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A vertically stacked filtration system meets many of the AguaClara Project constraints. First of all, both Both normal filtration and backwash back wash operations are gravity driven and requires require no electricity. It In additon, it is an open system. The required construction material, materials(PVC pipes, sand, concrete, and rebar) are relatively cheap and abundant in Honduras. Most importantly, sand filtration gives us the ability is proven to consistently lower the effluent NTU standard to below 1 NTU.
A stacked rapid sand filtration unit is preferable to a conventional rapid sand filtration unit as it requires less water to backwash back wash since the filters are stacked, and use the same water for all filter layers to backwashback wash. In addition, a stacked filtration system requires less planned area to achieve the same level of filtration.
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Keep up to date with Reflection Reports and Future Challenges. for the Stacked Filtration Team!
Current Research
Bench Scale Rapid Sand Filtration Model Design
Stacked Rapid Sand Filter Back Wash
Stacked Rapid Sand Filter Performance Comparison
Stacked Rapid Sand Filter Head Loss Measurement
VI. Future Research
Stacked Rapid Sand Filter Performance Study
A number of parameters will need to be studied in order to find the optimal design conditions for a filtration unit. The variables to be studied experimentally include diameter of sand grains, depth of each filter layer, horizontal spacing between pipes, and filtration velocity. A robust study of all these variables will require a long period of time, and it is thus recommended that we proceed with the design and installation of a filtration unit with conservative design parameters. Once full-scale performance can be analyzed, we can proceed with an exhaustive performance study to further optimize the stacked rapid sand filtration unit design.
Stacked Rapid Sand Filtration Unit Design
Model Design
To test the theory behind the stacked rapid sand filter, a bench scale model was designed and built. Using this model, a series of experiments will be conducted to test the filtering capacity of a stacked rapid sand filter.
Back Wash
The most important challenge to prove is the ability to clean the stacked rapid sand filter by the back wash method. In order to back wash the filter, we must prove that the individual filter layers can be fluidized in sequence, and thus result in a fully fluidized filter. Once fully fluidized, the filter can then be back washed, and required significantly less water than a comparable conventional single layer filter.
Performance Comparison
In order to learn if the flow distribution in the stacked rapid sand filter system is working reasonably well, it is necessary to conduct an experimental comparison to a conventional single layer filter of the same depth as a single layer of our stacked filter (20 cm).
Head Loss Measurement
The head loss across the filter over time is the governing factor that determines the effective filtration time. The build up of head loss ultimately determines how often the filtration unit will require back washing.
Future Research
Filtration Unit Design
It is necessary to design a filtration unit that can be built at our existing water treatment plants, and be incorporated into future plant designs. This includes a hydraulic analysis to calculate the critical elevations for how the filter fits in with the sedimentation tank effluent weir and the distribution tank. This analysis must be done using the approach for the design tool with correctly named variables, and with the standard inputs of flow rate and then automated design of the filter system. This automated design must include full hydraulic design of all of the piping required.
Plane Assumption Test
In our initial design, we assumed that as long as the spacing between the filtration tubes in each layer is small in comparison to the depth of a layer of sand, then the flow of water out of the tubes will converge and form a plane of filtration. For our purposes, we assumed that a ratio of 1 to 10 of tube spacing versus filter depth was small enough. Laboratory experiments should be conducted to find the depth which will still allow the tubes to be modeled as a plane. The The future challenge for the Filtration Team is to test the validity of our three assumptions mentioned above. We need to test the efficiency of 20 cm of sand with regards to clogging time and filtration efficiency. Our modeling of the area of filtration as a plane instead of a row of tubes needs to be tested as well. Finally, we need to find out the implications of back washing the sand filter with water that is normally used as an influent. Does the filtration efficiency eventually decrease over time?
Our immediate goal would be to design and build a bench scale model of our filtration unit as shown in figure 6. Instead of a square unit, we will use a 4 inch diameter test tube. We are currently working on the actual specifications of this model but everything would essentially be scaled down except the 20cm layer of sand. Once this prototype is built, we can push water 5-10 NTU water through and check to see if the effluent NTU is lower than 1. We can also measure the clogging time during this experiment and model the effectiveness of filtration versus time. Using this model, we can also simulate backwash with 5-10 NTU water and then rerun the first experiment to see how backwashing with the unfiltered water effect the filtration efficiency as backwash cycles are repeated. Finally, we can also vary the number of inlet and outlet tubes per plane can also be varied to see if our modeling of the layer of tubes as planes of filtration is accurate. We can slowly decrease the space between the tubes to see what find the necessary ratio between the sand layer depth and the space necessary spacing between the tubes needs to be to that will allow us to model the layer of tubes as a plane.
Performance Study
A number of parameters will need to be studied in order to find the optimal design conditions for a filtration unit. The variables to be studied experimentally include diameter of sand grains, depth of each filter layer, horizontal spacing between pipes, and filtration velocity. A robust study of all these variables will require a long period of time, and it is thus recommended that we proceed with the design and installation of a filtration unit with conservative design parameters. Once full-scale performance can be analyzed, we can proceed with an exhaustive performance study to further optimize the stacked rapid sand filtration unit design.
Figure 1: Plane Area Concept
Figure 6: Bench Scale Model