LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
9­
1
9.0
Second
Stage
Filtration
9.1
Introduction
The
LT2ESWTR
40
CFR
141.728(
c)
describes
second
stage
filtration
as
the
use
of
a
rapid
sand,
dual
media,
granular
activated
carbon
(
GAC),
or
other
fine
grain
media
unit
process
applied
in
a
separate
stage
following
rapid
sand
or
dual
media
filtration.
Applying
an
additional
layer
of
media,
such
as
a
GAC
cap,
on
an
existing
single
stage
filtration
unit
does
not
qualify
for
this
credit.

This
chapter
is
organized
as
follows:

9.2
LT2ESWTR
Compliance
Requirements
­
discusses
criteria
and
reporting
requirements
that
systems
must
meet
to
receive
Cryptosporidium
removal.

9.3
Toolbox
Selection
Considerations
­
discusses
issues
specific
to
second
stage
filtration
that
water
systems
should
consider
when
selecting
toolbox
options.

9.4
Design
and
Operational
Considerations
­
discusses
hydraulic
issues,
backwashing,
and
turbidity
monitoring
for
systems
that
integrate
a
second
stage
filtration
in
their
treatment
train.

9.2
LT2ESWTR
Compliance
Requirements
9.2.1
Credits
Under
the
LT2ESWTR,
a
system
that
employs
a
second,
separate
filtration
stage
meeting
the
following
criteria
may
receive
0.5
log
credit
for
Cryptosporidium
removal
(
40
CFR
141.728(
c)).

°
The
first
stage
of
filtration
is
preceded
by
a
coagulation
step
°
The
second
stage
of
filtration
is
comprised
of
rapid
sand,
dual
media,
GAC,
or
other
fine
grain
media
°
Both
filtration
stages
treat
100
percent
of
plant
flow
Chapter
9
­
Second
Stage
Filtration
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
9­
2
Under
the
LT2ESWTR,
a
system
integrating
a
slow
sand
filtration
process
for
the
second
stage
of
filtration
meeting
the
following
criteria
can
receive
2.5
log
credit
for
Cryptosporidium
removal
(
40
CFR
141.728(
d)).

°
No
disinfectant
residual
is
present
in
the
influent
to
the
slow
sand
filtration
process
°
Both
filtration
stages
treat
100
percent
of
plant
flow
9.2.2
Reporting
Requirements
To
receive
Cryptosporidium
removal
credit
for
compliance
with
the
LT2ESWTR,
systems
must
report
the
following
monthly
(
40
CFR
141.730):

$
Verification
that
100
percent
of
finished
water
was
treated
by
two
stages
of
filtration.
Actual
data
or
information
required
to
report
is
determined
by
the
State.
EPA
recommends
plant
piping
schematics
be
initially
reported
followed
by
monthly
operator
certification.

Reporting
for
LT2ESWTR
does
not
take
the
place
of
the
IESWTR
and
LT1ESWTR
reporting
requirements.
Specifically,
the
turbidity
of
the
combined
and
individual
filter
effluent
from
the
first
filtration
stage
must
be
reported
as
required
by
the
IESWTR
and
LT1ESWTR
(
40
CFR
141.74,
40
CFR
141.174(
a),
40
CFR
141.551,
and
40
CFR
141.560).

9.3
Toolbox
Selection
Considerations
Plants
already
employing
a
second
unit
process
that
meets
the
requirements
for
this
toolbox
option
(
e.
g.,
GAC
columns
to
meet
dissolved
organic
or
taste
and
odor
treatment
goals)
are
in
the
ideal
position
to
seek
credit.
Other
plants
that
have
enough
excess
filtration
capacity
or
unused
filter
beds
(
e.
g.,
built
in
anticipation
of
unrealized
plant
expansions),
may
be
able
to
convert
piping
to
enable
these
filters
to
operate
in
series
for
relatively
low
cost.
However,
many
plants
will
find
that
integrating
second
stage
filtration
into
an
existing
treatment
train
poses
significant
additional
space,
capital,
and
hydraulic
requirements.
These
systems
may
want
to
consider
this
option
if
the
additional
treatment
provides
other
benefits.
For
example,
systems
that
use
chloramination
and/
or
ozone
could
run
the
second
stage
under
biological
filtration
conditions
to
reduce
assimilable
organic
carbon
(
AOC),
which
promotes
biofilm
growth
and
nitrification
(
for
chloraminating
systems)
in
the
distribution
system.

Additionally,
plants
experiencing
taste
and
odor
problems
or
dissolved
organic
contaminants
in
their
raw
water
might
consider
installing
GAC
columns
to
alleviate
these
problems
and
also
receive
the
Cryptosporidium
removal
credit.
Chapter
9
­
Second
Stage
Filtration
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
9­
3
Slow
sand
filtration
plants
who
wish
to
consider
this
toolbox
option
should
either
have
sufficient
excess
filtration
capacity
to
allow
filters
to
operate
in
series
(
with
possible
piping
modifications)
or
have
sufficient
land
area
to
build
additional
filters.

9.3.1
Advantages
The
advantages
of
a
second
stage
filtration
process
are
the
same
for
both
rapid
and
slow
sand
plants
and
include
operator
familiarity
with
the
process,
ease
of
operation,
and
potential
to
reduce
disinfection
byproducts.
For
plants
with
existing
processes
and
infrastructure
meeting
the
two­
stage
requirements,
implementation
costs
are
likely
to
be
relatively
low.

9.3.2
Disadvantages
The
disadvantages
associated
with
second
stage
filtration
apply
primarily
to
those
plants
that
do
not
have
existing
processes
in
place
or
cannot
easily
convert
built­
in
infrastructure.
In
addition
to
the
capital
cost
for
new
filters,
these
plants
may
need
the
following
improvements
to
integrate
a
second
stage
of
filtration:

°
Space
if
there
is
currently
no
room
for
expansion
in
the
existing
plant
grounds
°
Additional
pumping
to
compensate
for
head
loss
associated
with
an
additional
filtration
process
°
Increased
backwash
supply
and
treatment
For
those
plants
that
have
existing
infrastructure
available
for
a
second
stage
of
filtration,
they
still
may
have
to
account
for
an
increased
volume
of
backwash
and
loss
of
head
due
to
the
second
stage.

Systems
with
rapid
sand
filtration
plants
that
are
considering
integrating
slow
sand
filtration
into
their
treatment
process
should
be
aware
of
the
following
differences
in
operation
and
performance
of
slow
sand
plants
compared
to
rapid
sand
plants:

°
More
space
required
for
slow
sand
plants
°
Decreased
filtering
performance
with
cold
temperatures
°
Maintenance
of
filters
requires
draining
and
scraping
a
thin
layer
off
the
top
of
the
filter
Chapter
9
­
Second
Stage
Filtration
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
9­
4
9.4
Design
and
Operational
Considerations
The
design
of
the
second
stage
is
site­
specific
and
depends
on
existing
infrastructure
(
e.
g.,
some
systems
may
have
enough
filtration
capacity
to
operate
filters
in
series)
and
space
and
hydraulic
requirements.
EPA
does
not
specify
or
restrict
certain
configurations,
beyond
the
requirement
that
all
flow
must
be
treated
by
both
stages.
Systems
that
have
existing
filters
not
in
use
or
not
used
to
capacity
may
reconfigure
the
piping
to
operate
in
series.
Media
sizing
for
the
second
stage
is
also
not
specified;
however,
typical
design
standards
for
regular
or
deep
bed
filters
should
be
followed.
If
the
filter
effluent
from
the
first
stage
is
not
combined
prior
to
second
stage,
the
turbidity
monitoring
for
IESWTR
and
LT1ESWTR
may
have
to
be
conducted
on
individual
filters.
For
these
cases,
systems
need
to
consult
with
the
State
to
develop
a
new
IESWTR
or
LT1ESWTR
filter
effluent
monitoring
plan.

9.4.1
Hydraulic
Requirements
Additional
pumps
may
be
needed
to
provide
the
necessary
head
between
the
first
and
second
stages
of
filtration.
The
number
of
pumps
and
total
number
of
filters
should
allow
for
redundancy,
to
ensure
that
sufficient
treatment
capacity
is
in
place
to
treat
all
the
plant
flow
in
the
event
of
equipment
breakdown
or
maintenance.
However,
the
filter
loading
rate
to
the
second
stage
does
not
necessarily
need
to
be
the
same
as
for
the
first
stage.
The
water
influent
to
the
second
stage
should
be
significantly
cleaner,
and
may
enable
higher
loadings.
Final
design
loading
rates
should
be
determined
in
consultation
with
the
State.

If
the
filter
effluent
from
the
first
stage
filters
is
not
combined
and
sent
to
the
second
stage
filters
via
a
distribution
box
or
other
flow
equalization
device,
plant
operation
may
be
more
complex.
For
example,
if
the
effluent
from
one
first
stage
filter
is
sent
to
just
one
second
stage
filter,
then
as
the
flow
from
first
filter
decreases
(
or
headloss
through
it
increases),
flow
through
the
second
filter
will
also
decrease,
unless
automatic
effluent
control
valves
are
installed
on
the
second
stage
filter.
Also,
in
this
case,
whenever
the
first
stage
filter
is
backwashed,
the
second
stage
filter
will
also
be
out
of
service.

9.4.2
Backwashing
Consistent
with
the
Filter
Backwash
Recycling
Rule,
the
filter
backwash
from
the
second
stage
(
as
well
as
the
first
stage)
must
be
recycled
to
the
head
of
the
plant
if
it
is
recycled.
The
existing
backwashing
capacity
may
be
limited
and
need
to
be
increased.
There
may
be
insufficient
finished
water
storage
to
supply
backwash
water
or
there
may
need
to
be
additional
pumping
capacity,
depending
upon
the
design
of
the
additional
filtration
stage
(
e.
g.,
if
the
existing
filters
have
a
small
area
and
the
new
filters
are
significantly
larger,
the
existing
backwash
pumps
may
not
be
able
to
supply
water
at
a
high
enough
flow
to
properly
expand
the
filter
bed).
It
is
likely
that
the
second
stage
filters
would
need
to
be
backwashed
less
frequently
than
the
first
stage
ones,
due
to
the
lower
solids
loading.
Chapter
9
­
Second
Stage
Filtration
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
9­
5
Filter
ripening
and/
or
filter­
to­
waste
times
for
the
second
filtration
stage
will
most
likely
differ
from
the
first
stage.

9.4.3
Turbidity
Monitoring
EPA
recommends
monitoring
the
turbidity
of
the
individual
filters
in
the
second
stage
in
order
to
be
able
to
identify
any
possible
filter
upset
situations.
Depending
on
the
first
filtration
stage
effluent
quality,
it
may
be
difficult
to
see
a
significant
difference
in
the
second
stage
effluent.
If
the
combined
second
stage
filter
effluent
is
the
only
process
stream
monitored,
it
is
unlikely
that
an
upset
in
one
second
stage
filter
could
be
detected.
