Migration of ions and organic matter during electro-dewatering of anaerobic sludge

This paper reports the migration of ions and organic matter, initially present in anaerobically digested sludge taken from the effluent of an anaerobic digestion unit in Mikkeli Wastewater Treatment Plant (SE Finland) during electro-dewatering process, employing various experimental strategies such as freeze/thaw and polyelectrolyte conditioning and various sludge loading rates. It was found that a decrease in sludge loading rate (from 20 to 5 kg DS m^-2) resulted in an increase in the maximum current density (145-467 A m^-2). The principle component analysis (PCA) showed a significant correlation between the dry solid (DS) content in the final sludge cake, sludge loading rate, freezing conditions, energy consumption and maximum current density during electro-dewatering process. The decrease in sludge loading rate resulted in the reduced time to achieve the highest concentrations of Na⁺ and K⁺ in the removed water at the cathode. Moreover, concentration of Na⁺ and K⁺ was reduced by 51 and 78% in the sludge cake, respectively, in comparison to blank experiments. Fe ions, Ca²⁺ and Mg²⁺ concentrations were found lower in the sludge cake at the anode and higher at the cathode. According to the statistical analysis, Fe and Ca ion concentrations at the anode and sludge loading rate had a negative correlation with the volatile solids/dry solids (VS/DS) ratio in sludge at the anode. High P concentration at the anode was only observed in experiments using freeze/thaw conditioned sludge samples and highly depended on the initial sludge freezing temperatures. Furthermore, at the end of experiments, concentration of problematic elements in the sludge cake such as Cr⁶⁺, Zn²⁺ and Mn ions increased from 63 to 100% and decreased from 23 to 70% at the anode, respectively, in comparison to the blank experiment.