Coagulation in swimming pools is essential because filtration alone cannot adequately trap very small particulate pollution. The sand grain size used in these filters is typically 0.5-1.0mm, resulting in a pore size (gaps between sand grains) of approximately 50-70 microns (1mm = 1000microns). If a particle is too big to pass through the pores, it will be caught. But if it’s smaller, it might still make its way through unless it sticks to the upper-facing surface of a sand grain or gets adsorbed onto it.

But here’s the catch – the size of cryptosporidia oocysts is only about 3-5 microns. Hence, relying solely on sand filtration will not suffice to remove them. That’s why you need to focus on coagulation – the process of clumping small pollution particles together to form flocs.

When combined with the progressive process of filtration, coagulation makes it possible to remove particles smaller than 50-70 microns. As the filter traps more particles, the pores shrink, which is referred to as ‘filter ripening.’ Eventually, the filter can strain particles as small as 5-10 microns.

However, particles smaller than that, such as cryptosporidium cysts (3-5 microns), bacteria (1-5 microns), and colloidal matter (as small as 0.1 micron), can still slip through the filter even when it’s fully ripened.

That’s why retaining cryptosporidia within the filter is crucial, as chlorine cannot kill it in the pool. To achieve this, a coagulant is essential. It causes small particles in the pool water to bind together and form flocs, which are usually around 20-50 microns and can be trapped in the filter. The most commonly used coagulants for swimming pool water treatment in the UK are Polyaluminium Chloride (PAC), Polyaluminium Sulpho-silicate (PASS), Aluminium Sulphate (Kibbled Alum), and Sodium Aluminate.