the river is controlled by Falls
dam. See attached Technical Memorandum addressing the modeling effort.
As you are aware the majority of the wastewater treatment facilities in North Carolina are of a biological
nature and experience some amount of inhibition during extended period of cold weather.The majority
of these facilities and particularly in the Neuse River have been given winter limits in their NPDES
permits. The nitrification process is especially affected when temperatures drop into the lower teens
(C°). In order to account for this inhibition when appropriate, regulatory agencies on the federal and
state level have issued winter limits to prevent nuisance compliance issues.
Also in support of our request the following facts are indicative of water bodies in our temperature
zone. During the winter months the ambient dissolved oxygen levels in the river are higher and increase
the assimilative capacity of the water body. Lower temperatures also inhibit the growth of algae and
therefore reducing oxygen depletion in the river.
653 Highway 42 West•P.O.Box 879•Clayton,North Carolina 27520•[phone]•Fax[phone]

As requested by the permitting group, please see the attached graph showing the monthly averages for
the Little Creek WRF 2017-2020.The data shows the effects of the cooler temperatures in the ammonia
removal efficiency of the current facility.
The Town of Clayton requests NC DEQ DWR's assistance in this matter as we are currently on tight
schedule to have this facility operational in 2023 to meet our residential and industrial needs. Please
contact me at 919-553-1536 if you have any questions.
Sincerely )
ames Warren
Wastewater Operations Superintendent
Town of Clayton, NC

Dewberry.
MEMORANDUM
Date: September 18, 2020
To: Adugna Kebede- DWR, NC DEQ
Pam Behm- DWR, NC DEQ
From: Laura Crisman, P.E.- Dewberry
James Warren-Town of Clayton
Subject:
Neuse River QUAL2K Model: Falls Dam to Goldsboro, Winter Limits
For Little Creek WRF Expansion
[Addendum to Neuse River QUAL2K Final Model Report, 8/17/2020]
MEMORANDUM
Introduction
The Neuse River QUAL2K model was developed to support the Little Creek Water Reclamation Facility (WRF)
expansion for handling future industrial and residential loads. NC DEQ has issued speculative NPDES limits for the
WRF expansion based on the modeling results submitted August 17,2020.The speculative limits indicated additional
modeling was necessary to evaluate inclusion of seasonal, winter limits for BOD5 and NH3-N, both of which are
included in the current NPDES permit. This memo summarizes additional Neuse River QUAL2K models that were
developed to evaluate winter NPDES limits for Little Creek WRF's expansion from 2.5 MGD to 6 MGD and 10 MGD.
Approach
Four additional QUAL2K models were developed to evaluate the potential impact of Little Creek WRF's expansion on
the Neuse River under winter conditions and winter NPDES permit limits. The winter Neuse River QUAL2K models
discussed in this memo were developed from the calibrated QUAL2K models previously submitted.A summary of the
four Neuse River winter models is presented in the table below:
No. Model Description Little Creek WRF Discharge Neuse River Model Inputs
1 Low Flow,Winter: Current NPDES flow at 2.5 MGD, • NPDES discharge at winter,daily max limits
Current WRF • Tributary water quality(temperature,DO,
Current NPDES daily max,winter permit limits
phosphorus and nitrogen species)estimated
2 Low Flow,Winter: Future flow at 6 MGD from winter AMS data and winter NPDES
Expansion WRF daily max limits to tributaries.
6 MGD Current NPDES daily max,winter permit limits
3 Low Flow,Winter: Future flow at 10 MGD
Expansion WRF
10 MGD Current NPDES daily max,winter permit limits
4 Low Flow,Winter: Future flow at 10 MGD • NPDES discharge at winter,monthly average
limits
Expansion WRF Current NPDES monthly average,winter permit
10 MGD limits • Tributary water quality estimated from winter
AMS data and winter NPDES monthly
average limits to tributaries.
Neuse River QUAL2K Model:Winter Limits Memorandum I 1 of 5

it
I Dewberry.
MEMORANDUM
Model Inputs
The QUAL2K modeled geography and hydraulic inputs reflect those of the previously submitted models. Descriptions
of inputs to the winter models are below:
1. Low(7Q10)flow at the headwater was used in the winter models as Falls Dam typically regulates to a lower
flow during winter.' The 7010 flow was also used in the summer models as a conservative model of drought
conditions. Thus, flow estimates in the winter models are the same as in the calibrated, low flow summer
models.The flows for tributaries and diffuse sources were calibrated using a December 2012 model, thus, it
is expected this is a good estimate of low,winter flow conditions in the Neuse River.
2. Water quality parameters(temperature, DO, nitrogen species and phosphorus)of the headwater and the 25
tributaries were derived from ambient monitoring station(AMS)data and NPDES dischargers to the tributaries.
Specifically,AMS data along the river was determined for November-March for the last four years of available
data,within the past ten years(attached to this memo).Where tributaries have NPDES dischargers,the water
quality of the tributary was estimated by a flow-weighted average of the nearest AMS and the NPDES permit
limits.
3. Sediment oxygen demand(SOD), ammonia and phosphorus sediment flux were estimated from data used in
the previously submitted models(EPA Project 18-0362).The field values were temperature corrected for the
winter models using a Van't Hoff Equation and an average winter water temperature at the nearest AMS,using
3-4 years of data in the past ten years. There is little data on winter SOD rates in the Neuse River. Some
studies have indicated that SOD rates may be three times higher in the summer compared to winter (Eric
Akomeah,2017).This is in line with the temperature corrected estimate of winter SOD rates compared to the
summer model estimate.
4. Rate parameters in the water column (e.g. reaeration rate, BOD oxidation, nitrification, denitrification, algae
growth and respiration)were estimated as the same inputs as previously developed calibrated low flow model
rate inputs. Note:the rate values are not the same in the winter model due to the lower temperature.QUAL2K
contains an internal Arrhenius correction for temperature for the first order rate reactions.
5. Ambient air temperature was estimated from winter weather data at the Raleigh RDU weather station. The
dew point was estimated at 80% relative humidity.
6. Wind speed and cloud cover were conservatively assumed to be zero and the calibrated inputs for shade
along the river were used in the winter model(45-60%shade).
7. Temperature input for diffuse sources used AMS data from the nearest station,estimating temperature as 95th
percentile temperature of winter data at the nearest AMS.The DO for diffuse sources in the winter is estimated
at 8.5 mg/L(90%of DO saturation at 18C).
8. NPDES point sources used winter permit limits (daily max for three models and monthly average for one
model) for flow and water quality inputs. For temperature and other water quality inputs that do not have
specific NPDES permit limits,discharge monitoring report data was used from the winter, low flow calibration
model.
I U.S.Army Corps of Engineers,in agreement with the Town of Smithfield,is required to release a minimum of 100 cfs(64.6 MGD)
from the dam during the summer months(April-October)and 65 cfs(42 MGD)during winter months(November-March).
Neuse River QUAL2K Model:Winter Limits Memorandum 12 of 5

1
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Dewberry.
Y
MEMORANDUM
Complete details on the model approach, geography, hydraulic and kinetic inputs may be found in the previously
submitted report.All inputs and notes specific to the winter models may be found in the QUAL2K model files submitted
with this memorandum.
Calibration
Details of model calibration may be found in the previous report, specifically for the Dec 2012 flow calibration model.
Results and Discussion
Figure 1 contains a summary graph of DO results for the four 7Q10,winter models of the Neuse River(attached).
The general shape of the DO curve from previous Neuse River models is also observed in these winter model DO
profile results. DO is increasing at the location of Little Creek WRF; in fact the models predict an increase in DO from
the WRF expansion for about 3 km downstream of the discharge.Additionally, as in previous model results,the winter
models do not predict a discemable,characteristic DO drop due to the Little Creek WRF.Again, likely due to effects of
overriding discharge and withdrawals modeled near the discharge that dwarf the impact of Little Creek WRF near the
point of discharge.The maximum difference in DO prediction with the expansion is observed 75 km downstream of the
discharge:where the DO is predicted to be 4.55 mg/L in the 2.5 MGD model;4.45 mg/L in the 6 MGD model;and 4.34
mg/L in the 10 MGD model.These model predictions assume all the point sources between Falls Dam and Goldsboro
discharge at their winter, daily maximum NPDES limits and that the river is at 7Q10 low flow, which is a deliberately
conservative scenario with highest potential for maximum DO suppression. At these conditions, the QUAL2K model
predicts the DO is above the minimum instantaneous in stream water quality standard of 4.0 mg/L.
Interestingly,the winter model of daily maximum NPDES limits results in a lower absolute minimum DO(4.34 mg/L, 10
MGD model)compared to the previously run, higher temperature model (4.92 mg/L, 10 MGD model). One generally
expects a higher DO profile during lower temperature, winter months in a waterway. However, the Neuse River has
dam-controlled flow, thus the low flow conditions may, and often do, occur during the winter months. Thus, it is due to
the increased nutrient load from the winter NPDES limits that QUAL2K predicts minimum DO lower in winter than in
the higher temperature model under the 7Q10 flow conditions.
Little Creek WRF is one of four major municipal WWTP's along the Neuse River between Falls Dam and Goldsboro,
and it is currently the smallest.2 The expansion to 6 MGD would bring it to the same size as the next largest WWTP,
Smith Creek RRF. The summer and winter discharge limits for flow, ammonia, phosphorus, and DO from the four
WWTPs are shown in tables below. The QUAL2K model predicts the river is impacted by the WWTPs' increased
maximum nutrient loading in the winter. Notably,WWTP nutrient removal is sensitive to low winter temperatures where
nitrification slows; thus it may be expected that at higher winter temperatures the WWTPs' actual (not modeled)
discharge may be nearer to the summer NPDES limits. In this way,the QUAL2K winter model is a worst case scenario
of highest winter temperatures,to reflect conditions where DO saturation value is lowest,and maximum winter NPDES
discharges to the River. These two conditions are not likely to occur simultaneously in the field, but are modeled as
most potentially limiting to DO in the Neuse River.
2 A total of fifteen NPDES dischargers were modeled to the Neuse River.A complete list of their summer and winter permit limits
may be found in the previous model report,as well limits for NPDES dischargers to Neuse River tributaries.
Neuse River QUAL2K Model:Winter Limits Memorandum I 3 of 5

Dewberry.
MEMORANDUM
Though the winter model predicts DO could theoretically be lower than in the high, summer temp model, it takes an
extraordinary confluence of circumstances to reflect the model conditions of 7Q10 flow, highest winter temperature,
and maximum daily discharge rates. The ambient monitoring station data appear to reflect that DO in the River in the
winter months has not been subject to these extreme conditions simultaneously: the nearest AMS to the location of
model-predicted DO low point is J5250000, where the 5th percentile of winter DO data is 8.6 mg/L (2010-2013). The
DO graph in Figure 1 includes AMS data, average winter and 5th percentile of the last 3-4 years at each AMS for
comparative purposes.
Perhaps a more realistic representation of the Neuse River is the QUAL2K winter model of monthly NPDES discharge
limits.As shown on Figure 1, the DO in this model scenario remains above 6.5 mg/L at all points downstream of Little
Creek WRF, modeled at 10 MGD, maximum expansion. This prediction is above the daily average water quality
standard of 5.0 mg/L in the Neuse River for the entire model geography.
Summer
DO
Facility Name Flow BOD5, 20°C(mg/L) NH3-N(mg/L) TP(mg/L) (mg/L)
Min
Monthly Daily Monthly Daily Quarterly Daily
MGD Avg Max111 Avg Max111 Avg Avg
Smith Creek WWTP
(RRF) 6 5.0 7.5 1 3 2 5.0
Neuse River RRF 75 5.0 7.5 1 3 2 5.0
Little Creek WRF 2.5 5.0 7.5 1 3 2 6.0
Central Johnston County
Regional WWTF 13.5 5.0 7.5 1 3 2 6
Winter
DO
Facility Name Flow BOD5,20°C(mg/L) NH3-N (mg/L) TP (mg/L)
Min
Monthly Daily Monthly Daily Quarterly Daily
MGD Avg Max111 Avg Max11i Avg Avg
Smith Creek WWTP
(RRF) 6 10.0 15.0 2 6 2 5.0
Neuse River RRF 75 10.0 15.0 2 6 2 5.0
Little Creek WRF 2.5 10.0 15.0 2 6 2 6.0
Central Johnston County
Regional WWTF 13.5 10.0 15.0 2 6 2 6
Neuse River QUAL2K Model:Winter Limits Memorandum 14 of 5

111 Dewberry.
MEMORANDUM
Conclusions
The QUAL2K models predict the Neuse River DO would stay above the minimum instantaneous water quality standard
of 4.0 mg/L at conservatively critical conditions in the Neuse River(low flow, high winter temperature. and all NPDES
discharges at winter daily maximum limits).The models also predict the DO would stay above the daily average water
quality standard of 5.0 mg/L when NPDES dischargers are modeled at winter monthly average limits. Ambient
monitoring station data from winter months indicate the QUAL2K model predictions of DO are highly conservative(lower
than observed AMS data),which is as the models designed. Based on the results of the four winter models,the Neuse
River has sufficient assimilative capacity to accept the additional flow and loading from Little Creek WRF expansion
with winter NPDES limits; showing a relatively small impact between 2.5 MGD to 10 MGD discharge from the Little
Creek WRF(0.2 mg/L).
Attachments:
Figure 1. Neuse River QUAL2K Model DO Profile at 7Q10 Flow,Winter
Ambient Monitoring Station Data-Winter Monthly Summary
cc:
David Hill, NC DEQ
Mike Templeton, NC DEQ
Michael Montebello, NC DEQ
Rich Cappola,Town of Clayton
Neuse River QUAL2K Model:Winter Limits Memorandum 15 of 5

FIGURE 1 NEUSE RIVER QUAL2K MODEL DO PROFILE AT 7Q10 LOW FLOW,WINTER:
LITTLE CREEK WRF DISCHARGE AT 2.5,6,and 10 MGD
*Note 1:Only major tributaries(>0.25 MGD)and major NPDES sources are identified on the chart.More minor sources are included in the QUAL2K models.
`Note 2:NPDES point source dischargers are modeled at summer maximum daily discharge limits,unless otherwise noted.
12.00
•LCWRF 2.5 MGD-winter model
•
•LCWRF 6 MGD-winter model
• • •
•
• • • LCWRF 10 MGD-winter model
• 10.00
•
• • • •Winter AMS Data:5th Percentile
•
• ` •Winter AMS Data:Average
•
• •LCWRF 10MGD; 800 J
• no
NPDES sources @ Monthly Limits E
••••A®i+r► • O
•••••••• ic
6.00
orisor
In
Crabtree Creek
Little Creek WWTP(NPDES)
i Neuse River WWTP(NPDES) -___ - ohnston County WTP Lee Steam Electric Plant(NPDES) 4.00
Smith Creek WWTP
i I Smithfield WTP
Poplar Creek
Smith Creek Harris Creek i Central Johnston County Regional
Poplar Branch WWTF(NPDES) Goldsboro WRF(NPDES)
_ _ _ __ l _ __ �7 2.00
f I Mocassin Creek; jJ I
I Marks Cree ``` Swift Cree, I I
Walnut Creek /".
11\11 Falling Creek
{I!
Polecat Branch
/ Black Creek '' 1 little River3i Mill Creek _
0.00
180.00 170.00 160.00 150.00 140.00 130.00 120.00 110.00 100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00
Neuse River Location-Distance from Model Endpoint in Goldsboro(km)
Dewberry Neuse QUAL2K Model-Winter 9/17/2020

Little Creek WRF,Clayton NC
NC0025453
Effluent Nh3-N BOD
Minimum Monthly Monthly Effluent Ammonia/Temperature
Temperature AVG Average 30
°C mg/L
Jan-17 14 0.04 2.28
Feb-17 16 0.05 2.47
Mar-17 14 0.29 2.59
Apr-17 18 0.53 5.19
May-17 20 0.51 2.73
Jun-17 22 0.17 2.00
Jul-17 24 0.17 2.45
Aug-17 24 0.41 2.00
Sep-17 24 0.22 2.00
Oct-17 19 0.03 2.00 20
Nov-17 18 0.19 2.00
Dec-17 15 0.58 2.00
Jan-18 12 1.91 2.00
Feb-18 15 0.62 2.04 v
Mar-18 15 0.12 2.69
Apr-18 15 0.36 2.31 a
May-18 19 0.06 2.00
Jun-18 23 0.2 2.00
Jul-18 24 0.02 2.00
Aug-18 25 0 2.00
Sep-18 26 0.01 2.00 1p
Oct-18 20 0 2.00
Nov-18 17 0.14 2.00
Dec-18 15 0.03 2.00 •
Jan-19 14 0.53 2.08
Feb-19 15 0.01 2.00
Mar-19 15 1.12 2.74 5
Apr-19 15 0.66 1.05
May-19 21 0.26 2.84
Jun-19 24 0.1 4.21
Jul-19 25 0 2
Aug-19 25 0.03 2 a 1__1- . . 1. ■,..., ._,.i,_ I.
Sep-19 23 0.01 2 n r n n n n n n n n r. n ro ro co ro ro m m m m ro m 4 4 4 4 2 2 2
.� 4' 4' .m1 4' ti 4 mi ti mi 2 n 2 N N
Oct-19 21 0.16 2 . LL ` n > - °° 6. 6 S. w a ` $ > i m N 6 'o w 6 6. i C m 0. 6 'o .o ` a >
Nov-19 18 0.49 2
g a g 2. . 9{ 0 2 ❑ 2 g a g ., 0 2 ❑ g a g 2. . Y o 2 ❑ ,2 g a g
Dec-19 15 0.4 2 Axis Title
Jan-20 14 0.07 2 ■Effluent Minimum Temperature°C ■Nh3-N Monthly AVG mg/L
Feb-20 13 0.51 2
Mar-20 17 0.1 2.43
Apr-20 18 0.15 3.22
May-20 19 0.09 2.06
Jun-20 22 0.02