Typically, there are three different operational parameters associated with the solids content of the feedstock to the digesters:

• High-solids (dry—stackable substrate)

• High-solids (wet—pumpable substrate)

• Low-solids (wet—pumpable substrate)

High-solids (dry) digesters are designed to process materials with high-solids content between *25 and 40%. Unlike wet digesters that process pumpable slurries, high solids (dry—stackable substrate) digesters are designed to process solid substrates deposited in tunnel-like chambers with a gas-tight door. They typically have few moving parts, require minimal or no pre-grinding or shredding, and do not use water addition. Solid state digestion of cattle dung is a suitable technology in which fresh cattle dung is anaerobically digested. Solid degradation of about 40-48% is observed in the effluent slurry that provides easy flowability to the oulet slurry [9].

Wet digesters can either be designed to operate in high solids content, with a total suspended solids (TSS) concentration greater than *20%, or a low solids concentration less than * 15%. High-solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High-solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture.

Low-solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input. Low-solids digesters require a larger amount of land than high-solids due to the increase volumes associated with the increased liquid-to-feedstock ratio of the digesters. There are benefits associated with operation in a liquid environment as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances they are feeding off and increases the speed of gas yields.