Influence of salt, pH and polyelectrolyte on the pressure electro-dewatering of sewage sludge

This paper deals with the influence of pH, salt and polyelectrolytes on the electro-dewatering (EOD) of agro-industrial sludge at 3% w/w of dry matter. Initially, a selection of polyelectrolyte types and doses was carried out for mechanical dewatering tests. Subsequent EOD tests were carried out in a laboratory two sided filter press at constant electric current density of 80 A/m2 and at pressure of 5 bar. It was found that whatever was the initial value of pH, salt content or polyelectrolyte type, the EOD progressed always towards the same equilibrium point at around 50% w/w of dry matter. EOD rate and energy input was not affected by the presence of polyelectrolyte whatever was its charge density and molecular weight. However, EOD rate and specific energy consumption and repartition of liquid at anode and cathode sides were strongly influenced by the salt content (adjusted by Na2SO4) or by the initial pH (adjusted with H2SO4 or NaOH). EOD performed better at lower salt content and at slightly acid pH. In optimum conditions, the process (EOD) required 2 h to reach dry matter of 40% w/w with specific energy consumption of 0.25 kWh/kg of water removed for the treatment of conditioned sludge. For comparison, compression without electric field at 5 bar required 11 h to reach 22% w/w of dry matter. This work emphasizes and demonstrates that the electrolytic hydroxide and hydronium ions formed at the electrodes have considerable influence in the course of EOD.