Work out grey water reuse volume instantly with clear inputs, formula shown and shareable results.
Grey water from showers, baths and basins amounts to 50-70 litres per person per day, of which 10-20% is lost in treatment and backwashing. Toilet flushing demand of 30-40 litres per person per day is usually well matched to the available supply, which is why it is the standard reuse application.
Grey water balance
Usable = occupants x generation per person x (1 - treatment loss); demand = occupants x toilet demand
Grey water reuse systems require treatment, disinfection, clear labelling and backflow prevention under plumbing and water regulations. Cross-connection with potable supply is a serious health risk.
Kitchen water carries fats, food solids and detergent that foul treatment systems rapidly. Laundry water is sometimes included but its high pH, lint and bleach load complicate treatment.
Untreated grey water turns septic within 24 hours, so systems must treat and use it promptly or disinfect for storage. Long-term storage of untreated grey water is a health hazard.