26 and is lowest in the other header 21 where because the pipe 27 is open ended and offers little resistance in comparison to the module 15 to the flow of sludge 12 the pressure is only slightly above atmospheric pressure the value of the pressure as measured by the gauge 26 is therefore a measure of the pressure difference between the ends of the tubes 22 the degree of dewatering of the sludge 12 affects its viscosity and consequently the measured pressure provides an indication of the degree of dewatering operation of the apparatus 10 is therefore continued until the pressure indicated by the gauge 26 reaches a predetermined value and the tank 14 is then emptied of that batch of sludge 12 by means of the pump 38 treatment of another batch can then commence it has been found that this process enables sludges of widely varying particle sizes from less than 1 u03bcm to more than 2 mm widely varying concentrations from less than 1 wt to more than 20 wt and of different chemical compositions e g ferric hydroxide magnesium oxide or sodium diuranate to be dewatered to give products which are of substantially uniform viscosity and are suitable for immobilization by mixing with cement powder after a batch of sludge 12 has been dewatered as described above it is desirable to remove any sludge deposits from the inside walls of the tubes 22 by a cleaning operation before treating the next batch the cleaning operation entails partly closing the throttle valve 24 and opening the recirculation valve 25 so as to reduce the rate of flow of sludge 12.