Experiment 1: Replicate of the Previous Sand 40 Experiment

Purpose

This set of experiments is designed to ensure that the the new system and taller aerator replicate results of sand bed experiments from Spring 2009.

Procedure

For this experiment, the following parameters were used:

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 was measured to be: 225 ml/min

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 this 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. The flow rate was found to be 225 ml/min. (In order to minimize changes made to the system, measurements for future experiments will be taken at the system effluent tube.) Run the Process Controller method file [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 flow entering the sand filter column was adjusted so that a 50% expansion was met. 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 will go through refilling cycles. Initially, an air valve at the top of the bubble collector will open and the water effluent valve located at the bottom of the bubble collector will close, allowing the collector to fill like a sitting column of water. Once a maximum height is reached, the air valve will shut off and the water valve will open, resulting in a partial vacuum at the top of the collector. This causes the column of water in the bubble collector to be suspended. As bubbles enter the collector, gas in the bubbles fills the partial vacuum, allowing the water column to slowly leave the collector. Once the minimum water level in the collector is reached, the apparatus refills.

For each emptying period of the bubble collector, data is collected via Process Controller. Analysis of the data collected can be quantified as a gas removal rate (see below).

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.

[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 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].

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.]:

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. One of the possible explanations, as described below, might be the air pockets being trapped inside the sand filter.
Additionally, the results can be compared with the [bubble formation potential model|Experiment 1 REPLICATE TO BE DELETED^Dissolved atmospheric gases.xmcd], 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. The decreasing rates of gas removal found from each run 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. We plan to install a webcam at the top of the filter to observe any instances of clogging.

• We will try to avoid getting a new sand column with a greater diameter, to avoid additional changes to the system.
• We would like to find a minimum expansion such that clogging does not occur, and a permanent solution that would not require further changes to the system.
• In general, we do not want further changes to the system. Upgrading the plumbing in the system is undesirable because it would be too costly in terms of time and efficiency, as greater flow rates would decrease the residence time of the new aerator, which was put in specifically to allow greater residence time.