NC0026441_Response to Add Info Request (SOC Update)_20240723

H z n Hazen and Sawyer
a e 4011 WestChase Boulevard,Suite 500
Raleigh,NC 27607•919.833.7152
July 23,2024
Mr.Nick Coco,PE via email: nick.cocokdeq.nc.gov
ARPA Environmental Program Supervisor II
Division of Water Resources,NCDEQ
1611 Mail Service Center
Raleigh,NC 27699-1611
Re: Request for Additional Information
SOC Update and Path Forward
Town of Siler City
NPDES Permit NC0026411
Dear Mr. Coco:
On behalf of the Town of Siler City(Town),Hazen and Sawyer(Hazen)is responding to your letter dated
July 3,2024 requesting additional information regarding the Town's proposed interim measures("interim
improvements")planned for the Siler City WWTP(SCWWTP). These interim improvements are part of
a strategy developed in conjunction with the City of Sanford(City)to achieve compliance with the intent
of Special Order by Consent(SOC)WQ S22-033.As discussed in our meeting with NC Department of
Environmental Quality(NC DEQ) staff on May 14,2024,Hazen has been actively working to complete
the design of the interim improvements and anticipate submitting an Authorization to Construction(ATC)
to the Division of Water Resources(DWR)in August. Our responses to the questions posed in your July
3rd letter follow below.
Question I -It appears based on the language in the Update that the proposed 4-stage BNR
improvement is the result of a conversion of an existing structure.Please identify which structure is
to be converted and provide a description of how the conversion of that structure's function will
impact the treatment at the facility.
Response—The existing biological process at SCWWTP consists of two racetrack-style oxidation
ditches constructed in 1994. These structures each have a nominal capacity of 2.2 million gallons and
currently operate as fully-aerated basins with surface air jet aeration. This process was originally
designed to meet speculative limits issued in 1991 that did not include a limit for Total Nitrogen
(TN). TN limits included in the SCWWTP's most recent NPDES permit issued in September 2022
(permit NC0026441)will decrease to an annual load of 54,800 pounds effective January 1,2025.
hazenandsawyer.com

Hazen Mr.Nick Coco,PE
July 23,2024
An ATC has been issued by DWR for the construction of a 6.0 mgd expansion and upgrade project
which includes a new biological nutrient removal(BNR)process improvements intended to meet this
TN limit. These process improvements include completion of a new 2.0 mgd treatment train and
conversion of the existing 4.0 mgd biological process. This construction project has recently been
awarded and is scheduled for completion in the first quarter of 2027 based on the City issuing an
Notice to Proceed by August 12,2024. However,these process improvements will not be in service in
time for the SCWWTP to meet its TN limit in 2025-2026,nor does this schedule comply with
NPDES permit requirements to have the 2.0 mgd treatment train in service by August 31,2024 and
the entire 6.0 mgd expansion and upgrade project operational by December 31, 2024.
The proposed interim improvements will convert the biological process in both existing oxidation
ditches to a 4-stage Bardenpho process to improve treatment performance and reliability. The
proposed interim improvements will include process modifications that will remain in service until
the permanent improvements can be constructed and commissioned. These interim improvements will
not adversely impact the construction of the permanent improvements and will allow SCWWTP to
meet its annual TN limit between 2025 and completion of the expansion and upgrade in 2027.
Question 2-Please provide a narrative description of the scope of work associated with the interim
process improvement identified in the Update.
Response—The existing continuous aeration process in each oxidation ditch will be converted to a
plug flow reactor operating as a 4-stage Bardenpho biological process for enhanced nitrogen removal.
This will be accomplished with the creation of pre-anoxic, aerobic,post-anoxic, and reaeration zones
separated by new baffles constructed in each ditch. Aeration will be provided by new removable fine
bubble aeration grids connected to blower units with air headers and flexible hose connections.New
rental blower units with variable drives paced on dissolved oxygen(DO)measurements will provide
airflow to the aeration grids.A new nitrified recycle (NRCY)window pump with a variable drive will
be installed at the end of each aerobic zone and discharged back to the pre-anoxic zone in each ditch.
Existing floating surface aerators will be repurposed as mixers in the pre-anoxic and post-anoxic
zones of each ditch.
Process control will be achieved by using DO and nitrate analyzers in each ditch to regulate blower
airflow and NYCR recycle rates via a programmable logic controller linked to the blowers and NRCY
pumps. While process modeling performed by Hazen does not predict that supplemental carbon will
be required to meet the TN limits,provisions for dosing carbon with chemical metering pumps will be
provided for each post-anoxic zone.
Existing chemical phosphorus removal via polyaluminum chloride (PACL) fed downstream of the
oxidation ditches will remain in service.
Proposed modifications to the basins are being made using materials and methods to facilitate speedy
procurement and construction. Baffles creating the plug flow path and treatment zones will be
constructed of materials that can be readily installed and removed in a short duration shutdown to
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Hazen Mr.Nick Coco,PE
July 23,2024
minimize disruptions to the treatment process. Existing aeration piping requiring removal for the
upgrade and expansion project will be removed from service earlier to accommodate the interim
improvements. Diffusers will be installed on platforms fabricated outside of the tanks and positioned
within the tanks with cranes to avoid conflicts with existing features and expedite construction.No
existing features that are needed to construct the proposed permanent improvements will be impacted
by the interim improvements. The blowers,NRCY pumps, and air distribution and NRCY piping will
be removed when the permanent improvements are commissioned.
Each ditch will be removed from service in turn to complete the construction of the interim
improvements while the other ditch remains in service.While converting the process in the first ditch,
we anticipate adding drop-in diffuser pods to the second ditch to enhance current process
performance and provide supplemental air for nitrification.We will also leverage SCWWTP's 4.0
MG of equalization storage to shave peak day and peak hour flows,reducing diurnal stresses on the
process. The process improvements will be implemented following thorough planning and
coordination between the SCWWTP operators,general contractor, and Hazen staff to effectively
maintain plant operations.
Question 3-Please provide design calculations for the 4-stage BNR system,including all influent
design criteria and any corresponding blower and pump sizing calculations.
Response—Hazen is working to complete all design assumptions, calculations and process modeling
needed to support an Authorization to Construct(ATC)package and will submit these as part of an
engineer-certified package.We would welcome the opportunity to review the details of our process
modeling and design approach in a pre-submittal meeting with DWR staff to help expedite your
review.
Question 4-The Update appears to indicate that the 4-stage BNR system is designed based on 2023
flows.Please provide justification for this design decision,as the current facility is permitted for 4.0
mgd and the Town has noted plans to upgrade its facility to 6.0 mgd by 2027.
Response—The interim improvements have been designed to serve the Town through December
2027 if necessary to account for any unforeseen construction delays with the 6.0 mgd expansion and
upgrade project. The BNR process has been designed to accommodate an Average Annual flow of
3.48 MGD and Maximum Month flow of 4.0 mgd.
As part of the design effort,Hazen developed flow projections that considered 2023's reported
Average Annual flow of 2.67 mgd, a two percent annual increase in flows that is conservatively
consistent with projections from the Town's latest Local Water Supply Plan update, and planned
wastewater flows of 0.6 mgd from Wolfspeed's Siler City facility through 2027.
Question 5-Please provide an explanation as to how this design will sufficiently treat total nitrogen
to meet permit limits in each interim year preceding completion of expansion construction. The
Update notes that the requirements would be met in 2025,but does not speak to 2026 or 2027.
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Hazen Mr.Nick Coco,PE
July 23,2024
Response—The SCWWTP NPDES permit annual TN limit remains constant at 54,800 pounds after
January 1,2025. The interim improvements have been designed to reliably meet this TN limit through
2027, at which time the 6.0 mgd expansion and upgrade should be operational. The interim
improvements are expected to be in service prior to the end of 2024. The following conservative
assumptions were made when designing the interim process improvements:
• The interim process improvements can treat the permitted flow and pollutant loading from all of
Siler City's Significant Industrial Users (SIU). Even where a SIU is currently discharging less
than their permitted load, it was assumed that the entire permitted load would be discharged to the
Siler City WWTP during this time period.
• Mountaire Farms' new$40 million pretreatment system is currently under construction and will
not be operational before the completion of Siler City's interim improvements. Therefore,
Hazen's process modeling and design is based on Mountaire Farms current level of pretreatment
performance.
• It was conservatively assumed that all 0.6 mgd of wastewater discharged by Wolfspeed will
contain nitrogen. Based on information provided by Wolfspeed, only approximately 0.38 mgd of
wastewater discharged by Wolfspeed will contain nitrogen;the remaining 0.22 mgd of
wastewater will be cooling tower blowdown and should not contain nitrogen. Further it was
assumed that Wolfspeed will pretreat their wastewater to domestic strength prior to discharge.
Question 6-The proposed interim measure would include the construction of one(1)4-stage BNR
system. Should this system require maintenance,how would the Town ensure adequate treatment
to accomplish compliance with the loading limits identified in the current NPDES permit, active
SOC and in the Update itself?
Response—The current processes and equipment are incapable of meeting the current NPDES
permit's annual TN limit beyond 2024. The interim improvements will convert both oxidation ditches
to a 4-stage Bardenpho process, completely replacing the existing process in each ditch. The interim
improvements are being designed such that maintenance on equipment can be performed without
having to remove a basin from service,improving reliability when compared to the current system.
If the interim improvements are not implemented,it is projected that the SCWWTP will exceed its
annual TN limit by May of 2025 and 2026,even without additional wastewater flows from
Wolfspeed or other sources. These interim improvements would allow the Town to achieve
compliance with its TN limit more than two years sooner than would be possible solely with the
completion and commissioning of the 6.0 mgd expansion and upgrade.
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Hazen Mr.Nick Coco,PE
July 23,2024
We would welcome the opportunity to meet with you to address any further questions you may have to
help facilitate timely review and approval of our ATC. I will follow up with you to schedule that meeting.
In the meantime,please do not hesitate to contact me at 919.277.7919 or
jbbuckleykhazenandsawyer.com with any additional questions or comments.
Very truly yours,
HEN AND SAWYER, PC
gJ. Buckle ,P BCEE
AVice P ' nt
cc: Town of Siler City/Jack Meadows
Town of Siler City/Chris McCorquodale
Town of Siler City/Brittany York
City of Sanford/Victor Czar
City of Sanford/Paul Weeks
NC DEQ/Richard Rogers
NC DEQ/Julie Gryzb
NC DEQ/David Lambert
Hazen/TJ Lynch
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