A number of water purification technologies are available to remove or reduce contaminants that may be in your drinking water. It’s common for several of these processes to be used together in series to achieve maximum contaminant reduction. Carbon and ceramic filtration are perhaps the most common and economical technologies in use. However, depending upon the contaminants in your water and your level of concern, other technologies such as ion exchange, ultraviolet disinfection, reverse osmosis, or distillation may be more appropriate.  

Filtration

Most water filters use a bed of carbon to remove contaminants and impurities through chemical adsorption, although ceramic filters also are relatively common. Some ceramic filters (especially ceramic candle filters) incorporate a high-performance activated carbon core inside the ceramic filter cartridge.

When contaminant molecules in the fluid to be treated are exposed to the carbon, they’re trapped inside the pore structure of the carbon substrate. Typical particle sizes that can be removed by carbon filters range down to about 0.5 microns.

Many carbon filters also use secondary media, such as silver or KDF (Kinetic Degradation Fluxion) media, to prevent bacteria growth within the filter. Alternatively, the activated carbon itself may be impregnated with silver to provide this bacteriostatic property.
 
Carbon filters are most effective at removing sediment, chlorine and its byproducts, volatile organic compounds (VOCs), pesticides, taste and odor from water. Carbon filters with pore sizes of less than one micron can remove most pathogenic bacteria, protozoa and cysts. They are not effective at removing minerals, salts, heavy metals and dissolved inorganic compounds.

Water filters come in many sizes and shapes, depending on their intended use. There are very small, portable filters commonly used by backpackers and campers, gravity filters in various sizes, pitcher filters, faucet filters, counter-top filters, under-counter filters, shower filters, and whole-house filters. 

Ion Exchange

The ion exchange process percolates water through bead-like spherical resin materials (ion exchange resins). As the water passes through the resin layer, ions in the water are exchanged for ions on the beads. The most common ion exchange process is used for “softening” hard water that contains high levels of calcium and magnesium.  The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the "softened" water.

Depending upon the resin used, the process can remove nitrates, sulfates, fluoride, iron, magnesium, calcium, manganese and dissolved inorganics (minerals and metals). But it does not effectively remove microorganisms.

Ultraviolet Disinfection

Ultraviolet systems (UV) expose supply water to ultraviolet radiation which kills pathogenic bacteria and viruses (although generally not cysts). UV systems do not remove suspended particles or dissolved solids. 

Since UV is not a physical filter, suspended particles (or turbidity) in the water can “shade” pathogens from the direct rays from the UV source, and “live” bacteria and virus could pass through the system. For this reason good UV systems usually have pre- and final filtration.

Reverse Osmosis

Reverse osmosis is a water purification technology that uses a semipermeable membrane. In the normal osmosis process, when two solutions with differing concentrations are separated by a semi-permeable membrane, the solution with the higher concentration will infiltrate the solution with the lower concentration. In reverse osmosis, the process may be reversed by applying pressure to the side that holds the more concentrated solution, forcing it through a membrane that will remove the impurities from the water.

Such systems typically include a number of steps:
    • a sediment filter to trap particles, including rust and calcium carbonate
    • optionally, a second sediment filter with smaller pores
    • an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade thin film composite membrane reverse osmosis membranes
    • a reverse osmosis semipermeable membrane
    • optionally, a second carbon filter to capture those chemicals not removed by the reverse osmosis membrane
    • optionally an ultraviolet lamp for sterilizing any microbes that may escape filtering by the reverse osmosis membrane

Depending upon the quality of the semipermeable membrane, a reverse osmosis system can remove 90-99% of nearly all contaminants, including total dissolved solids, lead and other toxic heavy metals, pesticides, sulfate, calcium, magnesium, sodium, potassium, manganese, aluminum, chloride, nitrate, fluoride, boron, radium, most microorganisms and most dissolved organic chemicals.

However, reverse osmosis systems normally are not “water efficient”, and wastewater rejected by the system may be as much as 10 to 20 times the amount of usable water recovered. And because it lacks oxygen and minerals, it has a flat taste.



Distillation

Distillation is probably the oldest method of water purification. Water is first heated to boiling. Then the water vapor rises to a condenser where cooling water lowers the temperature so the vapor is condensed, collected and stored.

Most contaminants stay behind in the liquid phase vessel. However there can sometimes be what is called carry-overs found in the distilled water. Organics such as herbicides and pesticides, with boiling points lower than 100°C, cannot be removed efficiently and can actually become concentrated in the product water. Another disadvantage of distillation is cost. Distillation requires large amounts of energy and water and is very slow to produce clean water.

Distilled water also can be very acidic (low pH). And because it lacks oxygen and minerals, it has a flat taste.