media 2 backwash water upper layer sand filter applications advantages subsequent layers sand filters force water water passes quartzite sand allowing water low cost effective removal key advantages wide range particulate matter treat wastewater require coagulation biological processes waters polluted special materials bottom upwards air passes topdown direction granular materials filtered water substances retained remove impurities pool water clean discharge filters filled remove impurities particles sand traps particles water filtration system previously filtered substances popular choice due faster flow rate slow flow rate filtering part moving