Evaluation of previous experiments with the new system

Objective

This set of experiments attempted to replicate the grain size research performed with the previous experimental setup. Additionally, dissolved oxygen measurements were taken to assess the effectiveness of each of the components in the setup with respect to theoretical expectations. Theoretically, it was expected that the aerator under 2 atm of pressure would be able to supersaturate the water with 18 mL/L of dissolved gas by this theoretical model, which predicts the 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. The model calculates the theoretical bubble formation potential as a function of the air pressure that the water equilibrated with prior to returning to atmospheric pressure. With the current aerator, a major assumption made was that the dissolved gas concentration would equilibrate with the pressure in the aerator, resulting in 18 mL/L of dissolved gas in the influent water through sand filter. While no theoretical model of gas removal by the sand filter was developed, the team believed that the extra surface area provided by the sand would remove gas by increasing the nucleation sites for bubbles and also providing sites to which bubbles could adhere and grow.

General Procedure

For the two experiments listed below, the same procedure was used with varying sand grain sizes. Sand 40 (0.49 mm - 0.57 mm) and Sand 30 (0.59 mm - 0.84 mm) were used for experiments one and two, respectively. The details of the procedure are available here.

Results and Discussion

Experiments 1 and 2 were performed to assess the functionality of the new system in terms of its ability to collect good, consistent data and to ensure that previous sand grain experiments results were replicable using the new system. The control experiment with no sand was performed after it was observed that measured gas removal from the sand filter fell short of the amount that was theoretically possible to be removed.

[Experiments 1 and 2 - Replicates of the Previous Fluidized Bed Experiments]

• Experiment 1 was performed shortly after the installation of the new setup. Sand 40 was used with the purpose of replicating previous grain size results.
• Experiment 2 was performed after making modifications to the system to account for the issues found in the Experiment 1. Sand 30 was used with the purpose of testing the functionality again and replicating previous results.
• Control Experiment with no sand was performed to test the effectiveness of the setup in the absence of sand in the sand filter.

Dissolved Oxygen Measurements

• When it was confirmed that the measured gas removal values did not match theoretical values, concentrations of dissolved oxygen were measured at various points in the experimental apparatus.

General Conclusions

The experimental results for gas removal rate using Sand 40 and Sand 30 gave results of about 5.09 mL/L and 2.01 mL/L, respectively. While this data suggests that the sand filter is removing gas, the gas removal rate recorded was significantly lower than the expected. The discrepancy between laboratory results and theoretical predictions indicated that the current system was not functioning effectively. The dissolved oxygen measurements taken throughout the system (recounted below) indicated problems with the current setup or the ineffectiveness of the sand at removing dissolved air from water. After running a control experiment with no sand, it was found that sand did not facilitate gas removal but instead seemed to inhibit it.

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

Measurements indicate that the aerator is able to supersaturate the water with 15.5 ml/L of dissolved gas; however, this value is less than the expected 18 ml/L. Additionally, subsequent measurements of dissolved gas at that point reveal inconsistent levels of gas supersaturation. The inability to regulate the amount of dissolved gas in the influent water into the sand filter may have significant impact on the ability of the team to run controlled experiments. In order to address this issue, the team has decided to alter the pressurized aerator by replacing the single aeration stone with a junction of four cylindrical aeration stone that would displace gas into the water in finer bubbles that would be more easily incorporated into solution.

Dissolved oxygen measurements taken after the sand filter suggest a greater problem with the system. The effluent water from the sand filter appears to contain more dissolved gas than the influent water.

To correct these issues, the following design modifications have been advised or implemented:

It is also possible that there are a few other factors affecting the collection and removal of dissolved air. An example might be the case when the bubble collector is collecting the large bubbles but not the tiny ones (How could we retrofit the bubble collector to collect smaller bubbles? If there are small bubbles escaping at the end, what does this say about the process overall? Could this be why we have floating flocs in flocculation and sedimentation?). It is hoped that some of the modifications will help make the system more efficient, thereby yielding the results which would eventually converge to the theoretical predictions.