ANALYSIS AND REDESIGN OF A BIOSAND WATER FILTER THROUGH COMPUTER MODELING AND PHYSICAL TESTING
Initially, physical tests to determine porosity and hydraulic conductivity were conducted on the traditional Biosand filter material. A Microsoft Excel-based model was built to mimic the initial physical filter design using the physical test results to calculate vertical and horizontal velocities and total discharge of water passing through the filter. The model was further calibrated by performing flow tests with the filter system. The model was then used to calculate advection velocity, residence time, and half-life in the biolayer. These results were used to better understand the limiting factors for efficient filter operation. This understanding was used to design a smaller and simpler filter system (based on a standard 5-gallong plastic bucket) that would attempt to be as efficient.
After constructing the new filter, its performance was tested using both a relatively-inert tracer (food dye) and E. coli contaminated water. Prior to transport tests, flow tests were performed to confirm model predictions. Known E. coli concentrations were passed through the filter using the same methodology of the inert tracer test to determine the effectiveness of the contaminant reduction.
The computer models revealed the influence of the filter media on the overall effectiveness of the filter. It was affirmed that the least permeable media controls the discharge of the filter, and to a certain degree the underlying coarser media has no impact on the effectiveness of the filter. This knowledge stimulated the design of the smaller filter, and the transport test results showed that the new simple filter design was effective at removing E. coli. However, further work should be pursued in order to improve the design so this filter could be implemented during future Olivet Nazarene University mission trips.