Page History

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Research performed last semester focused on studying the effects of sand grain size on gas removal. We found that smaller sand grains provide more surface area per unit volume than larger grains, but grains that are too small are carried to the surface with bubbles and are washed away. Grain size also affects porosity pore size of the sand bed, which affects the distance a gas molecule must diffuse to reach a surface. This also may affect aggregation of gas molecules to form larger bubbles. Further study of optimum grain size will be explored through MathCAD modeling, or through experimentation, if that is deemed necessary.

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We would like to determine whether effectiveness of gas removed depends on the concentration of dissolved gases in the influent water. We plan to adjust the amount of supersaturation , probably by varying pressure in the aerator to decrease the dissolved oxygen concentration.

Experimental Design

The Most experimentation related to floating flocs is Experimentation will be conducted using a new sand filter setup modified from the Spring 2009 setup. A description of the set-up and the Process Controller methods used to conduct experiments can be found here.

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Using the setup detailed above, we plan to conduct this experiment by beginning with a small amount of sand in the column and running the apparatus on the "On" state (described in the description of the system here) in Process Controller to measure and record gas removal. We would repeat with increasing depths of sand until either a relationship can be clearly defined. We plan to test bed depths within the range of 10 cm - 50 cm of Sand 40 (grain diameter 0.42 - 0.59 mm) with 50% expansion.

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This experiment will attempt to examine the fluidized bed expansions for a filter media under varying flow rates, with other parameters held constant. The experiment will be conducted using the described testing apparatus and the "On" state in the Process Controller. As of now, we do not foresee any major changes to the experimental setup. We will adjust bed expansion by adjusting the flow rate through the sand filter. Expansion of a bed of sand affects the distance that the gas molecule has to undergo to reach a solid surface. Consequentially, this affects the bubble formation process. By conducting this experiment, the team hopes to quantify the relationship between the bed expansion at a certain flow rate and gas removal to find optimal conditionals of bed expansion in the sand filter. Presently, we are considering using Sand 40 with a grain diameter 0.42 - 0.59 mm. The range of expansions to be tested will be determined after the bed depth experiments are conducted. A bed depth that yields moderate gas removal will likely be chosen and the physical limitations of the sand column will govern the expansion range.

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Because of seasonal changes in temperature, the dissolved air concentrations of influent water at AguaClara plants may not be consistent throughout the year. Because of this, we are interested in determining if there is a relationship between the dissolved gas concentration and the rate of gas removal via the sand filter. This experiment is designed specifically to measure the changes in the gas removal rate of the sand filter as a direct consequence of changes in the dissolved gas concentration of the influent water.

For this experiment, the setup will be run on the "On" state on Process Controller described above. It it will be assumed we will assume that the residence time in the new aerator will allow the dissolved gas concentration to equilibrate with the pressure maintained in the aerator. Since dissolved gas concentration is a function of pressure, the pressure in the aerator will be adjusted to achieve different dissolved gas concentrations in the influent water while keeping all other variables constant. It is likely that the range of pressures tested will be between 20 kPa - 100 kPa. The pressure is adjusted within the aerator by adjusting the "Aerator Min Air Pressure and Aerator Max Air pressure setpoints" in the process controller configuration file. Presently, we are considering using Sand 40 30 with a diameter of 0.42 59 - 0.59 84 mm with a bed depth of 30 cm and 50% bed expansion.

The initial experiment will be run with the aerator pressure set to the 100 kPa 101 kPa gage pressure. The experiment will be left to run for a day to observe and record the behavior of dissolved gas removal and to determine an appropriate runtime for subsequent experiments. (This will be determined by the time it takes for the bubble collector to refill a certain number of times)

Our hypothesis is that the behavior of gas removal over time will change. The current relationship between the volume of gas removed over time considering sand grain size or bed depth is found to be fairly linear at a constant rate. When the dissolved air concentration of the water is adjusted, the gas removal rate probably will be the same initially, then will decrease or level off as the gas concentration in the water reaches saturation at atmospheric pressure. This hypothesis is based on the notion that water that is not supersaturated with respect to the local absolute pressure and the gas composition of potential bubbles atmospheric pressure will not form bubbles. We believe predict that, as the concentration of gases decreases, fewer bubbles will form and fewer will grow large enough to float away from the sand filter.

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