Grey Water Recycling Calculator

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.

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. The current design standard is BS EN 16941-2:2021 (on-site non-potable water systems - treated greywater), which replaced the now-withdrawn BS 8525-1:2010. Building regulations approval may also be needed.

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 - thermotolerant coliforms multiply rapidly, so the Environment Agency advises not storing untreated grey water for more than about 24 hours. More advanced systems use biological treatment for longer storage or irrigation use.

A 4-person household generates roughly 200-300 litres of grey water daily from showers and basins, while WC flushing uses approximately 100-120 litres daily. WC flushing accounts for around 22-30% of total household water use (Energy Saving Trust / Waterwise), and published studies show grey water reuse for toilet flushing typically reduces household consumption by about 29-35%, depending on how fully grey water supply meets WC demand.

Size the pump to PEAK demand, not the daily average. A single 6-litre WC cistern refills in roughly 1-2 minutes, so it needs around 3-6 litres per minute, and simultaneous use of more than one WC raises this further. The "average daily throughput" figure shown in the breakdown spreads the day’s flushing over a 10-hour window and is far below the instantaneous duty a pump must deliver - do not use it as the pump spec. Confirm the pump duty against the chosen cistern fill rate and the loading units in BS EN 806 / BS 8558.

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.