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

Objectives

The purpose of this experiment is to determine the optimal plate settler spacing by exploring the limiting design parameters. The goal is to consistently produce effluent water with turbidity of less than 1 NTU.

Methods

The experimental apparatus is designed to be as robust as possible allowing for various experiments to be conducted using the same setup. The setup is made up of three unit processes, the constant turbidity control, flocculation, and sedimentation. Our focus for the experiment is on the settling column, where we will be varying certain parameters. We optimized the turbidity control system and the flocculator to create ideal conditions for floc blanket formation and sedimentation in the settling column. Each process is controlled by process controller, which is also used for data collection.

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.
• Flou 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 of 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. The original single flocculator was improved by added another flocculator. However, this addition caused the flocs to break apart. As a result, we decided to use one flocculator with an additional length to the original single flocculator. The original flocculator formed very small floc. As a result, a second cylinder with tubing coiled around it was created. This improvement leaded to more visible floc in the flocculator. The flocculator increased the number and size of flocs formed. However, this improvement also produced a supplementary problem, which was occasionally occurrence of air bubbles in the flocculator. As a result, the system flooded due to the build-up of air bubbles. To eliminate these occurrences, each cylinder of the flocculator had a long tube extended to the atmosphere as drawn in the schematic. Furthermore, the initial flocculator was horizontal with the table. It was observed that the horizontal placement of the flocculator cause formation of air bubbles. To remove and prevent air bubble formation, the flocculator was inverted to a vertical position.          
         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. 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

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
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Residence Time
Unable to render embedded object: File (theta.png) not found.

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: 153*10^4

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.
Based on the allowable space in the settling column the floc blanket height will be 50 cm.

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

Based on the allowable space in the settling column the floc blanket height will be 50 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.

Where b is the tube diameter.

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 .304 m. The table below lists the parameters of the tube settler apparatus for this experiment.
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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.

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 mm tubes were immersed in the floc blanket, effluent turbidity remained notably low. 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. These observations may be of interest for future work.

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 mm inner-diameter.

The graph shows incredibly low turbidity of the effluent coming out of the sedimentation tubes. In fact, most readings were below .3NTU. Such a low turbidity effluent is promising because it reflects a system that is capable of achieving very low turbidity water. This low NTU for the 12 mm inner-diameter tubes is expected since a large diameter tube, such as the 12 mm, provides more horizontal distance over which the flocs may settle-out than do the 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.

The two recent trials of 9 mm inner-diameter tubes have revealed more variability and more turbid effluent than the 12 mm tubes. The graph below shows that, while many effluent readings are below 1NTU, these data for the 9 mm tubes exhibit notable variation and instability.

For example, there seems to be a peak around 5.6 hrs. However, there is no obvious reason for this peak. Perhaps something happened to the system that was not documented, or perhaps flocs collected around the tops of the tubes and were pushed into the manifold around that time. In fact, the flocs collect in the 9 mm tubes unlike in the 12 mm tubes. In the 12 mm tubes, nearly all of the flocs settle down the tube. In the 9 mm tubes, flocs appear more likely to rest stably on the tube surface without settling down. Notice in the picture below the immobilized floc 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 that cannot settle down the tube as easily as lighter, more independent flocs.

Obstacles

The main challenges that our team has faced so far this semester have been determining reasonable tube diameters to test, finding an adequate tube settler apparatus, and remedying problems that have presented during the setup of our apparatus.

The determination of the tube settler diameters is discussed in the Sedimentation section above. The design of our experiment, including determination of a tube settler piece superior to the straws used this summer, was facilitated by our collaboration with Matt Hurst and Professor Weber-Shirk. The design that we are using is based on Matt Hurst's current apparatus.

Some of the challenges that have presented in our system are: leaking tubes, water not flowing out of the top two tube settlers (Paul wisely advised us to move our effluent tube to the top of the manifold which solved this problem), unsteady flow through tubes (to be addressed with added buffers), rapid decrease of the water-level in the clay/water bucket (maybe we will add a pump), inconsistent flow of waste from above floc blanket, and air bubbles entering flocculator from aeration in bucket (to be addressed by moving aerator).

We look forward to addressing these and other challenges very soon.

Future Work

Immediate future work will include documenting the process controller program that is in the process of being developed. Plans for the experimental process and data analysis can be found in the design section.

Although we have great ambition to vary flow rates, floc blanket depth, location of the tube settlers in relation to the floc blanket, and other parameters in the future, our team is currently determined to gather good data for our three tube diameters.

We seek to efficiently document our system, run experiments, and exploit the potential of plate settler spacing.