h3. Brief History
h3. Pilot Plant Construction and Baffle Design (Uniform Spacing)
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{anchor:tank}[!Pilot Plant^Starting Tank.jpg|width=200px!|Pilot Plant^Starting Tank.jpg]
h5. Plastic Tank that was the starting design constraint for the vertical flow hydraulic flocculator.
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The construction of the tank was started during the Spring 2007 semester. The floc tank was designed to be contained in a polyethylene tank of dimensions 182.9 cm × 91.4 cm × 121.9 cm (length × width × height) with a wall thickness of about 0.8 cm.[#tank]The design goal was to divide the tank into 3 separate sections, basically condensing a long, narrow flocculation tank into a more compact space by snaking the flow back and forth. The initial design divided the total minimum mixing value (20,000) evenly among the three sections, with each section having an even velocity gradient (G) of 45 s-1.
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{anchor:section dividers}[!Pilot Plant^tank dividers.jpg|width=200px!|Pilot Plant^tank dividers.jpg]
h5. Plastic dividers that create three sections for a serpentine flow path through the tank.
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{anchor:flow path top view}[!Pilot Plant^tank flow path.jpg|width=200px!|Pilot Plant^tank flow path.jpg]
h5. The serpentine flow path can be clearly seen in the top view of the flocculator.
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In order to divide the tank into three sections, a divider system had to be built in the tank. Originally it was planned to purchase two plastic sheets to function as the dividers and weld them vertically into the tank. This option was soon rejected due to two major concerns: difficulties of welding inside the cramped spaces of the tank, and lack of strength in the welds. After much contemplation, the maintenance shop proposed a plan to build a structure that would provide support and flexibility. The sketch shows that the [#dividers]were welded onto a base slab and the completed module was placed into the tank. A port hole (hole through which water flows between sections) was cut in each divider prior to welding. The dimensions of the dividers are approximately 182.9 cm x 121.9 cm with a 0.6 cm thickness. The choice of material for the dividers as well as the base slab is high-density polyethylene and was specifically chosen for its non-reactive property in water treatment process, and most importantly its ability to be welded.
Below is a list of fixed parameters (or "givens") and the values of G to be used in the initial setup.
Givens:
* Tank dimensions: 182.9 cm × 91.4 cm × 121.9 cm
* Tank wall thickness: 0.8 cm
* Tank divided into 3 sections (serpentine flow path)
* Total minimum mixing value (Gθ) = 20,000
* Initially 1st, 2nd, and 3rd sections of tank to have velocity gradient (G) of 45 s-1
Variables:
With initial design constraints defined, a MathCAD program was used as a design/calculation tool to determine variables.
* Flow rate\- Q
* Number of baffles per section\- n
* Baffle spacing - b
** Baffle dimensions:
- Width x height
- Different heights for top and bottom baffles)
* Dimensions of openings in dividers
- Width x area of flow path
* Total head loss\- h
* Water level\- L
Calculations:
The purpose of adding baffles was to increase mixing (G) by acting as an obstacle and forcing water through a restricted flow-path. G is a variable that controls the upper limit of shear. There is a maximum G (and thus a maximum shear) that a floc can experience before being broken up. The value of G directly controls the baffle spacing needed for efficient floc formation and preservation. A flow rate of 120 L/min and an effluent depth of 76.2 cm were chosen for baffle configuration design. Presented below are the equations and values used for design calculations.
* Baffle Design
Total minimum mixing value (Gθ = 20,000) is a function of path dimension and residence time. The equations used to calculate baffle spacing are presented below:
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{include:G with substitutions (floc tank)}
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{include:Floc Baffle Spacing Alternative Eq.}
Variables:
where
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* G = velocity gradient
* V = flow velocity through the channel
* ν = viscosity of water
* Kmin = 3 × (number of 180 deg turns)
* f = corresponding friction factor f(Reynolds number, material)
* L = flow depth (water level)
* Ls = shear length = water level - 1.5b
* w = 1/3 the tank width, and
* b = baffle spacing
Since both V and f are functions of baffle spacing (b), the above equation cannot be solved explicitly, and thus multiple iterations were employed in MathCAD to solve for b within a 1% error tolerance. The resulting b value for the first configuration with identical G (45 s-1) is 6.45 cm with a total headloss of 11 cm. Given these b values and the corresponding flow velocity (V), the below equations were then used to calculate the number of baffles in the each section:
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{include:Residence Time (one section)}\\
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{include:Number of Baffles (one section)}
The number of baffles - n - that was calculated for each section was found to be 27.
After the spacing and number of baffles was designed next was the height of the baffles. The baffles needed to be large enough to control the flow of the water and produce mixing. However, if they were too large the turning radius of the water would be small and create unwanted shear, which could break up flocs. To control this, the flow expansion around the turn of the baffles needs to be large enough to minimize unwanted shear. In order to do this, a flow space of 1.5b × w is required. Here w is the width of one section of the flocculator. The equations below were used to determine the dimensions of both types of baffles. The two types of baffles are top and bottom baffles.
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{include:Top Baffle Height}\\
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{include:Bottom Baffle Height}
The figure shown below shows what is considered a top and bottom baffle.
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{anchor:baffles side view}[!Pilot Plant^vertical flow path.jpg|width=200px!|Pilot Plant^vertical flow path.jpg]
h5. Side view of the baffles. Top and Bottom baffles and flow path can be clearly seen.
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The resulting fabrication height of top baffle is 88.9 cm while that of bottom baffle is 71.1 cm.
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{anchor:baffles}[!Pilot Plant^baffle connections.jpg|width=200px!|Pilot Plant^baffle connections.jpg]
h5. Corrugated Baffles spaced and connected with the cpvc caps and pipes. Pressure connection holds baffles together.
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The baffles are cut from plastic corrugated roofing material which can be purchased at most local hardware stores such as Lowe's and Home Depot. It is cheap, accessible, and very similar to the material used in Honduras.
Since baffle design and dimensions depended on the divider design, baffles were not cut until the dividers [#dividers] were installed. Baffle [#baffles]dimensions were determined to be approximately 88.9 cm × 30.5 cm for the top baffles and 71.1 cm × 30.5 cm for the bottom baffles. Baffles were cut slightly wider than the section width (30.5 cm). This was done to tighten the fit of the baffles, provide support for the dividers, and to prevent leakage of water between the baffle and the tank walls.
Baffles were attached as a contiguous unit for each of the three sections and then slid into the proper section. To assemble these units, plastic caps were screwed into four locations on each baffle. Short lengths of PVC pipe were fitted into the caps. The arrangement is shown below in Figure 10. This design has various advantages. First, it allows for relatively easy assembly. It also allows for easy disassembly, which means that it is easy to take apart the units to change baffle spacing by adding shorter or longer pipe sections. The design also decreases the amount of material used. To achieve the baffle spacing of 6.4 cm we cut connecting pipes to about 5.3 cm, accounting for the thickness of the caps. A band saw was used to cut the pipes at this dimension. A lathe and a mill were both used to bore ¼" holes into the caps.
Four different ¼" (0.6 cm) screw holes had to be drilled into each individual baffle at the location of the connectors. To position the holes we placed one top baffle on a bottom baffle and marked the desired location at the center of the nearest concave to improve accuracy and get a clean cut. The average position is about 2.5 cm from the top and 2.5 cm from the side for those holes at the bottom of the bottom baffles.
To move from one section to the next, water must travel through a port in the divider. The dimensions of these ports were determined with the following equations:
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{include:Port Hole}\\
It should be noted that here w indicates the width of one section in the tank. (about 1ft)
To avoid short-circuiting of flow, the ports were cut in rectangular shape of lesser width than the corresponding baffle spacing for that section.
Excess pipe material (standard schedule 40 pipe) from the rapid mix unit was used to build the outlet pipe. The turns were composed of two elbows joined at the ends by a small cut of piping. Each length of pipe for the rapid mix was about a meter in length to provide the 2 m flow length. A sanitary tee-fitting serves as the inlet of the unit.
Note: All text in blue is currently under revision.
{color:blue}The operational effluent flow depth was set to 96.5 cm to ensure the flow over the bottom baffles would not cause floc break-up because of high shear levels. A MathCAD program (N:\Research and Development\Vertical Flocculator PPT\AutoCAD\Flowrate trouble-shoot (identical G).mad) was made to understand how the manipulation of G and Gθ can be achieved by altering Q. As both G and Gθ are functions of head loss (hl) and hydraulic residence time (theta), two sets of equations (for computing hl and theta) are used to assess their values. Total head loss can be separated into major and minor loss. Minor loss is a function of the friction factor (f), which can be obtained from the Moody's diagram for a known Reynolds number. For this particular baffle configuration, major loss only makes up for less than 1 cm of the total head loss (11 cm); majority of total head loss comes from corner-turning which is accounted for by minor loss.{color}\\
{include:Head Loss Across Flocculator}\\
{include:Hydraulic Radius (Floc Tank)}\\
{include:Head Loss Coefficient}\\
This equation was solved for Kminor. The following set of equations listed below is used to compute theta, and thus G and Gθ along with the previously calculated head loss of this configuration.
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{include:Floc Tank Volume}\\
{include:Residence Time (general)}\\
{include:G average (per section of flocculator)}\\
All the equations described above were input into the MathCAD program for generation of the graph in the graph which dictates the exact relationship between Q and G and Gθ. [#Q,G,Gtheta graph]\\
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{anchor:Q,G,Gθ graph}[!Pilot Plant^Graph - G, Q , Gθ.jpg|width=200px!|Pilot Plant^Graph - G, Q , Gtheta.jpg]
h5. Graph of the relationship between flow rate, velocity gradient and mixing factor (Gθ) for the uniform baffle spacing.
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Our initial experimental condition was to test flocculation at G of 45 s-1. According to graph above a flow rate of 138 L/min is required to attain a G value of 45 s-1. With this flow rate applied, however, Gθ will greatly exceed the original estimate of 20,000 at the tank outlet, instead reaching a value of 34,170 for Q = 138 L/min. A different function was programmed in the same MathCAD file mentioned in the previous paragraph to re-estimate the new location of where Gθ = 20,000 is reached in the tank. Knowing the values for the target Gθ (20,000) and average G (45 s-1), adjusted reactive volume can be computed with the equation below. To assess this volume change in terms of tank length, see the appropriate equation below. Location value indicates the new sampling port location measured from the influent end of tank. For this particular design, the sampling tube would have to be placed 320 cm (Location = 320 cm) in the flow-path away from the influent end of the tank to measure a turbidity value for Gθ = 20,000. In other words, since one section of the tank is 182.9 cm long, this sampling port would reside at 137.1 cm from the flow-entry end of the 2nd section in order to measure turbidity for a Gθ of 20,000.
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{include:Tank Volume in terms of G}\\
{include:Delta Location}\\
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Since 2004, Cornell University's AguaClara team has worked in conjunction with Engineers for a Sustainable World (ESW), and Agua Para El Pueblo (APP) to design and build four water treatment plants in Honduras. In addition to providing clean water to La 34 and Ojojona, a plant in Marcala is under construction and a design for a plant in Tamara is also being completed. The plant at Ojojona also functions as a pilot operation, demonstrating successes and potential problems for future plants.
In spring 2007, the Vertical Flow Pilot Plant sub-team worked with various Cornell University staff to design and build a larger-scale vertical flow flocculator at the Cornell University Water Treatment Plant (CUWTP), to facilitate testing under turbulent flow conditions. The new flocculator more closely models Ojojona's existing configuration, hopefully allowing for more practical testing. This experiment will also allow verification and possible reduction of the large Gθ range (20,000 to 150,000) recommended for community-based flocculation. Flocculation effectiveness is influenced by a number of factors, including coagulant dosage, mixing value, influent turbidity and velocity gradient.
h3. Maintenance Repairs
Constant communication with the Cornell Water Treatment Plant was needed to attempt to integrate our pilot plant into their facility. Some modifications would be necessary after transportation and set up in the tank at the plant. We fabricated a frame to hold the rapid mix in place on the side of the tank. The inlet pipe had to be redirected to fit into the rapid mix pipe. Directing the inlet pipe directly straight down into the top of the rapid mix pipe would give us the maximum flow from that pipe. The outlet pipe was reconstructed to prevent leakage. Water from the treatment plant is directed to us before treatment. After the water is treated in our system the effluent exits the tank and reenters the treatment plant at the same point through a nearby drain. Assembly and installation of the turbidimeters, sedimentation tubes, and alum feeder were completed in the same day. |
