RESIDUALSHANDLING)ANDSTORAGEATWINSTON0SALEM’S …
RESIDUALS
HANDLING
AND
STORAGE
AT
WINSTON--SALEM'S
THREE
WATER
TREATMENT
PLANTS
?
A
CASE
STUDY
ON
LAGOON
PERFORMANCE
WINSTON--SALEM/FORSYTH
COUNTY
UTILITY
COMMISSION
Bill
Brewer,
Water
Treatment
Superintendent
BLACK
&
VEATCH
INTERNATIONAL
COMPANY
Jeff
Coggins,
P.E.,
Project
Manager
Scot
Thompson,
P.E.,
Engineering
Manager
1 Introduction
Like
most
utilities,
the
Winston--Salem/Forsyth
County
Utility
Commission
(CCUC)
is
faced
with
the
repetitive
and
costly
task
of
handling
solids
from
the
operation
of
their
three
water
treatment
facilities.
At
each
water
treatment
plant
(WTP),
residual
storage
lagoons
are
utilized
as
the
primary
means
for
handling,
storing,
and
thickening
of
process
residuals
ahead
of
final
disposal.
While
the
lagoons
at
each
plant
were
sized
to
allow
for
a
full
year
of
operation
followed
by
a
full
year
of
offline
thickening,
drying,
and
solids
removal,
the
type
of
lagoons
and
the
quality
and
thickness
of
the
resulting
residuals
differs
greatly
between
the
three
operating
facilities.
2 Facilities
Description
The
R.W.
Neilson
WTP
is
a
48
million
gallon
per
day
(MGD)
plant
that
consists
of
conventional
flocculation--sedimentation
using
aluminum--based
coagulant
followed
by
conventional
filtration.
The
plant
was
originally
24
MGD
and
has
been
expanded
twice
to
the
current
capacity.
The
three
lagoons
(one
large
and
two
medium--sized)
at
the
Neilson
WTP
are
unlined
shallow
earthen
basins
positioned
well
above
the
groundwater
table.
Residuals
are
typically
filled
to
about
6
feet
of
depth.
A
photo
of
the
Neilson
WTP
lagoon
is
shown
below.
Figure
1:
Neilson
WTP
Lagoon
--
Filling
Figure
2:
Neilson
WTP
Lagoon
--
Drying
The
P.
W.
Swann
WTP
is
a
25
MGD
facility
consisting
of
conventional
alum--based
flocculation--sedimentation
followed
by
conventional
filtration.
There
are
two
shallow
lagoons
at
the
plant
having
an
irregular
quadrilateral
shape.
One
is
slightly
larger
than
the
other.
Both
lagoons
at
the
Swann
WTP
are
clay
lined
with
concrete
side
slopes
and
are
positioned
low
on
the
site
within
the
influence
of
the
groundwater
table.
Residuals
are
typically
filled
to
about
6
feet
of
depth.
A
photo
of
the
Swann
WTP
lagoon
is
shown
below.
Figure
3:
Swann
WTP
Lagoon
--
Filling
Figure
4:
Swann
WTP
Lagoon
--
Drying
The
urbanized
site
of
the
R.
A.
Thomas
WTP
is
more
compact
than
either
the
Neilson
or
Swann
WTPs
and
does
not
offer
the
needed
space
for
the
large
shallow
lagoons
like
that
of
the
Neilson
and
Swann
WTPs.
Although
the
18
MGD
facility
was
completely
rebuilt
in
2010--2011,
the
lagoons
remained
in
the
same
location
with
some
improvements.
The
bottoms
of
the
lagoons
are
crushed
recycled
concrete
while
the
side
slopes
are
unlined
except
at
the
concrete
inlet
apron.
Underdrain
piping
is
normally
closed
during
the
filling
cycle
but
is
opened
for
drying.
The
underdrains
flow
to
the
Equalization
Basin
for
treatment
with
the
washwater.
The
lagoons
are
above
the
normal
water
table
and
are
normally
filled
to
about
8
feet
of
residuals.
A
photo
of
the
Thomas
WTP
lagoon
is
shown
below.
Figure
5:
Thomas
WTP
Lagoon
--
Filling
Figure
6:
Thomas
WTP
Lagoon
--
Drying
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