Treating water: methods compared

How do I make water from an unsafe source drinkable?

Each method acts against only part of the possible problems: turbidity, bacteria, viruses, protozoa, chemicals and salt. The comparison shows what a method does and does not do – so that you can combine methods to suit the source. ‘Yes’ means when used correctly according to instructions, not under all circumstances.

Starting situation

Legend
Method	Turbidity	Bacteria	Viruses	Protozoa (Giardia, Cryptosporidium)	Chemicals	Salt	Needs clear water	Time until drinkable	Needs fuel or power
Boiling water	No	Yes	Yes	Yes	No	No	No	Depends	Yes
Disinfection with chlorine	No	Yes	Yes	Partly	No	No	Yes	Depends	No
Disinfection with chlorine dioxide	No	Yes	Yes	Depends	No	No	Yes	Depends	No
UV disinfection and solar water disinfection (SODIS)	No	Yes	Depends	Depends	No	No	Yes	Depends	Depends
Hollow-fibre and membrane filters	Partly	Yes	No	Yes	No	No	Partly	Depends	No
Ceramic filters	Partly	Yes	No	Yes	No	No	Partly	Depends	No
Activated carbon	No	No	No	No	Partly	No	Yes	Depends	No
Improvised sand and gravel filter (teaching model)	Partly	No	No	No	No	No	No	Depends	No
Distillation and solar still	Yes	Yes	Yes	Yes	Partly	Yes	No	Depends	Depends
Sedimentation and pre-filtering	Partly	No	No	No	No	No	No	Depends	No

Legend:● Yes◐ Partly◇ Depends○ No* = note available

Boiling water

Effort:: Low to moderate; fuel and time
Needs:: Heat-resistant pot, stove or fire, fuel

TurbidityNo
Turbidity remains; settle or pre-filter first.
BacteriaYes
VirusesYes
Protozoa (Giardia, Cryptosporidium)Yes
Including Giardia and Cryptosporidium.
ChemicalsNo
Does not remove chemicals; evaporation can concentrate them.
SaltNo
Does not remove salt; salinity rises as water boils off.
Needs clear waterNo
No – works with cloudy water too; pre-clarifying is still recommended.
Time until drinkableDepends
Rolling boil for at least 1 minute (above about 2,000 m: 3 minutes) plus heating and cooling.
Needs fuel or powerYes
Yes – fuel required (drawback); demand rises with volume and altitude.

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Disinfection with chlorine

Effort:: Low; precise dosing required
Needs:: Approved chlorine tablets or unscented bleach with official instructions, container of known volume

TurbidityNo
Does not clarify; turbidity weakens the effect.
BacteriaYes
VirusesYes
With correct dose and contact time.
Protozoa (Giardia, Cryptosporidium)Partly
Giardia only partly (long contact time, especially when cold); Cryptosporidium practically not.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterYes
Yes – clarify cloudy water first (drawback); authorities sometimes give a higher dose for it.
Time until drinkableDepends
At least 30 minutes or longer per manufacturer; longer in cold water.
Needs fuel or powerNo
No fuel or power needed.

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Disinfection with chlorine dioxide

Effort:: Low; plan for waiting time
Needs:: Approved chlorine dioxide tablets or drops, container of known volume

TurbidityNo
Does not clarify; turbidity weakens the effect.
BacteriaYes
VirusesYes
With correct dose and contact time.
Protozoa (Giardia, Cryptosporidium)Depends
Giardia and Cryptosporidium only with the long contact time stated by the manufacturer – often several hours for Cryptosporidium.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterYes
Yes – clarify cloudy water first (drawback).
Time until drinkableDepends
Per manufacturer; shorter for bacteria and viruses, often hours for Cryptosporidium; cold extends it.
Needs fuel or powerNo
No fuel or power needed.

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UV disinfection and solar water disinfection (SODIS)

Effort:: UV-C low; SODIS low but time- and weather-dependent
Needs:: UV-C device with charged battery, or clear PET bottles and sun

TurbidityNo
Does not clarify.
BacteriaYes
Only in clear water and with correct use.
VirusesDepends
UV-C devices yes per manufacturer; with SODIS less certain depending on virus type.
Protozoa (Giardia, Cryptosporidium)Depends
UV-C devices yes per manufacturer; with SODIS less certain depending on species.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterYes
Yes – effective only in clear water (drawback).
Time until drinkableDepends
UV-C device: short, per manufacturer. SODIS: 6 hours of full sun, two days when heavily overcast.
Needs fuel or powerDepends
UV-C: battery/power required (drawback). SODIS: sun only, no fuel.

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Hollow-fibre and membrane filters

Effort:: Low
Needs:: Intact, never-frozen filter; additional chemical disinfection if viruses are a risk

TurbidityPartly
Retains particles but clogs quickly with cloudy water.
BacteriaYes
VirusesNo
No for typical 0.1–0.2 µm filters; only models with an explicit virus rating.
Protozoa (Giardia, Cryptosporidium)Yes
Including Giardia and Cryptosporidium.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterPartly
Partly – works with cloudy water but clogs; pre-clarifying extends its life.
Time until drinkableDepends
Drinkable immediately; flow depends on model and fouling.
Needs fuel or powerNo
No fuel or power needed.

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Ceramic filters

Effort:: Moderate; pumping and cleaning
Needs:: Undamaged ceramic element; additional chemical disinfection if viruses are a risk

TurbidityPartly
Retains particles; the element needs more frequent cleaning.
BacteriaYes
Depending on pore size and product quality.
VirusesNo
No reliable virus retention.
Protozoa (Giardia, Cryptosporidium)Yes
ChemicalsNo
Plain ceramic no; models with an activated-carbon core reduce some organic substances.
SaltNo
Does not remove salt.
Needs clear waterPartly
Partly – cloudy water slows the flow; pre-clarifying recommended.
Time until drinkableDepends
Drinkable immediately, but slow flow.
Needs fuel or powerNo
No fuel or power needed.

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Activated carbon

Effort:: Low
Needs:: Activated-carbon cartridge within its capacity; plus a disinfection method

TurbidityNo
Clogs with turbidity; pre-clarifying required.
BacteriaNo
No reliable removal.
VirusesNo
No reliable removal.
Protozoa (Giardia, Cryptosporidium)No
No reliable removal.
ChemicalsPartly
Reduces chlorine, smell, taste and some organic substances; hardly any metals, nitrate or salts.
SaltNo
Does not remove salt.
Needs clear waterYes
Yes – turbidity clogs the carbon (drawback).
Time until drinkableDepends
Immediate; contact time per manufacturer.
Needs fuel or powerNo
No fuel or power needed.

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Improvised sand and gravel filter (teaching model)

Effort:: Moderate; building and flushing
Needs:: Bottle, cloth, clean sand and gravel; a disinfection method afterwards is mandatory

TurbidityPartly
Reduces turbidity – the model does nothing more.
BacteriaNo
No reliable removal; always disinfect.
VirusesNo
No reliable removal.
Protozoa (Giardia, Cryptosporidium)No
No reliable removal.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterNo
No – its purpose is to pre-clarify cloudy water.
Time until drinkableDepends
About 30 minutes to build, then flow-through; subsequent disinfection adds time.
Needs fuel or powerNo
No fuel or power needed.

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Distillation and solar still

Effort:: High; digging or high fuel use, low yield
Needs:: Clear sheet, collecting vessel, diggable ground and sun – or pot, condensing surface and fuel

TurbidityYes
BacteriaYes
With a clean set-up; contamination via sheet or vessel is possible.
VirusesYes
With a clean set-up; contamination via sheet or vessel is possible.
Protozoa (Giardia, Cryptosporidium)Yes
With a clean set-up; contamination via sheet or vessel is possible.
ChemicalsPartly
Non-volatile substances stay behind; volatile chemicals (e.g. fuels, solvents) can carry over.
SaltYes
The only method in this overview that removes salt.
Needs clear waterNo
No – cloudy water can be used.
Time until drinkableDepends
Solar still: many hours for small, uncertain amounts. Faster with a heat source.
Needs fuel or powerDepends
Solar still: sun only. Stove distillation: a lot of fuel (drawback).

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Sedimentation and pre-filtering

Effort:: Very low, but time-consuming
Needs:: Two clean containers, cloth or coffee filter; then a disinfection method

TurbidityPartly
Reduces turbidity; very fine particles hardly settle.
BacteriaNo
No reliable removal; disinfect afterwards.
VirusesNo
No reliable removal.
Protozoa (Giardia, Cryptosporidium)No
No reliable removal.
ChemicalsNo
Does not remove dissolved chemicals.
SaltNo
Does not remove salt.
Needs clear waterNo
No – its purpose is to clarify cloudy water.
Time until drinkableDepends
Settling about an hour to overnight; then disinfection.
Needs fuel or powerNo
No fuel or power needed.

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