Build a community slow sand filter as a watertight box of 2-4 square meters holding, from the bottom up, 15 centimeters of 20-40 millimeter gravel with a perforated drain pipe, 5 centimeters of 5-10 millimeter gravel, 5 centimeters of 2-5 millimeter gravel, and 80-100 centimeters of clean fine sand, with 50-100 centimeters of raw water standing above. Fill it with water first and add the sand in small batches, then run it at 0.1-0.2 meters per hour and do not drink the output until it has ripened for 2-4 weeks.
Part of Water Treatment at Scale
A community slow sand filter is a box of graded gravel and fine sand through which settled water seeps slowly downward, cleaned mainly by a living layer on the sand surface rather than by the sand alone.
A Slow Sand Filter Cleans By Biology, So Its Construction Protects The Biology
The carrier calls the slow sand filter the most proven community water treatment technology in history. It removes 90-99% of bacteria, most protozoa, and much of the turbidity and organic matter. The layers and dimensions are the carrier’s; the order of work and the checks are added here. Sand and gravel that are dirty, a box that leaks, or a bed that can drain dry each defeat the filter, so the construction is mostly about avoiding those.
What You Need
- A watertight box of concrete, brick or ferrocement (Ferrocement Tanks), with an area of 2-4 m², for example 1.5 m × 2 m
- Gravel in three grades: 20-40 mm, 5-10 mm and 2-5 mm, all washed
- Clean, fine sand with an effective size of 0.15-0.35 mm; not beach sand, which carries salt, and not quarry dust, which carries fines (Sand Filters)
- A perforated drain pipe, an outlet pipe and valves
- An inlet diffuser and an overflow, described in Inlet and Outlet
- Water for washing media, plenty of it
Method: Box, Drain, Layers, Fill, Ripen
Step 1: Build and test the box. It must hold water with no leaks; test it full for a day before any media goes in.
Step 2: Lay the drain. Place the perforated pipe on the floor, leading to an outlet through the wall.
Step 3: Wash the media. Rinse each gravel grade and the sand until the rinse water runs clear. Dirty media clogs the bed and clouds the first output.
Step 4: Add the gravel layers, coarsest first: 15 cm of 20-40 mm, then 5 cm of 5-10 mm, then 5 cm of 2-5 mm. Level each one.
Step 5: Fill with water first, then add the sand in small batches so it settles through the water. Pouring dry sand traps air and sorts the grains.
Step 6: Level the sand surface carefully. A bed that is not level lets water run through the low spots fastest.
Step 7: Fit the inlet diffuser so incoming water does not scour the surface, and set the outlet so the water level can never fall below the sand.
Step 8: Ripen it. The carrier says to run it to waste or follow with disinfection until it has ripened 2-4 weeks (Biofilm Maturation).
Worked Example: A 1.5 m × 2 m Filter
Inputs: area 1.5 × 2 = 3 m²; sand 80-100 cm.
- Gravel: 3 × 0.15 = 0.45 m³ (underdrain), 3 × 0.05 = 0.15 m³ (support), 3 × 0.05 = 0.15 m³ (fine).
- Sand: 3 × 0.8 = 2.4 m³ up to 3 × 1.0 = 3.0 m³.
- Depth of media: 15 + 5 + 5 + 80 = 105 cm up to 15 + 5 + 5 + 100 = 125 cm.
- Add 50-100 cm of water above the sand: the box interior must be 155 cm to 225 cm deep, plus some freeboard.
The carrier lists a total depth of 1.5 m, which is about the media alone (1.05-1.25 m) plus a shallow layer of water. Check the finished depth against the layers you actually use. If the box is too shallow, you must reduce the water above the sand, and that head is what drives the flow.
Flow: at 0.1-0.2 m/h through 3 m², the filter passes 0.3-0.6 m³/h, which is 7.2-14.4 m³/day. The carrier’s design flow of about 2,000 L/day is much less. For 2 m³/day = 0.0833 m³/h, the area needed is 0.0833 ÷ 0.2 = 0.42 m² at the top rate and 0.0833 ÷ 0.1 = 0.83 m² at the bottom rate. If you build 3 m² and take only 2,000 L/day, the rate through the sand is 0.0833 ÷ 3 = 0.028 m/h, below the 0.1-0.2 range. Either size the filter to the flow, or accept the extra capacity and set the outlet to the 0.1 m/h rate, storing what you do not use.
Common Mistakes
- Pouring dry sand. Air and size sorting ruin the bed.
- Using beach sand or quarry dust. Salt, or fines that clog it.
- Letting the outlet fall below the sand. The bed can dry out and the living layer dies.
- Chlorinating the water before the filter. Chlorine can kill the living layer; add it afterwards.
- Trusting the output on the first day. It is not safe to drink until the layer has ripened.
Safety
Weight, wet concrete and a deep box
A full box holds several tonnes of water and sand; build its walls for the load, and do not stand on the rim. Wet cement burns skin and eyes. A deep, roofed or drained filter box is a confined space: no entry without ventilation, a harness on a line held by a person outside, and a rule that the person outside never goes in after a collapsed worker. Do not test the air with a flame.
What’s Next
- Learn next: Filter Bed Layers — what each layer is for and how it fails
- Combine with: Inlet and Outlet — controlling flow without drying the bed