Stacked Rapid Sand Filtration

Introduction

A vertically stacked filtration system meets many of the AguaClara Project constraints. First of all, both normal filtration and backwash operations are gravity driven and requires no electricity. It is an open system. The construction material, PVC pipes, sand, concrete, and rebar are relatively cheap and abundant in Honduras. Most importantly, sand filtration gives us the ability 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 since the filters are stacked, and use the same water for all filter layers to backwash. In addition, a stacked filtration system requires less planned area to achieve the same level of filtration.

For more information, please see the explanation of stacked rapid sand filtration theory and unit design.

Keep up to date with Reflection Reports and Future Challenges.

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

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 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 the necessary ratio between the sand layer and the space between the tubes needs to be to allow us to model the layer of tubes as a plane.

Figure 6: Bench Scale Model