# Pool Plant Courses - Blog

## Filtration Rate

Posted by poolplantcourses.org on Sunday, December 18, 2011 Under: Coagulation & Filtration
The filtration rate is the rate (in metres per hour) at which the pool water moves down through the filter during normal operation. It is not to be confused with the circulation rate, which is the rate (in cubic metres per hour) at which water is moving through the circulation system.

The filtration rate is calculated by dividing the circulation rate by the surface area of the filter. For example:

Circulation Rate: 100 cubic meres per hour
Filter Surface Area: 5.72 metres squared
Filtration Rate: 17.48 metres per hour (100 divided by 5.72)

You should have a flow-meter fitted onto the circulation pipework in your plant room to tell you what the flow rate is. You can easily calculate what the filter surface area is by multiplying the radius by itself and then multiplying that value by pi (3.14). For example:

Width of filter: 2.7 metres (this is also the diameter)
Radius: 1.35 metres (half of the diameter)
Radius Squared: 1.82 metres squared (1.35m x 1.35m)
Radius Squared x pi: 5.72 metres squared (1.82m x 3.14)

You'll need to know the filtration rate in order to establish whether your filter is a high, medium or low rate filter.

High Rate: 25 to 50 metres per hour (small domestic pools only; to fast to deal with pollution in public pools).
Medium Rate: 10 to 25 metres per hour (recommended for public pools).
Low Rate: up to 10 metres per hour (very good filtration, but requires a vary large surface area).

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