NC0036277_Correspondence_19970821

WDES DOCUMENT SCAMMIM& COVER SHEET
NPDES Permit:
NC0036277
McDowell Creek WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
August 21, 1997
This document is printed on reuse paper -ignore any
content on the reverse side

AN Al(ao x2
BLACK &VEATCH
8604 Cliff Cameron Drive, Suite 164, Charlotte, North Carolina 28269, (7041 548-8461, Fox: [phone]
Charlotte -Mecklenburg Utility Department B&V Project 27461.023
McDowell Creek Wastewater Treatment Plant B&V File A
August 21, 1997
Mr. Russell S. Rozzelle
Mecklenburg County Department of Environmental Protection
700 North Tryon Street
Charlotte, North Carolina 28202
Subject: McDowell Creek Water Quality
Dear Mr. Rozzelle:
In response to a recent request from Mr. Paul Brigham of your office, we are
providing the following information to document how the water quality of
McDowell Creek is being protected relative to the wastewater treatment plant
expansion. The McDowell Creek Wastewater Treatment Plant is currently
undergoing an expansion and improvements project. This project will expand the
plant capacity from 3 to 6 MGD and provide improved treatment capabilities,
including nutrient removal.
An Environmental Assessment (EA) was prepared for the McDowell Creek
Wastewater Treatment Plant expansion and improvements project in 1994. A
Finding of No Significant Impact was issued by the North Carolina Department
of Environment, Health, and Natural Resources on November 1, 1994. The
question of how the plant expansion might affect water quality in McDowell Creek
was addressed in the EA. A copy of pages 19 and 20 which address this issue are
attached for reference.
According to Mr. Steve Bevington of the North Carolina Department of
Environmental Management (DEM), Water Quality Section, a detailed water
quality model on nutrients in the Mountain Island Lake watershed was not
completed. Prior to the current project, nitrogen and phosphorus limits had never
been applied to this facility. However, historical data on nitrogen and phosphorus
in the treatment plant effluent was available to determine the quantity of these

Mr. Russell S. Rozzelle
Page 2
B&V Project 27461.023
August 21, 1997
nutrients being discharged. Discharge limits were then established by DEM to
ensure that the maximum quantity of these nutrients would not exceed what had
historically been discharged. For phosphorus in particular, the discharge limit was
established to ensure the quantity being discharged would be reduced from
existing levels. The final monthly average NPDES limits for nitrogen and
phosphorus were established as 10.0 mg/L and 1.0 mg/L, respectively.
Also, according to Mr. Bevington, modeling was conducted on McDowell Creek to
predict dissolved oxygen levels after the plant expansion is complete. This model
assumes minimum stream flow conditions combined with maximum plant
discharge to calculate reductions in dissolved oxygen in the creek. The
NPDES limits for BOD5, NH3-N, total suspended solids, and dissolved oxygen,
were all confirmed using this model. The results of the modeling confirm that
dissolved oxygen levels in McDowell Creek should remain in the acceptable range
for plant flows up to 6 MGD. When the plant was. first constructed, the discharge
limits on these constituents were originally established at or near the lowest limits
in the state. As a result, very little change to the limits is required to ensure
continued protection for McDowell Creek as the plant flows increase. The final
monthly average NPDES permit limits for BODS, NH3-N, total suspended solids,
and dissolved oxygen were established as follows:
Summer Winter
BODS 5.0 mg/L 10.0 mg/L
NH3-N 2.0 mg/L 2.5 mg/L
Total Suspended Solids 30.0 mg/L 30.0 mg/L
Dissolved Oxygen ? 5.0 mg/L >_ 5.0 mg/L

Mr. Russell S. Rozzelle
Page 3
B&V Project 27461.023
August 21, 1997
I hope this information clarifies the efforts made to protect the water quality in
McDowell Creek. If you have any further questions, please call.
Very truly yours,
BLACK & VEATCH LLP
David M. Parker, P.E.
Project Engineer
CMUDA7/jhb
Attachment
cc (w/attachment): Steve Bevington, DEM
Barry Gullet, CMUD

Mitigative measures: None required.
Eutrophication of Receiving Waters. Water quality sampling by Mecklenburg
County and modeling by DEM confirm that eutrophication occurs at the confluence
of McDowell Creek and Mountain Island Lake. Both nitrogen and phosphorus are
believed to contribute toward increased eutrophication. As such, both of these
nutrients are of concern, especially in the late summer months when these nutrients
accumulate in the cove. During this time, non -point source pollution is Iowest
because ( rainfall is low) and the McDowell Creek VWV'I'P becomes the primary
source for i hese nutrients. More detailed computer modeling of this confluence may
help to confirm this assessment of the sampling data ( P Y b performed be to DEM in
late 1994).
Speculative limits assigned by DEM include both a total nitrogen and total
phosphorus limit for the expansion of the McDowell Creek plant to 6.0 mgd. These
new limits will improve water quality in the Mountain Island Lake cove, according to
preliminary modeling by DEM. --
Another requirement imposed on the expansion of the McDowell plant pertains
to a zoning regulation promulgated with the development of the "Mountain Island
Lake Watershed Protection Study" for DEP. Mecklenburg County zoning regulations
for the Watershed Protection Overlay District states that "Discharges associated with
the existing public utilities may be expanded, however the pollutant load shall not be
increased beyond presently permitted ile els." Expansion of the McDowell Creek
plant was anticipated when the Watershed Protection Study was performed, with the
assumption that pollutant loads from the WWTP would not change from existing
levels when the study was performed. The plant expansion was supported over other
alternatives (such as package plants) by local environmental groups participating in
the study. The intent of the zoning ordinance is to allow the' WWTP capacity to
increase, While regulating the facility such that the water quality in the - lake is
protected. This requirement will be satisfied by establishing more strict effluent limits
which will result in improved water quality in McDowell Creek and Mountain Island
Lake.
The current NPDES permit for the plant includes limits on BOD, NH3, and total
suspended solids (TSS). Preliminary modeling of McDowell Creek and the cove in
Mountain Island Lake has indicated that the permit level of these constituents is
already sufficiently low, even at double the existing capacity, to pose no significant
impact to the water quality. However, this modeling has also indicated that nitrogen
and phosphorus levels in the plant effluent will likely have a significant impact on
10/19/94
EA-RPT.WPS 19

water quality. These nutrients, which are not currently regulated, can contribute to
increased algae growth and eutrophication. Limits on total nitrogen and phosphorus,
therefore, have been proposed by DEM to protect and improve water quality.
Mitigative measures: The proposed PVWTP expansion shall be designed to comply
with all NPDES permit limits. The speculative limits imposed by DEM for nitrogen
and phosphorus are at the limits of technology for what can be achieved by
conventional treatment. This will require the construction of biological phosphorus
and nitrogen removal, and effluent denitrifying sand filters. A limit on chlorine
residual in the effluent was also included. Ultraviolet disinfection will be installed
as part of the expansion, and the effluent sand filters will provide total effluent solids
well below the speculative limits.
Forest Resources. There will be no impact to the woodlands and related
resources in the area because the proposed construction activity will take place on
land owned by CMUD, currently utilized by the WW'ITP and previously cleared. —
Mitigative measures: None required.
Erosion Control. McDowell Creek will not be directly impacted by construction;
but indirect impacts, such as increased runoff and siltation during construction, may
occur.
Mitigative measures: Appropriate measures will be taken to assure that earth
moving activities required for construction do not adversely impact surface water by
increasing overland flow and the sedimentation and scour that may result. Silt
fences, bales, and other devices will be utilized to prevent sediment from entering the
stream with runoff from the construction areas.
Control of erosion and sedimentation to minimize water quality impacts to
McDowell Creek and Mountain Island Lake will be included in the construction
project. An approved sedimentation and erosion control plan will be obtained from
the North Carolina Department of Environment, Health, and Natural Resources
(DEHNR) Land Quality Section prior to the start of construction.
10/ 19/94
EA-RPT.WP5 20