Electro-Dewatering of Anaerobically Digested and Activated Sludges: An Energy Aspect Analysis

Electro-dewatering experiments were carried out on a laboratory-scale device to analyze the energy consumption under both a constant voltage (C. V.) and a constant current density (C. C.). The purpose of this study was to determine the impact of electro-dewatering parameters (voltage, current density, pressure, and amount of sludge to be treated) on energy consumption. The results of the C. V. experiments showed a very strong correlation between the applied voltage, the final dryness of the sludge filter-cake, and the total energy consumption (Wh/kg(additional water removed)). The calculation of the instantaneous energy consumption allowed us to determine the range of dryness where the electro-dewatering process (EDW) was energetically more advantageous than thermal drying. The instantaneous energy consumption depends mainly on the reached dryness. It does not significantly depend on other process parameters such as the applied voltage, the applied current density (in A/m(filter) (2)(surface)), the applied pressure, or the initial amount of sludge. In fact, the change in electrical power consumption due to different test conditions was balanced by changes in kinetics. The comparison between two tests (one with C. V. conditions, the other with C. C. conditions) showed that the kinetics were similar when the electrical power consumption was the same. The results of this work lead to two main findings: the dewatering kinetics depends essentially on the current density; and the final dryness of the filter-cake depends mainly on the applied voltage.