Experiment 1: Replicate of the Previous Sand 40 Experiment

Procedure:

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

Sand Grain Size: Sand 40 (.42 mm - .59 mm)
Sand Bed Depth: 60 cm
Sand Bed Expansion: 50%
Aerator Air Pressure: 100 kPa
Flow Rate, measured manually: 225 ml/min

Results and Discussion

The results from the experiment indicate that the amount of dissolved air removed in the bubble collector decreased after each of the data collection periods.

Unknown macro: {float}

Figure 1. depicts the initial and final water level in the bubble collector during each of the data collection period ("runs"). Each run represents a time period during which the water level in bubble collector gradually sinks falls down from its maximum to the set minimum point. 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. More detailed information on the bubble collector setup can be found here.
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. We added fitted a line to each of the runs to see the rate of change of the water level inside the bubble collector when water runs through the sand filter. 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.

Unknown macro: {float}

Unknown macro: {float}

The value of the linear fit is very close to 1, indicating that the data can be modeled accurately using a linear relationship. If we multiply the slope of the line by the cross sectional area of the bubble column, we get the rate of change in the volume of water with respect to time:

Then we divide the volume rate of change by the flow rate to find out how many milliliters of dissolved gas are removed per liter of water sent upwards through the sand filter:

For these calculations we used following values:

• radius of the bubble collector = 1.9cm
• flowrate = 225 mL/min (measured manually)
  

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

Unknown macro: {float}

Data was recorded for Sand 40 with each of the parameters specified above. The results of the experiment can be downloaded here in the form of the Excel sheet. The content of the gas removed during the second run, 5.09 mL/L, is very similar to the result from the Fluidized Bed Experiment done last semester, when the measured content of gas removed was 5.07 mL/L. The fluidized bed experiment involved the same sand parameters: Sand 40, depth = 60cm, bed expansion = 50%, aerator air pressure = 100kPa, except for the flow rate, which was 345 mL/min.
While the data from the second run are comparable, the data for each subsequent runs show gradual decrease in the content of air removed. These decreasing rates of gas removal probably resulted from a clogging problem in the sand filter. Clogging in the filter occurs because of the diameter of the sand column is relatively small. Large bubbles form in the sand bed and push segments of sand up to the top of the filter. While we did not directly observe this problem during the experiment, sensor data collected through Process Controller indicates that clogging occurred.

Additionally, the results can be compared with the bubble formation potential model, which models the theoretical bubble formation potential as a function of the air pressure that the water equilibrated with prior to returning to atmospheric pressure.

The model predicts the theoretical bubble formation potential to be around 18mL/L for water that has been previously exposed to 1 atm gage pressure at temperature of 25 C. Since our data show the removal of only 5.09 mL/L of the air entrained in the system, it is probable that some of the system components might not be functioning properly. The sand filter might not be able to remove all the bubbles coming in. If the bubbles in the system encounter a region of lower pressure, they might become trapped and form a column of gas entrained inside. Additionally, bubbles in the sand filter may be concentrated on the fluid surface as floats, thus creating a gas-liquid interface. It is also possible that the aerator and bubble collector might not be working properly. It might be necessary to measure the oxygen concentration at various points in the system to see what might contribute to the lower content of air removed.

Conclusions

While the results collected were unexpected, the initial runs indicate a gas removal very similar to that found in the Spring 2009 experiment. Since conditions in the aerator were maintained throughout the experiment, it is likely that the new aerator works as needed. However, the decreasing gas removal found for each run is cause for concern. We plan to install a webcam at the top of the filter to observe any instances of clogging during the experiment.

While we attempt to find a permanent solution to the clogging problem, we will also determine a minimum expansion for which clogging does not occur in the sand column. Although obtaining a larger diameter sand column would probably fix the clogging problem, we hope to avoid that option since a wider sand column would require higher flow rates through the system. This would probably require significant upgrades to the plumbing in the system and would be too time-costly and inefficient, since higher flow rates would also decrease the residence time of water in the new aerator, which was installed specifically for greater residence time.