Methods to Purify Water

If water is collected from anything besides safe tap water, the basic process should be to:


  • This step is important first because disinfectants are less effective if the water is cloudy.
  • Large particles all the way down to viruses as well as many minerals, heavy metals, and VOCs depending on the quality and composition of the filter.
  • A simple activated carbon filter will get the water odorless and colorless.

      2. Boil

  • This will kill all living organisms.   They are the primary source of bad water.

      3. Disinfect

  • Chemical disinfection will ensure the water stays pure longer.

Water Purification

Wikipedia: Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for the most  critical of it's intended uses, usually for human consumption.   Substances that are removed during  the process of drinking water treatment  include suspended solids, bacteria, algae, fungi, viruses, minerals, molecules, other chemical pollutants such as fertilizers and volatile organic chemicals or (VOCs).   Basically anything that can float, suspended, or dissolve in water may be present.  There is even a category called "emerging contaminants".

Process for drinking water treatment

A combination selected from the following processes is used for municipal drinking water treatment worldwide.

Pre-chlorination -for algae control and arresting any biological growth.

Aeration - along with pre-chlorination for removal of dissolved iron and manganese.

Coagulation - for flocculation which removes insoluble particles (colloids) called flocs or flakes.

Coagulant Aids - also known as polyelectrolytes improve coagulation for thicker flocculent formation.

Sedimentation - for removal of suspended solids trapped in the flocculent.

Filtration - removing particles from water.

Desalinization - removing salt from water.

Disinfection - for killing bacteria.

There is no unique solution (selection of processes) for any type of water.   Also, it is difficult to standardize the solution in the form of processes for water from different sources.  Treatability studies for each source of water in different seasons need to be carried out to arrive at the most appropriate processes. 

Technologies for potable water treatment are well developed, and generalized designs are available that are used by many water utilities (public or private).  In addition, a number of private companies provide patented technological solutions.  Automation of water and waste water treatment  is common in the developed world.  Capital costs, operating costs available, quality monitoring technologies, locally available skills typically dictate the level of automation adopted.

Processes for personal drinking water treatment can include many of the same used for commercially produced drinking water.  It largely depends on the source of the water specific for that location.   Water from a bayou in Louisiana is much different than water from a mountain stream in Colorado.  Soil, plant material, animal waste, human waste,  introduction from farming or industrial sites, and rain composition is different everywhere.  Temperature and acidity also affects what will grow and suspend in the water.  Test and know your available water sources.  Develop a process that suits the water you will be using.  Have a way to test your results to verify the process.

  A person can only survive three days  without water.

A loss of 10-22% body weight as water is fatal.   The amount lost through urine, water vapor from the lungs, and perspiration averages 2.5 liters per day.   The minimum  recommended  amount is one gallon  per day of drinking water.   If it is very hot or very cold you should double that amount.   Children, nursing mothers and ill people will also require more than the normal amount.  Signs of dehydration include thirst, sleepiness, apathy, nausea, emotional instability, labored breathing and dizziness, delirium and finally death. 

How much should you store?   The government says three days.   Red Cross recommends two weeks.  You should store as much as you can depending on your situation.   Don't forget the pets.  They need it as much as you do.   I've been through several hurricanes and was trapped in my home for a week without power.   Blocked streets prevented me from driving to buy anything including water.   Fortunately the water supply remained safe.   How much you can keep depends on how much room  you have and how many will be using the water.  

You also have to consider a non-potable water supply for washing dishes, clothes, yourself, and flushing toilets.

How long will it last.   Bottled water should last two years if kept in a cool dark place.   Heating or freezing will cause chemicals to release from the container.   If the water is cloudy or has a chemical taste, don't drink it.   I personally do not use  bottled water because 40% of them are tap water from somewhere else.   Read the label.   Get BPA free containers and create your own purified water.   Keep some stored and have other containers that can be filled if a crisis occurs.   Have several methods available to purify water if the tap water becomes unsafe or non-existent . 


The best method to get the most pure drinking water in any situation is a filter.   Ideally you want one that works on gravity in case pressure is not available.  If your regular tap water is not ideal then a whole house or point of use filter may be needed in normal, everyday situations.   You want multi-media filters to maximize the substances that are filtered and lower the cost of replacements.  Calculate the cost per gallon and choose a filter that suits your needs and budget.  This is the most important prepping/survival item you will need to sustain your life.

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It's best to filter first especially to remove turbidity which improves the chemicals  effectiveness.  It is also useful to extend shelf-life of water you have purified.  This is called secondary disinfection .

Boiling:  Boiling kills most types of disease causing organisms and is the preferred method recommended by the government. Use a non-reactive pot like stainless steel, glass, or ceramic coated steel.   Boil the water for 1-5 minutes, then let it cool.  Make sure it is a full rolling boil.  If you are more than one mile above sea level boil 4 times longer.  Boiling effectively kills organic organisms and does not remove many other harmful contaminants that may be present.  Boiling too long will concentrate the remaining contaminants.  This a preferred method if you do not have access to a good filter.  It requires a lot of energy to boil water which makes it expensive as a large scale disinfection method.

Sodium Hypochlorite:  Bleach.  Disinfecting with household bleach kills some, but not all, types of disease causing organisms.  Don't use scented bleach, color-safe bleach, or bleach with added cleaners.  It has a usable shelf-life of 3 to 5 months if you store it around 70F.  Most household bleaches have 4 to 6 percent of available chlorine.  After 5 months this will begin to decrease.  The amount you add should be about 1/8 teaspoon (8 drops) of household bleach for each gallon of water or 2 drops for each liter.   Stir or shake it well and let it stand for 30 minutes.  Double the amount of chlorine if the water is cloudy, murky, or if it is extremely cold.  The water will be safe to drink for three days.   After that time all the chlorine will have released into the atmosphere.

Calcium Hypochlorite:  Swimming pool chorine.  This works in the same way as household bleach.  This is a powdered form and is preferred because unlike liquid chlorine, there is a long shelf-life if kept sealed and dry.  It is more concentrated (65%) and weighs much less.  Just make sure there are no other additives.  You need to learn, practice making, and using liquid concentrates.  This where a scale and a way to accurately measure liquid volumes becomes essential.  1 gram for every liter of water makes a disinfecting solution.  Then add 1 part of the disinfecting solution to every 100 parts of water to purify. 

Potassium permanganate (KMnO4):   This can be used to disinfect water.  It can be purchased from camping supply and pool treatment stores.  The water will be colored slightly pink.  Use 3 or 4 crystals in a quart or liter of water.  Let the solution stand for at least 30 minutes.  This is an emergency use solution only and should not be used for planned leisure activities.

Iodine:  Iodine kills many, but not all, of the most common pathogens present in natural fresh water sources.  Iodine  must be dissolved in a solution (usually 47% alcohol).  Add 5 to 10 drops per quart or liter of water.  There are 420 drops in an ounce which will purify about 20 gallons of water.  If the water source is a lake or some other still body of water you want to add closer to the 10 drop amount.  If the source is flowing, you can add fewer.  If the water is green, brown, or anything but clear, filter first.  The suspended particles causing the water not to be clear will interfere with e chemicals effectiveness.  The taste of iodine is too offensive for some people.  They would probably prefer less drops.  After five minutes you need to purify the mouth and rim of the bottle.  Turn the bottle upside down and slowly unscrew the cap until you see water appear around the inner rim.  This purifies the area that will touch your mouth when drinking.  Screw the top back on and wait 30 minutes.  The water is safe to drink, however, you can add ascorbic acid or citric acid to remove the iodine taste and color from the water.

Commercial tablets:  For commercially prepared chlorine or iodine tablets, follow the instructions that come with them.  If you don't have the instructions, use one tablet for each quart or liter of water to purified.  Most of these are just iodine and you pay much more than just liquid iodine would cost.  They are convenient and easy to store.  Some come with a second bottle of tablets to remove the iodine taste once it has done it's job.  It is usually ascorbic acid or vitamin C.

Solar Water Disinfection (SODIS):  This is a simple and effective method for purifying water if you have no other means.  Pour the water into clear plastic PET bottles and expose to direct sunlight for at least 6 hours.  This has been shown to be an effective method of disinfecting.

Ozonation:  Complex and expensive process by where the water is exposed to generated ozone.  Almost half the cost is in the electricity required to operate the system.  A secondary disinfectant (chlorine) is needed because the ozone does not stay in the water long. 

UV Radiation:  A special lamp is aimed at a stream of water and when it penetrates the cell wall of an organism, the cell's genetic material is disrupted and is unable to reproduce.  They effectively destroy bacteria and viruses.  It doesn't add anything to the water that could wake it worse.  It is unsuitable for cloudy water and does not kill organic cysts. 

Filtering Water

Filtering should be the first step in making potable water.  Filters can utilize sieving, adsorption, ion exchanges, biological metabolite transfer, and other processes.  The best filters utilize many media and can render water almost pure by themselves.  They have become the preferred method to remove the most harmful contaminants from drinking water.

Sieve:   A sieve can be anything from a sock, a piece of cloth, or a cotton ball.  Sieves are meant to remove large particles as a first step in filtering.  Other membranes have been developed to remove smaller particles although , still a sieve,  they remove microscopic particles.  The great thing about sieves is that they can be backwashed or cleaned and reused.

Adsorption:  This is generally the next step.  Activated carbon of some form is most widely used.  Contaminants are trapped in the structure of the charcoal  (adsorption).  This structure is what is left behind from the cellular structure of the plant used.  To visualize this structure,  Think of a pill-shaped vessel (oblong) stacked in columns side by side and arranged in rings.  It provides a lot of chambers available to catch other particles.  They fill up eventually and must be replaced.  Granulated activated carbon filters  (GACs) have less filterable surface area compared to carbon block filters.  These are comprised of powdered charcoal with a binder (resin) which increases the surface area over standard GAC filters.  Carbon  filters made from coconut shells have the smallest micropores (i.e. more surface area) and make the best carbon block filters.  Other components added to the  carbon matrix can improve the removal of additional contaminants. 

Ion Exchange:   Ion exchange resins are used to remove poisonous and heavy metal ions from the water.  Ions are elements or  molecules that have a either plus or minus charge.   A resin (usually polystyrene beads with a high surface area) are charged with innocuous ions like sodium or potassium which are then exchanged for  things like copper, arsenic, lead, or cadmium.  This media removes what most filters on the market won't.

The best filters utilize all three of these methods using several different media.  Several graduated sieves (microfiltration and ultrafiltration membranes), GAC, carbon block, and ion exchange, rolled up into one filter is going to give you the best results. In addition, the slower that water passes through a filter, the longer it will be in contact with the media and the better it will work.

Types of home use filters

"Point of use filters"

These are filters installed at faucets or showers where they are used.   They  can be any of the types previously mentioned.

They can be single or multi-staged.   Some are mounted under a sink or can be mounted on a faucet or shower head.

Reverse osmosis which utilizes several filters in sequence as well as a semi-permeable membrane the water has to pass through.   The membrane will allow anything as small as water molecules to pass through as well.  While they may remove many of the harmful contaminants in the water, they also waste 4-8 times the finished water they produce.  They produce water slowly and store it in a small canister.   They are usually mounted under a sink.   It works with the water pressure coming into your house which usually needs to be between 45 and 60 psi.   They only work with water pressure so if you don't have city water or the minimum pressure you will need a pump to provide it.  I really don't recommend these because they waste too much water in the process.

Whole house filters are usually installed where the main water line enters the house.

They  can be several stages of different media types.  They usually are sediment and iron filters.  They can also include ultra-violet light filters which are similar to those used by aquarium enthusiasts.  The right combination of filters can produce high quality drinking water.  They require water pressure to operate.  They filter everything  coming into your house including the water you water your plants with, wash your clothes with, wash your car with, and flush your toilet with.  A lot of water flows through these systems and filter replacement is costly.  For most people this is not necessary, however if you have high levels of chlorine in your water you may want to consider this option.  The reason is that the chlorine is in all the water in your toilet, washing machine, and all sinks and tubs.  When this water is present it is releasing chlorine into the air making it highly toxic especially in a well insulated home.  You may need to ventilate or exchange air occasionally.

Counter top filters  vary widely in size and filtering capability.  They are mounted on your existing faucet and can provide very pure water very quickly.  Again, it depends on the filter. 

Portable filters such as, Canisters, Pitchers, and bottles are mostly gravity fed systems.  They also vary widely in effectiveness based on their filter media.  They  will work when there is no tap water pressure.  You can take them with you on trips or bug-outs.  

Some filters work so well they are classified as purifiers.

You have to decide what you want/need for your particular situation.

They are the best way to create potable water in an emergency or off grid situation.  They don't add harmful chemicals to the finished product.  You only make what you need at the moment.  New technology that combines several media in one filter offer the best value.  You only  have to replace one filter instead of five.  Make your selection based on your needs and always consider the price per gallon of the filter that produces the water.  Divide the cost of replacement filters by the total gallons produced.