Before designing a water system, you need to understand how much water your household actually consumes and — critically — how much of it truly needs to be potable quality.
| Tier | Quality | Uses | % of Household Use |
|---|---|---|---|
| Potable | Drinking water standard | Cooking, drinking, teeth brushing | 3–5% |
| Clean | Filtered, non-potable | Showers, handwashing, laundry | 55–65% |
| Grey | Lightly contaminated | Toilet flushing, garden irrigation | 25–35% |
| Raw | Untreated | Outdoor cleaning, fire reserve | 5–10% |
Key insight: A family of four uses 150–200 m³/year, but only about 7–10 m³ needs to be true drinking water quality. This realization is the foundation of autonomous water management.
The formula is straightforward:
\[V_{annual} = P \times A \times C\]Where:
Runoff coefficients by roof type:
| Roof Material | Coefficient |
|---|---|
| Metal (zinc, steel) | 0.90 |
| Tile (clay, concrete) | 0.80–0.85 |
| Slate | 0.85 |
| Flat roof (bitumen) | 0.70–0.80 |
| Green roof | 0.30–0.50 |
For our reference home in central France:
Compare this to non-potable household needs: ~130–180 m³/year. Rainwater alone covers only 30–40% of total needs, but it covers most garden irrigation and can supply toilet flushing and laundry.
To increase collection, you can add:
With 160 m² total collection area: $800 \times 160 \times 0.85 = 108{,}800 \text{ L} ≈ 109 \text{ m³}$
This covers 60–70% of total household water needs.
The key challenge is matching variable rainfall to constant demand. France receives most rain in autumn/winter (October–March) but garden demand peaks in summer.
Rule of thumb: Store 3–6 weeks of non-potable consumption.
| Tank Size | Cost (buried concrete) | Cost (above-ground plastic) | Covers |
|---|---|---|---|
| 3,000 L | €1,500–2,500 | €300–600 | ~2 weeks non-potable |
| 5,000 L | €2,000–3,500 | €500–900 | ~3 weeks |
| 10,000 L | €3,500–5,500 | €800–1,500 | ~6 weeks |
| 20,000 L | €6,000–10,000 | — | ~3 months |
Recommendation: A 10,000 L buried concrete tank is the sweet spot for most autonomous homes. It provides enough buffer for dry spells while remaining cost-effective.
A complete rainwater collection system includes:
Total system cost (10,000 L, buried): €5,000–8,000 installed
If rainwater collection is insufficient, a well or borehole provides a complementary or primary water source.
| Type | Depth | Flow Rate | Cost | Permit Required |
|---|---|---|---|---|
| Shallow well | 3–10 m | 0.5–3 m³/h | €2,000–5,000 | Declaration (France) |
| Deep borehole | 20–100+ m | 1–5 m³/h | €5,000–15,000 | Authorization |
| Artesian well | Variable | Self-flowing | €8,000–20,000 | Authorization |
Groundwater quality varies enormously. Common issues include:
Always test well water before designing treatment. A comprehensive analysis costs €100–300 and tests for 30+ parameters.
A well pump typically consumes 500–1,500W. For a household using 400 L/day from a well:
To make rainwater safe for all household uses (including drinking):
| Stage | Technology | Removes | Cost |
|---|---|---|---|
| 1. Sediment filter | 20 μm cartridge | Sand, debris | €30–50/year |
| 2. Carbon filter | Activated carbon | Chlorine, taste, organics | €50–80/year |
| 3. Fine filter | 1–5 μm cartridge | Fine particles | €40–60/year |
| 4. UV sterilizer | 254 nm UV lamp (40W) | Bacteria, viruses | €80–120/year (lamp) |
| 5. Optional: RO membrane | Reverse osmosis | Dissolved minerals, heavy metals | €100–200/year |
Total annual treatment cost: €200–400 for full potable treatment
Equipment cost: €1,500–3,000 for a complete multi-stage system
UV sterilization is the most practical solution for autonomous homes:
Important: UV only works on clear water. Turbidity must be below 1 NTU. Always pre-filter before UV treatment.
Greywater (from showers, sinks, and laundry) represents 50–60% of household wastewater and can be recycled for:
A basic gravity-fed system:
Cost: €1,500–4,000 for a complete system
Water savings: 40–60 L/person/day = 58–87 m³/year for a family of four
For more thorough greywater treatment, a constructed wetland uses plants and microorganisms to purify water naturally:
The treated water output is suitable for subsurface irrigation but not for direct food crop contact.
Putting it all together for our reference scenario:
| Source | Annual Volume | Reliability |
|---|---|---|
| Rainwater (80 m² roof) | 52 m³ | Seasonal variation |
| Rainwater (additional roofs, 80 m²) | 54 m³ | Seasonal variation |
| Greywater recycling | 60–80 m³ | Consistent |
| Well (backup/supplement) | As needed | Year-round |
| Total available | 166–186 m³ | — |
| Use | Annual Volume | Source |
|---|---|---|
| Drinking & cooking | 7–10 m³ | Filtered rainwater or well (UV treated) |
| Showers & baths | 60–85 m³ | Filtered rainwater |
| Toilet flushing | 44–58 m³ | Recycled greywater |
| Laundry | 22–29 m³ | Filtered rainwater |
| Dishes | 15–22 m³ | Filtered rainwater |
| Garden (see Chapter 2) | 30–80 m³ | Rainwater + greywater |
| Total demand | 178–284 m³ | — |
With full rainwater collection (160 m² roof area) + greywater recycling, the system covers 85–100% of needs in a normal rainfall year. A well provides backup for dry years and peak summer demand.
📊 Quick Reference — Water System Costs:
| Component | Cost | Lifespan |
|---|---|---|
| 10,000 L buried tank | €4,000–6,000 | 50+ years |
| Gutters & first-flush system | €500–1,000 | 20 years |
| Pump & pressure system | €500–1,200 | 10–15 years |
| UV + filtration system | €1,500–3,000 | 15 years (consumables annual) |
| Greywater system | €2,000–4,000 | 20 years |
| Well (if needed) | €3,000–15,000 | 30+ years |
| Total water autonomy | €8,500–25,000 | — |
| Annual operating cost | €300–600 | — |
Compared to municipal water at €4/m³ (supply + treatment): a family using 200 m³/year pays €800/year. The autonomous system pays for itself in 12–25 years, with much better resilience and independence.
| ← Previous: Introduction | Next: Water for Growing Food → |