Water Treatment at Scale
Why This Matters
Individual water purification — boiling a pot at a time — doesn’t scale. When you’re supplying 10, 20, or 30 people, you need a treatment system that runs continuously or in large batches, with consistent quality. The methods here can treat hundreds to thousands of liters per day using materials you can source or build locally.
The Treatment Chain
No single treatment step handles everything. Use multiple barriers in sequence:
- Sedimentation — removes suspended solids (mud, silt)
- Filtration — removes remaining particles and many bacteria
- Disinfection — kills pathogens that pass through the filter
Each step protects the next. A slow sand filter works poorly with turbid water. Chlorination works poorly with organic-heavy water. The chain matters.
Sedimentation Basins
The simplest first step: let gravity do the work. Water sits in a basin long enough for particles to settle to the bottom.
Design parameters:
- Retention time: 2-4 hours minimum for basic sedimentation
- Surface area matters more than depth. A wide, shallow basin settles particles faster than a narrow, deep one
- Two basins in parallel let you clean one while the other operates
Construction: Dig a rectangular basin, 2m wide x 4m long x 1.5m deep (roughly 10,000L capacity). Line with concrete or puddled clay. Water enters at one end through a baffle (a wall with holes that distributes flow evenly) and exits at the opposite end through a pipe set 30cm below the surface.
Natural coagulants: Crushed Moringa oleifera seeds are a powerful natural flocculant. Crush 1-2 seeds per liter of turbid water, mix the powder with a small amount of clean water to make a paste, stir it into the basin water for 5 minutes, then let it settle for 1-2 hours. Particles clump together and settle much faster.
Tip
If Moringa isn’t available, crushed prickly pear cactus pads also work as a coagulant, though less effectively.
Slow Sand Filters
The slow sand filter (SSF) is the most proven community water treatment technology in history. It removes 90-99% of bacteria, most protozoa, and significantly reduces turbidity and organic matter. It works through biological action — a living biofilm (called the schmutzdecke) grows on top of the sand and devours pathogens.
Building a community SSF:
Dimensions for 20-30 people (treating ~2,000 L/day):
- Filter area: 2-4 m2 (e.g., 1.5m x 2m)
- Total depth: 1.5m
Layers (bottom to top):
- Underdrain: large gravel (20-40mm), 15cm deep, with a perforated pipe at the bottom to collect filtered water
- Support gravel: medium gravel (5-10mm), 5cm
- Fine gravel: (2-5mm), 5cm
- Filter sand: clean, fine sand (0.15-0.35mm effective size), 80-100cm deep — this is the critical layer
- Supernatant water: 50-100cm of raw water sitting above the sand
Flow rate: 0.1-0.2 meters per hour (measured as the rate the water level drops). Control this by adjusting the outlet valve. Too fast = poor treatment. Too slow = insufficient volume.
Ripening period: A new filter needs 2-4 weeks for the biological layer to establish. During this period, the filter removes particles but not pathogens effectively. Run it to waste or follow with disinfection until ripened.
Maintenance: When flow rate drops below acceptable levels (the schmutzdecke is clogging), drain the water to just below the sand surface and scrape off the top 1-2cm of sand. Rinse it and set aside. Refill with water and let the filter ripen for 1-2 weeks before relying on it alone. When sand depth drops below 60cm after multiple scrapings, add back the cleaned, stored sand.
Warning
Never let a slow sand filter dry out. The biofilm dies when exposed to air, and the filter must be ripened again from scratch. Keep the water level above the sand at all times.
Chlorination
Chlorine is the most practical disinfectant for community water systems. It kills bacteria and most viruses, provides residual protection in the distribution system, and is cheap.
Chlorine sources:
- Household bleach (sodium hypochlorite 3-8%): most commonly available
- Calcium hypochlorite (HTH pool shock, 65-70% available chlorine): stores better than liquid bleach
- Salt electrolysis: running electric current through salt water produces sodium hypochlorite. Small solar-powered units exist for this purpose
Dosing:
Target: 0.5 mg/L free chlorine residual after 30 minutes of contact time.
For household bleach (5% sodium hypochlorite):
- Add 1 ml (20 drops) per 10 liters of clear water
- For turbid water, double the dose
- Wait 30 minutes before use
- The water should have a very faint chlorine smell — if no smell, add more; if strong smell, wait longer or use less
For calcium hypochlorite (HTH, 65%):
- Dissolve 8g in 1 liter of water to make a 0.5% stock solution
- Add 10ml of stock solution per 10 liters of water
Contact time: Chlorine needs at least 30 minutes at room temperature to work. In cold water (<10 degrees C), allow 60 minutes. At the point of consumption, you want a measurable residual of 0.2-0.5 mg/L — this proves the chlorine has dealt with all the organic demand and still has killing power left.
UV Disinfection at Scale
Batch SODIS system: Rather than random bottles, organize it:
- Designate 50-100 clear PET bottles for SODIS
- Build a reflective surface (corrugated metal roof) angled to face the sun
- Fill all bottles each morning, lay them out, collect each evening
- Label bottles with date, use in order
- This can treat 100-200 liters per day reliably in sunny climates
Gravity-fed UV: Build a shallow, clear-bottomed trough (glass or clear polycarbonate) 2m long, 30cm wide, 5cm deep over a reflective surface. Water flows slowly through the trough, exposed to sunlight for its full length. At 0.1 L/min flow rate, water gets roughly 6 hours equivalent UV exposure.
Common Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Skipping sedimentation before filtration | Sand filter clogs rapidly | Always settle turbid water first |
| Letting sand filter dry out | Biofilm dies, 2-4 weeks to re-ripen | Keep water level above sand at all times |
| Overdosing chlorine | Bad taste, people avoid treated water | Measure carefully, test residual |
| Underdosing chlorine | Pathogens survive | Test residual — should smell faintly of chlorine |
| No trained operator | System breaks down or is run incorrectly | Train at least 2 people, document procedures |
What’s Next
- Water Distribution — getting treated water to users
- Community Water Testing — verifying treatment effectiveness
- Community Water System — the overall system design