Reevaluation of previous experiments with the new system
Objective
This set of experiments attempts to replicate the grain size research performed with the previous experimental setup. Additionally, the effectiveness of each of the components in the setup will be assessed with respect to theoretical expectations.
General Procedure
The procedure for each experiment in this set is fairly similar, with the exception of sand grain sizes, which are varied among the experiments. For the two experiments listed below, 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.
In Process Controller, configure the system so that the aerator air pressure is maintained at roughly 100 kPa. Fill the sand column with 60 cm of Sand 40 and adjust the flow rate on the pump forcing water through the sand filter to establish a bed expansion of 50%. For the first experiment, manual measurements of flow rate were performed by unhooking the influent water tube into the sand filter and allowing the influent to fill a large graduated cylinder over the course of a minute. In order to minimize changes made to the system, flow rate measurements for the second experiment were taken at the system effluent tube. The flow rates were roughly 225 ml/min and 485 ml/min for Sand 40 and Sand 30, respectively.
Run the Process Controller method file, given here on the "On" state. The "On" state regulates the air pressure in the aerator by releasing small amounts of air through a valve when the system exceeds the maximum aerator air pressure of 102 kPa. The water level in the aerator is controlled in a similar manner; however, the water wasting valve is also subject to a duty cycle in which the valve will open for a set period of time and close for a set period of time. If the "on" condition for the wasting valve is not met (that is, if the water level does not exceed the regulated height), the wasting valve will remain closed.
The water entering the aerator and leaving is maintained at a constant rate throughout the experiment via manually controlled pumps. The water is allowed to flow through the sand column, where bubbles can form. When bubbles grow large enough in the filter, they can float up to the top and out through a tube into the bubble collector. Throughout the duration of the experiment, the bubble collector goes through cycles of unfilling and refilling. Initially, an air valve at the top of the bubble collector opens and the water effluent valve located at the bottom of the bubble collector closes, allowing the collector to fill like a sitting column of water. Once a maximum height is reached, the air valve shuts off and the water valve opens, resulting in a partial vacuum at the top of the collector. This suspends the column of water in the bubble collector. As bubbles enter the collector, gas in the bubbles fills the partial vacuum, allowing the water column to slowly drain from the collector. Once the minimum water level in the collector is reached, the apparatus refills and the cycle begins again.
For each emptying period of the bubble collector, data is collected through Process Controller. Analysis of the data collected can be quantified as a gas removal rate by considering the cross sectional area of the collector and the flow rate through the system. Please see the results and discussion for each experiment by clicking the links below.
Results and Discussion
Experiment 1 - Replicate of the Previous Sand 40 Experiment
- This experiment was performed shortly after the installation of the new setup with the purpose of replicating previous grain size results and assessing the overall functionality of the setup.
Experiment 2 - Replicate of the Previous Sand 30 Experiment
- After making adjustments to the system to account for the issues found in the first experiment, this second experiment was performed to again test the functionality of the system and to replicate previous results.
- When it was confirmed that the new aerator was not saturating water as much as possible, concentrations of dissolved oxygen were measured at various points in the experimental apparatus.
General Conclusions
The two experiments gave results that were somewhat different from those produced in last semester's experiments. Because the data log shows that the system was generally working as is expected, and because the new results were more consistent than last semester's, we plan to use the new results. The first experiment indicates clogging in the sand filter, for which we are currently devising a solution.
The unexpected results come from measuring dissolved oxygen concentrations at various points in the system. The results indicate that the sand filter is actually oxygenating the water further, not removing large bubbles. This requires some major rethinking of the way the system works, especially in improving aerator efficiency and pressure maintenance in the sand filter.