DO Removal by Partial Vacuum and Aeration
Purpose and Principle
The purpose of this experiment was to observe and quantify the change in dissolved oxygen transfer out of supersaturated water while it was subject to slight aeration under a partial vacuum. This experiment evaluated the use of air bubbles as a catalyst to increase the rate of removal of dissolved oxygen from water. The difference in oxygen concentration between the bubbles and the supersaturated water should in theory draw excess dissolved oxygen into the bubbles and out of the solution.
Procedure
At the start of the experiment, both the pressure sensor and the dissolved oxygen probe were calibrated in the same manner mentioned on the Floating Floc Aeration Method page. While using EasyData to monitor the pressure and dissolved oxygen, water was pumped out of the apparatus until the desired pressure was attained. A partial vacuum was maintained while the solution was aerated and the flow of air into the container was regulated by a rotameter that takes either pressurized air or room air. Pressurized air was originally used to aerate the water. However for later experiments the air inflow tube was detached from the rotameter to allow air to be sucked into the apparatus rather than being forced in; this better simulated how the air would be sucked through perforations in an actual pipe that had free falling water. Theoretically, the source of the air should have affect the experiment they had different partial pressures of oxygen. Since bubble formation depends on the difference in DO between the air bubbles and the water, source air with a lower partial pressure of oxygen would be able to form larger bubbles because of the greater concentration gradient. After the water was aerated under partial vacuum for a varied amount of time ranging from about one to four minutes, the apparatus was again exposed to atmospheric pressure and data was recorded in EasyData for at least two to three minutes to allow for the dissipation of erratic dissolved oxygen behavior.
The water used for this experiment was aerated under pressures ranging from 90 kPa to 100 kPa for over 10 hours in order to induce supersaturation with regards to atmospheric pressure. Aerating within the pressure range mentioned above ultimately resulted in an initial DO concentration of around 16 mg/L. Calculations using gas laws indicated that for the pressure range listed above, the concentration would be in the ballpark of 15 mg/L to 16 mg/L; however, this number may have been inaccurate due to faulty probe readings further addressed in the Results section below.
Using the Tamara Plant's design values the water entering the grit chamber is going approximately 10 m/min. This is based on using four 6 in diameter pipes to carry 740 L/day of water into the plant. Thus in a 1.5 m long vertical pipe (which is the height of the entrance tank) with free falling water the water would be exposed to a partial vacuum for only 10 seconds.
Results and Discussion
The results of our experiments suggested that there were many problems with using a dissolved oxygen probe in our experiments. Sharp dips in the dissolved oxygen concentration (as seen in #Figure 1) were indicative of a problem with bubbles forming on the probe. When the bubbles on the probe grew large enough to float away, the dissolved oxygen concentration fluctuated significantly. We attempted to correct this problem by moving the probe closer to the stir bar but to no avail - the problem persisted. In addition to that, the DO probes proved to be unreliable after being exposed to the partial vacuum in the reactor, calling into question the accuracy of the actual numbers obtained.
The figures below represent the experiments that produced acceptable results. #Figure 1 depicts the behavior of the DO concentration while the reactor was being aerated under the negative pressure conditions profiled in #Figure 2. The pressure data stayed constant after the first minute but, as mentioned above, the DO concentration readings were very disturbed. After subjecting the reactor to about three and half minutes of negative pressure, the system was opened to the atmosphere. #Figure 3 depicts the behavior of DO levels post vacuum and under atmospheric pressure, as seen in #Figure 4. Again, even though the pressure was constant, the DO values fluctuated corresponding to bubble formation. #Figure 3 indicates that the DO concentration increased after the pressure was increased to atmospheric pressure. We believed this to be due to reincorporation of gases into the water from bubbles that were too small to float out of solution.
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