pump and air at same time 100kpa maintain for about 8 hours to get to 16 mg/L

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

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.

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.