Sand filtration is used for the removal of suspended matter , as well as floating and sinkable particles . The wastewater flows vertically through a fine bed of sand or gravel . Air bubbles are blown into the sand bed to make it swirl around . Filtered water then flows through the filter bed in the opposite direction . The main benefit of a sand filter is the simple system which in many cases , can be used to obtain considerable yield . It’s primarily used for the removal of suspended matter . The yield in this case varies between 50 & 99.99% depending on whether support aids are used .