...
Chemical
...
Dose
...
Controller
...
Abstract
At the Cuatro Comunidades plant, the operator sets the plant's
...
alum
...
dose
...
using
...
a
...
...
...
...
...
.
...
The
...
appropriate
...
alum
...
dose
...
varies
...
based
...
on
...
two
...
factors,
...
the
...
incoming
...
turbidity
...
and
...
the
...
plant
...
flow
...
rate.
...
The
...
CDC
...
is
...
designed
...
to
...
maintain
...
a
...
constant
...
dose
...
regardless
...
of
...
changes
...
in
...
the
...
plant
...
flow
...
rate.
...
The
...
CDC
...
at
...
the
...
plant
...
was
...
monitored
...
and
...
improvements
...
were
...
made
...
to
...
the
...
original
...
model.
...
The
...
actual
...
chemical
...
doses
...
were
...
compared
...
with
...
the
...
theoretical
...
doses
...
for
...
both
...
models.
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\\ {float:border=2px solid black|width=600px} !CDCCDCschematic.jpg|width=600px! h5. Figure 1a. CDC schematic 1b. Original Cuatro Comunidades CDC {float} \\ h2. Introduction and Objectives The original CDC used in the Cuatro Communidades plant was the first generation of the technology used by AguaClara, (Figure 1). The CDC was made up of three components--- a flow control module (FCM), lever arm and a sutro weir. The alum flow rate into the plant was set by the available head between the FCM inlet valve and outlet hose. The available head was changed by moving the outlet hose along the lever arm. Once the hose was positioned on the lever arm, the flow rate should have been constant regardless of the available head in the alum stock tank. The sutro weir was used to create a linear relationship between plant flow rate and the height of water in entrance tank. The lever arm united the first two components of the CDC. The outlet hose of the FCM was connected to one end of the lever arm while a float balanced the opposite end of the fulcrum. The float height varies with the water level in the entrance tank which was determined by the sutro weir. When the float rose, the opposite end of the lever arm with the FCM outlet hose fell increasing available head in the FCM and therefore the alum dose. The coordination of plant flow rate and alum dose created by the CDC decreased the number of variables involved in selecting an appropriate alum dose. \\ The alum dose is set by positioning the outlet hose of the CDC at a certain distance from the fulcrum of the lever arm. When water is flowing through the plant the end of the lever arm attached to the dosing hose is angled downward. By moving the dosing hose further away from the fulcrum, the hose outlet is also moved downward in relationship to the inlet of the FCM, increasing the head available to drive the alum flow rate. This relationship between distance along the lever arm and increasing alum dose should be linear. Over the course of the summer, the alum flow rate was measured and the CDC system was monitored to detect any failure modes. After observing the original CDC, an improved aluminum model was constructed. The theoretical and actual flow rates for both CDCs are compared below. h2. Methods The alum doses delivered by both the original lever arm the aluminum model were measured. The alum dose was determined by measuring the flow rate delivered by the CDC. The dose was then calculated using the equation: {latex} \large $$ C_{dose} = {{Q_{FCM}{C_stock} } \over {Q_{plant} }} $$ {latex} where: * C ~dose~ was the alum dose * Q ~FCM~ was the alum flow rate * C ~stock~ was the chemical stock concentration * Q ~plant~ was the plant flow rate This dose was then compared to the expected theoretical dose delivered by the flow controller. Two different method were used to measure the flow rate. For the PVC lever arm the flow was measured filling a graduated cylinder for thirty seconds. The alum flow rate of the original CDC was measured every time the operator changed the dose. The alum flow rate tests were performed over a series of weeks. For the aluminum CDC, a more accurate measurement method was implemented to obtain the flow rate data. A clear PVC pipe was inserted between the chemical stock tank and the flow control module (Figure 2). {float:border=2px solid black|width=600px} !measurementdevice.jpg|width=600px|height=350px! h5. Figure 2: Flow rate measurement device {float} \\ To measure flow rate, the first valve from the stock tank to the PVC pipe was opened and the PVC pipe was filled. The line bypassing the PVC pip was then shut off and the time for the cylinder to empty to obtain the actual flow rate was measured. {float:right|border=2px solid black|width=300px} !Alumsettling.jpg|width=300px! h5. Figure 3: Change in color indicates settling in solution after several hours without mixing {float} It should be noted that in both methods, change in stock concentration over time was not accounted actual alum dose. The operator mixes the stock multiple times daily but some alum settles out of the solution in between mixing (Figure 3). Therefore it is likely that the actual dose was lowered than calculated in these tests. \\ h2. Results and Discussion h3. Original CDC |
Introduction and Objectives
The original CDC used in the Cuatro Communidades plant was the first generation of the technology used by AguaClara, (Figure 1). The CDC was made up of three components--- a flow control module (FCM), lever arm and a sutro weir. The alum flow rate into the plant was set by the available head between the FCM inlet valve and outlet hose. The available head was changed by moving the outlet hose along the lever arm. Once the hose was positioned on the lever arm, the flow rate should have been constant regardless of the available head in the alum stock tank. The sutro weir was used to create a linear relationship between plant flow rate and the height of water in entrance tank. The lever arm united the first two components of the CDC. The outlet hose of the FCM was connected to one end of the lever arm while a float balanced the opposite end of the fulcrum. The float height varied with the water level in the entrance tank which was determined by the sutro weir. When the float rose, the opposite end of the lever arm with the FCM outlet hose fell increasing available head in the FCM and therefore increasing the alum dose. The coordination of plant flow rate and alum dose decreased the number of variables involved in selecting an appropriate alum dose.
The alum dose was set by positioning the outlet hose of the CDC at a certain distance from the fulcrum of the lever arm. As the plant flow rate rose the end of the lever arm attached to the dosing hose angled further downward. By moving the dosing hose further away from the fulcrum, the hose outlet was also moved downward with respect to the inlet of the FCM, increasing the head available to drive the alum flow rate. This relationship between distance along the lever arm and increasing alum dose should be linear. Over the course of the summer, the alum flow rate was measured and the CDC system was monitored to detect any failure modes. After observing the original CDC, an improved aluminum model was constructed. The theoretical and actual flow rates for both CDCs are compared below.
Methods
The alum doses delivered by both the original PVC lever arm and the aluminum model were measured. The alum dose was determined by measuring the flow rate delivered by the CDC. The dose was then calculated using the equation:
| Latex |
|---|
where:
- C dose was the alum dose
- Q FCM was the alum flow rate
- C stock was the chemical stock concentration
- Q plant was the plant flow rate
This actual dose was then compared to the theoretical dose. Two different methods were used to measure the flow rate. For the PVC lever arm the flow was measured by filling a graduated cylinder for thirty seconds. The alum flow rate of the original CDC was measured every time the operator changed the dose. The alum flow rate tests were performed over a series of weeks.
For the aluminum CDC, a more accurate measurement method was implemented to obtain the flow rate data. A clear PVC pipe was inserted between the chemical stock tank and the flow control module (Figure 2).
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{float:border=2px solid black|width=600px}
!measurementdevice.jpg|width=600px|height=350px!
h5. Figure 2: Flow rate measurement device
{float} |
To measure flow rate, the first valve from the stock tank to the PVC pipe was opened and the PVC pipe was filled. The line bypassing the PVC pipe was then shut off and the time for the cylinder to empty to obtain the actual flow rate was measured.
Results and Discussion
Original CDC
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{float:right|border=2px solid black|width=600px}
!OriginalCDCGraph.gif|width=600px!
h5. Figure 24
{float}
\\
|
As
...
can
...
be
...
seen
...
in
...
figure
...
4,
...
the
...
actual
...
dose
...
delivered
...
by
...
the
...
CDC
...
was
...
highly
...
variable
...
close
...
to
...
the
...
fulcrum,
...
and
...
the
...
CDC
...
was
...
consistently
...
under
...
dosing
...
further
...
along
...
the
...
lever
...
arm.
...
Thirty
...
centimeters
...
away
...
from
...
the
...
fulcrum
...
the
...
CDC
...
was
...
supposed
...
to
...
deliver
...
a
...
maximum
...
dose
...
of
...
60
...
mg/L.
...
However,
...
the
...
CDC
...
was
...
only
...
dosing
...
an
...
average
...
of
...
35
...
mg/L.
...
The
...
range
...
of
...
doses
...
provided
...
by
...
this
...
CDC
...
was
...
appropriate
...
for
...
most
...
incoming
...
turbidities
...
(<200
...
NTU)
...
to
...
the
...
plant.
...
However,
...
the
...
dose
...
was
...
not
...
adequate
...
for
...
the
...
high
...
turbidity
...
raw
...
water
...
coming
...
to
...
the
...
plant
...
after
...
a
...
rainstorm.
...
Additionally
...
the
...
variability
...
in
...
the
...
dose
...
made
...
it
...
impossible
...
to
...
associate
...
a
...
particular
...
distance
...
along
...
the
...
lever
...
arm
...
with
...
a
...
specific
...
alum
...
dose.
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{float:left|border=2px solid black|width=300px} !la_attachment.jpg|width=300px! h5. Figure 45: Attachment of FCM hose to PVC lever arm {float} |
Some
...
of
...
this
...
variability
...
is
...
inevitably
...
due
...
to
...
the
...
fact
...
that
...
the
...
measurements
...
were
...
taken
...
on
...
different
...
days
...
and
...
the
...
FCM outlet hose
...
may
...
have
...
been
...
clogged
...
when
...
the
...
measurements
...
were
...
taken.
...
However,
...
several
...
defects
...
in
...
the
...
lever
...
arm
...
design
...
were
...
also
...
noted,
...
and
...
a
...
new
...
model
...
was
...
made.
...
The
...
original
...
design
...
consisted
...
of
...
a
...
PVC
...
lever
...
arm
...
with
...
notches
...
cut
...
into
...
it
...
for
...
each
...
chemical
...
dose.
...
The
...
dosing
...
hose
...
and
...
a
...
pressure
...
break
...
tee
...
were
...
moved
...
up
...
and
...
down
...
the
...
lever
...
arm
...
by
...
a
...
loop
...
of
...
fishing
...
line
...
and
...
a
...
knot
...
that
...
could
...
be
...
attached
...
to
...
each
...
slit,
...
(figure
...
5).
...
The
...
notches
...
on
...
the
...
slit
...
meant
...
the
...
operator
...
could
...
only
...
choose
...
incremental
...
doses.
...
The
...
alum
...
dose
...
at
...
each
...
notch
...
should
...
have
...
been
...
constant.
...
However,
...
the
...
fishing
...
line
...
connection
...
could
...
be
...
adjusted
...
within
...
each
...
notch
...
so
...
the
...
dose
...
was
...
not
...
consistent
...
with
...
location.
...
Furthermore
...
the
...
lever
...
arm
...
was
...
not
...
located
...
at
...
a
...
sufficient
...
height
...
in
...
the
...
entrance
...
tank.
...
At
...
high
...
flow
...
rates
...
the
...
end
...
of
...
the
...
lever
...
arm
...
was
...
submerged,
...
(figure
...
6).
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{float:right|border=2px solid black|width=350px} !1lawater.jpg|width=350px! h5. Figure 56: The end of the lever arm is almost submerged {float} |
At
...
high
...
plant
...
flow
...
rates,
...
when
...
the
...
water
...
level
...
in
...
the
...
entrance
...
tank
...
rose,
...
the
...
pressure
...
break
...
tee
...
and
...
hosing
...
floated
...
above
...
the
...
water
...
level.
...
This
...
may
...
have
...
decreased
...
the
...
distance
...
between
...
the
...
hose
...
outlet
...
and
...
the
...
level
...
arm.
...
If
...
this
...
was
...
true,
...
then the
...
CDC
...
was
...
delivering
...
lower
...
doses
...
at
...
higher
...
plant
...
flow
...
rates.
...
Additionally,
...
the
...
sutro
...
weir
...
often
...
clogged
...
with
...
leaves
...
if
...
it
...
was
...
not
...
constantly
...
monitored
...
and
...
cleaned.
...
Leaves
...
and
...
grit
...
blocked
...
the
...
open
...
surface
...
area
...
of
...
the
...
sutro
...
weir
...
and
...
increased
...
the
...
height
...
of
...
water
...
in
...
the
...
entrance
...
tank
...
when
...
there
...
was
...
no change
...
in
...
the
...
plant
...
flow
...
rate.
...
Because
...
the
...
alum
...
dose
...
was
...
based
...
on
...
the
...
height
...
of
...
water
...
in
...
the
...
entrance
...
tank
...
and
...
not
...
directly
...
on
...
the
...
plant
...
flow
...
rate,
...
the
...
alum
...
dose
...
increased
...
as
...
well.
...
This
...
issue
...
can
...
be
...
fixed
...
by
...
better
...
monitoring
...
and
...
cleaning
...
on
...
the
...
part
...
of
...
the
...
operator.
...
To
...
eliminate
...
the
...
problem
...
for
...
the
...
operator
...
a
...
new
...
configuration
...
of
...
larger
...
holes
...
that
...
is not
...
easily
...
clogged
...
could
...
be
...
designed
...
or
...
a
...
prescreening
...
system
...
could
...
be
...
put
...
in
...
place
...
before
...
the
...
sutro
...
weir.
Aluminum CDC
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h3. Aluminum CDC {float:left|border=2px solid black|width=400px} !aluminumleverarm.jpg|width=400px|height=350px! h5. Figure 67 Aluminum CDC {float} |
A
...
new
...
lever
...
arm
...
was
...
constructed
...
and
...
monitored.
...
The
...
lever
...
arm
...
was
...
made
...
of
...
a
...
rust
...
resistant
...
bar
...
of
...
aluminum
...
(Figure
...
7).
...
The
...
FCM
...
outlet
...
hose
...
slid
...
along
...
the
...
lever
...
arm
...
and
...
fixed
...
in
...
place
...
by
...
a
...
screw.
...
This
...
sliding
...
system
...
gave
...
the
...
operator
...
more
...
flexibility
...
when
...
choosing
...
an
...
alum
...
dose.
...
The
...
hose
...
hung
...
from
...
a
...
short
...
aluminum
...
bar.
...
The
...
difference
...
between
...
theoretical
...
and
...
actual
...
chemical
...
flow
...
rates
...
was
...
measured
...
for
...
this
...
lever
...
arm
...
as
...
well.
...
The
...
actual
...
alum doses of this chemical doser can be seen in Figure 8.
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doses after improvements were made to the chemical doser can be seen in Figure 8. {float:border=2px solid black|width=600px} !aluminumgraph.gif|width=600px! h5. Figure 8 {float} |
The
...
data
...
in
...
figure
...
8
...
was
...
taken
...
after
...
the
...
FCM
...
hosing
...
had
...
been
...
cleaned
...
and
...
was
...
taken
...
over
...
the
...
span
...
of
...
several
...
hours
...
rather
...
than
...
several
...
weeks
...
as
...
was
...
the
...
case
...
in
...
figure
...
3.
...
Each
...
actual
...
dose
...
point
...
represents
...
the
...
average
...
of
...
three
...
trials.
...
The
...
results
...
show
...
that
...
the
...
dose
...
is
...
increasing
...
linearly
...
with
...
plant
...
flow
...
rate
...
as
...
expected.
...
However,
...
the
...
CDC
...
was
...
still
...
incapable
...
of
...
reaching
...
the
...
higher
...
end
...
of
...
the
...
design
...
flow
...
rates.
...
Again,
...
the
...
maximum
...
dose
...
did
...
not
...
reach
...
60
...
mg/L.
...
This
...
means
...
that
...
there
...
was too
...
much
...
head
...
loss
...
in
...
the
...
hosing
...
system
...
of
...
the
...
FCM.
...
The
...
length
...
of
...
the
...
hose
...
controls
...
the
...
head
...
loss
...
in
...
the system. Shortening this hose would increase the maximum flow rate. Another way to increase the maximum dose delivered by the CDC would be to lengthen the lever arm which would give the ability to position the outlet hose further below the inlet of the FCM.
From figure 8, it is impossible to say whether the variability in the chemical dose at each location on the lever arm improved. The system used to measure the flow rate was more precise. The data was only taken at one plant flow rate and the system had just been cleaned. The data taken with the original PVC lever arm was taken at multiple plant flow rates and the system was not necessarily clean when each data point was taken.
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system so shortening this hose would increase the maximum flow rate. Another way to increase the maximum dose delivered by the CDC would be to lengthen the lever arm which would give the ability to position the outlet hose further below the inlet of the FCM. \\ From figure 8, it is impossible to say whether the variability in the chemical dose at each location on the lever arm improved. The system used to measure the flow rate was more precise. The data was only taken at one plant flow rate and the system had just been cleaned. The data taken with the original PVC lever arm was taken at multiple plant flow rates and the system was not necessarily clean when each data point was taken. {float:right|border=2px solid black|width=250px} !hosefitting.jpg|width=250px|height=350px! h5. Figure 9: Connection of hose to lever arm {float} |
A
...
final
...
adjustment
...
was
...
made
...
after
...
this
...
test
...
to
...
reduce
...
variability
...
in
...
the
...
dose---
...
the
...
hose
...
was
...
directly
...
attached
...
to
...
the
...
lever
...
arm
...
and
...
the
...
short
...
aluminum
...
bar
...
was
...
removed
...
(figure
...
9).
...
The
...
short
...
aluminum
...
bar
...
could
...
be
...
attached
...
to
...
on position along the
...
lever
...
arm
...
at
...
an
...
angle
...
changing
...
the
...
distance
...
between
...
the
...
hose
...
and
...
the
...
FCM
...
inlet.
...
The
...
change
...
was
...
made
...
to
...
fix
...
the
...
distance
...
between
...
the
...
FCM
...
inlet
...
and
...
outlet
...
hose
...
at
...
each
...
position.
...
This
...
will
...
help
...
maintain
...
a
...
constant
...
dose
...
at
...
each
...
lever
...
arm
...
position.
...
Additionally,
...
the
...
change
...
increased
...
the
...
distance
...
between
...
the
...
hose
...
and
...
the
...
water
...
level
...
in
...
the
...
entrance
...
tank
...
so
...
the
...
hose
...
will
...
not
...
be
...
submerged
...
at
...
high
...
plant
...
flow
...
rates.
...
Conclusions
The CDC did show the linear relationship between outlet hose position along the lever arm and alum dose that was expected. However the system was consistently under dosing the maximum alum doses. This can be fixed by shortening the dosing hose or lengthening the lever arm. The connection between the dosing hose and the lever arm should be rigid in order to ensure that the distance of the hose outlet from the lever arm is constant as the hose is moved along the lever arm. Further investigation should be made to see if the variability of dose delivered at a specific position along the lever arm can be improved.