I am looking into grey water systems for a future house. In the course of researching, I learned that in states that have adopted the International Plumbing Code, you can use the water for flushing toilets as well as irrigation. If you use it for flushing toilets, you have to dye it with either blue or green food dye, as well as disinfecting it in some way.

My question is: Is there a standard for how much dye to use per volume of water or a standard for opacity or something? I am mainly concerned about blowing through a lot of dye to make the water colored.

I have done lots of research on this, but all the documents i have found merely state that the water has to be dyed, not by how much.

1 Answer 1


Probably enough that your guests can easily see that it's not clear when their dog is looking for a convenient drinking bowl, going for the most obvious inappropriate use of toilet flushing water as potable. Alternatively (and probably requiring a bit more dye) would you notice if you were handed a glass of the stuff that it was blue, or green, not clear?

I assume you'd handle this (and the disinfection) with dosing pumps. Non-toxic dye is available quite concentrated, so it probably won't take much volume. Looking at one example (dye manufacturer's recommended dosing rate), a pound of dry dye treats a million gallons, or a gallon of liquid treats 96,000 gallons (or 12,833 cubic feet, which may help grasp the relation to your current use, depending what your water is billed in.) The liquid would be easier to dose automatically.

You might be able to use a lower rate, but that rate should at least be adequate, and does not seem to imply "blowing through a lot of dye" for remotely reasonable water use. I suspect the standard in play is "does the inspector see that the water is dyed?"

  • Yeah, i was assuming it was up to the AHJ eventually. Never thought to look at the dye manufacturers page for dosing info. Thanks
    – sww1235
    Apr 10, 2015 at 20:29

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