Water Filtration Design Factors

Hourly flow rate

The hourly flow rate depends on the amount of time specified for the circulation of the all the water in the pool.

Most domestic pools are designed for a pool water turnover period of 6 hours. Hence a 10 x 5 x 1.5m deep pool will have a required pumping rate of 10 x 5 x 1.5 /6 = 12.5 cubic metres per hour.

Commnercial pools have far more stringent criteria and hence much higher pumping rates are required.

 

Speed of Filtration

Sand Filters

In the sand filters the maximum filtration speed is normally assumed to be 50 m/h or 50 m³ per hour for every square metre of filter surface area. Sand filters are normally circular and of 0.5, 0 .6, 0 .75 or 0 .9 m in diameter

So the required filter area is = the hourly flow rate / 50

So with a flow rate of 13 cubic metres per hour the required filter area is 13 / 50 = 0.26 m2.

The sand filter area is 3.14 x dia x dia / 4 and so the filter size required is 570 mm or a 0.6 m sand filter.

Speed of Filtration in Other Filter Types

Filter Type
Allowable Filtration Speed

Cartridge

2 m3 per m2 cartridge area
Diatomaceous Earth 5 m3 per m2 filter area
Anthracite 30 m3 per m2 filter area

Particle Size Capability of Filtration Types

Filter type
Particle size allowed through filter
Sand 20 to 40 microns
Cartridge 10 to 25 microns
Diatomaceous Earth 3 to 5 microns

Zeolite

Zeolite is the name of the new wonder filter material that makes pool water amazingly clear and sparkling but is very costly.

 

Glass Filtration Media

Crushed, graded glass works exceptionally well and is now nearly as cheap as sand and is being used far more frequently.