eli5 Is there a reason the waste water like from showers can’t be filtered through pipes and used for toilet water?
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It can, it’s called gray water recycling. You need a special plumbing system. You have to filter chunks out of the water, at least, and mix in clean water with dirty water, but it can be done.
There is a reason.
What you are refering to is called grey water. Black water is the toilet. Grey water could be used for the toilet. But it would be cloudy, might smell, might carry bacteria and viruses, and would stain the toilet.
Some of those can be partially or fully eliminated (like bacteria and viruses, you can add chlorine or use an UV lamp with various level of success).
Some can be eliminated by adding a full filtration system, like the cloudy water. This get expensive fast.
You then need a storage tank for that grey water, which take space.
And you need a pump. And some floats. And valves… In short a not that complex system, but one that will break every few years.
Clean water is cheap enough that it is not worth the trouble.
Interessingly enough, you would be better to reuse that grey water for irrigation instead. You only need a gross particulate filter (and not a complex one) as to not block the pump or the irrigation nozzles and pipes. Plants don’t care about bacteria and viruses, and most soaps are fine. And some of the junk in the water will ends up decomposing and fertilizing the ground. This is therefore a better solution. But you still need to deal with a tank that need cleaning from time to time and all the system failures along the years…
I believe there are toilets in Japan that do just this. You wash your hands and the water from that is used to flush the toilet, which is directly under the sink.
A lot in here are giving explanations of why you can’t use grey water for X. OP asked solely about flushing toilets. And when it comes to toilets, yes, you can absolutely use grey water to do so. The problem is more in the “how” though. In the previous example the sink is directly over the toilet. So that’s super easy. But let’s take a shower, for instance. How would you get the water into the toilet tank, which is higher than the shower drain? Now you need a pump, which uses electricity so any gain in water savings is negated by energy costs. It’s more a practicality than anything. In my house in order to get the dishwasher water to the toilet it’d need to run 20 foot horizontally and then up 3 feet. It just doesn’t make sense, given how unrelated the number of dishwasher runs to the toilet use are as well. Because now you need a reservoir tank elsewhere. Run the dishwasher 5 times before you need to flush? So the only practical use of grey water for toilets is what is already being done: use the hand washing sink above: It’s a 1:1 use (unless you’re one of my coworkers apparently) and gravity is in your favor. Logitistically nothing else makes sense. And for any other uses than toilet flushing, yeah. You obviously don’t want to use grey water, but OP seems to already know that.
I lived in a house where the toilet waste went in a sealed tank. The gray water was separate system that watered the yard
A big one is the existing expectation of it being clean water. Dealing with all the bits and stuff in the gray water would just make it all so much more difficult. I could just see apartments flooding cuz something in the upper tank got clogged or bound or something from some loose strands of hair out of the shower. Only place I could see something like this being viable would be in places that are extremely stressed for water and can afford to outfit all their with a more robust model.
Some Japanese toilets, upon flushing, refill the tank through a sink basin located on top of the toilet. Wash your hands immediately on flushing with water that goes to the toilet.
It can. Water enters the home as fresh, and probably treated water. If it’s used in the shower or sink, it can be classified as grey water, meaning it’s no longer safe to drink but low risk of having pathogens that are harmful to humans. Black water is toilet water, contains human feces and a very high risk of human pathogens.
In some areas grey water can be legally used to irrigate landscapes. Gray water generally isn’t a good idea for reuse, but it can be treated and used a “recycled” water, which is basically the water that comes out of a sewer treatment plant normally would be released back into a river.
My shower goes straight out to my bananas. My bathroom sink goes to the pineapples. We use all of our grey water for plants.
In Japan there are toilets where you wash your hands and it goes directly into the cistern.
In Australia many commercial buildings will reuse greywater from sinks and showers for irrigation and toilet flushing.
This involves having separate drainage and reticulation pipework and a rather expensive treatment systems. The water quality is usually quite good after treatment, you can’t drink it of course, but health risks are minimal if the system is correctly designed and maintained.
However these systems usually don’t save money. Water is cheap and energy and equipment (and ongoing maintenance) are expensive.
These systems are usually used to meet stringent ‘sustainability’ certifications like green star that the building owner can then market to prospective tenants or purchasers.
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