A wide range of biomass types canbe used as substrates for the production of biogas by AD. The most common biomass categories used in biogas production are listed below and in Table 1.7. Animal manure and slurry, agricul­tural residues and by-products, digestible organic wastes from food and agro industries, organic fraction of munic­ipal waste and from catering, and sewage sludge, etc. are best study sources for biogas production.

Recently, various novel feed stocks has been tested and introduced for biogas synthesis in many countries, the dedicated energy crops (DECs), crops grown specif­ically for energy and biogas production. DECs can be herbaceous (grass, maize, and raps) and also woody crops (willow, poplar, and oak), although the woody crops need particular delignification treatment before

AD.

In AD, substrates can be classified according to the following criteria: methane yield, origin, dry matter (DM) content, etc. Table 1.7 gives a summary on the
characteristics of some digestible feedstocks. Substrates with DM content less than 20% are used for what is called wet digestion (wet fermentation), which includes animal slurries and manure as well as various wet organic wastes from food industries. When the DM con­tent is as high as 35%, it is called dry digestion (dry fermentation), and it is typical for energy crops and si­lages. The choice of types and amounts of feedstock for the AD substrate mixture depends on their DM con­tent as well as the content of sugars, lipids and proteins.