I have a cabin that uses water directly from a spring. What's the best way to filter that water
Spring water is typically exposed to the surface at some point and is in danger of being contaminated with animal wastes or other sources of bacteria. For safe drinking water, it is best to use a combination of disinfection (with chlorine, peroxide, ozone or UV light) with a 1 micron ceramic filter or polyester cyst filter to filter out giardia and cryptosporidium cysts.
It is important to filter for cysts in addition to killing bacteria. We also have Ultra-Filtration membrane systems that filter down to 0.015 microns which removes all the bacteria and giardia cysts without pre-chlorination.
If there is odor or color in the water, a chlorine or peroxide or ozone injection system combined with activated carbon works really great to kill the odor, disinfect the water and remove tastes. A 1 micron giardia cyst is used in combination with this system to provide safe drinking water.
If there is no odor and the water is very clear, then a 1 micron filter or UF membrane system combined with a UV sterilizer can offer safe drinking water with very little maintenance required and no chemicals used.