Why did CAWST update the biosand filter design?
In laboratory studies by three independent research teams, the portion of the water that remained in the filter during the pause period had much better bacteria and virus removal compared to the water that passed directly through the filter without a pause period. This variation in filtered water quality is because the fill volume (or ‘reservoir’ volume: the amount of water that can be poured into the filter at one time) is greater than the pore volume of the filtration sand (the volume in the void spaces between the sand grains). In addition, we have learned that reducing the depth of the reservoir and decreasing the flow rate in the filter will further improve the filter’s effectiveness.
