What are Pre Filters?
They are also known as sediment filters.
How They Work
Pre filters catch larger particles in the water in stages and prevent the main filter becoming blocked and slowing the flow.
For example a typical pre-filter configuration could be: first in line is a 50 micron pre filter - this catches anything larger than 50 microns. Next in line is a 20 micron filter, followed by a 5 micron filter... and so on.
(To give you an idea, a human hair is 100 microns in diameter. 5 microns is usually as much as you need to filter for UK water - unless you are on a non mains water supply and want to filter out dangerous protozoa like cryptosporidium
Without pre-filters, the life of the main filter would be much shorter.
There are 3 main types of prefilters
Pleated Water Filters
These look like folded paper and are more popular than string types see below.
The folds give Pleated Filters a much better depth in surface area. eg they have a 2.5 bigger surface area than PP. This makes them last longer and perform well - they will typically last about 2x as long at opt performance. (Here's a picture of a pleated filter).
String Water Filters
These are made by winding string around a core. They used to be very widely used but have been replaced by PP filters see below.
People who already have a string filter will naturally want to replace like for like. But cannier users also know they can wash the string. They shouldn't really do this but people like to save a few pennies here and there. Washing makes the string filter last longer. But it's not really worth the effort.
Polypropylene (PP) Water filters
These have updated string filters. They are better technology, outperforming string filters and are lighter to transport. PP filters are excellent at holding in contaminants while allowing for high flow rates http://en.wikipedia.org/wiki/Polypropylene