As disposal options for sludges become more difficult to find and more expensive to operate, those relying on some form of thermal treatment are becoming more commercially attractive. The incentive, especially for more heavily populated countries, is the production of treated solids suitable for beneficial application to land. It is the level of treatment required to achieve this cost effectively which has been the focus of much of the research in this field. There are now several commercially available plants designed to thermally treat sludges and many have been in full scale operation for a number of years. One of the auxiliary claims often made by proponents of such treatments, is that the resultant sludges are more amenable to dewatering and therefore easier and cheaper to handle. In this work we have used a novel filtration rig to obtain complete sets of filtration data for different sludges. Historically sludge characterisation has been achieved by measurement of an empirical Capillary Suction Time (CST) parameter, however with the new filtration apparatus it is now possible to obtain fundamental sludge characteristics across a wide range of volume fractions in hours not days. We have applied this technology to characterise different sludge samples before and after thermal treatment under different sets of operating conditions (pH, temperature and pressure) to simulate some of the commercially available thermal treatment technologies. We have also examined the effects of various chemical oxidants (hydrogen peroxide and Fenton's reagent) used as pretreatments to the thermal process and attempted to compare their cost efficiencies. The results show that the physical structure of the sludge is irreversibly altered by decreasing the pH or heating to temperatures in excess of 150°C in a way which significantly enhances the dewaterability of the material. The treated sludge not only has a higher permeability but also has a higher final per cent solids (often in the autothermal region). In contrast the amounts of chemical additives used in pretreatment to achieve similar effects are cost prohibitive.