Floating Flocs Team Summer 2009 Research Proposal

Introduction and Theory

There is a problem in some AguaClara water treatment plants with flocs floating to the surface of the water in sedimentation tanks. One hypothesis is that the water entering the sedimentation tank is supersaturated, and because total dissolved gas pressure is greater than the local solution pressure, gas comes out of solution and forms bubbles. The bubbles form on floc particles and bring them to the surface, causing the flocs to float instead of settling to the bottom of tank. Thus flocs escape the sedimentation tanks, polluting the effluent water and degrading the quality of the drinking water produced by the plant. One solution may be to send the water through a sand filter before it enters the sedimentation tanks to remove excess gas in the water, thereby preventing bubble formation on flocs.

In the treatment plants, water would be sent upward through a bed of sand suspended in the grit chamber. The sand filter is intended to provide some surface area on which bubbles can form. The bubbles would accumulate, forming larger bubbles that could quickly float to the surface of the water in the grit chamber. The anticipation is that enough bubbles can form and leave the water in the grit chamber so that the water flowing into the sedimentation tanks would have a lower dissolved gas content and less potential to form bubbles.

Previous research has found that smaller grain sizes are more effective in gas removal than are larger grains in the same volume of sand, since small grains provide more surface area per unit volume on which bubbles can form. Other parameters, such as optimal bed depth and bed expansion, will be determined experimentally. Additionally, since dissolved gas content fluctuates in natural bodies of water, such as the sources of water for AguaClara plants, the dissolved air content of the influent water into the experimental system will be varied to explore its effects on gas removal. The purpose of this team is to test a variety of parameters (listed below) to design an effective sand filter that can be implemented in AguaClara plants.

Parameters

Size of sand grain

Research performed last semester focused on studying the effects of sand grain size on gas removal. We found that smaller sand grains provide more surface area per unit volume than larger grains, but grains that are too small are carried to the surface with bubbles and are washed away. Grain size also affects porosity of the sand bed, which affects the distance a gas molecule must diffuse to reach a surface. This also may affect aggregation of gas molecules to form larger bubbles. Further study of optimum grain size will be explored through MathCAD modeling, or through experimentation, if that is deemed necessary.

Bed depth (i.e., height of sand column before expansion occurs)

A range of depths will be explored to determine the relationship between bed depth and gas removal. Experiments have shown that increasing bed depth increases gas removal. Future experiments will explore the exact relationship to determine whether gas removal levels off at some bed depth, etc.
Bed expansion (which depends on the flow rate of influent water)—The expansion of the sand bed affects its porosity, which affects the distance a gas molecule must diffuse to reach a surface, and also may affect aggregation of gas molecules to form larger bubbles. We would like to see the effects on gas removal using different expansions while holding other parameters constant.

Dissolved air concentration

We would like to determine whether effectiveness of gas removed depends on the concentration of dissolved gases in the influent water. We plan to adjust the amount of supersaturation, probably by varying pressure in the aerator to decrease the dissolved oxygen concentration.

Experimental Design

The experimental setup used in Spring 2009 was modified at the beginning of the summer to include a taller pressurized aerator in order to increase the residence time of the bubbles and to establish more consistent supersaturation in the water going to the sand filter. The new aerator includes a valve to release water to maintain the appropriate water level. The hot and cold water valves present in the original set-up have been omitted in the new one, and the water source has been changed to a container with water whose temperature is maintained at 20 degrees Celsius. The flow that leaves the aerator and enters the sand filter is regulated by a valve that is controlled by the PID flow accumulator used in the previous setup. A temperature probe is used just to confirm that the water temperature is 20 degrees Celsius. From there, the water flows up through the sand filter, and then to the bubble collector, as in the original set-up. (A schematic of the original set-up is included for comparison.)

The Process Controller software is used to automate experiments and record data on Microsoft Excel spreadsheets. Two states have been developed for the experimental setup - an "On" and an "Off" state. Generally, experiments will be run with the system on the "On" state. However, as experiments are conducted, it may be necessary to increase the number of states. A rough draft of the Process Controller methods used can be found here.

In the "On" state, water from the temperature-regulated source flows into the aerator until the water level reaches a certain height that is yet to be established. This maximum height (set point: Aerator Max Water Level) is limited by the water level indicator system on the aerator, which measures the difference between the air pressure in the system and the water pressure. The water level should not reach the tube connecting the headspace of the aerator to the pressure sensor. If the maximum water level is exceeded, a valve is opened to release the excess water until a minimum set water level (set point: Aerator Min Water Level) is reached. (See set point: Aerator Water Wasting Control for method governing the water release valve.)

During experiments, the aerator is kept in the range of 100 kPa - 101 kPa gage pressure in order to achieve consistent dissolved gas concentrations. The air pressure regulator valve functions much like the water level regulating valve control for the aerator mentioned above. (See set point: Aerator Air Release Control for method governing the air release valve.) When the pressure is below the maximum set pressure (set point: Aerator Max Air Pressure - 101 kPa), the valve will remain closed. Once the maximum pressure is reached, the valve will open until the minimum set pressure (set point: Aerator Min Air Pressure - 100 kPa) is reached. To control the flow of air leaving the system, a rotameter is connected after the air release valve.

The flow of supersaturated water exiting the aerator is regulated by a valve that is controlled by the PID flow accumulator. The flow rate can be adjusted in Process Controller by editing the set point "Flow Rate", which is the target value of the PID control algorithm.

The water leaving the aerator is then sent up through the sand filter and out to the bubble collector, which measures the amount of gas removed by the sand filter. The bubble collector has two valves - a water outflow valve and an air valve. At the start of each experiment, the bubble collector is filled with water to a level that will be determined experimentally (set point: Bubble C. Max Water Level). The bubble collector uses the same sort of water level indicator system as the aerator; the water level in the bubble collector is limited by the location of the tube connecting the headspace to the pressure sensor. During the process of filling the bubble collector, the air valve is opened and the water outflow valve is closed, allowing for natural refilling of the apparatus. When the maximum water level is reached, the air valve is shut and the water outflow valve is opened, causing a partial vacuum at the top of the collector that holds the column of water up. As water flows through the bubble collector, gas bubbles in the influent water slowly increase the pressure in the bubble collector, causing the water level to decrease to a minimum level (set point: Bubble C. Min Water Level). Once this minimum level is reached, the system refills by the same process. (See set points: Bubble C. Water Outflow Valve and Bubble C. Air Valve for methods governing the water outflow valve.)

In the "Off" state, the water release valve and the air valve in the aerator are opened to prevent the build-up of pressure. The flow valve that allows water out of the aerator and into the sand filter is closed and the air valve and water-out valve in the bubble collector is opened to drain the bubble collector.
The new set-up will be used to test the parameters listed above via the following experiments:

Experiment: Bed Depth

The Spring 2009 Floating Flocs team conducted some preliminary research into the effects of varying bed depth (i.e., the amount or height of sand in the sand column). An experiment showed that a greater bed depth increased gas removal. This result was expected, as a greater surface area of sand should allow more bubbles to form and accumulate. We would like to conduct further research into the effectiveness of various bed depths to model the relationship between bed depth and gas removal.

Using the setup detailed above, we plan to conduct this experiment by beginning with a small amount of sand in the column and running the apparatus on the "On" state in Process Controller to measure and record gas removal. We would repeat with increasing depths of sand until either a relationship can be clearly defined. We plan to test bed depths within the range of 10 cm - 50 cm of Sand 40 (grain diameter 0.42 - 0.59 mm) with 50% expansion.

Research conducted in Spring 2009 suggested that gas removal increases with bed depth, but the sample was too small to make any definite conclusions about the relationship over a larger range. We predict that very small bed depths will be ineffective in gas removal. As depth increases within some yet unknown range, gas removal probably will increase. At very large bed depths, we believe, the amount of gas removal will begin to taper off; as the concentration of gases in solution decreases, fewer bubbles will form, and the rate of change of gas removal will decrease.

Experiment: Bed Expansion (and Flow Rate)

This experiment will attempt to examine the fluidized bed expansions for a filter media under varying flow rates, with other parameters held constant. The experiment will be conducted using the describsed testing apparatus and the "On" state in the Process Controller. As of now, we do not foresee any major changes to the experimental setup. We will adjust bed expansion by adjusting the flow rate through the sand filter. Expansion of a bed of sand affects the distance that the gas molecule has to undergo to reach a solid surface. Consequentially, this affects the bubble formation process. By conducting this experiment, the team hopes to quantify the relationship between the bed expansion at a certain flowrate and gas removal to find optimal conditionals of bed expansion in the sand filter. Presently, we are considering using Sand 40 with a grain diameter 0.42 - 0.59 mm. The range of expansions to be tested will be determined after the bed depth experiments are conducted. A bed depth that yields moderate gas removal will likely chosen and the physical limitations of the sand column will govern the expansion range.

Our hypothesis is that if the bed expansion is too low, the gas bubbles might not be able to form properly or might be trapped inside the sand bed. On the other hand, we think that if the bed expansion is too high the gas removal rate might decrease as a result of the increased distance that the gas molecules would have to travel to diffuse.

Experiment: Dissolved Gas Concentration

Because of seasonal changes in temperature, the dissolved air concentrations of influent water at AguaClara plants may not be consistent throughout the year. Because of this, we are interested in determining if there is a relationship between the dissolved gas concentration and the rate of gas removal via the sand filter.
This experiment is designed specifically to measure the changes in the gas removal rate of the sand filter as a direct consequence of changes in the dissolved gas concentration of the influent water.

For this experiment, the setup will be run on the "On" state on Process Controller described above. It will be assumed that the residence time in the new aerator will allow the dissolved gas concentration to equilibrate with the pressure maintained in the aerator. Since dissolved gas concentration is a function of pressure, the pressure in the aerator will be adjusted to achieve different dissolved gas concentrations in the influent water while keeping all other variables constant. It is likely that the range of pressures tested will be between 20 kPa - 100 kPa. The pressure is adjusted within the aerator by adjusting the "Aerator Min Air Pressure and Aerator Max Air pressure setpoints" in the process controller configuration file. Presently we are considering using Sand 40 with a diameter of 0.42 - 0.59 mm with a bed depth of 30 cm and 50% bed expansion.

The initial experiment will be run with the aerator pressure set to the 100 kPa - 101 kPa gage pressure. The experiment will be left to run for a day to observe and record the behavior of dissolved gas removal and to determine an appropriate runtime for subsequent experiments. (This will be determined by the time it takes for the bubble collector to refill a certain number of times)

Our hypothesis is that the behavior of gas removal over time will change. The current relationship between the volume of gas removed over time considering sand grain size or bed depth is found to be fairly linear at a constant rate. When the dissolved air concentration of the water is adjusted, the gas removal rate probably will be the same initially, then will decrease or level off as the gas concentration in the water reaches saturation at atmospheric pressure. This hypothesis is based on the notion that water that is not supersaturated with respect to atmospheric pressure will not form bubbles. We believe that as the concentration of gases decreases, fewer bubbles will form and fewer will grow large enough to float away from the sand filter.