Dissolved Oxygen Measurements
According to MathCAD modeling of the system, bubble formation potential in the water should be 18 mL/L. Our measured values for gas removal were 5.09 mL/L and 1.99 mL/L for Sand 40 and Sand 30, respectively. Because measured gas removal was so much lower than the theoretical value, we twice measured the dissolved concentration at each sampling port to determine which of the components was not functioning effectively.
Procedure
Sampling Points: Water Source, Aerator Effluent, Sand Filter Effluent, Bubble Collector Effluent
Dissolved oxygen probes were used to measure the concentration of dissolved oxygen in samples of water taken from the water source and effluents from the aerator, the sand filter, and the bubble collector. Two different probes were used in samples from each point to confirm results. After each probe was assembled, it was placed in a solution of sodium sulfite to ensure a zero reading. Both probes we used accurately read the dissolved oxygen concentration in the sodium sulfite as "0". To further ensure the probes' accuracy, they separately were placed in a sample of tap water, which should have a dissolved oxygen content near 8 mg/L. Though the readings were noisy, both probes read that the dissolved oxygen concentration in the tap water was within 0.5 mg/L of 8 mg/L.
In a large beaker, water was collected from a sampling port at the first point, just beyond the water source. The probe was inserted near the center of the water sample and kept stable with a ring stand. After the probe membrane came in equilibrium with the water, the dissolved oxygen reading was recorded, and the probe was returned to the sodium sulfite solution. The beaker was emptied and refilled with water from the next sampling port. This was repeated until water from all four sampling ports had been tested with one probe. For the measurements taken with the sand filter in place, the process was repeated with Probe 2.
With the sand filter in place, water temperature remained at 20.8 °C while measurements with Probe 1 were taken. We encountered a problem while using Probe 2. The system that controls water temperature was temporarily out of order, and we began measurements with the source water temperature at about 33 °C. As we continued measurements (from the sampling port just beyond the water source down the line), water temperature dropped to 21 °C. Measurements were taken with Probe 1 after the system had been running for several hours; for Probe 2, the system had been running for about 45 minutes when measurement began.
In the absence of sand, water temperature stayed at 21.4 °C during the experiment. The system had been running for several hours prior to taking DO measurements. This ensured that the water was supersaturated throughout the system.
Results and Discussion
The results were very unexpected. Table 1. shows the first set of DO measurements taken after the Sand 40 experiment was performed. Water temperature was 20.8 °C when Probe 1 was used. Water temperature varied from about 33 to 21 °C while Probe 2 was used to measure dissolved oxygen concentrations, which may have affected the measured dissolved oxygen concentrations for the first trial of the second probe.
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Table 1.: Dissolved Oxygen Concentrations (DO) Measurements with Sand.
Sampling Port |
DO (mL/L), Probe 1, Trial 1 |
DO (mL/L), Probe 1, Trial 2 |
DO (mL/L), Probe 2, Trial 1 |
DO (mL/L), Probe 2, Trial 2 |
|---|---|---|---|---|
Water Source |
9.8 |
10.2 |
8.7 |
12.1 |
Beyond Aerator |
15.5 |
14.2 |
11.8 |
15.2 |
Beyond Sand Filter |
17 |
16.3 |
11.9 |
15.3 |
Beyond Bubble Collector |
17.8 |
16.2 |
12.3 |
15.7 |
Visual observation of the samples taken support the dissolved oxygen concentrations measured. The samples were very cloudy with small bubbles, indicative of supersaturation. If the system were working properly, the dissolved oxygen concentration would decrease from the aerator to the bubble collector, and the water taken from the sand filter effluent and the bubble collector effluent would contain large bubbles. We observed that no large bubbles were present in either sample but very small bubbles were abundant in both. These results indicated a problem with the setup or the fluidized bed method. Pressure measurements were taken and the head loss through the sand filter was calculated to ensure that large head loss through the system in combination with pressure build up in the setup was not resulting in tiny bubbles being reincorporated into solution. The results of these calculations can be found in the Floating Floc Head Loss Calculations MathCAD file. The measured head loss through the system seemed to match up fairly well to theoretical expectations.
In order to assess the effectiveness of the fluidized bed method, a control experiment was performed without sand and dissolved oxygen measurements were taken at the end of the experiment before stopping the setup. The results can be found below in Table 2. Juxtaposition of the results from Table 1 and Table 2 indicate that the fluidized bed method was not effective at removing gas, because the presence of sand did not significantly decrease the dissolved oxygen concentration in the water.
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Table 2.: Dissolved Oxygen Concentrations (DO) Measurements in the Absence of Sand
Trial |
Flowrate (ml/min) |
Source water DO (mg/L) |
After Aerator DO (mg/L) |
After Sand Filter DO (mg/L) |
After Bubble Collector DO (mg/L) |
Temperature (C) |
|---|---|---|---|---|---|---|
1 |
530 |
12.0 |
15.3 |
15.6 |
16.1 |
21.4 |
2 |
530 |
11.6 |
16.0 |
16.0 |
16.0 |
21.4 |
3 |
530 |
11.4 |
16.0 |
16.0 |
15.9 |
21.4 |
Conclusions
The DO measurements confirm the results from the Control Experiment where it was shown that the sand is inhibiting the process of gas removal. DO measurements taken from various sampling points indicate that there is only a slight increase in the DO concentration as the water flows from the sand filter through aerator to the bubble collector. This might suggest that the bubble collector is collecting excess gas coming from the aerator.