Floating Floc Detailed Task List
Spring 2009 Team members
Tiffany McClaskey
Ling Cheung
Tanya Cabrito
Haley Viehman
Wenny Wu
This semester, it is imperative that we find a solution to the issue of flocs rising to the surface of the water in the sedimentation tank. The Marcala and Tamara plants are seeing a significant amount of particles rising to the surface. Our understanding is that the water is coming into the plants super-saturated with oxygen. Once the water enters the grit chamber and is under atmospheric pressure, bubbles of oxygen begin to form. Some of these bubbles attach to the sediment particles in the water, and flocs form around the bubbles that haven't left the water while in the grit chamber. Since bubbles form more quickly when they are attached to something (you may have noticed that your glass of faucet water develops bubbles on the sides), the proposed solution is to create surfaces on which the bubbles can form. Larger bubbles rise faster than small bubbles so the goal is to create the largest bubbles possible in the shortest amount of time. We will explore two methods to speed up the process of bubble formation: A) Seeding the supersaturated water with bubbles and reducing the pressure to less than atmospheric pressure to increase the driving force that is causing the bubbles to form; and B) Run the supersaturated water through a sand filter to provide ample surface area for dissolved oxygen accumulation and bubble formation.
Ideas for full scale implementation that we will be investigating at Laboratory Scale:
• Add air bubbles using suction through a small hole in the side of a down-flowing pipe right before the water enters the grit chamber. The additional air in the water will increase the bubble size as it joins the preexisting air pockets. The pressure in the pipe could also be maintained at a partial vacuum to accelerate the bubble formation process.
• Add sand to the bottom of the grit chamber simulating back-wash in a sand filter. The idea is that the dissolved oxygen in the water will form bubbles on the sand particles. Once the buoyant force is greater than the connection between the bubble and the sand particle, it will rise to the surface of the water.
• Add the equivalent of lamellas to the grit chamber. Bubbles will form on the underside of the lamella and eventually become large enough that they roll across the lamella surface and float to the surface of the water.
Two contraptions will be made that we will use to simulate different situations to help us determine the best course of action. The team will also be split into two subgroups so that each experiment can have the undivided attention of different team members. The members of each subgroup were decided based on class and work schedules. Tiffany and Tanya will be working on the aeration experiments and Haley and Ling will be focusing on the backwash sand filter. Wenny will be assisting each team interpret the data, contemplating other experiments that can be performed, attending meeting and assisting on written assignments.
MODEL 1: AERATION
Aerator Apparatus
The first will consist of a dissolved oxygen probe and a pressure sensor connected to a 4 inch PVC pipe with a suction outlet, a hole for influent water, an air stone and an air inlet line. To drain the system, the water inlet line can be detached over the sink at a valve that can be toggled shut near the source of the water. A partial vacuum can be maintained by pumping the air out of the cylinder via the suction outlet. The device also has a stir bar inside to help us model a complete mix system. As we adjust the pressure in the cylinder, the pressure sensor will allow us to know that the actual pressure is in the pipe. The DO probe will allow us to track the rate at which the oxygen leaves the water. The level of DO needed to keep the flocs from rising to the top of the sedimentation tank in the treatment plants has yet to be determined.
Several different experiments will be conducted with this set-up. In order to simulate the water conditions in the pipes of the Honduras plants on a smaller scale, all experiments will be conducted with tap water that is supersaturated with oxygen.
Experiment 1 will be attempting to simulate what is happening in the pipe as air bubbles are being sucked into it. With the stir bar running, the dissolved oxygen level in the cylinder will be continually measured as different flow rates of air are pumped into the tube. This will be done with the lid securely attached to the top of the apparatus to make it air tight so we can add varying amounts of negative pressure in the pipe. The pressure in the pipe will depend on how high up from the outlet we put the holes through which air will be sucked into the pipe at the plants. By changing the pressure in the cylinder we are simulating different locations of the air holes.
With this experiment, we are seeking to measure DO levels as a function of time, pressure, and air flow rate. Using the pressure sensor and a rotameter, the pressure and air flow rate will be maintained throughout each test run and recorded. In addition to this, we will also be observing bubble size and recording the approximate average diameters of the bubbles. The clear cylinder will allow us to observe the growth and behavior of the bubbles.
The program written last semester will be used to determine the different air flow rates that will be used for the experiments. The program determines the amount of air that will flow through a hole under specific conditions, which depends on the size of the hole, the dimensions of the pipe and the flow rate of water. We can also f the likely pressure in the pipe for any given situation. This way we can model the plant in Tamara without having to reproduce the plant's high flow rate.
One of the drawbacks of this experiment is that it is basically assuming plug flow. The depth of the water in the PVC tube will hopefully be a completely mixed body of water but since we will not have the water flowing through the pipe the setup will imitate what is happening to the water in a 10 inch section.
The length of each test run will be determined once we have the experimental device working. We currently do not know the kinetics of the bubble formation and thus we do not know how long the batch tests in the pressure/vacuum chaber will need to be. The data will be evaluated based on the level of dissolved oxygen in the water and how big the bubbles are. The goal is to determine the pressure and air flow rate that produce the biggest bubbles and lowest DO level. This should help us determine the optimum orifice size and its height above the outlet of the pipe.
Experiment 2 will model the interface between the pipe that is under negative pressure and the grit chamber that will be under atmospheric pressure. After air has been pumped into the sealed PVC pipe that is under negative pressure for a period of time (yet to be decided), we will turn off the inlet air and remove the lid to the device, exposing the water to atmospheric pressure. The dissolved oxygen level in the water will be continually measured one inch below the surface on the water as well as at the bottom of the cylinder. We hope to see more bubbles form and rise to the surface once the water is exposed to atmospheric pressure. This should help us determine the rate at which the dissolved oxygen level in the water will decrease under varying air flow rates and initial pressures. This will in turn suggest the retention time needed in the grit chamber to decrease the DO to an acceptable level.
We will measure DO levels at the top and the bottom of the water column as a function of exposure time for each pressure and flow rate tested. We will also observe bubble size, average bubble diameters, and note any changes in the behavior of the bubbles under atmospheric pressure.
As in the first experiment, the length of each test run will be determined once we have the experimental device working. The data will be evaluated based on the level of dissolved oxygen in the water and how big the bubbles are. The goal is to determine the exposure time and air flow rate that produce the biggest bubbles and lowest DO level. This should help us determine the optimum orifice size and retention time in the grit chamber.
MODEL 2: BACKWASH SAND FILTER
This experiment will model the effect that running water through a layer of sand will have on its dissolved oxygen content. We hope to see that the sand particles facilitate larger bubble formation by providing surfaces to which small bubbles can adhere and grow until they are large enough to float to the surface. If the method proves to be effective, we can implement this in current and future AguaClara plants by transforming the grit chamber so that water enters at the bottom and flows upward through sand. Through experimentation as well as extensive literature searches, we hope to see the effects of the water's upward flow rate, the sand particle size, and the sand layer depth on dissolved oxygen levels. With this data, we plan to determine the optimum conditions needed for dissolved oxygen removal.
Our experimental set-up consists of a 63 cm glass column with an inner diameter of 2.5 cm, with caps for each end that allow water inflow and outflow. We will fill the tube partially with sand and send super-saturated tap water through the tube in a continuous flow (tapwater pressure should be sufficient). The flow will be large enough to suspend the sand particles, as though backwashing a sand filter. Water and any air bubbles that form will flow out of the tube at the top to a collection container open to the atmosphere, where will place the DO probe. The DO probe will monitor the dissolved oxygen content of the out-flowing water to help us determine the effect of the sand filter on dissolved oxygen levels.
The data from both models will be compared to decide which method would be the best for each individual plant. Which, if either, system to be used for each AguaClara water treatment plant will depend on the current configuration of the existing plants, the ease of retrofitting these grit chambers, the retention time in the grit chambers and the plant flowrate. The evaluation will be based on the method that can reduce the DO content the most, the retention time needed to achieve the optimal DO content and practicality of implementation. If neither method is seen to be fit to eliminate the occurrence of floating floc in the sedimentation chamber other options will be explored.
We currently have that materials required for the experiments detailed above.