The speed at which pool water moves through the filter during operation is called the filtration rate. Don’t mistake it for the flow rate, which is the speed at which water travels through the entire circulation system. To calculate the filtration rate, you simply divide the flow rate by the filtration surface area. Of course, you need to know the flow rate first!
Filtration rate has an effect on how well the filter does its job (filter contaminants out of the water), so pool operators need to know it so that they can assess whether the rate of their filters meets industry guidelines (covered later).
Ideally, your plant room should already have a flow rate meter fitted onto the circulation pipework. If not, it’s high time you get one. Without it, calculating the turnover time would be a real challenge, let alone determining the filtration rate!
As for the surface area of the filter, you might be able to find it on an information panel stuck onto the side of the filter. Check out the image below for an example (click to enlarge).
If there’s no sticker, don’t panic. Here’s what you need to do. Work out the surface area of the filter. Then, take the flow rate and divide it by the surface area.
The filtration rate can fall into one of three categories: low, medium, or high. These rates are set out below, along with their corresponding filtration capacities:
- Low: Up to 10 m3/m2/hr
- Medium: 10 – 25 m3/m2/hr
- High: 25 – 50 m3/m2/hr
If you want to achieve the required turnover time, you need to ensure that the filtration isn’t too slow. On the other hand, for public pools, the recommended rate is medium. But watch out! A rate that’s too fast won’t give the filter enough time to cope with the pollution levels found in most public pools.