NC0047597_Plan of Action_20100208

DURHAM
CITY OF DURHAM
DEPARTMENT OF WATER MANAGEMENT
101 CITY HALL PLAZA • DURHAM, NC 27701
919.560.4381 • fax [phone]
www.durhamnc.gov
1869
CITY OF MEDICINE
February 8, 2010
Mr. Michael E. Templeton, P.E.
Jordan Lake Watershed Coordinator
NCDENR Division of Water Quality
1617 Mail Service Center
Raleigh, NC 27699-1617
ID Administration
560-4381
▪ Customer Billing Services
560-4412
[I Laboratory Services
560-4386
• Plant Maintenance
560-4388
ID Wastewater Treatment
North Durham — 560-4384
South Durham — 560-4336
O Water Supply & Treatment
Williams — 560-4349
Brown — 560-4362
Q Water & Sewer Maintenance
560-4345
Re: Nitrogen Optimization Plan for South Durham Water Reclamation Facility
NPDES Permit No. NC0047597
Dear Mr. Templeton:
The City of Durham is in receipt of NCDENR correspondence relative to planned
modifications to the City's NPDES permit for the South Durham Water Reclamation Facility
(SDWRF) to incorporate new requirements under the Jordan Lake Nutrient Management
Strategy and its Wastewater Discharge rule, T15A NCAC 02B .0270. The draft NPDES
permit includes a new special condition requiring the City to submit a Nitrogen Optimization
Plan for the SDWRF that, upon acceptance by the Division, is to be implemented until new
nitrogen limits become effective in 2016. The enclosed information constitutes the City's
Nitrogen Optimization Plan for the SDWRF in fulfillment of this special condition of the
permit.
Background
The City of Durham, North Carolina owns and operates the South Durham Water
Reclamation Facility, wI ich discharges to the New Hope arm of Jordan Lake.. The SDWRF
is currently permitted to discharge up to 20 million gallons per day (mgd) of treated effluent to
the Jordan Lake watershed, and was designed to meet a monthly average effluent BOD limit
of 5/7 mg/L (summer/winter), a monthly average effluent TSS limit of 30 mg/L, a monthly
average effluent ammonia limit of 1/2 mg/L (summer/winter), and a quarterly average total
phosphorus limit of 0.5/2 mg/L (summer/winter). The SDWRF is also required to limit annual
Good Things Are Happening In Durham

South Durham Water Reclamation Facility
Nitrogen Optimization Plan
discharge of total nitrogen to 334,075 lbs/yr. The SDWRF is operated under NPDES Permit
No. NC0047597, which expires on April, 30, 2011.
The Jordan Lake Nutrient Management Strategy will require that the SDWRF meet annual
effluent nutrient total nitrogen and total phosphorus mass limits of 185,345 lb/year and
14,053 lb/year, respectively. These mass limits correspond to effluent total nitrogen and total
phosphorus concentrations of 3.0 mg/L and 0.23 mg/L, respectively, at the 20 mgd design
flow. The total phosphorus limit became effective January 1, 2010 and the total nitrogen limit
becomes effective in 2016.
The SDWRF completed a major plant upgrade and expansion in 1994 to provides for
biological phosphorus and nitrogen removal using a 5-stage biological nutrient removal
process. The process has been in operation continuously over the past 15 years reducing
nitrogen and phosphorus discharges into the upper New Hope amr of Jordan Lake. the
SDWRF operations staff has conducted ongoing optimization efforts over that period.
Recent Plant Performance
The SDWRF has a current annual average influent flow of approximately 10.5 mgd. The
plant has operated since the mid-1990s in a process mode to biologically remove both
phosphorus and nitrogen, producing an annual average effluent total nitrogen concentration
of approximately 6.5 mg/L. In 2008 a total of 254,766 pounds of nitrogen was discharged,
and in 2009 approximately 283,851 pounds was discharged from the SDWRF. These
nitrogen Toads, while well within the current total nitrogen annual mass limits of 334,075
Ibs/yr, do not meet the 185,345 Ib/yr annual mass nitrogen limit that will become effective in
2016.
Optimization Evaluations
The City of Durham recently completed an optimization study for the South Durham Water
Reclamation Facility (Optimization Study for the North and South Durham Water
Reclamation Facilities, December 2009, Hazen and Sawyer), which involved a detailed
evaluation and comparison of both of the City's plants in order to evaluate current
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South Durham Water Reclamation Facility
Nitrogen Optimization Plan
performance and determine what measures could be taken to improve future performance.
This study was originally initiated in 2006 and completed in January 2008, and was updated
as additional data became available and the Jordan Lake Rules were finalized. The study
included historical data analyses, detailed nutrient profiles, and a calibrated BioWinTM
biological process model of the entire SDWRF. The model was used to optimize plant
performance under current flow conditions and to determine what strategies may be used to
comply with the Jordan Lake Rules. The study recommended that several operational
parameters should be adjusted for optimum performance and also concluded that the City
would have to make capital improvements in order to further optimize nitrogen removal at the
SDWRF. It is acknowledged that conducting process optimization and constructing capital
improvements may disrupt existing process performance resulting in short term increases in
total nitrogen discharged from the plant until optimization efforts and construction are
completed. The following operational changes are being implemented at the SDWRF:
• The BioWinTM model was used to determine the best process configuration to
maximize nitrogen removal for SDWRF given the current flows and loads. The
SDWRF has four larger aeration tanks (2.55 MG each) and four smaller aeration
tanks (1.71 MG each). The process model simulations indicate that operating with 3
large and 2 small tanks in service during winter months and 2 large and 2 small tanks
in service during the summer months will be the optimal configuration for nutrient
removal at current flow rates. The City is implementing this change immediately.
During the period until the new total nitrogen limits become effective, the City will
continue to evaluate flows and loads and use the model to determine the optimum
process configuration for the conditions.
• The City has conducted full-scale pilot testing of supplemental carbon feed to
determine its effect on the denitrification process at the SDWRF. This work indicates
that a supplemental carbon source can be used to improve total nitrogen removal at
the SDWRF. The City is evaluating the need for supplemental carbon storage and
feed facilities in the capital improvements plans for the SDWRF.
• High phosphorus loadings from the solids process recycle streams have historically
required that the City dedicate process volume for biological phosphorus removal that
could otherwise be used for nitrogen removal. The City has implemented any
operational change to precipitate phosphorus in the plant's reactor clarifiers. This
reduces and stabilizes the phosphorus loading on the plant, allowing a larger portion
of the influent carbon to be managed for denitrification to reduce total nitrogen.
• The biological process model predicted that increasing the nitrified recycle rate in the
existing plant would decrease effluent total nitrogen. The SDWRF has experimented

South Durham Water Reclamation Facility
Nitrogen Optimization Plan
with adjusting nitrified recycle rates to optimize nitrogen removal,however, the system
does not respond as would be expected with improved denitrification when recycle
rates are increased. The City is planning capital improvements that must be made in
order for the SDWRF to be able to optimize nitrified recycle rates for maximum
nitrogen removal.
The following capital expenditures have been made or are planned for the SDWRF in order
to improve plant performance and lower effluent total nitrogen:
• The SDWRF is in the process of replacing the plant's fine bubble diffusers with new
membrane -type fine bubble diffusers. This modification will provide for better
dissolved oxygen control at the end of the aerobic zone of the 5-stage biological
nutrient removal process. In addition, the configuration of the diffusers is being
modified to provide a taper of aeration density from the front to the back of the
aerobic zone of the process. This tapered air distribution should also help the City
lower the dissolved oxygen content of the nitrified recycle flow to the first anoxic zone
of the process, which in turn should improve nitrogen removal performance.
• The City is also modifying the existing alum feed pump system that is used for
chemical precipitation of phosphorus. The existing system depends upon a hydraulic
split of the chemical in a distribution header to distribute the chemical to the
secondary clarifier influent. Modifications will allow dedication of pumps to individual
clarifiers to provide better distribution? of chemical for phosphorus removal. This
optimization, while primarily for phosphorus removal, will have the positive effect of
reducing chemical usage and therefore reducing inert solids carried in the mixed
liquor. This will allow for an increase in volatile mixed liquor suspended solids
concentrations, which the process modeling indicates will improve nitrogen removal.
• Process investigations identified that backmixing of dissolved oxygen into the
upstream anoxic zones is occurring at the interface between aerated and unaerated
zones of the 5-stage biological nutrient removal process, reducing efficiency of the
denitrification process in the anoxic zones. Modifications will be made to the existing
baffle walls which will minimize backmixing and will enhance denitrification,
particularly in the first anoxic zone. When the baffle wall modifications are complete,
the effectiveness of increasing the nitrified recycle rate should be improved.
• The City plans to install online nutrient analyzers in the aeration basins. This will
provide operators with a tool to track real-time process trends and make operational
adjustments to optimize nutrient removal.
With these ongoing improvements, the SDWRF is expected achieve significant nitrogen
reductions from current levels. However, the City anticipates that the SDWRF may require
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South Durham Water Reclamation Facility
Nitrogen Optimization Plan
additional capital improvements to provide consistently reliable compliance with the Jordan .
Lake Rules for nitrogen removal. The City is still finalizing plans for capital improvements to
address long term compliance, including the need for supplemental carbon feed facilities. It is
anticipated that construction of improvements will be completed in advance of January 2016
to allow the City time to optimize operation of the new facilities before the compliance date.
Summary,
This nitrogen optimization plan evaluates the existing treatment facilities and operations at
the South Durham Water Reclamation Facility and assesses potential for operational
modifications to reduce effluent nitrogen concentrations. The City of Durham recently
conducted an optimization study for the SDWRF, which suggested several operational
modifications and capital improvements to achieve the required nitrogen removal
performance. The City is moving forward with phased implementation of these operational
and capital improvements. As improvements are completed, nitrogen reductions would be
anticipated in the period prior to the 2016 compliance date.
If there are any questions regarding this submission, please feel free to contact Robert
Dodson at [phone], ext. 35530.
Sincerely,
q� /�.
e "tidiatilte0
Donald F. Greeley, P.E., Director
C: Raleigh Regional Office, Surface Water Protection
Michael Douglas, Raleigh Regional Office, DEH
Steve Reid, Technical Assistance and Certification Unit
Robert Dodson, Superintendent, SDWRF
Vicki Westbrook, Assistant Director, Department of Water Management
Reginald Hicks, Superintendent, COD Regulatory Compliance
5

DURHAM
1869
* ***
CITY OF MEDICINE
CITY OF DURHAM
DEPARTMENT OF WATER MANAGEMENT
101 CITY HALL PLAZA • DURHAM, NC 27701
919-560-4381 • FAX [phone]
www.durhamnc.gov
January 25, 2011
Mr. Michael Templeton, P.E.
Jordan Lake Watershed Coordinator
NCDENR Division of Water Quality
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
Re: Nitrogen Optimization Progress Report
South Durham Water Reclamation Facility, NC0047597
Administration
919-560-4381
Customer Billing Services
919-560-4412
Laboratory Services
919-560-4386
Plant Maintenance
919-560-4388
Regulatory Compliance
919-560-4381
Wastewater Treatment
North - 919- 560-4384
South - 919-560-4336
Water Supply & Treatment
Williams - 919-560-
4349
Brown - 919-560-4362
Water & Sewer
Maintenance
919-560-4344
Dear Mr. Templeton:
This letter serves as a progress report documenting the nitrogen reduction
optimization activities at the South Durham Water Reclamation Facility,
NC0047597, during calendar year 2010. Listed below are steps taken to date
to optimize the process:
• As mentioned in the initial plan submitted on February 8, 2010, staff
has completed the change to membrane type diffusers on 7 of the 8
aeration basins. In addition, the taper of the air is being modified to
allow for better dissolved oxygen control of the nitrified recycle.
• An existing sodium hydroxide feed system has been temporarily
converted to a glycerin feed system to allow for additional carbon feed
for nitrogen removal. The discharge of the system is a common feed
to the first anoxic cell which is not the optimal feed point. However, it
does allow feed to the system and has resulted in a reduction in the
total nitrogen levels. The department is in the midst of developing a
master plan for both of the City's water reclamation facilities. As a part
to this planning effort, a permanent feed facility is being sited that will
allow feed to the second anoxic zone of each of the aeration basins.
This is a more efficient and effective feed point for the chemical.
• The original optimization plan identified a problem with backmixing of
dissolved oxygen between the aerated and unaerated zones of the 5-
stage biological nutrient removal process which reduces the efficiency
e denitrification process. Changes in design of the baffle walls will
ecessary to minimize the backmixing. The design of the baffle
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Mr. Michael Templeton
January 25, 2011
Page -2-
•
wall modifications is complete and the City is in the process of
securing a contractor to begin construction in early to mid 2011.
• The City has completed design and is pursuing a contractor to
purchase and install online nutrient analyzers in the aeration basins.
This will provide operators with a tool to track real-time process trends
and make operational adjustments to optimize nutrient removal.
The City is continuing implementation of the items identified in the
Optimization Study for the North and South Durham Water Reclamation
Facilities, December 2009, Hazen and Sawyer. It is anticipated that
construction of improvements will be completed in advance of the January
2016 to allow the City time to optimize operation of the new facilities before
the compliance deadline.
If you have any questions, please call me at [phone], ext. 35530.
Robert Dodson
Superintendent
cc: Donald F. Greeley, P.E., Director
Vicki Westbrook, Assistant Director
Reginald Hicks, Superintendent, Regulatory Compliance
Ronald Taylor, P.E., Hazen and Sawyer
•
Good Things Are Happening In Durham
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