Aeration Method

Abstract

The aeration methods uses the negative pressure in the pipe to create a vacuum, sucking air into the pipe through small holes while the influent is in free-fall hence aerating the water. Many air bubbles will be infused into the water which will in turn increase the gas transfer rate so that once the water enters the grit chamber large bubbles should form quickly and rise to the surface at a much higher velocity, resulting in a shorter required retention time of the water in the grit chamber.

This section of the research seeks to simulate the conditions in the pipe and the entrance into the grit chamber to determine how quickly the dissolved oxygen content in the water will decrease once it hits the tank that is under atmospheric pressure after it has been exposed to different conditions in the entrance pipe. If this is found to be a viable method to solve the floating floc problem the next step will be to determine how big the holes in the pipe should be for different plant flow rates and how long the retention time has to be in the grit chamber to reach the desired DO content.

Introduction and Objectives

It is common in laboratories to use gases like nitrogen to strip oxygen out of solutions. The aeration method is based off of this idea but attempts to use air to strip oxygen out of solutions. This process requires a large amount of air to be pumped into the system, causing the formation of many bubbles. The addition of the bubbles into the system increases the rate of gas transfer and rapidly creates bigger bubbles. The gas in the water will then rise to the surface more rapidly. The contact time between the air and the water required to allow all or most of the gas to rise out of the water would thus be decreased.

The method of aeration for gas removal would require a high flow rate of air to be injected into the water. Pumps for getting air into the water are impractical to use in the Honduran towns that have water treatment plants designed by AguaClara and are not sustainable. Instead, the properties of gases and liquids can be used to infuse the water with small pockets of air without using mechanical energy.

Henry's Law states:
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.

Henry's Law can be utilized to pump air into the beginning of the system. A small hole in the pipe headed to the grit chamber at a point where the water is in free fall would create a negative pressure difference between the inside of the pipe and the atmosphere thus causing an influx of air. The flow rate into the pipe is a function of the orifice size and the location of the hole on the pipe. The density and velocity of the water after passing this hole can then be calculated. A time estimate for the amount of contact time between the atmosphere and water that is needed for all or most of the gas to leave the water can be determined from experimental data.

A model of this process was derived last semester and this semester we are testing this theory in the lab. We designed and had built an apparatus that can be used to simulate both the conditions in the pipe and the grit chamber.

General Procedure

To test the aeration method, an airtight is used to simulate the interface between the distribution pipe exit and the grit chamber at the AguaClara plants. The container is filled with water and sealed off and water is pumped out of the lid causing a partial vacuum. The environment created is similar to that in segments of the transmission line bringing water to the plants. After the water is put under negative pressure, the pump clamp is released to open the container to atmospheric pressure, which simulates the grit chamber conditions.

While the water at the actual plants have dissolved oxygen in excess of the 8 mg/L saturation level at atmospheric pressure, the experiments performed have usually involved water that is 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

The data collected has proven that the method is ineffective at removing dissolved oxygen from supersaturated water.

We postulate that the major reason for the failure of the aeration method is that the air bubbles do not provide enough surface area for the magnitude DO removal required and that the air bubbles are not easily accessible to much of the DO volume in the solution.

The size of a dissolved oxygen molecule is on the order of angstroms, while the Aeration Apparatus has an inner diameter of 4 inches. To reach the air bubble catalysts, a dissolved oxygen molecule near the wall of the apparatus 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 leave the solution. As a result of the bubbles' inability to leave the solution, the pressure of the water may cause the dissolved oxygen 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 nanometers or micrometers. A large volume of the dissolved oxygen in the solution would not be affected by the bubble catalyst. In light of these of results, we have decided to move away from the Aeration Method and focus on the Sand Filter Method.