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.
Unable to find DVI conversion log file.
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 Dose
For 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
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.