Underground Construction - April 2021 - 25

ABOVE: The rehabilitation required bypassing a total of 48 million gallons per day of sanitary sewage
to upgrade and rehabilitate the bar screens, wet well, and screw pumps at the station influent structure.
LEFT: The rehabilitation of the Cross Creek Water Reclamation Facility Influent Pump Station required
careful planning, a thorough risk assessment - and most importantly, solid partnerships to get the job
done safely and effectively.

from Hazen and Sawyer also joined the
team. Together, these team members
worked closely to determine the best
approach to the job ahead of them.

Rehab challenges, risks
As with any sanitary sewer pump bypass
project, the teams encountered challenges
and potential risks when assessing the
required rehabilitation at the Cross Creek
WRF Inﬂuent Pump Station, both logistical
and environmental.
The main challenges were the submergence of a suction pipe south of the pump
station and the depth of the manholes.
The suction manhole in one location was
30 feet deep. Essentially, it was too deep
for the pumps to properly bypass; if the
pumps were set above the grade at the top
of the nearby berm, suction lift would not

be achievable due to net positive suction
pump (NPSH) limitations.
At the discharge location, the team
was faced with the challenge of diverting
ﬂow from one channel to another as work
progressed, without shutting down the
bypass. This was important for keeping
the plant in operation.
Another challenge was the need to
maintain a certain amount of submergence
level. Submergence became an issue
to upstream manholes being at lower
elevation than the suction location at
the plant. This was due to the plant being
at a higher elevation to protect it from
ﬂooding, as it sits along the Cape Fear
River. The lower elevation took surcharging
out of the equation, as it could very likely
cause a sanitary sewer overﬂow.
Lastly, the teams needed to consider
environmental factors, since the area
is in a low line and
prone to hurricanes that could lead to
ﬂooding. Should a hurricane hit during the
bypass, losing the pump station was simply
not an option. Therefore, it required the
teams to account for the highest amount
of ﬂows as possible to protect against
potential sewage overﬂow into the nearby
Cape Fear River. This was taken into
account by ensuring that the pumps could
handle the average, peak and up to two and
a half times the peak ﬂow.

Pumping solutions
The goal of the bypass solution was to
simultaneously activate three pump
locations with 100 percent redundancy.
These systems connected via level
transducers that monitored the sewer
levels and would automatically turn on the
pumps if they sensed ﬂuid levels above a
set height, as well as control RPM levels
during low ﬂow periods. The RPM control
also assisted with fuel eﬃciency. The
systems would solely operate the station,
meaning there was no room for failure.
The teams kept a backup pump on each
pump location site, along with additional
discharge lines, as added protection.
Overcoming the submergence of the
suction pipe required multiple meetings
between PWC, Hazen and Sawyer, Crowder
Construction and Sunbelt Rentals -
with the ﬁnal determination being made
to bench down the pumps to achieve
suction lift. Due to the upstream manhole
elevation, this was imperative in order to
not surcharge and overﬂow.
To implement this portion of the
project, the teams brought in surveyors to
determine the correct elevation heights,
and PWC evaluated and conﬁrmed them.
Subsequently, Crowder Construction
excavated the dirt down to 17 feet where
the pumps would be placed.
UCONonline.com | APRIL 2021

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Underground Construction - April 2021

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