Boiling:
In an emergency, boiling is the best way to purify water that is unsafe because of the presence of protozoan parasites or bacteria.

If the water is cloudy, it should be filtered before boiling.  Filters designed for use when camping, coffee filters, towels (paper or cotton), cheesecloth, or a cotton plug in a funnel are effective ways to filter cloudy water.

Place the water in a clean container and bring it to a full boil and continue boiling for at least 3 minutes (covering the container will help reduce evaporation).  If you are more than 5,000 feet above sea level, you must increase the boiling time to at least 5 minutes (plus about a minute for every additional 1,000 feet).  Boiled water should be kept covered while cooling.  From Drinking Water for Emergency Use (pdf file).  You can also look at recommendations of the EPA.

The advantages of Boiling Water include:
 Pathogens that might be lurking in your water will be killed if the water is boiled
    long enough. 
 Boiling will also drive out some of the Volatile Organic Compounds (VOCs) that
    might also be in the water. This method works well to make water that is
    contaminated with living organisms safe to drink, but because of the
    inconvenience, boiling is not routinely used to purify drinking water except
    in emergencies.

The disadvantages of Boiling Water include:
Boiling should not be used when toxic metals, chemicals (lead, mercury, asbestos,
    pesticides, solvents, etc.), or nitrates have contaminated the water. 
Boiling may concentrate any harmful contaminants that do not vaporize as the
    relatively pure water vapor boils off.  
Energy is needed to boil the water

 Distillation:
In many ways, distillation is the reverse of boiling.  To remove impurities from water by distillation, the water is usually boiled in a chamber causing water to vaporize, and the pure (or mostly pure) steam leaves the non volatile contaminants behind. The steam moves to a different part of the unit and is cooled until it condenses back into liquid water.  The resulting distillate drips into a storage container. 

Salts, sediment, metals - anything that won't boil or evaporate - remain in the distiller and must be removed.  Volatile organic compounds (VOCs) are a good example of a contaminant that will evaporate and condense with the water vapor. A vapor trap, carbon filter, or other device must be used along with a distiller to ensure the more complete removal of contaminants.

The advantages of Distillation include:
 A good distillation unit
    produces very pure  water. This is one of
    the few practical ways to remove nitrates,
    chloride, and other salts that carbon
    filtration can not remove. 
 Distillation also removes pathogens in the
    water, mostly by killing and leaving them
    behind when the water vapor evaporates.
    If the water is boiled, or heated just short
    of boiling, pathogens would also be killed.
 As long as the distiller is kept clean and is
    working properly the high quality of treated
    water will be very consistent regardless of
    the incoming water - no drop in quality over
    time.
 No filter cartridges to replace, unless a carbon filter is used to remove volatile
    organic compounds.

The disadvantages of Distillation include:
Distillation takes time to purify the water, It can take two to five hours to make a
    gallon of distilled water.
Distillers uses electricity all the time the unit is operating
Distillers requires periodic cleaning of the boiler, condensation compartment, and
    storage tank.  
Countertop Distillation is one of the more expensive home water treatment
    methods, using $0.25 to $0.35 of electrical energy per gallon of distilled water
    produced - depending on local electricity costs.  The cost of ownership is high
    because you not only have the initial cost of the distillation unit to consider, but you
    also must pay for the electrical energy for each gallon of water produced. If it cost
    you $0.25 to distill each gallon, and you purified 10 gallons per week, you would
    pay $130 for your 520 gallons of distilled water each year.
Most home distillation units require electricity, and will not function in an
    emergency situation when electrical power is not available.

You might want to check  NSF International to see if the distillation system you are interested in purchasing is certified.

I have had a number of questions asking if distilled water (or water with most of the ions removed by reverse osmosis or deionization) are either bad for a person's health or beneficial for health relative to purified water that still contains ions (usually calcium and magnesium).  Click here to view my response to that question.

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

The disadvantages of Reverse Osmosis include:
Point of Use RO units make only a few gallons of treated water a day for drinking
    or cooking.  
RO systems waste water. Two to four gallons of "waste" water are flushed down
    the drain for each gallon of filtered water produced.  
Some pesticides, solvents and other volatile organic chemicals (VOCs) are not
    completely removed by RO.  A good activated carbon post filter is recommended
    to reduce these contaminants.
Many conditions affect the RO membrane's efficiency in reducing the amount of
    contaminant in the water.  These include the contaminant concentration, chemical
    properties of the contaminants, the membrane type and condition, and
    operating conditions (like pH, water temperature, and water pressure). 
Although RO filters do not use electricity, they depend on a relatively high water
    pressure to force the water molecules through the membrane.  In an emergency
    situation where water pressure has been lost, these systems will not function.
     * However, if a high quality activated carbon filter is used for the post filter, it could
     probably be disconnected and used to siphon water through in an emergency to
     reduce many contaminants.
RO systems require maintenance.  The pre and post filters and the reverse
     osmosis membranes must be changed according to the manufacturer's
     recommendation, and the storage tank must be cleaned periodically.
Damaged membranes are not easily detected, so it is hard to tell if the system is
    functioning normally and safely.

You might want to check  NSF International to see if the Reverse Osmosis system you are interested in purchasing is certified.

A reverse-osmosis system is a good treatment option for people who have
    unacceptably high levels of dissolved inorganic contaminants in their drinking
    water which can not be removed effectively or economically by other methods.
    Water from shallow wells in agricultural areas that contains high nitrate levels is
    a good example of a situation where RO would make sense. Most people,
    however, who are using municipal water water that is subject to EPA regulations
    usually have acceptably low levels of nitrates. Maximum nitrate levels should be
    able to be determined by requesting a water quality report from your local water
    provider.

I have had a number of questions asking if distilled water are either bad for a person's health or beneficial for health relative to purified water that still contains ions (usually calcium and magnesium).  This question pertains equally well to water from a RO system., because most ions have been removed by the treatment. Click here to view my response to that question.

Water Filters:
The topic of water filters is complicated because there are so many models available (over 2,500 different models manufactured by more than 500 companies), and because there are so many types of filtration strategies and combinations of strategies used. The basic concept behind nearly all filters, however, is fairly simple. The contaminants are physically prevented from moving through the filter either by screening them out with very small pores and/or, in the case of carbon filters, by trapping them within the filter matrix by attracting them to the surface of carbon particles (the process of adsorption). 

There are two main types of filters (sediment and activated carbon), and
    sometimes they are combined into a single unit.  A third type, which will be
    considered as a separate topic, is reverse osmosis.  

You will read about micron or sub micron filtration. This is a measure of how good
    the filter is at removing particles from the water - smaller is better. A micron is a
    unit of measure - one micron is about 1/100 the diameter of a human hair. A
    filter that removes particles down to 5 microns will produce fairly clean-looking
    water, but most of the water parasites, bacteria, cryptosporidia, giardia, etc. will
    pass through the pores. A filter must trap particles one micron or smaller to be
    effective at removing cryptosporidia or giardia cysts. Viruses can not be effectively
    removed by any filtration method.  In theory, reverse osmosis will remove viruses,
    but a small flaw in the membranes would allow viruses to pass undetected
    into the 'filtered' water.
    Click here too view a figure that compares the relative size of several biological
    contaminants with the pore size of some common filters. 

  A benefit of all home filtration systems is that they are passive. That is, they
     require no electricity to filter the water, and normal home water pressure is used
     to force the water though the filter. The only routine maintenance required is
     periodic replacement of the filtration element. As long as the cost of the
     replacement filter elements is reasonable, owning a even a high-end water filter
     can be very inexpensive if you look at the long term costs and compare it
     with other solutions.

Sediment Filters - Solid Particles are Strained Out of the Water: Fiber Filters: These filters contain cellulose, rayon or some     other material spun into a mesh with small pores. If you take     a piece of cloth and pour water containing sand through it     you will get the picture. Suspended sediment (or turbidity) is     removed as water pressure forces water through tightly     wrapped fibers.  Some small organic particles that cause     disagreeable odors and taste may also be removed. These     filters come in a variety of sizes and meshes from fine to     coarse, with the lower micron rating being the finer. The finer     the filter, the more particles are trapped and the more often     the filter must be changed.  Fiber filters are often used as pre-filters to reduce the     suspended contaminants that could clog carbon or RO filters. Fiber filters will not remove contaminants that are dissolved     in the water, like chlorine, lead, mercury, trihalomethanes or     other organic compounds. Ceramic Filters: Ceramic filters are much like fiber filters     and use a process where water is forced through the pores     of a ceramic filtration media. This provides mechanical     filtration only. This type of filter can reduce asbestos     fibers,cysts (if the pores are one micron or smaller), some     bacteria (with pore sizes in the 0.2 - 0.8 micron range**) and     other particulate matter.  Ceramic filters will not remove contaminants that are     dissolved in the water, like chlorine, lead, mercury,     trihalomethanes or other organic compounds, nor will they     remove viruses. These filters may be used as a back-end to     an activated carbon filter to provide a more thorough removal     of contaminants.   ** NOTE - NSF does not certify filters for bacterial removal, and I am unaware of any independent certification process for their removal.

Activated Carbon Filters: Activated carbon (AC) is particles of carbon that have been treated to increase their surface area and increase their ability to adsorb a wide range of contaminants - activated carbon is particularly good at adsorbing organic compounds.  You will find two basic kinds of carbon filters Granular Activated Carbon (GAC) and Solid Block Activated Carbon (SBAC).  Contaminant reduction in AC filters takes place by two processes, physical removal of contaminant particles, blocking any that are too large to pass through the pores (obviously, filters with smaller pores are more effective), and a process called adsorption by which a variety of dissolved contaminants are attracted to and held (adsorbed) on the surface of the carbon particles. The characteristics of the carbon material (particle and pore size, surface area, surface chemistry, density, and hardness) influence the efficiency of adsorption.  AC is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption. One reference mentions " The equivalent surface area of 1 pound of AC ranges from 60 to 150 acres (over 3 football fields)".  Another article states, "Under a scanning electron microscope the activated carbon looks like a porous bath sponge.  This high concentration of pores within a relatively small volume produces a material with a phenomenal surface area: one tea spoon of activated carbon would exhibit a surface area equivalent to that of a football field." ( estimates of surface area vary from different sources - RJ) AC is made of tiny clusters of carbon atoms stacked upon one another. The carbon source is a variety of materials, such as peanut shells, coconut husks, or coal. The raw carbon source is slowly heated in the absence of air to produce a high carbon material. The carbon is activated by passing oxidizing gases through the material at extremely high temperatures. The activation process produces the pores that result in such high adsorptive properties.  An article about Activated Carbon states that "Activated carbon is one of the best tools which can be used to reduce risks to human health and provide an aesthetically pleasing product at reasonable cost."  The article also describes how AC works and has some of the best scanning electron microscope pictures of actual AC granules I have seen. (The information came from The Via Department of Civil and Environmental Engineering (CEE) at Virginia Tech and is no longer available) The adsorption process depends on the following factors: 1) physical properties of the AC, such as pore size distribution and surface area; 2) the chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it; 3) chemical composition and concentration of the contaminant; 4) the temperature and pH of the water; and 5) the flow rate or time exposure of water to AC.  Interesting article:  What is Activated Carbon, Adsorption, History of AC.   The effectiveness of carbon filters to reduce contaminants       is affected by the factors affecting adsorption listed above      and three additional characteristics of the filter, contact time      between the water and the carbon material, the amount of      carbon in the filter, and pore size.       The length of contact time between the water and the         carbon material, governed by the rate of water flow and the         amount/volume of activated carbon, has a significant effect         on adsorption of contaminants. More contact time results in         greater adsorption.        The amount of carbon present in a cartridge or filter         affects the amount and type of contaminant removed. Less         carbon is required to remove taste- and odor-producing         chemicals than to remove trihalomethanes.        Pore size characteristics will be discussed in greater detail         on the GAC and SBAC pages, but GAC filters contain loose         granules of activated carbon while in SBAC filters, the         activated carbon is in the form of very small particles bound         into a solid, matrix with very small pores.   Because of the filter characteristics discussed above, the most effective      Point of Use activated carbon filters are large SBAC filtration      systems, and the least effective are the small, pour-through pitcher filters. Activated carbon filter cartridges will, over time, become less effective at     reducing contaminants as the pores clog withparticles (slowing water flow)     and the adsorptive surfaces in the pores become filled with contaminants     (typically not affecting flow rate). There is often no noticeable indication     that a carbon filter is no longer removing contaminants, so it is     important to replace the cartridge according to the manufacturer's     instructions.  The overall water quality (turbidity or presence of other     contaminants) also affects the capacity of activated carbon to adsorb a     specific contaminant. It is important to note, particularly when using counter-top and faucet-mount     carbon filtration systems, that hot water should NEVER be run through     a carbon filter. I have seen warnings about possible damage to the filter     from hot water. Perhaps more importantly, hot water will tend to release     trapped contaminants into the water flow potentially making the water     coming out of the filter more contaminated than the water going in.     Granular Activated Carbon (GAC):         In this type of filter, water flows through a bed of loose activated carbon         granules which trap some particulate matter and remove some chlorine,         organic contaminants, and undesirable tastes and odors.  The three main         problems associated with GAC filters are: channeling, dumping, and         an inherently large pore size.  Most of the disadvantages discussed         below are not the fault of the activated carbon filtration media, rather, the         problem is the design of the filters and the use of loose granules of         activated carbon.        The advantages of GAC filters include:        Simple GAC filters are primarily used for aesthetic water treatment,            since they can reduce chlorine and particulate matter as well as improve            the taste and odor of the water.        Loose granules of carbon do not restrict the water flow to the extent of            Solid Block Activated Carbon (SBAC) filters.  This enables them to            be used in situations, like whole house filters, where maintaining a good            water flow rate and pressure is important.        Simple, economical maintenance.  Typically an inexpensivefilter            cartridge needs to be changed every few months to a year, depending            on water use and the manufacturer's recommendation.          GAC filters do not require electricity, nor do they waste water.        Many dissolved minerals are not removed by activated carbon.  In the            case of calcium, magnesium, potassium, and other beneficial minerals,            the taste of the water can be improved and some (usually small) nutrient            value can be gained from the water. The bottom line is that GAC filters are effective and valuable water treatment devices, but their limitations always need to be considered.  A uniform flow rate, not to exceed the manufacture's specifications, needs to be maintained for optimal performance, and the filter cartridge must be changed after treating the number of gallons the filter is rated for.        The disadvantages of GAC filters include:        Water flowing through the filter is able to "channel" around the carbon             granules and avoid filtration.  Water seeks the path of least resistance.              When it flows through a bed of loose carbon granules, it can carve a             channel where it can flow freely with little resistance.  Water flowing             through the channel does not come in contact with the filtration            medium.  The water continues to flow, however, so you do not realize             that your filter has failed - you get water, but it is not completely filtered.        Pockets of contaminated water can form in a loose bed of carbon             granules.  With changes in water pressure and flow rates, these            pockets can collapse, "dumping" the contaminated water through the            filter into the "filtered" flow.        Since the carbon granules are fairly large (0.1mm to 1mm in  one             popular pitcher filter), the effective pore size of the filter is relatively             large (20 - 30 microns or larger).  GAC filters, by themselves, can not             bacteria.        As described above, hot water should NEVER be run through a            carbon filter        Also, if you think of a bed of charcoal that traps an occasional             bacterium, picks up a bit of organic material, and removes the chlorine             from the water, you can see how these filters might become breeding             grounds for the bacteria they trap. You will see warnings about GAC             filters suggesting you run water through them for a few minutes each             morning to flush out any bacteria. If you are considering purchasing a             GAC filter be sure to ask the representative about what they            recommend you do about this potential problem (I was told by one             sales person, that if the filter was not going to be used for a few days,             I should remove the filtration element, place it in a plastic bag, and             store it in the refrigerator until I returned).        Unless the filter plugs up or you notice an odor in the "filtered water",             it may be difficult to know when the filter has become saturated with            contaminants and ineffective.  That is why it is necessary to change            filter cartridges according to the manufacturer's recommendation. Solid Block Activated Carbon (SBAC):         Activated carbon is the primary raw material in solid carbon block filters;         but instead of carbon granules comprising the filtration medium, the          carbon has been specially treated, compressed, and bonded to form a          uniform matrix.  The effective pore size can be very small (0.5 - 1          micron).  SBAC, like all filter cartridges, eventually become plugged or          saturated by contaminants and must be changed according to          manufacturer's specifications.  Depending on the manufacturer, the filters          can be designed to better reduce specific contaminants like arsenic,          MTBE, etc. (an example)        The advantages of SBAC filters include:        Provide a larger surface area for adsorption to take place than Granular            Activated Carbon (GAC) filters for better contaminant reduction.         Provide a longer contact time with the activated carbon for more             complete contaminant reduction.        Provide a small pore size to physically trap particulates. If the pore size             is small enough, around 0.5 micron or smaller, bacteria that become             trapped in the pores do not have enough room to multiply, eliminating a             problem common to GAC filters.         Completely eliminate the channeling and dumping problems            associated with GAC filters.        SBAC filters are useful in emergency situations where water pressure             and electricity might be lost.  They do not require electricity to be             completely effective, and water can even be siphoned through them.        SBAC filters do not waste water like reverse osmosis.        Many dissolved minerals are not removed by activated carbon.  In the             case of calcium, magnesium, potassium, and other beneficial minerals,             the taste of the water can be improved and some (usually small) nutrient             value can be gained from the water.        Simple, economical maintenance.  Typically an inexpensive  filter             cartridge needs to be changed every few months to a  year, depending             on water use and the manufacturer's recommendation. This combination of features provides the potential for greater adsorption     of many different chemicals (pesticides, herbicides, chlorine, chlorine      byproducts, etc.) and greater particulate filtration of parasitic cysts,      asbestos, etc. than many other purification process available. By using     other specialized materials along with specially prepared activated carbon,      customized SBAC filters can be produced for specific applications or to      achieve greater capacity  ratings for certain contaminants like lead,     mercury, arsenic, etc.        The disadvantages of SBAC filters include:        SBAC filters, like all activated carbon filters, do not naturally reduce the             levels of soluble salts (including nitrates), fluoride, and some other             potentially harmful minerals like arsenic (unless specially designed) and             cadmium.  If these contaminants are present in your water, reverse             osmosis would usually be the most economical alternative followed by             distillation.        As described above, hot water should NEVER be run through a            carbon filter        As SBAC filters remove contaminants from the water they gradually            lose effectiveness until they are no longer able to adsorb the            contaminants.  There is no easy way to determine when a filter is            nearing the end of its effective life except that the 'filtered' water            eventually begins to taste and smell like the unfiltered water.  The            manufacturer's guidelines for changing filter cartridges should always            be followed. You might want to check  NSF International to see if the SBAC filtration system you are interested in purchasing is certified. An example of a certified list of contaminants significantly reduced by a high-end SBAC filtration system (** at 200 percent of capacity - that's after filtering twice the rated volume of the filter**):  For Standard 042 - aesthetic    Chlorine Reduction, Class I     (actual chlorine reduction 99.9%)    Particulate Reduction, Class I     (actual particulate reduction down to 0.5 micron)    Chloramine Reduction     (actual chloramine reduction greater than 96%)    Taste and Odor Reduction  For Standard 053 - contaminants of health concern and measured percent reduction    Asbestos Reduction >99%    Chlordane Reduction >99.8%    Cyst Reduction 99.9%    Lead Reduction >98%    Mercury Reduction >99.3% (pH 8.5) >91.4 (pH 6.5)    MTBE Reduction 96.6%    PCB Reduction >98%    Toxaphene Reduction >93%    TTHM Reduction (Trihalomethanes) >99.8%    Turbidity Reduction >99%    VOC Reduction (volatile organic chemical) Reduction - you will see a    long list of specific VOCs (individual percent reduction for the various    VOCs can be found on the product certification sheet below, but    most are 98-99% or more). A few SBAC filtration systems have been certified for arsenic reduction:

Bottled Water:

Bottled water is simply water from some source that a company (or in the case of water vending machines, the consumer) has placed in a bottle for resale.  Bottled water can have minimal (or no) processing - as in natural spring or mineral water, or it can be completely filtered and demineralized to nearly pure water and then have minerals added back in to make it taste better.

But, how can you be sure the water in the bottle you purchased is any cleaner or
    safer than your tap water?  Just like with any of the other water treatment solutions, 
    you will find reputable companies that provide a safe, quality product, and you will 
    find companies that fill their bottles  using the local, municipal water.  Each 
    company must be evaluated individually (A Review of Bottled Water and a NRDC 
    Discussion of Bottled Water).  There are 2 regulatory organizations, 1 trade 
    association, and 1 independent certification organization which contribute to 
    insuring the safety and quality of bottled water.

According to the University of Illinois at Urbana-Champaign, College of Agriculture, Cooperative Extension Service, bottled water is regulated at the federal level, and in some cases, at the state level. Bottled water suppliers who are members of the International Bottled Water Association (IBWA) regulate themselves and are given unannounced inspections by NSF International.  Bottled water companies can also have their product certified by NSF International.

Federal standards - Bottled water is regulated by the Food and Drug Administration (FDA), which requires manufacturers to submit samples regularly for testing. In the United States, the concentration of contaminants found in bottled water must be less than the "allowable levels" set by the FDA which are essentially the same as the maximum contaminant levels that the EPA requires municipal water supplies to meet.

State standards - Individual states must enforce the federal bottled water regulations, but they can also pass stricter standards.

International Bottled Water Association. The bottled water industry regulates itself through the International Bottled Water Association (IBWA). Bottled water FAQs. The IBWA sets manufacturing requirements, which help to ensure that bottlers meet FDA health standards. Bottled water producers who are members of IBWA are inspected annually by an independent laboratory, the National Sanitation Foundation. Through unannounced inspections, members are evaluated on compliance with the IBWA's performance requirements and FDA Quality Standards.
Not all bottled water manufacturers are members of the IBWA. Of the 475 bottling plants in the United States (in 1994), 250 are members. The label may indicate whether a bottled water comes from a member company.

You might want to check  NSF International to see if the bottled water you are interested in purchasing is certified.  The NSF Bottled Water Certification Program is an annual, voluntary certification process that includes both extensive product evaluations as well as on-site audits of bottling facilities.  The NSF testing program provides for annual unannounced plant inspections covering every aspect of a bottler's operation, from the source of the water, through the disinfection and treatment process, and including the container closure process. The company also performs extensive product testing for over 160 chemical, inorganic, radiological, and microbiological contaminants.  An interesting article from Scientific American magazine on bottled water.

The advantages of Bottled Water include:
 An emergency source of water in the event your primary water source fails or
    becomes contaminated.
 A convenient source of usually safe water for drinking outside of the home.
 Bottled water, since it does not contain chlorine, and may contain a mix of minerals
    to enhance flavor, may taste better than untreated tap water.
 Most bottled water will contain fewer contaminants than untreated tap water.

The bottled water industry would like the public to believe that bottled water is purer
    and safer than tap water and water produced by other treatment methods.  The 
    actual quality of bottled water depends on the bottling company, and most is    
    probably of good quality.  High quality home water treatment methods (filtration, 
    reverse osmosis, and distillation) can usually produce water of equal or better 
    quality, however.

The disadvantages of Bottled Water include:
Cost - Bottled water costs anywhere from $0.25 a gallon for water from a vending     machine to $0.50 for generic water jugs you purchase in a store to $1.00 - $1.50 
    or more for water delivered to your home. 
    If you purchase 10 gallons of bottled water a week, you will spend $260 (at 
       $.50/gal), $520 (at $1.00/gal), $780 (at $1.50/gal), or more every year.
     Many of us are willing to pay extraordinary prices for the availability and 
       convenience of of certain products. That 16 ounce bottle of "designer water" 
       you just purchased at the gas station for $0.69 costs $5.52 per gallon, probably 
       more than twice as much per gallon as your gas! Purchasing one 16 oz bottle 
       per day (at $0.69 per bottle) will cost you $248 per year - that's for a total of 45 
       gallons. If you take a minute to fill your own well-cleaned bottle with filtered or 
       distilled water, however,  a 16 oz bottle of water would cost perhaps1 - 2 cents a 
       day, or less than $7.30 per year.
Convenience - Using bottled water requires moving and storing jugs or bottles of 
    water. Water weighs about eight pounds per gallon, or about 40 pounds per five 
    gallon bottle.  Failure of a small valve can lead to a big mess, as I discovered after 
    arriving home one evening after a long day at work to find 3 gallons of water 
    soaking into the rug.
Environmental Impact - Producing bottles uses resources, and unless they are 
    reused or recycled, they cause a waste disposal problem.  Recycle or reuse the 
    empty bottles, if at all possible.

Keep it Clean - According to the University of Illinois at Urbana-Champaign,
    College of Agriculture, Cooperative Extension Service,
   "If you choose bottled water after careful investigation, keep in mind that all of 
   your hard work will go to waste if you aren't careful about keeping your bottled 
   water clean. You have to be faithful in maintaining the hygiene of your bottled 
   water, or you may increase your exposure to bacteria. Bacteria grow best in
   warm, moist areas. The wet, warm, threaded cap of an unrefrigerated bottle of
   water is a perfect place for bacteria to grow; they will begin to grow as soon as
   you break the seal. If ingested, these bacteria can cause gastrointestinal
   problems and other health risks. The key is to maintain the cleanliness of your
   bottles and store them properly. Follow these hints: 
   1) Store the bottle in a refrigerator at a temperature above freezing but less than
        50 degrees Fahrenheit. 
   2) Wipe the seal with a clean cloth after each use. 
   3) Avoid any type of buildup in the bottle cap. 
   4) If your bottle is refillable, make sure it is well-cleaned and rinsed before 
       refilling. If possible, recycle the old bottle and obtain a fresh, sterile, sealed
       bottle."    
5 gallon water dispensers must also be kept meticulously clean to prevent
    bacteria from growing in the reservoir area and bubbling into the bottle.  Safety &
    Health Services cleaning recommendations. (pdf file)

The advantages of using UV include:
No known toxic or significant nontoxic byproducts introduced
Removes some organic contaminants
Leaves no smell or taste in the treated water
Requires very little contact time (seconds versus minutes for chemical disinfection)
Improves the taste of water because some organic contaminants and nuisance
    microorganisms are destroyed
Many pathogenic microorganisms are killed or rendered inactive.
Does not affect minerals in water

The disadvantages of using UV include:
UV radiation is not suitable for water with high levels of suspended solids, turbidity, 
    color, or soluble organic matter.  These materials can react with UV radiation, and 
    reduce disinfection performance. Turbidity makes it difficult for radiation to 
    penetrate water and pathogens can be 'shadowed', protecting them from the light. 
UV light is not effective against any non-living contaminant, lead, asbestos, many 
    organic chemicals, chlorine, etc.  
Tough cryptosporidia cysts are fairly resistant to UV light. 
Requires electricity to operate.  In an emergency situation when the power is out, 
    the purification will not work.

UV is typically used as a final purification stage on some filtration systems.  If you are concerned about removing contaminants in addition to bacteria and viruses, you would still need to use a quality carbon filter or reverse osmosis system in addition to the UV system.

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. 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.  More information on Hard Water and Softening.

The advantages of water softeners include:
The nuisance factor of hard water is reduced.
some other other cations like barium,radium and iron may be reduced depending
    on the manufacturer's specifications.

The disadvantages of water softeners include:
The process of regenerating the ion exchange bed dumps salt water into the
    environment.
The elevated sodium concentration of most softened water can affect the taste
    and may not be good for people on low sodium diets, although sodium
    concentrations are typically quite low relative to sodium levels in most food.
Cation exchange does not reduce the level of anions (like nitrates), or biological
    contaminants (bacteria, viruses, cysts) ; nor does the process reduce the levels of
    most organic compounds.
Typically, approximately 50 gallons of rinse water per cubic foot of resin is
    required to totally remove hardness and excess salt from the resin after
    regeneration.

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.

KDF "Filters":
KDF filters employ a matrix (generally small granules) of a zinc/copper alloy, which allegedly eliminates contaminants from water by utilizing electrochemical oxidation reduction. Chemical properties of KDF include its alleged ability to:
     Remove chlorine (actually changes free chlorine to a less active form)
     Kill algae and fungi
     Control bacterial growth in the filter
     Remove hydrogen sulfide, iron, lead, cadmium, aluminum, mercury, arsenic and
       other inorganic compounds
     Partially reduce hardness

Zinc and copper are the preferred metals used in the KDF alloy since both are relatively good reducing agents with respect to common inorganic contaminants (such as chlorine), and both can be tolerated in solution in moderate concentrations without adverse side effects.

The advantages of KDF filters include:
KDF is the only filter medium I am aware of that allegedly removes contaminants 
    from running hot water (unlike carbon filters where hot water can release trapped 
    contaminants into the water stream).  This makes them ideal for use in the shower.
The filters change the free chlorine some people are allergic to into a form (zinc c
    chloride) that is much more easily tolerated.

The disadvantages of KDF filters include:
KDF filters do not, by themselves, remove organic chemicals (pesticides, 
    disinfection byproducts, MTBE, etc.), or parasitic cysts (giardia and 
    cryptosporidium).  If you are concerned about removing any of these contaminants, 
    other strategies will be needed in addition to the KDF media.
KDF filters need to be backwashed periodically with hot water to remove the 
    insoluble contaminants.  This method wastes many gallons of hot water and there 
    is no way to prevent dislodged pollutants from coming out later with the supposedly 
    filtered water.

I just became aware of a web site that casts doubt on some the contaminant reduction claims made for KDF (or at least on the chemical processes put forward to explain the reduction).  It has been many years since I took a chemistry class, so I need to do more homework on the subject - RJ.

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.

Activated Alumina:
Activated Alumina is a granulated form of aluminum oxide.  In this process, water containing the contaminant is passed through a cartridge or canister of activated alumina which adsorbs the contaminant.  The cartridge of activated alumina must be replaced periodically.  Activated alumina devices can accumulate bacteria, so treated water may have higher bacteria counts than raw water.

The advantages of Activated Alumina filters include:
An effective way to reduce levels of fluoride, arsenic, and selenium

The disadvantages of Activated Alumina filters include:
The use of other treatment methods would be necessary to reduce levels of other
    contaminants of health concern

'Altered' Water: No discussion about water treatment would be complete these days without mention of what I call "altered" water - water that has been treated in some way to purportedly "enhance" it's health effects.  These treatments fall under a wide range of categories, including: oxygenation; various 'catalytic', vortex, magnetic, & photonic treatments; microclusters, super-ionization, etc.

Regardless of any alleged  health benefits, these products are extremely effective at separating customers from their money - I have seen some of this bottled water sell for more than $20 a gallon plus shipping (that's about 3,000 times more than the cost of tap water)!  In my estimation, the fact that these products appear to flourish and multiply is a sad testament to the inability of many consumers to effectively evaluate claims about a product's effectiveness.

Be aware of the following when evaluating these products:
None of these products is regulated by the government (over and above
    standard rules to ensure safety of bottled water - dis