Activated sludge is known to poorly dewater due to its high surface charge density and the extreme solids compressibility,even after polyelectrolyte conditioning.The application of an electric field during pressure dewatering of sludge can enhance the dewaterability by the electroosmosis effect.A comparative study was conducted to investigate the additional effect of an electric field,applied during the expression phase,on the dewatering course of polyelectrolyte conditioned sludge compared to mere pressure dewatering.It was found that application of the electric field markedly improved the dewatering kinetics for all sludge samples,regardless of the conditioning treatment.By means of on-line streaming potential measurements and fractionated filtrate electrophoretic mobility measurements,it could be demonstrated that even at high polyelectrolyte doses,leading to positively charged sludge floes,negative surface charges were still present inside the sludge matrix.During expression of the sludge cake,when liquid is forced to move through the floe inside pores,these negative surface charges hampered pressure dewatering,but enhanced electroosmotic dewatering.Electroosmosis is therefore an appropriate technique to remove the water fraction that is associated with these negative surface charges.
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