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In some AguaClara plants, a surface foam develops at the end of rapid mix. The first experiment goal was to understand initial focus of the research was on the chemical conditions required for this surface foam to develop at the end of rapid mix and the first baffle. The first test trials were conducted with then the focus shifted to the fluid mechanics that make this occurrence possible and the simple retrofit designs that can ameliorate these conditions. In the initial experiments, different chemical conditions were tested for using a series of jar mixers and one-gallon tanks that modeled rapid mix. The first few trials tests ran a constant supply of clay and with varying amounts of alum but these did not exhibit any form of surface foam formation. Subsequent trials included organic matter: humic acid, but these only produced large non persistent bubbles. It was not until a stronger surfactant, liquid soap, was added to the baffle spacing that a surface foam with strong persistent bubbles developed. From these experiments it was concluded that air entrainment along with a surfactant in the raw water are the main chemical factors behind surface foam formation. In Honduras, the raw water may contain decaying matter which decays to fatty acids acting as the surfactant while the waterfall at the LFOM creates- the perfect- condition for air entrainment (what about the waterfall does?). The process of air entrainment along with natural surfactants in the water allow for the formation of surface foam. With this in mind, the research is now focused on retrofitting AguaClara's designs so that no air entrainment occurs in the entrance tank and rapid mix chamber by eliminating waterfalls and or implementing hydraulic jumps
Upon observing that waterfalls, like the one found in the LFOM, created the ideal fluid dynamic conditions for air entrainment; the second half of the research focused on retrofitting the LFOM at current AguaClara plants. The four designs that were suggested either used a submerged orifice, a vertical surface area or an inclined plane to decrease the velocity of the incoming water through the LFOM. In testing the viability of each design option the three limiting parameters of foam formation from water jets were recognized and documented.
Introduction and Objectives
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- Attempt to recreate the foam in a laboratory setting that has been forming in many of the AguaClara plants in Hoduras.
- Design a way to retrofit the current plants to eliminate the foam
- Learn about the current design for a Nonlinear Chemical Dose Controller and then update MathCAD code for the controller and hopefully be able to build a fully functional prototype by the end of the summer.
These two goals (which two?) are very important to the overall goals of AguaClara for a number of reasons. The foam that forms in the current AguaClara plants both increases the amount of work that plant operators have to spend to keep the water clean, and reduces the overall effectiveness of the plants. Although the foam cannot flow very far in the plant itself, it can be blown around by wind to the surface of the sedimentation tank. Small bubbles formed by decreased surface tension in rapid mix could persist as far as the sedimentation tank, where they would come out, possibly causing the floating floc problem. We suspect that the surfactants and natural organic matter lower the surface tension energy requirements, possibly creating these small bubbles that stay in solution long enough to reach the sedimentation tank.
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Also, as we evolve to build larger plants, the Linear Chemical Dose Controller (LCDC) won't be able to provide a sufficient dose of chemicals to treat the larger flow rates. The CDC team from past semesters has found that the LCDC can only provide chemical flows up to 400mL/min, which is too low for larger plants. Linear dose control depends on a change in head in the plant inflow tank to move a float up and down, which changes the flow of alum into the water. Thus, a Nonlinear Chemical Dose Controller (NCDC), where flow will not vary linearly with water height like the LCDC, is needed for bigger plants. A discussion of the need for a NCDC can be found in the second paragraph on the NCDC page. This summer we will need to improve upon the initial design of a NCDC in order to make it more dependable and robust.
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Summer 2009 CDC Research Team's goals and meeting minutes.
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Current Research
Determining the Cause of Surface Foam Experiments
Retrofitting Plants to Prevent Aeration ExperimentsNon-Linear Chemical Doser