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Keywords: demonstration plant, uniform shear, obstacles, improve flocculation
History
Introduction and Objectives
The demo plant was originally designed as an education and demonstration apparatus. This is the third version that has been build and is the most accurate representation of the actual plants located in Honduras. The previous demo plants were harder to follow and were less efficient.
The current demo plant is gravity-powered. The flow of fluids into the plant is regulated by constant head regulators. These constant head regulators are identical to the regulators used in Honduras. They regulate flow by keeping constant the difference in elevation between the top of the fluid in the regulator and the feed point. While the flow of fluids can be adjusted by changing the height of the feed points, the demo plant is designed to handle 100 mL/min of fluids.
The flocculator is open-top and is made of clear corrugated plastic. Each corrugation forms a channel that has a cross-section 10 mm long by 5 mm wide. Slits are cut in the corrugation to allow flow to weave through the device. The flocculator leads to the sedimentation tank made of the same corrugated plastic material. The corrugations here serve as the lamella. The flow through each lamella is kept uniform by a small orifice at the top of each channel. The orifice creates a significant amount of head loss and negates any difference in head loss between lamellas. The schematic of the demo plant is shown in Figure 1 below.
The demo plant is of manageable size and is easy to operate. In addition, it is transparent and allows direct observation of the flocculation and sedimentation processes. These properties make the demo plant very suitable for flocculation experiments that must be carried out under controlled conditions, but in an apparatus that closely models the actual AguaClara water treatment plants.
Currently, most of the shear is provided by the 180° turns at the ends of the baffles. There is much less shear in the vertical sections between baffles. Thus, the difference between the maximum shear and the mean shear of the flocculator is significant. In addition, localized maximum shear at the 180° turns near the end of the flocculator is more than enough to break up flocs, although the mean shear is at a safe level. It is proposed that if the shear level across the vertical sections of the flocculator is more uniform, the flocculator will be more efficiently used and the size of the flocculator can be reduced. In addition, it will effectively reduce maximum shear near the end of the flocculator and reduce floc breakup.
In order to even the shear level, the flow path of water in the vertical section must be disrupted. One can introduce obstacles around which the water must flow. The initial approach was to experiment with disrupting flow at a small scale with the VFHF demonstration plantSnip snip.