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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. In larger plants, the frictional loss caused by the channel walls is usually neglected as its magnitude is much smaller that the expansion loss. However, the Demo Plant flocculator's channels have cross-sectional dimensions that are comparable to the baffle spacing. Consequently, the frictional loss is on the same order of magnitude as the expansion loss and cannot be neglected. Figure 1 below shows how the loss coefficient varies with the flow rate.
Figure 1. Loss coefficients vs. Plant flow
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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 conventional ratio between the forementioned clearance and baffle spacing is about 1.5. However, we used a ratio of 2.5 in our design to greatly reduced the effect of sedimentation on the bottom of the flocculator, and to ensure that the is no restriction of flow at the top of the baffles.
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The new flocculator has 31 channels and is about 35 cm long. The corrugated plastic dictates that each channel will be 0.7 cm x 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. 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 5.4 minutes. The flocculator is compatible with the existing sedimentation tank, but is incompatible with the existing flow control module due to the downward flow direction of the first channel.
Figure 2. AutoCAD rendering of flocculator
An ISO A1 construction drawing was also created for the flocculator and a similar drawing was released for construction.
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Most construction work was performed by Paul Charles of the Civil and Environmental Engineering Machine Shop.