How can you conserve poop?

The average person should produce about a kg of scat per day (super-models would produce that much in a week, probably, but those anorexic walking skeletons are not exactly average).

That means around 365 kg per year.

Or about 29 tonnes over a lifetime.

Now, what is so awesome about someone keeping the equivalent content of a very large dump truck worth of human manure around? You are trying to scare your neighbors away?

You want to attract flies and dung beetles?

Well poop is not alive it is material, probably the best way is to get it around Gel so as to keep it's shape and also avoid its smell. And the Gel should get hard as a glass after some time. Like the PVA glue or something harder better.

Of course there are a few tones of bacteria alive in the fresh sh!t but who cares. You might even give a good item for when humans are no longer existent and someone finds your sh!t.

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Any liquid would not do because the **** is not one mass, it is going to make the liquid sh!ty and it will not be "preserved" the way it is.

Already been done a long time ago

well do not flush the toilet for years.

Eat it.

Put it in a jar...