DO Removal by Partial Vacuum

Purpose and Principle

The purpose of this experiment is to determine the degree of dissolved oxygen removal from supersaturated water subject to a partial vacuum. Dissolved oxygen removal from the water occurs because the partial pressure of oxygen in space above the water is lowered by the partial vacuum. Thus, the dissolved oxygen will transfer out to the space above the water in order to restore equilibrium as stated in Henry's Law. Measuring the dissolved oxygen in the water over a period of time will allow us to observe the amount of dissolved oxygen removed and also to calculate the approximate rate of dissolved oxygen removal.

Procedure

While using Easy Data to monitor the pressure and dissolved oxygen, water is pumped out of the apparatus until the pressure reaches -50 to -70 kPA. Once the desired pressure is attained, the pump is stopped and the apparatus is allowed to sit for a short period of time. The period of time can be determined by finding the approximate time the water spends under partial vacuum in the transmission lines that bring water to the plants. After that period of time, the clamp on the pump is released to open the system to atmospheric pressure. The dissolved oxygen is monitored and recorded for two to three minutes after the chamber is opened to atmospheric pressure.

Results and Discussion

We wish to see a drop of at least 2 mg/L in that period of time. From our experiments, we have found that the change in dissolved oxygen that occurs over the span of a few minutes is less than desirable. We ran experiments that involved aerating water under a partial vacuum and compared the results to data obtained from experiments in which water was only subject to a partial vacuum with no aeration. We were expecting to see a greater change in the dissolved oxygen concentration; however, contrary to our initial belief, aerating the water had little affect on the change in dissolved oxygen. Because of this, we are doubtful that the aeration method will solve the floating flocs problem and have decided to consider alternate solution methods. While we search for other possible solutions, we will still continue to run quick experiments with the aeration method in order to verify our decision to move to an alternate solution.

Some of the major concerns about our data include discrepancies caused by erratic behavior of the dissolved oxygen probe under partial vacuum. We are still trying to understand what might be causing the discrepancies and to what degree the functionality of the probe is affected. We are concerned that after the probe is subject to negative pressure, data collected after pressurization may be faulty. In the mean time, we will be measuring dissolved oxygen before and after pressurization and aeration instead of during the process. Also, we were initially concerned about the gradual pressure increase in our system. So, we tested the apparatus to make sure that it was airtight by putting the container under positive pressure and holding it over night. It proved to be airtight enough for our purposes. We postulated that the change in pressure is mostly due to bubbles leaving the solution.