Testing of Uniform Baffle Configuration

Testing of Uniform Baffle Configuration
After the completion and installation of the flocculation tank in the water treatment plant, tube settlers are used to test effluent turbidity at different locations. The tube settlers were designed to mimic the sedimentation tank that would traditionally follow the flocculator. Tube settlers were chosen because they provide an inexpensive way to sample and create a minimal disturbance within in the tank. Using this method, different locations of the tank can be sampled. Data gathered can be used to assess how each stage of the tank is affecting the final effluent turbidity. The tube settlers were designed using the following equipment.
Equipment:
-          Glass tube settlers (3)
                Length: 60 cm
                Diameter: 2.5 cm
-          Peristaltic pump
-          Turbidimeters (3)
Once the equipment was gathered, the next step was to design the flow rate for the peristaltic pump. The following assumptions were used in the calculation for the flow rate:
Assumptions:
-          The optimum angle for tube settler is 60°
-          The critical velocity is 10 m/day

Sixty degrees is used because it is the angle at which the distance required for floc settling is minimized and still allows the solids that settled on the side of the tube to slide down. The critical velocity is taken from a range of accepted values and has been found to be the critical velocity in previous plants in Honduras. The flow rate was calculated using the following equations.
 
Schematic of the Data Collecting Settling Tubes.
Dimensions and Variables chosen:
-          b = diameter = 2.5 cm
-          L = length of the tube settler = 60 cm
-          alpha = optimum angle = 60°

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LaTeX Markup:

\large
$$
V_\alpha   = V_c \left( {{L \over b}\cos \alpha  + \sin \alpha } \right)
$$

The following equations were adopted from Shultz and Okun for determining critical velocity for up flow through a tube. The flow rate calculated for our initial configuration was 44 mL/min. There is a linear relationship between pump speed and the flow rate through the settling tube. The flow rate though the settling tube also has a linear relationship to the critical velocity of the sedimentation process in the tube. It is important to note that the critical velocity of the settling tubes is the same as the critical velocity that can be found in the sedimentation tanks at Ojojona.
The setup of the tube settlers in the tank was the next design step. Originally the tube settlers were to be hung from the edge of the tanks at designated locations. On further inspection however, when laid between the baffles on top of the connectors they are at the correct angle and so can easily be relocated and do not require any attachment to the tank.
The final design consists of the tube settlers nestled between the baffles, and then connected to the peristaltic pump. The peristaltic pump pulls water from the peristaltic pump at the correct velocity and the water is routed through a turbidimeter in order to measure the turbidity. Also installed is a turbidimeter that measures the influent turbidity of the water before it reaches the tank. This turbidimeter is gravity fed.
 Data Collection

Process Controller was used as our main data collection tool. MathCAD programs were then used to analyze the data that was collected. Process Controller is a software package that is used to control the raw water pump, the alum pump and data collection. For the raw water pump Process Controller only controls the on/off status of the pump. When the flocculator is running the raw water pump is turned on and the flow rate is controlled by a valve that can only be changed in increments. The flow rate was calculated by partially draining the flocculator to below the outlet pipe height. The valve was then opened to a noted location and the time it took for the water to rise 5 cm was recorded. To calculate the flow rate increase in the volume of water in the tank was divided by the time it took for that volume to fill in the tank. The volume was calcuated by multiplying the height the water rose in the tank by the cross sectional area of the tank. When this technique was used the head loss over the flocculator was small and did not affect the measurement. If the head loss increases then a new way to measure the head loss will need to be created.
 
This choice of location for the tube settlers was chosen originally to establish general information about how flocs were forming in each individual section of the flocculator.

The states utilized by process controller allow the flocculator to run continuously and data to be collected about how alum dose and changes in Gθ affect flocculation and settled water turbidity With the configuration of the baffles spaced evenly throughout the flocculator G remains constant through the 3 sections. However, by sampling at different locations the volume of the flocculator that the water travels through changes which changes θ and thus Gθ. The tube settlers were placed one at the end of each section. Original Tube settler set-upThis figure shows the configuration that was originally tested and used to collect data. This configuration was chosen to get a general understanding of how each section contributed to floc formation and final settled water turbidity.
 
The graph shows the data and derived equation for optimal alum dose based on raw water turbidity.
The alum dose currently being used is an equation derived from data that was collected and analyzed by students working on the AguaClara project through CEE 453 during spring 2005 (Wilson and Rog , 2005). It utilizes a log relationship and is shown below, the equation is included in the figure (Alum Dose Equation). The equation is only relevant for turbidities under 100 NTU, at higher turbidities another relationship will need to be derived. The general form of the equation is Y = A + B*log(NTU) where A and B are fit parameters, and typically set to both be 15.
In order to investigate alum dosing further, Process Controller was used to cycle the alum dose from 0 mg/L to 30 mg/L in increments of 5 mg/L. Each increment was run for two residence times of the flocculator. The residence time of the flocculator was calculated using a flow rate of 114 L/min and the equations below.
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LaTeX Markup:

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$$
Vol_

} \over Q}
$$

The first residence time was to establish a stable environment and ensure that the water being sampled was using the alum dose being recorded. The second was time during which the data that was analyzed was collected. During these tests tube settlers were left in specified locations. Unless there was a rainstorm or some other large disturbance the raw water turbidity remains relatively constant. It is hoped that this setup will allow the impact of different alum doses to be seen at a specific Gθ and turbidity.
The values for G (45 s-1) and Gθ (20,000) were calculated at a plant flow rate of 120 L/min. With these design parameters in mind it was calculated using the head loss equation show below that there should be a head loss of 11 cm from the inlet to the outlet.
Schematic of the placement and and purpose of the tubes used to measure head loss across the tank.
In order to get a more accurate reading of measured head loss two holes were drilled into the lower part of the tank and a tube connected to the holes. The tube acts as a manometer and the water in the tube reaches the same height as the water in the flocculator at the position of the hole. head loss measurement
The tube can be moved to different locations around the outside of the flocculator and still maintains the height of the water at the position of the hole in the flocculator. The water level in the tube can be compared to the water level at almost any position in the flocculator. Since the change in water level is so small this allows a more accurate measurement than simply measuring the height of water at each location and helps to minimize error.

Variables:
-          Width of section (w, cm)
-          Baffle spacing (b, cm)
-          Head loss (hl, cm)
-          Number of baffles - n
-          Baffle length (L, cm)
-          Velocity (V, m/min)
-          Friction factor (f)
Using the manometer method, with a hole at the inlet and a hole at the outlet, the measured head loss can be used to find G, Gθ and K can be back calculated. The equations above where used to back calculate to find the head loss coefficient K.

Results from testing of Uniform Spacing
A flow rate of 114 L/min was the closest flow rate to the target flow rate of 120 L/min that could be achieved. The residence time of the tube settlers was found to be 7.6 min, and the residence time of the flocculator was 18 min. In order to ensure that our measurement of the residence time of the flocculator was correct a test was run. This test consisted of turning off the alum and waiting till there was little or no floc formation. Then the alum was turned on and when there was observable floc it was timed to see how long it took to reach the end of the flocculator. When this measurement was done it was discovered that the residence time of the flocculator was about 35 min. This value was the value that was used in Process Controller. This and other evidence discussed in Data collection and troubleshooting below helped us to conclude that that was short circuiting of water in the flocculator. This evidence was used to make further adjustments to the tank, also discussed in detail in Data collection and troubleshooting.
At the beginning of the summer there were some minor adjustments made to the flocculator. The first is that the baffles at the end of the first section appeared to be rising up and away from the bottom of the flocculator causing baffle skipping. There was also some concern that water might be flowing under the dividers from the first section to the third section. To fix both these problems sand was added to the bottom of the flocculator at a thickness of about 5 cm.
The module in the last section of the flocculator also seemed to be drifting towards the end of the section. This was due to the force acting on the baffles causing them to drift with the flow. The force is due to the water level on either side of the baffle being slightly different due to head loss. This causes the pressure on one side of the baffle to be slightly higher at each point on the baffle than on the other side creating a force over the entire area of the baffle. The pressure on each baffle is translated through the PVC connector pipes from one baffle to another, until the last baffle and the final connector pipes are carrying the entire force. A MathCAD file labeled "Force on baffles" was created to assist with this calculation. Equations for head loss were used to find the head loss over just one section. It was found that there should be a head loss of 3.8 cm over each section. With this information, equations listed below were used to find the total force on each section and then the force each connector pipe would have to support.
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LaTeX Markup:

\large
$$
P = \rho gh_l
$$
$$
A_

$$
$$
F_

= {{\rho *g*h_l *L_s *w} \over
{N_

}}
$$

Variables:
-          Pressure (P): 372 Pa
-          Density of water (ρ): 998 kg/m ^3
-          Head loss (hl): 3.8 cm
-          Baffle length (L _s): 72 cm
-          Width of section (w): 30.5 cm
-          Baffle area (Abaffle): 2,168 cm ^2
-          Force (F): 80.7 N
-          Force on each PVC pipe (Fpipe): 20.2 N
-          Number of Pipes (Npipes): 4
With a head loss of 3.8 cm (which is the calculated head loss for each section of the flocculator) the force on the end connector pipes would be close to 81 N or around 20 N per connector pipe. This is a considerable forward force on the baffles and is the reason that the end connector pipes from the tank to the last baffle are instrumental in holding the baffles at the correct spacing from the end of the wall.
In the third section of the flocculator, the end connector pipes were cut to 6.7 cm which was causing the last baffle to push up against the exit pipe and deform the baffle as well as the flow area. The connector pipes that were cut at the end were not long enough to push against the walls of the flocculator. Measurements were taken and new connector pipes were cut and added on. The connector pipes are an important part of the design of the baffles because they transmit the force. If the pilot plant flocculator was not in three sections but in one long module like the design in Honduras the calculated force would be 242 N with each connector pipe supporting more than 60 N.
Another modification was a change in the exit pipe height. The calculations that were done in spring calculated the water height at the exit being 76.2 cm high. When first installed the pipe was cut to this height. However, there was an exit hole cut into the side of the flocculator and then an elbow installed where a pipe could be attached. This means that even with the pipe installed in the elbow it was already about 12.7 cm above the bottom of the flocculator. Therefore the pipe was re-cut to 63.5 cm to ensure a water level of 76.2 cm. However, even after the pipe was cut and installed the water level is about 81.3 cm. Future studies should again try to re-cut the pipe and maintain the water level at 76.2 cm.

Conclusions from the Testing of the Uniform Flocculation Configuration

After the initial design and construction of the flocculator was complete, attention focused on getting the flocculator running. The beginning of the summer was spent ensuring that all the individual parts of the flocculator and tube settler setup were working and that all Process Controller methods were setup. Once this was finished data collection started and it was at this time that flaws in the design became apparent. Attention then shifted to fixing those flaws. The design of the dividers was the most problematic portion of the tank. The dividers were made as a separate piece and then lowered into the tank. This created several spaces and large gaps between the tank and the divider where water could skip sections and head directly towards the outlet. Several steps were made to fill the gaps and stop the leaking caused by the gaps. This caused more problems as the dividers were not straight and easily deformed, which caused bowing and other deformations when the Kwik Foam was used. The divider design thus caused skipping around the end of sections as well as around baffles. The leaks in the tank were discovered through observation, and head loss measurements. The first leak was discovered by observation of the tank and the subsequent leaks discovered and fixed through use of the head loss tubes and sampling in the tank.Alum DoseFor the majority of testing, alum dose was set by equation 18. After the change of A from 15 to 10 was made this approach was effective for the low turbidities that the flocculator experienced this summer. Hopefully in the future the raw water turbidity will change enabling testing of higher turbidities. Through use of equation 18, observing the floc tank and conversations with the operators at the water treatment plant it has become apparent that there is still a lot of research that needs to be done regarding alum dose. Observing the floc tank was helpful in being able to identify different kinds of floc and what different alum doses looked like in the water entering the water treatment plant. The water treatment plant has now switched to a different coagulant but if they had to go back to alum they said they would use past experience and alum doses as well as jar tests to set their doses. This suggests that for each water treatment plant an equation, formula or at least a rule of thumb could be developed off of past water treatment for future dosing. If this formula would be translatable to other water treatment plants and different water types is uncertain. The run increment alum dose test should help to shed light on alum dosing as it allows the alum dose to be changed while at a relatively constant raw water turbidity. Hopefully the data from this test will show either an optimum dose or a small range of optimal doses for specific settled water turbidity.

Design Suggestions (for future plants)

If a serpentine flow is again used, future designs should include a way to make the dividers a more central part of the design, and a material that does not deform easily but holds its structure and can be sealed to the tank should be chosen. If this is not a possibility then re-enforcement to the dividers should be added to ensure that deformation does not cause problems with baffle skipping. This way the width of the sections can be easily controlled as well as locations where leaks could be problematic could be observed during installation. The problem with making the divider and the tank not one central piece is that it is difficult to make the two pieces fit together and make them water tight.
Another suggestion that would make maintenance of the tank easier would be to include an outlet that could be opened and closed nearer to the bottom of the tank than the outlet pipe currently is. The current design leaves water at a height of a few inches above the bottom that needs to be pumped out before the tank is fully drained. Even then with the design of the dividers the bottom of the tank can never be fully drained because the bottom piece of the divider covers most of the bottom of the tank. This is problematic if repairs to the caulk or Kwik Foam need to be made, as they seal best on dry surfaces.
When dealing inside the tank the current configuration of the modules is sturdy and provides a structure that allows the baffles to move as a whole maintaining baffle spacing. This is an advantage as it ensures that they are evenly spaced and that the value of G is constant throughout each module. One of the problems encountered this summer was that in the pilot plant configuration the inlet and outlet are both pipes that were added that decreased the space in the first and last section of the flocculator. If this design is to be replicated the space that the inlet and outlet occupy should be taken into account when designing the number of baffles. This is due to the fact that the connector pipes that are used to keep the baffles from drifting to the end could become caught on the exit pipe and cause the baffles to be pushed against the inlet. When lowering the modules into the tank they need to be lowered very carefully and each portion of the section needs to be lowered at the same time, necessitating at least two people, usually three. If the modules were bent the connector pipes would pop out of the caps and it would be hard to replace them. The connectors are important because of the forces that they carry.