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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.