Organic constituents of anaerobic feedstock may include monomers, natural polymers (such as starch, lipids, elastin, collagen, keratin, chitin, and lignocelluloses), and synthetic polymers (such as polyesters, polyethylene, and polypropylene). Monomers are relatively easy to breakdown under normal anaerobic process conditions; however, polymers take a longer time to break down as shown in Figure 1.4 due to their chemical structures therefore may require pretreatment to improve their amenability to biodegradation. Lignocellulose polymers are usually the major organic constituents of municipal and industrial residues.