Experiment 2: Replicate of the Previous Sand 30 Experiment

Procedure

For this experiment, the general procedure for this set of experiments was followed using the following parameters:

Sand Grain Size: Sand 30 (0.59 mm - 0.84 mm)
Sand Bed Depth: 60 cm
Sand Bed Expansion: 50%
Aerator Air Pressure: 100 kPa
Flow Rate, measured manually: 485 ml/min

Results and Discussion

Data from the experiment indicate that similar amounts of gas were removed during each data collection period. Overall, the results were consistent. After each period the content air removed floated within the range of 1.981 mL/L - 2.002 mL/L. This might suggest that the components of the system function reliably.

The method of analyzing data was similar to that in Experiment 1. Figure 1. depicts the initial and final water levels in the bubble collector during each data collection period ("runs").

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Each run represents a specific time period during which the water level in bubble collector gradually sinks down from its maximum to the set minimum. This period is represented on the graph when the line slants downward. Once the minimum water level is reached, the system has to refill with water in order to continue the runs. For this reason, the water outflow valve is closed until the water level reaches the set maximum point. This period is represented on the graph by the vertical lines. (This section is redundant, you have already told us how to analyze the data)
Figure 1. also shows that the bubble collector did not complete the run during the third period. The measured water level in the bubble collector governs when the bubble collector should start a new cycle by filling up with water. It is possible that the cause was occasional noise in data. Since the data averaging period is 10 seconds, there might have been spikes in data collected in this particular averaging period. The multiple spikes that lasted just for a few seconds could have caused the average to be lower than the minimum water level set point. Essentially, this would fulfill the condition for the process to change to a refill state. More detailed information on the bubble collector setup can be found here. (Possibly...if this is the case, I would make your bubble collector set-up more robust so this cannot happen in the future)

The initial data collection period was omitted from the analysis since because of the setup conditions the air might have been trapped inside the system. For the subsequent data collection periods, we calculated the content of gas removed per liter of water sent through the sand filter. For reference, the formulas can be found here. For these calculations we used following values:

• radius of the bubble collector column: 1.9 cm
• average flow rate: 485 mL/min

We fitted a line to each of the runs to see the rate of change of the water level inside the bubble collector. The value of the linear fit is very close to 1, indicating that the data can be modeled accurately using a linear relationship. Figure 2. and Figure 3. show the linear fit line for the second data collection period, and more detailed graphs can also can be found here.

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The calculations for the amount of gas removed during each data collection periods gave us the results summarized in Table 1. and Figure 4.:

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Data was recorded for Sand 30 with each of the parameters specified under Procedure. The results of the experiment can be downloaded here in the form of the Excel sheet. The amount of gas removed during each run was consistent. The uniform results might indicate reliable functioning of the components of the system. As opposed to Experiment 1, no clogging was witnessed at this time.
The amount of gas removed is lower than the result from the Experiment 1, which involved using Sand 40. This might suggest that the sand with larger grain sizes might be less effective at gas removal than the sand with smaller ones. Larger sand grains have relatively lower surface area to volume ratio and thus provide less surface area to which the bubbles can attach to in the sand column. Although the sand grain size is just one of the factors that affect the gas removal rate, the results indicate that perhaps using filter media with higher surface area to volume ratio might help facilitate the gas removal process under certain conditions.

However, the amount of removed gas is still a bit lower than the result from the Fluidized Bed Experiment done last semester, when the measured content of gas removed was 3.23 mL/L. The fluidized bed experiment involved the same sand parameters: Sand 30, depth = 60cm, bed expansion = 50%, aerator air pressure = 100kPa, except for the flow rate, which was 345 mL/min. The cause of the difference in results might be the experimental setup, which has been modified since last semester. The modifications include replacing the aerator with a new one. Perhaps the current aerator is not producing water that is supersaturated enough. That might affect the environment in which bubbles are formed, and thus indicate why only 2.00 mL/L of gas is removed. For further observation, we measured the dissolved oxygen content at three various points in the system. Detailed information can be found here.

Conclusions

The results of this experiment are quite consistent. Although the results are different from last semester's, we intend to use these results because of their consistency. We will probably have to adjust the data averaging time or check the set points for the valves, since one of the valves opened and cut short the third data collection period.

That the gas removal is lower than last semester's is unexpected and is something we will look into. The sand filter may not working efficiently. This may be because the parameters are not optimal in this experiment, but it is also possible that the sand filter is not working well enough.

The efficiency of the aerator will also have to be considered. Perhaps smaller aeration stones will allow for greater efficiency of gas dissolution. The lower gas removal, the inefficiency of the aerator, and the possible ineffectiveness of the sand filter may all be related.