Current research regarding the effects of lamella spacing on sedimentation is an extension of experiments completed during summer 2008: Summer 2008

Fall 2008 Research

Abstract

The following report discusses an experimental apparatus designed to test the effects of plate settler spacing on effluent turbidity. Turbid water flows through a tube flocculator, clarifying column, and tube settlers. Although our interest is varying tube settler diameter, this apparatus also allows for varying other parameters within the system such as floc blanket depth, flow rate through the flocculator, and flow rate through the tube settlers. Preliminary results indicate that the apparatus produces reliable data. The system has been run overnight on several occasions. Additionally, effluent turbidities under 1 NTU have been consistently recorded with the use of 12.7 mm inner diameter tube settlers.

Objectives

The goal of the plate settler spacing team is to develop a more complete understanding of the effects of plate settler spacing on the sedimentation process. We seek to purify turbid water as efficiently as possible by optimizing materials and minimizing costs. Currently, our experiments vary the inner-diameter of the tube settlers. Ultimately, we will explore the interdependency of floc blanket depth, flow rates, and tube settler inner diameter.

Methods

The experimental apparatus design that follows allows for the independent variation of several parameters including flow rates through the flocculator and tube settlers, floc blanket depth, and tube settler inner diameter. The main components of the system include initial turbidity control, flocculation, floc blanket formation, and monitoring of effluent turbidity. Each component of the apparatus is regulated by process control. Process control allows the system to operate independently and allows for continuos data collection of effluent turbidity.

Schematic

Click below for the following image.
Original System
Photo of Whole System

Schematic Map for the Setup

• Bucket 1 has the concentrated clay that is being stirred. The concentration of the clay is 5 g/L.
• Bucket 2: (1)The gold labeled tube evaluates that the flow is maintained at 100 Ntu by checking the flow with the turbidity meter.
• The gold colored tubing indicates the water and clay mixture is going through Pump 1 to the turbidity meter to maintain a constant flow of 100 NTU influent water.
• Blue labeled tubing indicates the main flow that leads to the ultimate goal: measuring the plate settling tubes' turbidity. The light blue tubing goes from Bucket 2>>>Pump 3>>>Flocculator>>>Flou Blanket Column >>>to the Plate Settler Tubes >>>Turbidity Meter >>> Waste.
• The Pressure Sensor in Bucket 2 is connected to the computer monitoring the water level in the bucket to control whether or not water is flowing into the system.
• The blue tube from Bucket 2 is where the water coming from the main water source comes into the setup.
• Pump 3 pumps the mixture from the blue tank to the flocculator. Originally, the tube entering the pump is a single tube. However before entering the pump, this single tube is split into two. This is done to increase the flow rate. The required flow rate is 545mL/min and the maximum flow rate that can be produce by this pump is 300 mL/min. Therefore, the single tube is splited into two tubes and being pump at 272 mL/min for each tube.
• The Alum Stock is where the Aluminum Sulfate is stored and mixed with the magnetic stirrer.
• The purple tubing shows the flow of Alum comes into the setup from the Alum column to Pump 4. It is mixed into the water and clay mixture before the additional cylinder and before entering Pump 3.
• The Floc Blanket Column receives the water mixture from the bottom of the column. Then the water mixture either goes into the plate settler tubes or the waste tube.
• Floc blanket in the column is control by the flow control weir. A black tubing from the column to the flow control weir is where the head loss occurs.
• The plate settling tubes are placed at sixty degrees with a metal splint that is fastened to the manifold of the plate settling tubes.
• The flocculator is in a vertical position with two coiled cylinder. The mixture of clay, alum, and water is pumped into flocculator by Pump 3. After flocculation, the mixture is transported to the column.

Pathways for each colored tubing:

• Pink
  • Bucket 1 >>>>>Pinch Valve>>>>>Bucket 2
  • Inlet for the clay mixture into the Bucket 2 to mix with the water.
• Red
  • Bucket 2>>>>>Waste
  • This tubing is used to control the water level in the bucket 2. Therefore, this tubing is essential to prevent flooding.
• Cyan
  • Air>>>>>Bucket 2
  • This is the bubbling system for Bucket 2. The bubbler aid in the mixing of water and clay in Bucket 2.
• Gold
  • Bucket 2>>>>>Pump 1>>>>> Turbidity Meter>>>>>Bucket 2
  • This tubing ensures that the flow in Bucket 2 is maintained at 100 NTU.
• Purple
  • Alum Stock>>>>>Pump 4>>>>>Blue Tubing
  • The purple tubing shows how and where the alum is mixed into the system.
• Blue
  • Water Tank>>>>>Bucket 2
  • This show how and where the water comes from.  This is essential to guarantee that there is water in the system.
• Bright Green
  • Flocculator>>>>>Atmosphere
  • The tubing plays an important role to extract air bubbles from the flocculator and the system.
• Jungle Green
  • Bucket 2>>>>>Computer
  • This tubing reads and records the pressure in Bucket 2. This measurement will assist the Control Process when to shut down to prevent problems to occur in the experiment.
• Orange
  • Column>>>>>Flow Control Weir
  • Flou blanket control occurs here.
• Light Blue
  • Bucket 2>>>>>Pump 3>>>>>Flocculator>>>>>Column>>>>>Settling Tubes>>>>>Manifold>>>>>Turbidity Meter>>>>>Pump 2>>>>>Waste
  • THIS IS THE MEASURING TUBING. This measuring tubing is vital to collecting data to investigate whether our design it is good or not to improve water condition.

Major Changes from the Previous Setup to the Improved Setup

       The water tank was originally sitting on the table top. This is original position of the tank caused many problems to the whole setup. To prevent excess head loss in the mixture tank (bucket 2 as labeled in the schematic), the whole water tank was raised up. Before the water tank was raised, the mixture tank would be drained of its water supply causing the experiment to continue without water. These occurrences leaded to a discussion of the height-dependence of water tank. We confirm that water tank needed to be raised. After raising that the water tank by fourteen inches, the mixture tanks did not dry up during the experiment. From our observations and data, it is confirmed that raising the water tank was the right decision.
       The second change is that a cylinder was added before the tube splits into two for Pump 3. This cylinder as labeled in the Schematic above is a hollow plastic cylinder that has the tube coiled around its circumference. The purpose of this cylinder is to improve the mixing of the alum with the dirty water. The mixing was improved because the cylinder with the coiled tubing increases the contact time that the alum had with the dirty water before flocculation.
       Aside from the additional cylinder, the flocculator was improved. To remove and prevent air bubble formation, the flocculator was inverted to a vertical position. An in depth decription of design of the new flocculator can be found in the design parameters section.
         Another change to the setup is the method used to control the floc blanket. The original method to control the floc blanket was to open a valve that was connected to the tube. This tube was directly connected to the column. However, this method was far from precise and the floc blanket level fluctuated. Therefore, a floc blanket formatted required a completely drained system. To end this fluctuation in the floc blanket level, a device called the flow control weir was attached. This flow control weir is joined by black tubing. Within this black tubing, the head loss occurs. By altering height of the flow control weir, the height of the floc blanket is shifted. Essentially, it has the same functions as the original method of opening the valve. However, this flow control weir controls the height of the floc blanket better. This is proven by observations and data collected from each experiment. As a result, a stable floc blanket was achieved. It was also determined the floc blanket must be below the settling tubes for optimum effluent turbidity.
         Apart from the floc blanket, the floc formation was improved by removing a mesh in the column. The initial setup has a mesh that was locked onto the top of the cone inside the column. For the improved setup, this mesh was removed to form larger floc. When the mesh is present, the mesh broke down the original floc into smaller size floc. Without the presence of the mesh, it was observed that bigger flocs were formed.
         Last major alternation is the addition peristaltic pump inserted. Instead for the sharing the same pump as the settling tubes, the alum stock utilizes the new additional pump. With this additional pump, the alum stock concentration was adjusted so that system can be run for extended periods of time. The concentration was to 15g/L for 4 liters tank. With this concentration and tank size, the alum stock could last for 1.68 days. The original concentration of the alum varied with the flow rate. As a result, the alum stock did not last for a long period of time. Along with the adjusted concentration, the flow rate for the alum stock was modified to maintain the same alum concentration in the system.

       In conclusion, the following major changes were made:

1. Raised water tan
2. The addition of a cylinder before flocculatio
3. Vertical flocculator with additional length
4. Addition of air tubes in flocculato
5. Flow control Wei
6. Removal of the mes
7. Change in alum stock concentration
8. Alternative peristaltic pump for alum stock

Setup for Computer System---Process Controller

For detail on the setup for Process Controller, click on the link below.
Computer System---Process Controller

Checklist before Running the Setup

[Checklist Before Running the Setup]

Design Parameters

Flocculator design

The use of a tube flocculator allows for a more controlled flocculation environment as compared to flocculation with baffles. The flocculation design parameters are based on creating optimal conditions for floc blanket formation. The flow rate through the system the original experimental apparatus

• L: 17 m
• d: 3/8 in (inner tube diameter)
• G: 71.4 1/s (average velocity gradient)
• θ:125.4 s (residence time)
• Gθ: 9519

These values are found using the following equations:

Velocity Gradient

Residence Time

Several flocculator designs were used in the trial runs of the experiment. The initial flocculator was oriented horizontally across the work bench. This orientation led to a numerous small bubbles trapped throughout the system. It also made the flocculator difficult to fill. The first change therefore, was to use a vertical flocculator orientation. This allows bubbles in the flocculator to flow upwards towards the top where a t- joint and tube open to the atmosphere collected any bubbles in the flocculator.

Upon observation, it was clear that the flocs being produced in the flocculator were too small. It was originally thought that this was due to the high flow rate through the system. Again the flow rate through the flocculator is based on an upflow velocity of 100m/day for proper floc blanket formation in the settling column. The flow rate was then halved by adding another flocculator. Since the flow through pump 3 was already split this was adjustment was quite simple, each of the split flows was led directly to a flocculator. The flow out of the flocculator was then joined into a single tube flowing into the settling column using a t-joint. Floc formation was still less than satisfactory, with the average floc size in the flocculators less than 1mm. It was later noted that the combining of the two flows (each with a flow rate of approximately 272 mL/min) created shear valves far exceeding the average shear values in the flocculators (71.4 /s), which likely resulted in the breaking up of flocs formed in the two flocculators. It is important to not at this point the system was producing floc blanket, however, it was not very dense and the effluent turbidities were still quite high.

Current design

The current flocculator set-up contains one continuous flocculator placed on two adjacent columns. The flow rate through the system remains at 545 mL/min, however, the length of the flocculator has increased to 27 m. As seen in the schematic above there are now two flocculator columns with the water spiraling upward to the top of the first flocculator where there is the first bubble release t-joint. The flow then crosses over to the top of the second column where it spirals downward reaching another bubble release t-joint right before heading into the bottom of the settling column. The increased residence time in the system allows for larger floc formation, and an increase in the total number flocs forming in the flocculator. Improved floc formation allows for the formation denser floc blankets.

Final flocculator parameters

• d: 3/8 in *
• Q: 545 mL/min *
• G: 71.4 1/s *note these parameters stayed the same
• L: 27 m
• θ: 214.6 s
• Gθ: 15300

The high G value and long residence time helps create the large amount of flocs needed for the initial formation of the floc blanket. The flow rate through the flocculator (545 mL/min) is separated into two equal flows of 272 mL/min, based on the maximum flow through the peristaltic pumps.

There are still some improvements to be made to the flocculator. Suggestions for improvement are discussed in the future goals section

Uniform Flow

Uniform flow in the settling column is key for having laminar flow in the tube settlers as well as the creation of the floc blanket. As seen in the image to the right, a uniform flow is needed at the inlet of the settling tube so the flow becomes laminar before reaching the end of the tube. All design assumptions and parameters for the column are based on a laminar flow through the settling tubes.

Flow in to the system from the bottom of the settling column creates a jet in the center of the column, which is far from uniform flow. A typical dissipation ratio is 1:10; therefore, it would take on the order of 10 m for jet to dissipate in a column with a diameter 10 cm, as in the current setup. A cone with a ratio of 4:6 is used a diffuse to maximize the lateral spread of the jet. In addition to the cone, there is a mesh directly on top of the cone made of a 1 cm thick plastic sheet with uniformly distributed holes of 0.5 cm diameter. The purpose of the mesh is to break up the single jet coming from the column inlet into several smaller, weaker jets that dissipate quicker. The preliminary stages of the experiment tested the effectiveness of the cone and mesh system in dissipating the jet and allowing for optimal floc blanket formation and maintenance. The mesh was eventually removed due the breaking up of flocs. Further dicussion about testing the effectiveness of the mesh can be found in the future work section.

Based on the allowable space in the settling column the floc blanket height will be no more than 60 cm. The alum dosage of 45 mg/L is also important for the creation of the floc blanket. In previous experiments it was thought that 45 mg/L was too high increasing the "stickiness" of the tube settlers and leading to clogging. The same chemical properties of the aluminum sulfate actually aid in the creation and maintenance of the floc blanket, creating a stick mesh of suspended flocs.

Sedimentation

The sedimentation process, which occurs directly after flow through the floc blanket, is the final separation technique in our water purification system. By the time the water enters the tube settlers, uniform flow has been established and the floc blanket has already dramatically reduced the water turbidity. The design of the sedimentation section of the apparatus requires serious consideration of flow rates, tube geometry, and other parameters.

The design of our column allows for independent control of the flow rates through the floc blanket and the tube settlers. The independence of these flow rates is maintained by the additional waste outlet located above the floc blanket (see figure below). The water flows up through six tube settlers and a manifold, through a turbidity meter (where turbidity data is collected by process control), and is then wasted. The same pump circulating alum to the system controls this flow. In order to maintain a steady flow, a buffer may be utilized.

The flow rate (Q) through the tube settlers is calculated based on a desired upflow velocity (Vup) of 100 m/day through the tube settlers. We note here that the tube settlers are angled at the standard sixty-degrees from the horizon in order to optimize floc settling and accessibility for cleaning. We also note that flow through the tubes is assumed to be laminar, resulting in a parabolic velocity profile with the maximum velocity (Va) occurring at the center of the tubes.

Below are the equations used to calculate the flow rate, regulated by a pump, through the tube settlers:
Upflow Velocity

Velocity through tube settlers

Flow rate through tube settlers

Where Ased is the cross-sectional area of one tube.

As seen above, the flow rate is based on the inner diameter of the tube settlers, upflow velocity, tube length, and angle. Additionally, the critical velocity (Vc) is dependent on the inner diameter of the tube settlers (see equation below). The critical velocity is the velocity of the flow that facilitates particle settling.

For these experiments, a modified Vc equation is used that takes into account the circular geometry of the tube settlers
K is the efficiency factor: 1.33 for circular tubes according to Shultz and Okun.
Lu (the effective relative depth where laminar flow can first be considered) is defined by the following equations


Lr: relative depth
Lt: 12 in length of settling tubes
b: tube diameter
Re: Reynolds number (~280) (Shultz and Okun)

Evidently, analysis of the inner diameter of the tube settlers is important to understanding other key parameters of the system such as flow rate and critical velocity. Due to the codependency of these and other parameters, determination of the optimal diameter is multi-faceted.

First of all, the ratio of the vertical height of the tube to the tube diameter must be at least twenty-four in order to allow enough distance for the flocs to settle out of the flow (this value is based on the critical velocity and tank dimensions). In addition, a decrease in diameter allows for a decrease in plate length and, consequently, a decrease in sedimentation tank depth. However, if the diameter is too small, the resultant shear force may rupture the flocs. Also, previous research (see Summer 2008 above) indicates that various problems that are difficult to identify and quantify, such as clogging, exhibit at small diameters.

Since flow through the tube settlers is laminar, the shear stress is greatest at the tube surface where Va is zero. Below is a calculation of the minimum diameter based on the shear stress at the tube surface. We assume that floc integrity begins to suffer under shear stresses of 1 Pa.

The velocity gradient equation:

yields the equation for maximum shear stress:

Substituting the head loss equation:

into the maximum shear stress equation yields:

Since substituting our parameters into this equation yields an unreasonably small minimum diameter of 20 µm, shear stress is not the limiting factor for tube settler diameter. Therefore, the diameters tested in this experiment are based on floc size, the length/diameter ratio of twenty-four mentioned above, and material availability. The minimum diameter was set to be at least twice the diameter of a floc (estimated at 2 mm), and the length of the tubes was set to be 0.304 m. The table below lists the parameters of the tube settler apparatus for this experiment.

Results and Discussion

The results gathered from the above method are indicative of a system that is capable of producing consistent and quality data. However, more trials must be run and the system must be made more robust for the data to be as credible as possible. At this point, we offer discussion regarding the data gathered up to this point and the current design of the experiment.

Although the interest of these experiments is to determine the impact of plate settler spacing on effluent turbidity, floc and floc blanket formation are crucial elements for attaining the low effluent turbidities that are desired. Both floc and floc blanket formation are priorities of this system and must be discussed briefly. One improvement that was made to this system after the earliest trials was to ensure that the alum solution is introduced to the system by its own pump (it had previously been run on the same pump as the sedimentation tubes). This independence facilitates increased control over both the effluent and alum flow rates. Also, the rapid-mix chamber, discussed in the apparatus setup section above, and the over twenty-seven meter long flocculator help to ensure optimal floc formation. Another adjustment made to guarantee floc blanket formation was the removal of the mesh (placed in the bottom of the column to hasten flow distribution in the column) because it was ripping apart the flocs.

The graph below illustrates the importance of the floc blanket in attaining low effluent turbidity. The graph is the effluent turbidity for the first hour and a half of running the system.

Graph 1. Effluent turbidity with 12.7 mm inner diameter tube settlers before floc blanket is fully developed.

The drop in turbidity as the floc blanket forms correlates with previous findings by Matt Hurst that the floc blanket is important for attaining a low effluent turbidity.

The floc blanket depth is regulated by a flow-control weir, as discussed previously in the apparatus setup section. Thus far, the floc blanket has been maintained just above the weir control outlet and about 10cm below the sedimentation tubes. The top of the floc blanket is about 53 cm above the cone that is located at the bottom of the column. Presumably, the floc blanket extends into the cone.

During the first experimental trials, great attention was paid to floc blanket depth. Though the data below are based on the top of the floc blanket sitting below the sedimentation tubes, we observed that when the openings of the 12.7 mm tubes were immersed in the floc blanket, effluent turbidity remained under 1 NTU. This persistently low turbidity was visually dramatic since the tubes immersed in the floc blanket were significantly more filled with flocs than the tubes located above the floc blanket. More tests would need to be run in order to draw conclusions about the effect of tube/floc blanket location on effluent turbidity. However, these preliminary observations are noteworthy.

We now turn to the sedimentation tubes and the data gathered thus far. Below is a graph of the effluent turbidity over time of sedimentation tubes with 12.7 mm inner diameter.

Graph 2. Effluent Turbidity: 12.7 mm inner diameter tube settlers.

Graph 2 above shows incredibly low turbidity of the effluent from the sedimentation tubes. In fact, most readings were below 0.3 NTU. Such a low effluent turbidity is promising because it reflects a system that is capable of achieving very low turbidity water. This low turbidity for the 12.7 mm inner diameter tubes is expected since 12.7 mm is a fairly large spacing and does, therefore, provide more horizontal distance over which the flocs may settle-out than do smaller diameter tubes. Also worthy of brief mention here is the influent turbidity versus effluent turbidity. The data from all of the tests run so far show that even with a twenty-NTU variation in the influent turbidity, effluent turbidity remains constant. This indicates that the flocculation and sedimentation parts of this system are resistant to shifts in influent turbidity.

The two recent trials of 9.5 mm inner diameter tubes have revealed more variability and more turbid effluent than the 12.7 mm tubes. The graph below shows that, while many effluent readings are below 1 NTU, these data for the 9.5 mm tubes exhibit notable variation and instability.

Graph 3. Effluent Turbidity: 9.5 mm inner diameter tube settlers.

For example, there seems to be a subtle increase in effluent turbidity around 5.6 hrs. However, there is no immediate reason for this peak. Something may have happened to the system that was not documented, or maybe flocs collected around the tops of the tubes and were pushed into the manifold around that time. In fact, the flocs were observed to collect in the 9.5 mm tubes. In the 12.7 mm tubes, nearly all of the flocs settle down the tube. However, in the 9.5 mm tubes, flocs are more likely to rest stably on the tube surface without settling down the tube. The picture below illustrates immobilized flocs on the sides of the 9 mm tubes:

This may occur because the smaller diameter allows flocs to collect, coagulate, and form a greater mass. Consequently, this greater mass may be unable to settle down the tube as easily as lighter, more independent flocs.

A final comparison between the 12.7 mm and 9.5 mm tubes is their different critical velocity values. Recall that critical velocity is dependent on tube diameter:

where Lu is a function of the tube diameter b.

This means that when all parameters in the equation above are held constant except for tube diameter, the critical velocity will vary according to tube diameter. Therefore, the critical velocity through a narrower tube will be smaller than the critical velocity through a wider tube. This means that flocs should settle out of the 9.5 mm tubes more readily than they settle out of the 12.7 mm tubes. The data do not corroborate this hypothesis.

Conclusion

The current data does not support the hypothesis that the partile removal of the settle tubes improves as the critical velocity decreases. There is not enough supporting data, however, to make any conclusions about the effluent turbidity and its correlation to critical velocity. From observations it is clear that both the 12.7 and the 9.5 mm tubes were able to remove the particles effectively. However, the 9.5 mm did not drain as well as the 12.7 mm tubes at the same Va. This could lead us to limiting parameter in the performance of smaller tube settler spacings.

Varying the velocity could better show how the Va though the system effects the performance, thus further testing the predicted critical velocities. This experiment along with several others to be performed next semesters are disscused in the section below.

Some of the other success of the semester were the formation of a stable floc balnket, control of the floc blanket height using the flow control weir, the development of a system that produces consistant and accurate results, and consistant effluent turbidities of less than 1 NTU, with 0.07 being the lowest consitant value.

The final working system provides much opportunity for experimentation as we attempt to determine the optimal plate settler spacing.

Future Work

Continued Data collection

Data collection is still in the preliminary stages. The settling tubes with the smallest diameter, 4mm, have yet to be tested. The experiment as explained above should continue based on the observations and preliminary data collected.

Additional Experiment

• Jet dissipation
  It is still not clear what is happening in the cone as far as mixing and jet dissipation. Determining the fluid mechanics of this system could improve floc blanket formation. With the mesh the floc blanket formed quicker, however, the mesh resulted in the breaking up of flocs due to the low hole diameter of 0.5 cm. A mesh should be made with hole diameter of 1 cm to test the jet dissipation and floc maintenance. In addition to the mesh experiment, Several experiments can be performed solely on optimizing the jet dissipation.

• Tube spacings
  The velocity of the affects the particle removal, as previously mentioned. Varying the flowrates on each of the tube size could result present some interesting data. To collect these values one would only have to use the process controller increment method to vary the flowrate of the water pulled off the top of the settling column (pump 2).
  It would also be worthwhile to vary the floc blanket depth using the floc blanket height flow control unit. This should include testing the particle removal of settling tubes with the tubes submerged in the floc blanket.

Changing the alum dosage may also provide some interesting data concerning floc blanket performance.

Improvements to the System

• Making the system more robust
  Certain parts of the system are fragile, difficult to fix or correct and could use some design improvement. For example, the tube settler could use a more stable connection.
  One of the most unstable parts of the setup are the flocculators that sit on the table. They are very susceptible to being knocked over and have been several times! Next semester the team should speak with Paul in the shop and try to design some sort of structure to hold and stabilize the flocculators. This structure would also include some sort of support for the two bubble release tubes currently attached to the ceiling.

The goals and challenges for next semester are outlined in the Spring 2008 Challenges section.