NC0020354_Instream Assessment_19880812

NPDES DOCYHENT SCANNING COVER SHEET
NPDES Permit:
NC0020354
Pittsboro WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
Approval
LInstream
Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
August 12, 1988
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content on the reYerse side

MEMORANDUM
TO:
THRU:
DIVISION OF ENVIRONMENTAL MANAGEMENT
Robert Van Tilburg
David Vogt l�
Trevor Clejn
Steve Tedder
August 12, 1988
FROM: Mike Scoville AVS -
SUBJECT: Instream Assessment for the Town of Pittsboro WWTP
NPDES No. NC0020354
Chatham County
Summary
The Town of Pittsboro has requested a first-time SOC to increase their waste -
flow during the upgrade of their facility. The SOC has been requested because
the average monthly wasteflow from the WWTP has exceeded its NPDES permit limita-
tions for six of the last twelve months. The plant is currently on moratorium
for additional flow. Large amounts of infiltration and inflow (I&I) have been
experienced during rainfall events, leading to excessive flows in the winter and
spring months. i&I problems should be reduced during the upgrade through the
sewer renovation program required by the SOC. The facility is permitted for a
design capacity of 0.25 MGD and is expanding to 0.75 MGD. The SOC request is for
a 0.1224 MGD increase of domestic wastewater flow that warrants the facility
expansion.
Pittsboro discharges to Roberson Creek, a stream with a zero 7Q10. There-
fore, the impact of additional wastewater can not be reliably predicted since the
model results for zero flow streams are dependent on effluent concentration only
(i.e. identical results are produced for all wasteflows with the same wastewater
characteristics). Accordingly, no recommendation of SOC approval or denial can
be made by Technical Support. The assessment of instream impacts from any addi-
tional wastewater with the proposed limits will have to be based on best profes-
sional judgement.
A Level-B analysis was performed, however, to determine if the proposed waste
concentration; is likely to impact the aquatic biota of the receiving stream.
The results of the analysis indicated that the EPA suggested lower threshold of
3.0 mg/1 DO for fish propagation and survival would be violated at the waste con-
centration recommended in the SOC. However, at reduced effluent limits (B0D5=8
mg/1, NH3-N=2 mg/1> it is not expected that the 3.0 mg/1 threshold will be viol-
ated. A change to seasonal limits is recommended as a result of this analysis.
Toxic impacts were not assessed since the additional wastewater is of domestic
composition.

Analysis and Discussion ;'
The Pittsboro WWTP discharges into Roberson Creek (Class C) about 6.75 miles
from it's mouth at Jordan Lake. At the point of discharge, the average flow is
11.0 cfs, the summer 7Q10 is zero, and the 3002 is positive. Since the discharge
is greater than 0.1 MGD, it is assumed that the facility has a continuous dis-
charge that results in constant flow instream. Therefore, the model's steady-
state assumption should not be violated. Also, because the stream is being
modeled with no natural flow, the predicted impact to the stream is expected to
be approximately equal for different volumes of wastewater with equal waste con-
centrations.
The instream assessment request from the Raleigh Regional Office suggests
using the average flow for the past 12 months to arrive at an interim flow limit
for this facility. However, it appears inappropriate to use a 12 month average
when the infiltration and inflow problems are clearly seasonal. During the hot,
dry part of the year that is most critical to water quality (July - September),
infiltration and inflow are negligible. During the wet winter season, however,
the problem is somewhat severe. It is necessary to consider the two seasons sep-
arately and to evaluate the different conditions with seasonal models. Separate
sets of limits should be given for summer and winter based on past facility per-
formance, past monthly average flows, and the degree of impact to Roberson Creek.
The maximum wasteflow discharged by the Pittsboro WWTP during the summer
months (April - October) was 0.3694 MGD in April 1987. While the maximum
observed wasteflow provides a useful basis for interim flow limits by accounting
for excessive infiltration and inflow, in determining the effects of additional
waste on Roberson Creek the maximum flow should be compared with the average dis-
charge which occurs during critical low flow periods. The average discharge of
the Pittsboro WWTP from July through September for the last three summers is
0.2025 MGD. For the winter months (November - March) the maximum discharge was
0.4042 MGD in November 1985. The average winter discharge for the last three
winters is 0.3139 MGD.
Level-B analyses were performed to evaluate impacts under maximum and average
wasteflow conditions. These analyses compared the mean July - September dis-
charge at a design temperature that represents the 75th percentile of summer
monthly average temperatures with corresponding conditions for the maximum flow
month (April, 1987). The headwater DO concentration is based on the average DO
saturation at the facility's upstream monitoring site. Refer to Table 1 for a
summary of wasteflow assumptions and model inputs.
The model was run using the limits suggested in the SOC request (BOD5=8.0
mg/1, NH3-N=4.0 mg/1, D0=6.0 mg/1). The predicted instream DO for the critical
summer months sagged to 2.7 mg/1, violating EPA criteria (3.0 for warm waters)
for minimum DO to protect for fish survival. Based on treatment performance,
however, it appears unnecessary to give an NH3-N limit of 4.0 mg/1 since the
effluent NH3-N concentration has not exceeded 1.0 mg/1 for the past 3 years. A
2.0 mg/1 NH3-N limit, as opposed to the 4.0 mg/1 recommended, results in a pre-
dicted minimum DO of 3.8 mg/1. Actual DO minimum values are expected to be
higher. Using the highest NH3-N value in the past 3 years, the model predicted a
minimum DO of 4.2 mg/1. The summer effluent limitations recommended for incorpo-
ration with the SOC are listed in Table 1. It should be noted that a recent WLA
performed for the expanded permit wasteflow of 0.75 MGD resulted in final NPDES

limits of 5 and 2 mg/1 for GODS and NH3-N, respectively.
The Level-B analysis of winter conditions predicted no DO problems and no
violations of EMC criteria. The sum of the observed maximum winter flow and the
additional 0.1224 MGD was used as the model wasteflow and accounts for substan-
tial infiltration and inflow. The design temperature is the highest consecutive
seven day average temperature for November (1985-1987). The headwater DO concen-
tration is according to the facility's upstream monitoring data. Although no
water quality problems are predicted, it is unnecessary to assign a winter NH3-N
limit more than twice the summer NH3-N limit, especially if treatment capabili-
ties are not a limiting factor. Therefore, a 4.0 mg/1 NH3-N interim wintertime
effluent limit is recommended, for which the Level-B model predicts a DO minimum
of 6.0 mg/l. Refer to Table 1 for a summary of model inputs and results.
cc:
Chuck Wakild
Kent Wiggins
Steve Reid
Bill Kreutzberger

TABLE 1. Instream Assessment Summary for the Town of Pittsboro
Wasteflow Assumptions
Design Capacity
Average Flow (6/85 - 5/88)
Maximum Flow (6/85 - 5/88)
Additional SOC Flow Requested
Average Flow + SOC Flow
Maximum Flow + SOC Flow
Summer
0.2500 MGD
0.2025 MGD
0.3694 MGD
0.1224 MGD
0.3249 MGD
0.4918 MGD
Model Input Summary
Headwater Conditions:
Winter
0.2500 MGD
0.3139 MGD
0.4042 MGD
0.1224 MGD
0.4363 MGD
0.5266 MGD
Summer Winter
Drainage Area (sq. mi.) 10.30 10.30
7Q10 (cfs) 0.0 0.4
Qavg (cfs) 11.0 11.0
Design Temperature (C) 25.0 19.0
CBOD (mg/1) 2.0 2.0
NBOD (mg/1) 1.0 1.0
DO (mg/1) 5.52 7.79
Wastewater Inputs: Summer Winter
Flow (MGD) 0.3249 0.5266
CBOD (1.5*BOD5) (mg/1) 12.0 24.0
NBOD (4.5*NH3-N) (mg/1) 9.0 18.0
DO (mg/1) 6.0 6.0
Recommended Seasonal SOC Limits Summer Winter
Flow (MGD) 0.5000 0.5500
BODS (mg/1) 8.0 16.0
NH3-N (mg/1) 2.0 4.0
DO (mg/1) 6.0 6.0
Model Output Summary
•
Average Flow (July -September)
Mean Flow + SOC Flow
Max. Winter Flow + SOC Flow
Flow
(MGD)
0.2025
0.3249
0.5266
DO min.
(mg/1)
3.8
3.g
6.00
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