Animal manures represent a huge methane biogas potential. As estimated, 106 million dry tons of animal manures are produced each year in the USA, with approx­imately 87 million dry tons being available for methane biogas production [69]. Given a BMP of 200-400 m3 CH4/dry ton [8], the amount of animal manures available for AD provides a potential of 17-35 billion m3 of CH4 per year in the USA. The animal manures produced from confined animal feeding operations (CAFOs) offer one of the most abundant single feedstocks available for large-scale methane biogas productions. The composition and physical features (e. g., water contents) of animal manures vary widely from species to species and from oper­ation to operation [58]. In general, animals manures have relatively high water contents, ranging from 75% (poultry manure) to 92% (beef cattle manure). Most of the animal manure is organic matter, with VS contents ranging from 72% (poul­try manure) to 93% (beef cattle manure) of TS. Inorganic nutrients, including N,

P and K, are rich in animal manures, especially poultry manure. Because most of the readily degradable substances, especially carbohydrates, have been digested and absorbed by the animals, animal manures have very little readily fermentable sub­strates. Additionally, animal manures have high concentrations of amino nitrogen such as urea and ammonia and a large pH buffering capacity against acids. Thus, the fermentative acidogenesis during AD of animal manures typically does not result in significant pH decline, but high concentrations of ammonia can result, causing toxicity to methanogens, especially in thermophilic digesters where methanogens are very susceptible [43]. Furthermore, animal manures contain large amounts of microbial biomass, including bacteria and methanogens. Consequently, AD reac­tors digesting animal manures, especially livestock manures, can be started without the addition of external digested sludge as a start culture or inoculum.

Because of the relatively low contents of readily degradable substances, the methane biogas production from animal manures is generally slow. Thus, when digested alone a long retention time is needed. Co-digestion with nitrogen-poor yet carbohydrate-rich feedstocks, such as food-processing wastes and OFMSW, can substantially enhance CH4 production and stabilize the AD process of animal manures [59, 94]. Some animal manures, especially dairy cattle manure, contain sand from the sand bedding [42], which settles in AD reactors and can cause oper­ational problems if not dealt with properly. Due to the large differences in many physicochemical characteristics and degradability, different manures may require different AD technologies for efficient and cost-effective AD. Here the AD tech­nologies suitable for beef manure, diary manure, swine manure, and poultry litter will be discussed.