Thermal treatment of sewage sludge by ohmic heating

Electrical conductivity measurements and heating trials have been performed on three representative samples of dewatered sewage sludge to assess the feasibility of heating sewage sludge ohmically to the temperatures required for pasteurisation or sterilisation. The very high viscosity of the sludge necessitated the use of a three-terminal cell, the properties of which are detailed, for conductivity measurements. The electrical conductivity of the sludge samples ranged from 0.2 to 0.6 S/m at room temperature, increasing approximately linearly to 0.8-2.3 S/m at 90 degrees C. These values are well within the range of conductivities that are ideal for ohmic heating. It was demonstrated that sewage sludge could be ohmically heated from room temperature to boiling point rapidly, uniformly and at energy efficiencies greater than 98%. Impedance measurements showed that the efficiency of the process is a function of the applied voltage, improving as the voltage is increased. The ohmic heating process appears ideally suited to the thermal pasteurisation or sterilisation of sewage sludge.