The boiling method is used for killing living contaminants (bacteria, viruses and protozoa) found in a water supply. It does not sterilise the water, but instead, damages living pathogens enough to kill or prevent them from replicating.
Water must be heated to boiling point 100℃ at sea level then have a rolling boil for at least one minute, and three minutes if above 2000 metre altitude. Since water boils at a lower temperature at higher altitudes, the extra time accounts for the difference in temperature.
Advantage: no specialised equipment required since billy and stove is already part of usual multi-day bushwalking gear.
Disadvantages: Water takes time and fuel to treat, and time to cool down enough to drink or decant to other containers. Hard to do efficiently in large quantities quickly. Extra fuel is needed.

Chemical treatments are effective against viruses, bacteria, protozoa. Examples of chemical treatments include
Iodine denatures proteins in any living matter. 2% formulation five drops per litre, ten drops per litre if cloudy. Allow to stand for 30 minutes before drinking.
Advantages: Likely to be already carrying this in your first aid kit. Lightweight.
Disadvantages: takes time to be effective. Unpleasant chemical taste, potential adverse health effects if used for prolonged periods.
Chlorine Dioxide products sterilizes water by disrupting critical processes by living matter. Micropur tablets – use one tablet for 1 litre of water and take 30 minutes for bacteria and viruses, 2 hours for Giardia in clear water. Aquamira drops require two solutions to be mixed and take 15 minutes to treat water.
Advantages: Generally accepted to be more effective than iodine, better tasting and better for health. Lightweight.
Disadvantages: Must be careful to pack the tablets so that they don’t get crushed. Takes time for water to be safe to drink (30 minutes – 2 hours). May have to pre-filter water if organic substances are present because they will inactivate many chemical treatments.

Radiation treatment is effective against viruses, bacteria and protozoa. UV light sterilizes water by damaging the nucleic acids of pathogens that are involved in reproduction. Cellular RNA and DNA absorb UV energy, which breaks necessary bonds in these molecules. Without the ability to reproduce the pathogens cannot spread and infect. So UV light do not kill or remove pathogens, but rather de-activates them.
The steripen can sterilise 0.5 L water in a minute. It is generally most effective against bacteria and viruses, and less so against Protozoa because of their thicker cell wall. The water source is preferably as clear as possible because any organic matter in the water reduces the effectiveness of the UV treatment by absorbing UV light. Users must remember to recharge for every trip, and carry spare batteries for extended trips.
Advantages: Water immediately ready to drink; doesn’t taste funny.
Disadvantage: Need to carry backup water purification methods in case of machine failure or batteries running out in the field.

Filtration can be used to remove macroparticles using cloth fabric or microparticles using specialised pumps.

1. Macro-filter using fabric
Most simply, fabric-filtering removes large sediments include floating leaves, sticks, mud and other debris, but no harmful pathogens. Water can be effectively filtered through a clean t-shirt, handkerchief, bandana, coffee filter paper or a commercial pre-filter.
Advantage: no specialised equipment required other than clothes!
Disadvantage: doesn’t remove harmful pathogens

2. Microfilter using a pump
Microfilter pumps remove bacteria, protozoa, cysts and sediments, and sometimes but not always viruses. In general, water is pumped through pores of 0.2-0.3 µm diameter depending on the model, leaving bacteria, protozoa, cysts and sediments behind. The Katadyn Hiker Pro does 1 L/min. Filters and cartridges may need regular replacement depending on how dirty the water is.
Advantage: Water is ready very quickly (the “Katadyn Hiker Pro” does 1 L/minute).
Disadvantages: Need to carry backup water purification methods in case pump breaks in the field; does not necessarily remove viruses. 

3. Microfilter by sucking
A much cheaper form of filtration is the LifeStraw, where the user sucks clean water through a filter before drinking. This method removes bacteria, protozoa, cysts and sediments, but not microscopic minerals, chemicals and viruses. Water is pumped through pores (0.2-0.3 µm diameter depending on model) leaving bacteria, protozoa, cysts and sediments behind. This lasts for 1000 litres (approximately a year of drinking water).
Advantages: Water immediately drinkable. Filter lasts for approximately a year.
Disadvantages: Doesn’t remove microscopic minerals, chemicals, or viruses. Only can be used by one person at a time. Short straw length means that you need to be at the water source to drink. That is, clean water cannot be carried to camp. Alternatively, stick it into a wide-brimmed water bottle, but this can be fiddly. An alternative is the Sawyer bottle.

While humans cannot drink salt water straight, it’s possible to use a de-sal unit to make the water palatable.
Portable reverse osmosis units are available and can be used in emergency situations. They work by manually pumping water through a semipermeable membrane that removes enough salt to make it palatable. The size of the pores are so small that extremely pure drinking water is produced, but the process is slow. Katadyn claims to have the world’s smallest desal unit called the Survivor 06, which produces 0.89 litres of water per hour at a pumping frequency of 40 pumps per minute. While this is achievable in an emergency situation, it’s not that practical on a multi-day coastal bushwalk.