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Green Basics

Gray Water

Reduce Water Use by Irrigating with Gray Water


Reuse water, don’t waste it

To collect and use gray water for irrigation, drain lines from bathroom sinks, tubs, showers, and the washing machine empty to a centrally located holding tank, typically in the basement or crawl space. A filter at the holding tank screens out particles. Water is either drained or pumped from the tank to the irrigation lines.

A properly designed system has overflow protection for both the holding tank and the irrigation lines. The overflow valve for the holding tank feeds water directly to the sewer line if the filter clogs. Overflow protection for the irrigation lines can take two forms: a diverter valve directs water to secondary irrigation loops or, if the irrigated area becomes saturated, to the sewer line.

Plumbing codes vary. Options for reusing gray water vary according to local plumbing codes. If the building inspector allows, you can decide between systems that capture all of a house’s gray water or just the drain water from specific sources.

Although the bacteria in gray water are generally aerobic, gray water can’t be stored for longer than 24 hours without using up all of the oxygen in the water and encouraging the growth of smelly anaerobic bacteria.

That’s why the easiest and least expensive use for gray water is irrigation. The irrigation lines should be in the biologically active portion of the soil — no more than 9 in. to 12 in. below the surface.

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  1. Anonymous | | #1

    Ground water recharging with treated waste water
    What is the disinfection treatment level and specifications of treated water for the purpose mentioned?

  2. Anonymous | | #2

    removal of pathogens from treated wate water
    waste water treatment specification for ground water recharging

  3. Anonymous | | #3

    Gray water dumping
    I live on the Big Island of Hawaii - Kona side. We have a cess pool. We never ever had a cess pool before and we have a number of questions.
    Today we are cleaning with Dawn dish washing detergent and Borax.
    Is it safe to dump the dirty water in the yard or is it better I dump the water in the toilet?
    What about bleach?
    What about use of a garbage disposal?
    Toilet paper best kept out and then what to do with it burn it or land fill it? (We here mexicans in mexico figured this one out long ago - apparently they do not to use toilet paper but if not then what?

    We are trying to remediate mold without using hash cleaners but have reverted to bleach known for treating mold/mildew. We are looking into assessing what the best ventilation could be for our situation while we are cleaning. Any recommendations?

    What is the best envornmentally sensitve approach as to what to put in and not put in a cess pool?
    We are wondering if septic would be more environmentally sound practice?

  4. kentthompson | | #4

    A Different Perspective
    I'd like to express my sincere appreciation for this site! It's such a wonderful resource on so many topics. In that spirit, here are my comments on this article:

    I've been a greywater installer in California for the past 5 years. This article is either a little out of date or represents the views of manufacturers and installers of some of the more complicated greywater systems.

    For the single family home, systems that have a tank or filter are usually a bad idea, unless simpler options have been examined and discarded. For larger scale residential, these more complicated systems can make sense.

    Let's consider filters. As compared to your potable water supply, greywater is comparatively much dirtier and filters need to be changed monthly to avoid clogging. Do homeowners want to clean a nasty filter once a month? Most do not. This results in a system failing prematurely. A much more robust system results from piping the unfiltered greywater directly to landscape where it can be treated by the soil.

    Why have a tank? Better to let the greywater flow to the landscape and be stored in the soil or temporarily in a mulch basin prior to infiltration. It avoids the cost of a tank and the potential smells of partial drainage or sitting too long.

    Bacteria and parasites, while a concern in designing and installing the systems, are in practice not a problem if the greywater is released subsurface. To my knowledge there are no known cases of properly designed greywater systems making people sick.

    For additional reading, I recommend Laura Allen's website and books:

    Or Art Ludwig's website or book:

    Check out the 'laundry to landscape' or 'branched drain' type systems. They are a great place to start.

    To answer some of the (quite old) questions in the prior comments...the main pollutants you'd want to exclude from greywater are sodium, chlorine, and boron. Unfortunately these are in many common household products. Either changing products or using your system's diverter valve can preserve water quality.

    My understanding is that soil treatment is quite effective. Check out Art's site for more and better information:

    Thanks for hosting an article on greywater!

    Kent Thompson

  5. GBA Editor
    Martin Holladay | | #5

    Response to Kent Thompson
    Thanks for posting some helpful suggestions and links.

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