Aeration Method

Abstract

Two different methods for reducing the dissolved oxygen content in the influent water before it leaves the grit chamber are being explored. One of the methods utilizes 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 small 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.

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

Many of the AguaClara water treatment plants are having the problem of flocs rising to the surface of the water in the sedimentation. This is caused by the formation of air pockets on and inside the floc particles. The current hydraulic retention time in the grit chamber is not sufficiently long enough for all of the bubbles to form and rise to the surface. One solution to this problem is to make the bubbles form and rise faster.

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 connecting a perforated vertical segment of pipe to the transmission line that brings water to the plant to the bottom of the grit chamber. The free-falling influent water would cause a negative pressure difference between the interior and exterior of the pipe, naturally drawing air into the pipe via the perforations.

Following the development of theoretical models for this mechanism (See Theoretical Modeling of Aeration Method), 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 distribution of the perforations. 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 volume of water within the pipe with the measured factor being the required aeration exposure time necessary for the removal of the excess gas from the 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 water 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 generally the effect of aeration appeared to be insignificant. We postulated that the method failed because a large volume of the dissolved gas was unable to reach the bubbles introduced into the solution.

The maximum distance a gas molecule would have to travel to reach a bubble is on the order of centimeters, so we calculated the time it would require for a molecule of dissolved oxygen to travel 1 cm - 10 cm. It was found that with a suspending fluid of water at 20 C, a molecule would take about 34 days to travel a distance of 10 cm, while molecules just 1 cm away from the bubbles would require about 8 hours. In either case, it was confirmed that the method as designed would not be feasible. Thus, we have shifted our focus to the sand filter method.

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 Water in laboratories is often aerated to get gases out of the liquid. This process requires a large amount of air to be pumped into the system, causing many little bubbles. The addition of more small bubbles to 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.solution, resulting in an influx of bubbles into the water. Theoretically, the bubbles introduced into the system would facilitate gas transfer out of solution by expanding the gas-liquid interface and reducing the time required for the dissolved gases concentration to come to equilibrium with the partial pressure of the gases in the atmosphere.

Because pumps are not sustainable in Honduran towns with water treatment plants designed by AguaClara, another mechanism for providing The method of aeration for gas removal would require a high flow rate of air to be injected into the influent water was required. 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 energyThe 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. By Henry's Law, the negative pressure in the interior of the pipe would naturally cause an influx of air into the influent water to aerate the system.

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 The flow rate of 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. Henry's Law can then be applied to calculate the flow rate of air into the water. The density and velocity of the water after passing this hole can then be calculatedwould be a function of the orifice size and the location of the holes on the pipe. A time estimate for the amount of contact time needed between the atmosphere and water that is needed for the removal of all or most of the excess gas was to leave the water can be calculated determined from those values.A model of this process was derived last semester and this semester we are testing this theory experimental data. Following the development of a theoretical model (see Theoretical Modeling of Aeration Method), the physical feasibility of the method was tested in the lab. We To do this, we designed and had built an airtight apparatus that can be (seen in #Figure 2) that would be able to safely withstand pressure changes of about 100 kPa. The apparatus was used to simulate both the conditions in the pipe interface between the vertical segment and the grit chamber at the AguaClara plants.

Procedures

The apparatus for the aeration method is mainly a segment of clear PVC pipe that is about 9.75" long and has an inner diameter of 4". One end of the pipe is connected to a ---- base that has four evenly placed metal rods attached to it. The rods run freely along the length of the clear PVC pipe and penetrate an ---- lid, which is held in place with butterfly screws at each rod. There are five adapters in the contraption. An adapter in the center of the lid joins the apparatus to a pump via a 3/8" tube. Water can be pumped out from this location when the container is completely full to cause a partial vacuum or air can be pumped in to pressurize the container. Two adapters are located near the base of the contraption that function as a water inlet and air inlet. The air inlet also has an air stone connected to it on the interior of the pipe. The pressure in the contraption is measured with a pressure sensor attached to an adapter near the base, and the dissolved oxygen probe is connected at the bottom of the apparatus near a magnetic stir bar to prevent bubble formation on the probe. O-rings are used to seal the contraption at each adapter location and at the interface between the pipe and the lid.

This contraption 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 distribution pipes of actual 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.

Two types of experiments have been run, thus far. The first involves creating a partial vacuum in the container without aeration and observing the effects of the vacuum on dissolved oxygen and bubble formation. The procedure for this experiment is relatively simple. While using Easy Data to monitor the pressure, water is pumped out until the pressure reaches -50 to -70 kPA. The apparatus is allowed to sit for a short period of time and is then opened to atmospheric pressure and the dissolved oxygen is monitored and recorded for no more than two minutes. We wish to see a drop of at least 2 mg/L in that period of time. The second involves maintaining a partial vacuum in the container with slight aeration. The flow of air into the container is regulated by a rotameter that takes either lab air or room air. Originally, lab air was being used; however, later experiments involve detaching the air inflow tube into the rotameter and allowing air to be sucked into the apparatus as it would be through the holes in the actual pipe. After the water is aerated under partial vacuum for a period of time, the apparatus is again exposed to atmospheric pressure and data is recorded in the same manner as mentioned before.

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.

General Procedure

The reactor was filled with water and left open to the atmosphere in order to zero the pressure sensor used in the experiment. After calibrating the pressure sensor, the reactor was bubbled furiously for five to ten minutes. Following this, the dissolved oxygen probe was calibrated to read about 8.7 mg/L (near saturation level for pure water under atmospheric conditions) at this dissolved oxygen concentration. The reactor was then sealed off and water was pumped out to create a partial vacuum. The environment established was similar to the conditions that would have existed in the vertical segment of pipe. 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. Bubble formation was observed throughout the procedure and a dissolved oxygen probe was used to study the behavior of the dissolved oxygen concentration in reactor throughout the experiments. Please see #Figure 1 for the flow diagram of the aeration system.

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[!Aeration Flow Diagram^AerationDiagram.png|width=500px!|Aeration Flow Diagram]
h5. Figure 1: An aeration method flow diagram also indicating components of the aeration system. Click to see larger version.
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[!BubbleSystem.jpeg|width=175px!|Aerator Apparatus]
h5. Figure 2: Aeration Apparatus. Click for description and AutoCAD document.
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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 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 concentrationwished to see a drop of at least 2 mg/L in that period of time; however, contrary to our initial belief, expectations, the results from our experiments indicate that aerating the water had little affect on the change in dissolved oxygen. Because of this, we are doubtful determined that the aeration method will would not 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.

Conclusion

.

We postulated that the major reason for the failure of the aeration method was that the air bubbles were not easily accessible to much of the dissolved gas volume in the solution. The size of dissolved gas molecule is on the order of 10 ^-10 m, while the Aeration Apparatus has an inner diameter of 10.12 cm. To reach the air bubbles infused into the reactor, the bulk of the dissolved gas molecules in reactor would have had to travel over a few centimeters. To validate this reasoning, we used the equation presented below to predict the time an oxygen molecule would need to travel a distance of 1 cm - 10 cm.

$$
x \approx \sqrt {D_m t} 
$$

where
x = distance traveled (in m)
t = time required
D _m = molecular diffusion coefficient of the dissolved gas

For the given conditions of the water in the reactor (that is, temperature at 20 degrees and water viscosity of around 10 ^-3 kg/m/s), the diffusivity of an oxygen molecule is around 3.4E-9 m ² /s. Substituting that value into the equation and setting x = 10 cm, yielded a required time of approximately 34 days, which was clearly not acceptable. If we assumed that the bubbles introduced into the reactor would decrease the distance to 1 cm, it would still require about 8 hours. In light of these results, we have decided to shift our focus to the sand filter methodNo conclusions have been made yet. We will be able to asses the situation better once we have test set up working properly. Current results are not promising but problems with the DO probe prevents us from drawing any definitive conclusions yet.