ok, I try:
1) what's the benefit you want to assess (a) disposal/"getting rid" of the products? Or (b) recycling?
2) for (a), create a waste treatment process to model the treatment of the waste products; you can also have a sequence of waste treatments (1st waste treatment has waste as output which then is treated in another, 2nd waste treatment process), etc..
Basically, it is just reverse production, with flow type waste flow.
Hope this helps, Andreas