*** This version of Confluence is for testing only and contains a copy of content from June 29th 2026. No changes will be preserved. ***
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There is a problem in some AguaClara water treatment plants with flocs floating to the surface of the water in sedimentation tanks. One hypothesis is that the water entering the sedimentation tank is supersaturated, and because total dissolved gas pressure is greater than the local solution pressure, gas comes out of solution and forms bubbles. The bubbles form on floc particles and bring them to the surface, causing the flocs to float instead of settling to the bottom of tank. Thus flocs Since flocs are swept out with the "clean" water and escape the sedimentation tanks, polluting they eventually pollute the effluent water and degrading . As a result, the quality of the drinking water produced by the plant is degraded. One solution may be to send the water through a sand filter before it enters the sedimentation tanks to remove excess gas in the water, thereby preventing bubble formation on flocs.
In the treatment plants, water would be sent upward through a bed of sand suspended in the grit chamber. The sand filter is intended to provide some surface area on which bubbles can form. The bubbles would accumulate, forming larger bubbles that could quickly float to the surface of the water in the grit chamber. The anticipation is that enough bubbles can form and leave the water in the grit chamber so that the water flowing into the sedimentation tanks would have a lower dissolved gas content and less potential to form bubbles.
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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. 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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The current relationship between the volume of gas removed over time considering sand grain size or bed depth is found to be fairly constant. 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 will not form bubbles. We 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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Additional
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- Measure the oxygen concentration in the water after the supersaturator using a dissolved oxygen probe. Take a sample (install a manual sample valve if necessary) and then measure the dissolved oxygen concentration. Compare this with theoretical values.
- Measure the oxygen concentration after the bubble collector. You can use the MathCAD file I sent to calculate the resulting equilibrium concentration of oxygen. Note that the oxygen concentration will not be the same as the concentration we would get from equilibrium with the atmosphere.
Comments
The theoretical bubble formation potential is about 18 mL/L for water that was exposed to 1 atm gage pressure at 25 C. We are only removing about 5 mL/L. We need to determine if the supersaturator is producing water that is in equilibrium with 1 atm gage pressure by measuring the dissolved oxygen concentration.
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Tasks
A mathematical model of the system suggests that 18 ml of gas should be able to be removed by the sand filter system. However, experiments performed last semester indicated a removal of 5.09 ml/L with Sand 40 (grain size 0.42 mm - 0.59 mm) and even lower removal with larger sand grains. While it is likely that the conditions of the sand filter may not be suitable to remove 18 ml/L, it is also likely that other components of the system are not functioning ideally. It was assumed that the dissolved gas concentration in the aerator had enough time to equilibrate with the 2 atm of absolute pressure in the apparatus; however, that may not be a completely valid assumption. Additionally, the bubble collector may not be collecting all the bubbles that leave the sand filter.
In order to better understand what is happening in the system in term of gas removal, we plan to measure the oxygen concentration in the water before and after the pressurized aerator using a dissolved oxygen probe. Additionally, the oxygen concentration will be measured before and after the sand filter and the bubble collector. The values collected will be compared to the theoretical model to better assess the functionality and efficiency of the system.