Dissolved Oxygen Measurements
Dissolved Oxygen measurements were performed two times in order to fully assess the functionality of the system. 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. These discrepancies led us to measure the DO concentration to examine the functionality of the various components in the setup.
Procedure
Sampling Points: Water Source, Aerator Effluent, Sand Filter Effluent, Bubble Collector Effluent
Water Temperature: 20.8 °C for the first probe, 21 °C for second probe
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 probes were used in samples at each point to confirm results. After each probe was assembled, it was placed in a solution of sodium sulfite to ensure a zero reading. To test the probe's accuracy, it was placed in a sample of tap water, which should have a dissolved oxygen content near 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.
Results and Discussion
The results were very surprising. Table 1. shows the first set of DO measurements taken after the Sand 40 experiment was performed.
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Table 1.: Dissolved Oxygen Concentrations (DO) at Sampling Ports in the System.
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 |
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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 speculate that high pressure in the bottom of the sand filter dissolves smaller bubbles into solution.
Table 2. shows the second set of DO measurements taken after the Sand 30 experiment was performed.
Table 2.: Dissolved Oxygen Concentrations (DO) at Sampling Ports in the System.
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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 |
Dissolved oxygen measurement after the bubble collector do not agree with the removal rate recorded by the bubble collector in Process Controller. A review of pressure readings taken through Process Controller during Evaluation Experiment 2 confirms that the pressure in the bubble collector is atmospheric, which is to be expected from such an open system. The bubble collector might be inefficient at removing small bubbles. Inside, there is not enough residence time for tiny bubbles to rise to the top, especially when water level is relatively low. Therefore, it is difficult to remove small bubbles, which subsequently are swept with water to the waste.
Conclusions
To confirm suspicions that pressure in the sand filter is too high, measurements will have to be taken at several bed expansions, including at zero expansion. However, the measurements may only serve to quantify the obvious need for a different sand filter design. It is clear from the dissolved oxygen measurements that the sand filter is not doing its job of removing dissolved gas. We will probably design and build a sand filter that is open to its environment so that atmospheric pressure is maintained.