h1. DO Removal by Partial Vacuum and Aeration
h2. 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.

h2. Procedure

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 holes into an actual pipe that had free falling water. Theoretically, the source of the air should affect the experiment if they have different concentrations of dissolved oxygen. Since bubble formation depends on the difference in DO between the air bubbles and the water, source air with a lower amount of DO would 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 minute to about four minutes, the apparatus was again exposed to atmospheric pressure and data was recorded in the same manner as mentioned before for two to three minutes. 
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 meter 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.
 
h2. Results and Discussion

The data from this experiment shows that under partial vacuum and slight aeration there was a problem with bubbles forming on the DO probe. This is indicated by the sharp dips in the data which corresponds to a bubble rising off of the DO probe as seen in [#Figure 1]. Overall there was not much change in the DO over time but we found the DO probe to be unreliable when it was pressurized and was further complicated by the bubbles that formed and stuck to the probe under pressure as well as vacuum. The stir bar did not keep bubbles from forming on the probe even after we moved the DO probe closer to the bottom of the container.

The figures below represent the experiments that produced ok numbers. The first two graphs shown are for an experiment where the water was both under *negative?* pressure and being aerated.  
*[#Figure 2] shows the pressure values in the system during this experiment. The DO content during this time is shown in [#Figure 1]. The pressure data stayed constant after the first minute, but as mentioned above the DO data was all over the place because of the bubbles on the probe. These sets of data were collected while the system was closed and under pressure. Using the gas laws, we calculated the dissolved oxygen saturation level to be around 14.5 mg/L at a pressure difference of -80 kPa and and calibrating for the zero level DO to be 8 mg/L. The 8 mg/L was used to calibrate the DO probe but further research suggests that the average freshwater dissolved oxygen concentration is around 9 mg/L. This difference does not impact our experiments since the probe was calibrated to 8 mg/L for all of the experiments. Based on the numbers used, pressurizing the system did super saturate the system*


{anchor:Figure 1}
{float:left|border=2px solid black|width=600px}[!bubbles neg pressure system DO.png|width=600px!|DO Removal by Partial Vacuum and Aeration]
h5. Figure 1: The concentration of dissolved oxygen as a function of time while the system was under negative pressure and not being aerated
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{anchor:Figure 2}
{float:left|border=2px solid black|width=600px}[!bubbles closed system pressure -.png|width=600px!|DO Removal by Partial Vacuum and Aeration]
h5. Figure 2: The pressure inside of the container while it was closed and being aerated
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[#Figure 3] and [#Figure 4] show the system once it had been opened. Again, even though the pressure was constant the DO values fluctuated corresponding to bubble formation.

{anchor:Figure 3}
{float:left|border=2px solid black|width=600px}[!bubbles open system DO.png|width=600px!|DO Removal by Partial Vacuum and Aeration]
h5. Figure 3: The Dissolved Oxygen inside of the container after the system had been opened
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{anchor:Figure 4}
{float:left|border=2px solid black|width=600px}[!bubbles open system pressure -.png|width=600px!|DO Removal by Partial Vacuum and Aeration]
h5. Figure 4: The pressure inside of the container after the system had been opened
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