Large sediments are any visible biological material in or floating in the water source. These include leaves, sticks, mud; and other organic debris such as plant and animal matter.
Removing sediments or other large organic matter may make subsequent water treatment more effective. This is particularly true for chemical treatments where the active chemical particles tend to bind to organic matter, thus reducing their effectiveness.
An effective way of removing sediments is to filter the water through cloth. Collect water in a large billy and filter it into smaller containers. Clothing fabric filters well because they are strong enough to catch organic material but still allows the much smaller water particles through the holes in the fabric. A hat or bandana works well.
Filtered water still contains pathogens, so a decision needs to be made if further treatment is required.

Pathogens can make humans sick by challenging the immune system. Some pathogens kill cells or disrupt cell function, and others produce toxic waste products. Sometimes pathogens multiply so quickly that they crowd out the host tissues, disrupting normal function. The immune system fights back with chemical secretion, antibodies and fever, a heat response aimed at killing pathogens.
Consuming too many pathogens can cause mild to severe illness in humans and can lead to death, particularly in parts of the world where people do not have access to clean drinking water^[1].
Pathogens most harmful to humans include microscopic viruses, bacteria and protozoa as well as larger organisms such as worms.
Viruses are much smaller than bacteria or protozoa, and need to occupy another living cell to replicate. They are infectious agents that can attack a variety of living things from bacteria and protozoa through to animals and plants.

Bacteria are single-celled prokaryote organisms that are a few micrometres in length. Bacteria were one of the first forms of life to appear on earth, and are renowned for being able to survive in soil and water as well as extreme conditions such as radioactive waste and acidic springs.

Protozoa are single-celled eukaryote organisms that are generally bigger than bacterial cells and contain different cell contents.

Parasitic worms are more evolutionarily complex than viruses, protozoa and bacteria being multi-celled eukaryote organisms. Although they can survive independently, many species use another host organism to reproduce and spread. They can enter the human body in cyst or adult form via food and drink or bites from insects like mosquitoes and often reside on the skin or in the intestines. Humans suffer discomfort and illness from such infections including diarrhoea, fatigue and skin rashes.

Pathogens can generally be removed by:
Micro-filtering (e.g. microfilter by pumping, or LifeStraw
Chemical inactivation (e.g. iodine, micropure)
Boiling and
UV treatment.

Toxins can make people really unwell by poisoning the body. Sometimes, toxins only need to be in small concentrations to cause problems. In other cases, it’s the accumulation of toxins over an extended period of time that causes health problems. This is particularly a concern for people ingesting water from industrial run-off in areas that do not have adequate regulation of clean drinking water.
Toxins of concern include fertilisers, made of nitrogen, phosphorous and potassium. These can cause serious environmental damage and health risks to humans when found in waterways. Once fertilisers reach waterways, they enrich nutrients so much that some plant populations explode in quantity. Large amounts of blue-green algae is a sign of this Eutrophication process, and this algae produces toxins which go on to poison animals and humans alike.
Heavy metal runoff is another toxin of concern. For example, mercury runoff into water systems occurs via industry processes including coal and gold mining, Chlor-alkali plants and Trash incinerators. Mercury poisoning in humans can lead to itchy skin, swelling and skin discolouration, and at extreme levels of exposure, muscular weakness and mental confusion. Mercury poisoning is usually caused by consumption of aquatic produce where mercury has bioaccumulated. Mad Hatter disease was a chronic syndrome among hat makers who had prolonged exposure to mercury vapours. The “Mad Hatter” from Lewis Carroll‘s Alice’s Adventures in Wonderland is renowned for his eccentric behaviour.
Toxins cannot be removed by chemical or UV treatment, and boiling will only concentrate the toxins further. Instead, chemicals must be filtered out, a process that requires special equipment as it requires filtration at the atomic scale. While some hiking filters may claim to reduce toxins such as mercury, they are unlikely to remove such contaminations completely.
From a practical sense, if pushed for water supplies, drinking low levels of water containing toxins is probably not going to have ongoing effects, but continual use is unwise. It’s dangerous to consume fish or other aquatic food from these systems because toxins tend to bioaccumulate in aquatic organisms to levels that are very dangerous.

Saltwater typically refers to ocean water with high concentrations of sodium chloride. Saltwater is unpalatable and drinking it will lead to dehydration. But drinking water also contains other types of salts at palatable levels.
Tap water contains some salts, which varies with the source. This is why tap water tastes funny when drinking from a source that’s different to usual. In appropriate quantities, the following salts are not a problem:
- Calcium and magnesium – these describe the hardness of water
- Sulphates such as magnesium sulfate, calcium sulfate or sodium sulphate
- Iron from rock deposits and
- Chlorides, either naturally occurring or from industry.

As with toxins, salts are challenging to remove in the field as the process requires atom-sized filtering systems. Small hand-held units use portable reverse osmosis to purify water. The Katadyn Survivor 06, which produces 0.89 litres of water per hour at a pumping frequency of 40 pumps per minute. Given the effort required, it’s better to select a freshwater source where possible and only use desal unit in an emergency situation.