Thus far, we have mostly discussed anaerobic digestion of diluted farm-based wastes (~3%- 8% TS in the reactor), such as slurries of swine waste and dairy manure. However, “dry fermentation” has found a niche in the bioenergy industry as well (~ 18 %—3 5 % TS in the reactor), especially when energy crops are used as feedstock. These systems are most often operated at thermophilic conditions to take advantage of the superior hydrolysis rates (Richards et al. 1991; De Baere 2000). The advantage of high-solids digestion compared with low-solids digestion is that high-solids digestion requires smaller reactor volumes than low — solids digestion due to high VMPRs. However, possible toxicity of metals and ammonia must be taken into consideration (Jewell et al. 1993). Dry fermentation has also been referred to as anaerobic composting, dry digestion, or high-solids digestion (Jewell et al. 1993; Chyi and Dague 1994; De Baere 2000). Systems that are operated with a TS content between ~10% and 18% have been referred to as semidry digestion (Mata-Alvarez et al. 1993). Numerous pilot — scale and full — scale plants with different designs have been built and operated, and excellent performances have been reported. For example, a volumetric biogas production rate of 9.2L/L/day (this means that 10 times as much biogas is produced than the volume of the digester itself) was reported for a full-scale dry anaerobic composting (DRANCO) process for which the TS in the system averaged 31.3% (De Baere 2000).