---
titulo: Slow sand filter and charcoal biofilter
descripcion: Household (biosand) and community slow sand filter, with biological layer, sizing and cleaning; granular charcoal bed.
nivel: 1
estado: borrador
requisitos: []
relacionados: [01-agua-y-saneamiento/potabilizacion/hervido-del-agua, 01-agua-y-saneamiento/potabilizacion/filtros-de-ceramica-y-carbon, 01-agua-y-saneamiento/potabilizacion/desinfeccion-quimica-del-agua-con-cloro-y-yodo, 01-agua-y-saneamiento/potabilizacion/desinfeccion-solar-sodis]
fuentes:
  - "Huisman, L. y Wood, W.E. (1974). Slow Sand Filtration. Organización Mundial de la Salud, Ginebra."
  - "Visscher, J.T., Paramasivam, R., Raman, A. y Heijnen, H.A. (1987). Slow Sand Filtration for Community Water Supply: Planning, Design, Construction, Operation and Maintenance. IRC Technical Paper 24, La Haya."
  - "Thames Water y University of Surrey (2005). Slow Sand Filters (water-e). Guía de diseño, construcción y operación; resume a Huisman y Wood 1974 y a Visscher et al. 1987."
  - "CAWST (2009). Household Water Treatment and Safe Storage Fact Sheet: Biosand Filter; y Biosand Filter Manual, versión 10. Calgary."
  - "CAWST (2012). Biosand Filter Construction Manual (filtro de hormigón versión 10). Calgary."
  - "Global WASH Cluster. Compendium of Water Supply Technologies in Emergencies: fichas T.9 Slow Sand Filtration y H.5 Biosand Filtration. emergency-wash.org"
  - "Pennsylvania Department of Environmental Protection. Drinking Water Operator Certification Training, Module 17: Slow Sand Filtration, unidad 2."
  - "Kearns, J.P. et al. (2021). «Biochar Water Treatment for Control of Organic Micropollutants with UVA Surrogate Monitoring». Environmental Engineering Science. PMC8165476."
  - "FAO (1983). Simple Technologies for Charcoal Making. FAO Forestry Paper 41, Roma, cap. 5 y 6."
  - "Ferguson, R.I. y Church, M. (2004). «A simple universal equation for grain settling velocity». Journal of Sedimentary Research 74: 933-937."
  - "Pillai, J., Mathew, K., Gibbs, R. y Ho, G.E. (1999). «H2S paper strip method: a bacteriological test for faecal coliforms in drinking water at various temperatures». Water Science and Technology 40(2): 85-90."
  - "OMS (2011). Guidelines for Drinking-water Quality, 4.ª ed., cap. 7 (tablas 7.7 y 7.10)."
  - "Howard, G. y Bartram, J. (2003). Domestic Water Quantity, Service Level and Health. OMS, Ginebra."
  - "Sphere Association (2018). The Sphere Handbook, 4.ª ed., capítulo de agua, saneamiento e higiene."
idioma: en
traduccion: automatica
---
# Slow sand filter and charcoal biofilter

## Summary

A slow sand filter passes water at a very low velocity (on the order of 0.1 m/h) through a fine sand
bed 0.5 to 0.9 m deep. On the surface a biological layer (*schmutzdecke*) grows that traps and digests
most bacteria, protozoa and helminths and leaves the water clear (turbidity below 1 NTU). This article
covers the intermittent-flow household version (*biosand* type, 24 to 72 L/day per filter) and the
continuous-flow community version (1 m² of bed gives about 2400 L/day). After the sand there can be a
granular charcoal bed that improves taste, odor and color and retains part of the organic contaminants.
**Neither one replaces final disinfection**: the filtered water is boiled, chlorinated or exposed to the
sun before drinking.

## Prerequisites

- **Knowledge:** measure volumes and times with a graduated container and by counting seconds; disinfect
  the filtered water (see [Boiling water](hervido-del-agua.md) and the related articles on chemical and
  solar disinfection). Making charcoal: there is no article yet; section F gives a minimal method.
- **Materials (household filter):**
  - 1 watertight container 90 to 100 cm tall and 25 to 35 cm inside diameter: unglazed fired-clay jar or
    one with lead-free glaze, a wooden barrel, a concrete or food-grade plastic bucket.
  - Sand: about 40 L already sieved and washed (start with 60 to 80 L raw: washing and sieving remove
    part of it).
  - Separating gravel (0.7 to 6 mm): about 4 L. Drainage gravel (6 to 12 mm): about 4 L.
  - 1 outlet pipe 1 to 2 cm inside diameter: bamboo or reed cane with no internal nodes, fired-clay,
    copper or plastic pipe.
  - Sealant for where the pipe passes through the wall: clay with resin or beeswax (N1), lime mortar
    (N2) or cement (N3).
  - 1 diffuser: thin board, fired-clay plate or sheet metal, with holes about 3 mm.
  - 1 lid and 1 clean container with lid to collect the filtered water.
- **Materials (household charcoal biofilter):** 25 to 30 L of already crushed, sieved and washed
  hardwood charcoal; 1 container of 35 to 40 L with the same type of raised outlet pipe.
- **Tools:** buckets of 10 to 20 L (N1), woven-fabric or basketry sieves (N1), mortar or stones for
  crushing (N0), ruler or knotted cord with knots every 1 cm (N1), counting seconds (N0). For the
  community version: hoe and shovel (N1), clay (N1) or masonry (N2), sluice gate or wooden plug (N1).
- **Time and people:** household filter, 2 people for 1 to 2 days (mostly sieving and washing the
  sand), plus 2 to 4 weeks of ripening. Community filter of 2 m², 6 to 10 people for 1 to 3 weeks
  depending on the terrain, plus ripening.

## How it works

**What the sand retains.** Water crosses the gaps between grains (in fine sand, a few tenths of a
millimeter). Particles are caught by straining at the surface, by settling inside the pores and by
sticking to the grains. That retains turbidity, helminth eggs and protozoan cysts, but lets through many
bacteria and almost all viruses.

**The biological layer does the fine work.** In the top 1 to 2 cm a sticky film of algae, bacteria,
protozoa and organic matter forms over weeks (*schmutzdecke*, German for “dirty layer”). There pathogens
get stuck, are devoured by protozoa or die from lack of food. That layer needs:

1. **Contact time:** WHO/IRC recommends designing for **0.1 m/h**, with an accepted range of **0.1 to
   0.4 m/h** (Visscher et al. 1987; Global WASH Cluster). Faster than that, pathogens cross the bed.
2. **Oxygen:** it arrives dissolved in the water or, in the household filter, by diffusion through a
   layer of standing water **5 cm** deep (CAWST 2009). More water on top suffocates the layer; less, and
   it dries out from heat.
3. **Not drying out or being poisoned:** if the bed is left without water or chlorinated water is poured
   in, the layer dies.

**Grain size.** The **effective size** (d10) is the sieve opening through which 10 % of the weight
passes; the **uniformity coefficient** is CU = d60/d10. For a slow sand filter: **d10 = 0.15 to 0.30 mm**
and **CU < 5, preferably < 3** (Visscher et al. 1987; Pennsylvania gives 0.15 to 0.35 mm and CU 1.5 to
3). Coarser sand: water passes too fast. With many fines (silt, clay): it clogs within days. Too little
uniformity: fine grains fill the gaps between coarse ones and the flow rate collapses.

**Why the outlet rises above the sand.** The discharge point sets the minimum water level inside the
filter (communicating vessels). If it is above the sand, the bed never empties and the layer never dries
out. But if the pipe drops too far outside it can **siphon**: it keeps pulling water down below the
elbow's level and leaves the sand exposed to air. This is the part most often done wrong.

**What the charcoal does.** Charcoal has an enormous internal pore surface where organic molecules are
**adsorbed**: those that give taste, odor and color, pesticide residues and other organics. It also
**destroys free chlorine**. The higher the carbonization temperature and the finer the grain, the
greater the capacity: charcoal made at ≥ 850 °C in a gasifier worked well, though commercial activated
carbon outperformed it by a factor of 2 to 34 (average 15) (Kearns et al. 2021).

| Contaminant | Does the charcoal bed retain it? |
|--------------|---------------------------------|
| Taste, odor, color | Yes, well |
| Residual free chlorine | Yes (that's why disinfection happens **after** the charcoal) |
| Pesticides and other organics | Partly; depends on the charcoal and it gets exhausted |
| Bacteria, viruses, protozoa | **Not** reliably; bacteria can grow inside the bed |
| Salts, nitrates, fluoride, hardness | **No** |
| Metals (arsenic, lead, etc.) | Generally **no**, or very little |
| Turbidity | Little, and it clogs: that's why it goes **after** the sand |

**Order of the treatment train:** (settling if the water is turbid) → sand → charcoal → disinfection →
covered storage.

**What to expect from a ripened sand filter.** A 1-log reduction is 90 %, 2-log is 99 %, 3-log is
99.9 %.

| Pathogen or parameter | WHO 2011, table 7.7 (slow filtration) | Community (Global WASH Cluster) | Biosand (CAWST 2009) |
|----------------------|----------------------------------------|-----------------------------------|----------------------|
| Bacteria | 0.2 to 2 log | > 95 % | up to 96.5 % in the laboratory; 87.9 to 98.5 % in the field |
| Viruses | 0.25 to 4 log | > 95 % | 70 to > 99 % in the laboratory |
| Protozoa | 0.3 to > 5 log | > 99 % | > 99.9 % in the laboratory |
| Helminths | — | — | up to 100 % (assumed by size) |
| Outlet turbidity | — | < 1 NTU | 95 % reduction, down to < 1 NTU |

WHO's lower figures correspond to poorly operated filters (unripened, freshly scraped, too fast):
retention drops a lot. Even 95 % lets through 50 bacteria out of every 1000: it greatly reduces
diarrhea, **it does not guarantee safe water**. Hence the need for final disinfection.

**Inlet limits:** recommended turbidity up to 10 NTU, with peaks of up to 50 NTU in the community
filter (Global WASH Cluster); the biosand accepts up to 50 NTU (CAWST 2009). Above that, settle it
first.

## Procedure

### A. Sizing

1. **Calculate demand.** If capacity is scarce, filter only drinking and cooking water: **7.5 L per
   person per day**. If all household water is filtered, basic access is around **20 L per person per
   day** (Howard and Bartram 2003). *Success criterion:* a liters/day figure written down.
2. **Choose the type.** A biosand gives 24 to 72 L/day in 4 batches of 12 to 18 L (CAWST 2009).
   - Up to ~70 L/day: **1 biosand** (at 7.5 L/person/day, up to 9 people: 9 × 7.5 = 67.5 L).
   - Between 70 and ~290 L/day: **2 to 4 biosands in parallel**, one per 70 L/day (4 × 72 = 288 L).
   - More than ~300 L/day or collective use: **continuous-flow community filter**.
   *Success criterion:* type and number of filters decided.
3. **Community filter area.** Area (m²) = flow rate (m³/h) / velocity (m/h), at 0.1 m/h and 24 h/day.
   So, 1 m² gives 0.1 m³/h = 2400 L/day: 120 people at 20 L/day or 320 at 7.5 L/day.
   **Worked example:** village of 200 people at 20 L/day.
   - Demand: 200 × 20 = 4000 L/day = 4 m³/day = 4 / 24 = 0.167 m³/h. Area: 0.167 / 0.1 = 1.67 m².
   - **2 identical units** to clean one without cutting off supply: 2 × 0.9 m² (0.95 × 0.95 m). Under
     normal operation they run at 0.167 / 1.8 = 0.093 m/h; with one stopped, the other runs at
     0.167 / 0.9 = 0.19 m/h, within 0.1 to 0.4 m/h.
   - Flow rate to measure: 167 L/h = 2.8 L/min total, 1.4 L/min per unit: a 10 L bucket in about 7 min.
   - For drinking and cooking only (7.5 L): 1500 L/day = 0.0625 m³/h → 0.63 m² → 2 × 0.32 m²
     (0.57 × 0.57 m).
   Visscher et al. (1987) give 5 to 200 m² per unit: the 0.3 to 0.9 m² units are below what's typical.
   They work, but are scraped from the edge, without stepping inside. *Success criterion:* area, units
   and flow rate per unit recorded.
4. **Household filter flow rate.** Maximum load **600 L/h per m²** (CAWST 2009): maximum flow rate
   (L/h) = area × 600. Container of 30 cm: area = 3.14 × 0.15² = 0.0707 m²; maximum = 42 L/h = 0.7 L/min.
   The CAWST v10 filter works at **0.4 L/min** maximum with a full reservoir. Since 0.7 L/min is 1.75
   times that value, **aim for 0.4 to 0.5 L/min**. *Success criterion:* target flow rate for your
   container recorded.
5. **Household filter heights** (bottom to top; layers according to CAWST 2012 and Global WASH
   Cluster):

   | Layer | Thickness | Material |
   |------|---------|----------|
   | Drainage gravel (surrounds the pipe mouth) | 5 cm | 6 to 12 mm |
   | Separating gravel | 5 cm | 0.7 to 6 mm |
   | Filter sand | 45 to 50 cm | < 0.7 mm, no fines |
   | Standing water layer | 5 cm (4 to 6 cm) | — |
   | Gap to the diffuser | just enough that it doesn't touch the water (5 cm in the calculation) | — |
   | Reservoir above the diffuser | whatever a 12 L batch occupies | — |

   For a 30 cm diameter, 12 L occupies 12,000 cm³ / 707 cm² ≈ 17 cm. Total ≈ 5 + 5 + 50 + 5 + 5 + 17 + 5
   cm of freeboard ≈ **92 cm**. *Success criterion:* a scale drawing with the measurements, including the
   outlet elbow's height = sand surface + 5 cm.
6. **Check the pore volume.** Each batch must fit within the sand's pores so it stays inside the bed
   during the pause. Porosity of silica sand = 1 − bulk density / grain density = 1 − 1560 / 2650 ≈ 0.41
   (values from Thames Water 2005). Pore volume = 0.0707 m² × 0.50 m × 0.41 = 0.0145 m³ = 14.5 L ≥ 12 L.
   *Success criterion:* batch ≤ pore volume; if not, reduce the batch size.

### B. Obtaining, sieving and washing the sand and gravel

1. **Choose the source.** In order of preference (CAWST): **crushed rock**, dry quarry sand and river
   sand taken high on the bank. River sand usually carries organic matter and pathogens from excrement.
   Look for hard quartz grains that don't crumble when rubbed. Avoid beach sand (salt and shells), sand
   near latrines, dumps or mine tailings, and clayey sand. *Success criterion:* a squeezed handful of
   damp sand does not form a ball that keeps the imprint of the fingers (if it does, it carries too much
   clay).
2. **Sieve out the coarse material** through a sieve of about **0.7 mm**. Without proper sieves: cloth,
   mosquito netting or regularly woven basketry; opening ≈ (10 mm / threads per cm) − thread thickness
   (10 threads/cm with 0.3 mm thread → 0.7 mm). Also a thin board or leather perforated with hot wire of
   known thickness. Keep what is retained for the gravels. *Success criterion:* the sieved material
   passes the measurement in step 3 and, at the end, the flow-rate test (C.7).
3. **Measure the grain size without sieves.** Line up grains touching each other along 1 cm and count
   them: average diameter ≈ 10 mm / number of grains. Look for typical grains of 0.2 to 0.5 mm (20 to 50
   grains per cm), consistent with d10 = 0.15 to 0.30 mm and CU < 3. Do this with 3 samples from
   different spots in the pile. *Success criterion:* all 3 give 20 to 50 grains/cm, without two very
   different populations (dust plus coarse grains).
4. **Remove fines by decantation.** In water at 20 °C, a natural quartz grain falls at about **7.5 mm/s
   if it measures 0.1 mm** and at about **10 mm/s if it measures 0.12 mm** (Ferguson and Church 2004).
   Practical rule, valid for any container: **after stirring, wait 1 s for every cm of water above the
   sand** (30 cm → 30 s) and pour off the turbid water. What settles at less than 10 mm/s is removed:
   grains smaller than about 0.12 mm, below the target d10. In water at 10 °C the viscosity is 30 %
   higher and grains fall more slowly: wait 36 to 40 s per 30 cm.
   Procedure: fill the container with one third sand and the rest water (Thames Water 2005); measure the
   height of water above the sand with the knotted cord; stir vigorously for 10 s; wait according to the
   rule; pour off without dragging the sand off the bottom; repeat. *Success criterion:* after stirring
   and waiting, the water lets you see the sand at the bottom and no longer carries dust when poured off.
5. **Check the silt content without over-washing.** Bottle test (Pyper and Logsdon 1991, in Thames Water
   2005): 100 mL of sand in a clear jar, water up to 200 mL, shake hard and let it settle for 20 min.
   Turbid water, a silt band and the sand form. Silt (%) = silt height / sand height × 100; wash it if it
   exceeds 1 % (Visscher et al. 1987). In the biosand, don't wash more than necessary: sand that is too
   clean lets water through too fast, and the final call is made by the flow-rate test (C.7). *Success
   criterion:* silt band barely visible, ≤ 1 % of the sand height.
6. **Prepare the gravels.** Sieve what was retained in step 2 into two fractions: **0.7 to 6 mm**
   (separating) and **6 to 12 mm** (drainage), checking the largest grains in each pile with the ruler.
   Wash both until the water runs clear. *Success criterion:* wash water clear on the first pour; no flat
   or elongated stone larger than 12 mm.
7. **Store the fractions** separately and labeled, covered, on a clean surface. *Success criterion:* 3
   identified piles, free of soil, leaves or animal droppings.

### C. Building the household filter (biosand, N1)

1. **Prepare the container.** Fired clay: fill it with water for 24 h to check for leaks. Wood:
   untreated and non-toxic, let it swell with water for 2 to 3 days until it stops seeping. *Success
   criterion:* no visible leaks and the water level does not drop more than 1 cm in 24 h.
2. **Place the outlet pipe.** Two ways:
   - **Internal (preferable):** a vertical pipe inside the container, with its lower mouth 1 to 2 cm from
     the bottom, running up against the wall, passing through it at the height “sand surface + 5 cm” and
     exiting through an elbow.
   - **External:** a hole near the bottom and a pipe that runs up outside to that height and then exits.
   **Avoid siphoning:** after the elbow, the spout must drop only a little (**5 cm at most**) or have a
   **small hole (2 to 3 mm) at the top of the elbow** to let air in. Seal the pass-through in the wall
   with clay and resin or wax (N1), lime mortar (N2) or cement (N3), inside and out. Cover the lower
   mouth with cloth or mesh so gravel cannot get in. *Success criterion:* with the container full of
   water, water flows out of the spout and stops when the inside level reaches exactly the elbow's
   height, without dropping further over the next 10 min; no leaks from the seal after 12 h.
3. **Fill with water first and pour the media into the water**, so as not to leave air pockets. Mark the
   levels on the wall beforehand. Pour in 10 to 15 cm of clean water, add the drainage gravel up to 5 cm,
   level it, and add the separating gravel on top up to 5 cm. *Success criterion:* flat layers (± 1 cm)
   with water always above them.
4. **Add the sand** by handfuls inside the water, adding water as needed, up to 45 to 50 cm. Level it
   with your flat hand, without pressing. *Success criterion:* flat surface 5 cm (4 to 6 cm) below the
   elbow.
5. **Place the diffuser**, with holes about **3 mm** spread over its whole surface, resting on a ledge,
   on wedges or on the wall, **above the standing water layer** without touching it. *Success criterion:*
   when a 10 L bucket is poured all at once, the sand shows no crater or channels.
6. **Put on the lid** and, under the spout, the collection container on a stand, covered around it with a
   clean cloth. *Success criterion:* the stream falls inside without splashing or touching the rim.
7. **Flow-rate test.** Fill the reservoir with the full batch (12 L). As soon as it starts to come out,
   time how long it takes to fill a glass of known volume. Flow rate (L/min) = volume (L) × 60 /
   seconds. Example: 1 L in 150 s → 0.4 L/min. *Success criterion:* 0.4 to 0.5 L/min in a 30 cm container
   (≤ 0.4 in a CAWST v10), and no less than half that value so the family doesn't tire of waiting.
   - **Too fast:** sand is coarse or over-washed. Replace the top 5 to 10 cm with finer or less-washed
     sand and repeat.
   - **Too slow:** too many fines. Remove the top 5 to 10 cm, wash them more (B.4) and repeat.
8. **Check the standing water layer.** When water stops coming out, measure the height of the water above
   the sand. *Success criterion:* 5 cm (4 to 6 cm; CAWST 2009). If not, correct the sand's height (not
   the pipe's); if it drops below 4 cm with the sand well leveled, the pipe is siphoning (see Common
   mistakes).

### D. Ripening and daily use

1. **Use the filter every day** with the same water source from day one; changing sources reduces
   performance (CAWST 2009). *Success criterion:* daily log of batches (one mark per batch).
2. **Ripen the biological layer:** up to **30 days** in the biosand (CAWST 2009); in the community
   filter, up to **3 weeks** with new sand (Visscher et al. 1987), longer in cold water. During that time
   the water comes out clear but **is not treated**: always disinfect it. *Success criterion:* the flow
   rate drops somewhat and stabilizes, and a slightly sticky brown or greenish film appears on the sand.
3. **Respect the pause between batches:** **at least 1 h** from when water stops flowing; 6 to 12 h is
   recommended and 48 h is the maximum (CAWST 2009). Typical pattern: 4 batches per day. *Success
   criterion:* it's never refilled while still dripping; never more than 2 days pass without a batch.
4. **Pretreat turbid water.** Above 50 NTU, strain through a folded cloth or let it settle (CAWST 2009).
   Without a turbidimeter, **CAWST's bottle test (2009)**: fill a clear 2 L plastic bottle with the water
   and set it over large printed letters; if they can be read looking down through the bottle from above,
   it's probably below 50 NTU. *Success criterion:* the letters can be read; if not, settle it first.
5. **Never pour in chlorinated water, soap or hot water**: they kill the biological layer. *Success
   criterion:* the incoming water does not smell of chlorine.

### E. Maintenance

**Household filter: “swirl and dump”** (CAWST). Only when the flow rate no longer meets the household's
needs or drops below **0.1 L/min** (CAWST manual), not on a schedule.

1. If the filter is empty, pour in about **4 L** of water (CAWST). Remove the lid and the diffuser. Wash
   your hands.
2. Stir in circles with your palm, only the sand surface, without pushing your fingers down toward the
   gravel.
3. Scoop out the dirty water with a cup and dump it away from the source and the filter. Repeat until the
   flow rate recovers.
4. Level the sand, put the diffuser and lid back on. Clean the spout **on the outside** with water and
   soap or chlorinated water, without letting it get into the filter.
*Success criterion:* the flow rate returns close to that of the initial test. For a few days the filter
retains less well while the layer rebuilds (CAWST 2009): disinfect as always.

**Community filter: scraping**

1. **When:** when, with the outlet fully open, the flow rate falls below the design value or the water
   reaches the overflow. This ranges from several weeks to a year depending on the water (Global WASH
   Cluster).
2. **Partially drain it:** close the inlet and lower the level to about **10 cm below the sand surface**
   (Global WASH Cluster). *Success criterion:* the visible top sand is moist, not dry.
3. **Scrape** the top **1 to 3 cm** with a flat shovel or board (Thames Water 2005), in strips and evenly,
   removing it in baskets. On units smaller than about 2 m², work from the edge; on larger ones, step
   only on boards. Do it in a single day. *Success criterion:* clear, uniform sand appears underneath.
4. **Refill from below** with **already filtered** water: open the connecting pipe to the twin unit (H.6)
   and let the water rise through the drain. If there is no filtered water, do not fill from below with
   settled water (it seeds pathogens into the drain): fill from the top instead, slowly, pouring onto the
   flat stone (Thames Water 2005). *Success criterion:* the level rises a few cm per minute, without
   bubbles or craters, until it covers the sand.
5. **Re-ripen:** after scraping, allow **at least a few days**, and more after adding new sand (Global
   WASH Cluster; Visscher et al. 1987 gives 1 to 2 days after scraping and up to 3 weeks with new sand).
   Meanwhile, dump the water or disinfect it. *Success criterion:* outlet turbidity and flow rate back to
   their usual values.
6. **Wash and store the scraped sand** (B.4) in a covered pile.
7. **Recharge** the bed when it drops to **0.5 to 0.6 m** (Visscher et al. 1987; Global WASH Cluster),
   every few years. **Trench technique** (Thames Water 2005, from Huisman and Wood 1974): remove the old
   sand down to the gravel in a strip, put washed sand at the bottom and the old sand on top, and continue
   strip by strip. The old sand, already colonized, ends up on top and shortens re-ripening. *Success
   criterion:* bed back to 0.8 to 0.9 m, level, and design flow rate recovered.

### F. Preparing the charcoal for the filter

This also serves for the charcoal cartridge of household ceramic filters.

1. **Wood.** Hard, dense and dry (holm oak, oak, beech, fruit trees or coconut shell). Never treated,
   painted, particleboard or varnished. *Success criterion:* firewood with no rotten bark that sounds
   dry when struck.
2. **Carbonize it.** The best charcoal for water comes from a **TLUD gasifier** (*top-lit updraft*: lit
   from the top, with rising draft; sheet-metal drum, N2-N3): the one made at **≥ 850 °C** adsorbed much
   better than low-temperature charcoal (Kearns et al. 2021). Without sheet metal, **minimal N1 method in a pit or
   mound** (FAO 1983, chap. 5 and 6):
   - Stack the firewood tightly, filling gaps with thin sticks, in a pit or around a central pole. Cover
     with leaves and **10 to 20 cm of sandy soil**; leave a gap at the top and air inlets at the base (6
     to 10 in a 4 m diameter mound).
   - Light it by dropping embers through the top gap. **Thick white smoke**: the fire has caught. Cover
     with soil any crack from which flame escapes.
   - When the smoke **decreases and turns bluish and then almost transparent** (days, in a 4 m mound),
     carbonization is finished and what is burning is now the charcoal itself: **seal** all inlets and the
     top gap with soil or clay.
   - Let it cool sealed (2 to 3 days in that mound). Good-practice yield: 1 kg of charcoal per 4 kg of
     air-dried firewood.
   This charcoal is made at a lower temperature (about 550 °C in an improved mound, according to FAO) and
   unevenly: **it adsorbs less than gasifier charcoal and will remove less taste and odor**. Set aside
   the under-carbonized pieces from near the smoke outlet. *Success criterion:* pieces black inside, with
   no brown core, that ring metallically when struck, leave little soot, and break with a shiny fracture.
3. **Cool without air.** If you put it out with water, do so outdoors and away from the steam (see
   Safety). Spread the charcoal in the shade. *Success criterion:* plunging a hand into the pile shows no
   heat anywhere, and none the next day either; only then bag it.
4. **Crush it.** Dampen the charcoal so it doesn't raise dust and crush it in a mortar or between stones
   inside a sack. *Success criterion:* almost all of it in grains the size of a pea or smaller.
5. **Sieve it.** Aim for grains of **4 to 8 mm**. Kearns et al. (2021) model a full-scale adsorber (bed 58
   cm in diameter and 30 cm deep) with **4.5 mm** grains and **2.5 h** of contact time; Thailand's
   community system, in use since 2008, uses **7.5 mm** grains with about **18 h** of contact time. Finer
   grain adsorbs more (going from 0.165 to 0.059 mm multiplied capacity by 3.5 to 3.9 in the laboratory),
   but it clogs and is lost during washing. Crush again what doesn't pass through 8 mm and discard the
   dust that passes through a mosquito net. *Success criterion:* grains of similar size, with no dust when
   a dry handful is rubbed.
6. **Wash and saturate.** In a bucket with water, stir, wait 30 s and pour off the black water along with
   whatever floats. Repeat until it comes out nearly clear. Leave it soaking at least overnight so water
   fills the pores. *Success criterion:* rinse water light gray and almost all the charcoal at the
   bottom; discard whatever keeps floating.
7. **Decolorization test** (useful for comparing charcoals and for monitoring the bed, G.6): in two
   identical glasses put 200 mL of water with 1 drop of vegetable ink or strong tea; add to one a handful
   of the charcoal being tested and to the other an equal handful of reference charcoal; shake for 10 min
   and let it settle. *Success criterion:* the good charcoal leaves the water noticeably clearer than the
   reference, or the same if the reference is new charcoal.
8. **Steam activation (optional; when in doubt, skip it).** Industrial activation works between 700 and
   1000 °C and opens more pores by burning part of the charcoal. There is no reliable data on how much it
   improves charcoal already made at high temperature: if it comes from a gasifier, it probably isn't
   worth it. If you do it (N2), read Safety first:
   - Use an iron container, never clay, filled with grains up to 2/3, with a lid pierced by a short
     **vent** and by an **iron pipe at least 1 m long with a funnel on top** to add water far from the
     fire.
   - Heat until the container is red-hot (cherry red to orange, ≈ 800 to 900 °C, verificar) and keep it
     there for 1 to 2 h (verificar). Light the gas coming out of the vent.
   - Pour very little water through the funnel each time (about 50 mL every 5 to 10 min, verificar), with
     your face turned away from the vent and the funnel, because steam comes out in bursts.
   - **Stop adding water at least 15 min before finishing** and keep heating until **there is no flame at
     the vent** (gas exhausted). If you sealed it with gas still inside, air entering as it cooled could
     ignite the mixture, blow off the lid and throw embers.
   - Only then remove it from the fire, **seal the vent and the funnel with clay or damp sand** and let it
     cool.
   - **Do not open it until you can rest your hand on the lid.**
   *Success criterion:* lighter, more porous grains (weigh before and after) that, once washed, outperform
   the same unactivated charcoal in the decolorization test (step 7).

### G. Assembling and using the charcoal biofilter

1. **Place it after the sand**, never before. *Success criterion:* the water entering the charcoal is
   already clear.
2. **Size it by contact time.** The beds described (model and field) work with 2.5 to 18 h of contact
   (step F.5). In the intermittent filter, make sure **each full batch stays inside the charcoal during the pause** (6 to
   12 h): bed pore volume ≥ batch size. Measure the porosity between grains: fill a 1 L jar with
   already-saturated charcoal, add water to the brim and measure how much went in; porosity = liters of water
   / 1 L. Example with 0.45: 12 / 0.45 ≈ **27 L of charcoal**, which in a 30 cm container occupies
   27,000 cm³ / 707 cm² ≈ 38 cm. *Success criterion:* porosity measured, bed calculated and container
   chosen.
3. **Assemble the container like the biosand, but simpler:** 5 cm of drainage gravel around the pipe
   mouth, the charcoal on top and the outlet elbow above the surface of the charcoal, with the same
   precaution against siphoning (C.2). On top, a cloth or perforated plate. Fill with water first and pour
   the charcoal in. *Success criterion:* with the filter at rest, a visible layer of water remains above
   the charcoal and nothing floats.
4. **Rinse before use:** run through 2 or 3 batches and discard them. *Success criterion:* it comes out
   with no gray color or particles.
5. **Always disinfect after the charcoal**: boil, chlorinate or SODIS. If you chlorinate, do it **at the
   outlet** of the charcoal, which removes chlorine. *Success criterion:* the stored water smells slightly
   of chlorine, or was boiled.
6. **Replace the charcoal** when the taste or odor it used to remove comes back, or when, in the
   **monthly decolorization test** (F.7) with a handful taken from the top layer of the bed, it
   decolorizes clearly less than freshly prepared new charcoal. There is no published interval: Thailand's
   system has gone through several replacement cycles, but Kearns et al. (2021) do not give months. Do not
   reuse charcoal used for water without reactivating it; burn it outdoors, away from people. *Success
   criterion:* loading date and result of each monthly test recorded on the container.

### H. Building the continuous-flow community filter (N1-N2)

1. **Choose the site:** above the flood zone, downhill from the intake and uphill from the reservoir, and
   at least **30 m** from latrines and cesspits (Sphere 2018), farther from animal pens if the terrain
   drains toward the filter. *Success criterion:* water flows from the intake to the filter and to the
   reservoir by gravity.
2. **Excavate or build the tank.** Height: **0.3 to 0.5 m** of drain and gravel + **0.8 to 0.9 m** of sand
   + supernatant water + **0.2 to 0.3 m** of freeboard (Visscher et al. 1987; Thames Water 2005). WHO/IRC
   recommends **1 m** of supernatant water; with less it can still work, but there is less head to push
   the water through and it must be scraped more often. Waterproof it with puddled and compacted clay in
   layers (about 5 cm per layer, verificar) or with masonry set in lime mortar (N2). *Success criterion:*
   full of water, the level does not drop more than 1 cm/day aside from evaporation; if it drops more, add
   another layer of clay.
3. **Drain:** pipes 6 cm in diameter spaced 1 m apart (Visscher et al. 1987), of fired clay laid end to
   end with the joints slightly narrower than the finest stones of the gravel layer covering them, or a
   channel of flat stones, with a slight slope toward the outlet. *Success criterion:* water poured in at
   the high end reaches the outlet without pooling.
4. **Support gravel**, bottom to top (Visscher et al. 1987, in Thames Water 2005): **16 to 23 mm, 15 cm**;
   **4 to 5.6 mm, 10 cm**; **coarse sand 1 to 1.4 mm, 10 cm**. Wash them until the water runs clear.
   *Success criterion:* flat layers; a handful of filter sand poured with water onto the top layer does
   not disappear between the grains.
5. **Sand:** 0.8 to 0.9 m initial thickness (Visscher et al. 1987), poured into the water as in C.3-C.4.
   Mark the initial and minimum level (0.5 to 0.6 m) on the wall. *Success criterion:* flat surface
   (± 2 cm).
6. **Outlet and connection between units:** the drain leads to a chamber whose weir sits **slightly above
   the sand surface** (Huisman and Wood 1974, in Thames Water 2005), so the bed never empties; in front, a
   wooden plug or gate regulates the flow. Connect the drains of the two units with a **6 cm pipe with a
   plug** so one can be filled from below with filtered water from the other. *Success criterion:* with
   the inlet closed, water stops coming out and stays above the sand; with the plug closed, no water
   passes from one unit to the other.
7. **Inlet:** an overflow in the wall at the maximum supernatant water level returns the excess to the
   stream; the inlet pours onto a slab or flat stone. *Success criterion:* with the inlet fully open, the
   level stabilizes at the overflow and the sand does not move.
8. **Initial filling from below** with already-filtered water (from the twin unit through the connecting
   pipe, or from another filter poured into the chamber with the gate open), until it covers the sand; if
   only unfiltered water is available, fill from the top over the flat stone (Thames Water 2005).
   Afterward open the normal inlet. *Success criterion:* the level rises a few cm per minute, with no
   bubbles.
9. **Set the design flow rate** with the gate, measuring with a bucket and a clock (A.3: 10 L in about
   7 min per unit). Check it **every day** (Visscher et al. 1987) and open it up little by little as the
   layer slows the flow; a large sudden adjustment worsens the water quality (Thames Water 2005). *Success
   criterion:* the bucket fills in the design time at the daily check.
10. **24 h operation** toward a **covered reservoir** of at least **40 %** of daily production (Visscher
    et al. 1987; in the example, 1.6 m³), from which water is drawn with a tap. During ripening, dump the
    water or disinfect it. *Success criterion:* the reservoir never runs empty and the filter never stops.
11. **Cover the filter** with a small roof of branches, reed matting or boards: light favors algae, which
    clog the filter sooner (Pennsylvania manual). *Success criterion:* no floating algae or excrement in
    the water.

## Verification

**With N1 means, no laboratory:**

- **Clarity:** the filtered water looks clearly cleaner than the inlet water. Sharply seeing a black mark
  through 30 cm of water indicates improvement, **not** that 1 NTU has been reached; a turbidimeter is
  needed for that.
- **Odor and taste:** no rotten or moldy smell. If it smells rotten, the layer is oxygen-starved: check
  the standing water layer (5 cm) and the maximum pause (48 h).
- **Flow rate:** record it every week. A slow decline is normal; a sudden increase indicates a disturbed
  bed, a crack or a channel next to the wall (water is not being filtered).
- **Standing water layer** of 5 cm (4 to 6 cm) in the biosand; sand always covered in the community
  filter.
- **Charcoal:** monthly decolorization test (F.7).
- **Health:** diarrhea decreasing in the household or village compared with before (a delayed but real
  indicator).

**With more resources (N3-N4):**

- **Turbidity** of the outlet < 1 NTU with a turbidimeter.
- **Hydrogen sulfide test** (paper strip in a 20 mL jar): incubate at **25 to 35 °C** and read at 24 and
  48 h; if it turns black, fecal contamination is likely. Below about 20 °C it takes much longer and can
  give false negatives (Pillai et al. 1999).
- ***E. coli*** by membrane filtration or chromogenic substrate: WHO's target is 0 in 100 mL (WHO 2011,
  table 7.10), which in practice **is only achieved with final disinfection**.
- **Charcoal exhaustion:** pour water that smells of chlorine **directly onto the charcoal bed**, never
  through the biosand (D.5). If the outlet of the charcoal smells of chlorine, it is exhausted.

## Common mistakes

| Symptom | Likely cause | Solution |
|---------|----------------|----------|
| Water comes out almost as fast as it's poured in | Coarse or over-washed sand; channel next to the wall | Replace the top 10 cm with finer sand; pack damp sand against the wall |
| Very slow flow rate from day one | Sand with too many fines or clay | Remove the top layer and repeat decantation (B.4) |
| Flow rate drops within a few days | Turbid inlet water | Settle or strain it first; “swirl and dump” or scrape |
| Sand with dry patches or cracks | Outlet below the sand surface | Raise the elbow or the weir above the sand |
| Standing level < 4 cm despite well-leveled sand | The pipe is siphoning: the spout drops too far and keeps pulling water | Shorten the spout's drop to ≤ 5 cm or open a 2 to 3 mm hole at the top of the elbow |
| Water smells of rotten eggs | Oxygen-starved layer: more than 5 cm of standing water, pauses longer than 48 h | Adjust the sand height; use it daily |
| Crater in the sand under the stream | Diffuser missing, broken or submerged | Reinstall the diffuser above the water layer |
| Improves and worsens depending on the day | Change of water source; chlorinated or soapy water | Always use the same source; don't add chlorine or soap |
| Sand in the outlet water | Separating gravel absent or too coarse | Redo the gravel layers |
| Gray water after the charcoal | Poorly washed charcoal dust | Wash more; discard the first batches |
| The charcoal floats and comes out | It wasn't saturated with water | Soak it until it sinks before loading it |
| The bad taste comes back | Exhausted charcoal | Replace the charcoal |
| Diarrhea despite the filter | Unripened filter, recent scraping, recontamination during storage, or no disinfection | Always disinfect; use a covered collection container with a tap or spout |

## Safety

- **False sense of safety (the main risk).** A ripened filter retains most pathogens, not all of them;
  during ripening and after every cleaning, much less. Charcoal does not retain pathogens and can grow
  them. **Always disinfect the filtered water.** With water from an obviously fecal source (a river below
  a settlement, a pond with livestock), an undisinfected batch can transmit cholera, typhoid fever,
  hepatitis A or rotavirus, which kill small children through dehydration.
- **Chemical contaminants.** Neither sand nor charcoal removes salt, nitrates (dangerous for infants under
  6 months), fluoride, arsenic or most metals. If the source is near mines, tailings, heavily fertilized
  fields or industries, or there are tooth stains or frequent skin lesions in the area, look for another
  source. Exhausted charcoal can release what it has adsorbed: replace it.
- **Containers.** Old or traditional glazed pottery can leach lead: use unglazed clay or lead-free
  glazes. Do not use drums that held fuel, pesticides or chemical products, nor treated wood.
- **Dust.** Sieving dry sand raises silica dust, which causes silicosis over time: sieve it wet or
  outdoors, with a damp cloth over nose and mouth. Charcoal dust irritates eyes and lungs: crush it damp.
- **Carbonization and freshly made charcoal.** The mound and the pit give off smoke with carbon
  monoxide: work upwind (with the wind at your back) and never near homes. Stepping on a lit mound can
  sink you into embers: don't climb on it. Putting out red-hot charcoal with water releases steam and some
  carbon monoxide and hydrogen: do it outdoors and upwind. Charcoal can reignite hours later and burn down
  storage: don't bag it hot. Natural-fiber clothing, water on hand and an area clear of dry leaves.
- **Steam activation (optional: when in doubt, don't do it).** Risks: steam bursting out and burning the
  face; combustible and **toxic** gas (carbon monoxide and hydrogen); flame flashback if air enters with
  gas still inside, blowing off the lid and throwing embers. Therefore: only outdoors; water through a
  pipe at least 1 m long and little at a time, never with your face above the vent or the funnel;
  stop adding water 15 min before and heat until there is no flame before sealing (F.8); do not open until
  you can touch the lid; never in clay containers: they burst. Keep children and animals away.
- **Carbon monoxide poisoning.** **It has no smell and cannot be seen.** Symptoms: headache, dizziness,
  nausea, fatigue, confusion. At the first sign, **move upwind, out of the smoke**, and move others away
  too. If someone is unconscious but breathing, place them in the **recovery position** outdoors; if not
  breathing, give **rescue breathing** (and chest compressions if you know how) and call for help. **No
  one returns to the area until the fire is out and it has been ventilated.**
- **Community filters and wells.** A tank with water or a pit near the river is a drowning risk for
  children: fence it or cover it. In sandy ground, pit walls collapse: don't go into a pit deeper than
  waist height without shoring or sloping it. To scrape it, use boards and never work alone.
- **Maintenance hygiene:** the water from “swirl and dump” and the scraped sand carry pathogens. Dump that
  water and that sand away from sources, wells and raw-vegetable gardens, and wash your hands with soap.
- **Frost.** Frozen water kills the biological layer and cracks clay containers: protect or bury the
  filter.

## Variants

- **With fewer resources (N0):**
  - **Riverbank well (bank filtration):** dig a shallow pit in sandy ground 5 to 10 m from the channel
    (verificar) and draw the water that seeps in; the ground acts as a filter. It greatly improves
    turbidity, but flow velocity is not controlled: always disinfect. Fence it or cover it and watch for
    collapse (see Safety).
  - **Bark containers, basketry waterproofed with clay, or leather:** work the same if the 5 cm water
    layer, the raised siphon-free outlet and the maximum flow rate are respected; they don't last as long.
  - Without charcoal: the improvement in taste and odor is lost, not the removal of pathogens (which the
    charcoal never provided anyway).
- **With more resources:**
  - **N2:** masonry tank with lime mortar, glazed clay pipes, perforated sheet-metal diffuser,
    sheet-metal TLUD gasifier for the charcoal, iron container for activating it.
  - **N3:** concrete filter with a steel mold (CAWST v10, 70 to 75 kg, service life of more than 30 years
    according to CAWST 2009); standardized sieves for d10 and CU; **gravel prefilters** for more turbid
    water.
  - **N4:** commercial granular activated carbon (on the order of 15 times the capacity of good charcoal;
    Kearns et al. 2021); turbidity and *E. coli* monitoring; molded plastic filters (about 3.5 kg).
- **Scaling up:**
  - **Family:** 1 biosand (24 to 72 L/day) + 1 charcoal bed of ~27 L + disinfection + container with a
    tap.
  - **Village (100 to 1000 people):** 2 or more continuous-flow units; 5 to 200 m² per unit is typical
    (Visscher et al. 1987), though small ones under 1 m² serve small groups if scraped from the edge.
    Covered reservoir, a trained caretaker who measures the flow rate daily and staggers the scrapings.
  - **City:** dozens of beds (the municipal slow sand filter has been used since 1829 in London), with
    prior settling, mechanical sand washing, final chlorination and a laboratory. The slow sand filter
    takes up **up to 10 times** more area than a rapid one (Global WASH Cluster): beyond a certain size,
    coagulation with rapid filtration takes over.

## Sources

- Huisman, L. y Wood, W.E. (1974). *Slow Sand Filtration*. OMS, Ginebra. Capa biológica, vertedero de salida,
  raspado y recarga por zanjas.
- Visscher, J.T. et al. (1987). *Slow Sand Filtration for Community Water Supply*. IRC Technical Paper 24.
  0,1 m/h, d10 0,15 a 0,30 mm, CU < 5, lecho 0,8 a 0,9 m (mínimo 0,5 a 0,6 m), 1 m de agua sobrenadante,
  5 a 200 m² por unidad, gravas de soporte, dren, depósito del 40 %, re-maduración, control diario.
- Thames Water y University of Surrey (2005). *Slow Sand Filters* (water-e). sswm.info. Tabla de criterios,
  apdo. 4.2.11 (gravas), densidades de la arena, prueba de limo, raspado de 1 a 3 cm, zanjas, llenado inverso.
- CAWST (2009). *Biosand Filter* (ficha y manual v10). 0,4 L/min, 600 L/h/m², lámina de 5 cm, pausa de 1 a
  48 h, maduración de hasta 30 días, eficacias, 50 NTU y prueba de la botella de 2 L, «remover y tirar»
  (4 L, umbral de 0,1 L/min), orden de preferencia de la arena.
- CAWST (2012). *Biosand Filter Construction Manual*. Arena < 0,7 mm, gravas de 0,7 a 6 y 6 a 12 mm, prueba de
  caudal.
- Global WASH Cluster. *Compendium of Water Supply Technologies in Emergencies*, fichas T.9 y H.5. Velocidad
  de 0,1 a 0,4 m/h, turbidez, eficacias, recarga a 0,6 m, vaciado a 10 cm bajo la arena, re-maduración,
  superficie frente a filtro rápido.
- Pennsylvania DEP. *Drinking Water Operator Certification Training, Module 17*, unidad 2. d10, CU, cubierta.
- Kearns, J.P. et al. (2021). *Environmental Engineering Science*, PMC8165476. Carbón de gasificador ≥ 850 °C,
  adsorbedor modelo de 4,5 mm y 2,5 h, sistema de Tailandia de 7,5 mm y 18 h, tamaño de grano, carbón activado.
- FAO (1983). *Simple Technologies for Charcoal Making*. Forestry Paper 41, cap. 5 y 6. Montículo y fosa:
  color del humo, sellado, enfriado, rendimiento, temperatura.
- Ferguson, R.I. y Church, M. (2004). *Journal of Sedimentary Research* 74: 933-937. Velocidad de caída.
- Pillai, J. et al. (1999). *Water Science and Technology* 40(2): 85-90. Prueba de H₂S según temperatura.
- OMS (2011). *Guidelines for Drinking-water Quality*, 4.ª ed., cap. 7: tabla 7.7 (reducción por filtración
  lenta) y tabla 7.10 (*E. coli* 0 en 100 mL).
- Howard, G. y Bartram, J. (2003). *Domestic Water Quantity, Service Level and Health*. OMS. 7,5 L/persona/día
  para beber y cocinar; unos 20 L como acceso básico.
- Sphere Association (2018). *The Sphere Handbook*, 4.ª ed. Distancia mínima de 30 m entre letrinas y fuentes.