Demo Plant Fall 2007 Report

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The transparent demonstration plant is a good platform to perform [flocculation] experiments on, due to its manageable size and close approximation of an actual AguaClara water treatment plant. Experiments were done to investigate how more uniform shear in the flocculator - created by obstacles inserted in the vertical sections between bends - improves flocculation. Initial results showed that more uniform shear at the beginning of the flocculator caused effluent turbidity to decrease. Yet when the plant was modified to make it more robust and to make the results more repeatable, those initial results could not be replicated. Certain systematic errors in the initial experiments were eliminated and new errors were introduced. Subsequently, the plant was automated using [Process Controller] to make it more time-efficient, and the desktop turbidity meter was replaced by an inline turbidity meter to eliminate unintended bias in readings. However, the results still showed that the performance of the flocculator did not improve by making shear more uniform. It is probable that the performance of the unmodified plant is already at the theoretical limit, and that improving flocculation at the beginning of the plant cannot decrease the turbidity of the effluent any further.

Keywords: demonstration plant, uniform shear, obstacles, improve flocculation

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 plant.