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\begin_body

\begin_layout Title
Flocculation and Sedimentation Optimization
\end_layout

\begin_layout Author
Sarah Bolander, Marianne Collard, Mary John
\end_layout

\begin_layout Date
November 30, 2012
\end_layout

\begin_layout Part*
Introduction
\end_layout

\begin_layout Standard
As the Flocculation and Sedimentation Optimization Team, our goal for this
 semester is to improve the performance and efficiency of the flocculation
 and sedimentation process by incorporating floc recycle into the system.
 By increasing the number of particles in the influent water, there is a
 possibility that the coagulant will adhere more to the incoming particles
 than to the flocculator walls – resulting in a more efficient and economical
 use of coagulant.
 Also, the introduction of more particles into the flocculator should increase
 collisions between particles, thus prompting better capture of the minuscule
 particles so as to decrease the effluent turbidity.
 By recycling floc, in cases of influent raw water with low turbidity, the
 supplemental floc may help the development of a floc blanket as well.
 The Flocculation and Sedimentation Optimization Team is not new to Agua
 Clara, but this is the first time the team will implement and test the
 benefits of a floc recycle.
\end_layout

\begin_layout Part*
Literature Review 
\end_layout

\begin_layout Standard
There have already been multiple studies of the flocculator and components
 of flocculation such as coagulant dosage and type, turbidity, and residence
 time.
 Karen Swetland, in her paper on predicting the performance of flocculator
 designs, develops a model of performance that is based on these parameters.
 Incorporated into the model of performance is the efficiency of the coagulant
 in covering the clay particles, and one sink of coagulant that Swetland
 mentions is the walls of the flocculator.
 In figure 2 of her paper - a graph of the fraction of coagulant that sticks
 to the particles' surface as a function of influent turbidity- it can be
 seen that the amount of coagulant that does not stick to the clay particles
 is lower when the influent turbidity is greater than about 150 NTU (Swetland,
 2011).
 In this sense, floc recycle could aid in the efficient use of coagulant
 by increasing the effective turbidity of the influent raw water.
 In selecting coagulant dosage, another aspect to keep in mind is the effect
 of coagulant stacking on top of itself (which does not improve surface
 coverage of the influent turbidity, and is prone to occur at high coagulant
 levels)..
 In addition, as the influent turbidity increases, the floc volume fraction
 (the volume that the flocs and coagulant take up in the suspension) increases
 as well, which affects floc collisions.
\end_layout

\begin_layout Standard
Another possible benefit of incorporating floc recycle into the water treatment
 process is the easier production of a floc blanket.
 Matt Hurst, in his paper on floc blanket performance, discusses the effects
 of changing the upflow velocity, which, if too large, will result in flocs
 being swept through the plant without being able to settle out, but if
 too small will result in detrimental sedimentation (Hurst, 2010).
 For 100NTU influent turbidity, at which we are first building up the floc
 blanket, Hurst found the optimum upflow velocity to be between 1.0 and 1.3
 mm/s.
 At an influent turbidity of 100 NTU, Hurst found the alum dosage of 45mg/L
 to be optimal, and that additional alum was not particularly beneficial.
 The residence time of floc in the sedimentation tank has a possible effect
 on the performance of the floc blanket as well, in the sense that the prolonged
 time in the blanket could cause the breakup of weakly formed floc from
 the stress of collisions with other particles.
 Through the use of floc recycle, a possible goal is to achieve optimum
 values for operating a floc blanket in a treatment plant without the use
 of excess construction materials.
\end_layout

\begin_layout Section*
Flocculator
\end_layout

\begin_layout Standard
The current experimental flocculator incorporates rapid mixing to induce
 the formation of flocs from the particles in the influent water.
 By coiling PVC tubing around a cardboard tube, differing velocities are
 created in the influent water and coagulant.
 The resulting shear forces mix the coagulant and influent so that the particles
 in the water are coated with coagulant and become “sticky.” In the flocculator,
 these particles experience laminar velocity gradients, and collide with
 other particles to form larger flocs.
 Although the recycled flocs would be added into the influent water before
 the addition of coagulant, it is expected that their influence will be
 in the flocculator where the increased floc volume fraction will increase
 particle collisions.
\end_layout

\begin_layout Section*
Coagulant
\end_layout

\begin_layout Standard
The turbidity of the effluent water, as well as the range of the turbidity
 of the effluent water, significantly decreases with the addition of coagulant
 followed by flocculation.
 However, the cost of coagulant requires that it be used efficiently.
 Even with previously tested optimal concentrations, some of the coagulant
 (due to its “stickiness”) is lost to the walls of the PVC tubing of the
 flocculator.
 By increasing the coagulant’s interaction with particles, there may be
 a decrease in that amount of coagulant that is lost to wall effects.
\end_layout

\begin_layout Section*
Hypothesis
\end_layout

\begin_layout Standard
By introducing flocs that have already been formed in the floc blanket to
 the beginning of the flocculator, it is thought that performance of the
 floc blanket and sedimentation tank will increase.
 These flocs will be taken from three separate locations, all of which could
 be optimal places for the flocs to be drawn, depending on the performance
 of the floc blanket.
 The first location is the floc weir, where the already “wasted” flocs are
 coming from the top of the floc blanket in the sedimentation tank, the
 second location is from the floc blanket in the middle of the sedimentation
 tank and the third location is near the inlet at the very bottom of the
 sedimentation tank.
 
\end_layout

\begin_layout Standard
A recycle flow rate of 10% of the flow into the sedimentation tank, Q ,
 is used to transport flocs from the floc draw location back to the beginning
 of the system where recycle is introduced to the influent water before
 the coagulant is added.
 The introduction of already formed flocs to the turbid water should increase
 the formation of more flocs earlier on in the flocculator, thus capturing
 more clay particles, earlier on in the system.
 
\end_layout

\begin_layout Standard
It is possible that recycle of flocs may reduce the amount of coagulant
 to be used with certain turbidity levels.
 Our experiment starts with 45mg/L alum for 100NTU water, which is thought
 to be an appropriate ratio to be used in our system.
 However it is possible that there is a point where too much coagulant actually
 limits the efficiency of the system to build flocs.
 One of our goals is to reduce the coagulant to clay ratio to see if a lower
 coagulant dose can still be effective in turbidity removal.
 
\end_layout

\begin_layout Part*
Methods
\end_layout

\begin_layout Standard
Within the first experiment, the first cycle will build a floc blanket with
 100 NTU water, 1mm/s upflow velocity and 45mg/L alum for one or more solids
 residence times.
 The second cycle will keep upflow constant (1mm/s) but step down both influent
 turbidity and alum dose to 30NTU and 13.5mg/L alum respectively (note: the
 ratio between NTU and alum is constant for each cycle in a respective experimen
t) with the goal of maintaining a floc blanket at the lower turbidity and
 dose for two solids residence times.
 The third cycle will keep consistent upflow velocity, turbidity and alum
 doses as the second cycle (in this example 30NTU and 13.5mg/L) but add in
 floc recycle at the beginning of the flocculator for two solids residence
 times.
 Floc recycle flow will be 10% of the upflow velocity in the sedimentation
 tank (1mm/s).
 
\end_layout

\begin_layout Standard
The second and third experiments are set up in the same way, everything
 is kept constant except for the alum dose.
 For the second experiment alum dose will be 30mg/L for the first cycle
 when the floc blanket is building at 100 NTU, and will drop to 9.0mg/L as
 the turbidity drops to 30NTU during the second cycle and then remain at
 the lower dose for the third cycle.
 Again, the ratio between alum dose and turbidity remains constant throughout
 one experiment.
 The third experiment will have 15mg/L alum for 100NTU water during the
 first cycle, followed by 4.5mg/L alum and 30NTU water for the second and
 third cycles.
 
\end_layout

\begin_layout Section*
Cleaning the Apparatus
\end_layout

\begin_layout Standard
The first step taken in an effort to research the effects of floc recycle
 on the optimization of flocculation and sedimentation was to make sure
 there was a reliable baseline to compare the data to at the end of the
 experiment.
 First, the flocculator tubing was filled with a solution containing bleach
 and water and left in the tubing for 3 days.
 When the bleach solution was drained, the flocculator was left clean and
 free of the clotted flocs that were initially lining the length of the
 flocculator tubing.
 Likewise, a bleach solution was left sitting in the rest of the apparatus,
 mainly the sedimentation tank and floc hopper, in an effort to get rid
 of mold that had started growing inside and to clean the walls of the tubing
 that initially had a cloudy appearance.
 This bleach was left in the apparatus for 4 days and once drained, the
 apparatus contained little to no mold inside, however the walls of the
 sedimentation tank were still relatively unclear and cloudy.
 Clean water was run through the whole apparatus once more to wash out any
 additional loose debris.
 Overall, the apparatus ended up much cleaner than before, despite the cloudy
 walls of the sedimentation tank; however, it was concluded that the clearness
 of the pipes should not affect the data.
 This is just an aesthetic nuisance that is impairs seeing the floc blanket
 clearly.
 
\end_layout

\begin_layout Standard
After each experiment throughout the rest of the semester the turbidity
 meters were cleaned out with distilled water and kimwipes so the effects
 of the previous experiment on our influent and effluent data were not included.
 In addition, clean water and small pieces of sponge were run through the
 flocculator to remove mold and collected flocs on the walls of the flocculator
 tubing.
 
\end_layout

\begin_layout Section*
Set Up
\end_layout

\begin_layout Standard
All team members learned how to use process controller so that everyone
 could run the apparatus and understand the technology behind the system.
 Additionally, the floc recycle outlet and inlet was rearranged so that
 flocs would leave the sedimentation tank at the bottom of the floc hopper
 as indicated in the following picture.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename AguaClara RR pics.pdf
	width 12cm
	height 12cm
	BoundingBox 0bp 0bp 720bp 540bp

\end_inset


\end_layout

\begin_layout Standard
Another change we made to the set up was to create an automatic floc waste
 outlet so that waste can be removed automatically when the system is run
 for long periods of time; which is necessary since our the full cycle of
 one experiment is 39.4 hours.
 This time length was calculated using
\begin_inset Formula 
\[
\theta=\frac{V*C_{r}}{Q*C_{in}}
\]

\end_inset

 to determine floc residence time with V being the volume, 
\begin_inset Formula $C_{r}$
\end_inset

 the concentration of floc in the sed tank, Q the upflow velocity and 
\begin_inset Formula $C_{in}$
\end_inset

 the concentration of flocs coming into the sed tank.
 Since the system needs to be run for a large amount of time, the floc weir
 must be drained so it does not fill up and overflow.
 Therefore the waste outlet was made, which pulls waste water out of the
 floc weir when floc recycle is not on.
 The waste is pulled out at 10% of the upflow rate.
 To compensate for this adjustment, the raw water flow rate was increased
 by 10%, that way the desired flow rate Q, is present in the tubular sedimentati
on tank.
 This also leads to more comparable results between the control and floc
 recycle, because .1Q will always be being pulled out and added in.
 As shown in the picture, either the floc recycle, or waste water is being
 pulled out at .1Q.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename RRpic2.pdf
	width 12cm
	height 12cm

\end_inset


\end_layout

\begin_layout Standard
To clean the workspace and avoid problems, we changed all of the unused
 solenoid valves to manual valves and set them to the closed position.
 It turned out that some solenoid valves were not operating correctly and
 proved to be open when they were set at closed, so they were labeled as
 faulty in order to alert future users.
 
\end_layout

\begin_layout Section*
Trials
\end_layout

\begin_layout Section*
Trial 1
\end_layout

\begin_layout Standard
The first trial was started on October 5th.
 The trial was programmed so that the plant would run for one residence
 time during the build up phase at 100NTU with a coagulant dose of 45 mg/L.
 Once that time cycle was completed the influent concentration was decreased
 to 15NTU with a coagulant dose of 6.73mg/L and input for two residences
 times.
 Once that sequence was complete the system switched over to floc recycle
 and ran for two residence times.
 Floc recycle was drawn from the bottom of the floc weir.
 Effluent turbidity data was reviewed and the apparatus was cleaned out
 to get ready for the next trial.
\end_layout

\begin_layout Standard
Upon reviewing the data and checking the apparatus it is uncertain whether
 or not a floc blanket was ever formed and definite that a floc blanket
 was not maintained at the 15NTU stages of the experiment, which was confirmed
 by visual observation.
 Another issue with the first trial was that the influent NTU was not consistent
ly kept at 15NTU.
 After looking into the situation, the team concluded that there was too
 great of a lag time between the process control's reading of the influent
 turbidity and the drip valve that controls the turbidity of the water,
 which was set at 10 second intervals.
 Therefore, when the NTU dropped below 15NTU, more clay was added, and continued
 to be added until the next reading of the influent NTU took place.
 The result of the long lag time was that the drip valve was open until
 the NTU increase was eventually observed and, the NTU jumped much too high.
\end_layout

\begin_layout Standard
Before running more experiments, the apparatus must be working properly
 in order to get accurate data.
 What needs attention the most is the building of a floc blanket.
 The floc blanket is an essential component of the sedimentation system
 and needs to be maintained in order to reach low effluent NTU values.
 Our hypotheses of why a floc blanket was not created include:
\end_layout

\begin_layout Standard
1) 15NTU is too low of a turbidity to maintain a floc blanket
\end_layout

\begin_layout Standard
2) Upflow velocity of 1mm/s is too high.
\end_layout

\begin_layout Standard
3) Coagulant dosing is too high/low.
\end_layout

\begin_layout Standard
4) Floc blanket build up stage is not run for enough time
\end_layout

\begin_layout Standard
An experiment was run with the sole purpose of producing a floc blanket.
 The experiment was supposed to build and maintain a floc blanket for over
 a day, however two hours into the experiment, the influent turbidity was
 near 800 NTU.
 This brings forth warning signs of several different issues
\end_layout

\begin_layout Standard
1) Process Controller is not programmed correctly or working as expected.
\end_layout

\begin_layout Standard
2) The stock of clay mixture was improperly made and is too concentrated.
\end_layout

\begin_layout Standard
3) The drip valve going into the small clay stock is not functioning correctly.
\end_layout

\begin_layout Standard
4) The turbidity meter is not functioning properly.
\end_layout

\begin_layout Standard
5) The plant flow rate is too high, possibly not being regulated properly.
\end_layout

\begin_layout Standard
Other issues may exist, but these are the most probable.
 It was found that the vial in the influent turbidity meter was very dirty,
 as shown in the figure below, mostly due to such a high turbidity, however
 it may have gotten very dirty at the beginning of the experiment and then
 as the actual influent NTU decreased it may have kept reading high NTU.
 Nevertheless, the influent turbidity meter is now clean.
 It was also found that the tubing leading into the influent turbidity meter
 was very dirty and clogged with clay and this was leading to much higher
 turbidity when the system began running.
 These tubes were all cleaned out and replaced to fix this problem.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename Dirty Turbidity Meter.jpg
	lyxscale 25
	width 5cm
	height 5cm

\end_inset


\end_layout

\begin_layout Section*
Trial 2
\end_layout

\begin_layout Standard
On October 29th a new experiment was started, the main goal of this experiment
 was to build a floc blanket successfully.
 During the 100NTU build up stage, a floc blanket was built; however, the
 floc blanket was not maintained when the experiment switched to stages
 3 and 4 with an influent NTU of 15.
 This can be seen in the graph below of the effluent turbidity versus time,
 and it is apparent that at .81 days, which is where there was a change in
 stages, the effluent NTU spiked up from less than 1 NTU to 6 NTU.
 This suggests that the floc blanket was not maintained when the influent
 NTU decreased from 100 to 15.
\end_layout

\begin_layout Standard
After looking over the graph, it seemed that 15NTU was too low to maintain
 a floc blanket, therefore it was decided to change the influent NTU in
 stages 3 and 4 (the control and floc recycle stages) to 30 NTU.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 15NTU 45mgl with table.pdf
	scale 50
	BoundingBox 0cm 9cm 612bp 792bp
	clip

\end_inset


\end_layout

\begin_layout Section*
Trial 3
\end_layout

\begin_layout Standard
On October 31st another experiment was performed.
 This time with an influent NTU of 30 in stages 3 and 4 and the alum dose
 to NTU ratio was set at 45mg/L per 100NTU.
 Floc recycle was drawn from the bottom of the floc weir.
 A shown in the graph below, there did not seem to be a change in the effluent
 NTU between stages.
 A floc blanket was observed, so it is known that a floc blanket can be
 maintained at 30NTU.
 However, the floc blanket was not very dense or high, so it wasn't the
 most effective floc blanket.
 Since the floc blanket height did not reach the floc hopper, any floc recycle
 would not be expected to have a significant effect.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 30ntu 45mgl with table.pdf
	scale 50
	BoundingBox 0bp 7cm 612bp 792bp
	clip

\end_inset


\end_layout

\begin_layout Section*
Trial 4
\end_layout

\begin_layout Standard
On November 6th, an additional experiment was started.
 This time the alum dose was set at 30mg/L per 100NTU and the influent NTU
 in stages 3 and 4 was 30NTU.
 Floc recycle was drawn from the bottom of the floc weir.
 Once again the results are inconclusive on the effects of floc recycle
 and the effluent NTU does not seemed to have changed much between stages.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 30NTU 30mgl with table.pdf
	scale 50
	BoundingBox 0bp 8cm 612bp 792bp
	clip

\end_inset


\end_layout

\begin_layout Section*
Trial 5
\end_layout

\begin_layout Standard
The last experiment for the 30NTU influent was conducted at a ratio of 15mg/L
 of alum per 100NTU.
 Floc recycle was drawn from the bottom of the floc weir.
 This experiment was cut short due to a power outage, however from the data
 obtained, the effluent NTU seems to have a significant difference between
 the stages completed - the effluent turbidity seems to increase with the
 introduction of floc recycle.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 30NTU 15mgl with table.pdf
	scale 50
	BoundingBox 0bp 8cm 612bp 792bp

\end_inset


\end_layout

\begin_layout Section*
Floc Recycle from the middle of the floc blanket
\end_layout

\begin_layout Standard
The next experiments involved drawing flocs for recycle from the middle
 of the sedimentation tank.
 The decision to place floc recycle draw here was based on the visual observatio
n that flocs are not being recycled from the end of the floc weir because
 the floc blanket isn't building up high enough for flocs to enter the weir.
 It is hypothesized that the probability of recycling a full stream of flocs
 is higher at a lower position in the system where there will definitely
 be flocs.
 By placing the floc recycle in the middle, flocs will be drawn directly
 from the floc blanket (assuming the floc blanket height is above this position).
 This should minimize room for error and ensure that flocs are being recycled.
 One concern is that by recycling from the center the mechanics of the floc
 blanket may be disrupted, also the floc concentration in the blanket will
 be lower than what would occur if the floc blanket height reached the floc
 hopper and flocs settled in the hopper.
 By drawing flocs from the middle of the sedimentation tank the upflow velocity
 in the remaining section of the sedimentation tank will be decreased by
 the flow velocity of the floc recycle line.
 In our experiments the floc recycle flow rate is 10% the upflow velocity
 in the sedimentation tank.
 After the floc recycle draw, the upflow velocity in the remaining section
 of the sedimentation tank will be 90% of what the original upflow coming
 into the sedimentation tank was.
 
\end_layout

\begin_layout Section*
Trial 6
\end_layout

\begin_layout Standard
The next set of experiments was done with floc recycle placed in the middle.
 Instead of taking flocs from the end of the floc weir as before, now the
 flocs were taken from the middle of the sedimentation tank, about halfway
 up.
 This trial was done with an influent NTU of 30NTU in the control and floc
 recycle stages with a dosage of 13.5mg/L alum.
 As shown in the graph, there is a decrease in turbidity by roughly .5 NTU
 when floc recycle is introduced.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 30NTU 45mgl with table FR mid.pdf
	scale 50
	BoundingBox 0bp 8cm 612bp 792bp

\end_inset


\end_layout

\begin_layout Section*
Trial 7
\end_layout

\begin_layout Standard
The next trial was done with an influent NTU of 30NTU in the control and
 floc recycle stages with 9.5mg/L alum.
 Here again the floc recycle was done in the middle.
 From the graph below, it can be concluded that floc recycle was not effective
 in this trial.
 The effluent NTU actually increased by about 1 NTU when floc recycle was
 introduced to the system.
 
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename 30NTU 30mgl with table.pdf
	scale 50

\end_inset


\end_layout

\begin_layout Part*
Conclusions
\end_layout

\begin_layout Standard
After having tested floc recycle at an influent turbidity of 15 NTU and
 not being able to maintain a floc blanket, the Flocculation and Sedimentation
 Optimization team ran the floc recycle tests on influent water with a turbidity
 of 30 NTU in order to be able to better discern the effects of floc recycle.
 While the change in influent turbidity allowed the formation of a floc
 blanket, not enough flocs were making it to the floc weir (where we initially
 pulled floc for floc recycle), so the placement of the floc recycle was
 moved to the middle of the sedimentation tank in order to actually have
 flocs in the floc recycle stage.
 The results obtained from the ensuing experiments were rather mixed as
 to whether floc recycle was beneficial however, as, for example, trial
 6 with an alum dosage of 13.5 mg/L saw a decrease in effluent turbidity
 with the introduction of floc recycle, trial 7 with an alum dosage of 9.5
 mg/L saw an increase in effluent turbidity.
 This could mean that a low alum dose does not coincide well with increasing
 the influent turbidity with floc recycle.
 On the other hand, these results suggest the need for further experimentation
 – although there is a chance that floc recycle makes the effluent water
 more turbid, there is now some data that supports that floc recycle can
 improve the system, and as a result this opportunity should be further
 evaluated.
\end_layout

\begin_layout Part*
Future Work
\end_layout

\begin_layout Standard
Future work for the Flocculation and Sedimentation team should continue
 the study of floc recycle.
 Many problems within the system were encountered throughout the semester
 and all of the tests necessary to fully understand the effects of floc
 recycle in the flocculation and sedimentation system were not completed.
\end_layout

\begin_layout Standard
Immediate work can include running the same experiments as described above
 with floc recycle placed at the bottom of the sedimentation tank.
 Timing this semester did not allow us to test the recycle draw from this
 location but if would be beneficial to be able to compare the results of
 floc recycle at three different locations and three different alum doses.
 It is important that the optimal location for the floc recycle to be drawn
 from is found so other variables can be tested.
\end_layout

\begin_layout Standard
Other variables in the system to consider once the location is solidified
 is the best alum dosage to use with specific turbidites of water.
 Three alum doses have been tested but they span a wide range and the dosing
 should be refined to a smaller range.
 The optimal alum dose will affect the formation of flocs and the floc blanket
 and ultimately where floc recycle can occur.
\end_layout

\begin_layout Standard
There are many variable to consider and the flocculation and sedimentation
 team next semester should persevere and continue the study of floc recycle
 to see if floc recycle can benefit the AguaClara plants.
 
\end_layout

\begin_layout Bibliography
\labelwidthstring References
\begin_inset CommandInset bibitem
LatexCommand bibitem
key "key-2"

\end_inset

 Hurst, Matt, Monroe Weber-Shirk, Leonard W.
 Lion.
 
\shape italic
Parameters affecting steady-state floc blanket performance.
\shape default
Journal of Water Supply: Research and Technology-AQUA.
 Vol.
 59.5, pg 312-323.
 2010.
\end_layout

\begin_layout Bibliography
\labelwidthstring References
\begin_inset CommandInset bibitem
LatexCommand bibitem
key "key-3"

\end_inset

 Swetland, Karen A.
 
\shape italic
Predictive performance model for hydraulic flocculator design with polyaluminum
 chloride and aluminum sulfate coagulants.
 
\shape default
Thesis.
 Cornell University.
 2011
\end_layout

\end_body
\end_document
