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Evaluation of previous experiments with the new system

Objective

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h6. The aerator at the beginning of Summer 2009
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This set of experiments attempts attempted to replicate the grain size research performed with the previous experimental setup, to assess both the functionality of the new system and the validity of the Spring 2009 results. Additionally, dissolved oxygen measurements were taken to evaluate the effectiveness of each of the components in the setup will be assessed with respect to theoretical expectations.

General Procedure

. According to the theoretical model of the bubble formation potential, it is predicted that around 18 mL of bubbles will be formed for 1 L of water that has been previously exposed to 1 atm gage pressure at temperature of 25 ˚C. See Figure 1. for the relationship between the theoretical bubble formation potential and initial air pressure with which the water reached equilibrium before returning to atmospheric pressure. For the current aerator, it was assumed for the sake of calculation that the dissolved gas concentration would reach equilibrium with the pressure in the aerator, resulting in 18 mg/L of dissolved gas in the water flowing into the sand filter. While a theoretical model of gas removal by the sand filter has not been developed, the team believed that the abundant surface area provided by the sand would remove gas by providing ample nucleation sites for bubbles, and sites at which bubbles could adhere to a surface and aggregate.

General Procedure

The procedure for each experiment in this set is fairly similar, with the exception of sand grain sizes, which are varied among the experiments. For the two experiments listed below, the same procedure was used with a different size of sand grain used in each. Sand 40 (0.49 mm - 0.57 mm) and Sand 30 (0.59 mm - 0.84 mm) were used for experiments one Experiments 1 and two2, respectively. The details of the procedure are available here.

In Process Controller, configure the system so that the aerator air pressure is maintained at roughly 100 kPa. Fill the sand column with 60 cm of Sand 40 and adjust the flow rate on the pump forcing water through the sand filter to establish a bed expansion of 50%. For the first experiment, manual measurements of flow rate were performed by unhooking the influent water tube into the sand filter and allowing the influent to fill a large graduated cylinder over the course of a minute. In order to minimize changes made to the system, flow rate measurements for the second experiment were taken at the system effluent tube. The flow rates were roughly 225 ml/min and 485 ml/min for Sand 40 and Sand 30, respectively.

Run the Process Controller method file, given here on the "On" state. The "On" state regulates the air pressure in the aerator by releasing small amounts of air through a valve when the system exceeds the maximum aerator air pressure of 102 kPa. The water level in the aerator is controlled in a similar manner; however, the water wasting valve is also subject to a duty cycle in which the valve will open for a set period of time and close for a set period of time. If the "on" condition for the wasting valve is not met (that is, if the water level does not exceed the regulated height), the wasting valve will remain closed.

The water entering the aerator and leaving is maintained at a constant rate throughout the experiment via manually controlled pumps. The water is allowed to flow through the sand column, where bubbles can form. When bubbles grow large enough in the filter, they can float up to the top and out through a tube into the bubble collector. Throughout the duration of the experiment, the bubble collector goes through cycles of unfilling and refilling. Initially, an air valve at the top of the bubble collector opens and the water effluent valve located at the bottom of the bubble collector closes, allowing the collector to fill like a sitting column of water. Once a maximum height is reached, the air valve shuts off and the water valve opens, resulting in a partial vacuum at the top of the collector. This suspends the column of water in the bubble collector. As bubbles enter the collector, gas in the bubbles fills the partial vacuum, allowing the water column to slowly drain from the collector. Once the minimum water level in the collector is reached, the apparatus refills and the cycle begins again.

For each emptying period of the bubble collector, data is collected through Process Controller. Analysis of the data collected can be quantified as a gas removal rate by considering the cross sectional area of the collector and the flow rate through the system. Please see the results and discussion for each experiment by clicking the links below.

Results and Discussion

Experiment 1 - Replicate of the Previous Sand 40 Experiment

• This experiment was performed shortly after the installation of the new setup with the purpose of replicating previous grain size results and assessing the overall functionality of the setup.

Experiment 2 - Replicate of the Previous Sand 30 Experiment

• After making adjustments to the system to account for the issues found in the first experiment, this second experiment was performed to again test the functionality of the system and to replicate previous results.

Dissolved Oxygen Measurements

• When it was confirmed that the new aerator was not saturating water as much as possible, concentrations of dissolved oxygen were measured at various points in the experimental apparatus.

Results and Discussion

Experiments 1 and 2 were performed to assess whether the new system collects consistent data and to ensure that previous sand grain experiments results are replicable, and thus are valid. We performed the control experiment (with no sand) after we observed that the measured rate gas removal in the sand filter was less than 18 mg/L, the theoretical rate of gas removal. When we realized that gas removal was less than expected, we took the dissolved oxygen measurements to determine which component of the system was not functioning effectively.

Reruns of the Previous Fluidized Bed Experiments

• Experiment 1 was performed shortly after the new setup was installed. Sand 40 was used to evaluate previous grain size results.
• Experiment 2 was performed after the system was modified to account for problems that arose in Experiment 1. Sand 30 was used with the purpose of testing the functionality again and evaluating previous results.
• A control Experiment (i.e., one with no sand) was performed to evaluate the sand filter's effectiveness by measuring gas removal in the absence of sand.

Dissolved Oxygen Measurements

• When we had confirmed that the measured gas removal values were less than theoretical values, we measured the concentrations of dissolved oxygen at various points in the experimental apparatus. We found that the measurements taken at a point just past the sand filter showed dissolved oxygen concentrations that were higher than that of water going into the sand filter. This suggested a problem with either the setup or the fluidized bed mechanism. Pressure through the sand filter was calculated to explain that large head loss through the system in combination with pressure build up in the filter was not resulting in tiny bubbles being reincorporated into solution. The results of the headloss calculations can be found here. With the control experiment, we confirmed that the sand filter was not suitable for gas removal because the material properties of the sand are not conducive for removing gas.

General Conclusions

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h6. Figure 1: Theoretical bubble formation potential
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Experiments using Sand 40 and Sand 30 showed rates of gas removal rates of about 5.09 mL/L and 2.01 mL/L, respectively, while the control experiment showed a rate of 7.47 mL/L. These results suggest that the sand inhibits gas removal, rather than facilitating it. In addition, the concentration of dissolved oxygen measured at each of the four ports in the system (tabulated below) support the notion that the sand filter did not provide a suitable mechanism of gas removal, as the measurements indicated higher oxygen concentrations in the water at points past the sand filter.

Literature concerning bubble formation and behavior indicate that rough, hydrophobic surfaces are most suitable for bubble formation. (See "Fundamentals of Bubble Formation during Coagulation and Sedimentation Processes" by P. Scardina and M. Edwards on the Floating Floc Annotated Bibliography page.) The sand filter may be unsuitable because sand is not hydrophobic.

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h6. The aerator with four aeration stones
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Concerning, the dissolved oxygen measurements, the results indicate that the aerator is able to supersaturate the water with 15.5 mg/L of dissolved oxygen. This is less than the theoretical rate 18 mg/L. Please see the Table 1. below for the dissolved oxygen concentrations and click here for more details and subsequent DO measurements in the absence of sand. Also, further measurements at that port reveal inconsistent levels of gas supersaturation. This inability to regulate the amount of dissolved gas in the influent water to the sand filter may have a significant impact on our ability to run controlled experiments. In order to address this issue, the team has altered the pressurized aerator by replacing the single aeration stone with a junction of four cylindrical aeration stones that would displace gas into the water in finer bubbles, which would be incorporated into solution more easily.

Table 1: Dissolved Oxygen Concentrations (DO) at sampling ports in the experimental setup with a sand bed.

It is also possible that the bubble collector, which is used to measure gas removal, may not be effective at capturing small bubbles. Although we had initially suspected that the small bubbles traveling through the system originated in the sand filter, we are now unsure whether small bubbles formed in the sand filter itself. During the controlled experiment, we observed that small bubbles were present throughout the system. Because of this, we suspect that the source of the small bubbles is a component other than the sand filter--perhaps the aerator. Before the team redesigns the bubble collector, we plan to modify the flow accumulator so that any small bubbles coming from the aerator will be collected so that water entering the sand filter is free of bubbles. If small bubbles are formed in the filter and the bubble collector must be redesigned to capture these bubbles, a new design for the bubble collector will be developed.

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