Water And Hygiene · Volume 5
Building the Gravity System, and Treating What Comes Out of It
Introduction
These are working notes rather than an essay: what to buy, how to assemble it, the order of operations in camp, and how to treat the filtered water so it is still safe three days later.
🔴 Read the caution at the end before relying on any dosing figure here. The numbers below come from EPA and CDC emergency-disinfection guidance and are reproduced in good faith, but guidance changes and this page is not the authority. Check the current EPA page before a trip that depends on it.
5.1 Why build rather than buy
Complete gravity kits exist and work. The reason to assemble one is that it lets each part be chosen on its merits — and on this build one of those merits outranks the others.
🔴 Freeze tolerance is the selection criterion in Michigan. A used filter is wet inside; one below-freezing night cracks the fibres, and it still passes water afterwards while no longer filtering. Most compact hollow-fibre filters — including the ones inside most complete kits — are not freeze-tolerant. ⭐ The Platypus QuickDraw and the MSR Guardian are engineered to survive it, which makes the QuickDraw the sensible heart of a shoulder-season system.
Building also means the dirty bag, the tubing and the clean container can be replaced individually, rather than binning a kit because one bag split.
5.2 The parts list
Table 1 — The parts list
| # | Part | What to get | Approx. |
|---|---|---|---|
| 1 | Filter element | ⭐ Platypus QuickDraw — hollow-fibre, 0.2 µm, freeze-tolerant, standard 28 mm bottle threads | ~$40 |
| 2 | Dirty reservoir | 4–10 L. A purpose gravity/dirty bag, or a roll-top dry bag with a bulkhead fitting, or simplest — a hydration bladder, which already has a hose port | ~$20–35 |
| 3 | Tubing | Food-grade vinyl or silicone, ¼ in ID, ~4 ft. Buy twice what you need | ~$8 |
| 4 | Inline shutoff | A pinch clamp or small ball valve, so the flow can be stopped without unhooking anything | ~$5 |
| 5 | Clean container | ⭐ A water-spec Rotopax, or a lidded bucket kept solely for clean water | owned |
| 6 | Hang kit | Carabiner + 6 ft of webbing or paracord | ~$10 |
| 7 | Backflush syringe | Usually supplied with the filter — keep it with the kit | incl. |
| 8 | Pre-filter | Nylon paint strainer bag (5 gal size) — see vol 4 | ~$5 |
| Total | ~$90–105 |
⚠ Check the thread or barb sizes match before ordering tubing. The QuickDraw uses 28 mm bottle threads; adapters to hose barbs are cheap but they are a separate purchase and it is annoying to discover that at camp.
5.3 Assembly
1. Fit the dirty bag’s outlet. A hydration bladder already has one. A dry bag needs a bulkhead fitting through the lower side wall — drill undersize, fit the gasket both sides, snug it, and test with plain water at home.
2. Cut the dirty-side hose short — about a foot from bag to filter. Long dirty hose is dead volume and one more thing to kink.
3. Filter in line, oriented correctly. Hollow-fibre filters are directional. The arrow points to the clean side. Fitting it backwards will still pass water, which is exactly why it needs checking.
4. Cut the clean-side hose long enough to reach the container comfortably — three feet is plenty — and fit the shutoff clamp on this side.
5. Colour-code the two hoses. 🔴 Tape, heat-shrink, marker, anything. Dirty and clean must never be swapped, and under a headtorch at dusk two identical grey hoses will be.
6. Cap the clean end. A short length of tubing folded and clipped, or a proper cap. It must never touch the ground, the lake, or the outside of the dirty bag.
7. Test the whole thing at home with tap water, timed. You want to know its real flow rate and its real quirks before it matters.
5.4 The field procedure
1. Choose the source. 🔴 Flowing over still. Upstream of camps and roads. Never anything with scum, paint-streaking, pea-soup colour or a musty smell — no filter on this page makes a bloom safe.
2. Draw from mid-depth into the dirty bucket. Do not stir the bottom; do not skim the surface.
3. Settle 30 minutes if the water is anything but clear. Decant the clear top two-thirds; tip the dregs away from the source.
4. Pour through the paint strainer into the dirty bag.
5. Hang the bag at least four feet above the clean container. ⭐ Flow scales with head — a bag hung at shoulder height runs noticeably faster than one on a tailgate.
6. Purge the air. Open the clamp with the clean hose held low and let it run to waste for a few seconds. ⚠ An air lock in the clean line will stop a perfectly good filter dead, and it is the commonest reason a new setup “does not work.”
7. Let it run. Roughly 1.5–1.75 L/min — about 11 minutes for 5 gallons. Go and do something else.
8. Backflush at the end of every session, not when flow gets bad. Syringe clean water backwards through the filter until it runs clear. This is the single maintenance habit that determines whether the cartridge reaches its rated life.
9. Treat the clean container for storage — see below.
10. Protect it from freezing. 🔴 The used filter comes into the cab overnight in shoulder season. It does not stay in the bed.
5.5 Treating the filtered water
⭐ THE PRINCIPLE: filtration and chemical treatment do different jobs, and each covers the other’s weakness.
The filter removes Cryptosporidium — which is notoriously chlorine-resistant — along with Giardia, bacteria and sediment. 🔴 What it does not leave behind is any residual. The moment water enters the container it is defenceless against anything the funnel, the spout, a hand or the air introduces.
The chemical provides that residual. It is poor against Cryptosporidium, which does not matter here because the filter has already removed it.
⭐ That symmetry is the whole reason to do both, and it also means the long Cryptosporidium contact times quoted on tablet packaging do not apply to already-filtered water. You are dosing for residual, not for primary treatment.
5.5.1 Option A — unscented household bleach ⭐ recommended for bulk
Cheap, available anywhere, and the right tool for dosing gallons at a time.
Per EPA emergency-disinfection guidance, for clear water:
Table 2 — Per EPA emergency-disinfection guidance, for clear water
| Volume | 6% bleach | 8.25% bleach |
|---|---|---|
| 1 gal | 8 drops (⅛ tsp) | 6 drops |
| 2 gal | 16 drops (¼ tsp) | 12 drops |
| 2.5 gal | 20 drops | 15 drops |
| 5 gal | 40 drops (⅝ tsp) | 30 drops (≈½ tsp) |
| 10 gal | 80 drops (1¼ tsp) | 60 drops (≈1 tsp) |
Then: stir, and let it stand 30 minutes. ⭐ It should have a faint chlorine smell afterwards. If it does not, repeat the dose and wait another 15 minutes — that smell is the confirmation the dose was adequate.
🔴 Double the dose if the water is cloudy, coloured or very cold. Filtered water should not be cloudy, which is another argument for filtering first.
⚠ The bleach itself matters as much as the dose:
- Unscented only. No colour-safe, no added cleaners, no scent.
- 6% or 8.25% sodium hypochlorite — check the label; concentrations vary.
- 🔴 Bleach degrades. Use stock less than a year old, stored cool and dark. Old bleach is weak bleach, and it will not announce that.
- Carry it in a small dropper bottle, labelled, dated.
5.5.2 Option B — chlorine dioxide tablets
Katadyn Micropur MP1 and similar: one tablet per litre. Against unfiltered water the label times are 15 minutes for viruses and bacteria, 30 minutes for Giardia, and 4 hours for Cryptosporidium. ⭐ On filtered water only the short times are relevant.
Better tasting than bleach and far more convenient in small volumes — but at roughly a dollar a tablet, dosing 5 gallons means about 19 tablets. ⚠ That is not a bulk-storage solution.
⭐ Carry both: bleach for the containers, a strip of tablets in the day pack and as the backup if the filter fails.
5.5.3 What not to use
Iodine — poor against Cryptosporidium, tastes bad, and is not appropriate for extended use or for people with thyroid conditions.
UV pens — genuinely effective on clear water, but leave no residual at all, so they solve nothing for storage. They also need batteries and clear water.
Boiling — completely effective and the fallback if everything else fails. ⚠ A rolling boil for one minute is the standard. It leaves no residual, and it costs fuel.
5.6 Storage discipline
- Sanitise containers before the season: a bleach solution, rinsed, air-dried.
- Re-dose water stored more than a few days — chlorine dissipates.
- 🔴 Dispense from a tap or by pouring. Never dip anything into the clean container.
- Keep containers out of the sun. Warm water in a translucent container grows things faster and drives off chlorine.
- ⚠ Label them. Water containers, water only, permanently.
5.7 🔴 Before this is relied on
These are one owner’s working notes, not a public health document. The dosing figures reproduce EPA and CDC emergency-disinfection guidance as published, and 🔴 that guidance is periodically revised — there is active academic criticism of the current EPA bleach recommendations in the literature.
Check the current EPA “Emergency Disinfection of Drinking Water” page before any trip that depends on these numbers, and follow the manufacturer’s instructions on whatever tablet or filter is actually in the kit over anything written here.
⭐ And the point that outranks every number on this page: none of this makes a bad source safe. Filtration, carbon and chlorine together do not remove cyanotoxins, PFAS, or dissolved metals. Choosing where to draw from is the primary control. Everything here is the second line.
References
- US EPA — Emergency Disinfection of Drinking Water: 8 drops of 6% or 6 drops of 8.25% unscented household bleach per gallon of clear water; double for cloudy, coloured or very cold water; stir and stand 30 minutes; slight chlorine odour expected. Use bleach less than one year old.
- CDC — Making Water Safe in an Emergency: consistent guidance; boiling one minute at a rolling boil as the primary fallback.
- Peer-reviewed criticism of current EPA bleach dosing exists in the environmental-science literature — a reason to check the live guidance rather than a reason to disregard it.
- Katadyn Micropur MP1 — chlorine dioxide, 1 tablet per litre; 15 min bacteria and viruses, 30 min Giardia, 4 hours Cryptosporidium.
- Platypus QuickDraw — 0.2 µm hollow-fibre, freeze-tolerant, 28 mm bottle threads.
- Water and Hygiene vols 1–4, this project — payload, filtration limits, the shower, and pre-filtering.
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