Lignocelluloses are often a major or sometimes the sole components of
different waste streams from various industries, forestry, agriculture and municipalities.
Hydrolysis of these materials is the first step for either digestion to biogas (methane) or
fermentation to ethanol. However, enzymatic hydrolysis of lignocelluloses with no
pretreatment is usually not so effective because of high stability of the materials to
enzymatic or bacterial attacks. The present work is dedicated to reviewing the methods
that have been studied for pretreatment of lignocellulosic wastes for conversion to
ethanol or biogas. Effective parameters in pretreatment of lignocelluloses, such as
crystallinity, accessible surface area, and protection by lignin and hemicellulose are
described first. Then, several pretreatment methods are discussed and their effects on
improvement in ethanol and/or biogas production are described. They include milling,
irradiation, microwave, steam explosion, ammonia fiber explosion (AFEX), supercritical
CO2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv
processes, wet oxidation, ozonolysis, dilute- and concentrated-acid hydrolyses, and
biological pretreatments.