Current Research

Procedure

The experimental setup consists of a vertical clear PVC pipe that is 125 cm long with a 2.5 cm diameter. This is the filter column, and it is partially filled with the filter media. The filter media used were size 50 glass beads and sands with sieve sizes 30 and 40. These are summarized in [#Table 1] at the right.

Tap water is sent upward through the filter column. The tap water cannot be guaranteed to be super-saturated with gases already, so the aeration apparatus previously used in the aeration method has been implemented to supersaturate the cold water before it is fed through the filter column. The aeration chamber is kept under 1 atmosphere of pressure while the water is bubbled by an aeration stone. Air is allowed to leave the aeration apparatus through a valve controlled by [Process Controller] and a rotameter, which restricts the air flow to maintain pressure inside the chamber.

From the bubbling chamber, the super-saturated cold water joins hot tap water and flows through 1/4" tubing into the flow accumulator, which uses a pressure sensor, temperature probe, and two valves (one for hot water, one for cold water) controlled by [Process Controller] to regulate the water's flow rate (which is altered to achieve the desired level of suspension of the filter media in the vertical column) and the water's temperature (which is set to 20°C, and can be changed to mimic water temperatures in Honduras).

The mixed water passes from the flow accumulator and into the filter column through 1/4" tubing, suspending the filter media in the column. After flowing through the suspended filter media, the water and any bubbles that formed in the process pass through 3/8" tubing into the bubble collector.

A pressure sensor is located in the 3/8" tubing between the filter column and the bubble collector. This sensor measures the pressure inside the system as cm of head. For the setup to truly mimic the open-to-atmosphere conditions of a grit chamber, the height of the final outlet of water from the system must be adjusted until this pressure sensor reads zero.

The bubble collector is made of a 3.8 cm- diameter PVC pipe that is sealed at both ends. Water and bubbles from the glass filter column enter the chamber through 3/8" tubing that connects the top of the glass filter column to the bottom of the bubble collector. Inside the chamber (which is initially filled with water before each experiment), bubbles float to the surface while the water flows out through 3/8" tubing attached the bottom of the bubble collector. A 1/4" tube attached to the top of the bubble collector allows air to enter and leave. This tube as well as the water outflow tube at the bottom are both controlled by valves that are opened and closed by [Process Controller], which uses a pressure sensor to monitor the water level inside the chamber. The water level inside the chamber can be visually monitored through an additional 1/4" clear plastic tube that is attached to the top and bottom of the bubble collector.

At the start of an experimental run, the bubble collector chamber is nearly filled with water, the air valve at the top of bubble collector is shut, and the water outflow valve at the bottom of the bubble collector is open. As bubbles enter the bubble chamber and gather at the top, the water level in the chamber slowly sinks. When enough bubbles have entered the chamber to lower the water level as far as possible, the outflowing water valve is shut and the air valve is opened, allowing the chamber to refill. When the water level in the chamber reaches the maximum level again, the air valve shuts and the water valve opens, and the process begins again. The chamber can be drained by opening both valves at the same time. The system continues running during all of these processes in order to keep conditions as constant as possible.

The Process Controller Method we are using to run this setup can be downloaded [here|Current Experiments^SandFilterConfiguration.pcm].

The rate at which the water depth changes during a run is the same as the rate that air is being added to the collector, and so this is proportional to the rate that dissolved gases are being removed from the super-saturated water. Our data can be used to find the volume of dissolved gases removed per liter of water that flows through the system, allowing us to compare the relative effectiveness of each sand size, flow rate, and bed depth combination.

Results and Discussion

For each sand grain size, the change in water level was recorded over a period of time. Usually, the experiment was left to run for several hours. Water level vs. time was plotted for each of these runs. [#Figure 1] serves as an example, showing the raw data for a run using glass beads as the filter media.

We saw that the data behaves fairly linearly, so a linear trendline was fitted to the data in Excel for each sand grain size tested. This is demonstrated by the red segment of line in [#Figure 1].

The slopes for each filter medium tested are summarized in [#Table 2], along with the R ² value of the linear fit. All of our R ² values are very close to one, which means that the data is in fact very linear and can be accurately represented by the equation given by Excel. The slope of the Water Level vs. Time trendline represents the rate of change of the water level in the bubble collector for each experiment. For each run shown in the table, the filter bed was 60 cm deep and held at 50% bed expansion.

This rate of change in water level was converted to an equivalent rate of change in gas volume using measurements of the bubble collection chamber. The slope (change in height) was multiplied by the cross-sectional area of the bubble collector, as shown below:

To determine how many milliliters of gas were removed per liter of water flowing through the system, that volume rate of change was then divided by the flow rate of water in L/min. This was done for each sand, and the results were plotted in [#Figure]

After determining the volume of air removed per liter of water flowing through the system,
Flow rate glass beads work better although the slope isn't the steepest.