Greywater — the relatively clean wastewater from sinks, showers, baths, and washing machines — represents 50–80% of a household’s total wastewater output. Most of it disappears down the sewer after a single use. Capturing and recycling even part of this stream for toilet flushing or irrigation can cut household water demand by 25–35%, without collecting a single drop of rain.
Greywater is wastewater from all household sources except toilets:
Blackwater is toilet wastewater. Blackwater contains fecal pathogens and must go to the sewer or septic system — it is never recycled in residential systems.
Kitchen sink greywater is a grey area (literally): it contains fats, oils, food particles, and detergents at relatively high concentrations. In most systems, kitchen sink water is excluded from the greywater collection and directed to the sewer. This simplifies treatment requirements considerably.
For a 4-person household:
| Source | Per capita (L/day) | Household (4 persons, L/day) |
|---|---|---|
| Bathroom sinks | 15–25 | 60–100 |
| Showers | 50–80 | 200–320 |
| Baths | 10–25 (averaged daily) | 40–100 |
| Washing machine | ~9 (household, avg) | 35 |
| Kitchen sink (excluded) | 10–15 | — |
| Total (excl. kitchen) | ~95–130 | ~335–555 |
As a rule of thumb: 60–120 L of collectable greywater per person per day from shower-heavy households.
Potential offset for toilet flushing: A 4-person household with dual-flush toilets uses ~84 L/day for flushing. With 335+ L/day of greywater available, toilet demand is fully offset — with surplus for laundry or irrigation.
Greywater is cleaner than blackwater but not clean:
| Parameter | Typical greywater range | WHO drinking water limit |
|---|---|---|
| BOD (biochemical oxygen demand) | 50–300 mg/L | N/A (sewage indicator) |
| Suspended solids | 30–300 mg/L | N/A |
| E. coli | 10²–10⁴ CFU/100mL | 0 |
| Total coliforms | 10³–10⁶ CFU/100mL | 0 |
| pH | 6.5–8.5 | 6.5–8.5 |
| Detergents (surfactants) | 10–100 mg/L | — |
| Phosphates (from detergents) | 5–50 mg/L | — |
| Sodium | 20–300 mg/L | — |
Key hazards:
Important constraint: Untreated greywater degrades rapidly. Anaerobic bacteria multiply within 24 hours at room temperature, producing sulfide gases and pathogens. Untreated greywater must be used within 24 hours of collection or treated for storage.
The simplest system. Washing machine outlet water is diverted directly to subsurface drip irrigation, without storage or treatment.
Greywater from showers and sinks flows by gravity through a branched pipe network to subsurface irrigation zones. No pump, no storage.
Greywater is collected in a small surge tank (50–200 L) and pumped to toilet cisterns or irrigation within a few hours. No biological treatment.
Greywater passes through a constructed wetland or planted filter bed. Biological and physical processes remove BOD, suspended solids, and pathogens to levels suitable for toilet flushing and restricted irrigation.
Greywater passes through a multi-layer filter medium (sand, gravel, geotextile, activated carbon), removing suspended solids and biological load. Combined with UV or chlorination for disinfection.
High-end biological treatment with an ultrafiltration membrane. Output approaches secondary treated effluent quality — suitable for toilet flushing, laundry, and restricted surface irrigation.
The surge tank must hold peak greywater generation until it can be used.
Peak generation scenario: In a 4-person household, the morning peak (6–9 AM) might generate:
Peak generation rate: ~360 L in 3 hours = 120 L/hour
For a 2-hour surge (allowing the pump to distribute before the surge tank overflows):
Surge tank = 120 × 2 = 240 L
In practice, 200–400 L surge tanks are appropriate for most single-family homes.
Irrigation with greywater requires attention to sodium content. Sodium degrades soil structure (causes clay dispersion and surface sealing) and can damage plants.
Sodium Adsorption Ratio (SAR):
SAR = [Na⁺] / sqrt(([Ca²⁺] + [Mg²⁺]) / 2)
All concentrations in mmol/L (millimoles per liter).
| SAR value | Soil suitability |
|---|---|
| <3 | Excellent for all soils |
| 3–6 | Suitable for most soils |
| 6–9 | Potential risk for clay soils |
| >9 | Significant soil structure risk |
Typical household greywater SAR: 2–8 (highly dependent on detergent brand and formulation).
Recommendation: Use low-sodium, plant-compatible detergents for laundry and personal care if greywater will be used for irrigation. Avoid detergents containing boron, which is toxic to plants even at low concentrations.
| Jurisdiction | Greywater status |
|---|---|
| France | Regulated under arrêté 2009: permitted for subsurface irrigation and toilet flushing with ministerial approval; reuse study required |
| Germany | State-dependent; generally permitted for toilet flushing with BioMedia or DIN 1986-100 compliant systems |
| UK | Permitted under water reuse guidance; BS 8525 provides technical standard for domestic greywater |
| Australia | State-dependent; NSW, Vic, and SA have specific approvals for Level 1 (laundry-to-landscape) and higher systems |
| USA | State-dependent; California, Arizona, Texas relatively permissive; some eastern states restrict or require permits |
| Spain | Regulated under Real Decreto 1620/2007 for water reuse |
Always check local authority requirements before installation. Regulations change more frequently than this book.
Greywater contains pathogens. Safe installation and use requires:
Children and immunocompromised individuals should not have contact with greywater-irrigated soil.
Previous: Chapter 6 — Filtration and Water Treatment