Several types filtration media is used to remove suspended solids. Silica sand is the most common. When water flows downward through the bed, solids form a mat on surface and filtration occurs in the top few inches. The sand is cleaned by upward water flow. Air scour will be used to loosen bed for efficient backwash.
Pressure filters are classified in to three types; Pressure Sand Filters, Dual Media Filters and Activated Carbon Filters
Important design factors for sizing filters are influent water quality, velocity, effective size of media, uniformity coefficient and density.
Pressure filters are closed cylindrical vessels with media accompanying piping and valves for routing water flow during operation and cleaning. Tanks will be made with mild steel with or without protective coating, stainless steel and fiber reinforced plastic. Standard filter media depth is 800-1000 mm and grading according to the work conditions. Differential pressure is generally 0.1-0.3 bar during start and require cleaning at 1.5-2 bar. The filtration rate goes up to 20 m/hr and backwashing is 1.5 times higher.