Reverse Osmosis (RO)

Water pressure is used to force water molecules through a membrane that has extremely tiny pores, leaving the larger contaminants behind. Purified water is collected from the "clean" side of the membrane, and water containing the concentrated contaminants is flushed down the drain from the "contaminated" side.  The average RO system is a unit consisting of a sediment/chlorine pre filter, the reverse-osmosis membrane, a water storage tank, and an activated-carbon post filter.  They cost from about $150 to over $1,500 for point of use systems.

The advantages of Reverse Osmosis include:
1. Reverse osmosis significantly reduces salt, most other inorganic
material present in the water, and some organic compounds. With a quality carbon filter to remove any organic materials that get through the filter, the purity of the treated water approaches that produced by distillation.

2. Microscopic parasites (including viruses) are usually removed by properly
functioning RO units, but any defect in the membrane would allow these
organisms to flow undetected into the "filtered" water - they are not
recommended for use on biologically unsafe water. 

3. Though slower than a carbon or sediment water filter, RO systems can typicallypurify more water per day than distillers and are less expensive to operate and maintain.

4. Reverse Osmosis systems also do not use electricity, although because they require relatively high water pressure to operate, they may not work well in some emergency situations.

Water Softeners and Deionizers

Water softeners operate on the ion exchange process (specifically a cation exchange process where + ions are exchanged).  In this process, water passes through a media bed, usually sulfonated polystyrene beads. The beads are supersaturated with sodium (a positive ion). The ion exchange process takes place as hard water passes through the softening material. The hardness minerals (positively charged Calcium and Magnesium ions) attach themselves to the resin beads while sodium on the resin beads is released simultaneously into the water.  When the resin becomes saturated with calcium and magnesium, it must be recharged. The recharging is done by passing a concentrated salt (brine) solution through the resin. The concentrated sodium replaces the trapped calcium and magnesium ions which are discharged in the waste water. Softened water is not recommended for watering plants, lawns, and gardens due to its elevated sodium content.

Several factors govern the efficiency of a cationic softener:

• Type & quality of resin used;
• Amount of salt per cubic foot of resin for regeneration;
• Brine concentration in the resin bed during regeneration;
• Brine flow rate through the resin bed (contact time) during regeneration;
• Raw water hardness; 
• Raw water temperature - softeners perform better at higher temperatures; and
• Optimal flow rate of hard water through the resin bed.

Although not commonly used, potassium chloride can be used to create the salt brine for softeners designed to use KCl. In that case potassium rather than sodium is exchanged with calcium and magnesium. Before selecting an ion exchange water softener, test water for hardness and iron content. When selecting a water softener, the regeneration control system, the hardness removal capacity, and the iron limitations are three important elements to consider. 

Water Deionizers use both Cation and Anion Exchange to exchange both positive and negative ions with H+ or OH- ions respectively, leading to completely demineralized water.  Deionizers do not remove uncharged compounds from water, and are often used in the final purification stages of producing completely pure water for medical, research, and industrial needs.

A potential problem with deionizers is that colonies of microorganisms can become established and proliferate on the nutrient-rich surfaces of the resin. When not regularly sanitized or regenerated, ion-exchange resins can contaminate drinking water with bacteria.

Ozonation

The formation of oxygen into ozone occurs with the use of energy. This process is carried out by an electric discharge field as in the CD-type ozone generators (corona discharge simulation of the lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultra-violet rays from the sun). In addition to these commercial methods, ozone may also be made through electrolytic and chemical reactions.

Ozone is a naturally occurring component of fresh air. It can be produced by the ultraviolet rays of the sun reacting with the Earth's upper atmosphere (which creates a protective ozone layer), by lightning, or it can be created artificially with an ozone generator. 

The ozone molecule contains three oxygen atoms whereas the normal oxygen molecule contains only two.  Ozone is a very reactive and unstable gas with a short half-life before it reverts back to oxygen. Ozone is the most powerful and rapid acting oxidizer man can produce, and will oxidize all bacteria, mold and yeast spores, organic material and viruses given sufficient exposure.  

The advantages of using Ozone include:

• Ozone is primarily a disinfectant that effectively kills biological contaminants.  
• Ozone also oxidizes and precipitates iron, sulfur, and manganese so they can be filtered out of solution.  
• Ozone will oxidize and break down many organic chemicals including many that cause odor and taste problems.
• Ozonation produces no taste or odor in the water. 
• Since ozone is made of oxygen and reverts to pure oxygen, it vanishes without trace once it has been used.  In the home, this does not matter much, but when water companies use ozone to disinfect the water there is no residual disinfectant, so chlorine or another disinfectant must be added to minimize microbial growth during storage and distribution.

The disadvantages of using Ozone include:

• Ozone treatment can create undesirable byproducts that can be harmful to health if they are not controlled (e.g., formaldehyde and bromate).  
• The process of creating ozone in the home requires electricity.  In an emergency with loss of power, this treatment will not work.
• Ozone is not effective at removing dissolved minerals and salts.

Caution - The effectiveness of the process is dependent, on good mixing of ozone with the water, and ozone does not dissolve particularly well, so a well designed system that exposes all the water to the ozone is important.

In the home, ozone is often combined with activated carbon filtration to achieve a more complete water treatment.