Filtration Rate and Area

Filter area and flux

Filtration rate is filtrate flow divided by the effective area currently in service. Use bed area for a granular filter and active membrane area for a membrane system. For example, 100 m³/h over a 20 m² bed gives a superficial rate of 5 m/h. This is not the actual water velocity inside the pores.

Using the result

The area mode divides flow by a selected operating rate. A flow of 6 m³/h at 30 L/(m²·h) requires 200 m² of active area. The flow mode multiplies area by rate. Filter downtime during cleaning, backwash water, temperature and pressure loss are outside this simple relationship. Verify the selected flux against the process conditions.

Use the area actually in service

An example plant delivers 60 m³/h of filtrate through two filters with a combined active area of 24 m². Superficial velocity is 60 / 24 = 2.5 m/h. If one filter is taken out of service and active area falls to 12 m², velocity at the same flow becomes 5 m/h. These numbers illustrate the effect of area, not acceptable operating limits.

Halving velocity at unchanged flow requires twice the active area. That extra area must actually be in service. Including installed but idle filters in the denominator understates the operating flux. For membranes, use the defined effective membrane area rather than the external housing area.

Check both volume and time conversion factors when converting litres and cubic metres. A result in m/h expresses superficial volumetric flux as length divided by time. The same numerical flux does not imply the same operating conditions for every filter type. Fouling and increasing pressure loss cannot be determined from the area ratio alone. Review filtrate quality and cleaning cycle data separately when assessing the result.

Frequently asked questions

How do you calculate the effective filtration area?

For example, a flow of 100 m³/h at a selected superficial filtration rate of 5 m/h requires 20 m² of effective area. Divide flow by rate, noting that this preliminary estimate does not establish a suitable operating rate or assess backwashing outages, pressure drop or water quality.

Calculate with these values