LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
1
5.0
Presedimentation
5.1
Introduction
Presedimentation
is
a
preliminary
treatment
process
used
to
remove
gravel,
sand,
and
other
material
from
the
raw
water
and
dampen
particle
loading
fluctuations
to
the
rest
of
the
treatment
plant.
This
toolbox
option
is
applicable
to
new
sedimentation
basins
only;
systems
with
existing
presedimentation
basins
that
are
required
to
conduct
source
water
monitoring
for
Cryptosporidium
must
collect
samples
after
the
basins
for
the
purposes
of
bin
classification
(
40
CFR
141.726(
a)).

Sedimentation
processes
are
common
in
the
water
treatment
process
and
much
design
and
operational
information
is
available.
However,
the
use
of
an
additional
sedimentation
basin
in
series,
or
a
pre­
sedimentation
basin
at
the
head
of
the
treatment
plant
is
not
as
common
as
the
standard
sedimentation
basin,
and
little
information
is
available.
Therefore,
the
guidance
provided
in
this
chapter
is
based
on
the
design
and
operational
principles
of
sedimentation
processes.

This
chapter
on
presedimentation
is
organized
as
follows:

5.2
LT2ESWTR
Compliance
Requirements
­
This
section
describes
the
criteria
presedimentation
basins
must
achieve
in
order
to
receive
Cryptosporidium
removal
credit.

5.3
Toolbox
Selection
Considerations
­
This
section
assists
systems
in
determining
whether
the
presedimentation
toolbox
option
is
a
viable
and
beneficial
option
for
meeting
the
LT2ESWTR
bin
requirements.

5.4
Types
of
Presedimentation
Basins
­
This
section
compares
several
sedimentation
basins
and
clarifiers
in
terms
of
structure
and
factors
affecting
settling
efficiency.

5.5
Design
and
Operating
Issues
­
This
section
discusses
typical
design
and
operational
issues
including
redundancy,
short
circuiting,
sludge
removal,
and
coagulant
addition.
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
2
Systems
with
existing
presedimentation
basins
must
monitor
for
Cryptosporidium
after
the
presedimentation
basin
and
prior
to
the
main
treatment
plant
for
the
purpose
of
determining
bin
assignment
and
cannot
receive
presedimentation
credit
towards
Cryptosporidium
removal
to
meet
the
bin
requirements
(
40
CFR
141.704(
b)).
5.2
LT2ESWTR
Compliance
Requirements
5.2.1
Credits
Presedimentation
basins
with
coagulant
addition
may
receive
0.5
log
Cryptosporidium
removal
credit
under
the
LT2ESWTR
if
they
meet
the
following
criteria
(
40
CFR
141.726(
a)):

°
The
presedimentation
basin
must
be
in
continuous
operation
and
must
treat
all
of
the
flow
reaching
the
filters.

°
A
coagulant
must
be
continuously
added
to
the
presedimentation
basin
(
or
prior
to)
while
the
plant
is
in
operation.

°
The
presedimentation
basin
must
achieve
0.5
log
(
68
percent)
turbidity
reduction
on
an
average
monthly
basis,
for
at
least
11
of
the
12
previous
months.
For
those
systems
not
operating
year­
round,
the
0.5
log
turbidity
reduction
must
be
met
for
all
but
any
one
of
the
operating
months,
based
on
the
last
12
consecutive
months.

5.2.2
Monitoring
Requirements
Systems
must
measure
presedimentation
basin
influent
and
effluent
turbidity
at
least
once
per
day,
or
more
frequently
as
determined
by
the
State
(
40
CFR
141.726(
a)).

5.2.3
Calculations
For
compliance
with
the
LT2ESWTR,
the
log
turbidity
reduction
must
be
calculated
as
a
monthly
mean,
from
readings
collected
daily,
according
to
the
following
equation
(
40
CFR
141.726(
a)).

Log
Reduction
=

Log
10(
Monthly
Average
Influent
Turbidity)
­
Log
10(
Monthly
Average
Effluent
Turbidity)
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
3
Or
if
calculated
as
a
percent,

Percent
Reduction
=

(
Monthly
Average
Influent
Turbidity)
­
(
Monthly
Average
Effluent
Turbidity)
x
100
(
Monthly
Average
Influent
Turbidity)

Example
Calculation
Average
influent
turbidity
=
16.3
NTU
Average
effluent
turbidity
=
4.2
NTU
Log
Reduction
=
Log
10(
16.3)
­
Log
10(
4.2)
=
0.59
Percent
Reduction
=
(
16.3­
4.2)/
16.3
=
74.2%

5.3
Toolbox
Selection
Considerations
The
purpose
of
this
section
is
to
assist
systems
in
determining
whether
the
presedimentation
toolbox
option
is
a
viable
and
beneficial
option
for
meeting
the
LT2ESWTR
bin
requirements.
There
are
two
general
aspects
for
systems
to
evaluate
when
considering
this
toolbox
option:

1)
Can
the
turbidity
removal
requirements
be
met
consistently
over
the
expected
range
of
raw
water
conditions?

2)
What
are
the
advantages
and
disadvantages
of
installing
a
presedimentation
basin?

For
presedimentation,
the
first
question
is
driven
by
source
water
particle
load
and
how
much
of
that
load
a
proposed
sedimentation
basin
would
remove.
Before
researching
potential
presedimentation
designs,
a
system
should
determine
if
their
source
water
has
a
high
enough
turbidity
on
a
consistent
basis.
Section
5.3.1
discusses
the
source
water
characteristics
necessary
to
meet
the
compliance
requirements.
Section
5.3.2
discusses
the
advantages
and
disadvantages
of
adding
a
presedimentation
process
to
the
treatment
train.

5.3.1
Source
Water
Quality
To
meet
the
0.5
log
turbidity
removal
requirement,
the
source
water
should
have
consistently
high
turbidity.
When
influent
turbidity
is
low,
most
presedimentation
basins
will
have
difficulty
achieving
0.5
log
reduction.
For
example,
if
a
system
has
an
average
of
10
NTU
source
water
turbidity
for
a
few
months
of
the
year,
the
average
effluent
turbidity
would
have
to
be
3.2
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
4
NTU
for
those
months,
which
could
be
difficult
for
some
systems
to
achieve.
Table
5.1
lists
influent
and
effluent
turbidity
values
that
yield
0.5
log
reduction.

Table
5.1
Influent
and
Effluent
Turbidity
Values
Resulting
in
0.5
Log
Reduction
Monthly
Average
Turbidity
(
NTU)
Monthly
Average
Turbidity
(
NTU)

Influent
Effluent
Influent
Effluent
2
0.6
50
15.8
5
1.6
60
19.0
10
3.2
70
22.1
20
6.3
80
25.3
30
9.5
90
28.5
40
12.6
100
31.6
5.3.2
Advantages
and
Disadvantages
of
Installing
a
Presedimentation
Basin
The
presedimentation
process
can
reduce
influent
fluctuations
in
particle
loading,
flow,
and
other
water
quality
parameters.
An
additional
sedimentation
process
in
series
provides
increased
operational
flexibility
to
handle
rapid
changes
in
influent
turbidity.
It
also
allows
for
enhanced
performance
of
subsequent
processes
in
the
treatment
plant.
Although,
if
the
presedimentation
effluent
turbidity
is
too
low,
the
second
sedimentation
process
may
not
be
able
to
provide
significant
removal
since
removal
performance
is
enhanced
by
increased
particle
load.

As
with
the
addition
of
many
unit
processes,
the
two
major
disadvantages
are
capital
costs
and
land
requirements.
The
requirement
of
coagulant
addition
may
increase
chemical
costs,
although
the
amount
added
in
the
next
stage
could
be
reduced.
Whether
these
chemical
costs
offset
each
other
is
site­
specific.

5.4
Types
of
Sedimentation
Basins
There
are
several
types
of
sedimentation
basins
(
also
called
clarifiers)
used
for
drinking
water
treatment.
Selection
of
a
basin
for
presedimentation
should
be
based
on
turbidity
removal
capability
and
meeting
the
flow
and
space
requirements
of
the
facility.
The
focus
of
this
chapter
is
on
guidance
for
complying
with
the
LT2ESWTR,
therefore
the
discussion
in
this
section
is
limited
to
factors
affecting
settling
efficiency,
as
measured
by
turbidity
removal.
Further
information
on
design
can
be
found
in
the
following
literature:
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
5
°
Water
Quality
and
Treatment
 
A
Handbook
of
Community
Water
Supplies,
5th
ed.
(
AWWA
1999)

°
Integrated
Design
and
Operation
of
Water
Treatment
Facilities,
2nd
ed.
(
Kawamura
2000)

Table
5.2
provides
a
comparison
of
several
sedimentation
basins
and
clarifiers.
It
is
likely
that
only
horizontal
clarifiers
would
be
chosen
for
presedimentation,
since
they
are
less
complex
in
operation
compared
to
the
others
(
i.
e.,
upflow,
high
rate,
reactor,
and
ballasted
sand
clarifiers).
The
table
includes
the
additional
types
since
some
plants
that
choose
to
employ
the
presedimentation
toolbox
option
may
elect
to
use
their
current
sedimentation
basin
for
presedimentation
and
construct
a
new
basin
for
primary
sedimentation.
The
performance
advantages
and
disadvantages
listed
in
the
table
relate
to
settling
efficiency
or
indications
for
potential
process
upset.
These
were
derived
from
Integrated
Design
and
Operation
of
Water
Treatment
Facilities
(
Kawamura
2000)
and
are
characteristic
of
sedimentation
processes,
not
specifically
presedimentation
processes.
The
remainder
of
this
section
provides
short
descriptions
of
different
clarifier
types.
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
6
Table
5.2
Comparison
of
Sedimentation
and
Clarifier
Types
Type
Performance
Advantages
Performance
Disadvantages
Applicable
for
Presedimentation
and
Sedimentation
Horizontal
Flow
(
general)
­
Easy
to
operate
and
maintain
Rectangular
Basin
­
Tolerant
to
shock
loads
­
Good
for
handling
large
flows
­
Subject
to
wind
and
density
currents
(
causing
short­
circuiting)
­
Designs
with
trays
have
shown
poor
settling
efficiency
Circular
Basin
­
Easy
sludge
removal
­
Can
obtain
high
clarification
efficiency
­
Greater
potential
for
hydraulic
imbalance
in
comparison
to
rectangular
basin
(
not
good
for
removing
alum
flocs)

Applicable
for
Sedimentation
Upflow
Clarifier
(
general)
­
High
clarification
efficiency
­
Need
constant
flow
rate
and
water
quality
­
Limitations
on
size
Center
Feed
­
Easy
sludge
removal
­
Short
circuiting
Peripheral
Feed
­
Good
for
source
water
with
high
solids
­
Potential
short­
circuiting
High
Rate
Settlers
(
horizontal
flow
or
upflow)
­
Increases
the
hydraulic
load
capability
and
settling
efficiency
of
horizontal
flow
basins
and
clarifiers
­
Can
form
scales
(
calcium
carbonate)
which
clog
flow
­
Poor
flocculation
possible
Reactor
Clarifiers
(
general)
­
Good
clarification
due
to
seeding
effect
­
Need
constant
flow
rate
and
water
quality
­
Requires
greater
operator
skill
High
recirculation
and
mechanical
sludge
plow
­
Tolerant
to
shock
loads
­
Dependent
on
one
drive
unit
­
Limitations
on
size
Sludge
blanket
zone
and
mechanical
sludge
plow
­
Good
turbidity
removal
­
Very
sensitive
to
shock
loads
­
Requires
2­
4
days
to
build
sludge
blanket
Ballasted
sand
­
Can
handle
higher
flows
with
very
low
detention
times
(
on
the
order
of
minutes)
­
Can
handle
shock
particle
loads
without
increasing
coagulant
dose
­
Quick
process
startup
­
Short
detention
time
means
not
much
time
for
process
adjustments
Note:
Adapted
from
"
Integrated
Design
and
Operation
of
Water
Treatment
Facilities."
Kawamura
(
2000).
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
7
Sedimentation
processes
can
be
categorized
in
three
general
types:
horizontal
flow
basins
or
clarifiers,
upflow
clarifiers,
and
reactor
clarifiers.
High
rate
settlers
are
modified
horizontal
or
upflow
clarifiers
with
plate
or
tube
modules
placed
into
the
basin
to
increase
the
settling
area.
An
additional
design
described
in
this
chapter
that
differs
from
the
three
general
types
is
ballasted
sand
or
high­
rate
microsand
process
(
a
proprietary
design).

5.4.1
Horizontal
Flow
5.4.1.1
Rectangular
In
rectangular
sedimentation
tanks
the
water
flows
in
one
end
and
ideally
proceeds
through
the
basin
in
a
plug
flow
manner.
A
uniform
distribution
at
the
inlet
is
an
important
design
factor.
Rectangular
basins
can
be
susceptible
to
density
currents
that
cause
short
circuiting.
These
basins
are
easy
to
operate,
have
low
maintenance
costs,
offer
predictable
performance
under
most
conditions,
and
are
most
tolerant
to
shock
loads.
High
rate
settlers
can
be
easily
installed
to
improve
settling
efficiency.
Rectangular
basins
are
particularly
well
suited
for
large
systems
compared
to
circular
basins
that
require
additional
space
and
yard
piping
for
equivalent
flow.

5.4.1.2
Circular
The
flow
in
circular
basins
is
more
commonly
from
a
center
feed
well,
radially
outward
to
the
peripheral
weirs.
In
comparison
to
rectangular
basins,
circular
basins
will
have
more
land
between
the
basins
and
also
require
more
yard
piping.
Circular
basins
have
easy
sludge
removal,
can
obtain
high
clarification
efficiency,
and
are
adaptable
to
high
rate
settling
modules.
However,
if
flow
distribution
from
the
inlet
is
not
uniform,
the
settling
efficiency
will
be
hindered.
These
basins
are
not
as
hydraulically
stable
as
rectangular
basins.

5.4.2
Upflow
Clarifier
In
upflow
clarifiers
the
influent
enters
at
the
bottom
and
clarified
water
flows
upward
while
the
solids
settle
to
the
bottom.
As
with
horizontal
flow
basins,
upflow
clarifiers
can
also
be
modified
with
high
rate
settling
modules.
Upflow
clarifiers
can
provide
higher
clarification
efficiency
than
horizontal
flow,
however,
they
are
more
sensitive
to
shock
loads
than
horizontal
flow
basins.

5.4.3
Reactor
Clarifier
Reactor
clarifiers
use
the
seeding
concept
to
improve
settling.
The
water
flows
through
the
sludge
layer
so
particles
can
coalesce
with
already
formed
flocs.
Two
common
designs
of
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
8
reactor
clarifiers
are
slurry
recirculation
and
sludge
blanket
clarifiers.
Both
operate
on
a
center
feed
system
with
built­
in
flocculation
zones.
The
process
is
more
complex
than
traditional
horizontal
or
upflow
clarifiers.
Reactor
clarifiers
can
provide
high
clarification
efficiency
but
at
the
cost
of
flexibility
C
the
source
water
quality
and
hydraulic
loads
must
be
constant.

5.4.4
High
Flow
Rate
Designs
High
rate
settlers
are
modules
of
inclined
tubes
or
plates
that
are
installed
in
horizontal
flow
(
plates
only)
or
simple
upflow
clarifiers.
They
provide
increased
surface
area
for
particles
to
settle
and
reduce
settling
time.
Kawamura
(
2000)
noted
poor
performance
occurred
when
flow
distribution
was
uneven
and
flocculation
was
poor.

5.4.5
Ballasted
Flocculation
Ballasted
flocculation
is
a
high­
rate,
physical­
chemical
clarification
process
that
uses
sand
to
improve
the
settling
of
flocculated
particles.
The
floc
attaches
to
the
surface
of
a
sand
particle,
which
has
a
settling
time
20
to
60
times
faster
than
an
alum
floc
(
Kawamura
2002),
thus
creating
a
high­
rate
settling
process.
Because
of
the
increased
settling
rate,
the
space
required
is
much
less
than
other
clarifiers.

5.5
Design
and
Operational
Issues
5.5.1
Redundancy
As
stated
earlier,
for
compliance
with
the
LT2ESWTR,
all
flow
must
be
treated
by
the
presedimentation
process
to
receive
Cryptosporidium
treatment
credit
(
40
CFR
141.726(
a)).
Systems
should
consider
the
need
for
redundancy
in
the
design
of
a
presedimentation
process.
Smaller
systems
or
systems
with
a
demand
much
lower
than
the
design
capacity
may
be
able
to
shut
down
the
water
treatment
plant
for
presedimentation
basin
maintenance
activities
and,
thus,
not
require
additional
basins
for
redundancy.
However,
systems
that
operate
on
a
continuous
basis
do
not
have
that
flexibility
and
should
have
a
plan
for
staying
in
compliance
while
a
basin
is
shut
down.

5.5.2
Short
Circuiting
A
common
issue
that
must
be
considered
in
the
design
and
operation
of
presedimentation
basins
is
short­
circuiting.
If
a
portion
of
flow
does
not
receive
close
to
the
intended
treatment
(
in
this
case,
detention
time),
then
the
effluent
turbidity
is
likely
to
be
higher
than
anticipated.
Several
factors
affect
short­
circuiting
including
even
distribution
of
flow
at
the
inlet,
density
or
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
9
temperature
differentials
between
influent
and
basin
water,
surface
currents,
and
basin
cleaning
and
sludge
removal.

A
proper
design
of
the
inlet
is
one
of
the
most
important
design
factors.
In
addition
to
flow
short­
circuiting,
a
poorly
designed
inlet
can
lead
to
overall
hydraulic
instability
in
the
settle
zone.
Installation
of
perforated
baffles
is
a
simple
and
effective
method
for
even
flow
distribution
from
the
inlet
to
the
basin.

Temperature
differentials
and
high
wind
velocities
could
induce
circular
currents
in
the
vertical
direction
of
the
basin.
Influent
water
warmer
than
the
basin
water
will
rise
to
the
surface
and
reach
the
outlet
of
the
sedimentation
basin
much
faster
than
the
intended
detention
time
of
the
basin.
Influent
water
colder
than
the
basin
water
will
dive
to
the
bottom
of
the
basin
and
flow
along
the
bottom
of
the
basin
and
rise
to
the
top
of
the
basin
at
the
outlet,
thereby
reaching
the
outlet
of
the
sedimentation
basin
much
faster
than
the
intended
detention
time
of
the
basin.
Above
ground
tanks
built
of
steel
are
more
susceptible
to
temperature
differentials
from
exposure
to
the
sun
and
heat
transfer.

The
degradation
of
effluent
water
quality
due
to
wind
is
more
noticeable
in
circular
or
square
sedimentation
basins
of
diameters
greater
than
100
B
115
feet.
When
using
long,
shallow
rectangular
settling
basins,
effects
of
wind
induced
currents
can
be
minimized
by
ensuring
that
the
longitudinal
axis
of
the
basin
is
perpendicular
to
the
prevailing
wind
direction.
In
addition
to
causing
flow
short­
circuiting,
currents
can
also
scour
settled
solids,
causing
resuspension
of
settled
solids
and
increasing
effluent
turbidity.

5.5.3
Sludge
Removal
Sludge
build­
up
in
the
tank
decreases
the
volume
of
the
sedimentation
basin
and
reduces
the
settling
time
in
the
basin.
Additionally,
as
sludge
builds
up,
particles
become
more
susceptible
to
resuspension
during
sludge
removal,
increasing
the
effluent
turbidity.
Sedimentation
basins
with
high
rate
settlers
accumulate
sludge
rapidly,
and
therefore
require
continuous
sludge
removal.

5.5.4
Coagulant
Addition
and
Dose
Ranges
of
Common
Coagulants
Current
operational
practice
of
presedimention
processes
often
focus
on
mitigating
shock
loads
in
the
raw
water
supply
(
such
as
turbidity
spikes
due
to
precipitation
in
river
source
waters).
However,
during
periods
of
low
influent
turbidity
less
attention
may
be
given
to
the
actual
performance
of
the
basin,
resulting
in
less
than
0.5
log
turbidity
reduction
through
the
basin.
To
receive
the
credit,
the
presedimentation
basin
may
need
to
be
operated
more
stringently,
including
the
addition
of
coagulant.
The
coagulant
dose
required
to
treat
an
influent
stream
depends
on
the
chemical
composition
of
the
influent,
the
characteristics
of
the
colloids
and
suspended
matter
in
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
10
the
influent,
the
addition
of
a
coagulant
aid,
the
water
temperature,
and
mixing
conditions.
Coagulant
dose
and
other
water
chemistry
parameters
of
the
coagulation
and
sedimentation
processes
are
system­
specific.
Jar
test
procedures
for
evaluating
the
appropriate
coagulants,
dosages,
and
other
chemical
attributes
for
a
treatment
train
are
provided
in
AWWA's
Operational
Control
of
Coagulation
and
Filtration
Processes.
Chapter
5
­
Presedimentation
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
5­
11
References
AWWA.
2000.
Operational
Control
of
Coagulation
and
Filtration
Processes,
AWWA
Manual
M37,
Second
Edition,
pp.
1­
34.

Kawamura,
Susumu.
2000.
Integrated
Design
and
Operation
of
Water
Treatment
Facilities.
John
Wiley
&
Sons,
Inc.

USEPA.
1998.
Optimizing
Water
Treatment
Plant
Performance
Using
the
Composite
Correction
Program,
pp.
233­
236.
