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    Safe water in the wild

    Water 4 h 20 slides

    Drink safely from any source

    Learning objectives

    By the end of this module you'll be able to:

    • Identify the pathogens in natural water and choose the right treatment (filter, UV, chemicals).
    • Understand why a 0.1 μm mechanical filter does not stop viruses and when to combine methods.
    • Apply the UV protocol (clear water first) and the WHO-validated chemical doses for each situation.
    • Read a natural water source and draw water from the right spot before any treatment.
    • Calculate your water needs by activity and plan safe survival rationing.
    • Maintain filters and containers so they stay reliable over time.

    Slide 1 of 20

    Field microbiology

    Biological risks of water in the wild

    What can make you ill — and how to get rid of it

    PathogenSizeRemoved by 0.1 μm filterRemoved by UVRemoved by chlorineSymptom onset
    Bacteria (E. coli, Salmonella)0.2-10 μm✓ Yes✓ Yes✓ Yes6-48h (severe gastroenteritis)
    Protozoa (Giardia, Cryptosporidium)2-15 μm✓ Yes (0.1 μm)✓ Yes⚠️ Chlorine-resistant1-4 weeks (persistent diarrhoea)
    Viruses (norovirus, hepatitis A)0.02-0.1 μm✗ No (too small)✓ Yes (254 nm)✓ Yes12h-6 weeks
    Heavy metals (Pb, As, Hg)Dissolved ions✗ No✗ No✗ NoChronic (not acute)
    Nitrates (NO₃⁻)Dissolved ions✗ No✗ No✗ NoChronic (except for infants)
    Turbidity (clay, organic matter)Variable✓ Partly⚠️ Reduces UV effectiveness⚠️ Reduces effectiveness—

    The critical rule: a mechanical filter alone (LifeStraw, Sawyer) does NOT protect against viruses. In western Europe the viral risk in fresh water is very low. Outside Europe (Asia, Africa, South America) or with very turbid water: filter + UV are a must.

    Knowledge check

    Why is a 0.1 μm mechanical filter not enough outside Europe?

    Slide 2 of 20

    Filter technology

    Filter technology — how it really works

    Mechanical filtration by pore size — the physics behind the product

    TechnologyFiltration thresholdMechanism2026 productsCapacity
    Hollow-fibre filterField standard0.1 μmPore-size exclusion in hollow fibresSawyer Squeeze, MSR TrailShot378,000 L (Sawyer)
    Ceramic filter0.2-0.3 μmExclusion by ceramic porosity + adsorptionKatadyn Hiker Pro, Doulton1,500-2,000 L
    Activated carbon filterNo bacterial thresholdChemical adsorption (taste, colour, chlorine)Only as a complement300-1,000 L
    Reverse osmosis0.001 μm (desalination)Pressure → semi-permeable membraneKatadyn PowerSurvivor1.5 L/h
    Sawyer backflushing — critical maintenance — Every 5-10 uses: flush it in reverse with the syringe supplied. A filter that is not backflushed loses 50-80% of its flow in a few days. Never let a wet filter freeze (it breaks the fibres → the filter is useless).
    Filter integrity test — Submerge the filter in water and tilt it. If bubbles come out without squeezing = damaged filter (broken fibres). A cracked filter no longer filters anything. Test after every drop or freeze.

    Slide 3 of 20

    SteriPen UV

    SteriPen UV — physics and how to use it

    The UV lamp destroys the DNA of micro-organisms — but only if the protocol is followed

    Lethal UV dose for pathogens

    Dose (mJ/cm²) = Irradiance (mW/cm²) × Time (s)

    16 mJ/cm²=Minimum dose for 99.99% of E. coli (4 log)

    40 mJ/cm²=Dose for viruses and cryptosporidium (4 log)

    0.9 L of clear water=Maximum volume per SteriPen Adventurer cycle (90s)

    Ex.SteriPen Adventurer: 4,000 μW/cm² × 90s = 360 mJ/cm² — well above the thresholds for every pathogen in CLEAR water
    The water must be CLEAR — absolute rule — UV is absorbed by suspended particles. Turbidity > 1 NTU: effectiveness drops by 40-70%. Filter mechanically first (0.1 μm or a fine cloth), then treat with UV.
    SteriPen protocol — 1. Clear water in a clean container. 2. Switch on. 3. Immerse and stir slowly for 90 seconds (1L). 4. Green light = treatment confirmed. 5. Drink within 2h (the treatment does not protect against recontamination).
    Managing CR123 batteries off-grid — The SteriPen Adventurer uses 2× CR123. One pair treats 100 L. Always carry at least 2 spare pairs for a 2-week trek. Store the batteries separately (condensation and knocks).
    Battery alternative: USB-rechargeable models — SteriPen Ultra (USB-C): 50 treatments per charge. Works with a power bank. Ideal if you have a solar panel or hub dynamo.

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