Due to tightening environmental regulations, the volume of solid residue from wastewater treatment is increasing. This wastewater solid residue or in other word sludge is an odorous semisolid material which contains organic, inorganic and microorganism and needs to be treated to reduce the adverse effect of its constituents to the environment (Eshtiaghi et al. 2013).

Anaerobic digestion is one of the most important processes for reducing sludge volume by reducing of about 30% of the organic load (part of the solid organic matter is converted into gas) and for producing biogas. The process operates either at 35-37 C or 55-58 C, and sludge needs to be heated by external heat exchangers or direct steam injection. Also, every 3 or 4 h the anaerobically digested sludge inside the digester is mixed with raw sludge which most likely will be at a different temperature compared to digested sludge (Sanin et al. 2011). Thus the temperature-dependant rheological characteristics of sludge are important in design and operation of digesters.