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 using tube settlers and a tube flocculator.

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

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

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 is completed directly after the floc blanket, is the final separation process of water purification. By the time the water enters the tube settlers, the turbidity has been dramatically reduced by the floc blanket. As seen in the figure below, the apparatus allows for independent control of the flow rates through the floc blanket and the tube settlers because of the additional waste outlet just above the floc blanket. The water flows up through six tube settlers and manifold, through a turbidity meter, and is wasted. This flow is controlled by the same pump circulating alum to the system. In order to maintain a steady flow, a buffer may be utilized.

The flow rate is based on a desired upflow velocity of 100 m/day through the tube settlers (this number is based on previous pilot plant research). Note that the tubes are at the standard sixty-degree angle from the horizontal. Below is the calculation of the flow rate, regulated by a pump, through the tube settlers.

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, flocs may be impacted by shearing at the tube surface. Also, previous research (see Summer 2008 above) shows that various problems that are difficult to identify and predict quantitatively, such as clogging, exhibit at small diameters.

Since flow through the tube settlers is laminar, the shear stress is greatest at the tube surface. 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 this calculation 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 twelve inches. Table 2 below lists the parameters of the tube settlers 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