Grout and thinset mortar are types of concrete and should never be rinsed down drains. That includes the muddy water rinsed from the buckets and trowels used to handle these materials. Remember concrete contains sand and can actually harden underwater, which makes it perfect for clogging pipes.
Most people find it easiest to wash up their buckets and trowels outdoors using a water hose instead of indoors using a sink. However, you still need to collect as much of the concrete in the rinse water as possible and dispose of it as solid waste for several reasons:
Sometimes weather will make it necessary to do all your work indoors, including concrete clean up, and so you might as well know how to clean up buckets and tools without using gallons and gallons of water to dilute the concrete and wash it into the lawn.
Also, if you mix up concrete frequently, the calcium in concrete rinse water can build up and kill vegetation by raising the pH of your soil. (I have used concrete water to balance the pH of my compost heap, which is highly acidic due to all the decaying organic matter.)
Rinsing Buckets and Trowels with Minimal Water
Use the trowel to shovel and scrape as much left-over concrete as possible out of the bucket and into a lined trash can.
Use old plastic grocery bags and newspapers to wipe out the bucket as cleanly as possible. Add 1 cup of water at a time. Stir the bags and newspapers around the bucket with a trowel. Dump the moist dirty mess into the trash. Repeat.
Do the same for your trowel.
It is possible to use this method to get your buckets and trowels sufficiently clean to do a final rinse in a sink without fear of harming plumbing in any way.
What To Do With “Muddy” Water
However, sometime things will not go to plain either during the mosaic work or the clean up. Sometimes your hand-rinsing bucket or some other bucket will end up with a lot of dirty water with concrete on the bottom.
Waste slurries such as this should never be poured down a drain. Even if you dilute the concrete so much that it could never form a bond, the sand in it will remain a solid and accumulate in low spots in the piping.
There is a solution that doesn’t involve throwing away the whole bucket: Stir up the slurry so that it can be poured out into a disposable basin such as an old plastic milk jug with the top cut off or a plastic trashcan lined with a garbage bag. The concrete will harden over night, and you can pour off the water in the morning and discard the disposable container in the trash as solid waste.