Effect of ultrasound on particle surface charge and filterability during sludge anaerobic digestion

The goal of this study was to follow and compare dewaterability parameters of untreated and “pre-sonicated” sludge during semicontinuous anaerobic digestion by particle charge density and sludge filterability measurements. Floc disintegration due to sonication resulted in a significant increase of soluble COD and colloidal charge density. It is interesting to note that in all the tests despite the higher specific charge density and soluble COD values of the sonicated feed, the digested sludge at steady state present in both reactors statistically comparable values, independently on the pretreatment. The biological hydrolysis of the untreated sludge causes a large release of dispersed charged fines whereas the digestion of the sonicated sludge is characterized by a significant removal of fines and colloids already present in the pretreated feed. The amelioration of the filterability during the digestion of the sonicated sludge in test #2 suggest that anaerobic digestion may be very effective for degrading fine dispersed particles produced by mechanical pre-hydrolysis. Nevertheless, ultrasound pre-treatment does not improve the dewaterability of the digested sludge with respect to the untreated one. Optimisation of both disintegration degree and hydraulic residence time is necessary to improve the dewaterability of the digested sonicated sludge.