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Flocculation Model

A [theoretical model|^Demo Plant Flocculation Model.xmcd] is used to model hydraulic flocculation in the flocculator. The model was first developed by Dr. Monroe Weber-Shirk and was subsequently updated as more empirical data became available. The model predicts the amount of mixing, which is the product of shear and residence time, required to achieve a target floc size. The calculation is based on the concentrations of kaolin clay and aluminum sulfate in the feed, and an efficiency factor that accounts for the fact that not every collision between two particles cause them to stick together. In addition, the model calculates the maximum shear that a floc can withstand as a function of how much mixing it has gone through, based on the empirically-determined shear strength of aluminum sulfate flocs.

Since the new demo plant will be constructed out of the same corrugated plastic as the existing demo plant, it will have 10 mm by 7 mm channels. It has been experimentally observed that the biggest flocs that the existing demo plant can create is about 0.7 mm in diameter. Hence, the target floc size was set to 1 mm. The feed clay concentration was set to 500 mg/L, with a corresponding turbidity of 180 NTU. The [aluminum sulfate dose|^Demo Plant Alum Dosage.xmcd] was set to 45 mg/L. Finally, the efficiency factor was experimentally determined to be 0.2.

Flocculator Design

The [flocculator design program|^Demo Plant Flocculator.xmcd] was initially written in Fall 2007 as part of a CEE 454 final project, to create a design with varying baffle spacing such that the flocculator is as compact as possible. Since corrugated plastic with uniform corrugations will be used, the program was altered to accept a preset, uniform baffle spacing instead of determining them.

The program starts designing the flocculator from its exit. The dimensions of the last channel is determined from the baffle spacing, channel width and the given height of water. The degree of mixing provided by each channel is calculated from the plant flow and the dimensions of the channel. The number of channels required is then obtained by dividing the required amount of mixing calculated by the flocculation model by the degree of mixing provided by each channel. The maximum shear in each channel is also calculated using the channel dimensions and plant flow. The maximum shear is constant for all the channels since the channel dimensions are uniform.

The head loss in each channel is calculated by summing the frictional loss caused by the channel walls, and the expansion loss caused by the 180° turn at the end of the channel. The head loss is incremented every channel to determine the hydraulic profile. A given headroom is added to the height of the water in the first channel to give the height of the flocculator. The clearance between the end of each baffle and the water surface or flocculator floor is 2.5 times the baffle spacing.

The new flocculator has 31 channels and is about 35 cm long. The corrugated plastic dictates that each channel will be 0.7 cm by 1 cm. The clearance between the end of each baffle and the water surface or flocculator floor is 2.5 cm. The plant flow is 100 mL/min and the height of the water in the last channel is 25 cm, in order for the new flocculator to be compatible with the existing sedimentation tank. The height of the water in the first channel is about 26 cm and the headroom is set to be 2 cm. The flocculator is therefore 28 cm tall. The maximum shear in the flocculator is 45/s and the total mixing is 5300. The residence time is about 5.43 minutes.

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An [ISO A1 construction drawing|^Demo Plant Flocculator.pdf] was also created for the flocculator and a similar drawing was released for construction.

Materials

In building the plant, we used corrugated plastic for the lamella. The corrugated plastic we chose was 0.7 cm thick and had a flute thickness of 1.0 cm. The plastic was cut to size, and alternate ridges were cut out from the inside of the material on each end to form the body of the flocculator. This was mounted onto an acrylic cylinder. The stands were then made out of two plastic pieces and a screw, as shown in the picture.

Finished Product

A photograph of the final design of the flocculator is shown below. Most construction mentioned on this page was done by Paul and Tim in the Machine Shop in Hollister Hall, Cornell University.

Picture of Flocculator.