Pretreatments include mechanical size reduction, heat, steam, steam explosion, autohydrolysis, acid hydrolysis, alkali treatment, ammonia, chemical pulping, solvent extraction, and various combinations of these separate processes. The purpose of pretreatment is to maximize subsequent bioconversion yields and minimize the formation of inhibitory compounds. Classification processes Air classification is used to separate mixed MSW into light and heavy fractions. It was developed to produce streams for paper recovery and combustion. In this process, a shredded waste stream is fed into a cyclone with air injected from the bottom.
Magnetic separation is used to remove ferromagnetic materials.
Shredded waste passes through a magnetic separator on a continuous conveyor belt. Hand sorting is used to recover high value materials such as aluminum and some plastics. Recycling is best done by the consumer at the source. It is far easier to keep recoverable materials separate than it is to mix them and have to separate them later. Mechanical size reduction Requirements for mechanical size reduction vary with the feedstock.