Purifying water

I’ve previously written posts on finding water and filtering water, for this post I want to take a look at different ways of chemically purifying water using commercial, shop bought products.

Broadly speaking, for chemical purification we have 3 chemicals at our disposal:

  • Iodine,
  • Chlorine, and
  • Chlorine dioxide.

There are many products on the market and whilst I mention some, and have included photos of some, in this post don’t take that as me endorsing any particular one, I’ve used them because it represents a product within one of the groups listed above.

Everything in this post is to the best of my knowledge, but take the time to check for yourself.

A note on terminology

I want to mention a difference in terminology between the UK and the US.  In the UK we use ‘filtering’ to mean the act of removing particles from the water that will affect how it looks and smells; essentially removing bits of mud, leaf etc.  In the US this is sometimes referred to as ‘purifying’.

However, in the UK when we talk about ‘purifying’ we’re talking about making the water safe to drink.  In the US this is sometimes called ‘disinfecting’.

The important point here is that the word ‘purify’ is used in different ways, so always check what your reading to understand how these words are being used.

To be clear, in this post I’m using ‘purify’ to mean ‘to make safe to drink’.

Waterborne nasties

Fundamentally we can group the nasties that might contaminate our water into 5 broad groupings:

  • Chemicals,
  • Parasites,
  • Protozoa,
  • Bacteria, and
  • Viruses.

Sometimes protozoa are grouped under parasites, which makes sense as the protozoa we need to be aware of are parasitic protozoans (not all protozoans are parasitic).

Below are what I believe to be examples that sit within each grouping, but check for yourself, I’m not a biologist!

Parasites – Liver flukes, specifically the larvae of liver flukes
Protozoa – These are single celled animals such as cryptosporidium and giardia
Bacteria – Cholera, typhoid, salmonella, E. colli, Legionnaires disease, dysentry
Viruses – Hepatitis A, hepatitis E, polio virus, rotavirus, echovirus (meningitis)

I’m not going to discuss chemical contaminants here as none of the treatments below will remove them; if you believe the water is chemically contaminated, don’t drink it.


Iodine used to be a common buy from the chemist, but nowadays (certainly in the UK) you’re more likely to buy it online.  Typically for purifying water you want 2% iodine tincture as, although there are plenty of iodine tablets on the market, the tincture generally has a longer life span.  In fact some iodine tablets have a life of only a year once the container has been opened.

purifying water | bushcrfat | Kent | London | south east

If you buy iodine tincture you might find that it’s in a glass bottle, so if that’s the case swap it to a robust plastic bottle with a dropper.  Make sure you clearly label the bottle.

Some iodine tablets come with a second tablet that will neutralise the iodine taste.

purifying water | bushcrfat | Kent | London | south east

To use the iodine tincture you want to add 5 drops to 1 litre of water.  Leave the water for a minimum of half an hour before drinking, periodically shaking the water bottle.  If you haven’t filtered the water beforehand, leave for an hour before drinking or use more iodine.  Also be aware that iodine tincture takes longer to work in cold conditions, in very cold weather you want to give the iodine tincture a couple of hours to do its work, or use more.

Iodine will take out most contaminants that are alive, with the exception of cryptosporidium.  Another great advantage of iodine tincture is that you can also use it to disinfect a wound.

Be aware that some people are allergic to iodine (often people who are allergic to shellfish can be allergic to iodine too, but not always).  Also note that pregnant women, women over 50 and people with thyroid problems or taking lithium should avoid using iodine for water purification.

Chlorine tablets

Chlorine is the most common method used worldwide for treating water and is a favourite of many aid organisations.  Typically chlorine tablets are made from troclosene sodium (NaDCC), which dissolves in water to produce chlorine.

You need to look at the labelling of chlorine tablets carefully; I have some that say the tablets will destroy harmful bacteria, but make no mention of parasites, protozoa or viruses.  I have other chlorine tablets that say they will kill most viruses and bacteria, but again no mention of parasites or protozoa.  I knew that chlorine tablets won’t kill cryptosporidium, but was interested whether they would work against giardia.

purifying water | bushcrfat | Kent | London | south east

So I contatced Hydrachem, the manufacturers of ‘Oasis Water Purification Tablets’ and asked them if their product kills giardia; they were very helpful and their Technical Director provided information stating that it takes 3mgs of chlorine per litre of water to kill giardia with an exposure time of 10 minutes, along with references to the studies that the results were derived from.  He also told me that

“Each OASIS 8.5mg NaDCC tablet delivers 5ppm available chlorine in 1 litre of water (5 mg available chlorine per litre of water).  This concentration is the concentration recommended by the World Health Organisation (WHO) for the treatment of water for emergency use.” 

There’s a table on the Hydrachem website that gives the amont of NaDCC in their other products

So whilst the Oasis tablets will kill giardia, some might not, so check with the manufacturer if it’s not clear.

Generally with these tablets you drop 1 into a litre of water and leave for half an hour, but they can vary so read the instructions and make sure you follow them.

They can have a ‘swimming pool’ taste to them and some on the market come with a second tablet to neutralise the taste.

Chlorine Flocculent Powder

Often referred to as Chlor-Floc, these generally come in powder form.  The ‘flocculent’ part of the name refers to the fact that it can reduce cloudiness in the water, although I would recommend that you filter before using chlor-floc.  It will kill bacteria and viruses, but again check to see if it has a high enough concentration to kill giardia.

purifying water | bushcrfat | Kent | London | south east

Always read the instructions and make sure you follow them.

Chlorine dioxide tablets

Chlorine dioxide tablets are able to kill some parasites, cryptosporidium, giardia, bacteria and viruses.  Again, follow the instructions given on the packaging.

purifying water | bushcrfat | Kent | London | south east

In general chlorine dioxide tablets have little taste.


Once you’ve purified your water, it’s worth tipping your water bottle upside down and undoing the lid slowly until the thread fills with water and then leaving in this position for a few minutes to clean the threads.


Similar to having several ways of lighting a fire with you, carry more than one way to purify your water.

Here’s a summary of what various treatments will or won’t kill.  Find out which nasties are likely to be present in the water where you’re heading and get the appropriate treatment.

Liver fluke Cryptosporidium Giardia Bacteria Viruses
Iodine Yes No Yes Yes Yes
Chlorine tablets No No Some Yes Some
Chlor-floc No No Some Yes Yes
Chlorine dioxide tablets Yes Yes Yes Yes Yes

Finding, filtering and purifying water is something we cover on our  2 Day Bushcraft Course, 5 Day Bushcraft Course, Weekend Survival Course, 5 Day Survival Course and the Institute for Outdoor Learning Bushcraft Competency course.  You can see photos from those courses, as well as all of our others, on our Facebook page.

About Gary

Lead Instructor at Jack Raven Bushcraft, teaching bushcraft, wilderness and survival skills to groups and individuals.

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