Media Filters / Tank-Type Pressure Filters

There are several types of so-called pressure filters available, each performing a specialized task. A single description of the equipment mechanics is sufficient to understand the principal.

A typical filter consists of a tank, the filter media, and valves or a controller to direct the filter through its various cycles – typically service, backwash and rinse.

Easily the most critical aspect of pressure filter performance is the relationship of flow rates to filter bed area and bed depth. This relationship is the primary cause of trouble and poor performance in filter systems. If problems develop, the most common reason is that many filters are inaccurately “sized” for the job. The nominal flow rate in the service cycle depends on bed area available and generally should not exceed a nominal rate of 5 gallons (18.8 L) per minute (gpm) per square foot of bed area (12.15 m3/h per m2), with at least a 30-inch (76.2 cm) filter bed depth.

Another important design criterion is backwash flow rate. Backwash flow rates are a function of backwash water temperature, type, size, and density of media, and the specific design of the pressure filter. Media with densities of 90-100 lb/ft3 generally require 12 to 16 gpm/ft2 of bed area. Less dense media may use lower backwash rates. Very cold water uses somewhat lower backwash rates, and warmer water requires higher rates. The table below expresses this relationship as a function of tank diameter. There are many types of filter media but all of them should follow the flow rate guidelines in Table 6.