Pine-branch water filter

This filter uses a thin disc of pine branch to clean water by gravity, with no electricity. It belongs in highland Kenya where pine is actually available. It is not certified drinking-water treatment. Boil or chlorinate drinking water if you can. This does not remove chemical poisons such as fluoride or arsenic.

Time	2 hours of work plus 1 hour of soaking; 1–2 extra days if you dry the discs
Difficulty	Medium
Cost	KSh 200–800 without alcohol; 500–1,400 with food-grade ethanol
Location	Highland counties with plantation pine, for example Nyandarua, Nakuru, Kiambu, Kericho, Elgeyo Marakwet

Materials

One straight, green pine branch about 4 cm across (Pinus patula or another pine). Use fallen wood or prunings, with the landowner's permission.
Two cleaned 1.5 L plastic bottles
About 1 m of plastic hose
Two hose clamps, or wire and rubber tape that can make a tight seal
A clean cotton cloth to catch mud before the wood
Hot water at 60–65 °C for 1 hour
Food-grade ethanol or pharmacy isopropyl if you want to store dry discs — not methylated spirit
A clean, covered vessel for the filtered water

Tools

A sharp knife or small saw
A knife to peel bark
A pan and stove to hold hot water (not a rolling boil)
A marker for the top and bottom of the branch

Safety

This is not an approved water filter. Do not treat the output as safe to drink. If you can, boil after filtering, or use household chlorine as labelled.
Use pine, or a cypress hedge only as an unproven stand-in. Do not use flowering trees such as eucalyptus, mango, or grevillea: their vessels are too long and a short disc will not filter.
Do not use yew or other poisonous trees. If you cannot name the tree, do not build this.
Methylated spirit contains methanol. Never put it on a drinking-water filter.
If you used ethanol, discard the first flush. Do not drink it.
Wear gloves when cutting. Do not fell a live tree without permission.
A wetted disc rots in a few days. If it smells bad, turns dark, or flow dies, throw it away.

Steps

Choose a straight green pine branch about 4 cm across. Mark the end that pointed toward the tip of the branch.
Cut the branch and put it in water at once so it does not dry.
Peel the bark and the soft layer under it until pale outer wood shows all the way round. Do not use the dark inner heartwood.
Cut one or more discs about 1 cm thick, square across the grain — not on a slant.
Soak the discs in water at 60–65 °C for 1 hour. Do not boil; boiling damages the membranes inside.
If you will use a disc today: keep it in cold water until you mount it. If you will store it: soak it in food-grade ethanol or isopropyl for 24–48 hours, then dry it on edge in shade or at under 45 °C. Do not use methylated spirit.
Cut the base off one bottle to make a dirty-water tank. Join the neck to the hose. Set the tank about 1 m above the filter.
Clamp the disc in the hose so the wood fills the opening. Water must go through the wood, not around it. Test with clean water first; if it leaks, tighten and try again.
Cover the tank opening with cotton cloth. Pour settled water through the cloth, not heavy mud straight onto the wood.
Discard the first water through the filter (especially after ethanol). Then collect filtered water in a clean, covered vessel.
Replace the disc every 1–4 days, or as soon as the flow dies. A wetted disc is not reused after it rots; a treated dry disc can wait up to two years before first use.

Local adaptations

Plantation pine is a highland tree. On the coast or in lowlands without pine, do not swap in a local flowering tree. Cypress hedges (Cupressus lusitanica) are non-flowering and common in towns, but the published tests used pine and ginkgo, not Kenyan cypress; treat that swap as an experiment. Very muddy water, such as rain ponds, still needs the cloth and may clog a disc quickly. In areas with high fluoride in borehole water, this filter does not help.

Sources

Ramchander, Hegde, Antony, Wang, Leith, Smith and Karnik, "Engineering and characterization of gymnosperm sapwood toward enabling the design of water filtration devices", Nature Communications, 2021 (lab tests and field work in India).
Boutilier, Lee, Chambers, Venkatesh and Karnik, "Water filtration using plant xylem", PLOS ONE, 2014.
MIT News, "MIT engineers make filters from tree branches to purify drinking water", 25 March 2021.
Open fabrication notes: http://www.xylemwaterfilter.org/

Before you start

MIT lab tests reported more than 99% removal of E. coli and rotavirus in spiked test water, and lower coliform counts in spring, tap, and groundwater in India. That is not a guarantee for your kitchen. Tree species, disc thickness, leaks, and how dirty the water is all change the result.

This does not remove chemicals (fluoride, arsenic, farm pesticides), and it has not been certified as a WHO household treatment for everyday use. Boiling is still the more reliable method in most Kenyan homes.

How it works

Pine pulls sap through narrow conduits crossed by membranes with very small pores. Bacteria are larger than those pores. A disc of green sapwood can therefore hold microbes back — if the water cannot sneak around the wood. Flowering trees have long vessels; a short disc will not stop them.

Untreated dry wood seals those membranes and flow dies. A hot-water soak plus ethanol is how the researchers stored discs dry. Once a disc is wetted and used, it clogs and then rots, which is why it lasts days, not months.

Using and replacing

Raise the tank so gravity does the work. The prototype rate was about 1 litre per hour, not a kitchen tap.
Keep filtered water in a clean vessel. The filter does not help if you dip a dirty cup into it.
When flow dies, the disc is finished. Do not recut it to "open" it; cut a new disc.
Do not dry a used disc to use it again.

Common problems

Water races through. It is bypassing the wood. Tighten the clamps; recut the disc if it does not fill the hose.
No flow. The disc dried without treatment, is too thick, or has clogged. Start from green wood or a treated disc; keep thickness near 1 cm.
Bad smell. It has begun to rot. Throw it away, wash the bottles and hose, cut a new disc.
You used eucalyptus. Do not drink that water. Start again with pine.