Slow sand filter
Clean drinking water with no chemicals, no power and no moving parts — the highest lives-saved-per-effort structure on this site.
Home: Make water safe, Level 0 · Triage, level 1 of 3 in its track. Serves 3 steps across 2 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
This is not really a filter. Straining water through sand removes turbidity and very little else; a pore that stops bacteria would clog in an hour.
What actually does the work is a living layer — the schmutzdecke — that establishes itself on the top few millimetres of sand over two to four weeks. It is a biological community that consumes, traps and outcompetes the organisms in the incoming water. Properly ripened and properly operated, a slow sand filter removes the great majority of bacteria and protozoan cysts, and it does so with no chemicals, no power, no consumables and no moving parts.
The implications for how you run it are the whole point, and they are counter-intuitive:
- It must never be allowed to dry out or drain. The layer is alive. Let the water level drop below the sand and it dies, and you are back to two to four weeks of ripening with no protection in the meantime.
- It must run slowly. Fast flow tears the layer and pushes water through untreated.
- It must be fed clear water. Turbid water blinds it in days. Settle first.
- A new filter is not a filter. For the first weeks it looks and behaves identically to a working one, while doing almost nothing.
Build it early, ripen it before you need it, and boil in the meantime.
Section through the filter
Fig 1
Masonry / brickLoose fill / grainGravel / aggregateWaterFire / hot zoneVoid / air
Media grading and flow rate
Fig 2
Loose fill / grainGravel / aggregate
Keyed parts
| 1 | Inlet with a diffuser plate | Breaks the fall of incoming water so it does not scour a crater in the sand bed. |
| 2 | Supernatant water, 100–150 mm standing | Constant head above the sand. Also gives the biological layer its dissolved oxygen. |
| 3 | Schmutzdecke — the living layer | The top few millimetres. This is the filter. Everything else is plumbing. |
| 4 | Filter sand, 600–900 mm | Washed, graded, sharp. Depth is the safety margin against short-circuiting. |
| 5 | Graded gravel support, 100–150 mm | Coarse at the bottom, fine at the top, so sand cannot migrate into the underdrain. |
| 6 | Underdrain | Perforated pipe or a channel of stones collecting filtered water across the whole floor. |
| 7 | Outlet weir above the sand surface | The single most important detail. It makes draining the bed physically impossible. |
| 8 | Cover | Keeps out light, leaves, insects and animals. Light grows algae, which blinds the bed. |
| 9 | Scum drain | For skimming floating debris off the supernatant without disturbing the bed. |
Specifications
| Bed area | 1.0 m² | Scale by area, never by making one filter faster |
| Sand depth | 0.90 m | 0.6 m minimum. Depth is your margin against short-circuiting and scraping losses |
| Sand grain size | 0.15–0.35 mm effective | Sharp river or pit sand, thoroughly washed. Not soil, not silt |
| Gravel support | 120 mm, 3 graded layers | 15–30 mm, 5–15 mm, 2–5 mm bottom to top |
| Supernatant depth | 100–150 mm | Constant. Provides head and dissolved oxygen to the biological layer |
| Filtration rate | 0.1–0.3 m/h | ≈ 100–300 L/h per m². Exceeding this defeats the whole mechanism |
| Ripening period | 2–4 weeks | Longer in cold water. Do not rely on the filter during this time |
| Raw water turbidity | < 10 NTU ideally | Settle or roughing-filter turbid water first, or the bed blinds in days |
| Scraping interval | 1–3 months | When the flow falls off. Remove only the top 10–20 mm |
| Re-sanding | when depth < 0.6 m | After perhaps 10–20 scrapings |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Tank or basin | 1 m² × 1.4 m deep | Masonry, rendered brick, ferrocement, a lined pit, or a food-grade drum. Must be watertight and opaque. |
| Filter sand | ≈ 0.9 m³ | Sharp, graded, and washed until the rinse water runs clear — this takes far longer than expected and is not optional. |
| Graded gravel | ≈ 0.12 m³ | Three sizes, washed. |
| Underdrain | ≈ 3 m | Perforated pipe, split bamboo, or a stone-covered channel across the floor. |
| Outlet pipe and weir box | 1 | Arranged so the lip stands above the sand surface. This detail is the whole safety design. |
| Diffuser plate | 1 | A perforated board or stone slab under the inlet to stop scouring. |
| Cover | 1 | Timber, thatch or slab. Excludes light, leaves, insects and animals. |
Tools: Shovel and buckets · Sieves or screens for grading · Trowel · A measuring vessel and something to time with · Clean container for the washed sand
Construction
Site above and away from contamination
Uphill and well clear of any latrine, animal pen or washing point, and at least 30 m from any of them. Site it so filtered water can be drawn without anyone walking on or reaching into the bed.
✓ Check: Nothing upslope within 30 m that could drain toward it.
Build the tank and set the underdrain
Watertight tank with a level floor. Lay the underdrain across the whole floor so water is collected evenly — a drain at one corner draws all the flow through one part of the bed and leaves the rest idle. Plumb the outlet now, with its lip above where the sand surface will be.
✓ Check: Fill with water and leave overnight. No measurable drop.
Wash the sand. Then wash it again.
This is the tedious step that decides everything. Wash in batches, stirring and pouring off, until the rinse water runs genuinely clear. Silt and clay left in the sand will blind the bed within days and cannot be removed once it is placed.
✓ Check: A jar of washed sand shaken with clean water settles clear within a minute.
Place gravel and sand in water
Lay the graded gravel coarse-to-fine. Then part-fill the tank with clean water and place the sand into the water, a little at a time. Placing sand dry and then flooding traps air pockets that create channels straight through the bed. Level the surface gently; do not compact it.
✓ Check: No visible air bubbling up as the last sand goes in.
Fit the diffuser and set the flow
Fit the diffuser plate under the inlet. Start the flow and set it to 0.1–0.3 m/h by timing the outlet into a measured vessel. For a 1 m² bed that is roughly 2–5 litres per minute. Adjust at the outlet, not the inlet.
✓ Check: Measured outflow within range, and the sand surface undisturbed under the inlet.
Ripen it — and boil in the meantime
Run continuously for two to four weeks before trusting it. During ripening the filter looks and behaves exactly like a working one while removing very little, which is precisely why this period is dangerous. Boil all drinking water until it is ripe.
✓ Check: A visible brownish biological layer has developed on the sand surface, and outflow is consistently clear.
Run it continuously, forever
The layer is alive and must be kept wet and fed. Continuous flow is far better than intermittent. If you must stop, keep water standing above the sand — the outlet weir arrangement does this automatically, which is why it is plumbed that way.
✓ Check: Water always standing above the sand, every day, without exception.
Scrape, do not dig
When the flow falls below usable, drain the supernatant to just above the sand, and scrape off only the top 10–20 mm. Do not dig into the bed, do not stir it, do not replace the whole surface. Refill gently and expect a shorter re-ripening of a few days.
✓ Check: Flow restored; sand depth still above 0.6 m.
Keep two filters
The single most valuable operational decision. Two beds in parallel means you can scrape and re-ripen one while the other supplies water, and you are never forced to choose between a blocked filter and an unripe one.
✓ Check: Both beds ripe, and a written record of which was last scraped and when.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Flow rate | 0.1–0.3 m/h | Faster tears the biological layer and pushes water through untreated, with no visible change in the output. |
| Sand depth | ≥ 0.6 m at all times | Below this, water can short-circuit and pathogens pass. |
| Water above sand | always > 0 | The layer is living. Dry it out and it dies, and there is no warning that it has. |
| Sand cleanliness | rinse runs clear | Residual silt blinds the bed permanently within days. |
| Grain size | 0.15–0.35 mm | Coarser and pathogens pass; finer and it blocks almost immediately. |
| Scraping depth | 10–20 mm only | Deeper removes bed depth needlessly and lengthens re-ripening. |
How this fails
Trusting a new filter. For two to four weeks it looks identical to a working one and removes very little. This is the most dangerous failure here precisely because it is invisible — boil during ripening.
Letting the bed drain. Kills the biological layer, silently. The outlet-above-sand arrangement in Fig 1 makes this physically impossible, which is why it is worth the extra plumbing.
Running it too fast because more water is wanted. Output looks the same and treatment collapses. Add a second filter instead; never speed up an existing one.
Feeding it turbid water. Blinds the surface in days. Settle in a tank or run a roughing filter first.
Unwashed sand. Blinds the bed on commissioning and the only fix is to remove and wash everything.
Digging over the bed when it slows. Destroys the layer and mixes fines through the depth. Scrape the top only.
Recontamination after filtering. Filtered water put into the dirty bucket that carried the raw water. Keep raw and treated vessels permanently separate and marked.
Treating it as complete protection. It removes most bacteria and cysts, not all, and it does not remove dissolved chemicals. Where there is doubt, boil or chlorinate after filtering.
How you know it worked
VerificationOutflow runs consistently clear at the design rate, a visible biological layer has established, and nobody drinking from it has had diarrhoeal illness over a full season. Where you can test, treated water should show a large and consistent reduction against the raw source.
Scaling and variants
Household: the same design in a 200 L drum with a 0.25 m² bed serves a family at roughly 25–75 L/h. Everything about the operating rules is identical, including the ripening period.
Village: scale by area, in multiple beds — never by increasing flow through one. Two or three beds in parallel also give you the rotation that makes scraping painless.
Turbid sources: add a roughing filter upstream — a horizontal box of graded gravel, coarse to fine, that takes the load off the sand. In a river with a seasonal silt load this will be the difference between a filter that runs for months and one that blinds every week.
Cold climates: biological activity slows sharply near freezing, so ripening takes longer and performance drops. Insulate and cover, and keep boiling as the winter fallback.