Pilot Plant- Sedimentation Sludge Blanket Tank

Abstract

A sedimentation tank was connected in series with the flocculator at the pilot plant. This sedimentation tank allowed us to isolate the process in the AguaClara design that needs to be optimized for achievement of our overriding goal of consistent production of water, under 1 NTU. The addition of the sedimentation tank enabled us to test not only the flocculation process but the sedimentation tank design for efficiency. Testing will include not only the traditional lamella design of sedimentation tanks but also the alternative floc blanket based sedimentation tank designs. These two processes will be tested side by side in parallel sedimentation tanks.

Introduction

The pilot plant sedimentation tank is a vertical flow style tank and uses only a floc blanket as its removal mechanism. This tank was the first close to full scale sludge blanket sedimentation tank that the AguaClara team has constructed. As with other AguaClara designs, the tank will be run by the elevation head in the flocculator tank and will not require electricity. There are several design restraints due to the current set up at the pilot plant. There was 33.25" of available water head at the end of the flocculator to power the flocculator. The piping connection between the flocculator tank and sedimentation tank also cannot have a shear value that exceeds the shear in the last baffle section ( G _cell = 24 /s), or else the flocs made in the last section will be broken up upon entrance to the sedimentation tank.

Floc Blanket Sedimentation Tank Design

The design of the tank was completed during the Spring 2008 semester. The sedimentation tank was designed to be contained in a polyethylene tank of dimensions 24" x 24" x 36" (length × width × height) with a wall thickness of about 5/16". The design goal was to have enough area in the tank to create a floc blanket. With the available water level height, approximately the same height as the water level in the floc tank, we did not have enough room to include plate settlers. Initially we wanted to split the plant flow rate (110 L/min) in half, allowing us the possibility of installing two parallel sedimentation tanks for experimentation purposes, but we found that a flow rate of 55 L/min into the tank would have required a 6 inch pipe to transport water from the floc tank to the sedimentation tank to avoid breaking up flocs. Due to cost restraints (a six inch bulk head fitting would have cost about $300) we limited this pipe to being 4 inches; this was done by lowering the flow rate of the sedimentation tank down to 24.5 L/min. This constraint on the inlet pipe and flow rate required us to switch to a smaller tank than originally planned. This change was necessary so our tank would have an upward velocity of 100m/day. 100m/day is the upward velocity of full scale plants in Honduras, keeping this parameter the same made the two designs comparable. This low flow and smaller tank set-up allowed for parallel testing of tanks containing different sedimentation processes.

Given Variables:

• Length (L) = 58.7 cm (23.125")
• Width (W) = 58.7 cm (23.125")
• Height (H) = 91.4 cm (36")
• Water Level (WL) = 84.5 cm (33.25")
• Flow Rate (Q) = 24 L/min

Below is an AutoCAD drawing of the proposed sedimentation tank design.

Below is a table of the calculated pipe dimensions and other calculated design parameters. The method behind each number can be found in the following design.

Upward Velocity:
The upward velocity parameter determines what size of floc particles are removed from the sedimentation tank. Thus in order for our design to be comparable to existing sedimentation tank design V _up needs to be the same. V _up was set to be 100 m/day. From this we calculated the flow rate through the tank to achieve this V _up.
Given that there are no lamella the calculations for this are fairly straight forward.

From this equation the flow rate was determined to be 24 L/min.

Inlet Pipe Calculations:
To determine the diameter of the inlet pipe we used the constraint that the maximum shear due to minor losses had to be less than G _cell in the last section of the flocculator (G _cell = 24/s). The predominant minor losses from the inlet pipe will occur at the exit point and the elbow. The minor loss coefficient for the elbow is 0.9 and the exit is 1.0 so K was set to be 1.0 for this design. The equation to calculate shear due to minor losses in a pipe is shown below. This equation is solved for D, the diameter of the pipe that would provide shear equal to G _cell. The equation used is very similar to the equation used to find the baffle spacing for the flocculator.

Variables:
K = 1.0 (minor loss for an exit)

• This equation was derived from a series of substitutions that can be seen above.
• The head loss term that is found in the above derivation is assumed to include only minor losses. The minor head loss for this pipe is assumed to be the dominating factor because the pipe is designed to be relatively short in length and the bends and exit are our major sources of shear, the primary locations where floc break up would occur.
• The velocity equals Q/A for the pipe.
• The residence time term is found to be the volume over the flow rate through that volume.
• The volume used was assumed to be the cross sectional area of the pipe times 2 diameters of the pipe.

Note: Initially during preliminary design with a large tank, we were using 55 L/min as the plant flow rate, and the required pipe diameter was 5.11 to achieve the desired G _cell value. Due to the cost of 15.24cm (6") bulkhead fittings (nearly $300), we had to find an alternative design. We decided to lower the flow until a pipe with inlet diameter equal to 10.16cm (4") was achieved. We found that the maximum flow for these conditions is 24 L/min, so we changed the flow rate of the sedimentation tank. The option of multiple inlets was considered but this was discarded because then 2 - 4 bulk head fittings would be needed. Thus, the overall flow of the tank was lowered and a smaller tank was chosen.

Launder Calculations:

Pipe Diameter
The effluent launder will span the length of the tank about 10.16cm (4") off of the center of the tank. The launder is placed 5.08cm (2") beneath the water level. This value was chosen so that enough water head would be available to allow for relativley equal flow through the lauder orifices and so that a majority of the tank height would be available for the floc blanket. The diameter of the effluent launder was calculated by iterating through pipe diameters to find the existing diameter of pipe that gave the desired flow rate and orifice holes. The diameter was selected based on the following equation:

• Qratio is assumed to be 0.92, this indicates that flow through the first orifice will be 92% of the flow through the last orifice.
  The best pipe diameter is 3.81cm (1.5") inches. The number of orifice holes was tweaked until the orifice hole diameter would match up with a drill bit size and still gave the proper amount of head. (equations for this can be seen in the following section) This head loss was defined to be about 5 cm, or the head available above the launder. If the head loss through the launder orifices exceeded the available head, we would have encountered issues because the water in the outlet system would have had less energy than the surrounding system, which would have effected the flow rate and caused air to collect in the outlet pipe. The total number of orifices was determined to be 15.

Orifice Diameter
The orifice equation was used to determine the necessary area of the orifice that gives the correct amount of head and matches with an existing drill bit size. The flow rate used in this equation was the total plant flow rate divided by the number of orifices. The minor loss coefficient for an orifice is assumed to be 0.63. The head loss term in the equation is total head available less the head loss in the manifold pipe less the velocity component of head loss through the pipe.
This area was then converted into a diameter and rounded to the next smallest drill bit size (0.7541cm or 19/64").

Orifice Head Loss
The head loss due to the orifice is an important parameter because it effects how evenly the water will flow through the orifices. Thus we back calculated the actual head loss achieved given the diameter adjustment that was made to achieve an available drill bit size.
The head loss through the orifice was calculated by using the following equation:

D is the diameter of the orifice (0.7541cm or 19/64"), the minor loss coefficient of the orifice is 0.63, and the flow is the flow through one of the orifices. The minor loss for an orifice was calculated to be 4.54 cm. The head loss through an orifice is parallel to other orifices, so you do not add the head loss in each orifice. So the water level in the plant leveling tank will be approximately 4.54 cm since the launder orifice head loss will be significantly larger then the losses in the piping system between the two tanks.

Secondary Outlet from the Flocculator
The flow rate of the sedimentation tank is designed to be 24 L/min, but the flocculator flow rate is designed to be as high as 110 L/min. This means that excess flow needs to bypass the sedimentation tank and go directly to the existing outlet in the flocculation tank. A large outlet weir is our proposed design for this alternative exit. Because the head loss out of the sedimentation tank is relatively high (5 cm through the manifold and 13.7 cm out of the plant leveling tank) a small height variation (on the order of mm) of the water in the floc tank will have a negligible effect on the sedimentation tank flow rate.

Since the flow rate into the floc tank has been measured to be anywhere between 70 L/min to 110 L/min, It was important to check whether the height of water in the floc tank could change enough to effect the flow rate to the sedimentation tank. If the accessory weir is required to take in a smaller flow rate, the water height in the floc tank will lower. This is the equation that describes the water height above the weir as a function of the flow rate.

Where

where H is the height of water above the weir and Pw is the weir height), Q weir is the plant flow rate minus the sedimentation flow rate, and the available perimeter was set to be 120 cm. This perimeter values was chosen arbitrarily to attain a factor of safety of about 10 between the head loss coming out of the sedimentation tank and a head loss over the alternative weir in the flocculation tank. A factor of ten was not feasible because this would have required the head loss over the alternative weir to be 0.5cm which was not feasible. At 120 cm the head loss is 0.75cm. In order to minimize changes in the floc tank, the perimeter will be established by cutting a pipe laterally and letting water flow over the cut out, down into the trough and then out the existing 7.62cm (3") outlet. When this was calculated we got a water height above the weir of 7.6 mm, which is small enough when compared to the head loss in the launder to assume that it will not affect the flow rate into the sedimentation tank.

Hopper Design for Floc Blanket
Given the calculated amount of sludge the tank will create, a continuous sludge drainage system was created. This was done to decrease the size of the hopper in the tank. It was desired to keep the hopper small so that there would be minimal disturbance in the tank. φ _floc is the specific volume of the floc, it was found to be 0.016. Most of the sludge produced is assumed to be made of alum. The alum concentration is approximately 10 mg/L. This alum concentration is used in the φ _floc determination. The rate of sludge build-up is the sedimentation tank flow rate times φ _floc.

When calculated, the sludge volume accumulated over two days was determined to be 1.105 x 10^3 L. We created this hopper with a conical funnel with flex-hose connected at the bottom. This small funnel will continuously drain sludge out of the tank and into the outlet panel. The hose will be 2m long and 1.27cm (1/2") in diameter. The diameter was determined using the manifold equation used in the design of the effluent launder design. If no sludge consolidation occurs, then the flow rate for the sludge drain must equal φ _{floc~Q ~tank}, or approximately 380 mL/min.

Plant Leveling Tank

The plant leveling tank will handle the outflow from both the floc blanket tank and the plate settler tank. Using one leveling tank and identical inlet and outlet piping for the two tanks ensures identical flow (assuming that differences in the loss coefficients through the tanks themselves are negligible). We decided to make the head loss through the leveling tank the dominant head loss through the system, which then sets the flow rate through the sedimentation tanks. Our design for the outflow system was a surface piercing 7.62cm (3") pipe with an orifice cut into the pipe below the water level. It was desired that the flow rate through the sedimentation tank would remain constant at 24 L/min no matter the flow rate coming into the flocculator (as long as it was at least 24 L/min). In order to ensure this the head loss out of the plant leveling tank (which controls flow in the sedimentation tank) had to be significantly greater than the head loss over the alternative exit in the flocculator. We chose a factor of safety of 18, thus the head loss out of the plant leveling tank would be 18 times the head loss out of the flocculator. the depth of the orifice hole beneath the water surface was determined to be 13.673cm (18 times the head loss over the flocculator outlet weir). Then using the orifice equation, with the flow set at 24 L/min, this head was used to determine the orifice size (2.223cm or 7/8"). The orifice equation used was the same as used for the flocculator outlet but converted to a circular perimeter from a rectangle.

Tank Drainage Manifold
A tank drain system was designed to drain in 30 minutes (Q _drain = 20 L/min. The design is similar to the effluent launder except this manifold is located at the bottom of the tank.

By the same method as was used for the tank effluent manifold the manifold diameter was determined to be 1.905cm (0.75") and the orifice diameter is 0.3572cm (9/64").

• The available head the available head for the sludge ports is the total water depth (84.455cm or 33.25") minus the head loss in the manifold and minus the velocity head at the end of the manifold.
• Kminor was assumed to be 0 in the manifold and 1.0 for the pipe entrances and exits.
• The constant for the determining orifice diameter was found to be 0.63
  *Qratio = 0.90
• 15 orifices were assumed to provide a similar layout to the effluent manifold.
  The hole size in the drain manifold was increased to 1/4" to allow for faster tank drainage.

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.