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 desired pressure is reached. 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

The graphs above indicate a change in dissolved oxygen of about .3 mg/L over a minute to two minutes for water subject solely to partial vacuum. For the first test, the water was subject to a pressure drop from atmospheric to approximately -70kPa. A total pressure drop of -70kPa was used in the second test also; however, during the experiment the water was kept at approximately -37kPa for a period of time in order to observe the effect of pressure on dissolved oxygen. It can be seen by juxtaposing the two curves on the same plot that the behavior of dissolved oxygen after vacuum are fairly similar. The first test was performed with a higher initial dissolved oxygen content in the water, so the graph is positioned slightly higher than the second test curve.

The change in dissolved oxygen under a range of pressure is fairly consistent. Using this data as a baseline, a second experiment was run in which the water was aerated under partial vacuum and is described in DO Removal by Partial Vacuum and Aeration