How to Find and Purify Water
Water advice tends to arrive as a list of methods — boil it, filter it, use tablets — as though they were interchangeable. They are not. Each one has a specific gap, and the gap is what makes people sick.
Chlorine tablets are excellent against bacteria and viruses and poor against Cryptosporidium. A typical hollow-fibre hiking filter removes protozoa and bacteria and does nothing about viruses. Boiling handles everything biological and nothing chemical. Choosing between them requires knowing what you are actually up against.
This article is the comparison table nobody prints, plus the water that is already inside your house and how to find it before you go looking for a creek.
The 10-minute version
- Boiling is the most reliable single method. Bring water to a rolling boil for one minute — three minutes above about 2,000 m. It handles bacteria, viruses and protozoa including Cryptosporidium.
- Filters are only as good as their pore size, and pore size is often marketed dishonestly. You want an absolute rating of 1 micron or smaller for Cryptosporidium and Giardia, ideally 0.3 micron or smaller, and certification to NSF/ANSI 53 or 58 for cyst reduction. Nominal ratings are not the same thing.
- Filters do not remove viruses. A 0.3 micron filter stops bacteria and parasites but viruses are smaller. If viruses are a realistic concern, boil, or filter and then chemically treat.
What is actually in the water
Three categories of biological risk, and they behave differently.
| Threat | Examples | Size | Boiling | Chlorine / iodine | Filter (≤1 µm absolute) | UV |
|---|---|---|---|---|---|---|
| Protozoa (cysts) | Cryptosporidium, Giardia | ~1–15 µm | Yes | Crypto resists chlorine; Giardia needs long contact times | Yes | Yes, in clear water |
| Bacteria | E. coli, Campylobacter, Salmonella | ~0.2–5 µm | Yes | Yes | Yes | Yes, in clear water |
| Viruses | Norovirus, hepatitis A, rotavirus | ~0.02–0.1 µm | Yes | Yes | No — too small for most filters | Yes, in clear water |
Two things to take from that table.
Cryptosporidium is the reason chemical treatment alone is not a complete answer. It is notably chlorine-resistant, which is why municipal boil-water notices exist even in chlorinated systems, and why “just add a tablet” is incomplete advice.
Viruses are the reason a hiking filter alone is not a complete answer in water that may contain human sewage — flooded urban water, for instance. Filters are excellent for a clear mountain creek and inadequate for a flooded street.
Chemical contamination is a separate problem. Boiling, filtering and chlorinating do nothing about fuel, pesticides, heavy metals or industrial runoff — and boiling actually concentrates them. Floodwater is presumed chemically contaminated, and no camping-scale treatment fixes that. Do not drink it.
The methods, in order of reliability
1. Boiling
The default, because it is the hardest to get wrong.
Bring it to a rolling boil and hold it for one minute. Above roughly 2,000 m, make it three minutes. A rolling boil means actual sustained bubbling, not the first few bubbles on the base of the pot.
- Pre-filter cloudy water through a cloth or coffee filter first — sediment shields organisms and makes the water unpleasant.
- Let it cool in the same covered vessel to avoid recontaminating it.
- Flat-tasting boiled water is normal; it has lost dissolved air. Pouring it between two containers restores the taste.
- An electric kettle that switches off at boil is sufficient for household boil-water notices in normal circumstances.
2. Filtering, done properly
Filters are convenient and fast, and the marketing around them is a minefield.
- Absolute versus nominal. An “absolute” 1 micron rating means essentially nothing larger passes. A “nominal” 1 micron filter can pass 20–30% of 1 micron particles — which is a real failure rate against exactly the organisms you are filtering for. Buy absolute.
- Look for NSF/ANSI Standard 53 or 58 with “cyst removal” or “cyst reduction” wording. That is a tested claim rather than a marketing one.
- 0.3 micron or smaller removes bacteria and parasites. Viruses still pass.
- Pre-filter turbid water. Silt clogs cartridges quickly, and a clogged filter is a dead filter.
- Never let a used filter freeze. Ice damages the internal membrane invisibly, and the filter then passes everything while appearing to work normally.
- Back-flush according to the instructions. Flow rate dropping is the warning sign.
3. Chemical treatment
Tablets and drops — chlorine dioxide, sodium dichloroisocyanurate, iodine.
- Excellent against bacteria and viruses. **Poor against Cryptosporidium.**
- Contact time matters and increases in cold or cloudy water — read the packet rather than guessing, and give it the full time.
- Iodine should not be used long-term, in pregnancy, or by anyone with thyroid disease.
- Chlorine dioxide performs better against protozoa than plain chlorine, with a longer contact time.
- Light, compact, cheap, and the right thing to pair with a filter: filter first, then treat, and you have covered every biological category.
4. UV
SteriPen-style devices and solar disinfection.
- Effective against bacteria, viruses and protozoa in clear water only. Turbidity creates shadows and organisms survive in them.
- Battery-dependent, and useless in cloudy water without pre-filtering.
- Solar disinfection (clear PET bottles in full sun for a day) works as a last resort in clear water, in strong sun. It is slow and weather-dependent.
5. Distillation
The only method that addresses salt and most chemical contamination. Boil water and condense the steam on a cool surface. It is slow and fuel-hungry, and it is the answer when the water is saline or you suspect chemical contamination.
Never drink seawater or urine. Both cost you more water than they provide.
Where the water already is
In a household emergency, you almost certainly have more water on hand than you think.
Safe with no treatment:
- Stored bottled water
- The hot water system — typically 100 to 300 litres. Turn off the power or gas to it first, then drain from the bottom valve into clean containers. Water will be rusty at first.
- Ice in the freezer
- Tinned fruit and vegetable liquid
Usually safe, treat if unsure:
- The toilet cistern — the tank, not the bowl — if there are no chemical blocks or blue cleaner in it
- Rainwater tanks, if the roof catchment is clean and there is no lead flashing or bird load. Boil it if anyone is immunocompromised.
- Water in pipes: open the highest tap in the house to let air in, then collect from the lowest.
Non-potable, useful anyway:
- Swimming pool water, for washing and flushing only. The chlorine and stabiliser concentrations are not designed for drinking, and boiling concentrates them rather than removing them.
- Bath water you filled before the event, for toilet flushing.
Do not drink: floodwater, water near industry, agriculture or roads, stagnant water with algae or a visible film, or anything with a chemical smell.
How much you actually need
3 litres per person per day for drinking, minimum, and more in heat or for anyone pregnant, breastfeeding or unwell. Three days is the standard planning figure.
| Household | 3 days | 7 days |
|---|---|---|
| 1 person | 9 L | 21 L |
| 2 people | 18 L | 42 L |
| Family of 4 | 36 L | 84 L |
Then add washing, cooking and toilet water on top. That can be non-potable, and a filled bathtub covers a surprising amount of it.
What actually goes wrong
People treat clear water as clean. Clarity says nothing about Giardia or a virus. Cold, clear, fast-running water from a beautiful creek with a dead animal or a cattle paddock upstream will make you very unwell.
They buy a filter and stop thinking. Filters do not remove viruses. In flood or sewage-affected water, that matters enormously.
They rely on tablets against Cryptosporidium. Chlorine-based treatment is the weakest common method against exactly the organism most likely to be behind a boil-water notice.
They cut the contact time. Cold and cloudy water needs longer, and “close enough” is where infections come from.
They let a filter freeze, then keep using it. There is no visible sign of the damage.
They recontaminate treated water by pouring it back into the bottle they collected it in, or by handling it with unwashed hands.
They ignore the hot water system, then go looking for a creek — with 200 litres of drinkable water sitting in a cylinder in the laundry.
What it costs
| Tier | Spend | What it covers |
|---|---|---|
| Free | $0 | Boiling on the stove; knowing how to drain the hot water system; filling the bath and every container when a warning is issued; a cloth for pre-filtering |
| Solid | ~$30–60 | A box of chlorine dioxide tablets ($15), a 10 L collapsible container ($15), a couple of 6 L bottles rotated every six months |
| Good | ~$120–200 | The above plus a certified absolute-rated filter with cyst-reduction certification, and a 40 L storage drum with a tap |
| Remote or long-term | $400+ | A gravity-fed filter system for a household, or a filter plus chemical treatment plus a means of distillation if salinity or chemical contamination is a realistic concern |
The free tier does most of the work in a household emergency. Filters matter most when you are away from a stove.
The drill: half an hour, twice a year
- Find and drain a litre from your hot water system. Turn off its power or gas first. Locate the valve, learn the technique, put the litre on the garden. Doing this once means knowing where 200 litres is when you need it.
- Boil a pot properly and watch what a rolling boil actually looks like versus the first bubbles. Most people stop far too early.
- Pre-filter muddy water through a cloth, then boil it. Note how much sediment comes out.
- Read your filter’s actual rating. Absolute or nominal? Micron size? NSF certified? Many people discover their filter is not what they assumed.
- Check your tablets’ expiry date and the contact time on the packet. Write the contact time on the container in marker.
- Rotate stored water. Date the bottles, six-monthly.
Frequently asked questions
How long do you actually have to boil water?
Bring it to a rolling boil and hold it for one minute — three minutes above about 2,000 m elevation. A rolling boil, not the first bubbles.
Does boiling remove chemicals?
No, and it concentrates them as water evaporates. Boiling addresses biological contamination only. For chemical contamination the answer is a different water source, or distillation.
Will a hiking filter make floodwater safe?
No. Filters do not remove viruses, and floodwater carries both sewage and chemical contamination that no camping-scale treatment addresses. Do not drink floodwater.
Are purification tablets enough on their own?
Usually not. They are strong against bacteria and viruses but weak against Cryptosporidium. The robust combination is filter first, then chemically treat — or simply boil.
Can I drink rainwater from my tank?
Generally yes if the roof catchment is clean, there is no lead flashing, and the tank is maintained. Boil it if anyone drinking it is very young, elderly, pregnant or immunocompromised, or if the tank has been disturbed by flooding or fire.
Is pool water drinkable in an emergency?
Use it for washing and flushing, not drinking. Pool chemical concentrations are not designed for consumption, and boiling concentrates them further.
How much water should I store at home?
Three litres per person per day for drinking, at least three days — 9 litres per person. A family of four needs about 36 litres, which is roughly one supermarket shop. Store washing water separately.