This answer to a question about a toilet/sink combo makes me wonder:

What design considerations need to be taken into account if I was to modify my existing sink & toilet so that the sink waste water supplies the toilet tank?

I'm specifically thinking about

  • relative heights
  • traps
  • overflow
  • under-supply (i.e. how do you provide for the situation where the tank needs more water but nobody is using the sink -- does it automatically fill with water from a separate supply line, or do you have to run water in the sink?)
  • sanitary requirements

but there may be other considerations that I haven't thought about.


3 Answers 3


For modification of your existing water fixtures to use grey water to fill and flush a toilet, putting the sink over the toilet, or even any modification of the fixtures themselves, is unnecessary. Modification is still possible, many pedestal sinks do not actually require the pedestal, and can be mounted directly to the wall. This could allow mounting over a toilet if desired, though the options will vary depending on the specific fixtures used.

For the proceeding example, I will assume that you are working with a system that you have complete control of, and authority over, such as a single family home. You may be able to discuss these options with the community, if you live in an apartment or similar system, as implementation would require group approval, but the benefits would still apply.

You would want to start by changing the drainage path from any grey drains, sinks/tubs/washing machine for example, to drain into a holding cistern, rather than into the septic/sewage system. You could divert downspouts from your rain gutters to deposit into the grey water cistern as well. While a single bathroom sink may not have enough overall drainage to supply a single toilet with adequate quantities of water, the grey cistern will have a much greater chance of meeting the demand. This will leave you with only black water drains entering the septic/sewage system, thereby providing use of the same water twice before discarding, and as such greatly reducing your water usage.

You may (should?) be able to collect more in the grey water cistern than is needed to provide a supply for your toilet needs. This cistern may also be tapped for other uses such as watering the lawn and garden, washing the vehicles or building exterior, and so on. Grey water could ferment, if left in the cistern too long, so a drainage option should be included as well.

I would suggest finding reading specifically relating to grey water systems before implementing a project, to better familiarize yourself with the setup, as well as potential hazards. Jay Bazuzi recommended Art Ludwig's Create an Oasis with Greywater, and from the description it appears to be a good place to start. There are many books on the topic, and the more you read the better you will be prepared. You might also be interested in researching 'Humanure' while you are at it, as a way to recycle the black water drains as well, providing a possible third use of the same water; as well as increasing food production, and further reducing septic/sewage load.

Some other things to consider are the possible impact of the following:

  1. Fecal matter reaching the grey water, e.g. cloth diapers in the washing machine.
  2. Urine reaching the grey water, e.g. urinating in the shower.
  3. Cleaning products in the grey water, and their affect on plant life, if using to water the lawn/garden.

Toilets use gallons of water per flush, and are often one of the top water consumers in the home, so it's worth thinking about reducing water usage here.

A cheap and simple option is a bucket.

  1. Disconnect the drain from under the sink.

  2. Put a cap on the sewer line to keep sewer gasses out of your home. Careful: even an unglued cap can stick very tightly. A screw-on cap is one way to avoid that problem.

  3. Put a 5-gallon bucket under the sink. You may be able to find these as waste from restaurants or grocery stores.

  4. When you want to flush the toilet, pour water from the bucket, instead of using the fresh water in the toilet tank.

Some limitations:

  • Small children won't be able to handle the heavy bucket.

  • You need to keep an eye on the bucket to make sure it doesn't get too full, if you wash hands a lot between flushings.

  • Pouring too much water too fast into the toilet bowl might cause it to splash out - yuck.

One great advantage to this approach is that you don't have to worry about the toilet overflowing if it's clogged -- you can just stop pouring!

If you have a cabinet under the sink, you might want to remove the doors so it's easier to monitor the bucket, and to haul it out. A small curtain is a nice alternative.

A more radical solution is Humanure. It uses almost 0 water, converts waste into a valuable resource, and reduces load on septic/sewer systems.

  • 2
    While a bucket might be a simple way to achieve the goal, as you point out there are numerous limitations. Perhaps a better way would be to plumb the facility so that grey water is stored, rather than discarded. The toilet could then be plumbed to fill from the grey water tank, with an alternate path to draw potable water if the grey tank is empty. This would allow using rain catchment and all grey water drains to feed the water hungry toilets; as well as significantly reducing septic load, to only black water waste.
    – Prymaldark
    Jan 30, 2013 at 2:45
  • @Jay: I had plumbing in mind when I asked the question, but this is a useful answer for the simple approach. In the end, I need something that does not have the limitations you mention.
    – bstpierre
    Jan 30, 2013 at 3:30

Maybe a pump with tilt switch for bucket and cistern to fill and empty if either get too full, on a timer in case not used to keep water moving and not letting it ferment?

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