ABSTRACT
Solid-state anaerobic digestion (SS-AD) generally occurs at solid concentrations higher than 15%. In
contrast, liquid anaerobic digestion (AD) handles feedstocks with solid concentrations between 0.5% and
15%. Animal manure, sewage sludge, and food waste are generally treated by liquid AD, while organic
fractions of municipal solid waste (OFMSW) and lignocellulosic biomass such as crop residues and
energy crops can be processed through SS-AD. Some advantages of SS-AD include smaller reactor
capacity requirements, less energy used for heating, and no processing energy needed for stirring. Due to
its lower water content, the digestate of SS-AD is much easier to handle than the effluent of liquid AD.
However, SS-AD systems also have disadvantages such as larger amounts of required inocula and much
longer retention time.
The principles and applications of the SS-AD process are reviewed in this paper. The variation in
biogas production yields of different feedstocks is discussed as well as the need for pretreatment of
lignocellulosic biomass to enhance biogas production. The effects of major operational parameters,
including C/N ratio, solids content, temperature, and inoculation on the performance of SS-AD are
summarized. While an increase in operating temperature can improve both the biogas yield and the
production efficiency, other practices such as using AD digestate or leachate as an inoculant or
decreasing the solid content, may increase the biogas yield but have negative impact on production
efficiency. Different reactor configurations used in current commercial scale SS-AD systems and the
impact of economics on system selection are also discussed.