Sedimentation is the first treatment step: raw water stands still in a basin for at least 2-4 hours so that mud and silt sink to the floor before the water goes to the sand filter. A 2 meter by 4 meter basin 1.5 meters deep holds 12 cubic meters, and for clay that will not settle by itself, crushed moringa seed stirred in as a coagulant makes the particles clump and drop out in 1-2 hours.
Part of Water Treatment at Scale
Sedimentation is the removal of suspended solids from water by letting gravity pull them to the bottom of a quiet basin, the gentlest and cheapest step in a treatment chain.
Standing Water Drops Its Mud, And That Protects Every Later Step
Particles denser than water sink when the water is still. A basin gives them the time. The carrier’s design parameters are a retention time of 2-4 hours at minimum, a wide shallow basin settling faster than a narrow deep one, and two basins in parallel so that one can be cleaned while the other works.
Mud matters beyond appearance. It clogs the sand filter, shields germs from chlorine and sunlight, and uses up disinfectant. Settling is the cheapest way to take it out.
What You Need
- A rectangular basin, dug and lined with concrete or puddled clay
- A baffle at the inlet, a wall with holes that spreads the flow evenly
- An outlet pipe set 30 cm below the surface at the far end
- Moringa seeds, or another natural coagulant, for clay
- A second basin, so that cleaning does not stop the supply
Method: Dig, Line, Fill, Clean
Step 1: Dig a rectangle, for example 2 m wide, 4 m long and 1.5 m deep (carrier design). Line it with concrete or puddled clay so it neither leaks nor lets groundwater in.
Step 2: Fit the baffle at the inlet end and the outlet pipe 30 cm below the surface at the other end, so floating scum and the settled sludge both stay behind.
Step 3: Run the water through slowly. It should look clearer at the outlet than at the inlet.
Step 4: For clay-cloudy water, add moringa. Crush 1-2 seeds per liter, mix the powder with a little clean water into a paste, stir it into the basin water for 5 minutes, then let it settle for 1-2 hours. The particles clump and sink. If you cannot get moringa, crushed prickly pear cactus pads work less well.
Step 5: Clean regularly. Sludge builds up on the floor. Drain one basin and clear it while the other runs.
Worked Example: How Big Is Enough?
The basin volume is 2 × 4 × 1.5 = 12 m³ = 12,000 liters; the carrier rounds this to roughly 10,000 L.
- Retention time = volume ÷ flow. For the carrier’s filter flow of about 2,000 L/day, which is 2,000 ÷ 24 = 83.3 L/h, the basin holds water for 12,000 ÷ 83.3 = 144 hours, 6 days.
- To get just 2-4 hours at that flow, only 83.3 × 2 = 167 L to 83.3 × 4 = 333 L is needed.
So the carrier’s basin is far larger than the minimum for a village of 20-30. Size is then set by other things: room to enter and remove sludge, and room for the baffle to spread the flow (Settling Basins). A basin a few meters long is easier to build and clean than one holding 333 L. The same basin would give 4 hours at 3,000 L/h, which is 72,000 L/day, for a much larger community.
Moringa by the seed does not scale: 1-2 seeds per liter in a 12,000 L basin is 12,000-24,000 seeds. Use it for batches in a drum. Other pages in this wiki give the dose as powder, 50-100 mg/L and 50-150 mg/L, so work out the right dose with a jar test and do not rely on seed counts (Settling Basins).
Common Mistakes
- No baffle. The inlet jet stirs the basin and the settled mud comes back up.
- Never cleaning it. The sludge fills the basin and the settling time falls.
- Overdosing moringa. It adds organic matter that raises the chlorine demand later.
- Treating settled water as safe. Settling removes mud, not germs.
Safety
A drained basin is a confined space and a drowning hazard
A deep basin, drained or full, has sludge that can release gas and walls that cannot be climbed out of. Do not enter to clean it without a harness tied to a line held by a person outside, ventilation, and a rule that the person outside never enters after a collapsed worker. Do not test the air with a flame. Fence basins against children and animals.
What’s Next
- Learn next: Basin Design — sizing the basin from flow and particle size
- Combine with: Filter Construction — the step that follows