A biosand filter is a bucket or drum packed with graded sand, kept permanently wet, with a living layer of microorganisms growing on the sand surface. Water goes in the top, comes out clean enough to look at without flinching, and the filter keeps working for years on nothing but gravity. The playbook's water page tells you why stored water runs out faster than anyone expects. This is the thing you build so it stops mattering.
It is not a survival gadget. It is the same slow sand filtration that supplied Victorian British cities, shrunk to a household.
What this filter does, and what it does not
What it is genuinely excellent at is the job that exhausts every other method: taking water that is cloudy, smelly and full of silt, and making it clear. Clear water is what chlorine needs to work at all. Bleach dosed into muddy water gets used up on the mud rather than the pathogens, which is why the bleach ratios double for cloudy water and why they still cannot be trusted on genuinely dirty input. Run the water through sand first and your bleach goes four times further.
So the honest way to think about it: the filter buys you clarity and takes out the big organisms. Disinfection after it takes out the rest. Two steps, always, in that order.
What you need
The single most important number there is the sand grain size, and it is the one people get wrong. Builders' sharp sand from a UK merchant is usually too coarse and lets bacteria through; play sand is often too fine and clogs within weeks. You want something close to 0.15 to 0.30 mm, and the practical test is that it should feel like caster sugar rather than granulated, and should not have visible stones in it.
Wash the sand until the rinse water runs clear. This takes far longer than you expect, often ten or fifteen changes of water, and skipping it is the most common reason a new filter runs cloudy for a month.
Building it
The geometry that makes this work is not obvious, so it is worth understanding rather than copying.
- Drill the outlet low, run the pipe high. The outlet leaves the container at the very bottom, then the pipe turns and rises up the outside, finishing exactly 5 cm above the level the sand will reach. Water can only leave once it has risen that high, which is what holds the 5 cm standing layer in place when the filter is at rest.
- Lay the drainage gravel first, 6 to 12 mm, about 5 cm deep, covering the inlet of the outlet pipe completely.
- Then the separating gravel, 0.7 to 6 mm, about 5 cm deep. Its only job is to stop the fine sand washing down into the coarse gravel and out of the pipe.
- Then the washed filtration sand, 45 to 50 cm deep. Level it flat.
- Fit a diffuser plate above the sand: a perforated tray or a plate with small holes, sitting a few centimetres below where the water surface will be. Without it, pouring water in gouges a hole straight through the sand surface and destroys the biolayer you are about to spend a month growing.
Charging it and growing the biolayer
A new filter is just sand. It filters by physical straining alone, which is worth something but nothing like the finished article. The biological layer on the sand surface, sometimes called the schmutzdecke, is what does most of the pathogen removal, and it has to be grown.
Run the filter normally, 10 to 12 litres at a time, once or twice a day. Expect 7 to 30 days before it performs properly, and understand that on poor source water full maturation can take several months. During this period treat the output as merely clearer, not cleaner. Disinfect it as you would the raw water.
The rhythm matters more than the volume. The biolayer feeds on what the water brings it and then needs a rest period to consume it, so a filter used in one daily batch outperforms one trickled through continuously.
Running it day to day
Pause between batches: 6 to 12 hours is the target. One hour is the minimum. Forty-eight hours is the maximum before the biolayer starts to starve.
Pour a batch in, let it run through, leave it. Come back that evening or the next morning and pour the next. That is the whole operation.
Two habits worth building:
- Pour gently onto the diffuser, never straight onto the sand. One careless pour undoes weeks of maturation.
- Keep the lid on. An open filter is an invitation to mosquitoes, which turns your water supply into a breeding site.
When it slows down
Flow dropping off is normal and expected. It means the biolayer and trapped sediment are doing their job and gradually blocking the surface.
The fix is called wet harrowing, and it is deliberately gentle:
- Drain the water down to just above the sand.
- Stir only the top centimetre or two of sand with your hand or a stick, enough to cloud the water.
- Scoop out the dirty water.
- Refill and run as normal.
Flow returns immediately. The biolayer is partly disturbed, so give it a few days to recover before trusting it again.
Do this when the flow becomes annoying, not on a schedule. If harrowing stops restoring the flow, or the sand bed has dropped noticeably over the years, dig out the top 5 cm, wash it thoroughly, and put it back. A well-built filter runs for a decade or more before it needs anything more drastic than that.