Appendix
C
Measuring
Ozone
Residual
Accurate
ozone
residual
data
will
allow
the
calculation
of
correct
log­
inactivation
values
and
maintain
optimized
performance.
Ozone
residual
measurements
might
be
inaccurate
if
sampled
or
measured
incorrectly.
Residual
measurement
Quality
Assurance
(
QA)
issues
include:

1.
Configuration
of
the
ozone
sample
collection
lines
within
the
contactor,
2.
Stability
of
the
indigo
trisulfonate
reagent
when
analyzing
grab
samples,
and
3.
Standardization
and
maintenance
of
on­
line
ozone
analyzers.

C.
1
Sample
Collection
The
ozone
residual
in
water
decays
rapidly.
The
half­
life
ranges
from
less
than
1
minute
to
more
than
20
minutes.
Ozone
contactors
are
sealed
vessels
with
sample
lines
that
penetrate
the
walls
or
roof
structure
of
the
contactor.
The
detention
times
in
the
sample
lines
should
be
as
short
as
possible
in
order
to
minimize
ozone
residual
decay
(
loss)
in
the
sample
lines.

The
ozone
residual
profile
in
a
contactor
will
vary
significantly
depending
on
the
method
of
operation,
water
quality
and
water
flow
conditions
(
e.
g.,
HDT).
A
separate
sample
port
located
at
the
outlet
of
each
chamber
within
the
contactor
allows
maximum
flexibility
for
sampling
ozone
residual
over
variable
operating
conditions.
Sample
ports
located
at
the
outlets
of
diffusion
chambers
should
be
placed
to
ensure
the
diffusers
do
not
interfere
with
the
collected
sample.

The
inlet
to
the
sample
pipe
inside
the
ozone
contactor
should
be
located
directly
in
the
main
flow
stream,
such
as
shown
in
Figure
C.
1.
The
inlet
should
extend
into
the
contactor
sufficiently
in
order
to
obtain
a
representative
sample
(
i.
e.
about
¼
to
½
of
the
contactor
width).
Gas
bubbles
might
be
carried
into
the
sample
inlet
and
cause
errors
in
the
residual
measurement.
A
sample
inlet
tube
that
is
flared
and
that
is
turned
either
upward
or
opposite
the
flow
of
the
water
(
depending
on
the
location)
reduces
the
potential
for
entrapment
of
gas
bubbles.
However
in
highly
turbid
waters,
a
vertical
inlet
and
flared
configuration
might
result
in
clogging
due
to
solids
deposition
inside
the
line.
In
these
cases
a
compromise
is
to
position
the
sample
line
such
that
the
inlet
is
horizontal
rather
than
vertical.
Figure
C.
1
Example
Sample
Locations
in
an
Over/
Under
Baffled
Bubble
Diffuser
Contactor
Minimizing
the
travel
time
through
the
sample
line
is
important,
especially
when
the
ozone
decay
rate
is
high
(
i.
e.,
ozone
half­
life
is
short).
It
is
desirable
to
minimize
the
travel
time
so
that
the
ozone
decay
is
<
10
percent.
Figure
C.
2
shows
the
relationship
between
simulated
sample
line
travel
time
and
ozone
residual
loss
for
various
ozone
half­
life
values.
For
example,
the
travel
time
in
the
sample
line
should
be
less
than
10
seconds
if
the
ozone
half­
life
is
one
minute,
in
order
to
maintain
the
ozone
residual
loss
at
or
below
10
percent.
Figure
C.
2
Relationship
Between
Ozone
Residual
Loss
and
Detention
Time
through
the
Ozone
Sample
Line
for
Various
Ozone
Half­
Life
Values
The
sample
line
diameter
should
be
large
enough
(
minimum
3/
8th
inch
inside
diameter
and
preferably
½
­
in
to
¾
­
inch)
to
minimize
clogging
of
the
line
with
suspended
solids.
Sample
pipe
diameter
and
flow
rate
should
be
selected
in
order
to:

1.
Maintain
consistent
flow
without
plugging
2.
Minimize
detention
time
in
the
sample
line
3.
Meet
flow
rate
requirements
of
an
on­
line
analyzer
installed
at
that
location
Gravity
flow
is
all
that
is
necessary
to
meet
sample
flow
requirements
in
most
locations.
In
other
cases,
pumping
is
necessary.
Sample
lines
might
contain
some
gas
bubbles
as
well
as
liquid.
It
is
important
to
ensure
that
lines
are
vented
in
high
spots
where
gas
binding
might
occur.
Gaseous
ozone
in
high
concentrations
is
hazardous
to
breathe.
Sample
line
vents
and
drains
should
be
directed
away
from
occupied
areas.

Some
of
these
points
are
touched
upon
in
Section
O.
3.2
of
Appendix
O
of
the
SWTR
Guidance
Manual
(
1991).
C.
2
Ozone
Residual
Measurement
Ozone
residual
is
determined
using
the
Indigo
Method
(
Standard
Methods
4500­
Ozone
­
20th
Edition,
1998)
when
analyzing
grab
samples.
The
method
assumes
that
high­
purity
reagents
are
used.
Since
the
publication
of
the
20th
Edition,
several
reports
(
Gordon
et
al.,
2000a
and
2000b;
Rakness
et
al.,
2001;
Rakness
and
Hunter,
2001;
and
Rakness
et
al.,
2002)
have
been
published
discussing
a
potential
biasing
in
the
Indigo
Method.
The
potential
biasing
involves
the
value
of
the
so­
called
"
sensitivity
factor",
f,
as
defined
in
the
Standard
Method.
In
short,
these
reports
suggest
that
the
actual
sensitivity
factor
might
be
lower
than
the
Standard
Method's
value,
and
hence
the
calculated
ozone
concentration
will
be
undervalued.

The
Standard
method's
proportionality
constant,
f,
(
0.42
L
mg­
1cm­
1)
that
is
used
to
calculate
the
ozone
residual
is
based
on
an
indigo
trisulfonate
molar
absorbance,
 ,
of
20,000
M­
1
cm­
1.
These
recent
reports
suggest
that
f
may
not
be
constant
and
may
depend
on:

1.
The
source
and
age
of
the
neat
indigo
trisulfonate
solid
2.
The
age
and
handling
of
the
indigo
stock
solution
that
is
prepared
as
part
of
the
method
Briefly,
these
reports
indicate
that,
due
to
either
of
the
above
aspects,
f
can
be
substantially
lower
than
0.42
L
mg­
1cm­
1.
In
other
words,
the
molar
absorbance
can
be
much
lower
than
20,000
M­
1cm­
1.
Gordon
et
al.
(
2000a
and
2000b),
Rakness
et
al.
(
2001),
Rakness
and
Hunter
(
2001),
and
Rakness
et
al.
(
2002)
reported
that
the
apparent
molar
absorbance
of
some
indigo
stock
solutions
might
be
as
low
as
11,000
M­
1cm­
1,
and
in
an
extreme
case
6,000
M­
1cm­
1.
The
authors
suggest
that
the
ramifications
of
applying
an
f
value
of
0.42
L
mg­
1cm­
1
when
the
solution
has
a
lower
true
f
value
are
the
underestimation
of
the
ozone
concentration.

These
issues
are
not
completely
resolved
at
the
time
of
the
writing
of
this
guidance
manual.
However,
the
evidence
is
suggestive
enough
to
warrant
a
new
recommended
QA
control
concerning
the
quality
of
the
indigo
stock
solution.
Should
changes
in
the
Standard
Method
be
approved
prior
to
issuing
the
final
version
of
this
guidance,
those
changes
will
be
discussed.

The
gravimetric
indigo
trisulfonate
method
is
fairly
easy
to
apply
in
the
field
and
is
accurate.
It
should
be
noted
that
the
method
described
herein
is
somewhat
different
than
the
20th
Edition
of
Standard
Methods
in
that
the
volume
of
both
the
blank
and
the
samples
are
determined
gravimetrically.
The
procedural
steps
include:

1.
Prepare
indigo
stock
solution
as
described
in
Standard
Methods
2.
Prepare
Reagent
II
solution
(
for
ozone
residuals
greater
than
0.05
mg/
L),
as
described
in
Standard
Methods.
3.
Prepare
flasks
for
sampling.

3.1.
Clean,
dry
and
label
several
125
mL
Erlenmeyer
flasks
(
enough
for
each
sample
plus
one
blank).

3.2.
Obtain
the
tare
weight
of
each
flask.

4.
Add
10.0
mL
of
Reagent
II
solution
to
each
flask.

5.
Add
approximately
90
mL
of
distilled
water
to
one
or
two
flasks
and
use
these
flasks
as
the
blank
(
i.
e.,
use
value
from
one
blank
or
average
of
values
from
two
blanks).

6.
Collect
ozone
sample.

6.1.
Thoroughly
flush
sample
line
to
be
used.

6.2.
Do
not
run
sample
down
the
side
of
the
flask,
as
this
will
cause
ozone
off­
gassing.

6.3.
Fill
flask
with
sample,
gently
swirling
flask
until
a
light
blue
color
remains.
Do
not
bleach
completely
or
the
residual
value
will
be
incorrect.

7.
Wipe­
dry
the
outside
of
sample
and
blank
flasks.

8.
Weigh
sample
and
blank
flasks.

8.1.
Total
weight
for
sample
is
tare
weight
of
flask
plus
10
mL
indigo
plus
added
sample.

8.2.
Total
weight
for
blank
is
tare
weight
of
flask
plus
10
mL
indigo
plus
added
distilled
water.

9.
Prepare
the
spectrophotometer
for
measuring
absorbance.

9.1.
Identify
the
cell
path
length
(
e.
g.,
1­
cm,
5­
cm,
etc.).

9.2.
Set
the
wavelength
to
600
nanometers.

10.
Measure
absorbance
of
blank
and
samples
within
four
hours.

10.1.
Follow
instructions
for
spectrophotometer
concerning
zeroing
the
instrument.

10.2.
Record
absorbance
of
each
sample
and
each
blank.

11.
Complete
calculations
­
see
example
below.

Example:

A
10
mL
aliquot
of
Reagent
II
solution
was
added
to
a
125
mL
Erlenmeyer
flask
that
was
used
for
the
blank.
The
flask
had
a
tare
weight
of
83.62
g.
The
final
weight
of
the
flask,
plus
the
10
mL
aliquot
of
reagent,
plus
the
added
distilled
water
was
179.77
g.
The
total
volume
of
the
10
mL
Reagent
II
aliquot
plus
added
distilled
water
was
determined
by
subtracting
the
bottle's
tare
weight
from
the
total
weight,
assuming
that
1
mL
of
liquid
weighs
1
g
(
96.15­
mL
=
[
179.77­
g
­
83.62­
g]
*
1­
mL
/
1
g).

The
spectrophotometer
had
a
path
length
of
1
cm.
The
absorbance
reading
of
the
gravimetric
blank
was
measured
as
0.234
cm­
1
at
wavelength
of
600
nm.
This
reading
must
be
corrected
for
the
difference
in
the
volume
of
the
blank
used
in
order
to
check
the
quality
of
the
reagent.
The
calculated
absorbance
of
a
1:
100
blank
dilution
can
be
determined
using
Equation
C­
1.
In
this
case,
the
1:
100­
absorbance
value
was
0.225­
cm­
1,
which
is
greater
than
or
equal
to
0.225­
cm­
1.
This
means
that
the
indigo
trisulfonate
solution
was
considered
acceptable.

(
C­
1)

The
125­
mL
flask
that
was
used
for
the
ozone
sample
had
a
tare
weight
of
94.10
g.
Sample
water
was
directed
into
the
10­
mL
of
Reagent
II
solution
until
a
light
blue
color
remained.
The
final
weight
of
the
flask,
plus
the
10­
mL
aliquot
plus
the
sample,
was
167.39
g.
The
absorbance
reading
at
a
path
length
of
1
cm
was
0.159.
The
volume
of
the
water
sample
was
63.29­
mL
(
63.29­
mL
=
[
167.39­
g
­
94.10­
g
­
10­
g]
*
1­
mL
/
1­
g).
The
ozone
residual
was
calculated
using
Equation
C­
2,
which
resulted
in
a
value
of
0.41
mg/
L.

(
C­
2)
(
)(
)
b
V
f
V
A
­
V
A
=
mg/
L
S
T
S
B
B




where
A
B
=
absorbance
of
the
blank
(
as
measured,
not
as
corrected
by
equation
C­
1)

A
S
=
absorbance
of
the
sample
V
B
=
volume
of
the
blank
plus
indigo,
mL
V
T
=
total
volume
of
the
sample
plus
indigo,
mL
V
S
=
volume
of
the
sample
(
total
weight
­
tare
weight
­
10)

f
=
0.42
b
=
path
length
of
cell,
cm
C.
3
On­
line
Ozone
Residual
Analyzer
Calibration
On­
line
ozone
residual
analyzers
are
available
that
can
continuously
monitor
ozone
residual
in
the
water.
This
makes
it
possible
to
automate
the
disinfection
credit
calculation
using
the
plant's
computer­
control
system.
However,
the
analyzers
must
be
maintained
properly
and
their
calibration
must
be
checked
periodically
so
that
readings
match
grab­
sample
results
that
are
based
on
the
indigo
trisulfonate
procedure.
Generally,
probe­
type
monitor
readings
tend
to
drift
downward
over
time
due
to
weakening
of
the
electrolyte
solution.
Calibration
checks
should
be
conducted
regularly,
such
as
at
least
once
per
week.
This
section
describes
a
calibration
check
protocol
which
involves
collecting
grab­
samples
and
analyzer
readings
simultaneously
and
comparing
the
values.

The
calibration
check
should
consist
of
collecting
at
least
three,
and
preferably
five,
ozone
residual
grab
samples
and
corresponding
analyzer
readings.
The
following
calibration
protocol
has
been
used
successfully
at
operating
ozone
facilities.

1.
Collect
three
to
five
grab­
sample
ozone
residuals.
Obtain
an
analyzer
reading
while
the
grab
sample
is
being
collected.
Wait
15
seconds
to
30
seconds
between
each
pair
of
grab
sample
and
analyzer
reading.

2.
Measure
the
ozone
residual
concentration
in
the
grab
samples
using
the
indigo
trisulfonate
method.

3.
Calculate
the
average
grab­
sample
ozone
residual
value
and
the
average
analyzer
ozone
residual
value.

4.
Compare
the
average
of
the
on­
line
analyzer
to
that
of
the
indigo
grab­
samples.
The
average
of
the
on­
line
analyzer
cannot
deviate
more
than
10%
or
0.05
mg/
L
(
which
ever
is
largest)
from
the
grab­
sample
average.
If
the
average
of
the
on­
line
analyzer
deviates
more
than
this,
then
adjust
the
meter
reading
per
the
manufacturer's
instructions.
Note
that
this
QA
control
is
two­
sided.
It
is
especially
important
that
the
on­
line
analyzer
not
record
more
than
10%
or
0.05
mg/
L
greater
than
the
grab
samples.
However,
a
negative
deviation
(
negative
bias),
while
not
effecting
public
safety,
may
also
be
useful
as
an
indication
of
a
malfunctioning
unit.

5.
Allow
the
analyzer
to
stabilize
for
a
period
of
30
minutes
after
adjusting
the
meter
reading
and
repeat
steps
1
through
4
until
the
difference
calculated
in
step
4
is
<
10%
of
the
grabsample
average
and
<
0.05
mg/
L.
