Some products should leaves some waste or byproducts when produced in a factory, for example like the process from iron rod to screws, some iron fragments should be produced as a byproduct from the cutting process. Some other more complicated or varied byproducts should be generalized into one kind of resource like "waste", "scrap", "garbages" etc, which can be recycled into a new reusable resource with a new processor building.
Waste management and processing is one of the most important part in a factory.