The practice of recycling solid waste is an ancient one. Metal implements were melted down and recast in prehistoric times. Today, recyclable materials are recovered from municipal refuse by a number of methods, including shredding, magnetic separation of metals, air classification that separates light and heavy fractions, screening, and washing.
Another method of recovery is the wet pulping process: Incoming refuse is mixed with water and ground into slurry in the wet pulper, which resembles a large kitchen disposal unit. Larger pieces of metal and other non-pulpable materials are pulled out by a magnetic device before the slurry from the pulper is loaded into a centrifuge called a liquid cyclone. Here the heavier noncombustible, such as glass, metals, onto a glass- and metal –recovery system; other, lighter materials go to a paper-fiber-recovery system.
The final residue is either incinerated or used as a landfill. Increasingly, municipalities and private refuse- collection organizations are requiring those who generate solid wastes to keep bottles, cans, newspapers, cardboards and other recyclable items separate from other waste.