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{float:right|border=2px solid black|width=300px}
!sedtanks.jpg|width=300px!
h5. Figure 1: Sedimentation Tanks at the CuartoCuatro Communidades plant
{float}
h2. |
Introduction
...
and
...
Objectives
...
The
...
Cuatro
...
Communidades
...
plant
...
has
...
two
...
sedimentation
...
tanks
...
designed
...
for
...
an
...
upflow
...
velocity
...
of
...
70
...
m/day.
...
The
...
tanks
...
are
...
shallower
...
than
...
those
...
of
...
previously
...
constructed
...
Aguaclara
...
plants.
...
Like
...
the
...
other
...
plants,
...
the
...
tanks
...
use
...
lamella
...
to
...
increase
...
their
...
efficiency.
...
The
...
sedimentation
...
tanks
...
were
...
designed
...
to
...
accommodate
...
a
...
floc
...
blanket.
...
A
...
floc
...
blanket
...
is
...
suspension
...
of
...
flocs
...
that
...
are
...
too
...
large
...
to
...
rise
...
to
...
the
...
top
...
of
...
the
...
tank
...
and
...
two
...
small
...
to
...
settle
...
out.
...
A
...
floc
...
blanket
...
should
...
act
...
as
...
a
...
filter
...
trapping
...
flocs
...
as
...
they
...
rise
...
to
...
the
...
top
...
of
...
the
...
sedimentation
...
tanks.
...
In
...
the
...
laboratory,
...
...
...
have
...
been
...
show
...
to
...
greatly
...
increase
...
effluent
...
water
...
quality
...
in
...
a
...
flocculation-sedimentation
...
system.
...
The
...
sedimentation
...
tanks
...
in
...
the
...
Cuatro
...
Communidades
...
plants
...
are
...
a
...
much
...
larger
...
less
...
controlled
...
system
...
and
...
it
...
was
...
unclear
...
whether
...
it is
...
possible
...
to
...
form
...
and
...
maintain
...
a
...
floc
...
blanket
...
in
...
the
...
plant.
...
No
...
previous
...
observations
...
of
...
the
...
tanks
...
showed
...
conclusive
...
evidence
...
of
...
a
...
floc
...
blanket.
...
The
...
Cuatro
...
Communidades
...
plant
...
was
...
designed
...
with
...
a
...
plant
...
flow
...
rate
...
of
...
380
...
liters
...
per
...
minute but
...
the
...
available
...
head
...
in
...
the
...
conduction
...
line
...
set a
...
plant
...
flow
...
rate
...
of
...
only
...
190
...
-270
...
liters
...
per
...
minute.
...
At
...
these
...
flow
...
rates
...
no
...
effects
...
of
...
a
...
floc
...
blanket
...
were
...
observed
...
and
...
the
...
upflow
...
velocity
...
may
...
not
...
have
...
been
...
great
...
enough
...
to
...
keep
...
flocs
...
in
...
suspension.
...
One
...
tank
...
was
...
shut
...
off
...
to
...
increase
...
the
...
upflow
...
velocity
...
through
...
the
...
sedimentation
...
tanks
...
and
...
to attempt
...
to
...
form
...
a
...
floc
...
blanket.
Methods
It should be kept in mind that during these tests, the main purpose of the plant was to provide potable water for the communities. These tests were a lower priority and this created several initial challenges to shutting off one sedimentation tank. When all the flow passed through one tank, the head loss through the original effluent launders was high enough to flood the plant. New effluent launders were created to run one tank. They were designed with a head loss of four centimeters using the equation:
| Latex |
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\large
\[
h = \left( {\frac{Q}
{{K A}}} \right)^2 \frac{1}
{{2g}}
\]
|
where:
- Q is the plant flow rate
- A is the total area of the effluent launder orifices
- K is a minor loss coefficient
- h is the head loss
To increase the area and decrease the head loss the effluent launder holes were increased from 5/16 in to 7/16 in.
Another problem was that in the first three attempts to run one tank the effluent turbidity increased from below 10 NTU to above 20 NTU. At this point it was unacceptable to send the effluent to the distribution line and the water was sent to waste. This procedure could only be maintained for 2-3 hours during the day without draining the water storage tank for the communities. No improvement in effluent quality was observed in these trials. The plant was recently cleaned before the second and third of the three trials. During these initial trials the incoming turbidity was above 100 NTU.
In order to shut off the plant for a longer period of time, the fourth trial was run at night when the communities were not consuming water. The sedimentation tank was cleaned at 9 pm before the plant was started. The plant was successfully running by 12:00 pm with an influent turbidity of 40 NTU and effluent turbidity of 4 NTU. It should be noted that the three hours in between were spent cleaning the sedimentation tank and filling the plant. After thirty six hours, the lamella were removed from the tank to see if there was floc blanket beneath them. After the lamella were replaced, the plant continued to run with only one sedimentation tank for the next four days. The incoming turbidity was consistently between 20 and 40 NTU.
Results and Discussion
As was mentioned, in the first three attempts, the effluent turbidity spiked initially. The trials could not be run for longer than three hours without disrupting the communities water supply. In the third of these initial trials, it was hard to visually distinguish between the water at the end of the flocculator and the end of the sedimentation tank closest to the flocculator after an hour. The turbidity measured at the end of the flocculator was 119.1 NTU and in the back of the sedimentation tank the turbidity was 102.4 NTU indicating that flocs were directly rising as they entered the back of the sedimentation tanks. However, close to the effluent channel the turbidity measured in the sedimentation tank was 27.12 NTU indicating that most of the flocs were rising in the back of the tank. This third test was only one and a half hours and the effluent quality from the sedimentation tank was consistently decreasing. It is unclear whether this upflow velocity was too high for the tanks or if, given more time, a floc blanket would have formed.
In the fourth test, there was no spike in effluent as had been expected from the first three trials. The plant was run for thirty six hours with no significant changes in effluent quality (figure 2). No evidence of a floc blanket was seen in the plant sludge judge.
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h2. Methods It should be kept in mind that during these tests, the main purpose of the plant was to provide potable water for the communities. These tests were a lower priority and therefore there were several initial challenges to shutting off one sedimentation tank. When all the flow passed through one tank, the head loss through the original effluent launders was high enough to flood the plant. New effluent launders were created to run one tank. They were designed with a head loss of four centimeters using the equation: where: * Q is the plant flow rate * A is the total area of the effluent launder orifices * K is a minor loss coefficient * h is the head loss To increase the area and decrease the head loss the effluent launder holes were increased from 5/16 in to 7/16 in. Another problem was that in the first three attempts to run one tank the effluent turbidity increased from below 10 NTU above 20 NTU. At this point it was unacceptable to send the effluent to the distribution line and the water was sent to waste. This procedure could only be maintained for 2-3 hours during the day without draining the water storage tank for the communities. No improvement in effluent quality was observed in these trials. The plant was recently cleaned before the second and third of the three trials. During these initial trials the incoming turbidity was above 100 NTU. In order to shut off the plant for a longer period of time, the fourth trial was run at night when the communities was not consuming water. The sedimentation tank used was cleaned at 9 pm before the plant was started. The plant was successfully running by 12:00 pm with an influent turbidity of 40 NTU and effluent turbidity of 4 NTU. It should be noted that the three hours in between were spent cleaning the sedimenation tank and filling the plant. After thirty six hours, the lamella were removed from the tank to see if there was floc blanket beneath them. After the lamella were replaced, the plant continued to run with only one sedimentation tank for the next four days. The incoming turbidity was consistently between 20 and 40 NTU. h2. Results and Discussion As was mentioned, in the first three attempts, the effluent turbidity spiked initially. The trials could not be run for longer than three hours without disrupting the communities water supply. In the third of these initial trials, it was hard to visually distinguish between the water at the end of the flocculator and the end of the sedimentation tank closest to the flocculator after an hour. The turbidity measured at the end of the flocculator was 119.1 NTU and in the back of the sedimentation tank the turbidity was 102.4 NTU indicating that flocs were directly rising as the entered the back of the sedimentation tanks. However, close to the effluent channel the turbidity measured in the sedimentation tank was 27.12 NTU indicating that most of the flocs were rising in the back of the tank. This third test was only one and a half hours and the effluent quality from the sedimentation tank was consistently decreasing. It is unclear whether this upflow velocity was too high for the tanks or if, given more time, a floc blanket would have formed. In the fourth test, there was no spike in effluent as had been expected from the first three trials. The plant was run for thirty six hours with no significant changes in effluent quality (figure 2), although the plant was not monitored the second night. No evidence of a floc blanket was seen in the plant sludge judge. {float:border=2px solid black|width=600px} !sedgraph.gif|width=600px! h5. Figure 2 {float} |
Although
...
the
...
plant
...
was
...
not
...
monitored
...
for
...
the
...
second
...
night, it did not rain,
...
no
...
changes
...
in
...
the
...
plant
...
conditions
...
were
...
observed
...
in
...
the
...
morning
...
and
...
the
...
influent
...
conditions
...
were
...
constant
...
before
...
and
...
after
...
the
...
test.
...
It
...
is
...
reasonable
...
to
...
assume
...
that
...
the
...
influent
...
and
...
effluent
...
did
...
not
...
significantly
...
change
...
in
...
the
...
night.
...
Finally
...
to
...
determine
...
if
...
there
...
was
...
a
...
floc
...
blanket,
...
water
...
was
...
siphoned
...
from
...
different
...
heights
...
in
...
the
...
sedimentation
...
tank.
...
The
...
turbidity
...
was
...
consistently
...
below
...
12
...
NTU
...
below
...
the
...
lamella.
...
However
...
the
...
exact
...
height
...
of
...
the
...
hose
...
was
...
never
...
determined
...
because
...
the
...
hose
...
was
...
slightly
...
buoyant
...
and
...
could
...
not
...
be
...
seen
...
below
...
the
...
lamella.
...
If
...
the
...
test
...
if
...
repeated,
...
the
...
hose
...
should
...
be
...
weighted
...
down
...
or
...
attached
...
to
...
a
...
rigid
...
pole
...
to
...
measure
...
the
...
turbidity
...
at
...
an
...
exact
...
depth.
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\\ {float:left|border=2px solid black|width=300px} !halflamella.jpg|width=300px! h5. Figure 3: half the lamella were removed from the sedimentation tank {float} |
After
...
thirty
...
six
...
hours,
...
the
...
lamella
...
were
...
removed
...
from
...
the
...
tanks
...
(figure
...
3).
...
Other
...
than
...
the
...
initial
...
disturbance
...
of
...
removing
...
the
...
lamella
...
from
...
the
...
tank
...
the
...
water
...
was
...
fairly
...
clear.
...
It
...
was
...
possible
...
to
...
see
...
to
...
the
...
top
...
of
...
the
...
plates
...
resting
...
in
...
the
...
bottom
...
of
...
the
...
sedimentation
...
tanks.
...
Flocs
...
that
...
had
...
settled
...
in
...
the
...
channel
...
entering
...
the
...
sedimentation
...
tanks
...
were
...
swept
...
into
...
the
...
tank
...
in
...
an
...
attempt
...
to
...
see
...
where
...
they
...
were
...
going.
...
It
...
was
...
not
...
possible
...
to
...
see
...
what
...
happened
...
to
...
these
...
flocs.
...
However,
...
in
...
general
...
it
...
seemed
...
as
...
though
...
more
...
flocs
...
were
...
rising
...
in
...
the
...
middle
...
of
...
the
...
sedimentation
...
tanks
...
than
...
the
...
sides.
...
When
...
the
...
lamella
...
were
...
replaced
...
and
...
one
...
tank
...
was
...
shut
...
off
...
for
...
the
...
following
...
four
...
days,
...
there
...
was
...
no
...
measured
...
improvement
...
in
...
effluent
...
quality.
...
During
...
this
...
period
...
of
...
time
...
the
...
plant
...
influent
...
was
...
consistently
...
below
...
40
...
NTU
...
and
...
the
...
flocculator
...
was
...
working
...
well.
...
These
...
results
...
indicate
...
that
...
flocs
...
were
...
not
...
reaching
...
the
...
sedimentation
...
tanks
...
but
...
settling
...
out
...
in
...
the
...
flocculator,
...
and the
...
channel
...
in
...
between
...
the
...
flocculator
...
and
...
the
...
sedimentation
...
tanks
...
during
...
the
...
thirty
...
six
...
hour
...
test.
...
Figure
...
4
...
shows
...
the
...
sludge
...
build
...
up
...
underneath
...
the
...
plates
...
in
...
the
...
sedimentation
...
tanks
...
after
...
a
...
month
...
without
...
cleaning.
...
Flocs
...
were
...
clearly
...
reaching
...
the
...
sedimentation
...
tanks
...
when
...
the
...
incoming
...
turbidity
...
was
...
higher
...
in
...
the
...
first
...
few
...
trials.
...
It was
...
unclear
...
whether
...
or
...
not
...
a
...
floc
...
blanket
...
could
...
have
...
formed
...
at
...
high
...
incoming
...
turbidities.
...
One
...
tank
...
should
...
be
...
shut
...
off
...
at
...
night
...
when
...
higher
...
turbidities
...
are
...
coming
...
to
...
the
...
plant
...
to
...
see
...
if
...
a
...
floc
...
blanket
...
can
...
formed
...
in
...
a
...
system
...
as
...
complex
...
as
...
the
...
Cuatro
...
Communidades
...
sedimentation
...
tanks
...
given
...
more
...
time.
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{float:border=2px solid black|width=300px} !sludge.jpg|width=300px! h5. figure 4: sludge settled below the plates in the bottom of the sed tank {float} |
However,
...
if
...
a
...
floc
...
blanket
...
can
...
be
...
formed
...
at
...
this
...
plant
...
given
...
ideal
...
conditions,
...
it
...
may
...
unreasonable
...
to
...
expect
...
that
...
one
...
can
...
be
...
maintained
...
at
...
this
...
plant.
...
The
...
amount
...
of
...
flocs
...
reaching
...
the
...
sedimentation
...
tanks
...
would
...
need
...
to
...
be
...
increased
...
to
...
ensure
...
that
...
the
...
floc
...
blanket
...
could
...
be
...
sustained
...
at
...
low
...
incoming
...
turbidities.
...
Also
...
the
...
plant
...
often
...
needs
...
to
...
be
...
shut
...
off
...
for
...
cleaning
...
or
...
minor
...
malfunctions,
...
at
...
this
...
point
...
the
...
floc
...
blanket
...
would
...
collapse.
...
If
...
the
...
effluent
...
turbidity
...
must
...
initially
...
spike
...
before
...
improving,
...
the
...
first
...
three
...
attempts
...
to
...
run
...
one
...
tank
...
show
...
that
...
it
...
takes
...
longer
...
for
...
the
...
floc
...
blanket
...
to
...
form
...
than
...
the
...
residence
...
time
...
of
...
the
...
communities'
...
water
...
storage
...
tank
...
and
...
their
...
water
...
demand
...
allows.
...