Aeration Method

Abstract

The aeration method attempted to use air bubbles as a catalyst to facilitate gas removal from supersaturated water by increasing the gas-liquid interfacial area. The proposed design for the aeration mechanism in AguaClara plants involved a vertical segment of pipe connecting the transmission line that brings water to the plant to the bottom of the grit chamber. A small orifice would be drilled into the pipe, and a negative pressure difference between the interior and exterior of the pipe due to the free-falling influent water would naturally draw air into the pipe via the orifice.

Following the development of theoretical models for this mechanism (See [LINK TO THEORETICAL MODELING]), research was performed to test the physical feasibility of the method and to determine the optimal design for the vertical segment. The parameters of interest regarding the design of the pipe were the height of the segment and location of the orifice. Of the two parameters, required height was determined to be the major factor governing feasibility. Since there was no practical way to directly test a range of heights in the lab, a reactor was designed to model a segment of water within the pipe with the measured factor being the required aeration exposure time necessary for excess gas to leave the volume of water.

After running many experiments, it was found that the aeration method would not be suitable for our purposes because the rate of gas removal from solution occurred too slowly. Experiments were run with the influent subject solely to a partial vacuum and also subject to partial vacuum with slight aeration. Although there were discrepancies with data collection, it was found that while aeration affected the transfer rate of gas out of solution, the change appeared to be insignificant.

Introduction and Objectives

It is common in laboratories to use gases like nitrogen to strip oxygen out of solutions. The aeration method was based off of this concept but attempts to use air to strip gas out of solutions. This process required a large amount of air to be pumped into the solution, resulting in an influx of bubbles into the influent water. Theoretically, the bubbles introduced into the system would have facilitated gas transfer out of solution by expanding the gas-liquid interface. The time required for the dissolved gases to come to equilibrium with atmospheric pressure would thus be reduced.

Because pumps are not sustainable in Honduran towns with water treatment plants designed by AguaClara, another mechanism for providing a high flow rate of air into the influent water was required. The proposed design for the mechanism was a segment of vertical pipe with a drilled orifice that would have a negative pressure difference between the interior and exterior of the pipe caused by the free-falling influent water. The pressure difference would natural cause an influx of air into the influent water to aerate the system as stated by Henry's Law recited below.

Henry's Law
At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

The flow rate of air into the pipe would have been a function of the orifice size and the location of the hole on the pipe. A time estimate for the amount of contact time needed between the atmosphere and water for the removal of all or most of the excess gas was to be determined from experimental data. Following the development of a theoretical model (see [LINK TO THE MODEL]), the physical feasibility of the method was tested in the lab.
We designed and had built an apparatus that was used to simulate both the conditions in the pipe and the grit chamber.

General Procedure

To test the aeration method, an airtight reactor able to safely withstand pressure changes of about 100 kPa was used to simulate the interface between the transmission line pipe exit and the grit chamber at the AguaClara plants. The reactor was filled with water, sealed off and water was pumped out to create a partial vacuum. The environment created was similar to that in the vertical segment connecting the transmission line to the grit chamber. After the water was put under negative pressure for different periods of time, the reactor was open to atmospheric pressure, simulating the grit chamber conditions.

The water at the actual plants have dissolved oxygen in excess of the 8 mg/L saturation level at atmospheric pressure. The experiments performed usually involved water that was originally around saturation level or slightly below.

We have decided that this is acceptable, since water under negative pressure has a lower DO saturation level so the water is supersaturated with respect to the lower saturation concentration.

Experimental Methods and Results

DO Removal by Partial Vacuum

• A partial vacuum is created in the container and the effects of the vacuum on dissolved oxygen and bubble formation are observed.

DO Removal by Partial Vacuum and Aeration

• A partial vacuum is maintained in the container while the water is slightly aerated throughout each trial. The effects of the vacuum plus the aeration is observed and recorded.

General Conclusion

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 wish to see a drop of at least 2 mg/L in that period of time. Results from experiments that involved aerating water under a partial vacuum were compared to results 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 focus on the sand filter method.

We postulate that the major reason for the failure of the aeration method was that the air bubbles were not easily accessible to much of the DO volume in the solution. [DISTANCE TO AN INTERFACE]

The size of a dissolved oxygen molecule is on the order of 10 ^-10 meters, while the Aeration Apparatus has an inner diameter of 10.12 cm. To reach the air bubble CATALYST, a dissolved oxygen molecule must travel a few centimeters to over an inch. While gas bubbles may form on the wall of the apparatus due to supersaturation of the water, the bubbles that form are usually tiny and are often too small to float to the surface. As a result of the bubbles' inability to leave the solution, the pressure of the water may cause the gas in the bubbles to be reincorporated into solution. Effectively, only molecules in close proximity to the bubbles get incorporated into the bubbles. We would approximate that the influence region where incorporation of dissolved oxygen into the bubble occurs would be on the order of -

A large volume of the dissolved oxygen in the solution would not be affected by the bubble CATALYST.