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The goal of our research is to evaluate the feasibility of a siphon-aided backwash design to be used for a sand or mixed-media down-flow filter. The benefits of siphon-aided backwash are:
1) The height of the clear-well does not need to be equal to the the head pressure required to attain 30%-50% expansion.
2) The filter bed can be right next to the clear-well, connected via an open pipe without a valve. Flow of water into and out of the clear-well will be controlled by valves on the influent and backwash manifolds.
3) Using 55 gallon HDPE barrels, we will create a system of parallel filters surrounding a shared clear-well. By backwashing the filters out of phase, we will reduce the demand for a large clear-well. Additionally, the downflow filters will be filling the clear-well while the other filters being backwashed draw from it. This allows faster clear-well recharge rates and shortens the time between consecutive backwash cycles.
We are discontinuing research of the Sipon-Aided Backwash for the following reasons. It is a pressurized system which prevents the operator from properly observing the operation of the system. It is not economically feasible because it requires either the construction of one large clear well or multiple smaller clear wells. The filtration system itself would require the construction of one large or multiple small filtration systems.
Foam Filtration
Currently, an AguaClara plant can produce effluent water after sedimentation with a turbidity of about 5 NTU. Our goal is to reduce the effluent turbidity to less than 1 NTU. One potential method of accomplishing this is adding a filtration unit to the AguaClara plants.
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We based our initial design on empirical granular filtration equations from existing literature on granular filtration. A bench-scale model of our system was built and tested to observe the discrepancy between the actual and the calculated fluidization velocity required to achieve a target bed expansion. We noted an increasing difference between those two sets of values that prompted us to hypothesize that, if we were to base our design on these empirical equations, we need to implement a significant safety factor. Further experiments are needed before we can conclude whether or not our design is feasible.
Figure 1: Clear Well Filtration Design
We are discontinuing research of the Sipon-Aided Backwash for the following reasons. It is a pressurized system which prevents the operator from properly observing the operation of the system. It is not economically feasible because it requires either the construction of one large clear well or multiple smaller clear wells. The filtration system itself would require the construction of one large or multiple small filtration systems.
Stacked Filtration System
So far, we have created a preliminary design for a stacked filtration system for the Agalteca Plant. It is a granular, down flow filtration system consisting of four separate rapid sand filtration units. The basic concept of our design is to make the backwash flow rate equal the filtration flow rate in order to use the effluent water from the sedimentation tank to backwash. We have chosen this concept because of the following benefits:
1) Relatively small size of the entire filtration system compared to other granular filtration systems researched.
2) Availability of the materials that would be involved in its construction.
3) Relative ease of operation.
4) If given full plant flow, the ability to conduct backwash while maintaining normal filtration operations in at least half capacity.
Each separate unit consists of 5 planes of sand filtration stacked vertically on top of each other. Each plane consists of a set of inlet tubes, 20 cm layer of sand for filtration, and a set of outlet tubes. Currently, for each layer of tubes except for the bottom layer, we have selected eighteen ½ inch PVC tubes separated by a distance ½ inches. This is most likely more PVC tubes than necessary for reasons that will be explored more in detail in the Theory section. We have selected the bottom layer of tubes to be 1 ½ inch in diameter. All of the manifold that connects to these filtration tubes except for the bottom one will be 3 inch in diameter while the bottom manifold and the rest of the pipe system of the filtration system will consist of 6 inch PVC pipes. All pipes and tubes used are schedule 40. This filter is designed for sands with typical characteristics of D60 of 0.55mm, porosity of 0.4, and specific gravity of 2.65. It will filter at rate of 1.4 mm/s and backwash at 14 mm/s with expected 30% bed expansion. It has the following dimensions as shown in Figure 1 Preliminary Design of Vertically Stacked Filtration.
The design is based on a couple of assumptions that still need to be validated with experiments.
Specifically, we still need to test the following experiments: efficiency of effluent sedimentation water to backwash the filter, efficiency of 20 cm layer of sand to filter water, clogging time of 20 cm of layer when exposed to typical sedimentation effluent water, the effect of the number of tubes per plane on filtration efficiency, bench scale model to test backwash effectiveness with regards to bed expansion and effectiveness of our backwash procedure. All of these will be covered more in detail in the Future Challenge Section.
Future Challenges
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Foam Filtration sub team
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