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Created by
user-7814f, last updated on Aug 04, 2009
3 minute read
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. Then the process was repeated, with measurements taken with the other probe.
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.
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.
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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 |
In addition, water taken from sampling ports beyond the sand filter and the bubble collector were very cloudy with small bubbles.
-------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 effluents of the sand filter and the bubble collector would contain large bubbles. We think the sand isn't working. -------
Table 2. shows the second set of DO measurements taken after the Sand 30 experiment was performed.
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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 |
11.0 |
14.3 |
16.2 |
15.0 |
21.4 |
3 |
530 |
11.6 |
16.0 |
16.0 |
16.0 |
21.4 |
4 |
530 |
11.4 |
16.0 |
16.0 |
15.9 |
21.4 |
The DO concentrations are quite consistent, with the exception of the Trial 2. It is possible that this might be a faulty reading that occurred only during this particular trial.
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.