IDI was developed as a process for improving sludge settleability while also reducing the quantities of waste activated sludge produced by activated sludge treatment plants. Thermal pre-treatment has been reported to improve the stabilization and enhance the dewatering of sludge, reduce the number of pathogens and reduce foaming problems, and can be realized at relatively low costs. The biological/thermal pre-treatments have received particular attention for industrial application due to their efficiency and relatively low cost compared to chemical-physical methods that require greater capital investment and higher operating costs. Thermal pre-treatment has been applied to full-scale wastewater treatment plants and is the most widely used technology to achieve sludge reduction. The thermal treatment of sludge changes the rheological properties of waste activated sludge from non- to pseudo-Newtonian. Although thermal hydrolysis reduces apparent sludge viscosity by orders of magnitude, the viscosity of digested sludge is independent of the presence or absence of pre-treatment.