Internal recycling is the reuse in a manufacturing process of materials that are a waste product of that process.
Internal recycling is common in the metals industry.
External recycling is the reclaiming of materials from a product that has been worn out or rendered obsolete. An example of external recycling is the collection of old newspapers and magazines for the manufacture of newsprint or other paper products. In some areas, industries are required to pretreat wastewater before it is funneled into a waterway. Many gardeners recycle organic, biodegradable kitchen scraps by mixing them with leaves and grass clippings in a compost mound. There the organic refuse decomposes and is biochemically transformed into usable soil humus.