We put very fine sand in the bottom layer. On the second layer, we put steel and on the top layer, we put big gravels. After we test our prototype, the water that we filtered by our prototype was very clean, and it was very transparent. We wanted to put charcoal on the second layer but we couldn’t prepare the charcoal. So, we put compressed steel wool. But next day, the steel wool got rusty.
When we making the fine sands, it took very long time. every on our team worked on making fine sand. Because we did that, our prototype filtered water very clean.