Pilot Plant- Sedimentation 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 require 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 in bulk head fitting would cost about $300) we limited this pipe to being 4 inches; it 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 will allow 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.

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.

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.

Design of the Sedimentation tank with Lamella

A preliminary design was done for the lamella tank. This design is still under review to determine if this algorithm was truly the best way to calculate the length of the lamella. It is intended that another design will be created that does not assume a set b (baffle spacing) value.

The same tank size was used for the lamella design as was used for the floc blanket design. The lamella were designed to be constructed in a grid because a material with a set baffle spacing of 11mm was thought a possible material. In this suggested grid set-up a series of channels of 11mm by 7mm in cross section would be set into the tank, this was done to ensure that a V _up of close to 100 m/day was met. If the lamella were adjusted to be parallel plates that extended the width of the channel (the current agauclara design) then the baffles would have to placed much closer to each other to guarantee a V _up of 100 m/day.

The inlet design, the effluent launder design, tank drain design will all be the same as for the floc blanket sedimentation tank. The only difference is that the inlet will have to enter the tank below the lamella and thus two 22.5 degree angle bends are to be used to lower the inlet pipe level from where it exits the floc tank to where it needs to enter the sedimentation tank. This tank was designed to be versatile and the lamella placed close enough to the top of the tank to allow for the possibility that a floc blanket could be formed underneath the lamella if desired.

Lamella Design
The lamella were designed out of corrugated plastic. Flow was designed to move upward through the grid of channels in parallel. The dimensions of the material proposed for use are listed below.

• b = 10.5 mm - the distance between corrugations
• w = 7 mm - the width of the channel (same as the width of the material)
• thickness = 0.40 mm - the thickness of the plastic material
• α = 60 deg

The first step in the lamella design was to determine the size of the lamella needed. A method very similar to the one used to determine the correct pipe size for the effluent launder was used to find length of the lamella. The equation for lamella length is implicit and thus the correct length was iteratively determined.
The implicit equation used can be seen below:

The following two equations were substituted into the above main equation.
Number of lamella sheets needs to fill the tank:

The number of cells in on sheet of lamella given the tank size:

The target V _up value was 100 m/day. This was left constant instead of being updated with each iteration. This was done because the program would not converge otherwise. After the program returned the converging value of Lamella Length, the active upward velocity and critical upward velocity needed to be calculated. The active upward velocity is different from the target upward velocity because the dead zone created from the lamella angle has to be taken into account. The active length of Lamella, active upward velocity and critical velocity were calculated using the following equations:

Insert equation for active length of lamella.

The initial program of used to determine the lamella length returned the total length of the material placed at an angle of 60 degrees. For construction uses and determination of the inlet position we needed to determine the vertical height of the lamella. For this calculation the follow was used:

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.