Dissolved Oxygen Measurements

According to MathCAD modeling of the system, bubble formation potential in the water should be 18 mL/L.In the second experiment we conducted during the summer, which is described here, gas removal was only 2 mL/L. To achievea better understanding of how the system functions, we measured dissolved oxygen concentrations at various points in the system.

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 mL/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 measured dissolved oxygen concentrations at each of four points in the system.

Table 1: Dissolved Oxygen Concentrations (DO) at Sampling Ports in the System.

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. The sand filter is further aerating the water, instead of removing dissolved gases.

Since increased pressure facilitates dissolution of gases into water, we suspect that pressure in the sand filter is too high. However, the inadequacies of the bubble collector are more elusive. 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.

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. Such measurement has already begun and will be completed when a mechanical malfunction with the sand filter is resolved. However, the measurements will 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.

The question of what is wrong with the bubble collector is still unanswered. We must further examine how the bubble collector behaves before proposing a solution.