A lab experiment studied how the depth of sand in a slow sand filter affects its ability to clean wastewater. The filter removed around 80% of turbidity, 46% of COD, and 35% of color from the water. The top layer of sand works like a sedimentation tank, removing most of the impurities. The middle and bottom layers act like an aeration tank and settling tank, further cleaning the water. The best results were seen with 800mm of sand with grain sizes of 0.4-0.6 mm. This shows that slow sand filters can effectively treat wastewater, similar to a secondary treatment system.