I'm looking to supply my toilet tank with rain water and grey water from, say, the washing machine, but if that's not enough water to fill up the tank, I want to use water from the water mains to top it off. Preferably, this would be an automatic solution, so that I can use as much waste water as possible but never end up with an underfilled tank.

I don't have room for a large underground cistern, preferably I'd just modify the toilet's tank so that it takes in the water from both the mains and the waste water directly.

I've been working on a design (some frankensteinian monster involving two valves and float arms but I can't figure out how to get it to use the "waste" valve first and only activate the mains valve when there's no more water coming from the waste valve and the tank isn't filled to capacity yet.

Anyone have a good design for inspiration?

  • 1
    I've thought about doing something like this myself... two options come to mind: have both valves on one float arm, with the pressure on the main valve set lower (so both open at the same time, but most water will come from gray water side). Or, have the gray water spray on top of the main water float arm, holding that valve closed while pumping.
    – LShaver
    Commented Feb 20, 2017 at 17:15
  • You can use the grey water for yard watering, and go for a Composting toilet. This could save you a lot of plumbing :)
    – J. Chomel
    Commented Feb 21, 2017 at 7:21
  • Alas, I live in an area where the yard is watered regularly by the sky, automatically. :)
    – Bas
    Commented Feb 21, 2017 at 9:40
  • I like the idea of having the gray water spray close the main water valve. Only problem I can see is the water pressure from the mains would either be too high for the spray to keep the valve closed, or I'd have to reduce the mains water pressure so far down that it'll slow to a trickle... Still, worth experimenting.
    – Bas
    Commented Feb 21, 2017 at 10:31

1 Answer 1


Assuming you can get the appropriate variance or exception from municipal and state health and building codes to put drain water into the toilet feed at all (which is unlikely in the United States), the low cost solution uses a combination of height (which fluid systems use all the time to create pressure differentials) and a second tank. There are a few criteria to consider.

  1. You don't want your washer to leave water in the basin at the end of a spin cycle.
  2. You don't want to wear out your washer pump prematurely.
  3. You don't want the utility water at your faucets, ice makers, and shower heads to be contaminated with washer drainage.

Here's the lowest cost and highest reliability design:

  • A large upstream tank receives the washer drainage.
  • The upstream tank has a large overflow outlet into a trap (U shaped pipe) leading to the sewer line.
  • The upstream tank must be on strong legs or attached securely to a strong wall such that its bottom is far enough above the top of the toilet tank(s) fill level so that the connecting pipe can decline according to code toward the toilet.
  • You may need (or want) an additional drain pump to take the burden of raising the water to above the fill level of the upstream tank off the washer drain pump. (If so, care must be taken to ensure the pump starts VERY soon after the spin cycle of the washer begins.)
  • The upstream tank also has a buoyancy controlled toilet valve fed by the utility system water, with an inline feed valve as is usual, but no flap.
  • The drain at the bottom of the upstream tank feeds the normal toilet valve through a filter that is designed to be easy to change.

The normal toilet tank will fill slowly, but this system does not require electrically controlled valves and the design of pump controllers. To get it to fill faster, you can raise the height of the upstream tank (to perhaps the attic, which would definitely require a secondary pump to relieve the burden on the washer drain pump.

  • I'll have to look into local codes (not in the US) but you raise a lot of good points. I'm thinking of simplifying the design to use only rainwater. There's plenty of it available over here and this takes away the need of a lot of pumps. Also, the downstairs toilet is directly under a terraced roof - I'm thinking of letting the roof's drain go through a filter that fills up a holding tank directly above the toilet, and letting gravity fill the toilet's tank when necessary.
    – Bas
    Commented Feb 21, 2017 at 10:28

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