All content on this website is provided for information only, and everyone is responsible for ensuring that they abide by their local laws and regulations that pertain to the subject matter presented here.

Source material: This design is largely based on the design outlined in this YouTube video: https://youtu.be/232xA2e8RiQ?si=ESzpjDkyJ8PPNP75. I have also modified the design slightly with the additional step of UV purification from the sun, which is simple to do provided plastic bottles are readily available. ) I have removed the charcoal bucket due to the risk of using charcoal if it is not properly managed, and instead, I have modified the design to ensure about 5 cm of water remains above the sand layer so that the biofilm can stay active.

Goal of this webpage:

To provide all the information required on one webpage so that anyone can create clean, safe drinking water using simple, low-cost materials and no electricity. This design filters out large particles, bacteria, and viruses through gravel, sand (with biofilm), and solar disinfection (SODIS: Solar Water Disinfection). This system reduces large and small particles and pathogens. To make water safe to drink, always disinfect the filtered water using SODIS or boiling or a similar treatment method that is shown to completely eliminate pathogens (viruses, bacteria, etc). Feel free to print this webpage for anyone you think could benefit from it, and if they do not speak English, use Google translate or similar to translate the webpage prior to printing it out.

Step-by-Step: Building the Filter System

Materials Needed:

• Buckets: 3 total – all the same size. A recommended size is a typical 20 litre bucket or larger; food-safe recommended. Try to get a size that is at least the height of a typical 20 L (5 gallon) “builder’s bucket.” These are typically about 37 cm (14.5 inches) tall. It is also beneficial for the sides to be slightly tapered.
  • 1 × Top gravel pre-filter bucket
  • 1 × Inner sand/biofilm bucket (with small holes in the bottom)
  • 1 × Outer bucket (sets the standing water level; outlet holes on its side)
• Collection container with lid (for clean water)
• Gravel (that includes the sizes below)
  • Large gravel (6–12 mm) – drainage + spacing (outer bucket base, inner bucket bottom layer, side gap)
  • Small gravel (1–6 mm) – separation layer inside the inner sand bucket
• Sand (target: feels like table salt/fine sugar)
• Cloth (old cotton T-shirt or similar) for:
  • Covering outlet holes inside the inner sand bucket
  • Optional pre-filter cloth on top of the gravel pre-filter bucket for very dirty water
• Mesh / sieves (any you can obtain; use alternatives if not available)
  • 10 mm – 12 mm (½”) or whatever is closest to 12 mm – to remove oversized stones (gravel)
  • 6 mm (¼”) or whatever is closest – to split large vs small gravel
  • ~0.7 mm (#24) or whatever is closest – recommended coarse screen to keep pebbly grains out of the sand (keep what passes)
  • ~0.10 mm (#150) or whatever is closest – ideal to remove powdery fines (very small particles) from sand (discard what passes); if you don’t have #150, use the jar-wash method to rinse out fines (the very fine parts in the sand are extremely important to remove, or the sand can cement together because of them and completely block the water flow through the filter)
• Small plastic tab (~2 × 4 cm) and Glue to make the drip lip under the outer bucket outlet holes
  • If glue isn’t available, use any local method so water drips into the container (tie on a small stiff leaf/twig, wire a thin scrap of plastic/metal, or position the container close under the outlet).
• Straps/ties/rope to secure buckets to the frame
• Scrap wood/bricks (frame/stand)
• Basic tools: knife or nail/awl, drill (or hot nail), marker, bucket-safe glue (optional), file
• PET clear bottles (for SODIS), or fuel/heat source for boiling

Water Collection Tips (Best to Worst):

✅ Best Sources:

• Rainwater (collected from clean roofs or tarps)
• Flowing surface water (streams, rivers — away from livestock or human activity)

⚠️ Acceptable (with caution):

• Lakes or ponds with clear water and little algae
• Wells or springs (unprotected)

❌ Worst (use only in emergencies):

• Stagnant puddles, muddy water, water with visible scum or debris

If only stagnant or muddy water is available:

1. Let it settle: Place in a container and leave undisturbed for 4–12 hours.
2. Scoop from the top: Avoid collecting water from the surface film or bottom sediment.
3. Strain through cloth: To remove visible debris before pouring into the gravel layer.
4. Always use SODIS after filtering this type of water.

Build the Filter:

Step 1: Prepare Your Buckets

1. Clean all of the buckets well.
2. In the bottom of two of the buckets, drill 6-8 small holes (about 2–5 mm in size). DO NOT add any holes to the third bucket yet.
3. Cover holes from inside (for the two buckets) with cloth folded 2–3 times or fine mesh.
   • Prevents sand and gravel from falling through the small holes.
   • Glue or tie the cloth on securely.
4. Optional: For the gravel bucket (top), holes may be uncovered if gravel is coarse enough that they won’t fall through the holes.

Step 2: Sort and Wash the Gravel

The gravel bucket has two layers of gravel:

1. Bottom layer = larger gravel (6–12 mm, about the size of a pea or fingernail)
2. Top layer = smaller gravel (1–6 mm, about the size of coarse sugar crystals or lentils)

This setup makes sure:

1. The large gravel supports drainage and keeps outlet holes from clogging.
2. The smaller gravel catches finer dirt and debris before the water moves down into the sand bucket.

How to Sort the Gravel:

Option A – If you have sieves or mesh:

1. Use ~12 mm (½”) mesh to remove oversized stones (discard the stones that remain above the mesh)
2. Use ~6 mm (¼”) mesh to separate large gravel (kept for the bottom layer) from small gravel.
3. Use ~1 mm mesh to remove fine sand/dust.

Option B – If you don’t have mesh:

1. Pick out and discard stones much bigger than your thumbnail (too large).
2. Save stones about pea-size (6–12 mm) for the bottom of the bucket.
3. Save smaller gravel about the size of rice grains to lentils (1–6 mm) for the top of the bucket.
4. Rinse away all fine powder or dust until the water runs clear.

Filling the Gravel Bucket:

1. Cover outlet holes with folded cloth to prevent washout. If the holes are small enough, and the gravel large enough that it will not fall through the holes, this step can be skipped.
2. Add the large gravel first, filling the bucket bottom ⅓ to ½ full.
3. Add the small gravel on top, filling the rest of the bucket up to about ¾ full.

Step 3: Sort and Clean the Sand (for the Sand/Biofilm Bucket + Outer Bucket Setup)

The sand is the most important part of the filter. It removes very small particles and helps grow a thin living biofilm on top of the sand that eats germs.

Target sand quality:

• Effective Size (ES): the “typical grain size.” Aim for sand about the size of table salt or fine sugar (about 0.15–0.20 mm). If too coarse, germs pass through. If too fine, water flow can stop.
• Uniformity Coefficient (UC): tells how similar the grain sizes are. Aim for UC 1.5–2.5 (most grains about the same size). This gives steady flow and fewer clogs.

If you cannot measure ES/UC: that’s okay. Do the following so the sand still works well:

• Remove big pieces (pebbles).
• Wash away powder (the very fine “flour” that makes water cloudy).
• Keep sand that feels like table salt or fine sugar between your fingers.

A) Sort the Sand — With Sieves or Mesh (best if available)

1. Remove big pieces: Pour sand through a ~0.7 mm sieve (about like window screen). Throw away what does not pass through (pebbles, big chunks).
2. Remove powder: If you have a finer sieve or cloth, wash or sieve to remove very fine powder (roughly <0.1 mm).
3. Rinse well: Put sand in a bucket, add clean water, stir, and pour off the milky water. Repeat until the rinse water is only slightly cloudy.

(For people with more sieves: a #24 (0.71 mm) sieve helps remove oversize; a #150 (0.10 mm) helps remove powder. Keep the sand mostly between 0.10–0.70 mm, with the “feel” close to salt/sugar.)

B) Sort the Sand — No Sieves (Jar-Wash Method)

Use this if no mesh is available. It works well with just buckets or jars.

1. Load: Put about ¼ bucket of sand into a clean bucket.
2. Fill: Add clean water until the bucket is ~¾ full.
3. Stir: Mix strongly with a stick or by hand for 20–30 seconds to lift powder into the water.
4. Short settle: Let sit 5–10 seconds only. The good sand (heavier) falls quickly. The powder stays floating.
5. Decant: Carefully pour off the top dirty water into another container (do not pour out the settled sand). Discard the dirty water away from the work area.
6. Repeat: Do steps 2–5 3–6 times (or more) until the poured-off water, after a 5–10 second settle, looks only slightly cloudy.
7. Feel test: Rub a pinch of sand between your fingers. It should feel like table salt or fine sugar (not powdery like flour, not gritty like couscous). If still powdery, wash again.

C) Load the Sand/Biofilm Bucket (Inner Bucket)

This is the inner bucket that will hold the sand and biofilm. It has holes in the bottom, so water must pass through the sand to get out.

1. Cover outlet holes: Inside the bottom of the sand bucket, cover the drilled holes with 2–3 layers of folded cloth so sand cannot fall through.
2. Add gravel base layers (inside this sand bucket):
   • Add 5 cm of large gravel (6–12 mm, pea size) on the bottom.
   • Add 5 cm of small gravel (1–6 mm, rice–lentil size) on top of the large gravel.
3. Mark the inside of the bucket to prepare for adding the sand (refer to image below):
   • Line A — Top of small gravel: mark (scratch or permanent marker) the inside wall at the top of the small gravel layer (this should be at about 10 cm, per the above step).
   • Line B — Sand surface: If your bucket is tall enough, mark the sand surface at least 30 cm above Line A (more is fine). If it isn’t tall enough, simply measure down 7 cm from the top rim and mark that—that’s Line B. The 7 cm leaves about 5 cm standing water above the sand (for Line C) and ~2 cm freeboard (space between the top of the water and the top of the bucket). (On a typical 20 L bucket this gives ~20–22 cm of sand.)
   • Line C — Standing water line: mark 5 cm above your chosen sand surface line. This is the target water level that must remain above the sand after draining.
   • Field tip (no ruler): 5 cm ≈ the width of two adult fingers; 30 cm ≈ a forearm length from wrist crease to mid-elbow (use consistent local references).
4. Add water first: Pour clean water into the bucket until it is about half full.
5. Add sand into water: Slowly pour the prepared sand into the water (never into a dry bucket). This prevents air pockets and keeps layers even. Level the sand surface to your sand line (Line B).
6. Vent: Make 2–3 tiny vent holes near the top rim/lid so air can escape as water flows down.

D) Set the Standing Water Level (Outer Bucket + Outward-Drip Outlet)

This is the outer (3rd) bucket. It creates a “water line” that keeps about 5 cm of water above the sand inside the inner bucket, so the biofilm stays alive — no hoses needed.

Do these steps in this order (inner bucket removed when drilling):

1. Prepare the outer bucket base: Put 4–6 cm of large gravel in the bottom.
2. Mark the outlet height:
   • Place the inner sand bucket on top of this gravel (just to measure).
   • Look at Line C inside the inner bucket (the 5 cm standing water mark).
   • On the outer bucket wall, make a mark that lines up exactly with Line C. This height sets the standing water level.
3. Remove the inner bucket to avoid puncturing it during drilling.
4. Drill outlet holes in the outer bucket:
   • Drill 2–3 holes (about 6 mm) side by side at the mark from Step 2.
   • Keep the holes level. The lowest edge of these holes sets the 5 cm waterline above the sand.
5. Make an outward drip lip (so water doesn’t run down the wall):
   • Cut a small plastic tab (about 2 × 4 cm) from a bottle or lid.
   • Glue it just below the outlet holes so it sticks out 1–2 cm like a tiny shelf; or glue the head of a zip-tie under the holes as a drip break.
   • Let it set. Test with a little water to confirm the drip falls cleanly outward.
   • If glue isn’t available, use whatever material is available to create a lip either below the drainage holes or out of each hole so that filtered water drips directly into the collection bucket, and does not run down the side of the outer bucket (this introduces the risk of contamination and algae growth)
6. Re-insert the inner bucket: Set it back on the outer bucket’s gravel base.
7. Stabilize the side gap: Drop large gravel down the gap between the two bucket walls to keep the inner bucket centered and stable. Stop below the outlet holes so you don’t block them.
8. Seal the top gap: At the top gap between the buckets, stuff cloth or plastic strips (or press in clean mud) to keep insects and dirt out. Leave the outlet holes and drip lip clear.

Why this works:

• Water must travel down through the sand and gravel and out the bottom holes of the inner bucket into the outer bucket.
• The outlet holes in the outer bucket stop drainage at sand surface + 5 cm, leaving a small water layer on top of the sand to protect the biofilm (it will die if it dries out.)

Step 4: Complete Filter Construction and Start Using the Filter

A) Check the water height above the sand

1. Pour water into the sand/biofilm bucket and let it drain.
2. Look inside: there should be about 5 cm of calm water above the sand surface when draining stops.
   • If not, adjust sand height, gravel height in bottom of outer bucket, or the outlet-hole height in the outer bucket until you have ~5 cm of water depth above the sand surface.

B) Assemble the full filter

1. Place the gravel pre-filter bucket on top of the sand/biofilm bucket.
2. Strap or tie the buckets so they cannot tip.
3. Place the collection bucket so that it collects the water from the drip lip.
4. If water is very dirty, lay a cloth on top of the gravel when pouring, and rinse/replace the cloth as needed.

C) Gentle start-up use (first 1–2 weeks)

1. Pour gently into the gravel bucket. 2–4 L once or twice per day. Wait 8-12 hours before the second pour.

D) Normal use (after the first 1-2 weeks)

1. Gradually increase to your usual fill (for 20 L buckets, up to ~8 L per cycle is typical).
2. Keep pouring gently, allow rest time between cycles, and maintain the 5 cm water layer above the sand.

Why the “rest time” between pours matters

• Oxygen during the pause helps the biofilm eat and trap germs.
• No pause (continuous pouring) = worse treatment.
• Overlong pauses (several days) can let the top layer go stagnant.

Simple rules of thumb for normal use

• Ideal rest: about 8–12 hours between pours (e.g., do a pour in the morning, and one in the evening).
• Wait at least 1 hour between pours, and do at least one pour every 1-2 days
• If you won’t use it for a couple of days, keep it covered, shaded, and don’t let the sand dry out. If it does dry out, scrape the top 1–2 cm of sand and pour water new water into the filter
• Keep the 5 cm water layer above the sand at all times.

E) Final safety step (required; see next step for details)

• Always disinfect the filtered water with SODIS or boiling before drinking.
• The biofilm adds extra protection over time, but disinfection by SODIS or boiling is what makes the water safe.

Step 5: Solar Disinfection (SODIS: Solar Water Disinfection)

1. Fill clear PET plastic bottles with the filtered water.
   • Use only PET bottles (look for the “1” recycling symbol).
   • Do not use colored or glass bottles — they may block UV light.
2. Lay bottles flat in full sun on a reflective surface (metal sheet, corrugated roof, etc.).
3. Leave in sunlight for at least 6 hours (or 2 days if cloudy).
4. The water should now be safe for drinking.

Alternative to the above step: Bring water to a rolling boil for 1 minute (or 3 minutes at high altitudes, such as those above ~2,000 meters / 6,500 feet) to ensure it is safe to drink.

When Not in Use

• Cover buckets to prevent contamination.
• Store SODIS bottles in shade when they are not being used for the solar disinfection stage.

Maintenance Instructions

A) After Each Use (Quick Care)

1. Make sure water is dripping cleanly outward into the collection container (not down the wall).
2. Let the unit rest (no water added) for at least one hour before the next pour.

B) As Needed — Clean the Top Gravel Bucket (only if dirty or flow slows)

When to do this:

• You see mud, slime, algae, or leaves on/in the gravel.
• The top cloth turns brown/slimy quickly.
• Water pools on top of the gravel instead of flowing through.
• Overall flow slows even after using clear source water.

How to rinse (keep layers separate):

1. Remove top cloth (if used). Wash or replace.
2. Lift out the small-gravel layer (1–6 mm) into a wash bucket.
   • Add clean water, stir 10–20 s, let settle 5–10 s, pour off dirty water.
   • Repeat 3–5× until almost clear.
3. Lift out the large-gravel layer (6–12 mm) and rinse the same way.
4. Dispose of rinse water away from your water source.
5. Rebuild the bucket: large gravel back in first (bottom ⅓–½), then small gravel on top (to ~¾ full).
6. 5 cm standing-water check (since the gravel bucket is off):
   • Fill the sand/biofilm bucket, let it drain, confirm ~5 cm of calm water above the sand.
   • If not, adjust sand to your mark or re-check the outer bucket outlet height.
7. Replace the gravel bucket; strap/tie the stack.
8. (Optional) Lay a clean cloth on top of the gravel if source water is very dirty.

Tip: If you’re short on time, at minimum wash or replace the top cloth and scoop/rinse just the top few centimeters of small gravel.

C) If Flow Slows or Stops

1. Remove the top gravel bucket. Clear leaves, algae, or debris from the top.
2. Pour water through the gravel bucket and confirm that water flows through it okay. If it does not, do procedure B above.
3. Check that vent holes near the sand bucket rim are open and not blocked.
4. Check that there is 5 cm of standing-water above the sand (gravel bucket is off—do it now).
5. If the gravel bucket was not the issue, it is likely the sand bucket: do a light surface refresh
   • With a clean spoon/stick, gently scrape 1–2 cm from the top of the sand.
   • Top up with a little prepared sand if needed and level the surface to your mark.
   • Gently pour some water in to the sand bucket and check that the water drains to the 5 cm depth level.
   • Do the 5 cm water depth check again and adjust as necessary to get the water depth around 5 cm.
   • Let the unit rest 1 day before adding more water.

D) Deep Cleaning (Every 6–12 Months, or if water smells/looks bad)

1. Disassemble: Remove the top gravel bucket. Lift out the inner sand bucket from the outer bucket.
2. Clean parts: Wash lids, cloths, and bucket walls. Clear the outer bucket outlet holes and wipe the drip lip.
3. Gravel and sand
   • Gravel (inner bucket base layers and top gravel bucket): rinse until water runs clear.
   • Sand: if badly fouled, remove; jar-wash in batches to flush out powdery fines; keep sand that feels like table salt/fine sugar.
4. Rebuild the inner sand bucket (use your marks)
   • Cloth over the bottom holes (inside).
   • 5 cm large gravel (6–12 mm), then 5 cm small gravel (1–6 mm).
   • Add water, then pour in prepared sand to 30–50 cm depth (your sand surface mark). Level it.
5. Set standing water: Place the sand bucket back into the outer bucket; fill and confirm ~5 cm of water above sand. If not, adjust sand level or verify outer-bucket outlet holes are at sand + 5 cm.
6. Reassemble: Put the top gravel bucket back; secure the stack.
7. Start-up: For the next 7–14 days, use gentle, smaller pours. Remember to wait at least 1 hour between pours, and do at least one pour every 1-2 days.

E) If the Sand Dried Out or Was Contaminated

1. Remove the top gravel bucket.
2. Take off and discard the top 2–5 cm of sand (dried/crusted layer).
3. Jar-wash or sieve sand for replacing the sand that was removed.
4. Add clean water so that it is above 5 cm level mark.
5. Refill with sand to your sand surface mark. Level the sand. (You will put the new sand into the water.)
6. Let the water drain and check that there is 5 cm of standing water before putting the gravel bucket back on top.
7. Reassemble: Put the top gravel bucket back; secure the stack.
8. Start-up: For the next 7–14 days, use gentle, smaller pours. Remember to wait at least 1 hour between pours, and do at least one pour every 1-2 days.

F) When to Replace Things

• Cloth screens: clean weekly; replace when torn or slimy.
• Sand: only when persistently foul-smelling, contaminated, or after many deep scrapes (re-prep with jar-wash or sieved sand).
• Gravel: rarely needs replacing; rinse well.
• Drip lip: replace if the water starts creeping down the wall.

Pathogen Removal Estimates (provided for reference only)

Note: These are estimates for guidance only. Actual performance varies with source water quality, sand preparation, and consistent operation. Always finish with SODIS or boiling to make water safe for drinking. One of the best ways to check how clean the filtered and treated water is by testing it, but of course this may not be feasible in many regions.

🟡 Filter Only — Biofilm Growth Timeline (from new to fully mature)

Pathogen Type	Estimated Removal %	Notes
Protozoa (e.g., Giardia, Cryptosporidium)	90–99%+	Improves as biofilm matures (days → weeks). Larger organisms are well removed by sand + biofilm.
Bacteria (e.g., E. coli, Cholera)	70–99%	Lower at first use; rises with biofilm development and steady pause periods.
Viruses (e.g., Norovirus, Rotavirus)	40–90%	Variable; biofilm helps but not reliable for full virus removal.
Particulate Matter	~100%	Gravel + sand remove turbidity and fine particles.

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🟢 Filter + Final Disinfection (SODIS or Boiling) — Best Results

Pathogen Type	Estimated Removal %	Notes
Protozoa (e.g., Giardia, Cryptosporidium)	99%+	Sand/biofilm handle most protozoa; SODIS/boiling finishes the job.
Bacteria (e.g., E. coli, Cholera)	95–99.9%+	SODIS in strong sun (≥6 h) or boiling provides high kill; use as required step.
Viruses (e.g., Norovirus, Rotavirus)	90–99.9%+	SODIS greatly improves virus inactivation; boiling achieves full inactivation when done properly.
Particulate Matter	~100%	Mechanical removal by gravel + sand.