Multi Grade Filter / Pressure Filters Depth filters for removal of suspended solids and turbidity using coarse and fine media mixed together in a fixed proportion. The specially graded sand media produces a filter bed with adequate pore dimensions to retain both large and small suspended particles. This filter performs at a substantially higher specific flow rate than conventional filters. Specific flow rates of 15-30 m/h have been successfully obtained for treating water containing 25-50 ppm suspended solids to produce filtrate with less than 5 ppm.