Upflow sand roughing filtration is a simple and efficient way to clean water before using it in slow sand filtration. Researchers compared upflow and downflow methods and found that upflow allows for longer filter runs and better use of bed depth. Surprisingly, using polluted water for backwashing can improve turbidity removal, possibly due to biological processes. Further experiments showed that using raw or polluted water for backwashing is just as effective as using tap water. This suggests that raw water backwash can be a good way to clean upflow roughing filters.