Interactive lesson

Drinking water, step by step

Interactive lesson: from the lab test to the glass — what can be in water, how to take a sample and read the results, filters, boiling, chlorine, UV and storing water.

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This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. What can be in the water

Bacteria, nitrate and arsenic don't change the colour, smell or taste of water.

Clear isn't clean — Bacteria from manure and septic tanks have no colour and no smell. Clear water from a well is not yet safe drinking water.
Septic tank and manure — They are the source of E. coli, enterococci and viruses: diarrhoea, vomiting, jaundice. What helps is protecting the source, then chlorine, UV or boiling.
Nitrate from fields — It comes from fertiliser, manure and septic tanks and is dangerous for babies. Drinking water may contain at most 50 mg/L.
Arsenic from deep rock — In parts of the US and Canada, and locally in the UK and Ireland near old mining areas, groundwater can naturally contain arsenic, clear and tasteless. The limit is 10 µg/L: with a drilled well or borehole, ask for an arsenic test.

Why: "We've drunk it all our lives" doesn't mean the water is safe. Adults often get used to poor water, with the odd upset stomach, but a baby, a pregnant woman or an elderly person can't.

A lab test is the only way to know whether water is safe to drink.
If the source is constantly contaminated, no filter will make it safe in the long run. Protecting the source comes first (H04-02).

2. Taking a sample

A badly taken sample either passes bad water or fails good water.

When to sample — Test for bacteria at least once a year, ideally after heavy rain in April or October, when the risk is highest.
Sterile bottle — Get the bottle for bacteria from the lab. Don't open it before sampling, and don't touch the inside or the cap.
Prepare the tap — Remove the aerator and any attachments. Clean the tap and wipe it with alcohol or flame it briefly with a lighter, metal taps only.
Run, fill, chill — Run the water for 3–5 minutes, fill the bottle to the mark without overfilling and close it at once. Label it, chill it without freezing and deliver it the same day.

Why: Test before the first drink from a new source, for bacteria once a year, and at once after a flood, work on the well, or a change in colour, smell or taste. A private well or private supply package is offered by accredited or state-certified labs; your local authority or county health department can point you to one.

Take the sample from the tap you drink from, or the one closest to the well. Not a hose, not a dripping tap, not a tap with a filter.
After chlorinating a well, wait 7–14 days and until the smell of chlorine has completely gone.