A filter is any substance, such as cloth, paper, porous porcelain, or a layer of charcoal or sand through which liquid is passed to remove suspended impurities or to recover solids.
There are two types of filters used for water filtration:
- Sediment filters trap and contain large particles and heavy sediments (silt, sand, etc).
- Activated carbon filters come in a variety of shapes and models from ion filters enclosed in stainless steel columns to stubby plastic domes. There are in-line counter top and faucet attached units. Activated carbon itself is found in three forms: powdered, granulated and solid. Activated carbon filtration is used for polishing water, that is, removing odor and taste causing contaminants. Activated carbon filters are effective at removing chloroform, chlorine, some pesticides and organic chemicals.
Soluble materials such as salts and metals in ionic forms are not removed by filtration but rather by semi-permeable separation such as reverse osmosis.
Osmosis is the passage of a solvent (water) through a semi-permeable membrane. A semi-permeable membrane is a membrane which traps the solute (material dissolved in the water) and allows water to pass through giving clean water.
When a pressure greater than the osmotic pressure is applied to an aqueous solution, water molecules can be driven through a semi-permeable membrane. The solution is concentrated and pure water is obtained on the other side of the membrane. By utilizing tap water under its own pressure, the process of reverse osmosis has the ability to separate pure water from pollutants and contaminants.
Reverse osmosis has the ability to remove:
- Pyrogen (a substance resulting from dead bacteria which can cause fever)
- Amoebic cysts
- Nuclear contaminants
- Sodium based salts
To effectively remove low molecular weight hydrocarbons such as trihalomethanes, it is recommended that a granulated activated carbon filter be used in combination with a reverse osmosis system.
Water softeners work on the principle of ion exchange with salt free ions (sodium and potassium) substituting for calcium and magnesium ions. Once the calcium and magnesium are removed, the water is no longer hard.
Water softeners, while neutralizing the hardness concentration of magnesium and calcium, increase the salt content of water. For every calcium and magnesium ion removed, two salt ions are added.
It is better to use sodium chloride as a regenerate rather than potassium chloride due to the high cost associated with potassium, but any softener can use either sodium or potassium as a regenerate.
All softened water is not recommended for drinking, therefore a secondary removal system such as reverse osmosis should be used if taste or odor are still a problem.
Water softeners contain a resin that will remove soluble iron on an ion-exchange basis, the same way as calcium and magnesium are removed in water softening. Depending on the size of the unit, removal up to 10 ppm (parts per million) of soluble iron can be removed.
Iron is not considered a health problem, but it can be very objectionable if present in amounts of greater than 0.3 ppm.
Water Hardness Problems
Pipe scaling occurs when hard water deposits a rock-like scale, makes pipes leak and restricts water flow.
In water heaters, heated water forms a rock-like scale faster. The scale builds up on the heating element, reducing efficiency and shortening the life span of the water heater. Softening water can save up to 22% in water heater costs (electricity and maintenance).
Hard Water Scale
Hard water scale clogs and erodes working parts of plumbing fixtures. It also forms on skin and dulls the complexion while contributing to hair problems. Soap curds with hard water increasing the use and cost of soap and household cleaners. The increased soap usage also strains septic and municipal waste systems. Hard water also dulls colors in clothes and makes them stiff and scratchy.
Soluble iron can leave stains on clothing, kitchen sinks, dishwashers, bathroom sinks, tubs, showers and toilets.