Sedimentation Tank Methods and Results

Results

The first sedimentation tank containing the floc blanket has been constructed. Flocs enter the tank and do not appear to be breaking up, so it has been determined that our entrance pipe was large enough. The hydraulics are working correctly and the water levels are as calculated. We also measured the flow rate of the sedimentation tank and the sludge hopper, and both were very close to the design rates.

Over the past week we have been observing how the floc blanket has been forming. Sludge has been piling up slowly and a suspension of flocs has begun to form. By the end of the first week it was very clear that a thin floc blanket had formed in the tank. Although some floc were noticed to still be making it up to the effluent manifold a large number of flocs were being captured by the floc blanket. The flocs forming the blanket were relatively large, up to 5 mm in diameter. The top of the blanket was forming around the level of the sludge hopper. This tells us that the hopper is successfully controlling the level of flocs in the tank. Hopefully with more time for development the floc blanket performance will improve as the floc blanket thickens, and the smaller flocs noticed toward the top of the tank will be captured.

It was observed that upward flow through the tank seemed slightly faster in the front corner of the tank by the effluent manifold. Upon tank draining, it was observed that the sludge at the bottom of the tank formed a conical shape. The evenness of the conical shape demonstrates that flow seems to be flowing uniformly out of the inlet and up through the tank, and thus the slight increase in flow noticed at the top of the tank is not significant, and it is not leading to a large amount of scour at the bottom of the tank. Flow irregularity is a concern because it is one of the main causes of blanket instability and poor blanket formation.

At the end of June an inline turbidity meter was installed to compare the floc formation in the floc blanket tank to the results coming for the tube settlers at the end of the flocculator. The turbidity meter was set up to draw samples from the plant leveling tank set-up after the launder out of the sludge blanket tank. After this set up was completed the tank was run at up-flow velocities of 30, 50, 70, and 100 m/day. At all of these velocities the tube settlers out of the flocculator performed better than sludge blanket removal system. Graphs of the outlet turbidity versus time for the sludge blanket tank and the final tube settler in the flocculator can be seen below.

At 30 m/day there was very little suspension of the floc blanket, it was mostly settled sludge. All floc that weren't heavy enough to settle on their own rose through the tank and exited the tank through the launder.

At 50 m/day the tank performed decently. A good floc blanket formed in the tank and as long as the blanket was kept a few inches below launder. The suspension was thick and fairly stratified, thicker at the bottom and progressively thinner closer to the top. Still some fairly decently sized floc were escaping to the top. It was suspected that this flow rate was too low and that the shear in the blanket could possibly be breaking up flocs. This was a suspicion that was difficult to prove.

At 70 m/day the sludge blanket was thinner than at 50 m/day but still well developed and thick at the bottom. The floc rising through the blanket were about the same size as the in the 50 m/day tank and the results were in consistent. Sometimes better than the 50 m/day but also worse.

At 100 m/day the blanket was stratified and well formed but much thinner toward the top in comparison to the 50 m/day tank. The floc that were rising up through the tank were much smaller than at the other velocities suggesting that there was less shear in the blanket at this higher velocity and that flocs were not being broken up and rising through the blanket. The 100 m/day velocity needs to be re run to confirm the low shear theory given that changes were made to the flocculator during the testing of various velocities.

These results show that a sludge blanket alone is not as effective a method for settlement removal as the lamella simulated by the tube settlers. Thus now it is being explored if results can be improved by combining the technologies of sludge blankets and lamella. The design scheme for this combination tank can be seen in a following section.

Originally the sludge blanket height was adjusted by draining through the sludge hopper that was set at the desired height for the sludge blanket. Slow draining through the main tank drain at the bottom of the tank was also used for sludge blanket draining. The preferred method of sludge draining to maintain a stable blanket was drainage from the bottom of the tank. When the sludge is drained the blanket rolls over on it self as it re-stabilizes. The blanket was less disrupted when drainage was done from the bottom. When samples of the blanket were taken the bottom portion was found to be darker in color and denser than the upper portions of the blanket. This darker color and the anaerobic septic smell that was found during total tank drain lead to the conclusion the bottom of the blanket is older and susceptible to anaerobic digestion. Draining from the bottom would also help to remove this septic sludge from the bottom of the blanket.

Issues for full scale implementation

It is important to consider possible issues pertaining to the implementation of a floc blanket into the AguaClara treatment plants. One concern is the length of time it takes to develop a fully functional floc blanket, and what to do with the water that goes through this sedimentation tank, but probably has a high turbidity during the formation period. It needs to be determined how long this could take, and explore the option of using a combined floc blanket and lamella design to continue sedimentation when the floc blanket is not fully functional. Another issue is the reliability of the floc blanket. If there is a period of time when either the influent water is clean, or for some reason alum is not being added, it is important to know if the floc blanket will be able to sustain itself, or what might happen. If the floc blanket does not sustain itself, and a new floc blanket has to be created, then this could make a substantial period of time when the sedimentation tank is not producing clean water. This is also a reason that a combined floc blanket and lamella tank may be superior.

When and how excess sludge in the floc blanket is drained is an additional issue that will need consideration. It seems unlikely that a continuous drain, like the one we are currently using, will be a good idea, because there is likely to be too much water wasted. An adequately sized hopper will be necessary and hopefully located in an area that will minimize the disruption of uniform upward flow. It will also be important to determine what may happen if the hopper is not emptied in the designated time frame. There should be an adequate safety factor before the floc blanket rises to the point that it affects the output of clean water.

Ojojona is currently using a sludge judge to monitor the sludge level, but even with the use of the "sludge judge" it may still be difficult to see all that is happening with the floc blanket, and because floc blankets seem to be fragile, it will be very important that plant operators adequately understand how to maintain floc blankets.

Future Research

The goal of this project is to determine the most efficient method of sedimentation. The two technologies that have been considered are floc blankets and lamella. There are some concerns about building a sedimentation tank that relies solely on a floc blanket. It takes time to build up a floc blanket that works well, especially if the incoming water has a low initial NTU. Also if an improper alum dose if given to the flocculator for any amount of time the floc blanket may disappear, removing the only method of sedimentation. Having some addition settling mechanism such as lamella would help with with settling during the times when the floc blanket is not performing well. It is the goal of this project for this summer and the upcoming semester to determine the reliability of floc blankets and determine how difficult would be to maintain a floc blanket even during times when turbidity is low, if alum is not being dosed correctly, or if only small flocs are being produced.

Other research projects include developing methods that would encourage faster formation of floc blankets. Other ideas included adding a mesh fabric filter at the top of the tank just below the launder to capture an tinp floc that escape above the floc blanket. This mesh would also allow for higher V _up values to be used the nesh would act a secondary capture for the smaller floc not captured in the blanket. Also given that φ _floc is the major factor affecting sludge blanket formation, techniques to affect φ _floc could also created stronger and better floc blankets.

Possible modifications of the floc blanket tank include: lowering the continuous flow sludge removal rate to minimize water waste, optimizing floc blanket performance by modifying the blanket height and installing a clay solution feed to increase inflow turbidity to better mimic conditions in Honduras.

After construction of the lamella sedimentation tank, research will be done to see if anything can be done to improve setting in this design. We also want to analyze the possibility of forming a floc blanket underneath the lamella. The combination of these technologies seems like a viable option.

Inline Turbidity meters can hopefully be procured to test the outgoing turbidity from each sedimentation tank. This will allow testing how well sedimentation tanks perform over time and during various conditions, such as performance of the flocculator and raw water turbidity.