Cuatro Communidades Flocculator
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h1. Cuatro Communidades Flocculator {float:right|border=2px solid black|width=240px}!flocculator.JPG|width=240px! h5. Figure 1: Flocculation Channels at the Cuatro Communidades Plant {float} h2. Abstract The flocculator at the Cuatro Communidades plant uses a vertical set of baffles to mix alum with contaminants in the raw water. The Cuatro Communidades flocculator has three channels. The baffles are arranged in a tapered configuration so that flocs are exposed to higher energy dissipation rates initially and lower rates as they grow. In this study, tube settlers were used to measure residual turbidity at different points along the flocculator. Tube settlers were placed along the flocculator to determine how much flocculation was occurring in the last flocculator channel. The results were compared at different raw water turbidities. h2. Introduction and Objectives During flocculation alum is mixed with raw water and in the process aggregates of contaminants called flocs are formed. Flocs settle out of water more easily than the contaminants themselves because they are heavier. Alum is a coagulant used to help floc formation. Conventional flocculators typically use a long slow mechanical stirring process to mix alum and contaminants together. This long process slowly builds large flocs. To avoid the use of electricity, flocculation occurs as water snakes through a vertical set of baffles. The energy dissipation rate through these baffles is set by the distance between the baffles. This rate is a measure of how much mixing is occurring. The Cuatro Communidades plant has a tapered set of baffles. The baffles are close together and the energy dissipation rates are high early in the flocculator. When the flocs have grown and are more fragile the energy dissipation is lower. It was unclear at what point in the flocculator the flocs were large enough to settle out in the sedimentation tank. Tube settlers were used to measure residual turbidity along the flocculation channels. Tube settlers were placed at different locations in the flocculator to determine if adequate flocculation had occurred before the end of the flocculator and if the last channel in the flocculator was improving residual turbidity. h2. Methods Each tube settler functioned in as an isolated pair of lamella in the sedimentation tanks. In order to mimic the design of the lamella in the Cuatro Communidades plant the tube settlers were designed to rest at a sixty degree angle, alpha, and with a critical velocity, V ~alpha~, of 10 m/day. The critical velocity of the tube settler determines the size of the smallest floc that is able to settle out in the tube settler. The critical velocity is calculated by the equation: |
Abstract
The flocculator at the Cuatro Communidades plant uses a vertical set of baffles to mix alum with contaminants in the raw water. The Cuatro Communidades flocculator has three channels. The baffles are arranged in a tapered configuration so that flocs are exposed to higher energy dissipation rates initially and lower rates as they grow. In this study, tube settlers were used to measure residual turbidity at different points along the flocculator. Tube settlers were placed along the flocculator to determine how much flocculation was occurring in the last flocculator channel. The results were compared at different raw water turbidities.
Introduction and Objectives
During flocculation alum is mixed with raw water and in the process aggregates of contaminants called flocs are formed. Flocs settle out of water more easily than the contaminants themselves because they are heavier. Alum is a coagulant used to help floc formation. Conventional flocculators typically use a long slow mechanical stirring process to mix alum and contaminants together. This long process slowly builds large flocs. To avoid the use of electricity, flocculation occurs as water snakes through a vertical set of baffles. The energy dissipation rate through these baffles is set by the distance between the baffles. This rate is a measure of how much mixing is occurring.
The Cuatro Communidades plant has a tapered set of baffles. The baffles are close together and the energy dissipation rates are high early in the flocculator. When the flocs have grown and are more fragile the energy dissipation is lower. It was unclear at what point in the flocculator the flocs were large enough to settle out in the sedimentation tank. Tube settlers were used to measure residual turbidity along the flocculation channels. Tube settlers were placed at different locations in the flocculator to determine if adequate flocculation had occurred before the end of the flocculator and if the last channel in the flocculator was improving residual turbidity.
Methods
Each tube settler functioned in as an isolated pair of lamella in the sedimentation tanks. In order to mimic the design of the lamella in the Cuatro Communidades plant the tube settlers were designed to rest at a sixty degree angle, alpha, and with a critical velocity, V alpha, of 10 m/day. The critical velocity of the tube settler determines the size of the smallest floc that is able to settle out in the tube settler.
The critical velocity is calculated by the equation:
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{latex}
\large
$$
V_c = {{DV_\alpha } \over {\sin \alpha (L\sin \alpha \cos \alpha + D)}}
$$ {latex}
|
where:
...
- V c is the critical velocity
- D is the tube settler diameter
- L is the length of the tube settler
Wiki Markup {latex}\large $$ \alpha $$ {latex}
...
is
...
- sixty
...
- degrees
Wiki Markup {latex}\large $$ V_\alpha $$ {latex}
...
is
...
- the
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- velocity
...
- in
...
- the
...
- tube
...
- settler
...
The
...
tube
...
settlers
...
were
...
designed
...
to
...
siphon
...
water
...
from
...
the
...
flocculator.
...
Water
...
settled
...
out
...
in
...
a
...
PVC
...
pipe
...
and
...
passed
...
into
...
a
...
small
...
flexible
...
tube
...
from
...
which
...
it
...
was
...
collected.
...
The
...
flow
...
rate
...
was
...
controlled
...
by
...
the
...
height
...
difference
...
between
...
the
...
tube
...
settler
...
outlet
...
and
...
the
...
water
...
level
...
in
...
the
...
flocculator
...
using
...
the
...
equation:
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{latex} \large $$ h = {{V_{siphon}^2 } \over {2g}} - {{V_\alpha ^2 } \over {2g}} + h_l $$ {latex} |
where:
...
- h
...
- is
...
- the
...
- height
...
- difference
...
- used
...
- to
...
- control
...
- the
...
- siphon
...
- V siphon is the exit velocity of the clear tube
Wiki Markup {latex}\large $$ V_\alpha $$ {latex}
...
is
...
- the
...
- velocity
...
- in
...
- the
...
- tube
...
- settler
- h l was the head loss in the entire system
The tube settlers were placed at various locations in the last two flocculator channels and the effluent turbidity from the tube settlers was measured.
The turbidity was measured every thirty minutes at each location and the results were averaged. Results were only considered valid when the raw water turbidity and the alum dose were constant throughout the entire trial. It should be noted that the flow rate was not constant in every trial. This affects the results because decreasing the flow rate decreases the energy dissipation rate through the baffles. A table of flow rates and trials can be seen below.
Table 1
Raw Water Turbidity (NTU) | Flow Rate (L/min) |
|---|---|
115 | 255 |
72 | 285 |
35 | 255 |
30 | 210 |
Results and Discussion
Residual turbidities were measured at four different incoming turbidities along the flocculator shown in figure 3.
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* h ~l~ was the head loss in the entire system The tube settlers were placed at various locations in the last two flocculator channels and the effluent turbidity from the tube settlers was measured. The turbidity was measured every thirty minutes at each location and the results were averaged. Results were only considered valid when the raw water turbidity and the alum dose were constant throughout the entire trial. It should be noted that the flow rate was not constant in every trial. This affects the results because decreasing the flow rate decreases the energy dissipation rate through the baffles. A table of flow rates and trials can be seen below. h5. Table 1 ||Raw Water Turbidity (NTU)||Flow Rate (L/min)|| |115|255| |72|285| |35|255| |30|210| h2. Results and Discussion Residual turbidities were measured at four different incoming turbidities along the flocculator shown in figure 3. {float:border=2px solid black|width=600px}!flocculatorgraph.gif|width=600px! h5. Figure 3 {float} |
The
...
results
...
are
...
not
...
very
...
comparable
...
as
...
different
...
incoming
...
turbidities
...
were
...
measured
...
at
...
different
...
points
...
along
...
the
...
flocculator
...
and
...
at
...
different
...
flow
...
rates.
...
However,
...
the
...
results
...
show
...
a
...
1-3
...
NTU
...
decrease
...
in
...
residual
...
turbidity
...
in
...
the
...
end
...
of
...
the
...
third
...
flocculator
...
channel—
...
between
...
baffles
...
45
...
and
...
59.
...
Flocs
...
must
...
be
...
growing
...
much
...
more
...
slowly
...
here
...
compared
...
to
...
the
...
first
...
two
...
channels
...
where
...
floc
...
growth
...
reduces
...
the
...
residual
...
turbidity
...
more
...
rapidly.
...
Although
...
this
...
final
...
reduction
...
seems
...
small,
...
it
...
is
...
not
...
insignificant.
...
In
...
the
...
case
...
of
...
the
...
30
...
NTU
...
raw
...
water
...
turbidity
...
trial—
...
the
...
trial
...
that
...
saw
...
the
...
largest
...
decrease
...
in
...
turbidity
...
in
...
the
...
final
...
channel—
...
the
...
turbidity
...
decreases
...
to
...
4.5
...
NTU,
...
sixty
...
percent
...
of
...
the
...
7.7
...
NTU
...
that
...
entered
...
the
...
final
...
channel.
...
While
...
the
...
improvements
...
in
...
turbidity
...
in
...
the
...
final
...
channel
...
are
...
small
...
they
...
are
...
still
...
a
...
significant
...
percent
...
of
...
the
...
incoming
...
residual
...
turbidity.
...
It
...
should
...
be
...
noted
...
that
...
the
...
flow
...
rate
...
was
...
much
...
lower
...
in
...
the
...
30
...
NTU
...
trial
...
therefore
...
the
...
energy
...
dissipation
...
rate
...
was
...
lower
...
as
...
well.
...
Because
...
of
...
the
...
difference
...
in
...
energy
...
dissipation
...
rates,
...
flocculation
...
was
...
probably
...
occurring
...
later
...
at
...
this
...
low
...
flow
...
rate.
...
This
...
would
...
explain
...
the
...
larger
...
decrease
...
in
...
residual
...
turbidity
...
in
...
the
...
final
...
channel.
...
The
...
flow
...
rate
...
through
...
the
...
plant
...
was
...
variable
...
and
...
it
...
was
...
not
...
uncommon
...
to
...
see
...
plant
...
flow
...
rates
...
as
...
low
...
as
...
210
...
L/min.
...
The
...
third
...
flocculator
...
channel
...
was
...
probably
...
more
...
beneficial
...
when
...
the
...
plant
...
flow
...
rate
...
was
...
lower.
...
A
...
peak
...
in
...
turbidity
...
is
...
seen
...
after
...
baffle
...
36
...
in
...
the
...
middle
...
of
...
the
...
second
...
flocculator
...
channel
...
in
...
the
...
30
...
NTU
...
raw
...
water
...
turbidity
...
trial.
...
An
...
earlier
...
trial
...
(not
...
shown)
...
indicated
...
this
...
peak
...
might
...
be
...
after
...
baffle
...
29.
...
This
...
trial
...
is
...
not
...
shown
...
because
...
plant
...
conditions
...
were
...
changing
...
as
...
data
...
was
...
being
...
taken.
...
This
...
peak
...
is
...
due
...
to
...
flocs
...
that
...
large
...
enough
...
to
...
block
...
the
...
light
...
from
...
the
...
turbidimeter
...
but
...
too
...
small
...
to
...
settle
...
out
...
quickly.
...
This
...
peak
...
was
...
not
...
measured
...
in
...
the
...
other
...
three
...
trials.
...
If
...
the
...
location
...
of
...
this
...
peak
...
is
...
of
...
interest
...
more
...
measurements
...
should
...
be
...
taken
...
along
...
the
...
second
...
flocculator
...
channel.
...
The
...
location
...
of
...
this
...
peak
...
would
...
be
...
another
...
way
...
to
...
determine
...
the
...
effects
...
of
...
lower
...
plant
...
flow
...
rates
...
and
...
lower
...
energy
...
dissipation
...
rates
...
on
...
flocculation.
...
Conclusions
Most floc growth is occurring in the first two channels of the flocculator. There is only a 1-3 NTU decrease in residual turbidity in the third flocculator channel. This decrease is small but still significant on the way to producing water under 1 NTU. The third channel probably has more of an impact on residual turbidity when the plant flow rate is lower.