Grey water is wastewater from showers, baths, hand basins, and washing machines. It excludes water from toilets (black water) and kitchen sinks (which contain food waste and grease). Grey water can be treated and reused for WC flushing, garden irrigation, and laundry.

Is grey water recycling legal in the UK?

Yes, grey water recycling is legal and encouraged in the UK. Systems must comply with the Water Supply (Water Fittings) Regulations 1999, which require backflow prevention (no cross-connection with mains water) and clearly marked non-potable pipework. Building regulations approval may be needed.

What treatment does grey water need?

Minimum treatment for WC reuse: coarse filtration (hair/debris), fine filtration, and disinfection (UV or chlorine). Without treatment, grey water deteriorates within 24-48 hours. More advanced systems use biological treatment for longer storage or irrigation use.

How much water can I save?

A 4-person household generates approximately 200-300 litres of grey water daily from showers and basins. WC flushing uses approximately 100-120 litres daily. Grey water can supply most or all WC demand, saving approximately 30-40% of total household water consumption.

What maintenance does a grey water system need?

Monthly: check and clean pre-filter, check pump operation, test UV lamp function. Annually: replace UV lamp, service pump, clean tank, test water quality. Systems with automatic cleaning and monitoring require less manual maintenance.

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Grey Water Recycling Calculator

Size a grey water system for shower/basin to WC reuse

Number of Occupants

Shower Volume

Showers per Day (each)

How We Calculate This

Enter the number of occupants, shower and basin usage patterns, and WC flushing demand. The calculator determines daily grey water supply and WC demand, then sizes the storage tank based on daily production and buffer requirement. Pump flow rate, system components, and annual water savings are calculated to help specify a complete grey water recycling system.

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.