Introduction to Coagulation
Filtration alone is not sufficient to trap very small particulate pollution. In swimming pool filters, the size of the sand grains is usually 0.4 - 1.5mm. 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 – 6 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.