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A couple of articles encourage the use of rainwater to flush toilets. (article 1, article 2)

However, would it be safe to do so if the rainwater in your particular area has a pH that is considered as acidic or akaline? (The risks of which are outlined on this page and reproduced below) enter image description here

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    Chemical remark: the pH of rain water may be sour or alkaline (depending on local air quality and e.g. the type of roof it was collected from), but it will have hardly any buffer capacity. The actual amount of acid or base that is in there will be small. Rainwater is typically very soft (basically no Ca²⁺ or similar kations). – cbeleites unhappy with SX Jul 4 at 12:14
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    Note that if you are in an area served by public water/sewer this may not be legal, especially if the sewage bill is based off the water meter. – Ecnerwal Jul 4 at 19:02
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    There are likely a million homes in the US which derive their entire water supply (including drinking water) from "cisterns" which are refilled using collected rainwater (from the roof). In many parts of the US this is far preferable to using well water. Bear in mind that rainwater is essentially indistinguishable from "distilled" water. – Hot Licks Jul 4 at 20:31
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    @HotLicks depending on where you happen to live, rainwater may as well be pretty much distinguishable from "distilled" and even not safe to drink. But I am yet to see a rainwater not safe to flush the toilet. – fraxinus Jul 4 at 23:08
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    Would seem weird for any piping system primarily intended to carry human waste, especially urine, to be sensitive to rain water. – Nat Jul 5 at 5:37
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It is doubtful the pH+ of rainwater anywhere, other than directly downwind of an erupting volcano, would be unsafe in terms of damaging plumbing fixtures or pipes.

That said, there is concern in how the rainwater is stored. It must be kept so as not to be a breeding place for mosquitoes, toxic algae or other noxious critters, nor should the container cause entrapment of wildlife. There also must be air breaks to ensure that the untreated rainwater does not enter and contaminate potable water. There may be local regulations regarding use of rainwater or "greywater".

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    There are worse things in a toilet than rain water! – Steve Wellens Jul 4 at 4:26
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    "It is doubtful the pH+ of rainwater anywhere, other than directly downwind of an erupting volcano, would be unsafe in terms of damaging plumbing fixtures or pipes." I remember reading about some pretty terrible cases of industrial pollution that might have gotten that bad. Probably not something you'd have to worry about in the developed world, but it might be something you'd have to worry about in nations with weak or nonexistent environmental laws like China. – nick012000 Jul 4 at 12:40
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    @nick012000 Although pollution can get pretty bad, you're not going to get rainwater that can corrode plumbing before other very serious problems occur, like rain burning your skin or having toxic humidity in the area. It's unlikely for your plumbing to be your only problem if you get pollution that bad. – Nelson Jul 6 at 3:48
  • In the event that a sizable percentage of the population is suddenly using rainwater to flush their toilets, regulations regarding such matters are likely to be suspended or at least not enforced. – Eric Hauenstein Jul 6 at 13:29
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    I know in my area, it is illegal to direct any storm or ground water to the sanitary system. Technically that is what this amounts to but doesn't seem to relate to the reason for this:to avoid the cost of treating the storm water and avoiding overwhelming the system. But unless the regulations are modified, you might be in violation of codes, which could be an issue when renovating or selling the home. – JimmyJames Jul 6 at 17:58
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Yes, you can do this, but you must make sure that the supplies to the wc are separated from the rest of the supplies to the taps, showers etc.

We planned this in the plumbing for our house so that we could easily separate the washing machine and 3 wc from the other items - plumber was not happy but we got what we wanted.

That meant we only needed a simple filter from the rain water tank as it never gets into the potable system.

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    Interesting, how often do you have to change that “simple filter”? Also, what do you do when it hasn’t rained for a while? (Honestly curious for possible future inclusion in my hone) – statueuphemism Jul 4 at 10:51
  • Simple filter is to remove leaves etc check out coanda screens. As for rainfall you have to plan - water use how much, when and compare to annual rainfall and when then you calculate how much storage you need. Nature is fickle so you need to factor in variation you do not get the same rainfall each year and also wster use in the home changes. Good exercise on a spreadsheet. Do you know how much water you use each day? – Solar Mike Jul 4 at 10:58
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    I was thinking about keeping the lines separate and planning for typical use/rainfall, but then adding a “just in case” feeder to the tank from the city water lines via an air gap. The city water lines would only kick on to feed the rainwater tank when a float valve gets to low enough (basically, reusing the toilet tank fill design). I feel like this would add some piece of mind in case my family comes down with a stomach bug during the hottest and driest summer days without overbuilding for that rarest of scenario. – statueuphemism Jul 4 at 11:58
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    @statueuphemism: over here (Germany) such as rainwater system usually has such a feeder. Of course, you can also go for the low-tech solution and feed via garden hose when the cistern runs empty (cisterns here are typically put underground in the garden) – cbeleites unhappy with SX Jul 4 at 12:09
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Rainwater is regularly captured and used by rural domestic properties here in New Zealand for everything in a typical household - from drinking to flushing toilets, with minimal treatment.

Its pretty much the norm for people in rural locations (if you are a few miles outside a built up area, you wont have mains water or sewerage) - capture rainfall runoff and store it in large tanks for use in the home. The alternative is drilling a borehole (expensive and not always possible) or regular water deliveries (again, expensive).

Pretty much the only treatments the water goes under is to kill microbes and filter large detritus.

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  • Same for Australia. – Kingsley Jul 6 at 2:47
  • One cool tip I've learned about collecting rain water in NZ - after long periods of draught, disconnect the inlet so that all the bird shit washes off. Keeps it cleaner. – dzh Jul 7 at 9:27
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What can a rainwater contain? Something that was either in the air or on the roof.

  1. Acids: sulfuric or nitric, coming from the air pollution. If the air is more or less safe to breath, water raining from it is acceptable for the purpouse intended in regard to the acidic content. Steel pipes may suffer some corrosion, but they are rarely used today. Plastic pipes are OK.

  2. Bases: the only more or less gaseous base is ammonia. See about acids. Corrosion is less likely. You will pretty much know if there is unsafe level of ammonia in the air.

  3. Dust: it may be dominated by soil, soot, tyre flakes or something else. May settle at the bottom of the container. Easy to filter out.

  4. Microbes, tree leaves, bird feces and feathers, insects and other organic life-related things. Not always easy to filter out. May make water unsafe or unpleasant to drink. May clog something as well. The toilet will be still happy to flush with it.

Life may try to develop in the container. In order to prevent it, filter the input as much as practical, keep the container dark and covered. UV lamp may also help, but may as well damage a plastic container.

All things considered, go for it.

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Virtually any water is safe for toilet water use as long as it has, at minimum, a "sand filter" of sufficient size to keep out sand and particulates. Even fresh water from a lake, river, or stream can suffice, with its entrapped microorganisms and impurities.

Ocean-going vessels utilize seawater for numerous functions, including firefighting and the supply plumbing of the urinals and toilets. This necessitates the use of stainless steel piping and fixtures for the toilets, but enables the use of the "free" water source that they float upon. (Granted, they may not be "flush" toilets onboard a ship, but the analogy is otherwise valid.)

For residential-grade installations, water quality will affect the looks and longevity of your piping and fixtures. If you are willing to use collected rainwater, then you can expect to suffer some additional costs of maintenance, filtration, storage, and possibly pumping that your neighbors wouldn't incur. Your tank and bowl might grow algae or become discolored without extra cleaning. An addition of copper to the holding tank or within your piping would help mitigate the algae. Even still, none of these factors affect the safety of the toilet: it will still work and not harm you.

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Yes, using rainwater can be a great way to preserve water from potable and drinkable sources. However, I do agree that some contents could cause for problems if they are present both for restrictions, erosion, and etc.

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