Posted by on Monday, December 19, 2011 Under: Coagulation & Filtration
Straining is one of the processes taking place during filtration and is perhaps, the one most pool operators will be familiar with. The process involves dirty water passing through the filter media (usually sand) and particles of pollution becoming trapped in the small gaps (pores) between the grains of sand because they are too large to pass through.

Take a look at the picture (click to enlarge). In swimming pool filters, the size of the sand grains is usually 0.5 - 1.0mm. This results in a pore size of approx. 77microns (1mm = 1000microns). Anything to big to pass through the pores will become trapped, anything smaller will pass through. If you consider that the size of bacteria is 1 - 5microns, you will realise that sand filtration on its own will not 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 is known as floc. 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 much smaller than 77microns, in fact there is no specific bottom limit to the size of particle that can be removed.

In : Coagulation & Filtration 

Tags: "straining" "micron" "sand"