Water Treatment Thoughts for Outdoor Activities

Updated February 2009

The information here is, to the best of my knowledge, accurate.  Any information here can and should be checked against other sources before making any decisions about water treatment.  Do a little research of your own and be your own judge.  If you find something not true or think I am missing an important fact feel free to contact me, I am always open to suggestions and corrections.

You can go out and buy what ever filter a friend or a sales person recommends to you.  Or you could choose to get a little information first, then make an informed decision.  A decision you will feel confident about.  If you choose the later, I hope this will help point you in the right direction.

People commonly think the water around them on an outdoor adventure is safe because it is in the great out doors.  They picture a guy hiking along a stream where no man has gone before (so they think).  The man takes out his cup and scoops up some good clean mountain water.  If you do this chances are you will be just fine, but there is also a chance of getting very sick.  The chances of getting sick are much higher than you may think.

I chuckle when I hear someone say they are in an area where no one goes so they are safe from contamination.  Truth is people go everywhere.  It is rare a person will hike or pack somewhere without getting some information on the area first.  Where do they think this information came from?  Most people want to hike or backpack in an interesting place so we all pick similar places.  It would be very difficult to find a place to hike or pack where no one ever goes.  Taking people out of the equation, think of all the animal feces upstream and on the banks of the stream (where rain water can wash over it and run into the stream).

Protozoa (E. Coli or Salmonella), bacteria (Cryptosporidium or Giardia), viruses (Polio, Hepatitis A and Norwalk Virus), chemical (pesticides etc) and metals (metals and minerals in high concentrations) can all be spread through water.  Protozoa, bacteria and  viruses get into the water when feces containing these bacteria viruses come in contact with the water.  Rain water running over the feces and then running into the water can also get the bacteria into the water you want to drink.  Think of all the animals out there that may shitting in the water upstream from you.  Think of all the animals that have shit on the hill side where rain water has washed across their feces then into the stream you want to drink from.  Freezing does not kill all bacteria, so that fresh snow melt you are drinking from can carry bacteria in it as well.

I have found dead animals lying next to or in streams.  They have either fallen from a cliff or been killed by a predator like a mountain lion.  Imagine walking in a beautiful place and getting thirsty.  You walk over to the stream scoop up some water and have a drink.  It tastes great.  You start hiking up stream and find a dead animal laying in the water about 100 feet up stream from where you just scooped up that water.  Doesn't feel to safe now does it?

Chemical contamination is also something to be concerned with.  I have hiked near a stream that looked inviting enough to drink from.  After looking further up stream I changed my mind.  I found the stream started from a spring inside an old lead mine.  You never know what is up stream.  The lead was not put there by the miners, it is naturally occurring since it’s what they were after.  With that in mind you can have naturally occurring chemicals or metals in the water you drink as well.

The Narrows at Zion National Park is a good example of possible bacterial, viral or chemical pollution. If you decide to travel the entire Narrows in Zion, you will find the trip starts by traveling through someone's farm land (Chamberlains Ranch) with cattle and meadows.  A stream runs through it and into the Narrows.  Any chemicals used on that farm land have the potential to run downstream into the water you will want to drink on your trip.  Any bacteria of viruses in the feces of the cattle will also likely end up in the water as well.

The bottom line is, the water you are visiting could very easily be contaminated, so keep that in mind and do something to protect yourself.

What are we trying to filter out of the water?


Protozoa are usually single celled organisms and are usually a eukaryote (has a nucleolus).  Protozoa are the largest of the things we are trying to filter out, aside from large debris in the water like dirt particles.  They are said to range from 2 microns in size and larger.  Chemical treatment will take care of some but not all protozoa.  Boiling is very effective at killing protozoa.  Filtering is effective at removing protozoa with a recommended absolute pore size of 1 micron or less.   In the protozoan group are Giardia and Cryptosporidium.  These are the two major things you will hear about that need to be filtered out in this category.

Cryptosporidium (single celled parasite) was very rarely found in humans in the U.S. until the 1980's.  Now animals, that's another story.    There are a lot of different species of Cryptosporidium.  The one that is found in infected humans is Cryptosporidium Parvum, or C. Parvum.  It has been documented that Cryptosporidium has spread through municipal water systems, even though those systems met the federal water treatment standards.  1987 in Carrollton, Georgia and 1993 in Milwaukee, Wisconsin.  Low levels of the Cryptosporidium Oosysts (Oh-oh-syst) are present in municipal drinking water systems since the methods used, only get rid of most but not all of the Cryptosporidium.  It is not known the number of Oosysts needed to cause infection but the low levels  normally found in municipal drinking water systems is not usually something to be concerned with.

Cryptosporidium lives in the intestines and is excreted in the feces.  It is spread in a lot of ways.  The most commonly thought of method is feces coming in contact with the water we drink or rain water running across contaminated feces then running into the water we will drink.  Anytime you get it in your mouth you can be infected, even if the amount of Cryptosporidium is minute.  If manure was used to fertilize the veggies you are eating and you did not wash them well, there may be Cryptosporidium present that can infect you.  Drinking untreated or poorly treated water can infect you.  You could swallow a small amount of water in a chlorinated swimming pool and be infected since chlorine is not effective at killing it.  It can spread from person to person through contact like changing a diaper or shaking hands with someone that has touched feces.  It is surprising to think of all the ways you could get a very small amount of this to your mouth.  Washing your hands and general personal hygiene is extremely important when trying to prevent the infection.

Cryptosporidium is very resistant to most forms of chemical treatment like Iodine or Chlorine.  Boiling water is effective but must be maintained at a rolling boil for about 2 minutes.  Although the size of the protozoa group is 2 microns and above it is recommended that a filter with a pore size of 1 micron or less be used to remove it.  The pore size must be absolute not nominal.  Nominal pore size means approximate or average pore size.  Absolute pore size means the pores will be no larger than the size stated.  Freezing the water has not proven to be a reliable way to kill Cryptosporidium.

The illness caused by Cryptosporidium is called Cryptosporidiosis and is similar to Giardiasis.  Symptoms are watery diarrhea, abdominal cramps, nausea, low fever, and dehydration.  The symptoms usually develop 4 to 6 days after infection but have been known to develop between 2 - 10 days.  Healthy individuals can be ill for several days usually not more than 2 weeks and some don't get ill at all.  Cryptosporidium can be deadly to those with a weakened immune system rather it be from HIV, chemotherapy, drugs that degrade the immune system, etc.  Cryptosporidium cannot be diagnosed by symptoms alone since the symptoms are similar to a lot of other conditions.  The medical profession says Cryptosporidium has no cure other than wait and let it pass.  There have been some documented successes with certain antibiotics but not enough to say they have a quick fix for the problem.  There are some herbal cures out there that claim to get rid of Cryptosporidium.

Cryptosporidium Oosysts are present in most surface bodies of water in the U.S. and are more abundant after heavy rains.  According to the EPA, Cryptosporidium is the leading cause of waterborne illness in the United States.

Giardia or Giardia Lamblia (a single celled parasite) has been around for a long time.  I did not run across any information about how long Giardia has been present in the U.S. but it has been here for quite some time.  In Europe it is sometimes referred to as Lamblia Intestinalis.  Giardia lives in the intestines and is spread through feces just as Cryptosporidium.  It is protected by an outer shell that allows the Giardia to live outside the body for long periods of time.  Giardia is spread the same ways that Cryptosporidium is and the same precautions with personal hygiene should be observed to help prevent infection.

Giardia can be treated with chemicals such as chlorine and iodine.  Boiling water is effective at killing Giardia and filtering will remove it as well.  The 1 micron pore size recommended for filtering Cryptosporidium will remove Giardia as well.

The illness caused by Giardia is known as Giardiasis (sometimes known as Beaver Fever because Giardia is thought to be wide spread among beavers) and is the most frequent cause of non-bacterial diarrhea in North America.  It is an infection of the small intestine.  Symptoms are watery diarrhea, stomach cramps, upset stomach and dehydration.  Some people show no symptoms at all.  Symptoms usually begin 1 to 2 weeks after infection.  The illness usually lasts for 2 to 6 weeks but has been known to last for months or years.  Chronic cases are hard to treat.  There are a few strains of Giardia and there is more than one that can infect a human.  Different people will have different symptoms and different levels of severity when exposed to the same strain.  One strain is not more severe than another it varies from person to person.  Unlike Cryptosporidium where low levels are tolerable it is possible for one Giardia cyst to infect you.   Giardia cannot be diagnosed by symptoms alone since the symptoms are similar to a lot of other conditions. There are drugs your doctor can give you to treat Giardia.  Some individuals exhibit some immunity after being exposed.

Giardia is more prevalent in children and is common in a lot of day care centers especially those that do diaper changes.  One study from backpacker magazine (Dec. 1996) looked at 10,000 samples from streams all across the U.S. and Giardia was found in everyone.  The incidents in the U.S. population are estimated to be around 2%.


Bacteria are usually a single celled organisms and are usually a prokaryote (has no nucleolus).  Fecal Coliforms is a collective term referring to all bacteria, as a whole, found in fecal matter from humans and animals. The size of bacteria is said to range from 0.2 microns and larger.  Larger bacteria can be filtered out with a micro filter but to remove the smaller bacteria a sub micron pore size is required.  Bacteria are responsible for diseases like Cholera, Typhoid Fever, E. Coli and Salmonella.

E. Coli or Escherichia Coli is a bacteria with hundreds of different strains.  It may be surprising for you to hear most strains are harmless and are naturally present in most animals and humans.  E. Coli is one of the dominant species of bacteria found in human feces.  The E. Coli bacteria help prevent the growth of other harmful bacteria in the intestines.  E. Coli also produces appreciable amounts of vitamins.  When you hear of E. Coli, harmless is not a word that usually comes to mind.  That's because we never hear of all the good strains of E. Coli, all we hear about are the bad strains and how they made someone sick.  There are very few strains of E. Coli that cause illness in humans.  One of the harmful strains of E. Coli produces powerful toxins that can severely damage the lining of the intestines causing a severe illness.  That is the strain usually referred to when you hear about E. Coli.  This strain is called E. Coli O157:H7 and is one of the more rare strains.  It was first recognized as a cause of illness in 1982.

E. Coli. lives in the intestines like Cryptosporidium and spreads through feces as well.  So it is spread the same way as Cryptosporidium.  Healthy cattle can carry it in their intestines, so needless to say meat can become infected during slaughter.  Ground beef is especially susceptible to being contaminated since E. Coli. can be mixed in during the grinding process.  Infected meat can smell and look normal.  Raw milk can be contaminated by the bacteria being on the cows nipples or on the equipment used to milk the cow.  Pasteurized milk is a good choice since the pasteurizing process kills the E. Coli..  Water contaminated from sewage can also be a source of infection, and that is what makes this something to be aware of when camping.  If an animal can shit in the water you are drinking it is possible for you to be contaminated with E. Coli.  The amount of E. Coli. needed for infection to occur is not known but thought to be very small.

Boiling the water for more than 1 minute, chemicals such as chlorine and filters (with an absolute pore size of 0.2 microns or less) are all effective at eliminating the risk from E. Coli.

The disease caused by E. Coli O157:H7 is called Hemorrhagic colitis.  Hemorrhagic colitis usually starts with abdominal pains and watery diarrhea which can produce grossly bloody diarrhea.  Usually there is little or no fever and some individuals have no symptoms at all.  The illness usually resolves itself in 5 to 10 days without antibiotics.  Antibiotics have not been shown to improve recovery and it is thought that the antibiotics may increase the chances of kidney problems.  Anti diarrheal agents should be avoided as well but I haven't found an explanation as to why.

Infection with E. coli O157:H7 is diagnosed by detecting the bacterium in the stool.  Most laboratories do not test for E. Coli 0157:H7 so you need to make a special request to test for it.  If you have bloody diarrhea you should ask for your stool to be tested.  The method to ask for when testing is sorbitol-MacConkey (SMAC) agar.  It is thought that 2% to 7% of infections lead to Hemolytic Uremic Syndrome, in which the red blood cells are destroyed and the kidneys fail.  This usually occurs in young children and the elderly.  In the United States Hemolytic Uremic Syndrome is one of the leading causes of acute kidney failure in children and most cases of it are caused by E. Coli 0157:H7.  Hemolytic Uremic Syndrome has a death rate of 3% to 5% when the patient is in intensive care and about 1/3 of people with Hemolytic Uremic Syndrome have abnormal kidney function later in life.  About 8% of the people with Hemolytic Uremic Syndrome develop other lifelong complications like high blood pressure, seizures, blindness, and paralysis.

Hemorrhagic colitis is not a common occurrence, but the actual rate of occurrence is not known.  People with severe symptoms will seek medical help.  People with less sever or no symptoms are much less likely to seek help and the condition only lasts a few days in some cases.  In the Pacific Northwest, E. coli O157:H7 is thought to be second only to Salmonella as a cause of bacterial diarrhea.

Salmonella is a rod shaped bacteria that is wide spread in birds and pigs.  It was first discovered in the intestine of a pig in 1885 by Daniel E. Salmon.  Environmental sources of salmonella are water, soil, animal feces, raw meats and any surface that may have come in contact with these things.  Since salmonella is a bacteria, lives in the intestines and is present in fecal matter the precautions and methods of spread are the same as for Cryptosporidium or E. Coli.

Boiling the water for more than 1 minute, chemicals such as iodine and filters of the proper size are all effective at eliminating the risk from Salmonella.  I have not yet found a source that tells what size a Salmonella bacteria is.  The only reference I can find is the advertising of water filters saying they take care of Salmonella because they filter down to 0.2 microns.  Take it for what it is worth, just remember it is advertising.

Salmonella can cause typhoid and other intestinal diseases, some of which will have a typhoid like fever.  The strain that causes typhoid is Salmonella Typhi.  Typhoid fever is rare in the United States today but other illnesses caused from Salmonella are common and are collectively called Salmonellosis.  Other symptoms are nausea, vomiting, abdominal cramps, diarrhea and headache.  3 to 4 weeks after the symptoms cease there is sometimes a period of arthritic symptoms.  The time from infection to symptoms is usually 6-48 hours.  The infective dose is around 15 to 20 cells.  Symptoms usually last 1 or 2 days but can go on longer.  It is estimated that 2 to 4 million cases of salmonella occur annually in the United States.

Campylobacter Jenuni or C. Jenuni or Campy is another bacteria.  Healthy humans do not usually carry C. Jenuni but it is often found in cattle, chickens and other animals.  Flies can also carry C. Jenuni.  The disease caused by C. Jenuni is called Campylobacteriosis and is often called campylobacter enteritis or gastroenteritis.  The symptoms are diarrhea (bloody or non-bloody), nausea, headache and muscle ache.  These symptoms are just about the same as a lot of other viruses so diagnosis cannot be done by simply looking at the symptoms.  The disease usually lasts 7 to 10 days.  About 25% of cases involve a relapse.  The disease will go away on its own with out antibiotics but some antibiotics can help reduce the time.

According to surveys referenced in the FDA's Bad Bug Book C. Jenuni is the leading cause of bacterial diarrheal illness in the United States.  A study in the 1980's found 23% of patients showing up at a clinic near Grand Teton National Park were infected with Campy as compared to 8% infected with Giardia.  In 1996 a clinic reported seeing about 2 cases of Campy infection for every 1 case of Giardia.


Viruses are the smallest microorganisms and some cannot be removed by filtering.  They are said to be 0.004 microns and larger.  Wilderness studies suggest that 60% of all back country illnesses are actually caused by bacteria and viruses.  Viruses can cause Polio, Hepatitis A and Norwalk Virus. Boiling and chemical disinfecting can effectively kill viruses.  In general viral infections from back country waters are very few compared to bacterial infections.  Viruses that effect humans are usually only present in water that has come in contact with human feces.  A virus will usually have a short life span after it is outside the human body.  A class we were put through at my work informed us that Hepatitis is actually more of a danger than HIV.  While HIV lives outside the body for a few minutes or hours, Hepatitis can live outside the body for up to 5 days.  Lets face it a large percentage of places you will back pack in have been visited by other humans.  This in itself indicates there is a risk of viral infection when you are in the wilderness.  So even though the chance of viral infection is low it still is a possibility and should be considered when making your decision on a water treatment method.

Chemicals and Metals:

I have not looked into chemicals or metals at this point and am not sure I will.  But they are listed just as a reminder that they do exist. If anyone has information that might be of interest in relation to the topic of this page relating to chemicals or metals please email me and share. Protozoa, bacteria and viruses can cause major problems when exposed to a very small amount because they get into your body and begin multiplying.  Chemicals and metals in the waters may enter the body in small amounts but do not multiply.  A camper may come in contact with low concentrations of metals or chemicals.  Extended exposure to these may develop problems, but short term exposure, like that of a camping trip, to low the levels of the metals and chemicals in the water usually do not cause complications.  That is the main reason I have not put any effort into looking for information for removing metals or chemicals.

Chemicals and metals in your water are definite possibilities when camping.  I have done a pack trip through the Narrows in Zion National Park.  The trip starts out by traveling through someone's farm land (Chamberlains Ranch) with cattle and meadows.  A small stream runs through this farm land and is part of the water that is makes up the water in the Narrows.  Any chemicals used in that farm land have the potential to end up in the water you would filter down stream for drinking.  I have also done some hiking in areas I would enjoy going back to for an over night since it was beautiful and had a nice stream.  Hiking further up the stream made me not want to camp there.  The water was coming from a spring that was inside an old lead mine.  Both of these water sources would appear safe if you did not know what was up stream.

What are the Water Treatment Choices?


Boiling the water is very effective at killing living things from the water like protozoa, bacteria and viruses but does not neutralize chemicals or physically remove the bacteria. I do not like the idea of boiling water on a back packing trip unless its the only option. 

If you do boil water as a method of treating water, the common recommendation is to keep the water at a rolling boil for at least 1 minute at sea level and add 1 minute for every 1,000 feet above sea level.  Adding this time can add to lengthy boil times and add to the amount of fuel that needs to be carried along.  The reason for adding time is the temperature of boiling water changes with atmospheric pressure (altitude).  At sea level water boils at 212°F.  At 7000 feet the temperature drops to below 200°F.

Chemical Treatment

Chemical treatment is usually done with chlorine or iodine.  It has the advantage of being one of the lightest most compact methods.  All you need is a small bottle of liquid or tablets.  Chlorine is not effective against some viruses so is not the most popular method.  Iodine works well against viruses, protozoa and bacteria but usually doesn't kill Cryptosporidium.  The down side to chemical treatment is the time involved in waiting for your water to be safe.  You put the chemical in your water and wait for the prescribed time and you are good to go, usually 30 minutes (up to 4 hours or more for CryptoSporidium).  Iodine is not recommended for continuous use.  If used too much it is actually a poison.

UV light Treatment

Treating water with UV light has proven effective as well.  Advertising for products like the SteriPen say it kills viruses and bacteria (including cryptosporidium) in seconds.  Other sources I have found on the subject say things like mostly effective and somewhat effective. 

Using this method consists of putting water into a container and inserting a UV light source in the container to disinfect the water by irradiating it with UV.  The amount of water to be treated at any one time should be kept small so the UV light can reach all of the water.  If more that 0.5 or 1 liter of water is desired, the water must be treated in batches.

While there are no filters or cartridges to change, batteries are required.


A filter is considered to be 1 to 4 microns, and a micro filter is considered to be 0.2 to 1 microns. A micron is one one millionth of a meter or 0.0000394 inches.  Filtration has the advantage of taking less time than boiling or chemical treatment, is usually lighter than the fuel carried for boiling water and unlike boiling or chemical treatment, filtration completely removes the contaminants in question.  The down side is the pore size of even the best filters are rarely below 0.2 microns, which is not small enough to filter out some viruses.  Although you do not have to pack extra fuel as with boiling you do still need to carry the filter.  Filtering will meet almost all of your drinking water needs with the exception of some viruses.  Since infections due to viruses are not very common filters are considered to be all you need for safe back country water.

Filtering also increases the odds that you will filter out other things that boiling and chemical treatment will not, like metals or chemicals, depending on the size of the particles.  It is not the intent of filters to remove chemicals or metals but depending on sizes some of these may be removed as well.

When selecting a filter look for the smallest pore size you can find.  Also look to see that the pore size is rated as absolute and not nominal.  Nominal means it is the average size.  If the average size is 1 micron then there are holes that are larger and smaller than that size.  Not comforting when you are depending on that pore size to protect your health.


Purification seems to be a term no one will agree on a definition for.  Some say filtering below 0.004 microns is purification, others say it takes a 2 step process to be called a purifier, filtration then another form of treatment to eliminate other risks.  I subscribe more to the 2 step definition. But in reality pure would mean an absence of everything except water.  Each water filter company seems to have it's own view of what purification means.

To me pure means the absence of foreign material (impurities). Chemical treatment renders the water SAFE not PURE. Filtration removes most but not all (depending on the size) impurities so it too makes the water SAFE not PURE.  Purification when thinking about what you do to your water in the back country is usually considered a 2 step process, filter then some other form of treatment.

Most of the purifiers I have found that use filtration and another process are bottle types.  A bottle you fill up with a cartridge in it that filters the water through a straw you drink from.  These involve a filter, some other form of killing things (usually a proprietary name for the process so you don't know exactly what the process is) and sometimes a carbon filter as well.  tadyn makes a few of these if you wish to look some of them up.  The problem I have with the bottle type of purifiers is they are only easy to use if you only need the amount of water the bottle will hold.  If you want to fill up a 100 ounce water bladder it won't be a fun project.

There are two systems I have run across that are mistakenly called purifiers.  These two systems use a chemical process to kill any protozoa, bacteria and viruses present.  The dead microorganisms are still in the water.  This makes the water safe to drink, but in my opinion should not be called purified water.  The two systems are Micropur MP1 tablets by Katadyn and the MIOX purifier by MSR.  These are some good systems but are obviously chemical treatments.

The Katadyne Micropur MP1 tablets are used like iodine and are effective against all living things in the water.  The water is safe from everything except Cryptosporidium (under the category of cycts) in 30 minutes.  Cryptosporidium is said to take 4 hours.  Their literature indicates that it may take less time but the EPA requires them to list the time for worst case scenario which is cysts in cold and dirty water.

The MSR chemical treatment system called the MIOX purifier,  uses regular salt and transforms it into a combination of antioxidants through an electrolysis process.  The antioxidant blend is poured into the water and effectively renders it safe from protozoa, bacteria and viruses by killing them.  Your water is safe from everything except Cryptosporidium i in 30 minutes but you have to wait up to 4 hours to kill Cryptosporidium.  Just like MP1 the time is most likely less but is posted as worst case scenario.  I like this unit due to it's small size and that it kills everything living and it uses ordinary table salt and weighs in at 8 oz.

General Ecology makes a purifier called the First Need Purifier that uses filtration (0.4 microns absolute pore size, 0.1 nominal pore size) then a proprietary non-chemical three step process to remove a lot of other things from the water and is set up like a standard pump style water filter.  The process is called Structured Matrix.  I called to find out more of how Structured Matrix works and was told it is a proprietary trade secret but the web site gave a quick run down.  For the explanation go to the Q&A section on their site and scroll down to "What is structured matrix".  First Need removes (not kills) the protozoa, bacteria and viruses.  The Structured Matrix removes e even more.  It also removes some of the more common pesticides, PCB's and other natural and non-natural chemicals.  The other purifiers I have found do not go this far.  The only draw back is the absolute pore size of the filter is 0.4 microns.  If this were only a filter this would not be small enough.  Since the other phases of their system work on the water after the filter I felt safe with this system.

My Choice

I do not like chemical treatments.  They are light but require wait times and are marginally effective against cryptosporidium.  Using the chemical treatments to excess can also be bad for your health.  I am not yet fond of the UV light treatment method.  I am not yet confident about it's effectiveness in killing cryptosporidium.  The advertising says it works great in only seconds.  Other sources describing it's effectiveness in not so confident terms.  I am also not fond of the batteries depleting while on a trip.  Both chemical and UV treatment leave the offenders in the water even though they are killed.

Boiling water can be very effective but requires taking time to set up the stove and boil the water.  Along with the time to boil the water is the extra fuel that needs to be carried.  Boiling water, like chemical and UV treatment leaves the offenders in the water even though it is dead.  I reserve the boil method for use when my purifier is not available.

Filtering removes the bacteria and protozoa from the water.  No need to try to kill it, just remove it.  Unfortunately viruses are generally too small for a filter to remove.

Purification is my choice for water treatment.  Purifying water has no accepted definition.  I prefer the definition of filtration then an additional method(s) of removing more.  The MIOX and MP1 systems are advertised as purifiers but are just better versions of chemical treatments.  Purification removes what it can by filtration then kills and removes more through other proccesses.

The First Need Purifier by General Ecology is in my opinion the best choice.  The unit weighs 15 ounces which is similar to the weight of most pump style filters.  The First Need Purifier is pump style so there is no need to carry a power source (batteries).  The purifier is field cleanable with a back washable cartridge and can attach directly to most bottles including an MSR Dromedary bag which eliminates the need for an output hose (I have always hated dealing with two hoses for the filtering process).  The water is ready to drink immediately, everything is killed or removed and there is only one step to the process.  The Purifier also removes some chemicals, pesticides and PCB's.  Pesticides my not be a very common thing to worry about but it is possible and as is finding water contaminated with metals.  Another nice feature of the First Need is it can be gravity fed as well.  You can attach the input hose to a bag of water supported over the First Need and walk away while it filters large amounts of water for you.  This would be a good feature for groups of people or for just one person if you are not in a hurry for your water and don't feel like pumping.

Choosing a Safe Water Source

If possible choose water that is clear and still.  Some bacteria like Giardia will sink to the bottom in still water.  Avoid collecting water near areas that have obvious human or animal activity.  Trail crossings, meadows with cow pies, camp sites near by, animals around the water, active or inactive mines near by (mines can leave chemicals or metals in the water), etc. 

If you are collecting snow and find it has a pinkish water melon color (watermelon snow) stay away from it.  It is caused by an algae that thrives in a cold wet environment.  There are several species of this algae but the most common one is Chlamydomonas nivalis.  While filtering will remove the algae, it will not remove the toxins that it leaves behind.  Another note for snow is that freezing can not kill some bacteria so after the snow is melted it should still be treated.

Hopefully this will help point you in the right direction for making your own choices on water treatment.

Sources to recognize in no particular order