Eli5: Why do we use clean, potable water in toilets?
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There is a trade-off between the amount of plumbing complexity and the amount of water saved and reused. In most countries, we use what is basically the simplest system: one tier of water, used from everything from toilets, to irrigation, to drinking. This means everyone needs one set of water delivery pipes, the streets need only one set of water delivery pipes, etc. The downside is that a lot of water (about 680L per person per day) needs to be processed up to drinking standard, while only about 2 litres of that is drunk. In my city, they charge NZD$237 per year for water, and even in Las Vegas, it’s only NZD$2.37 per 1000L .
In some buildings, “grey water” from sinks, showers, and baths is recycled for use in toilets and irrigation. It can also be treated and recycled for sinks and showers. This can decrease water consumption by 30%, and water heating by even more. At current prices, it is often cheaper just to use potable water and not bother with a grey water system. Even when it is cheaper in the long run to install a grey water system, the developers don’t see the benefit because it will be someone else paying the water bill.
However especially in desert areas, a far cheaper and more effective water-saving measure would be to stop watering your garden and replace anything that dies with native plants specifically evolved for your neighbourhood conditions.
I can talk more about the plumbing complexity involved in a municipal grey water system, let me know if you’re interested.
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