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What is the grey water?

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4 views asked May 30, 2018 by Devan Talebi

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A. System description:

  • Gray Water systems refer 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, 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 washdown. 

  • Grey Water is, by definition, a non-potable water and extreme care must be exercised to ensure that it does not contaminate any drinking water supply.

B. System Components:

  • A separate gray water collection piping system. 

  • A primary waste treatment system consisting 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. 

C. 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.

  • The following issues should be considered in the design of any gray water system:

            a. The design flow is based on the number of people in a facility.

            b. Lavatory use is estimated at 0.25 gal/use

            c. Men use urinals 75% of the time and water closets 25% of the time.

            d. The average person uses a toilet 3 times a day.

  • Design estimates shall evaluate:

            a. Gray water supply

In an office building, fixtures such as lavatories, water coolers, mop sinks or service sinks, and coffee room  sinks, are estimated to generate 1 gal/day/person. For an office building with 200 employees, we would 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.

           b. Gray water demand

The gray water demand for an office building is estimated based on 3 water closet or urinal uses/day/person. For calculation purposes, assume the population is 50% male and 50% female and that men use urinal 75% of the time and water closets 25% of the time. Total gray water demand = 712.5 gal/day = 178,125 gal/yr.

answered May 30, 2018 by Devan Talebi

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