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One 100 mm pipe collects waste from two toilets. Lately I’m having problems; I could hear the waste flowing randomly so I decided to cut through the PVC.

Here is what I found (I broke this part off, this is what the bottom part the inside of the pipe is):

I was able to clean the pipes using an implement that plugs on my high pressure cleaner, but the pipes are still filled with those deposit.

What are they called, should I try to remove them, and if so, how can I remove them.

It’s really dense, and even smaller parts that broke off won’t just flow with the water.

The smooth part is the bottom part, and the rough one is the top one.

This does not react with vinegar: no foaming, no softening.

The cross section is similar to an oyster’s . You can see the stratums.

Also, the slope is wrong: there is one section of 5 meters that is perfectly horizontal, then a T junction, then 5 meters with a 1 cm / m slope..