WQ0041331_Narrative_20240422

Section G — Project Narrative
Johnston County Public Utilities
Selma Equalization & Pumping Facility
PURPOSE
APR 19 2024
NCDEQIDWR/NPDES
Currently, the Selma Equalization & Pumping Facility (SEPF) (WQ0041331) pumps wastewater to a
gravity sewer that leads to the Buffalo Creek pump station (PS) with treatment at the Central Johnston
County WWTP. The permitted capacity of the existing SEPF is 3.45 mgd (2,400 gpm). The County is,
however, constructing the new 210 Wastewater Treatment Plant (WWTP) and desires to pump the
wastewater from the SEPF to the Reedy Branch PS (WQ0042510) which then pumps the wastewater to
the 210 WWTP. For reference, the attached figures illustrate the current flow of wastewater from the
SEPF to the Central Johnston County WWTP and the proposed flow of wastewater from the SEPF to the
210 WWTP.
To accommodate pumping wastewater to Reedy Branch PS, the SEPF requires an upgrade and a force
main extension. Furthermore, the County intends to improve the SEPF by adding a mechanical screen to
protect pumps and grit removal to reduce solids loading to the existing equalization (EQ) tanks/lagoon
and the force mains to the Reedy Branch PS and Hwy 210 WWTP.
This narrative/application is specifically for the SEPF upgrades. The SEPF force main extension is being
designed by another engineering consultant and a separate application will be submitted for the force
main when its design is complete, estimated in 2024.
BACKGROUND
The SEPF receives wastewater from the Town of Selma. This facility, originally constructed in 1959, was
once Selma's WWTP (WPC-NC-44) and was later converted to a regional PS with EQ in 1976 as part of
the Smithfield -Selma 201 Facilities Plan. At that time, the facility ownership was transferred to Johnston
County.
Since 1976, there have been several upgrades and improvements. The most recent facility improvement
was a flood resiliency project (January 2020), which established the WQ number for the facility
(WQ0041331). A summary of the SEPF's historical improvements is outlined below:
Selma Equalization and Pumping Facility - History of Improvements
Selma Sewage Treatment Plant 1. Initial construction of treatment plant, including;
(December, 1959) a. Influent bar screen and grit collector (existing southern
headworks)
b. Primary clarifier, trickling filter, and secondary clarifier
c. Sludge digester and sludge drying beds
d. Outfall to nearbv creek
Smithfield -Selma 201 Area 1. Conversion to regional wastewater PS and transferred
Wastewater Facilities ownership to the County
Improvements 2. WWTP Outfall decommissioned
(November 1976) 3. Primary clarifier converted to Effluent PS (currently
(EPA Project No. 37056002) referenced as the existing secondary effluent PS)

4.
New 12-inch DIP effluent force main to the Buffalo Road
gravity sewer
S.
New 0.6 MG EQ concrete -lined lagoon
6.
Trickling filter converted to 0.4 MG EQ tank
7.
Secondary clarifier, return sludge PS, recirculation PS,
sludge digester, and sludge drying beds abandoned
Central Johnston County Regional
1.
New influent PS (currently referenced as the Main Effluent
WWTP Improvements to the Selma
PS)
Equalization and Pumping Facilities
2.
New manually cleared bar rack (existing northern
(May 2000)
headworks)
3.
New connection to 12-inch effluent force main (FM)
4.
Associated gravity sewer improvements to redirect onsite
sewers to the new PS
Selma Equalization Facilities
1.
New 1.5 MG concrete EQ tank
Improvements
2.
Associated piping improvements on site, including meter
(February 2003)
vault and flow meter on effluent 12-inch FM
(SRF CS370560-08)
1.
Replaced first 4,085 LF of 12-inch FM with 16-inch FM.
_
Selma EQ & Wastewater Pump
Station Facilities Flood Resiliency
Total FM length is 9,295 feet, all 16" diameter pipe to
Improvements Project
discharge manhole.
(Jan 2020 approval, WQ0041331)
2.
New electrical building
3.
The top of wet well and screen structure raised 2-feet for
flood protection
PROPOSED SEPF IMPROVEMENTS
Attached Sheet G-7 shows the existing process flow diagram along with facilities to be
abandoned/demolished, which are listed below:
1. Southern influent sewer headworks including mechanical step screen and aerated grit removal
system to be abandoned.
2. Northern influent sewer headworks, including manually cleaned bar rack to be abandoned.
3. Main effluent PS wet well and valve vault consisting of triplex 2,450 gpm submersible pumps
within concrete wet well to be abandoned.
4. Secondary effluent PS and EQ consisting of duplex 2,400 & 2,000 gpm pumps in a dry -pit, wet -
pit configuration within the basement of the old primary clarifier to be abandoned.
S. West area PS and EQ to be demolished.
6. Abandoned sludge PS to be demolished.
7. Abandoned recirculation PS to be demolished.
8. Main effluent PS electrical building to be abandoned.
9. Secondary electrical building to be abandoned.
10. On -site generator to be replaced.
Attached Sheet G-8 shows the post -construction process flow diagram. Once complete, the PS's
permitted capacity will be 15.5 mgd (10,765 gpm), consisting of 4.5 mgd (3,125 gpm) of effluent
pumping capacity to the Reedy Branch PS and 11 mgd (7,640 gpm) of EQ pumping capacity to fill onsite
EQ tanks/lagoon.

Upon completing construction, the SEPF will include the following assets in permit WQ0041331:
1. Onsite influent gravity sewer
a. Northern influent gravity sewer
L 75 LF 30-inch PVC sewer
b. Southern influent gravity sewer
L 533 LF 21-inch PVC sewer
c. Gravity sewer to accept South Selma PS force main
L 60 LF 20-inch PVC sewer
d. Headworks influent gravity sewer
L 20 LF 36-inch PVC sewer
e. Six manholes within the site
2. Headworks
a. Screen
i. 15.5 mgd mechanical screen with washer/compactor
ii. Manual bar rack in secondary channel for backup.
iii. Slide gates in the channels will provide the ability to isolate the screens.
b. Grit removal system
L Stacked tray grit separator with a peak capacity of 15.5 mgd
ii. Two 300 gpm recessed impeller grit slurry pumps
iii. Grit washer
iv. Grit washer/compacter
c. Grit system diversion pipe
L 120 LF 36-inch PVC diversion pipe to convey screened wastewater around the
grit removal system and directly to the wet wells, for use when performing
maintenance to grit removal system.
d. 70 LF 36-inch PVC effluent pipes to PS wet wells
3. Effluent / EQ Fill PS and valve vault structure
a. PS structure consisting of two wet wells separated by a concrete wall, each with three
variable speed submersible pumps, as follows:
L Effluent Wet Welland pumps:
1. Firm capacity of 3,130 gpm (4.5 mgd) with two duty pumps and one
standby pump.
2. Each pump's design point is 1,565 gpm at 237 feet of operating pressure
with discharge to the 36-inch gravity sewer that leads into the Reedy
Branch PS.
ii. EQ Wet Welland pumps:
1. Firm capacity of 7,639 gpm (11 mgd) with two duty pumps and one
standby pump.
2. Each pump's design point is 3,825 gpm at 61 feet of operating pressure
with discharge to the on -site flow distribution box for distribution to the
onsite EQtanks/lagoon.
b. Combined valve vault for check, isolation, and air release valves
c. Variable frequency drives will allow for continuous operation overall flow conditions.
Design allows for 2.4 cycles/hour based upon influent flow being 50% of ADF of 1.5 mgd.

4. Effluent force main to Buffalo Road gravity sewer
a. Total 9,295 LF 16-inch PVC/DI force main. However, once the SEPF force main extension
design is complete (by another consultant), the total force main force main from SEPF to
Reedy Branch PS will be 43,200 LF of 16-inch and 20-inch pipe.
b. Flow meter vault with 12-inch magnetic flow meter
c. Above grade bypass pumping connection
5. EQ force main to flow distribution box and tanks/lagoon
a. 26 LF 18-inch DI force main
b. 36 LF 24-inch DI force main
c. EQ flow meter vault with 18-inch magnetic flow meter
d. 170 LF 24-inch DI pressure pipe to tanks/lagoon
6. EQ Tanks/Lagoon Flow Distribution Box
a. Flow distribution box to distribute wastewater to either of the three EQ structures via
manually operated weir gates
7. EQ Tank Drain Flow Control System
a. Flow control vault with 12-inch magnetic flow meter to measure drainage flowrate from
EQ tanks/lagoon to Effluent Wet Well and 12-inch electrically actuated plug valve to
throttle flow to Effluent Wet well and keep within Effluent PS capacity of 4.S mgd.
b. 350 LF 16-inch DI EQ drain piping
c. 30 LF 12-inch DI EQ drain piping
8. Equalization facilities
a. 1.S MG concrete tank (existing to be reused)
b. 0.4 MG concrete tank (existing to be reused)
c. 0.6 MG concrete lined lagoon (existing to be reused)
9. Water system improvements
a. 1,050 LF 4-inch water service with yard hydrants
b. 3-inch water meter and reduced -pressure zone backflow preventer in an above grade
heated box
10. Electrical building
a. Audible and visual alarm on building exterior
b. Exterior diesel generator with automatic transfer switch within the electrical building
c. Cellular -based SCADA telemetry system
d. HVAC system with two air handling units and two air cooled condensing units
Proposed Pump Station Firm Capacity
An analysis was performed to establish the SEPF's capacity. The current average daily flow to the SEPF is
1.5 mgd. The projected future average daily flow is 2.8 mgd based upon information from the US 70
Sewer Basin — Phase 1 Modeling Report dated September 2022. Using the results from this report, the
SEPF was modeled to establish PS capacity while continuing the use of existing and/or proposed onsite
equalization. The model was based upon the following:
1. A discharge location at the Reedy Branch PS.
2. Use of existing/proposed force mains from SEPF to the Reed Branch PS as follows:
a. Reuse of the existing 16-inch diameter SEPF force main

b. 9,500 LF of 20-inch diameter force main extension to the soon -to -be -abandoned US 70
force main
c. Reuse of the 20-inch diameter US 70 force main with discharge to the 36-inch diameter
sewer that leads into the Reedy Branch PS.
3. Effluent pump selection assuming no more than 250 feet (108psi) of pressure at the pump to
ensure its integrity.
4. A maximum firm pump station capacity of 4.5 mgd to keep below the proposed maximum of
250 feet of pressure while using NC DEQ requirements for Hazen -Williams coefficients for PVC
and ductile iron pipe.
S. Continued use of equalization at the SEPF.
6. 30 years of historical rainfall data from the Raleigh -Durham Internation airport.
7. Wet weather flow monitoring data from March -May 2021.
The model results indicate the following:
1. The projected influent peak hourly flow to the SEPF is 15.5 mgd.
2. 4 MG of storage is required to reduce the potential of wet weather overflows.
3. A firm capacity of 11 mgd for the equalization pumps that feed onsite EQ tanks (15.5 mgd
influent peak hourly flow less the 4.5 firm effluent pump capacity).
Regarding existing EQ storage, the SEPF can currently store up to approximately 3 MG. The County
intends to design/construct additional equalization within the next 5 years to achieve a total storage
volume of 4 MG. To accommodate future additional EQ storage, the proposed SEPF design provides
pipe connections for a future EQ tank (i.e.: a connection for a future pipe from the EQ pumps to a future
tank and a connection from a future EQ storage tank to the proposed effluent pump wet well).
Based upon our interpretation of the 2T Rules and the Minimum Design Criteria for Pump Stations and
Force Mains, we believe that the proposed SEPF meets the requirements for redundancy and sizing a
pump station to accommodate the projected influent peak hourly flow).
Furthermore, the County continues to work with the Town of Selma to reduce its infiltration/inflow that
contributes to the high wet weather flows experienced at the SEPF. With an influent flow of 15.5 mgd,
the station is designed with peak hourly flow to average daily flow peaking factors of 10.33 (15.5
mgd/1.5 mgd) for current conditions and 5.5 (15.5 mgd/2.8 mgd) for future conditions. These values far
exceed the minimum peaking factor requirements established by NC DEQ.
Receiving Sewer
As mentioned, the SEPF will now pump wastewater to the Reedy Branch PS via discharge to the existing
36-inch diameter PVC sewer that extends 1,156 linear feet from a manhole at the intersection of
Highway 70 and Swift Creek Road to the Reedy Branch PS. This 36-inch sewer's slope ranges from a
minimum slope of 1.28% to 1.77%. It's full pipe capacity based upon minimum slope of 1.28% and a
Manning "n" of 0.013 is 48.90 mgd (33,960 gpm), which is far greater than the current and future
influent peak hourly flow to the Reedy Branch PS. Furthermore, the Reedy Branch PS will pump
wastewater directly to the County's new 210 WWTP headworks and, thus, does not flow through any
other gravity sewer or pump station before treatment.

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