NC0031836_Ammonia-Nitrogen Limitations_20200302

7 CITYof jtatek' e
NORTH CAROLINA.
February 25, 2020
Attn: Mr. GaryPerlmutter
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NCDEQ/ Division of Water Resources ReC
NPDES Complex Permitting MpR 42 202�
1617 Mail Service Center
Raleigh, North Carolina 27699-1617 14COE , ,t4PMs
RE: Action Plan for Ammonia-Nitrogen Limitations
City of Statesville— Fourth Creek WWTP
NPDES Permit NC0031836
To Whom It May Concern:
The Division of Water Resources included new, lower Ammonia-Nitrogen (NH3-N) limits in the
renewal of NDPES Permit NC0031836 for the Fourth Creek WWTP, as well as a schedule of
compliance. This Action Plan summarizes the strategy to achieve compliance with the final
Ammonia-Nitrogen limitations. The ongoing and proposed steps are summarized below:
1. The City continues to optimize the reduction of ammonia-nitrogen at the plant as it has
for many years.
a. Influent wastewater flows are strongly correlated with rainfall. This is likely
related to increasing proportions of I/I. Annual average flow has increased from
2.38 MGD in 2017 to 3.25 MGD in 2019.
Influent Flow and Loading 2017-2019
4.50 600
4.00
500
3.50
3.00 400
p 2.50
300 exo
2 2.00 i=
1.50 200
1.00
100
a 50
0.00 0
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lac �a� lJ� 49 °, lac lac 4a\ l•: °, lac a 4<a> 49 °,
-Monthly Average Flow Influent BOD ----Influent TSS Influent Ammonia
PO Box 1111 • Statesville, NC 28687 • ci.statesville.nc.us

February 25, 2020
City of Statesville— Fourth Creek WWTP— NPDES Permit NC0031836
Page 2 of 4
b. Wastewater strength has decreased over the past three years. The highest
average influent concentrations for BOD (176 mg/I), TSS (310 mg/I), and
ammonia (18.4 mg/I) from 2017 to 2019 all occurred in 2017.
c. Under the NPDES discharge limits currently in effect, none of the effluent
concentrations from 2017-2018 would have exceeded the limitations. Under the
final ammonia limits set to take effect on March 1, 2021, several exceedances
would have occurred. This is believed to have been due to runoff from the
uncovered biosolids storage area.
2. Recent and Proposed Improvements
a. The Fourth Creek WWTP has two (2) aeration basins, each with six (6)
mechanical surface aerators. In 2017, the City replaced two (2) surface aerators
with higher efficiency units that have a lower impeller to improve tank mixing.
b. In 2019, the City replaced two (2) additional surface aerators with new units.
c. The City plans to continue replacing two (2) surface aerators each year, as funds
are available, until all twelve (12) surface aerators have been replaced.
d. The belts in the belt filter press were recently replaced to improve performance.
3. The City recently contracted with an engineering firm to evaluate the NPDES permit
effluent limitations for ammonia-nitrogen at the 4 and 6 MGD flow rates. Steps 4 and 5
listed below are based upon a review of the WWTP performance and BioWin modeling
of the process at 4 and 6 MGD.
BioWin Model Flow Schematic
BOD Influent Aeration Basin 1
o w o
IJ
Filtrate
Aeration Basin 2 Outfall
> I >
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Digester Filtrate
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Cake

February 25, 2020
City of Statesville—Fourth Creek WWTP—NPDES Permit NC0031836
Page 3 of 4
Effluent Pollutant Concentrations at Current Flows(Dynamic Simulation)
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01/24/21 03/25/21 05/24/21 07/23/21 09/21/21 11/20/21
Outfall BOD-Total Carbonaceous Outfall Total suspended solids
Outfall N -Ammonia
4. As flows approach 3.6 MGD (or 90% of the permitted flow of 4.0 MGD), the City will
continue to further optimize ammonia-nitrogen reduction.
a. Aeration energy is currently reduced by 50%for 12 hours per day (night hours)
to promote settling. When ammonia-nitrogen values exceed 2.0 mg/I during
April through October, the City will increase the aeration energy during these
night hours to reduce effluent ammonia-nitrogen.
b. Activated sludge will be wasted at an increased rate and/or a more frequent
schedule to prevent sludge blankets from rising to levels that would cause solids
carryover. The current wasting is approximately 1.3% of the influent flow. The
wasting rate as a percentage of influent flow will be increased towards 2.0% or
higher.
c. Return activated sludge rates will be increased from the current average of 58%
of influent flow.
d. Belt press operations will be spaced out throughout the week to allow time to
for filtrate sidestream flows to work their way through the basins to minimize
the impact of this additional loading.

February 25, 2020
City of Statesville— Fourth Creek WWTP—NPDES Permit NC0031836
Page 4 of 4 1
5. As flows exceed 4.0 MGD, the City will continue implementing the steps noted above.
The chart below shows a 6-month simulation of effluent ammonia-nitrogen using actual
monthly temperatures from 2019 at 6.0 MGD with 100% RAS flow and wasting at 5% of
the influent flow. Any optimization strategies to improve aeration efficiency and
energy, clarifier settling, sludge wasting, and belt filter press capture rate will improve
the ability of the Fourth Creek WWTP to perform at higher flows.
Project Composite Parameters at 6 MGD
12;l—TVri -rl . _ . . . . r:rn.:1nu u 11uVIV� u u u u-:rrrr.-r.+.~.1 TJ
10..
9:
8:
off 7:
E NH3-N monthly winter limit
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0 Z 5.
.
4
NH3-N monthly summer limit
3.
2'
1 :
01/24/21 03/25/21 05/24/21 07/23/21 09/21/21 11/20/21
Outfall Composite BOD-Total Carbonaceous(flow weighted)
Outfall Composite Total suspended solids(flow weighted)
Outfall Composite N-Ammonia(flow weighted)
Please feel free to contact us to discuss this strategy moving forward.
Sincerely,
CITY • .TATESVILLE /
ANDY S ITH
Operations Manager