The bones
Your skeleton.
It is known as the anus or rectum.
our food and water, you eat it as a solid and it comes out as a solid, mostly. when you drink water it goes in as a liquid and comes out as a liquid.
If you think about this, you can figure it out. Why is it important to have a solid structure to your body? Without bones, you'd be a slug or a jellyfish.
The nucleus is the solid part of a comet.
where is solid waste collected in the body
If you mean what body system it's a part of the Digestive System
The anus is located at the end of the last part of the colon. The anus is the opening where solid waste leaves the human body.
The question itself needs to be more fundementally defined in its context as the human body contains many "solids". These include but are not limited to; blood (clots), bones, eyeballs, mucus (solidified), skin and of course not forgetting waste and other stuff.
Humans are part solid and part liquid.
Les Paul is a solid body and it is wonderful!
Its top is solid.