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The transparent demonstration plant is a good platform to perform [flocculation] experiments on, due to its manageable size and close approximation of an actual AguaClara water treatment plant. Experiments were done to investigate how more uniform shear in the flocculator - created by obstacles inserted in the vertical sections between bends - improves flocculation. Initial results showed that more uniform shear at the beginning of the flocculator caused effluent turbidity to decrease. Yet when the plant was modified to make it more robust and to make the results more repeatable, those initial results could not be replicated. Certain systematic errors in the initial experiments were eliminated and new errors were introduced. Subsequently, the plant was automated using [Process Controller] to make it more time-efficient, and the desktop turbidity meter was replaced by an inline turbidity meter to eliminate unintended bias in readings. However, the results still showed that the performance of the flocculator did not improve by making shear more uniform. It is probable that the performance of the unmodified plant is already at the theoretical limit, and that improving flocculation at the beginning of the plant cannot decrease the turbidity of the effluent any further.

Keywords: demonstration plant, uniform shear, obstacles, improve flocculation

The demo plant was originally designed as an education and demonstration apparatus. This is the third version that has been build and is the most accurate representation of the actual plants located in Honduras. The previous demo plants were harder to follow and were less efficient.

The current demo plant is gravity-powered. The flow of fluids into the plant is regulated by constant head regulators. These constant head regulators are identical to the regulators used in Honduras. They regulate flow by keeping constant the difference in elevation between the top of the fluid in the regulator and the feed point. While the flow of fluids can be adjusted by changing the height of the feed points, the demo plant is designed to handle 100 mL/min of fluids.

The flocculator is open-top and is made of clear corrugated plastic. Each corrugation forms a channel that has a cross-section 10 mm long by 5 mm wide. Slits are cut in the corrugation to allow flow to weave through the device. The flocculator leads to the sedimentation tank made of the same corrugated plastic material. The corrugations here serve as the lamella. The flow through each lamella is kept uniform by a small orifice at the top of each channel. The orifice creates a significant amount of head loss and negates any difference in head loss between lamellas. The schematic of the demo plant is shown in Figure 1 below.

INSERT FIGURE 1, SCHEMATIC OF DEMO PLANT, HERE

The demo plant is of manageable size and is easy to operate. In addition, it is transparent and allows direct observation of the flocculation and sedimentation processes. These properties make the demo plant very suitable for flocculation experiments that must be carried out under controlled conditions, but in an apparatus that closely models the actual AguaClara water treatment plants.

Currently, most of the shear is provided by the 180° turns at the ends of the baffles. There is much less shear in the vertical sections between baffles. Thus, the difference between the maximum shear and the mean shear of the flocculator is significant. In addition, localized maximum shear at the 180° turns near the end of the flocculator is more than enough to break up flocs, although the mean shear is at a safe level. It is proposed that if the shear level across the vertical sections of the flocculator is more uniform, the flocculator will be more efficiently used and the size of the flocculator can be reduced. In addition, it will effectively reduce maximum shear near the end of the flocculator and reduce floc breakup.
In order to even the shear level, the flow path of water in the vertical section must be disrupted. One can introduce obstacles around which the water must flow. The initial approach was to experiment with disrupting flow at a small scale with the VFHF demonstration plant.

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Flocculation is facilitated by velocity gradients, or shear. Shear causes suspended particles to collide. Chemical coagulants, such as aluminum sulfate (alum) and poly-aluminum chloride (PAC), neutralize the negative surface charge of the particles and encourage them to adhere to one another after collisions. The two work in tandem to form flocs. Thus, higher shear causes more collisions and results in larger flocs. The number of collisions is also proportional to the amount of time that the suspended particles spend in the region of shear. Thus, longer time results in larger flocs.
It is necessary to have a quantity that measures the amount of shear that the suspended particles have been subjected to, since that determines the size of the flocs. This quantity is the product of shear (G, s-1), and the amount of time spent in the region of shear (¿, s). The resulting dimensionless number (G¿) is a measure of mixing.

However, shear is also capable of breaking up flocs. Larger flocs, with greater cross-sectional areas, span larger velocity gradients and are subjected to higher forces. Thus, they are more susceptible to breakup than smaller flocs in the same shear. Therefore, the maximum shear that a floc can tolerate decreases with size. It is paramount to create high to promote flocculation without exceeding the limit that the flocs can tolerate.

The relationships between the relevant quantities for a vertical flow, baffled hydraulic flocculator are given below.

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Values

• Average shear ( , s-1)
• Gravitational acceleration (g, 9.81 m/s2)
• Head loss (hl, m)
• Flow rate (Q, m3/s)
• Kinematic viscosity of water (¿, 10-6 m2/s)
• Height of flow channel (h, m)
• Width of flow channel (w, m)
• Baffle spacing (b, m)

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Values

• Maximum shear created (Gmax, s-1)
• Minor loss coefficient (K)

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Values

• Maximum shear allowed (Gmax,design, s-1)
• Diameter of floc (dfloc, m)
• Shear strength of floc (¿floc, 0.8 Pa)
• Coefficient of drag of floc (CD)
• Density of water (¿w, 1000 kg/m3)

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Values

• Residence time of baffle (theta baffle, s)

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Values

• Degree of mixing of flocculator (G theta)

Hexagonal steel nuts, measuring 9 mm side-to-side and 3 mm thick, were tied into lines using dental floss. A wooden toothpick was tied to the end of each line to be used as the support to suspend the nuts in the flocculator. Each line had 13 nuts and 4 lines were made in total. The clay suspension was made by mixing 1000 mg/L of kaolin clay in tap water. The alum solution was made by dissolving 1000 mg/L of aluminum sulfate crystals in tap water.

The concentration of clay in suspension and the alum dose were the same as those used during public demonstrations of the plant. The clay concentration and alum dose were 1000 mg/L and 73 mg/L respectively. The flow of clay suspension was 100 mL/min as per plant design and the alum solution flow rate was 7.3 mL/min (Equation 3). The total plant flow was thus 107.3 mL/min.

The demonstration plant was set up with all valves closed. The 4 lines of nuts were inserted into the 5th to 8th channels of flocculator. The clay suspension and alum solution flow rates were set by inserting the feed tubes into the appropriate holes on the dosing apparatus. Next, the plant was filled with tap water and the weir at the outlet of the sedimentation tank was adjusted at the same time, such that the steady state level of water in channel 1 was near the top of the flocculator (Figure 3).

INSERT FIGURE 3 HERE

To start the first experimental run, all the feed valves were opened simultaneously and the plant was undisturbed for 12 minutes to reach steady state. A sample of the effluent was then taken and its turbidity was measured using a Hach 2100N desktop turbidity meter. Two turbidity readings were taken consecutively, and the sample was agitated between readings. Immediately after taking the sample, the clay suspension tank was stirred to homogenize the suspension. Samples were taken at 2, 4, 6, and 8 minutes after the initial sampling. The clay suspension tank was stirred after taking each sample.

After taking 5 samples, the run was ended and the feed valves were closed. To prepare for the next run, the flocculator and sedimentation tank were emptied and rinsed with tap water. The clay suspension and alum solution tanks were topped up. The line of nuts furthest from the flocculator inlet was removed and the run was repeated. The experiment was performed with 4 to 0 line of nuts suspended in the flocculator.

To replicate the results of the SSN experiments in a more rigorous manner, the experimental setup was extensively modified. Spherical Gremlin Green tin fishing sinkers were chosen to be used as obstacles. Nylon fishing line replaced dental floss since the former is stronger and more durable. The clay suspension and alum solution were fed to the plant using Cole-Parmer Masterflex® L/S peristaltic pumps (Figure 4) to ensure constant and more accurate flow rates. The clay suspension was also stirred throughout the experiment using a magnetic stirrer (Figure 5) to keep the turbidity consistent.

INSERT FIGURE 4 HERE
INSERT FIGURE 5 HERE

The fishing sinkers were tied onto nylon fishing lines. The protruding sharp ends of the sinkers were removed to make them more spherical. 4 lines were made with 11 submerged sinkers on each. An additional sinker was added to each line to be used as a clip to suspend the line from the top of the flocculator. The distances between consecutive sinkers on a line were very similar between lines Clay suspension was made by mixing 275 mg/L of kaolin clay in tap water to achieve a turbidity of 100 NTU (Equation 1). Alum solution was made by mixing 1000 mg/L of aluminum sulfate crystals in tap water.

The alum dose required to treat the 100 NTU suspension was calculated to be 45 mg/L (Equation 2). Therefore, the flow rate of alum was 4.5 mL/min (Equation 3). The clay suspension flow rate was 100 mL/min as per plant design, giving a total plant flow of 104.5 mL/min.

The alum solution was set atop a magnetic stirrer to dissolve any residual alum crystals prior to the start of each experimental run. The lines of sinkers were inserted into adjacent channels of flocculator, starting from the 3rd (Figure 4). The alum solution was then removed from atop the magnetic stirrer and the clay suspension was set atop the stirrer. The clay suspension would be constantly stirred throughout the run.

To start the run, the pumps were turned on simultaneously and the plant was undisturbed for 12 minutes to reach steady state. A sample of the effluent was then taken and its turbidity was measured using a Hach 2100N desktop turbidity meter. Two turbidity readings were taken consecutively, and the sample was agitated between readings. Samples were taken at 2 and 4 minutes after the initial sampling.
After taking 3 samples, the run was ended and the pumps were turned off. To prepare for the next run, the flocculator and sedimentation tank were emptied and rinsed with tap water. The clay suspension and alum solution tanks were topped up. The alum solution was set atop the magnetic stirrer in the meantime. The line of sinkers furthest from the flocculator inlet was removed and the run was repeated. The experiment was performed with 4 to 0 line of sinkers suspended in the flocculator.
An alternate experiment was also performed to reduce the contribution to flocculation by the baffles, in order to accentuate the contribution to flocculation by the obstacles. The experimental setup and procedures for the alternate experiment were identical to the original Suspended Tin Sinkers experiment, except that the clay suspension and the alum solution were fed to the 22nd channel of the flocculator (Figure 6). The lines of sinkers were suspended in alternate channels instead of adjacent channels, starting from the 23rd channel. The run time before the first sampling was reduced to 8 minutes.

INSERT FIGURE 6 HERE

The STSPC experiment was performed to verify the results of the STS experiment. The obstacles, clay suspension and alum solution used were identical to those used in the original STS experiment. Process Controller was used to automate the experiment for better time-efficiency and precision. A MicroTOL inline turbidity meter replaced the Hach desktop turbidity meter in order to remove the unintended bias in manual turbidity readings (Figure 7).

INSERT FIGURE 7 HERE

The residence time of the setup was measured by pumping tap water into the empty plant at 100 mL/min and measuring the time elapsed before water starts flowing out of the outlet of the inline turbidity meter. The measured residence time was 900 seconds (15 minutes), which was longer than the 12 minutes estimate used in previous experiments.

The demonstration plant was set up and connected to the host computer. Within Process Controller, the total plant flow rate was set to 100 mL/min, comprising of 4.5 mL/min of alum solution and 95.5 mL/min of clay suspension. The resulting alum dose was 45 mg/L. The alum solution was set atop a magnetic stirrer to keep any precipitates in suspension throughout the experiment. The clay suspension was stirred constantly by a mechanical stirrer to keep its turbidity constant.

Process Controller was programmed to perform the experimental run in 3 sequential steps. Each step (state) had its own operating parameters (setpoints) and logic (rules). Effluent turbidity was measured by the inline turbidity meter and recorded onto a spreadsheet every 5 seconds. State 1 was programmed to fill up the plant with 100 mL/min of tap water for 900 seconds. The stirrers were turned on to ready the alum solution and clay suspension. The connecting tubes were manually purged of air and the level control device was manually adjusted such that the steady state level of water in channel 1 was near the top of the flocculator. After 900 seconds, Process Controller proceeded to State 2. It shut off the tap water pump and turned on the alum solution and clay suspension pumps for 1350 seconds. The stirrers continued to stir the alum solution and clay suspension. Finally, Stage 3 flushes the plant with 100 mL/min of tap water for 900 seconds, allowing for further experimental readings without introducing an excess of clay. This was done to ensure that the effluent turbidity readings were not affected by the accumulation of clay in the plant. The procedure was repeated 5 times for each number of lines of sinkers.

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Enter your Conclusions here.

Here is an example table. I refer to the table by creating an internal link (an anchor) that will take the viewer to the top of the table. For example here I am talking about the analog wiring standard that we use in the AguaClara laboratory. Note that I position the floating table above the paragraph where it is first referenced so that it appears along the side of that paragraph. I haven't figured out an automatic way to set the width of the table. Currently I am doing it by trial and error. If someone figures out a better way, please edit this!

I recommend
An output control box designed and fabricated around the Basic Stamp® Microprocessors (Parallax 16 port BS2sx and 40 port BS2p BASIC Stamp® modules) is used for on/off control of up to six devices and for variable control of up to six peristaltic pumps.

The float macro keeps the graphic and the caption together and floats the figure on the page with text wrapping around it automatically. Because the top of the figure will align with the text that the float is above, I recommend insert the figure wrapped in the float macro immediately above the paragraph where the first reference to the figure occurs. This will place the figure along side the paragraph with the reference. Use anchors to refer to the figure just like you would use "Figure 11" refernces in a conventional manuscript. There is no way to implement auto numbering of the Figures so for now don't even bother to use numbers in the Figure. Instead, in the body of the report where you first reference the output control box add an anchor link that connects to the Figure. Use heading 5 for table and figure captions. This makes it possible to generate a list of tables and figures. Note that there is no numbered Figure reference in the caption. Also note that the image is a hyperlink to the full size original image. If the image is from a different source file the hyperlink should be to the original source file such as a MathCAD or Excel sheet.

In this example I set the size of the float and the size of the image to 200px. The viewer can see the full image by clicking on it.

You can also use the chart macro to create a chart dynamically within the wiki.