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

Schematic Map for the Setup

--Bucket 1 has the concentrated clay that is being stirred.
--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.
--Turquoise labeled tubing indicates the main flow that leads to the ultimate goal: measuring the plate settling tubes' turbidity. The blue tubing goes from Bucket>>>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.
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 2. It is mixed into the water and clay mixture 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.
--The plate settling tubes are placed at sixty degrees with a metal splint that is fastened to the manifold of the plate settling tubes.

Design Parameters (parameters/equations/length and diameter values/critical settling velocity values)

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. For this experimental apparatus the flocculator is 17 m and the inner tube diameter is 3/8 in. The average velocity gradient for the flocculator (G) is 71.4 1/s , the residence time (θ) is 125.4 s and the G θ is 9519.

These values are found using the following equations:

Velocity Gradient
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Residence Time
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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.

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.

Flocculator design

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 flowrate through the system remains at 545 mL/min, however, the length of the flocculator has increased to (FLOCCULTATOR LENGTH). 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
• Flowrate *note this parameter stayed the same
• G *the average velocity gradients also stayed the same
• Theta
• GTheta

There are still some improvements to be made to the flocculator. Suggestions for improvement are discussed in the FUTRUE GOALS SECTION.

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

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.