Classification of anaerobic digesters has been made by such as wet and dry, batch and continuous, single and multiple stages depending upon the desired outcome. Dry systems include the Dranco and Valorga processes. secondly, wet systems include single-stage systems based on the Waasa process, Linde process, and double stage systems based on Hitachi design, IBVL design, and the TERI’s enhanced acidification and methanation process. Earlier anaerobic digestion was employed to treat sewage and sludge, which eventually progressed to be used for degradation of industrial wastes on a much larger scale. The basic functioning of an anaerobic digester can be understood from The Draco Anaerobic Digestion Plant for Biodegradable Waste (See Appendix 1). Anaerobic digesters can be operated using a variety of inorganic compounds that act as terminal electron acceptors (Shammas, Liu &. Wang, 2009). Existing anaerobic digesters are mainly based on two principles, i.e. those that work on dispersed-growth bacteria and the others on attached growth bacteria (Polprasent, 2007). The attached-growth digesters are more efficient for higher organic loadings and lower retention time.
Anaerobic digesters utilize four main groups of bacteria namely, acid-forming (hydrolytic and fermentative) bacteria, acetogenic (acetate and H-producing) bacteria, acetoclastic (methane forming) bacteria, and Hydrogen-utilizing methane bacteria (Polprasent, 2007. p.153).
Anaerobic contact reactors: These function on high-rate processes retaining the sludge and concentrating solids in a separate reactor. subsequently the solid residual is returned to the influent. The most commonly used anaerobic contact reactors include the continuously stirred tank reactors (CSTR) and completely mixed contact reactors (CMCR). these are mostly employed in the treatment of municipal sludge. A more sophisticated form of contact reactor is the sand-bed filter reactor, mostly used for wastewater treatment. .