NC0047597_South Durham WRF Nutrient Optimization Report_20230526

**** WATER MANAGEMENT
CITY OF DURHAM
RECEIVE*
May 23, 2023 MAY 2 6 2023
NCDENR/DWQ/NPDES PROGRAMS NCDEQ/DWR/NPDES
Attn: Jordan Lake Watershed Coordinator
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
SUBJECT: South Durham WRF Nutrient Optimization Report
Please see the attached South Durham Nutrient Optimization Report provided per Section A. (8.)
Nutrient Optimization of the South Durham NPDES Permit NC0047597 issued January 1, 2023.
If you have any questions or need additional information,please email me at
[e-mail] or call me at 919-560-4386, ext. 35532
Regards,
Charles Cocker
Superintendent, SDWRF
6605 Farrington Road
Chapel Hill, NC 27517

South Durham Water Reclamation Facility Nutrient Optimization Report
Permit NC0047597
May 2023
The South Durham Water Reclamation Facility (SDWRF) has the following processes in place for
nutrient removal.
- A 2 MG Equalization Tank to even out the diurnal flow patterns through the treatment
processes.
- An Anaerobic Tank to help with luxury uptake of phosphorus.
- Eight (8) Aeration Basins, each with 7 cells for multiple configurations of anoxic and
aerobic zones.
- Side-Stream Treatment to aid in the removal of ammonia from the dewatered anaerobic
sludge filtrate.
- Tertiary Filters
- Sodium Hydroxide Feed Facility to add alkalinity to the process.
- Alum Feed Facility to aid in phosphorus removal.
- Carbon Feed Facility to add BOD to the anoxic cell following the basin's aerated cell to
aid in denitrification.
The 2023 average daily effluent flow was 9.7 MG. This compares with a 1990 average daily
flow of 8.4 MG. Even though the area of Durham that the SDWRF serves has seen a tremendous
amount of growth over the years, water conservation efforts appear to be working well.
The 2 MG Equalization Tank was put into service in June 2022. It is filled during the higher
incoming flows of the day and drained during the overnight hours when flow drops off. Filling
and drainingnare controlled by SCADA. Evening out the flow through the plant processes has
worked well for the plant. No changes are expected in this process.
The Anaerobic Tank has been used to aid biological phosphorus removal for over 25 years. The
primary effluent and RAS flows are combined in this tank. The contents are kept in suspension
with submersed propeller mixers. The Aeration Basin Influent Channel is also anaerobic. No
changes are expected in this process.
Eight Aeration Basins are available for use at the SDWRF. The plant has operated with three
basins for about 5 years with good results. In August 2022 cell 3, which had been anoxic, was
changed to aerobic. At the same time, the air to grid 3 of the aerated cell (cell 4) was choked
back to make the zone more anoxic. This moved the aeration zones back in the basins to be sure
ammonia was reduced and gave the process more secondary anoxic time to aid in
denitrification. The plant did see and continues to see a reduction of TN with this change. The

average effluent TN in 2021 was 6.9 mg/L. The 2022 average was 5.6 mg/L. Another aeration
basin will be put into service in June 2023 with the anticipation of more detention time leading
to reduced effluent TN and TP numbers.
A Veolia designed Side-Stream Treatment Facility went into service in September 2015.The
process uses a biofilm that reduces ammonia with aerobic nitrification and anaerobic ammonia
oxidation. When built, the process was guaranteed to reduce ammonia 70%. The system has
consistently achieved this. The average ammonia reduction so far in 2023 is 73%. The system
has an EQ tank that allows it to run every day, including long holiday events. Reduced ammonia
loading at a constant 24-hour low gpm rate through the plant processes is an improvement over
the high ammonia slug that went through the processes for a 7-hour period in the past. The
side-stream treatment system has lowered effluent TN approximately 1.5 mg/L. No changes are
expected in this process.
The Tertiary Filters remove most of the solids that come over the final clarifier weirs. Average
effluent TSS results are consistently below<2.5 mg/L. No changes are expected in this process.
The facility can add sodium hydroxide to the plant process when needed. To date, we have not
added this alkalinity to determine if it could improve nitrogen removal, especially during the
colder months. We intend on trying this out in the next winter season.
The facility adds aluminum sulfate (alum)to the process at the end of the aeration basins.This
chemical addition is fed daily to aid the biological phosphorus removal system. Alum feed
dosage is based on Lab results. Although higher feed dosages can result in lower effluent TP
results, our approach will continue with no changes for now.
A Carbon Feed Facility is in place to add additional food (BOD)to the aeration basins to aid in
nitrogen removal. The feed point for the glycerin will be the anoxic cell following the aerated
cell in each basin. The concept of nitrogen reduction was proved during the startup of the
facility about seven years ago. Carbon has not been fed to date. Carbon feed will be
implemented on an 'as needed' basis.
Planned Process changes.
As mentioned above, an additional aeration basin will be put into service in June 2023.The
expectation is that a longer detention time will result in improved nitrogen and phosphorus
removal. We will give this operational change several months to prove if there is value in it or
not.
The plant has an optional ammonia-based strategy for DO control which is already in the
plant's SCADA system. With ammonia-based DO control, air output to the aeration basins will
be based on ammonia concentrations. With just enough air provided for complete ammonia
oxidation,there will be very little free oxygen available for denitrification, forcing the organisms
to get oxygen from nitrate, thus reducing TN discharged.

The plant has purchased seven HACH ammonia probes which will be placed in key areas of the
process. The probes and associated signals to SCADA are planned to be in place sometime in
July 2023. Durham's consulting engineering firm reports successful nitrogen reduction at other
facilities using ammonia-based DO control. The same results are expected at the South Durham
Water Reclamation Facility.
Any comments or questions can be directed to Charles Cocker, Plant Superintendent, SDWRF,
6605 Farrington Road, Chapel Hill, NC 27517.
Email [e-mail].
Phone—919-560-4386, ext. 35532