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

Fall 2008 Research

Objectives

The purpose of this experiment is to determine the optimal plate settler spacing and by exploring the limiting design parameters. The goal is to consistently produce effluent water with an effluent 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 settling column. 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.

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.
--Turquoise labeled tubing indicates the main flow that leads to the ultimate goal: measuring the plate settling tubes' turbidity.

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 (see table). The average velocity gradient for the flocculator (G) is 71.4 s-1 , the residence time (θ) is 125.4 s and the G θ is 8.959*103.

These values are found using the following equations:

Average Velocity Gradient

Residence Time

The high G value and long residence time helps create the large amount of flocs needed for the initial formation of the floc blanket.
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. This mesh will break up the single jet coming from the column inlet, into several smaller, weaker jets that will dissipate quicker. The preliminary stages of the experiment will test the effectiveness of the cone and mesh system in dissipating the jet and allowing for optimal floc blanket formation.

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 manifold, through a turbidity meter, 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 reduce plate length. 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. The desired upflow velocity is dependent primarily on the critical velocity (Vc) necessary to allow flocs to settle out of the water:
Below is the calculation of the flow rate, regulated by a pump, through the tube settlers.

Where Ased is the cross-sectional area of one tube and Qsed is the flow rate through one tube settler.

As seen above, the flow rate is also based on the inner diameter of the tube settlers. The inner diameter of the tubes is analogous to the spacing between lamella in parallel plate sedimentation. The 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.

Results and Discussion

Obstacles

Flow into bottom bucket
Flow through

Graphs and results from trials

Future Work

Varying
Flow rate
Floc blanket depth
Location of tube settlers