h1. Aeration Method
h2. Abstract
The aeration method sought to design a vertical segment of pipe connecting the transmission line exit to the bottom of the grit chamber. The segment would have been subject to negative pressure due to the free-falling influent, and holes drilled along the length of the pipe would have allowed air to be drawn in to aerate the influent. The air bubbles would have increased the gas transfer rate by increasing the interfacial area.
This section of the research sought to simulate the conditions in the vertical segment and the entrance into the grit chamber to determine how quickly the dissolved oxygen concentration would have decreased once the water was open to the atmosphere in the grit chamber. The water was exposed to different conditions before it reached the simulated grit chamber to observe the effects of gas removal and to find the optimal conditions.
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.
h2. 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 idea but attempts to use air to strip oxygen out of solutions. This process required a large amount of air to be pumped into the system, causing the formation of many bubbles. The bubbles introduced into the system would have increased the rate of gas transfer and rapidly created bigger bubbles. The time required for the dissolved gases to come to equilibrium with atmospheric pressure 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 are not sustainable in the Honduran towns that have water treatment plants designed by AguaClara. Instead, a section of pipe that has negative pressure can be used to provide aeration.
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|Aerator Apparatus] that can be used to simulate both the conditions in the pipe and the grit chamber.
h2. 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 and 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 a period 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.
{panel} I would have expected all of your source water to be supersaturated with oxygen. {panel}
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.
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[!AerationDiagram.jpg|width=500px!|Aeration Flow Diagram]
*Figure:* Aeration flow diagram. Click to see larger version.
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[!BubbleSystem.jpeg|width=175px!|Aerator Apparatus]
*Figure:* Aeration Apparatus. Click for description and AutoCAD document.
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h2. Experimental Methods and Results
[DO Removal by Partial Vacuum | 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 | 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.
h2. General Conclusion
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[!AerationShortcomingExplanationDiagram.JPG|width=600px!|Aeration Method Shortcomings Diagram]
*Figure:* Diagram explaining shortcomings of the Aeration Method compared to Sand Filter Method. Click to see larger version.
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{panel} The diagram can be improved. The important length scale is the distance to an interface where the dissolved gasses could form bubbles. In the case of the reactor without any bubbles, the length scale is the diameter of the cylinder. In the case of aeration, the length scale is the distance between bubbles. In the case of the sand filter, the length scale is the pore diameter. These length scales are not all micrometers! {panel}
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.
{panel} I�d emphasize the distance to an interface. {panel} [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 - {panel} You can easily calculate the diffusion length scale. See the equation below. {panel}
{latex}
$$
x \approx \sqrt {D_m t}
$$
{latex}
A large volume of the dissolved oxygen in the solution would not be affected by the bubble CATALYST. {panel}Introduce the idea of catalyst. You could even show how the time for a significant reduction in dissolved gas concentration is related to the distance to a surface using the equation above. Thus you can think of these surfaces as catalysts. {panel} In light of these of results, we have decided to move away from the Aeration Method and focus on the Sand Filter Method.
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