Filtration alone is not sufficient to trap very small particulate pollution.
In swimming pool filters, the size of the sand grains is usually 0.5 – 1.0mm. This results in a pore size of approx. 50 – 70 microns (1mm = 1000microns). Anything too big to pass through the pores will become trapped, anything smaller may pass through unless they settle on the upper-facing surface of a sand grain, or they are stick to the surface of a sand grain via adsorption.
If you consider that the size of cryptosporidia oocysts are approximately 3 – 5 microns, you will realise that sand filtration on its own will not be adequate to remove it. This is why it’s very important to pay close attention to the process of coagulation, which clumps small particles of pollution together to form what are known as flocs.
This process of coagulation, combined with the fact that filtration is a progressive process (more and more pollution will be removed each time the water passes through the filter) means that it is possible to remove particles smaller than 50 – 70 microns, in fact there is no specific bottom limit to the size of particle that can be removed.
As the filter begins to trap particles, the size of the pores decreases. This is known as ‘filter ripening’. When the filter is fully ripened, it will be capable of straining particles the size of around 5 – 10 microns.
The size of cryptosporidium cysts are about 3 – 5 microns, bacteria is around 1 – 5 microns, and colloidal matter can be as small as 0.1 micron, so as you can see, even with a ripened filter, some of the pollution is too small and will pass through the filter.
Cryptosporidia is not going to be killed by the chlorine in the pool, so it is essential that it is retained within the filter. A coagulant is required, which works by causing the small particles suspended in the pool water to bind together to form what are known as flocs. The flocs are typically 20 – 50 microns, so will be large enough to become trapped in the filter. The most common coagulants used in swimming pool water treatment are:
- Polyaluminium Chloride (PAC)
- Polyaluminium Sulpho-silicate (PASS)
- Aluminium Sulphate (Kibbled Alum)
- Sodium Aluminate