Pool Filtration – the Ins and Outs

A swimming pool must be filtered to remove any insoluble particles and to keep the water is clean and clear. Clear water is not only wanted for visual beauty but also for hygiene and safety.

High Rate Pool Sand Filter

The most commonly used filter is a High rate sand filter on both Domestic and commercial pools.

The high rate pool sand filter has sand, either 16:30 or the more finer grading of 18:30 and the sand bed is 200-310mm deep, depending on the size of the filter tank.

Water is pressured through the sand bed at a high rate that a procedure of mechanical flocculation occurs. Along with an initial pass of filtration, the electrical charges are rubbed off which allows the dirt particles to agglomerate and create larger particles. On the next cycle through the filter these larger particles are collected.

The main benefit of the sand filter is that is simple. When the bed of sand is blocked with dirt, the pressure inside will register on a gauge which will let you know when you need to flush it out or backwash your sand filter.

This means switching off the pump and turning the valve to backwash. As soon as the pump is turned back on, the flow of water is reversed and washes up through the sand bed and removes the dirt. You hold also do a rinse after this stage. After this process, the valve can be turned back to the filter position for normal operation. All the dirty water from the filter is usually transferred into a sewer gully, gravel pit or storm water, depending on your state or council rules.

Sand filters will usually eliminate particles down to around 15 microns in size. The sand in the filter will usually only require changing about every 5 years, however this will depend on pool usage, size of pool etc.  You can now also purchase other forms of filter media such as glass and volcanic rocks but speak to your local pool technician.

One of the benefits of the cartridge filter is that the technique of cleaning makes it perfect for use in areas that don’t have a proper sewer system or where disposal of waste is a challenge.  They can also use a lower volume of water when cleaning compared to backwashing a sand filter.

The filtering aptitude of this type of filter is between sand and diatomaceous earth and will eliminate particles down to around 10 microns in size.

Diatomaceous Earth (DE) Pool Filter

Now less common in a residential environment but does provide high quality filtration. Diatomaceous earth is a powder made from finely crushed coral-like rock.

Inside the DE filter are several hollow plastic frames that can be square, tubular or round depends on the manufacturer. On top of these frames are stretched polyester covering or pad.

Water from the pool is pressured into the filter tank and through the pad. The water then continues through the center of the filter and loops back to the pool.

To start filtration, Diatomaceous Earth is assorted into a slurry and poured into the skimmer box with the pump running. It is consumed into the filter, forming a cake on the outside of the pad. As water goes through the pad, it must also go through the cake of DE where filtering takes place. The DE cake drops from the pad when the pump is stopped, then reforms again when the pump starts.

The Grains of DE are so fine that it can remove particles down to approximately 2-5 microns in size. When the pressure increases it is a warning that the DE cake is blocked and the filter needs backwashing.

This can be done by turning off the pump and turning the valve to the backwash position. You must keep in mind that used up DE cake is also washed away. So, a replenishment is required after every backwash.