When considering a system to treat your drinking water, it is valuable to have some understanding of the vocabulary terms often used—like filtration, purification, and disinfection. This knowledge, combined with an understanding of treatment methods, will help you compare systems based upon their strength and weaknesses.
While often used synonymously, there are important differences between filtered water, purified water, and disinfected water.
Disinfection is the first step in creating potable water, and is done to insure no pathogens (bacteria, virus, microorganisms) are living in the water you drink.
Boiling of water is the original form of disinfection. Often, after severe flooding or when bacteria have been found in the municipal supply lines, your community may issue a "boil water" alert until they can chlorinate the supply lines long enough to ensure the threat has been eliminated.
For personal use (e.g. backpacking) iodine is a familiar chemical used to disinfect lake or steam water . For municipalities, the primary disinfection chemical is chlorine.
Certainly, addition of disinfecting agents to water supplies has greatly reduced the incidence of waterborne disease in developed countries. Access to potable drinking water is still a primary concern for millions of people throughout the third world.
Chlorine (and related products like chloramines) are poisons. They are very effective when used properly, though they all produce nasty "disinfection by-products" which are often carcinogenic. Certainly consuming small quantities of a toxic substance like chlorine is preferred to developing a case of typhoid, cholera, or dysentery. But there is a growing body of scientific evidence showing chlorine in drinking water may actually pose significant long term dangers.
suspended vs. dissolved
A great way to understand the difference between suspended and dissolved solids is to consider what happens if you were to put salt and pepper into a glass of water and stir.
Very quickly, the salt will dissolve into the water. It won't be visible, but will be measurable.
The pepper will not dissolve and will remain visible. Small specs of pepper will be dispersed throughout your glass of water. The pepper represents suspended solids.
Webster's Dictionary defines filtration as: The act or process of filtering; the mechanical separation of a liquid from the undissolved particles floating in it.
Taken literally, this definition would limit filtration to removal of undissolved (suspended) solids. In the world of water treatment, the term is not so narrowly defined.
Technically speaking, carbon (either granular activated carbon or carbon block) acts as both a filter and purifier. Suspended particles are filtered out (depending on size), and dissolved organic substances (and some inorganics like chlorine) are removed by adsorption.
Ceramic filters, carbon block filters, and sediment filters act like mechanical sieves, effectively screening out suspended particles depending on pore size. For example, a .2 micron filter will effectively remove bacteria and cysts, but not virus or endotoxins.
Filtration is also subdivided by particle size removed. The classes of filtration include particle filtration, micro-filtration, ultra-filtration, and nano-filtration. Often you will find manufacturers of reverse-osmosis systems boast their membrane has multiple stages like reverse osmosis and ultra-filtration. Since all reverse osmosis membranes must be applied over some kind of substrate, the same way a film of Gore-Tex® must be applied to a substrate - usually some type of nylon fabric, saying a membrane has two unique stages is really nothing more than marketing hype. (The number of stages is no guarantee of effectiveness.)