Effect of electrolytes content on the electro-osmotic dewatering of agro-industrial sludge

The agro-industrial sludge, tested in this work, comes from a biscuit factory (France). Initially, it contains 3 % w/w of dry matter. The actual treatment by centrifugation dewaters it to 13 % w/w of dry matter. Electroosmotic dewatering under pressure (EOD) was investigated, as an alternative technology of mechanical treatment, to improve dewatering. This electrical treatment efficiency depends from the surface electrical charge of the sludge particles. Because, the electrolyte content may influence the particles electrical charge, this study aims at determining the effect of salt content and pH modifications on the biscuit factory sludge dehydration by EOD. The EOD under pressure carried out for 3 hours, leads to a conditioned sludge dry matter content of 40 % w/w for an electricity consumption of 0.2-0.3 kWh/kg of water removed, while a mechanical dewatering without electric field application carried out for 11 hours leads to a sludge dry matter of only 22 % w/w. The DC electric field generates the creation of an electroosmosis unidirectional flow toward the cathode. The increase of salt content was detrimental to EOD rate and started to be prejudicial for the specific energy consumption (in kWh/kg of water removed) above 8-15 mS/cm. This work emphasises that the electrolytic ions hydroxide and hydronium formed by electrochemical reaction at the electrodes have considerable influence in the course of EOD. These ions can change the charge of particles at anode and cathode and thus the intensity of the electroosmotic flux.