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What is a greywater system?

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223 views asked Apr 6, 2015 by Devan Talebi

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Grey Water is a non-potable water and extreme care must be exercised to ensure that it does not contaminate any drinking water supply. Gray Water systems refers to dual water supply systems which permit the reuse of “gray” wastes discharged from lavatories, bathtubs, showers, service sinks, laundry tubs, and other fixtures discharging dilute non-fecal wastes or a fully treated sewage affluent, as a secondary, non-potable water source.

Gray Water does not include waste water from water closets, kitchen sinks and dishwashers. Depending on the type and degree of treatment of collected gray waste, this water can serve as an appropriate source of supply for water closets, urinals, cooling towers, irrigation and hose bibbs for wash-down. 

Design Criteria

It is estimated that approximately 66% of the waste water discharged from a typical household in 1 day is gray water. The remaining waste water (34%) is black water from water closets. The discharge from the separate piping system supplying the gray water system should be sized based on the applicable plumbing code.

For designing the gray water system the following should be considered:

  1. The design flow is based on the number of people in a facility.
  2. Lavatory use is estimated at 0.25 gal/use
  3. Men using urinals for 75% of the time and water closets for 25% of the time while women just using the water closet.
  4. The average person uses a toilet for 2 times a day.

For example in an office building the fixtures such as lavatories, water coolers, and coffee room sinks, are estimated to generate 1 gal/day/person. For an office building with 200 employees, can expect to be able to recover 200 gal/day for gray water reuse. Based on 5 working days/week and 50 weeks/year annual use, 50,000 gal could be available for a gray water reuse.

For mop sinks, service sinks and washer (Laundry) need to add additional gallon. For mop sink 15 gpd, for domestic washer (laundry) 15 gpd can be considered.

System Components:

  • A separate gray water collection piping system. 
  • A primary waste treatment system consist of:

           a. Turbidity removal.

           b. Storage

           c. Biological treatment

           d. Filtering. 

  • Disinfecting systems consisting of ozone, ultraviolet irradiation, chlorine, or iodine.
  • Treated water storage and system distribution pressure pumps and piping. 
answered Apr 6, 2015 by Devan Talebi

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